1
Introduction
Carbon nanotubes (CNTs)
are structurally described as sheets of
six-membered carbon atom rings (i.e., graphene) rolled up into cylinders.
CNTs with only one layer are known as single-walled CNTs (SWCNTs),
and those with two or more layers are known as multiwalled CNTs (MWCNTs).
Cup-stacked carbon nanotubes and carbon nanohorns are also sometimes
called CNTs.
1−3
Currently, these very attractive carbon materials
and nanomaterials are a subject of vigorous product development in
a broad range of fields.
4−11
The reasons are that CNTs have useful electrical, thermal, and mechanical
characteristics, and their base material performance can be improved
by combination with other materials.
12−23
A recent industrial application of CNTs as an electrode additive
to lithium-ion batteries is based on their excellent electrical characteristics.
Addition of CNTs prevents battery deterioration and substantially
lengthens time to recharging. It is doubtless that the demand for
high-performance batteries will grow increasingly with multifunctionalization
of personal computers and mobile phones, development of new mobile
terminals, spread of electric vehicles, and other factors.
24−30
Composite materials with the excellent mechanical characteristics
of CNTs have already been used in sporting goods such as golf clubs,
tennis rackets, and bicycles. CNTs are also expected to have applications
that reduce the weight of aircraft and automobiles.
10,14,31−35
A wide variety of advantages are gained from the
use CNTs in precision parts as well. CNTs are also used in transistors
and memory devices, and enhance their efficiency. The use of CNTs
in various displays and TV screens continues to increase in rate.
CNTs are also widely used in products designed to prevent static electricity,
to shield electromagnetic waves, to store electricity, and for other
purposes.
36−45
Furthermore, Japan is now facing nuclear energy issues stemming
from the accident at Tokyo Electric Power Company’s Fukushima
No. 1 nuclear power plant. As a result, CNTs are expected to play
a major role in developing new energy sources such as solar photovoltaic
power generation and wind power generation.
46−52
In the medical field, extensive research activities are underway
to develop new CNTs biomaterials for use in the treatment and diagnosis
of disease. For example, application of CNTs to cancer treatment and
diagnosis, such as in drug delivery systems (DDSs) for treatment of
cancer, hyperthermia, and in vivo imaging, has been investigated.
53−57
In a study that aimed at applying CNTs to regenerative medicine,
CNTs were found to work excellently as scaffold materials for nerve
and bone tissue regeneration.
58−63
Furthermore, R&D activities are underway to improve the mechanical
strength and durability of implants by combining CNTs with existing
biomaterials.
64−67
Besides, numerous ideas have been put forth about how CNTs can be
used in the treatment of a variety of diseases.
Figure 1 shows the trend in the number of
articles found in the PubMed database (http://www.ncbi.nlm.nih.gov/pubmed/) (accessed
20 March 2014) by searches using “carbon nanotubes”
and “biomaterials” as keywords. The number has been
soaring since 2005, suggesting that CNTs research has become a highly
competitive field worldwide over the past few years. Of course, numerous
articles on the biological applications of CNTs do exist that cannot
be captured with these two simple keywords, and the graphic representation
of this trend is no more than an indicator of the increase in this
research over time.
Figure 1
Time trends for the number of articles found in the PubMed
database
(http://www.ncbi.nlm.nih.gov/pubmed/) (accessed 20 March
2014) by search using “carbon nanotubes” and “biomaterials”
as keywords. Recent years have seen a rapidly increasing number of
research articles on the application of CNTs to biomaterials; the
number has been soaring since 2005, suggesting that the application
of CNTs to biomaterials has become a highly competitive research field
worldwide over the past few years. This graph indicates only a time
course, and numerous articles on biological applications of CNTs do
exist that cannot be captured with these two keywords.
One reason for the intense competition to find
biomaterial applications
of CNTs and for the great potential of CNTs to advance medical care
is their small size (nanometers in diameter and micrometers in length),
which makes them suitable to react with living organisms.
68−70
Hence, the size of CNTs, which is at the cell organelle level, is
likely to facilitate their effect on living cells. Specifically, CNTs
are similar in thickness and length to microtubules, which make up
the cytoskeleton and mediate a wide variety of cellular activities
such as motor protein activity.
71
Biomaterials
containing CNTs of such size make reactions with cells more controllable
and make treatment and diagnosis that focus on target cells more feasible,
accurate, and less invasive to living organisms than conventional
approaches. The second benefit from biologically applying CNTs is
the ease with which they bind to a broad range of molecules thanks
to the extremely high reactivity of CNT surfaces.
72,73
CNTs serve as a platform for binding multiple molecular entities
such as drugs for therapeutic purposes, marker molecules, cell-binding
molecules, and molecules facilitating the transfer of drugs to target
tissues. Thus, CNTs can facilitate the diagnosis and treatment of
diseases by facilitating substance recognition, adhesion, and affection
to target cells. This potential is expected to lead to a groundbreaking
new technology with applications to cancer treatment and regenerative
medicine. In the near future, it is more likely that CNTs will be
used as biomaterials for treatment and diagnosis of various diseases
than for industrial purposes such as in batteries and aircraft. CNTs
are of paramount importance to future advances in medical care. On
the other hand, the small size and high surface reactivity of CNTs,
properties that underlie their advantage as biomaterials, can adversely
affect the human body. CNTs have not yet been used clinically (despite
the dramatically increasing amount of research into biomaterial applications
worldwide) because of safety concerns associated with implantation
of CNTs devices in the body.
74−77
Currently, the safety of CNTs (primarily the safety
of inhaled CNTs) is being investigated throughout the world.
78−83
Inhalation is the most likely route of external exposure of the
human body to CNTs used in industrial products, so that inhalation
toxicity must be determined first. It should be noted, however, that
the safety profile of CNTs as biomaterials differs completely from
that of inhaled CNTs.
68,69,84
Part of the safety evaluation of CNTs for biomaterial application,
unlike that for inhalation, must include studies of the biological
toxicity of implants in vivo. In many cases, biomaterial-specific
studies must include implant toxicity, cytotoxicity, carcinogenicity,
and genotoxicity studies. The safety of CNTs must be confirmed in
these toxicity studies before they can be used in biomaterials. For
this reason, the number of reports on the safety of biomaterials containing
CNTs has been increasing.
54,68,85−89
Although these reports have demonstrated the safety of these biomaterials,
researchers are still unable to reach a definitive conclusion. This
is because CNTs are essentially nanoparticles, and biomaterials containing
CNTs do not fall within the scope of biomaterials as traditionally
conceptualized.
68,90
Of course, CNTs are not a drug,
any other chemical substance, bulk material (as used herein, the term
bulk material/biomaterial refers to a nonparticulate bulky material/biomaterial),
or biodegradable material currently in use. Nanosized particulate
substances lacking high biodegradability have not been used in the
medical care field so far. Because of the nanosize of CNTs, many toxicity
factors associated with nanosize will need to be investigated. Factors
likely to impact the toxicity of CNTs and living organisms include
thickness, length, specific surface area, and surface chemistry, as
well as types of chemical modifications, defects in CNTs, and catalyst
left unconsumed in the manufacturing process.
72
Factors affecting the administration of CNTs to living organisms
include choice of dispersant, dispersant concentration, method of
in vivo exposure, and duration of in vivo exposure. Furthermore, organ
specificity, cell specificity, types and incidences of biologically
adverse events, in vivo distribution, and other factors must be examined.
91
Collectively, these facts seem to suggest that
developing biomaterial applications of CNTs will be difficult. Thus,
absolutely no clinical applications have been found to date despite
the rapid increase in the number of research articles dealing with
CNT biomaterials.
10,77
However, inasmuch as applying
CNTs biomaterials has potentially great benefits, the research must
continue. Now is the time to review the present status based on available
safety evaluation studies, to identify and resolve issues, and to
implement clinical applications. Essentially, the human body consists
principally of water and organic molecules, so life can be described
as being supported by carbon.
92
To date,
no problems have been reported from the use of materials consisting
of ultrapure carbon, such as pyrolytic carbon used in artificial heart
valves, carbon fibers used in Achilles tendon sutures, and the amorphous
diamonds used in artificial finger joints.
93−96
When reviewing, in detail,
research articles by a great many researchers,
it is evident that the major problem with the development of biomaterial
applications of CNTs has been the lack of a particulate substance
to serve as a biological safety reference material, and hence the
inability to establish criteria for evaluating biological safety.
This critical issue was first pointed out in 2009 by Auffan et al.
in Nature Nanotechnology,
68
and no appropriate reference has since been found. We consider that
nanosized highly pure carbon black particles are suitable as a reference
material for safety evaluation of CNTs.
97,98
This is because
no safety issues have appeared in the vast number of people who have
black tattoos, containing principally nanosized highly pure carbon
black. The use of carbon black as a reference is described in detail
in section 5. Provided that same reference
is used to conduct multifaceted extensive toxicity studies and provided
that international standards of safety evaluation are established,
it will be possible to apply CNT biomaterials in a wide range of clinical
settings in the near future.
This Review covers many recent
studies on biomaterial applications
of CNTs mainly published between 2005 and 2013, and gives an outline
of our published studies with new references. First, the findings
in these studies are comprehensively discussed to evaluate the safety
of CNTs as biomaterials. The way to realize safe clinical application
of CNT-based biomaterials in the future is then proposed clearly.
The challenge must always be kept in mind. Making the best use of
all talents and abilities of researchers worldwide, this research
will lead to a major revolution in the medical care field and benefit
patients greatly. In this Review, we submit a proposal of paramount
importance that we think will be the key to accomplishing this significant
goal.
2
Present Status of Research into the Application
of CNTs as Biomaterials
As is evident from the recent increase in the number of relevant
articles, research into application of CNTs as biomaterials is advancing
rapidly (Figure 1). CNTs have applications
to a broad range of fields, many of which, in addition, have top priorities
in clinical medicine today (Figure 2).
84,99−101
This section divides these applications
into five categories: cancer treatment, regenerative medicine, implants,
DDSs for noncancer targets, and other applications. Notably, many
technologies utilizing CNTs are applicable to more than one of these
fields. For example, the technology using CNTs as anticancer agent
delivery systems is also useful for drug delivery systems targeting
noncancer diseases. The technology for combining CNTs with other biomaterials
is the key to successful application in new highly functional implants
and in scaffolds used in regenerative medicine. Hence, this classification
system was chosen only because it facilitates organization of the
various published reports. In the future, classifying the studies
on CNTs biomaterials with a focus on important technologies for their
biological applications would be even more useful and expected to
accelerate advances in relevant research.
Figure 2
Biological applications
of CNTs encompass a broad range of fields,
many of which, in addition, represent themes of top priority in today’s
clinical medicine, such as cancer treatment and regenerative medicine.
Modified from ref (84), which is published under the Creative Commons Attribution
License.
All studies of biological applications
reviewed below highlight
at least one benefit of CNTs biomaterials, so these benefits are described
below. The importance of these benefits has stimulated the rapid emergence
and evolution of much research.
69
2.1
Benefits from Application of CNTs as Biomaterials
The
first benefit comes from the small size of CNTs. Although this
benefit may have a negative impact on safety, it by far outweighs
the possible risk. The following six capabilities can be attributed
to the small size of CNTs:
(1)
Reacting with cells by entering the
cells or adhering to cell surfaces
(2)
Acting on biological macromolecules
and cell organelles of similar size
(3)
Acting on parts of the body with fine
structures
(4)
Distributed
via the bloodstream after
intravenous injection and the like; thus they may be used in targeted
drug delivery systems and in vivo imaging
(5)
Rapidly eliminated from the body
(6)
Having effects when combined with
other biomaterials, for example, on fine structures to increase their
mechanical strength
Because capabilities
(4) and (5) assume that CNTs circulate
in the bloodstream, the possibility that the risk of accumulation
in particular organs and leading undesirable reactions to the organ
outweighs the benefits must be taken into account. It is necessary
to make the best use of these advantages, while minimizing the disadvantages.
This is also true for other nanobiomaterials currently under investigation.
Interactions between nanosized substances and living organisms will
be further elucidated in the future. Nanobiomaterials are going to
occupy an important position in nanomedicine, a research field that
has only recently been established.
102−105
The second benefit is the ease
of chemical modification. CNTs, because of their macromolecular size,
have high chemical reactivity.
106
It is
likely that the CNTs used in biological applications will be functionalized-CNTs
(f-CNTs). When used as particles, rather than as a composite material,
CNTs are likely to be f-CNTs.
73
CNTs can
serve as a platform for concurrent binding of drugs, peptides, high
molecular polymers, and other molecules that otherwise cannot be bound
to each other (Figure 3).
107−112
Thus, it would be possible to construct CNTs with multiple functions
that have not traditionally been co-occurrent, such as drug transport,
cell adhesion, biomembrane transport, and release at targeted sites.
For example, CNTs coupled with an anticancer agent and monoclonal
antibody can be used to target cancer cells.
113,114
Figure 3
CNTs
are capable of working as a platform for concurrently binding
drugs such as anticancer agents, proteins, and peptides such as monoclonal
antibodies, high molecular polymers, and other molecules that otherwise
cannot be bound to each other. Making the best use of this feature,
it would be possible to concurrently add to CNTs multiple functions
that have traditionally been unable to concur, such as drug transportation,
biomembrane passage, and release at targeted sites.
There are two types of interactions with CNT surfaces:
those based
on covalent bonds and those based on noncovalent bonds. Of course,
covalently bound substances (in contrast to noncovalently bound substances)
are unlikely to dissociate from CNTs, so the appropriate method of
binding must be chosen according to the target site and intended use.
CNTs synthesized using the chemical vapor deposition (CVD) technique
have open ends to which chemical modifiers can be bound specifically.
24
More interestingly, it is possible to transport
molecules, atoms, etc., that have been inserted into the cylindrical
hollow structure unique to CNTs. CNTs with such chemical modifications
are called peapods because of their shape.
115,116
As such, CNT peapods can transport drugs in encapsulated form, and
are expected to be increasingly investigated because of their potential
application as DDSs and in vivo imaging.
117−119
The third benefit derives from the chemical composition of
CNTs,
which is very pure carbon. Carbon has already been used in many implant
devices, including artificial heart valves, and no adverse effect
of such biomaterials on living organisms has been reported to date.
96
The following features of CNTs may be regarded
as advantages:
(1)
High biocompatibility
(2)
High strength-to-weight ratio
(3)
High tensile strength
(4)
Forming flexible nanofibers
(5)
High chemical reactivity
(6)
Conferring increased strength and
other favorable characteristics to other substances when combined
with them
(7)
Inducing
slow but significant biodegradation
(8)
Colored in black that is easily distinguishable
and detectable using a light microscope
The fourth benefit is the excellent electrical, magnetic, and thermal
characteristics of CNTs in biomaterials. In fact, studies have used
CNTs (because of their electrical characteristics) for nerve regeneration
112,120−122
and muscle actuation,
123,124
and (because of their magnetic characteristics) for cancer treatment
and DDSs.
55,125
Furthermore, CNTs (because of
their high photoenergy absorption capacity and thermal conductivity)
have been proven effective for cancer thermotherapy.
56,107,126−128
As stated above, CNTs (unlike conventional materials) can
serve
a wide variety of functions in tissues and cells of living organisms.
This great potential has stimulated research into the application
of CNTs as biomaterials in many fields. Overall, CNTs can be viewed
as a revolutionary tool that will advance the practice of medicine,
imposing expectations that biomaterials will be the main field of
application of CNTs.
2.2
Application to Cancer Treatment
Currently,
the most vigorously studied application of CNTs biomaterials is to
cancer treatment. A wide variety of methods have been used to treat
various cancers.
55,129−133
Detection of foci as early as possible and administration of an
effective treatment are of paramount importance in cancer treatment.
CNTs are expected to lead to innovative therapeutic and diagnostic
methods. Although many other ongoing studies are not included, the
following is an overview of the applications of CNTs to cancer treatment
that are currently attracting much attention. Thus, clinical application
of CNTs is a very promising field of study.
2.2.1
Biomarkers
and Imaging
There have
been recent dramatic technical improvements in methodology for the
early diagnosis of cancer, with remarkable advances being made in
tumor marker tests and the diagnostic imaging of cancer. Even now,
however, it is difficult to detect early asymptomatic cancer; cancer
is often detected only in the terminal stage. Against this background,
studies have been conducted to detect the expression of biomolecules
in the initial stage of cancer using CNTs as biomarker detectors.
The application of CNTs to the detection of a prostate cancer marker
(PSA), colorectal cancer markers (CEA, CA19-9), and a hepatocarcinoma
marker (AFP) has been reported.
134−137
Applicability is based on the
small size of CNTs that facilitates distribution in living organisms,
and some evidence showing direct detection of biomarkers in vivo has
been reported.
138−141
CNTs have been used in noninvasive imaging, including for
highly sensitive detection of very small tumors, using single CNT
molecules conjugated to contrast reagent for CT or MRI, a heavy metal
(gadolinium, etc.), and an antibody with high affinity for cancer
cells.
142−145
A study is also ongoing that examines the application of a heavy
metal encapsulated by the aforementioned peapod CNT to cancer imaging.
146
The most investigated imaging application is
MR molecular imaging, which is effective in early detection of cancer.
Furthermore, studies on the use of CNTs for photoacoustic molecular
imaging show that it enhances contrast and resolution necessary to
in vivo imaging. High resolution using a blend of SWCNTs and fluorescent
peptide as the contrast medium for photoacoustic imaging were obtained.
147
Tumor vascularization plays an important role
in cancer development and metastasis. For this reason, noninvasive
detection of vascularization activity is critical to cancer diagnosis
and assessment of patient responses to cancer treatment. A wide variety
of molecular targets relevant to tumor vascularization have been identified,
and can be used for tumor vasculature targeting and imaging. A method
of optical imaging using a new photoprobe with the optical properties
of CNTs has been developed to facilitate visualization of vascularization
events.
148,149
2.2.2
Drug Delivery Systems
for Cancer Treatment
Of the biological applications of CNTs,
DDSs for cancer treatment
have been the most vigorously investigated. In cancer chemotherapy,
adverse drug reactions are problematic, sometimes making it difficult
to deliver adequate amounts of drugs to target organs. Because of
their very large specific surface area that can bind many molecules
beneficial to cancer treatment, CNTs can be used for DDSs in cancer
treatment
89,129,150,151
and have been used as a platform
to facilitate targeted delivery of a drug, antibody, other protein
or peptide, lipid, polysaccharide, etc. (Figure 3). For example, a highly efficient
missile therapy consisting of
a combination of a hydrophilic group, a monoclonal antibody to cancer
cells, an anticancer agent, and other components has been reported.
117
Using a nanoscale vehicle such as CNTs, drugs
can be delivered to cancer cells that could not otherwise be delivered
by microscale vehicles.
152
This is because
thus-functionalized CNTs can pass through the cell membrane via a
mechanism for the cellular uptake of foreign substances, such as endocytosis.
CNTs with attached peptides or ligand bind to specific receptors on
the cancer cell surface, and enter the cancer cells where they release
the therapeutic agent more safely and efficiently. A DDS can be described
as ideal when it delivers the needed amounts of therapeutic agent
to the target in a timely manner, and CNT-based DDSs have the potential
to fulfill this requirement.
132,153,154
SWCNTs coupled with a tumor-specific monoclonal anti-CD20
antibody (rituximab) intravenously injected into mice after intramedullary
transplantation of a human B-cell lymphoma resulted in accumulation
of SWCNTs in the lymphoma.
110,155
Other researchers
attached a tumor-recognizing module to the surface of hydrophilic
f-SWCNTs to specifically bond with cancer cells, and then a prodrug
module of an anticancer agent (a taxoid with a cleavable linker) to
the surface of hydrophilic f-SWCNTs. They showed that the cytotoxicity
of this tumor-targeting DDS is mediated via intracellular migration,
drug release, and intracellular activation.
153
Moreover, the application of CNTs to gene therapy (i.e., as carriers
of genes to targeted cancer cells) has been studied.
156−160
Because CNTs-based platforms are infinitely variable and easily
designable, they are expected to lead to groundbreaking cancer treatment
systems.
Before thus applying CNTs for DDSs, their pharmacokinetics
after
topical or intravenous injection must be clarified. The disposition
of intravenously injected CNT–drug composite has been examined
extensively.
105,161−165
Factors that influence transport of the composite through the bloodstream
include thickness, length, and flexibility of CNTs as well as changes
in properties resulting from the binding of the drug. Of course, injection
of CNTs-based DDS into the tumor site directly is a safer approach.
Furthermore, the use of magnetized particles to facilitate efficient
uptake of CNTs in cancer tissue has been studied. For example, treatment
of lymph node metastasis by subjecting magnetic functionalized CNTs
to a magnetic field to promote their migration to lymph nodes has
been studied.
166,167
Treatment with gemcitabine (GEM)-loaded
magnetic functionalized CNTs subjected to a magnetic field resulted
in regression of lymph node metastasis and suppression of metastatic
growth both in vitro and in vivo.
55
In
addition, many anticancer agent-loaded CNTs-based nanoscale DDSs have
been developed.
101,128,168−170
2.2.3
Cancer Treatment Using
External Energy
CNTs absorb electromagnetic wave energy.
On the basis of this property,
the use of CNTs in cancer hyperthermia has been tested.
53,171−174
For example, cancer lesions were exposed to CNTs loaded with a tumor-specific
epitope (to be absorbed selectively), then to infrared rays, and cancer
tissue was specifically destroyed by the heat generated.
107
Another report showed the method for treating
peritoneal metastases from colorectal cancer consisted of rapidly
heating the cancer mass to 42 °C within 10 s in the presence
of oxaliplatin or mitomycin C using infrared rays absorbed by CNTs.
175
In a recently reported study, the generation
of heat and reactive oxygen species generated upon exposure of CNTs
to infrared rays for 10 min was harmful to human lung cancer cells.
Specifically, 45% of the cancer cells had been killed 24 h later.
56
The microwave absorption characteristic of CNTs
theoretically permits accurate heating; microwave thermotherapy for
cancer treatment is also a promising technology.
173
Meanwhile, various improvements have been made in
the targeting methods. Using anti-CD22 antibody coupled with SWCNTs
followed by exposure to laser radiation succeeded in shrinking B cell
lymphoma.
176
A study proposed that cancer
cells could be destroyed using bubbles generated by administering
CNTs and ethanol and exposing the cancer cells to laser light.
177
Recently, a nanosecond pulse electrical field
was used to kill the pancreatic cancer cell line PANC1 in the presence
of MWCNTs and resulted in a 2.3-fold reduction in cell survival as
compared to control cells.
178
In
other studies, effect of thermotherapy was mediated through
a CNT/DNA/IgG antibody composite bound to target cancer cells,
179
and the effectiveness of a CNTs/polyethylenimine/siRNA
composite was attributable to RNA interference and photothermal therapy.
128
Furthermore, cancer imaging and thermotherapy
was carried out concurrently by conjugating quantum dots to CNTs.
180
The variety of CNT applications has been increasing.
CNT peapods encapsulating iron nanoparticles and a chemical modification
that facilitates binding to cancer cells have been used in cancer
thermotherapy. The iron in the CNTs is highly biocompatible because
it is protected from reacting with the ambient environment, and the
electromagnetic wave thermotherapy is safe and effective.
118
In conclusion, investigations of thermotherapy
with CNT adducts of other materials are ongoing.
These cancer
treatments based on the ability of CNTs to absorb
external energy cannot be clinically applied before methods of electromagnetic
wave exposure are investigated. This is because the body rapidly absorbs
the energy. In the case of simple exposure, the utility of CNTs is
limited to accessible cancers. However, when used in combination with
an implanted energy source, the utility of CNTs extends to deep cancers.
171,181,182
Cancer thermotherapy involving
the clinical application of CNTs is currently a rapidly growing field
of research.
2.3
Application to Regenerative
Medicine
The aim of regenerative medicine is repair and regeneration
of human
body tissues and organs affected or lost because of disease, trauma,
and the like. Developments in embryonic stem cell (ES cell) research
and the development of induced pluripotent stem cells (iPS cells)
in 2007 further stimulated regenerative medicine research.
183,184
Tissue regenerative therapies use cells, growth factors, genes,
etc. Whichever means is used, no tissue can be regenerated without
a scaffold. Thus, the scaffold is of paramount importance in therapy,
and research aimed at developing CNTs as scaffold material has been
increasing.
185−191
2.3.1
Studies Assessing the Applicability of CNT
Composites to Regenerative Medicine
The use of CNT composites
in regenerative medicine has been vigorously investigated in vitro.
Results showed that a CNT/collagen composite could be used as a scaffold
for myocyte culture, and that a CNT/polyurethane composite could be
used as a scaffold for fibroblasts growth and biosynthesis.
192−194
A CNT/polyurethane composite used as a scaffold for culturing vascular
endothelial cells was effective in promoting their proliferation and
suppressing thrombus formation.
195
A CNT/poly l-lactic acid/hydroxyapatite composite increased the adhesion
and proliferation of periodontal ligament cells (PDLCs) by 30%.
196
Regenerated silk fibroin films incorporating
MWCNTs were shown to support the adhesion and growth of human bone
marrow stem cells.
197
SWCNTs nonwoven films
enhanced long-term proliferation of many cell types.
198
While in vitro studies examining the reactions between
cells and CNT composites used as scaffolds are numerous, there are
few in vivo studies.
125,174,185−188,190,199,200
It is hoped that in vivo animal
experiments based on in vitro findings will be carried out in the
future. The application of CNTs to bone tissue regeneration and nerve
tissue regeneration is of paramount interest.
2.3.2
Bone Tissue Regeneration
Regarding
bone tissue regeneration, a CNT/polylactic acid composite was shown
to promote osteoblast proliferation in vitro as early as in 2002.
58,201
Later, a CNT/polycarbonate urethane composite and a CNT/poly lactic-co-glycolic acid
composite were reported to enhance the
adhesion of osteoblasts.
202−204
In 2006, a study showed that
SWCNTs and MWCNTs promoted the proliferation of osteocytes and osteoblasts
when used alone.
205
This was followed by
in vitro studies showing the wonderful effects of CNTs on bone-related
cells.
66,186,206−212
In 2008, we showed for the first time that CNTs promote bone
tissue formation in vivo as well.
213
The
study employed an experimental system that used recombinant bone morphogenetic
protein-2 (rhBMP-2) to induce ectopic osteogenesis in mouse back muscle.
214
Bone formation on a collagen sheet was shown
to occur earlier in the presence rhBMP-2 attached to a scaffold of
MWCNTs than in the presence of rhBMP-2 alone (Figure 4). Later, other researchers
confirmed that osteogenesis was
promoted by CNTs in vivo. For example, a layer-by-layer assembled
carbon nanotube composite promoted osteogenesis and bone repair when
implanted in rat calvarial bone defects.
215
Carbon nanohorns, a type of CNT, were attached to a porous polytetrafluoroethylene
membrane by vacuum filtration, and rat calvarial bone defects were
covered with the membrane. The extent of osteogenesis was greater
under the membrane containing carbon nanohorns than under the membrane
without the carbon nanohorns, showing that carbon nanohorns accelerated
bone regeneration.
216
Figure 4
MWCNTs promote ectopic
osteogenesis by rhBMP-2 and collagen. (a)
A soft X-ray radiogram of newly formed bones extirpated 2 weeks after
placement of rhBMP-2/collagen/MWCNT composite (upper lane) or rhBMP-2/collagen
composite (lower lane) in mouse back muscle. Larger bones with more
intense opacity were formed when using collagen conjugated with MWCNTs
than without. (b) Bone mineral contents (BMCs) in bones formed at
2 weeks of implantation. A significantly higher BMC was observed in
bones formed at 2 weeks of implantation of collagen conjugated with
MWCNTs than without. Each error bar indicates the standard deviation
of the mean (n = 8); asterisk, P = 0.016 between samples treated with carbon nanotubes
and those
without (unpaired Student’s t test). (c) Histological
images of bones extirpated at 2 weeks. The trabecula was thicker and
denser when using collagen conjugated with MWCNTs than collagen alone.
The tissue around the implanted collagen–MWCNT conjugate was
found to have MWCNTs absorbed uniformly in the trabecula and bone
marrow. The MWCNTs were seen to have entered the trabecula and came
in direct contact with bone substrate. Hematoxylin-eosin staining.
Scale bars = 100 mm. Reprinted with permission from ref (213). Copyright 2008 John
Wiley & Sons, Inc.
Later, we attempted to elucidate the mechanism underlying
promotion
of bone tissue regeneration by CNTs. In 2009, we showed that CNTs
specifically suppressed the differentiation of osteoclasts as well
as expression of the transcription factor NFκB in osteoclasts.
217
In 2011, we showed that CNTs could serve as
the seed material for the crystallization of hydroxyapatite, the major
component of bone, and that CNTs attracted Ca ions and activated osteoblasts.
Another finding was that this activation was accompanied by the deposition
of hydroxyapatite around the CNTs, which was catalyzed by alkaline
phosphatase (ALP) released from osteoblasts.
218
These findings demonstrated that CNTs functioning as a scaffold
interact with the body to promote osteogenesis and thereby the process
of bone tissue regeneration. To date, no other scaffold has interacted
with the body in this way; CNTs are expected to be breakthrough materials
in regenerative medicine research as well.
2.3.3
Nerve
Tissue Regeneration
Currently,
brain injuries, spinal cord injuries, and large-gap peripheral nerve
defects are intractable, and their treatment is an important goal
of regenerative medicine. To enhance and stimulate the regeneration
of these injured nerve cells and fibers, application of a wide variety
of nerve conduits and synthetic guidance devices has been attempted
but has failed to yield satisfactory results.
219
Applying CNTs is expected to lead to the development of
new methods of nerve regenerative medicine and contribute to improvements
in patient quality of life.
122,220−224
Use of CNTs as a scaffold for neural cell growth has been
vigorously studied for more than 10 years and found to be useful for
neural cell adhesion and axonal growth.
117,225−227
CNTs promoted neurite elongation in a wide
variety of cultured neurons.
228−230
CNTs were also reported to aid
regeneration of Schwann cells.
231
Another
study found that CNTs were useful in the differentiation of embryonic
stem cells to nerve cells.
220
In these
studies, electric stimulation was often used to promote neural cell
growth, making the best use of the favorable electroconductivity of
CNTs.
120
Regenerative medicine for nerve
is an interesting research field that aims to apply in combination
the electrical and mechanical properties of CNTs to biomaterials.
2.3.4
Regeneration of Other Tissues
The
application of CNTs to the regenerative medicine of tissues other
than bone and nerves has also been investigated. Cartilage regeneration
was promoted by a composite of CNTs and polycarbonate urethane.
232
Other studies have examined the application
of CNTs to skeletal muscle regeneration
233,234
and heart muscle regeneration. For example, inducing differentiation
of mesenchymal stem cells to cardiomyocyte lineage cells by electrical
stimulation with CNTs was succeeded in vitro, and notably finding
evidence of electrically stimulated cross talk among these cells.
235
CNTs also promoted heart muscle maturity and
altered the electrical characteristics of heart muscle.
236
In the future, CNTs will be used to stimulate
the regeneration of many other tissues and organs. Regenerative medicine
is a field of applied medicine that capitalizes on the unique features
of CNTs such as nanoscale size, large specific surface area, and high
surface reactivity, as well as electroconductivity. Furthermore, unexpected
effects, such as the promotion of osteogenesis resulting from the
interactions of CNTs with the body, may be found in a wide variety
of tissues, so regenerative medicine is quite an interesting field
of applied research.
2.4
Application to Implant
Materials
Implant technologies have been used in many clinical
settings, such
as orthopedic surgery, dental and oral surgery, and craniofacial surgery.
Artificial valves and artificial blood vessels have been used in heart
and other surgeries. These implants are required to possess, in addition
to mechanical characteristics such as strength and durability, high
biological compatibility because they come in direct contact with
living tissue.
237,238
Many types of orthopedic implants,
in particular, have long been used in clinical settings in many patients.
Examples include artificial joints used to treat osteoarthritis and
rheumatoid arthritis, plates and screws used to treat bone fractures,
and cages and rods used for interbody fusion. Hence, many different
materials are used in orthopedic implants.
239−241
Metals are used for bone fracture treatment and in artificial joints,
including stainless steel, titanium alloys, cobalt–chromium,
and tantalum. Ceramics (mostly alumina and zirconia ceramics) are
used in artificial joints and artificial dental pulp. Ultrahigh molecular
weight polyethylene (UHMWPE) is used in the sliding parts of artificial
joints. Polyether ether ketone (PEEK) is often used for interbody
fusion.
Since 2003, we have been working to conjugate CNTs to
polyethylene for use in sliding parts and rotating parts of artificial
joints.
58,69
The sliding parts of a polyethylene artificial
joint wear away with long-term use, leading to the breakage of the
artificial joint and necessitating revision surgery.
242−245
With this in mind, we are developing more durable artificial joints
made of polyethylene and CNTs to reduce the amount of wear loss. The
sliding parts of artificial joints are sometimes made of ceramic instead
of polyethylene. Although ceramics are generally unlikely to wear,
alumina ceramics break easily, and zirconia ceramics are liable to
deform due to phase transition in vivo.
246,247
Hence, we are working to develop a new ceramic material (alumina
ceramics combined with CNTs) that is unlikely to break down and deform.
248,249
Although many difficulties exist, including homogenously blending
CNTs and ceramics, we have already obtained a blend with improved
fracture toughness values. The number of patients undergoing artificial
joint replacement surgery has been increasing each year worldwide;
accordingly, the number of patients undergoing revision surgery is
increasing steadily.
250
Clinical application
of CNT-based artificial joints would dramatically reduce the number
of patients undergoing revision surgery and allow use of artificial
joints by young patients.
Furthermore, we are developing a CNT/PEEK
composite for spinal
fusion cages used in interbody fusion surgery. Spine interbody fusion
cages of PEEK material have already been used clinically; however,
poor bone compatibility poses an obstacle to the bonding of the implant
and bone around it.
251,252
Hence, spine interbody fusion
cages made of a CNT/PEEK composite of high bone compatibility are
being developed by conjugating CNTs to PEEK, thereby utilizing the
bone induction potential of CNTs described in section 2.3.2: Bone Tissue Regeneration.
The development
of these artificial joints and spine interbody fusion cages is further
described in section 6.4.
In these composites,
the CNT content ratio is up to 10 wt %, often
about 5 wt %, with only a small amount of CNTs entering the body.
Furthermore, because they are composite materials, there is little
or no possibility that CNTs (that is particulate) will be directly
exposed to living organisms. For this reason, CNT composites can be
thought to be highly safe, with the reactions between CNT particles
and living organism rarely posing a problem. In view of biological
safety of CNT composites, we believe that the first application of
CNTs should be in implants in the form of composites as described
above.
253−255
Taking into account the above-described
utility of CNTs as a reinforcing
material and their safety as composites, it is expected that a wide
variety of CNT composite implants will be developed in the future.
Although technically difficult, conjugating CNTs to metals and ceramics
would produce great benefits. While this field has so far received
only scant attention, we hope that more R&D effort will be directed
to this field, where CNTs are most likely to find clinical applications.
2.5
Application to DDSs for Treatment of Noncancer
Diseases
As stated in section 2.2.2: Drug Delivery Systems for Cancer Treatment, CNTs have
large specific
surface areas, possess high surface reactivity, and therefore can
be conjugated with a wide variety of molecular species, including
low-molecular-weight compounds, genes, proteins, and vaccines, in
large amounts. In addition, because CNTs can be delivered to the small
structures in living organisms, they are expected to act as an ideal
DDS.
100,117,256−258
Research has recently been advancing rapidly toward the development
of more useful CNT-based DDSs for various diseases. Many improvements
have been made in the reactivity with the cell membrane, which is
particularly important to DDS applications. For example, SWCNTs bound
to an integrin monoclonal antibody were used to enhance their adhesion
to cells.
114
Bonding of a bilayer-forming
lipid to CNT surfaces was used to lessen the influence on the cell
membrane.
54,259
DDSs targeting a wide variety
of diseases other than cancer have also been investigated. Some examples
are described below.
As compared to alginate microspheres alone,
a composite of CNTs and alginate microspheres exhibited improved drug
encapsulation efficiency, resulting in decreased drug leakage. Hence,
the release of theophylline, a drug used to treat respiratory diseases,
was extended, suggesting a potential for application of this composite
to prolong the sustained therapeutic effects of encapsulated drugs.
260
Moreover, a study showed that CNTs successfully
coupled to a therapeutically active molecule could be delivered to
cells of a pathogenic organism.
261−263
In addition, because
of their distinct mechanism of action on resistant strains against
which existing antibiotics are ineffective, CNTs have the potential
to be an innovative therapy.
264
CNTs are
reported to suppress bacterial proliferation.
265−268
Attempts have been made to treat diseases by immune activation or
vaccination with modified CNTs. For example, a neutralizing B cell
epitope conjugated to CNTs induced intensive antipeptide antibody
responses to hand-foot-and-mouth disease virus, suggesting its potential
as an immunotherapy.
269
The use of
CNTs in gene delivery systems is also under investigation.
For example, DNA-wrapped MWCNTs prepared by sonication (because they
are well and stably dispersed by sonication) are likely to have applications
to gene therapy.
256
A composite consisting
of MWCNTs with biomolecules immobilized by the addition of a polyamidoamine
dendrimer was found to be a promising DDS for a wide variety of genes.
270
Regarding antisense therapy, two problems with
antisense nucleic acids, rapid decomposition and poor diffusibility
in the cell membrane, impose limitations on its application to clinical
treatment. When bound to SWCNTs, however, antisense-myc was readily
internalized by HL-60 cells and continued to control intracellular
genes.
271
Furthermore, more than one report
is available on the introduction of short interference RNA (siRNA)
in cells using CNTs as a delivery system.
272−275
According to a 2010 report, the gene transfer efficiency is high
at 95%, with no cytotoxicity observed. In conclusion, research aimed
at the application of CNTs to gene DDSs has increased dramatically.
While their application to gene therapy is expected, CNT-based gene
DDSs may also be an important tool in biological research.
2.6
Other Biological Applications
In
addition to the above-described applications for cancer treatment,
regenerative medicine, implants, and DDSs, CNTs are expected to have
biomaterial application in a wide variety of therapeutic settings.
276
CNTs have a great potential for use as
sensors and actuators in nanomedicine
89
and as sensors and stimulants in nerve tissue. Neuroblastoma NG108
and rat primary peripheral neurons produced high voltage-activated
currents when electrically stimulated through conductive SWCNT films,
demonstrating the electrical coupling of SWCNTs and neurons. This
finding suggests that SWCNTs can be used to effectively control nerve
tissue stimulation.
120
CNTs (because of
their electrical properties) may also serve as muscle actuators or
be directly applied to artificial muscles.
123,277
At present, it is technically impossible to use CNTs as a substitute
for muscles in living organisms, and we hope that these studies will
evolve into research on the application of CNTs as biomaterials. Furthermore,
a DNA actuator based on encapsulated DNA-MWCNT was designed using
a computer.
278
Another potential
application of CNTs is as an in vivo sensor to
measure glucose concentrations in diabetic patients using near-infrared
rays in vivo, bearing in mind that CNTs are capable of controlling
far-infrared luminescence.
279
Hence, specific
biomolecules adsorbed to CNTs and applied to in vivo sensors can be
used to monitor a wide variety of diseases. Application of CNTs to
nanosized devices injected into the body or medical nanorobots for
in vivo implantation
99,280
is also under investigation.
As stated above, the electrical, thermal, and mechanical characteristics
unique to CNTs are expected to give rise to new biomaterials that
do not fall within the scope of existing concepts. Furthermore, CNTs,
when brought into contact with various cells and tissues, may have
unknown in vivo characteristics. Research into application of CNTs
as biomaterials is expected to advance and lead to groundbreaking
therapeutic approaches.
3
Present Status of Research
into the in Vivo
Toxicity of CNTs Used as
Biomaterials
Currently available studies of the in vivo toxicity
of CNTs mostly
concern inhalation toxicity. Research into the toxicity of inhaled
CNTs has been advancing rapidly since the publication of two articles
by Takagi et al. and Poland et al. in 2008; the revelation that intraperitoneal
administration of CNTs causes inflammation and carcinogenesis attracted
worldwide attention.
281,282
These two studies used intraperitoneal
administration as a surrogate for mesothelial tissue reactions to
inhaled CNTs, bearing in mind that mesothelial tissue is present in
both the thoracic and the peritoneal cavities. What was always problematic
in these studies was that the CNTs were fibrous particles of similar
size to asbestos particles.
283−287
It should be noted, however, that the toxicities of CNTs (very pure
carbon particles) and asbestos (a mineral containing a large amount
of impurities) are distinct. CNTs are highly flexible, whereas asbestos
is rigid. Currently, intraperitoneal administration is often used
to explore the mechanism of mesothelioma development and for other
purposes,
80,288,289
and inhalation exposure or intratracheal administration is used
to assess inhalation toxicity.
79,82,290−296
Recently, inhalation exposure studies have shown increasing accuracy,
allowing extensive examination of gene expression in body tissues
and blood after exposure.
297
Following
these many studies, the Organization for Economic Co-operation and
Development (OECD), the U.S. National Institute for Occupational Safety
and Health (NIOSH), the National Institute of Advanced Industrial
Science and Technology (AIST) in Japan, and other organizations have
announced their findings.
298−302
Their reports showed that, as compared to asbestos, CNTs have much
lower inhalation toxicity. The currently projected goal of toxicity
assessment is to determine the threshold level of exposure triggering
inflammation in the lung. In the near future, international criteria
of exposure to inhaled CNTs will be established. Worldwide, the inhalation
toxicity of few other substances has been investigated and discussed.
In the context of production, use, and disposal of industrial products,
CNTs are believed to be handleable, provided that safety measures
based on the latest research findings are fully implemented, and that
any available numerical criteria are met.
303
With respect to inhalation exposure, researchers and manufacturers
of CNT-containing biomaterials should follow the same standards.
As stated in the section 1, the type of
toxicity to the human body differs completely between the inhalation
route and implantation route of exposure. Fewer studies have been
conducted on the in vivo toxicity of CNTs biomaterials than on the
inhalation toxicity of CNTs; however, the number of relevant reports
has recently been increasing.
77,91,191,304
Unfortunately, all of the reported
experiments assessing the in vivo toxicity of CNTs biomaterials lacked
reference materials.
68
Notably, many published
articles have suggested that the toxicity of CNTs biomaterials is
extremely low.
91,191,305,306
3.1
In Vivo
Implantation Studies
This
section reviews articles on implantation toxicity studies of CNTs
as biomaterials. Most reports on local reactions following implantation
of CNTs showed that mild inflammatory reactions occurred immediately
after implant placement but disappeared early. Examples of such research
include a study of subcutaneous implantation of alginate gel bound
to SWCNTs,
307
a study of subcutaneous implantation
of a poly(propylene fumarate) assembly bound to SWCNTs,
308
and a study of subcutaneous implantation of
two MWCNTs with different lengths.
309
None
of these studies found any indication of intense inflammatory reaction.
In our study of subcutaneous implantation of MWCNTs in mice, mild
inflammation persisted for about 1 week, resolved rapidly, and never
turned into chronic inflammation. Histological profiling identified
MWCNTs as phagocytosed by macrophages and remaining at the implantation
site for a long period of time.
58
Studies
of subcutaneously implanted CNTs by other researchers yielded similar
results representing the body’s characteristic reactions to
CNTs.
Although subcutaneous implantation studies are a representative
and convenient method of assessing the general biological compatibility
of biomaterials, it is also necessary to study CNTs biomaterials actually
implanted in organs.
191
We conducted a
bone implantation study of MWCNTs used as scaffolds for bone regeneration
and as biomaterials in contact with bone. After implanting MWCNTs
in bone defects artificially made in mouse tibias, we observed normal
bone repair, with incorporation of MWCNTs particles into repaired
bone substrate. Electron microscopy detected physical bonding of the
bone substrate hydroxyapatite in contact with CNT particles. These
results show that MWCNTs possess an extremely high compatibility for
bone tissue (Figure 5).
213
On the other hand, when SWCNTs and MWCNTs were implanted
in rat gluteal muscle, acute inflammation developed and progressed
to chronic inflammation.
76
Further investigations
will be needed to elucidate CNT–muscle compatibility. A wide
variety of interactions between in vivo implants of CNTs and various
organs can be observed in the bodies of living organisms, making it
possible to elucidate the reaction of living organisms to CNTs bound
to endogenous molecules (e.g., albumin, hemosiderin). We think that
a consensus has now been reached that the inflammatory reactions are
mild and disappear early after subcutaneous implantation. At the next
stage, other sites for clinical application of implants should be
investigated in detail along with the biological reactions at each
site.
Figure 5
MWCNTs exhibiting good bone compatibility as they are absorbed
in repaired bone without interfering with bone repair. (a) A histological
image of a tibia extirpated 4 weeks after surgery for implant of MWCNTs
in a pit drilled in tibial diaphysis after incising the anterior surface
of a mouse leg. Cortical bone and a medullary cavity were normally
formed to the extent of complete bone repair. The MWCNTs were found
to have been absorbed in the newly formed bone tissue and enclosed
in bone substrate. Hematoxylin-eosin staining. Scale bar = 100 mm.
(b) An electron microscopic image of MWCNTs absorbed in repaired bone
tissue at 4 weeks. The MWCNTs were found to be in direct contact with
bone substrate hydroxyapatite. Scale bar = 1 mm. Reprinted with permission
from ref (213). Copyright
2008 John Wiley & Sons, Inc.
3.2
In Vivo Kinetics
When applying CNTs
to biomaterials, it is important to study their in vivo kinetics.
304,310,311
Specifically, it is necessary
to determine whether CNTs circulate through the body via the bloodstream,
whether they accumulate in particular organs, what reactions take
place in the organ, and how they are excreted from the body. Of course,
in vivo kinetics is of direct relevance in DDSs and imaging where
localized accumulation of CNTs and distribution systemically via the
bloodstream is expected. However, CNT composites used as implants
do not enter the circulation, and even CNTs particles used topically
hardly ever enter the bloodstream. It can also be hypothesized that
small CNTs but not large CNTs enter the bloodstream to some extent.
The focus of in vivo kinetic studies has been on inhalation toxicity
rather than on the applicability of CNTs to biomaterials. CNTs adsorbed
to the lungs are thought to enter the bloodstream to some extent because
the lung is the organ responsible for blood gas exchange. Therefore,
it is necessary to examine the disposition of CNTs after they are
inhaled and enter the pulmonary circulation. Some reports are available
on the disposition of intravenously injected CNTs.
86,144,306,312−315
These studies provide valuable information on applications of CNT
biomaterials and topical applications of CNTs both involving their
entry and assumed entry into the bloodstream. Reported studies mostly
found that CNTs entering the bloodstream are nontoxic in individuals
and various organs.
191,310
For example, no sign of toxicity
was registered at least 90 days after intravenous injection of pristine
SWCNTs in mice.
312
No sign of acute toxicity
was registered after intravenous injection of SWCNTs or MWCNTs conjugated
with diethylenetriaminepentaacetic acid (DTPA) in mice.
306
Another study verified the safety of SWCNTs
24 h after intravenous injection.
305
No
toxicity was found in mice 4 weeks after receiving an intravenous
injection.
310
Variable findings have been
reported depending on the sites of accumulation of intravenously injected
CNTs in laboratory animals. Many studies found that most CNTs were
excreted in urine, with only a small amount accumulating in the liver
and spleen.
87,191,316
Intravenous injection studies notably found that both MWCNTs and
SWCNTs were most likely to accumulate in the liver and spleen.
310,317
Because CNTs enter capillaries and remain in various organs, it
can be thought that the liver and spleen, which are rich in blood
vessels, are the most likely organs of CNTs accumulation. The toxicity
of CNTs accumulated in the liver and spleen is thought to be low.
86,305,306,318,319
Other organs where CNTs accumulate
include the lung, urinary bladder, kidney, and gut. Although the doses
used in these experiments are variable, they are often up to 20 μg/kg
body weight. The solution used to disperse and inject CNTs is also
variable, with phosphate buffered saline (PBS) being the most commonly
used solution.
91
Historically, various
techniques for monitoring the migration of
radioisotope-labeled CNTs in the body have been employed in disposition
studies. 13C was used in 2002, followed by 14C.
320,321
In rats injected with 14C-labeled
MWCNTs, the liver accumulated most of the dose, followed by the lung,
spleen, and kidney. The MWCNTs were gradually cleared from these organs,
and quickly eliminated by excretion from the kidney. Analysis of the
in vivo distribution of 125iodine-labeled hydroxylated
SWCNTs showed rapid distribution throughout the body and then excretion
in urine and feces.
322
A study of intravenously
injected SWCNTs modified with 111indium-labeled DTPA and 99mTc-labeled MWCNTs found
that these composites were rapidly
removed from the blood via the kidney. In addition, electron microscopic
examination of collected urine samples containing CNTs showed that
the CNTs remained unchanged.
306,323
14C-Taurine-labeled
MWCNTs were administered via the intravenous route and oral route
using a stomach tube. By 10 min after intravenous administration,
a large amount of 14C-taurine-labeled MWCNTs had accumulated
in the liver, with smaller amounts accumulating in the heart and lung;
however, no accumulation was observed in any other organs. On day
90, retention of MWCNTs was found in the liver only. When administered
through a stomach tube, 14C-taurine-labeled MWCNTs were
detected only in the stomach, small intestine, and large intestine,
with no vascular migration observed. The technique for labeling CNTs
and tracking their migration used in these experiments is also applicable
to disposition studies following in vivo implantation.
310
Other methods of monitoring the disposition
of CNTs have been investigated.
The disposition of SWCNTs (possessing intrinsic Raman spectroscopic
signatures) can be monitored by Raman spectroscopy. Liu et al. quantified
intravenously injected SWCNTs in the blood circulation of mice, and
detected SWCNTs by Raman spectroscopy in various organs and tissues
including gut, feces, kidney, and urinary bladder, and their excretion
via the bile and kidney. Autopsy, histological examination, and blood
biochemistry did not reveal any sign of SWCNTs toxicity in mice.
86
A real-time technique for detecting CNTs in
the circulation uses photoacoustic flow cytometry.
324
Recently, echography was used to visualize CNTs and may
be used in future research into the disposition of CNTs.
139,276
The disposition of CNTs as biomaterials implanted in living
organisms
is a controversial issue, and some articles have suggested that SWCNTs
but not MWCNTs, which have larger diameters, enter the bloodstream.
152,155
While CNTs are mostly phagocytosed by macrophages at many sites
in the body, these macrophages do not return to the bloodstream; therefore,
the hypothesis that macrophages do not transport CNTs into the bloodstream
is convincing.
325
In 2011, CNTs were reported
to migrate from subcutaneous implants to other organs and to be associated
with inflammatory cytokine alterations. According to the report, CNTs
did not accumulate in the liver, spleen, kidney, or heart, and although
their migration to regional lymph nodes was slight, the lymph nodes
remained undamaged. Inflammatory cytokine levels initially rose slightly,
but then returned to their original levels. Accordingly, it was concluded
that CNTs do not affect the immune system.
326
Of course, special caution should be exercised when using CNTs in
particular sites, for example, the heart and lung. Their use in the
ovary and uterus, which lie within the abdominal cavity, should also
be avoided. In cases where CNTs are topically used at other sites,
little enters the bloodstream, and if a very small amount does enter,
no systemic toxicity would be expected. This is the current conclusion.
Conversely, when CNTs are used as DDSs or in imaging (where they
migrate via the bloodstream), SWCNTs may be more suitable than other
composites. In this case, the toxicity and accumulation of SWCNTs
in nontarget organs need to be examined in detail. For this reason,
the first use of CNTs biomaterials should be topical, and their systemic
use should be implemented with extreme caution.
Finally, an
in vitro study on the influence of intravenous CNTs
on microvascular endothelial cells, which serve as a blood–tissue
barrier, showed that CNTs might increase endothelial cell permeability.
The reasons for increased permeability include higher levels of ROS
and reconstitution of actin filaments, with possible involvement of
MCP-1 and ICAM-1.
327
Further research reflecting
these findings in vivo is expected.
3.3
Effects
of Chemical Modifications
In the in vivo implantation studies
and in vivo kinetic studies of
CNTs, attention should be paid to the difference between the body’s
reactions to chemically modified functionalized-CNTs (f-CNTs), which
can be a response to the binding partner molecule, and the body’s
reactions to pristine CNTs.
292,328
CNT is generally chemically
modified by oxidatively destroying a C=C bond in it, attaching
a carboxyl group, and reacting the carboxyl group with another molecular
entity.
91,329
The main purpose of the most commonly performed
chemical modification of CNTs, coupling with polyethylene glycol (PEG),
is to increase their water solubility, and many studies have found
that PEG alters the body’s reactions to CNTs. PEG bound to
CNTs was reported to stimulate immunocytes to produce inflammatory
cytokines.
109,330
A study concluded that the biological
toxicity of chemical modifications of PEG-CNTs is influenced by PEG.
Mice injected with SWCNTs modified by both PEG and another functional
group had higher neutrophil counts than mice injected with SWCNTs
modified by PEG alone.
87
In recent years,
however, an increasing number of studies have shown that bound PEG
reduces harmful effects.
77,331,332
A kinetic study of intravenous SWCNTs found that PEG conjugation
accelerated the removal of SWCNTs from the body.
324
Numerous chemical modifications other than PEGylation can
cause this phenomenon as well as a wide variety of changes in the
distribution of SWCNTs in the body. For example, attachment of paclitaxel
to SWCNTs resulted in increased localization in the gut and liver,
and attachment of rituximab to CNTs increased levels of accumulation
in the liver.
110,333
This observation is attributed
to differences in the affinity for or reactivity with a wide variety
of cell types in various organs depending on the molecule bound to
CNTs. Size of the binding functional group and the type of chemical
modification (whether covalent or noncovalent bond) can also influence
the biological toxicity.
88
Likely
reasons why appropriate f-CNTs are generally safer than pristine CNTs
include decreased toxicity due to the presence of functional groups
of high biocompatibility and increased dispersibility in water, thus
preventing their aggregation.
72,75,86,263,331,334−336
On the other hand, new forms of toxicity can emerge. In the application
of particulate CNTs, f-CNTs are used in almost all cases. For this
reason, it is necessary to build a library of data at least on representative
f-CNTs, and, in particular, on the differences in reactions in vivo
between chemically modified CNTs and pristine CNTs, which can be accessed
by researchers worldwide.
3.4
Carcinogenicity Studies
Few in vivo
studies have been conducted on the carcinogenicity of CNTs biomaterials
implants. In the intraperitoneal administration studies to investigate
inhalation-related mesothelioma carcinogenesis and its mechanism,
the abdominal cavity, where mesothelial tissue is present, was used
as a surrogate for the thoracic cavity.
281,282,288
Entry of intraperitoneally administered
CNTs biomaterials into the abdominal cavity is unlikely. Conversely,
use of CNTs in parts of the body from which entry into the abdominal
cavity is likely (e.g., uterus, ovary) should be avoided. Even when
CNTs biomaterials were implanted in common sites, nothing more than
very mild transient acute inflammation developed, with no finding
of carcinogenicity reported to date. Carbon, a substance of high biocompatibility,
is very unlikely to be carcinogenic. Carcinogenesis might result,
only if inflammation were persistent at the site of implantation.
Because CNTs are fibrous nanoparticles, they have not been used as
biomaterials. Subcutaneous implantation of CNTs has resulted in only
brief, very mild inflammation. Persistent chronic inflammation is
unlikely, provided that the site of implantation is appropriate.
58
However, it should be noted that the impurities
and chemical modifier molecules present in CNTs can be carcinogenic.
In fact, no methodology has been established to assess the in vivo
carcinogenicity of biomaterials whether they are particulate substances
like CNTs or bulk biomaterials. We developed a new tool for assessing
the carcinogenicity of CNTs involving subcutaneous implantation in
genetically modified cancer-prone mice.
98
No carcinogenesis was detected in these mouse recipients of subcutaneous
CNTs implants. This experimental study is described in detail in section 5.
3.5
Oxidative Stress
Because of its association
with apoptosis and carcinogenicity, oxidative stress is a good indicator
of toxicity. Whether CNTs induce oxidative stress is somewhat controversial.
In vivo studies have revealed CNT-induced changes in oxidative stress
markers. For example, intravenously injected SWCNTs induced high levels
of oxidative stress markers in the lung and liver,
312
and a study with the antioxidant vitamin E found that SWCNTs
played a major role in the induction of oxidative stress.
337
Hence, SWCNTs are likely to induce oxidative
stress.
191
On the other hand, gene expression
analysis in the liver and spleen found that intravenously injected
MWCNTs significantly raised the level of the oxidative stress marker
NAD(P)H in mice.
338
However, the prevailing
opinion is that MWCNTs do not induce very much oxidative stress.
339−341
Even if oxidative stress is induced and is due to an essential property
of CNTs, the underlying mechanism remains unclear. Metal catalysts
remaining in CNTs have been suggested to induce oxidative stress.
These facts are discussed in further detail in section 4.2.1 with a focus on cells.
3.6
Biodegradability
The biodegradability
of CNTs is currently a hot research topic. Carbon fibers, which in
the past were clinically used to reinforce the Achilles tendon, have
been shown to fragment over a long time. This is attributable to the
degradation of carbon fibers in the body.
96
The degree of biodegradability of any biomaterial is an important
toxicity issue. In the case of highly biodegradable materials, the
toxicity of their decomposition products must also be assessed. On
the other hand, if the material of interest is rapidly degraded in
the body, the carcinogenicity and other forms of toxicity that are
possibly exhibited by its original form will no longer be a concern.
In 2008, pioneer investigators showed that CNTs are biodegradable.
342
Since then, the biodegradability of CNTs has
been characterized as slight, and future advances in the relevant
research are expected.
343−348
Even if CNTs biodegrade, however, their biodegradation occurs at
extremely slow speeds; therefore, it can be thought that biodegradability
has no major impact on the safety of CNTs biomaterials except in special
cases such as where a single CNT fiber is used alone.
3.7
Other In Vivo Studies
In vivo studies
have been conducted to assess carbon nanotube uptake and toxicity
in the brain and spinal cord. A current focus is on migration of CNTs
to the central nervous system (CNS), particularly to the brain.
349
Advances are expected in the application of
CNTs as DDSs in the treatment of cerebral and spinal diseases. Accordingly,
studies assessing neurocompatibility have been conducted using CNTs
injected into the mouse brain and spinal cord.
70
However, research into CNTs interactions with the central
nervous system is still at the very initial stage.
99,350
Other studies found that CNTs caused allergic reactions,
351
and aggravated infectious disease rates.
352,353
Another study found that SWCNTs activate platelets and accelerate
thrombus formation in the microcirculation.
354
These biological reactions to CNTs biomaterials are important and
have to be examined extensively.
More recently, a nanoparticle-adhering
protein was reported to
possibly cover a part of the nanoparticle surface, reducing the targeting
activity of nanoparticles in the body.
355,356
This phenomenon
is called “protein corona formation” and discussed again
in section 4.3.
3.8
Body
Size Differences between Humans and Small
Animals
What should always be kept in mind in medical research
is that results from animal experiments can differ from actual clinical
findings.
357−360
Traditionally, small animals have been used in most animal experiments.
It remains unknown whether assessments of CNTs toxicity shown in vivo
in small animals are reproducible in humans, which have larger organs.
In particular, the toxicity of small particulate substances has not
been controversial and may be negligible as the body size increases.
Conversely, the effects on finer structures of individual organs may
increase the toxicity.
Differences in blood vessel thickness
depending on animal body size can impact the disposition of CNTs.
Most blood vessels are thicker in humans than in small animals. However,
the thickness and structure of the terminal microvessels are thought
to be nearly the same in different animal species. Hence, the migration
of CNTs from tissue to the bloodstream and the obstruction of blood
vessels by CNTs transported via the bloodstream are reproducible in
small animals. For this reason, CNTs biomaterials can be deemed safer
in humans because of the greater thickness of their central blood
vessels, provided that no problems have been revealed by in vivo kinetic
studies in small animals. Kinetic differences in the transport of
CNTs (used in DDSs and imaging) through blood vessels and its dependence
on animal body size must fully be taken into consideration.
Because cell size is the same in humans and small animals, the
relationship between CNTs and cells and the effects of CNTs on cells
are nearly the same. Therefore, even for basic body reactions to a
small particulate substance, the results of animal experiments are
considered to be highly representative.
Although these differences
depending on animal body size may be
resolved to some extent by conducting studies in larger animals such
as dogs, it is difficult to maintain constant experimental conditions,
making evaluation of a wide variety of CNTs impossible in large animals.
As with ordinary biomaterials, for which International Standards Organization
(ISO) and other standards are already available, it is reasonable
to commence clinical application of CNTs biomaterials, provided that
no problematic findings are obtained from assessments in small animals.
It should always be borne in mind, however, that adverse reaction
assessments can yield results inconsistent with findings from animal
experiments.
4
Present Status of Research
into in Vitro Toxicity
of CNTs for Biomaterials
Cells cultured to test for inhalation toxicity can be used to assess
the in vitro toxicity of CNTs biomaterials.
361−365
A large number of studies have examined the use of macrophages to
test for inhalation toxicity. Because macrophages play an important
role in the in vivo response to CNTs implants, inhalation toxicity
data obtained using this type of cell are relevant to toxicity assessment
of CNTs biomaterials.
155
Unlike drugs
and other chemical substances, CNTs are nanosized
particles possessing unique properties; therefore, special cautions
should be exercised when investigating CNTs in vitro. For example,
because CNTs are essentially hydrophobic and insoluble in water, a
surfactant must be used as a dispersant in culture experiments.
329
One article reported that the chemical properties
of such dispersants altered the toxicity of CNTs.
366−371
In addition, CNTs may adsorb phospholipids and albumin in the culture
broth, which are recognized by and interact with cells.
372−374
Furthermore, attention should be paid to possible reactions between
CNTs and test reagents.
91,191
One study concluded
that photometric methods were unsuitable because CNTs absorb light.
375−377
These factors affect the results of in vitro studies, making their
interpretation difficult.
4.1
Cellular Uptake of CNTs
Cellular
uptake of CNTs has been investigated in many types of cells by many
researchers, and different studies have reported widely variable results.
For example, SWCNTs have been reported to be absorbed by RAW264.7
cells in some studies and not in others.
340,364,372,378
Firme et al. studied the mechanism of CNTs passage (e.g., endocytosis/phagocytosis
and nanopenetration) through the cell membranes of many types of cells.
91
Endocytosis is a form of active uptake of small
extracellular particles (diameter ≤100 nm), and phagocytosis
is another form of active uptake in which relatively large particles
enter immunocytes such as neutrophils, macrophages, and dendritic
cells. On the other hand, nanopenetration is a form of passive uptake;
some authors have hypothesized that chemically modified or molecule-adsorbing
CNTs enter cells by nanopenetration.
75,107,157,379−384
We examined the cellular uptake of pristine CNTs, and reported
that the mechanism of this uptake depended on the type of cell and
choice of dispersant. We also reported that nonimmunocytes also actively
absorbed CNTs mainly through endocytosis/phagocytosis (Figure 6).
385,386
Other researchers likewise denied
the role of nanopenetration in cellular uptake of SWCNTs.
387
Adhesion to cell surfaces has been observed
even in cells that do not absorb CNTs; it remains unknown whether
the molecules that facilitate CNTs adherence to cells and those that
facilitate CNTs absorption are identical. It has been reported that
cell membrane proteins are involved in the cellular uptake of CNTs.
384,388
Furthermore, these membrane proteins may bind specifically to CNTs.
80,389
However, it will be necessary to investigate the influence of protein-containing
dispersants on this binding between membrane proteins and CNTs.
369,371,385
A recent report suggested that
exposure to electromagnetic waves promotes CNTs entry not only into
the cytoplasm of cells, but also into the nucleus.
390
In conclusion, much remains to be elucidated about the
cellular uptake of CNTs and its underlying mechanism.
Figure 6
Cellular uptake of pristine
MWCNTs varies depending on the type
of cell and the choice of dispersant. (a) Combined images from bright
field images and phase-contrast photomicrographs obtained 24 h after
exposure of human malignant pleural mesothelioma cells (MESO-1), human
bronchial epithelial cells (BEAS-2B), and human neuroblasts (IMR-32)
to carbon black (CB, 50 nm diameter) and MWCNTs. Both CB and MWCNTs
were absorbed in the MESO-1 cells and BEAS-2B cells, and localized
around the respective exposure sites, whereas in the case of the IMR-32
cells, both CB and MWCNTs adhered but failed to be absorbed. CB and
MWCNTs were added at 1 μg/mL for the treatment of BEAS-2B cells,
and 10 μg/mL for the treatment of the other cells. Scale bars
= 50 μm. Reprinted with permission from ref (384). Copyright 2011 Nature
Publishing Group. (b) A comparison of cellular uptake in BEAS-2B observed
1 and 24 h after exposure to MWCNTs dispersed using different dispersants.
Cellular uptake was determined in terms of the intensity of side scattered
light (SSC) from MWCNTs absorbed in the cells using a flow cytometer.
The MWCNTs dispersed in gelatin or 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)
were increasingly absorbed over
time, whereas those dispersed in carboxymethylcellulose (CMC) were
little absorbed in the cells. Reprinted with permission from ref (385). Copyright
2011 Dove
Medical Press.
To clarify the mechanism
underlying the cellular uptake of CNTs,
a wide variety of approaches have been developed. For example, light
scattering analysis was used to qualitatively assess the cellular
uptake of CNTs; a fluorescence detection technique was used to study
the cell trafficking of CNTs; and 3-D dark-field scanning transmission
electron microscopy was used to examine ultrastructural localization
of CNTs in appropriately prepared target cells.
368,391−393
Successful monitoring of the cellular uptake
and intracellular behavior of CNTs would clarify the reactions between
CNTs and cells in more detail. The mechanism behind the cellular uptake
of CNTs and their intracellular behavior not only has a bearing on
the cytotoxicity of CNTs, but also on their pharmacokinetics when
used in DDSs; thus, much more of this research is expected.
4.2
Mechanism Behind the Cytotoxicity of CNTs
Many studies
have assessed the cytotoxicity of CNTs. Some early
studies found that CNTs and asbestos have equivalent cytotoxicity
in macrophages and other cells.
75,76,394
Recent studies, however, found that CNTs have low cytotoxicity.
155
The reader of such in vitro cytotoxicity studies
should be alert to the fact that CNTs above a certain level dose-dependently
reduce cell counts regardless of cell type. This finding reflects
a natural reaction of living cells to contact with foreign particulates
such as CNTs. The issue is whether CNTs have a higher or lower degree
of cytotoxicity than biologically safe substances.
The objective
of the cytotoxicity study should also be noted. When safety is the
aim of the CNTs biomaterials evaluation, concentrations in the toxic
range (according to many reports; on the order of μg/mL) are
used, which are much higher than the likely actual concentrations
in vivo. Such high concentrations cannot occur in actual settings
and can lead to an unreasonable emphasis on the toxicity. Rather,
it would be more meaningful to determine the concentration at the
lower limit of cytotoxicity and whether this lower limit can occur
in vivo.
In addition, it should be well recognized that different
types
of cells can exhibit distinct responses even to the same kind of nanoparticles.
This phenomenon was recently named the “cell vision”
effect.
395
Exploring this effect will make
it possible to clarify the mechanism for cytotoxicity. Mahmoudi et
al. clarified the mechanism underlying this difference in cytotoxicity
among the different cell types, and investigated the detoxification
of nanoparticles.
396,397
In all cases, when applying
CNTs to biomaterials, their cytotoxicity
to living organisms should be as low as possible, and by establishing
the mechanism underlying their cytotoxicity, less cytotoxic CNTs can
be found. A wide variety of studies to elucidate this mechanism are
ongoing.
156,398,399
4.2.1
Oxidative Stress
Oxidative stress
is a focus of studies aimed at determining the mechanism underlying
the toxicity of CNTs in vitro as well as in vivo. Some articles but
not others have reported that CNTs may induce cytotoxic oxidative
stress.
400
This cytotoxicity from oxidative
stress has been attributed to the persistence of catalytic metals
(Fe, Co, Ni, etc.) used in producing CNTs. Many studies have found
that the cytotoxicity of CNTs increased with increase in metal content
ratio.
72,368,401,402
Some CNTs contain in excess of 10% (w/w) metallic
impurities, which can produce free radicals and thereby damage tissue.
263,400,403
This process can occur even
after CNTs are phagocytosed by macrophage. For example, NADPH oxidase
is intracellularly activated, and the resulting highly active superoxide
radical kills bacteria and other pathogens. Residual Fe activates
peroxides to produce hydroxyl (OH–) radicals leading
to oxidative effects on cellular proteins, lipids, and DNA. Residual
Co can produce chromosome anomalies. However, a study found that Ni
has no cytotoxic effects, but this finding needs to be investigated
further.
75,290,404
Oxidative
stress may be induced by aggregation of CNTs. Shvedova et al. found
that CNTs have low in vitro cytotoxicity provided they are properly
dispersed using appropriate procedures and their metallic impurities
are removed.
155
Our study concluded that
there was no correlation between the amount of oxidative stress from
CNTs with low residual iron content and cell proliferative response
or inflammatory reaction.
386,405
Carbon nanohorns,
a type of carbon nanotubes without metallic impurities, were reported
to be quite safe, with cytotoxicity less than 10% of the cytotoxicity
of dust from road pavement.
406
However,
it is unrealistic to expect that CNTs will contain absolutely no metallic
impurities. Accordingly, an article discussed the limit of metallic
impurity not affecting the redox properties of CNTs.
407
The susceptibility of CNTs to oxidation in the presence
of metallic impurities was also analyzed.
408
In all cases, the lower was the level of metallic impurities, the
lower was the level of induction of oxidative stress. Collectively,
these available reports lead to the judgment that carbon purity level
of 99% or more is not problematic.
On the other hand, it has
long been suggested that when cells absorb CNTs, long fibers are left
unabsorbed and induce oxidative stress.
281
This phenomenon is known as frustrated phagocytosis. A recent report
stated that CNTs that are shorter than a given length are absorbed
and not toxic, whereas longer CNTs are not absorbed but are toxic.
409−412
Consequences such as carcinogenesis may stem from prolonged inflammation
due to frustrated phagocytosis in the thoracic cavity lasting long
after CNTs are inhaled (Figure 7). Cytotoxicity
due to frustrated phagocytosis in the context of use of CNTs as biomaterials
is discussed in section 6.2.2.
Figure 7
A schematic diagram showing
a hypothesized mechanism of carcinogenesis
due to frustrated phagocytosis. If left unabsorbed, long CNTs in cells
can produce oxidative stress and induce inflammation. It has been
suggested that a long period of persistent inflammation in the thoracic
cavity following inhalation of CNTs can lead to carcinogenesis. Currently,
research into the inhalation toxicity of CNTs is facing a problem
with the determination of the margin of inhalation exposure that does
not cause persistent inflammation.
In September 2012, the National Institute of Standards and
Technology
(NIST) in the U.S. reported a finding that is completely inconsistent
with findings that SWCNTs protect DNA from oxidative stress.
413
Hence, no consistent conclusion has been reached
concerning oxidative stress. Collectively, previous studies using
many types of cells under a wide variety of conditions have led to
a near consensus that CNTs do not induce oxidative stress if their
aggregability and length are limited.
155
A recent study showed that chemical treatment with, for example,
triethylene glycol can reduce the likelihood of aggregation in biological
fluids and toxicity of even long CNTs.
414
4.2.2
Effects on Immunity
The second
issue concerns the interactions of CNTs with immunocompetent cells,
including cellular uptake and subsequent intracellular transport.
As such, immunocompetent cells bear a direct relationship to the safety
of CNTs in vivo. Of course, pristine CNTs (because they lack antigen-presenting
protein) do not cause immune reactions other than those to a foreign
substance. Hence, if localized inflammation is brief, immune reactions
should resolve. However, immunocompetent cells may absorb CNTs because
of their nanosize, may not absorb some CNTs completely because of
their fibrous form, and may orchestrate the development of an inflammatory
response to residual metals and other factors in CNTs. Keeping these
possibilities in mind, it is necessary to understand how immunocompetent
cells respond to CNTs. Many in vitro studies have reported no response
of immunocompetent cells to very pure and very short CNTs.
155,415
For example, CNTs did not have a remarkable effect on antigen-presenting
cells (APCs) such as mouse macrophages (RAW 264.7 cells) and mouse
bone marrow-derived dendritic cells (bmDCs).
416
An article reported that CNTs did not induce inflammatory cytokines
in macrophages, whereas residual metals did.
85,401,402
If CNTs are shown to escape
surveillance by immunocompetent cells, this finding will provide strong
evidence for high safety of CNTs as biomaterials. Of course, it is
theoretically impossible that pristine CNTs cause autoimmune disease.
4.2.3
Attempts To Lessen the Cytotoxicity
As
stated above, various methods for minimizing the cytotoxicity
of CNTs have been studied. For example, reducing nanotube cytotoxicity
through chemical modification to change physicochemical properties
and hence biological activity has been proposed. A library of 80 different
surface-modified nanotubes was screened for protein bindability, cytotoxicity,
and immune responses. Nanotubes had high biocompatibility, low protein
adsorption properties, low cytotoxicity, and low immunostimulatory
activity.
417
It has also been found that
some shapes of CNTs are not cytotoxic,
309,418
and change
of the graphitization temperature during CNTs synthesis alters their
biological activity.
405
Hence, expectations
are for the minimization of CNTs cytotoxicity. To this end and for
the above-described reasons, the cellular mechanisms of CNTs recognition
and the effects of the physicochemical properties of CNTs on cytotoxicity
need to be clarified.
396,397,419
4.3
CNT–Protein Interactions
CNTs
used in vivo are unavoidably exposed to proteins. For this reason,
successful application requires an understanding of both the adsorption
of proteins to CNTs and the resulting biological responses to protein-adsorbed
CNTs. While attempts to functionalize CNTs using antibodies and receptors
(that are peptides or proteins) are underway,
176,420,421
the influence of proteins on
pristine CNTs should be investigated. CNTs specifically adsorb fibrinogen,
apolipoproteins, and albumin from blood.
422
As such, albumin is a component of most CNT dispersants in common
use for toxicity experiments,
366−368,370
and it is necessary to determine whether CNTs toxicity assays actually
assess pristine CNTs toxicity or albumin-adsorbed CNTs toxicity. Examination
of the mode of adsorption to SWCNTs by plasma proteins fibrinogen,
γ-globulin, transferrin, and bovine serum albumin using an atomic
force microscope was reported, and protein binding reduced SWCNTs
cytotoxicity.
423
However, the SWCNTs used
in this experimental study contained many metals such as Cr, Fe, Mo,
and Co, and their effect must also be taken into account.
The
phenomenon in which various proteins coat the nanoparticle surface
has recently been termed “protein corona” formation.
424
The protein corona is influenced by a wide
variety of factors, including temperature, protein concentration,
gradient concentration, protein source, and physicochemical properties
of nanoparticles. The protein corona has also been reported to have
major impacts on the biological reactions of cells and living organisms.
For example, nanoparticles on cells and living organisms were shown
to lose activity when their surface is partially covered by protein.
355,356,425−429
As such, the protein corona may determine the fate of CNTs in living
organisms. In addition, changes on the nanoparticle surface caused
by formation of the protein corona can alter the effects of chemically
modified CNTs. Shannahan et al. compared the proteins coating MWCNTs
with SWCNTs, and those coating modified with unmodified, which revealed
a difference in protein composition between SWCNTs and MWCNTs and
an increase in the variety of component proteins as a result of modification
with COOH groups.
430
Functional deterioration
of chemically modified nanoparticles has been repeatedly shown to
occur; there is an urgent need to determine whether the same phenomenon
can occur in CNTs.
On the other hand, to explain the decreased
cytotoxicity of protein-bound
CNTs, a recent study hypothesized that the human body developed a
biological system mediated by protein binding to deal with exposure
to numerous nanoparticles (i.e., developed a defensive mechanism against
nanoparticles).
431
This suggests that CNTs
research may elucidate the body’s defensive mechanism, which
is unclear.
4.4
Mutagenicity, Genotoxicity,
and Apoptotic
Potential of CNTs
Assessments of the mutagenicity and genotoxicity
of CNTs are also important in vitro safety studies.
432−441
This is because the results from these assessments reflect the carcinogenicity
of CNTs. Relatively common approaches include the Ames test, comet
assay, and micronucleus test.
The Ames test, also known as the
reverse mutation test, is to quantify reverse mutation (i.e., restoration
of amino acid biosynthesis capability in bacteria originally deprived
of that capability through mutation). Ames test studies with Salmonella typhimurium
and other test strains have
often shown that neither SWCNTs nor MWCNTs are mutagenic. A mutagenesis
study showed that the frequency of mutations in mammalian cells (Chinese
hamster pulmonary fibroblasts) is not altered by MWCNTs.
438,442−445
The comet assay is a technique used to detect DNA damage in
individual
cells, enabling separate determination of early disorders induced
at the DNA level, repair kinetics, and residual disorders. For this
reason, comet assays have been performed on many types of cells exposed
to SWCNTs and MWCNTs. CNTs induced DNA damage in some studies but
not in others. The prevailing opinion is that any DNA damage caused
by CNTs is mediated by reactive oxygen species (ROS).
446−449
The purpose of the micronucleus test is to detect damage to
the
gene of interest in animal cells following administration of a test
substance. Cells containing micronuclei can serve as an index of gene
damage. Micronucleus test studies to assess the toxicity of SWCNTs
and MWCNTs in many types of cells have yielded mixed results.
399,404,442
Some studies of apoptosis
induction by CNTs found induction of
apoptosis signals in macrophages and other cells to induce apoptosis
signals, while others did not find any sign of apoptosis induction.
318,378,450−452
Many cells incorporating CNTs underwent G1 phase arrest.
430
We reported that iron-rich MWCNTs caused nonapoptotic
cell death.
453
On the other hand, other
experiments found that highly pure MWCNTs caused apoptosis-like cell
death, suggesting that the CNTs impurities have a major effect on
apoptosis.
386
In conclusion, the
mutagenicity and genotoxicity of CNTs remain
unclear; some studies judged CNTs to be mutagenic or genotoxic and
others did not.
89,432,437,443,454−456
Results varied and depended on the cell
type even within the same study.
442
In
cases where genotoxicity was observed, authors hypothesized metals-induced
oxidation of the DNA or suggested other hypotheses.
457
Variable results and conclusions are attributable to variable
test conditions such as the dispersibility of CNTs in solution and
the amount of CNTs used, as well as the amount of CNTs impurities,
but not the form of CNTs (all studies assessed particulate substances).
There is no current evidence in CNTs of high purity, although carcinogenicity
from mutagenicity or genotoxicity calls for vigilance.
155
Further investigation will be necessary in
different cell types to determine whether cells incorporating CNTs
undergo apoptosis.
4.5
Cellular Signaling Events
Microarray
or proteomics studies of cell signaling events induced by CNTs have
been reported.
458
In a microarray study
using human embryonic kidney cells exposed to SWCNTs for 2 days, decreased
expression of cyclins and cdks (a gene affecting
the G1 phase of the cell cycle) and increased expression of apoptosis-related
genes were demonstrated.
318
Other researchers
exposed foreskin cells to SWCNTs, and found that the expression of
HMOX1, HMOX2, ERCC4, and HSPE1 and that of ATM, CCNC, DNAJB4, and
GADD45A more than doubled when determined using stress and toxicity
arrays and RT-PCR, respectively.
459
Using
reporter gene assays of MWCNT-exposed bronchial epithelial cells,
MWCNTs activated the transcription factor NF-κB to induce increased
phosphorylation of p38, ERK1, and HSP27 in the MAP kinase pathway
and the production of inflammatory cytokines.
369
Activation of NF-κB in macrophages was also reported.
460
We examined the effects of MWCNTs on cellular
signaling events in osteoclasts and showed that MWCNTs suppressed
osteoclast differentiation by inhibiting the nuclear migration of
the transcription factor NFATc1.
217
In
conclusion, the influences of CNTs on cell signaling events are important
to the understanding of cellular function, and further research will
be needed.
Proteomics-based studies have been conducted using
keratinocytes and hepatoma cells. Results have shown changes in expression
of proteins related to metabolism, stress, redox, cytoskeleton formation,
apoptosis, etc., in both types of cell.
461,462
Our proteomics analysis under low-cytotoxicity conditions using
monoblastic leukemia cells that do not absorb MWCNTs confirmed these
changes in proteins (Table 1).
463
Such comprehensive analyses of cell signaling
events increase understanding of the essential features of cellular
change.
464
It is hoped that research activities
will identify the pathways on which CNTs have a direct impact, and
make major contributions to the assessment of the cytotoxicity of
CNTs.
Table 1
Proteins of Human Monoblastic Leukemia
Cells (THP-1) Changed by Exposure to CNTs As Determined by Proteomic
Analysisa
gene ontology term
proteins
biosynthetic process
heat shock protein β-1,
elongation factor 1-δ,
DNA mismatch repair protein Msh2, 6-phosphogluconate dehydrogenase
decarboxylating, triosephosphate isomerase
signal transduction/cell communication
elongation factor
1-δ, DNA mismatch repair protein Msh2, 14-3-3 protein γ, serine/threonine-protein
phosphatase 2A 55 kDa regulatory subunit B α isoform, protein DJ-1
carbohydrate
metabolic process
6-phosphogluconate dehydrogenase decarboxylating,
triosephosphate
isomerase, serine/threonine-protein phosphatase PP1-α catalytic
subunit, α-ketoglutarate dehydrogenase, neutral α-glucosidase AB
nucleobase,
nucleoside, nucleotide, and nucleic acid metabolic
process
DNA mismatch repair protein Msh2, 6-phosphogluconate
dehydrogenase
decarboxylating, triosephosphate isomerase, DNA damage-binding protein 1
protein metabolic
process
actin related protein 2/3 complex
subunit 2, serine/threonine-protein phosphatase PP1-α catalytic
subunit, serine/threonine-protein phosphatase 2A 55 kDa regulatory
subunit B α isoform, DNA damage-binding protein 1
catalytic process
6-phosphogluconate dehydrogenase decarboxylating, triosephosphate
isomerase, α-ketoglutarate dehydrogenase, DNA damage-binding protein 1
multicellular
organismal development
DNA mismatch repair protein Msh2,
triosephosphate isomerase,
14-3-3 protein γ, serine/threonine-protein phosphatase PP1-α
catalytic subunit
response to stress
heat shock protein β-1, DNA mismatch repair protein Msh2,
DNA damage-binding protein 1, protein DJ-1
cell differentiation
heat
shock protein β-1, DNA mismatch repair protein Msh2, 14-3-3 protein γ
cell
cycle
DNA mismatch repair protein Msh2, serine/threonine-protein
phosphatase PP1-α catalytic subunit, DNA damage-binding protein 1
transport
14-3-3 protein γ, protein DJ-1
cell death
heat shock protein
β-1, DNA mismatch repair protein Msh2
organelle organization and biogenesis
actin related
protein 2/3 complex
subunit 2, DNA mismatch repair protein Msh2
translation
heat shock protein
β-1, elongation factor 1-δ
lipid metabolic
process
triosephosphate isomerase
a
Adapted with permission from ref (463). Copyright 2011 Elsevier.
4.6
Choice of Cells
To date, cytotoxicity
studies have often been conducted using fibroblasts and macrophages
such as RAW cells. However, cellular reactions to CNTs depend on the
type of cell,
396,397
and it can be thought that the
reactions are specific for the organ bearing the target cells. For
example, a study comparing the cytotoxicity of CNTs in the liver,
spleen, and lung found that CNT-induced oxidative stress dose-dependently
increased toxicity in the liver and lung, but not in the spleen.
465
We must clarify the mechanism underlying the
reactions of different cell types and organs to CNTs. Because biological
reactions to CNTs vary among types of cells and organs, toxicity studies
using cells from likely sites of use will be needed before CNTs can
be clinically applied.
For example, in a study assessing CNTs
for use in nerve regeneration, human neuroblastoma cells and primary
mouse neurons were exposed to MWCNTs, and their reactions were examined
for effects on cell survival, oxidative stress, and apoptosis.
70
Another study examined the effects of CNTs on
heart cells, specifically on impulse conduction characteristics, myofibril
structure, and reactive oxygen species production in the patterned
growth strands of neonatal rat ventricular cardiomyocytes. CNTs particles
had much less effect than diesel exhaust particles and titanium dioxide
nanoparticles.
466
To assess the use CNTs
as a possible bone tissue regeneration scaffold, we examined in detail
their effects on osteoblasts (bone-forming cells) and osteoclasts
(bone-absorbing cells), as described in section 2.3.2.
217,218
5
Reference
Materials for Safety Evaluation of CNTs as Biomaterials
The safety of CNTs for biomaterial
application remains unknown
because toxicity studies have yielded inconsistent or even contradictory
results as stated above. Moreover, no nanoparticle reference material
has been shown to be safe to use in living organisms. All biomaterials
are essentially foreign to living organisms, and hence exhibit some
toxicity to living organisms. Of concern is the level of toxicity;
the biological safety of CNTs cannot be assessed without conducting
a toxicity study using as a reference substance that has already been
recognized as safe to use in living organisms.
For example,
in 2010, the cytotoxicity, genotoxicity, and apoptosis-inducing
potential of MWCNTs was examined in human fibroblasts. Physiological
saline admixed with a dispersant served as the only negative control.
Results showed that MWCNTs exhibited dose-dependent toxicity in all
dose groups as compared to the negative control, and that the cell
survival rate decreased dramatically due to DNA damage, triggering
pathways leading to programmed cell death. Hence, the conclusion was
reached that CNTs are highly toxic. It should be noted, however, that
it is scientifically incorrect to assess the toxicity of CNTs merely
by comparing the results obtained in the presence and absence of CNTs.
The solution (containing a dispersant) used in the reported study
cannot serve as a reference for toxicity assessment. This study showed
nothing more than that the experimental system used worked well, and
no conclusion regarding CNTs toxicity can be drawn.
For researchers
in this field, identification of an appropriate
reference material for toxicity studies, which is presently unavailable,
is a top priority. Kostarelos et al. pointed this out in 2009 in their
review published in Nature Nanotechnology.
68
The reference substance must be a nanosized
particulate with established biological safety. A substance can be
judged as safe to use in living organisms only if it is shown to be
equally or less toxic than its reference material. To render a judgment
on the functioning of an experimental system, a conventional chemical
substance can be used as a feasible alternative for the positive-control
reference material. However, no best negative-control reference material
has been found, so the safety of CNTs as biomaterials remains indeterminable.
5.1
Why Is There No Substance That Can Serve as
a Reference for CNTs?
Researchers have been seeking a substance
with many of the same properties as CNTs. Such references do not actually
exist. Without a reference, CNTs cannot be used as biomaterials. From
a broader viewpoint, any nanosized particulate substance should be
considered to be a reference candidate. In fact, reference materials
are specified for bulk biomaterials on the basis of this broad concept.
For example, in cytotoxicity testing of bulk materials, a high-density
polyethylene film serves as the negative reference material for the
extraction method, and a polyurethane film containing zinc diethyldithiocarbamate
(ZDEC) serves as the positive reference material. For the direct contact
method, a plastic sheet for tissue culture serves as the negative
reference material, and ZDEC-containing polyurethane serves as the
positive reference material. These substances are specified in the
ISO 10993-5 Biological evaluation of medical devices - Part 5: Tests
for in vitro cytotoxicity (2009).
467
Hence,
it is internationally accepted that a reference for a bulk biomaterial
should be a bulk material of totally different nature. There is no
rationale for viewing particulate materials as the only exception.
Essentially, the unfavorable criticism of nanosized fibrous particulate
substances is due largely to the fact that asbestos causes cancer
and other diseases. Because CNTs resemble asbestos in size and shape,
their toxicity has created a stir in the media.
281,282
It should be noted, however, the inhalation toxicity of CNTs is
distinct from the toxicity of CNTs biomaterials. Recently, inhaled
spherical titanium oxide particles were reported to be carcinogenic;
468
however, if the judgment is made on the basis
of shape and size only, no spherical nanoparticles could be used as
biomaterials, and almost all nanoparticles would be inapplicable to
biomaterials. It is obvious to everyone that this claim makes no sense.
Even if fibrous nature, thin and long shape, and large aspect ratio
are problematic, we should keep in mind that carbon fiber biomaterials
have long been used for Achilles tendon repair and other clinical
purposes with absolutely no coincidence of carcinogenicity.
96,469,470
In conclusion, the most reasonable
approach is to assess the toxicity of CNTs by focusing on biological
reactions to nanosized particulate substances.
5.2
Biomaterials
Comprising Artificial Nanosized
Particles
The second reason for the inability to find a best
reference material is that no nanosized particulate substance has
been used as a biomaterial. This issue bears not only on CNTs, but
also on a wide variety of nanoparticles, and research into biological
application of nanoparticles has recently been rapidly growing. Some
pharmaceuticals anchored to nanosized particles are already in clinical
application. For example, abraxane, a nanoparticle substance prepared
by conjugating the anticancer agent paclitaxel with albumin, degrades
in the body, releasing the anticancer agent. Such conventional nanosized
particles are specifically used as DDSs by making the best use of
their biodegradability, and cannot be viewed in the same way as nanoparticulate
biomaterials that are poorly degraded in the body.
301
To date, only four kinds of artificial materials
have been used in living organisms: chemical substances, materials
with biodegradability, bulk materials lacking biodegradability, and
micrometer-sized or larger particulate substances. Nanosized particulates
have not been used in the body. Chemical substances, biomaterials
with biodegradability, and bulk biomaterials have been used in the
human body since ancient times, and many such substances have proven
to be safe. For this empirical reason, researchers have been able
to use these substances as references. When these substances were
used as biomaterials for the first time, no scientific toxicity testing
was needed. Those substances found over time to be safe to use in
the human body remain in use today. Toxicity studies using some of
these substances as references have been conducted to demonstrate
the safety of other substances in the same category, and then using
the other substances thus judged to be safe as references, the safety
of still other similar substances has been demonstrated. Through this
process, numerous substances have been made available for clinical
application. The internationally accepted ISO standards dealing with
safety evaluation are currently serving very well and have also emerged
from this historical precedent.
467
The
standard reference materials are known biologically safe substances
rather than new reference materials evaluated to be safe for humans.
Micrometer-sized or larger particulate biomaterials, for example,
granular hydroxyapatite, have never posed a major problem even though
they were subjected to the same safety evaluation process as conventional
biomaterials.
471−473
Because CNTs and other nanosized particulate
substances fall into a different category of biomaterials than micrometer-sized
or larger particulate substances, the use of conventional bulk biomaterials
and hydroxyapatite particles as reference materials for them is controversial.
Because nanosized particulate substances have not been used in the
human body, there is no implicit reference with established safety.
474
For these reasons, obtaining a reference
with confirmed biosafety
in the human body for use in toxicity studies of CNTs appears to be
impossible. From a broader perspective, however, otherwise unknown
nanoparticles may be discovered. We considered that highly pure carbon
black could serve as a reference for CNTs, because it is the primary
component of the black ink used in tattoos, and also because black
tattoo inks have long been injected into human bodies and are currently
used by a tremendous number of people worldwide. Evidence showing
that black tattoo inks are composed of nanosized carbon black particles
is described below, with an overview of the biological safety of CNTs
using carbon black as a reference.
5.3
Safety
Evaluation of CNTs Using Nanosized
Carbon Black Particles as a Reference
5.3.1
Nanosized
Carbon Black Particles in Tattoo
Ink
Two commercially available black tattoo inks (Sumi-Black,
Unique Tattoos, Subiaco, Australia; Lining-Black, Classic Ink, Victoria,
Australia) were purchased and extensively analyzed for components.
Each was dried, and the resulting solid product was morphologically
examined by scanning electron microscopy (SEM); particles with a nearly
uniform diameter of several tens of nanometers were found to have
accumulated (Figure 8a). After SEM examination,
the particles were subjected to an elemental analysis using energy
dispersive X-ray spectroscopy (EDS). Results showed that both inks
had a C content of about 99.5 wt % and different impurity profiles,
with trace amounts of Na and S detected and attributable to the surfactant
added. A Raman analysis using common industrial carbon black (Vulcan
XC 72, Cabot, Boston, MA) as a control revealed that Raman shift of
both black tattoo inks was nearly the same as that of the control
(Figure 8b). Furthermore, transmission electron
microscopy (TEM) revealed that the particles in black tattoo inks
had nearly the same shape as those of ordinary carbon black (Figure 8c). These findings
identified the particles in tattoo
inks as pure carbon black (i.e., nanosized carbon particles) as with
MWCNTs.
97
Figure 8
Having historically been proven safe to
the human body, tattoos
comprise nanosized highly pure carbon black, and hence serve well
as a reference material for evaluating the safety of CNTs, which likewise
occur in the form of nanosized carbon particles. (a) Scanning electron
microscopy (SEM) images of tattoo carbon black-1 (TCB-1) and tattoo
carbon black-2 (TCB-2) prepared by drying two different tattoos. TCB-1
and TCB-2 were found to have accumulated in the form of generally
regular particles having a diameter of about 30–50 nm, and
generally irregular particles having a diameter of about 50 nm, respectively.
(b) Raman analysis of TCB-1, TCB-2, and ordinary carbon black. TCB-1
and TCB-2 exhibited nearly the same Raman shift pattern as with ordinary
carbon black. D-band, turbostratic amorphous; G band, graphite crystal.
(c) Transmission electron microscopy (TEM) images of TCB-1, TCB-2,
and ordinary carbon black. These three substances were found to have
nearly the same particle shape. Reprinted with permission from ref (97). Copyright
2011 Elsevier.
In 2012, on the other hand, a
report titled “Chemical Substances
in Tattoo Ink” was released from Denmark.
475
Concerning a research project implemented by the Danish
Technological Institute in cooperation with Bispebjerg Hospital and
the National Food Institute, Technical University of Denmark, the
report explicitly described carbon black as the principal component
of black tattoo ink, and toxicity assessments of carbon black found
no biological safety problem.
An extremely large number of humans
have received black tattoos
since ancient times, and this practice has caused no major problems;
tattoos are popular even today. Hence, carbon black can be described
as a biomaterial that has been proven by historical evidence to be
safe for use in the human body. As such, the nanosized carbon particles
used in black tattoos, as with CNTs, are very pure carbon black; thus,
carbon black should be considered as a good reference material for
CNTs.
5.3.2
Comparison of Characteristics of CNTs and
Carbon Black
To use the biologically safe carbon black tattoo
ink as a reference material for CNTs, both substances should share
some characteristics. Despite their considerably different characteristics,
current reference materials for bulk materials have been used as international
standards, and safety assessments have been conducted with no major
problems. This has become feasible because of the large amount of
data compiled throughout the long history of biomaterials research.
However, references for nanoparticle biomaterials remain to be found.
The accuracy of safety evaluation will be increased by using substances
with similar characteristics in the beginning.
The characteristics
(including composition, size, shape, and surface chemistry of the
reference material used for CNTs [i.e., carbon black]) were compared
to those of CNTs per se (Table 2).
476
Both substances are highly pure carbon particulates
of similar size (i.e., they are nanosubstances, entities internationally
recognized as being not less than 100 nm in one or more of the three
dimensions).
477,478
CNTs and carbon black have distinct
shapes: fibrous particles and spherical particles, respectively. Although
various classifications of surface chemistry are available, the most
common practice is to characterize surfaces as hydrophilic or hydrophobic.
The surfaces of CNTs are hydrophobic, and carbon black particles,
without surface treatment, are essentially hydrophobic. Hence, three
of the four representative characteristics of particulate substances
are shared; therefore, it is reasonable to use carbon black as a reference
for CNTs. Although some researchers may disagree based on the distinction
between fibrous and spherical particles, no reference can have exactly
the same characteristics as the test substance. Considering the absence
of any other appropriate reference, it is very fortunate that carbon
black with high similarity to CNTs has a long history of use in living
organisms and demonstrated safety in the human body. Problems stemming
from the fibrous nature of some nanoparticles are discussed in section 6.2. Despite
these problems, it can be concluded
that fibrous nanoparticles pose no hazard at sites of CNTs implantation
if inflammation is not persistent.
Table 2
Comparison of Characteristics
of CNTs
and Carbon Black
characteristic
CNT
carbon black
composition
high-purity carbon
high-purity
carbon
size
nanosized
nanosized
shape
fibrous
particle
spherical particle
surface chemistry
hydrophobic
hydrophobic
All experiments can
be performed using mass as an index because
CNTs and carbon black are both highly pure carbon. For particulate
substances, it is often difficult to measure the number and volume
of particles; therefore, the ability to use mass as the simplest index
is an obvious advantage in the evaluation of CNTs safety. Carbon black
can also serve as a reference for the evaluation of the biological
safety of other nanosubstances used as biomaterials. However, mass
cannot be used as an index for safety comparison when density varies;
therefore, another index such as particle count will have to be used,
making the procedure more complicated and difficult to perform, and
even reducing its accuracy.
Research into application of non-CNT
carbon-based nanomaterials
to biomaterials has also progressed steadily, although there are fewer
non-CNTs studies than CNTs studies. The use of fullerenes or graphene
for DDSs and imaging has been studied, and their biological safety
has been evaluated.
479,480
Additionally, nanosized carbon
fibers, which traditionally have not been nanosized, are now available
thanks to recent technical advances. Although carbon fiber products
are promising candidates as nanobiomaterials because of their history
of clinical use as biomaterials, their safety needs to be evaluated
because of their nanosize,
214,481
and carbon black can
serve as an appropriate reference.
5.3.3
Safety
Test
A skin implantation
test with MWCNTs was conducted using carbon black tattoo ink as a
reference material. Results showed that MWCNTs induced acute but mild
inflammation reactions in subcutaneous tissue, which resolved early.
The subcutaneously implanted MWCNTs were shown to be absorbed initially
by macrophages and remained in the macrophages for a long time. These
short- to long-time histological reactions to the MWCNTs were found
to be very similar to the histological reactions to the carbon black
tattoo ink particles (Figure 9). This finding
shows that when implanted in vivo, MWCNTs (as with tattoo ink particles)
exhibit good tissue affinity at the implantation site and stay intact
in macrophages for a long time.
97
Figure 9
Histological
reactions to subcutaneously implanted MWCNTs are very
similar to those to carbon black, showing good tissue compatibility.
Hematoxylin-eosin staining. Scale bars = 20 μm. TCB-1, Tattoo
carbon black-1; TCB-2, Tattoo carbon black-2 (see Figure 8). (a) Histological images
from a negative control
group (NC) of male ddY mice at 6 weeks of age receiving an injection
of 10 μL of physiological saline and a surfactant given to a
pocket created in subcutaneous tissue in the back. At 1 week of treatment,
the subcutaneous tissue had been repaired nearly completely. Repair
was complete at 4 weeks. No change was observed at 12 and 24 weeks.
(b) Histological images of subcutaneous tissue from a group receiving
an injection of 10 μL of MWCNT solution (4.0 mg/mL). Most particles
were found to have been absorbed in macrophages at 1 week. In the
areas around the injection site, accumulated fibroblasts, neutrophils,
and lymphocytes were found, with weak inflammatory reactions observed.
At 4 weeks, the MWCNTs remained incorporated in macrophages, and the
inflammatory reactions around the injection site had resolved. The
macrophages that had absorbed MWCNTs turned into multinucleated giant
cells, creating an appearance like foreign-body granuloma. The histological
profiles obtained at 12 and 24 weeks did not differ from the profile
obtained at 4 weeks. (c) Histological images of subcutaneous tissue
from a group receiving an injection of 10 μL of TCB-1 solution
(4.0 mg/mL). At 1 week, most particles were found to have been absorbed
in macrophages in the subcutaneous tissue, and as in the MWCNT group,
accumulated fibroblasts, neutrophils, and lymphocytes were found,
with weak inflammatory reactions observed. At 4 weeks, the inflammatory
reactions around the injection site had resolved as in the MWCNT group.
The histological profiles obtained at 12 and 24 weeks were similar
to the profile obtained at 4 weeks. (d) Histological images of subcutaneous
tissue from a group receiving an injection of 10 μL of TCB-2
solution (4.0 mg/mL). All histological profiles obtained at 1, 4,
12, and 24 weeks were similar to those obtained with the TCB-1 solution.
(e) Histological images of subcutaneous tissue from a group receiving
an injection of 10 μL of zinc dibutyldithiocarbamate (ZDBC)
solution (4.0 mg/mL). At 1 week, accumulation of many types of inflammatory
cells such as fibroblasts, neutrophils, lymphocytes, and plasma cells
was observed, and intense inflammatory reactions had been induced
over a wide area, with fat necrosis and nuclear debris formation observed.
No accumulation of macrophages was observed. Even at 4 weeks, inflammatory
cells remained and inflammatory reactions persisted, although the
inflammation was going to disappear. At 12 and 24 weeks, the inflammatory
reactions had resolved, and the subcutaneous tissue had been repaired
into scar tissue with fibrosis. Reprinted with permission from ref (97). Copyright
2011 Elsevier.
We then conducted a colony formation
assay to determine the in
vitro cytotoxicity of MWCNTs using carbon black tattoo ink particles
as a reference. Both MWCNTs and carbon black tattoo ink particles
inhibited colony formation in a concentration-dependent fashion. At
higher concentrations, colony counts were higher with exposure to
MWCNTs than with exposure to carbon black tattoo ink particles (Figure 10). These
findings demonstrated that the cytotoxicity
of MWCNTs was not greater than that of carbon black tattoo ink particles.
97
When assessing the cytotoxicity of nanoparticles,
the colony formation assay yields numerical results, and is currently
considered to be the best (most sensitive and reproducible) method
of toxicity assessment.
402
Figure 10
The cytotoxicity of
MWCNTs is not higher than that of carbon black.
TCB-1, Tattoo carbon black-1; TCB-2, Tattoo carbon black-2 (see Figure 8). (a) Appropriateness
of cytotoxicity assessment
in colonization test. The colonization capacity of V79 cells (Chinese
hamster lung fibroblast cell line JCRB0603) decreased as the concentration
of the positive control ZDBC increased. The concentration for 50%
colony count reduction (IC50, reference value range: 1–4
μg/mL) was found to be between 1 and 2 μg/mL, confirming
that the cytotoxic action of the test substance was properly assessed.
(b) Macroscopic photographs showing colonization test results. Colony
counts of V79 cells cultured using a culture broth alone and those
cultured in the presence of MWCNT solution, TCB-1 solution, and TCB-2
solution were compared. The concentrations in the solutions were 12.5,
50, 200, 400, 800, and 1600 μg/mL, respectively. (c) Colony
counts versus carbon concentrations in MWCNT, TCB-1, and TCB-2 solutions.
MWCNTs inhibited the colonization in a concentration-dependent fashion,
and TCB-1 and TCB-2 likewise inhibited the colonization in a concentration-dependent
fashion. When comparing colony counts, MWCNTs produced significantly
higher colony counts than TCB-1 at concentrations of 200 μg/mL
or more, and than TCB-2 at concentrations of 400 μg/mL or more.
Error bars indicate standard deviations (n = 6);
*, p < 0.001; **, p = 0.016.
Reprinted with permission from ref (97). Copyright 2011 Elsevier.
Furthermore, we conducted a carcinogenicity test of CNTs
in a trancegenic
rasH2 mouse
482−485
using tattoo carbon black tattoo ink as a reference material. The
rasH2 mouse has recently also been used in studies of bulk biomaterials.
486−488
We implanted MWCNTs or tattoo carbon black subcutaneously. Results
showed that no neoplasms were produced because of implantation of
MWCNTs. In the group with carbon black implanted as a reference, one
animal died but had apparent tumors on histopathological examination
(Figure 11, Table 3).
The 75 mg/kg dose of MWCNTs implanted in this study was considerably
higher than the doses that had been used in previous implantation
studies in ordinary mice.
89,91,155,304
In summary, the above-described
test for assessing the carcinogenicity of subcutaneously implanted
CNTs by in the trancegenic animals for the first time revealed no
carcinogenesis from CNTs as well as tattoo carbon black tattoo ink.
98
Figure 11
In a subcutaneous implantation test using cancer-developing
transgenic
rasH2 mice, MWCNTs were found to be not carcinogenic; their carcinogenicity
was determined to be not higher than that of carbon black. (a) Changes
over time in survival rate of rasH2 mice. All mice in the MWCNT group
were alive at 26 weeks. In the carbon black group, 1 animal died at
22 weeks, and at 26 weeks, 9 of the 10 animals were alive. In the
solvent group, all mice were alive at 26 weeks. In the N-methyl-N-nitrosourea (MNU)
group, 1 animal died
at 13, 14, 17, and 22 weeks each; 6 of the 10 animals were alive at
26 weeks. (b) Histological images of tumor masses in various organs
of cancer-developing mice. (A) A tumor mass was observed in the spleen
of 1 of the 10 mice in the MWCNT group that survived for 26 weeks;
this was identified as an inflammatory pseudotumor, not a neoplasm.
(B) A neoplasm developed in a lung of 1 mouse in the carbon black
group that survived for 26 weeks; this was diagnosed as a benign adenoma.
(C) All 10 mice in the MNU group had tumors. Proventricular tumors
developed in all 10 animals, with abnormal squamous epithelial growth
observed (upper left panel). Skin tumors developed in 6 of the 10
animals, which occurred as malignant skin tumors in the thigh (upper
right panel). Genital tumors developed in 4 animals (lower left panel).
A thymic tumor developed in 1 animal (lower right panel). Hematoxylin-eosin
staining. Scale bars = 10 μm. (c) Histological images of subcutaneous
implantation sites taken at 26 weeks. (A) In the MWCNT group, no neoplasm
developed, with macrophages found to have accumulated while phagocytosing
MWCNT particles. No inflammatory cells such as neutrophils and lymphocytes
were observed around the implantation sites. (B) In the carbon black
group, like in the MWCNT group, macrophages phagocytosed MWCNT particles,
with no neoplasm observed. Arrow, MWCNTs; arrowhead, carbon black.
Hematoxylin-eosin staining. Scale bars = 10 μm. Reprinted with
permission from ref (98). Copyright 2012 Nature Publishing Group.
Table 3
Neoplastic Changes in rasH2 Mice Implanted
with CNT, Carbon Black, Solvent, or N-Methyl-N-nitrosourea (MNU) Solutiona
total number
control
carbon black
CNT
MNU
organ
diagnosis
10
10
10
10
skin (back
area)
papilloma
0
0
0
2
keratoacanthoma
0
0
0
0
skin (face)
papilloma
0
0
0
3
keratoacanthoma
0
0
0
0
skin (thigh)
papilloma
0
0
0
1
keratoacanthoma
0
0
0
0
spleen
inflammatory pseudotumor
0
0
1
0
hemangioma
0
1
0
0
hematopoietic system
malignant lymphoma
0
0
0
2
epithelial
thymoma
0
0
0
0
kidneys
hemangioma
0
0
0
0
pancreas
hemangioma
0
0
0
0
lungs
adenocarcinoma
0
0
0
0
adenoma
0
1
0
1
hemangioma
0
0
0
0
forestomach
papilloma
0
0
0
10b
basal cell tumor
0
0
0
0
squamous cell carcinoma
0
0
0
0
perineal
papilloma
0
0
0
5c
a
Adapted with permission from ref (98). Copyright 2012 Nature
Publishing Group.
b
Significant
differences at p = 0.0000054125 (Fisher’s
direct method).
c
Significant
differences at p = 0.016254 (Fisher’s direct
method).
The aforementioned
tests showed that nanosized carbon black particles
(a tattoo ink component) could be used as a reference for safety evaluation
of CNTs. The in vivo implantation test, cytotoxicity test, carcinogenesis
test (in transgenic mice), and other tests all found that the toxicity
of CNTs is equal to or less than that of carbon black tattoo ink.
If a safety test using a substance with verified biological safety
as a reference material finds that CNTs exhibit a level of toxicity
equivalent to, or lower than, that of the reference material, then
it can be concluded that CNTs are safe. We are currently conducting
mutagenesis and genotoxicity tests with highly pure carbon black as
a reference material. So far, our results show that CNTs are as safe
as carbon black particles under the experimental conditions used in
the studies.
6
Discussion and Perspective
6.1
Available Safety Evaluations Relevant to CNTs
as Biomaterials
In this Review, studies on using CNTs as
biomaterials have been reviewed, and currently available in vivo and
in vitro studies on the evaluation of CNTs safety as biomaterials
have been described separately. It is clear that many benefits will
come from application of CNTs biomaterials to a wide range of important
medical services, including cancer treatment, regenerative medicine,
implants, and DDSs.
180,489−491
Making the best use of the findings of these application studies
would improve current clinical practices and ensure remarkable progress
of medical care. For this reason, research into application of CNTs
to biomaterials has been increasing rapidly (Figure 1); however, no clinical application
of CNTs has been realized
yet
77
because the evidence for the biological
safety of CNTs as biomaterials is not definitive. It is easy to say
that certain new materials pose risks to living organisms; when one
study raises a safety-related question, the test substance can be
said to be risky but cannot be said to be completely safe unless its
safety is demonstrated in all situations where it is likely to be
used. Safety cannot be assured without conducting numerous studies,
and this is practically impossible. Therefore, as many studies as
possible are needed to make a reasonable, acceptable consensus judgment
based on a comprehensive assessment of the findings. It would otherwise
be impossible to realize the clinical application of a new biomaterial.
The accumulated research into the application of CNTs biomaterials
is already sufficient to make such a judgment. The primary objective
of this Review is to logically determine whether CNTs can be safely
used as biomaterials in clinical settings.
Many pioneering studies
have evaluated the safety of CNTs biomaterials. A wide variety of
CNTs have been used in many different ways, and studied using various
methodologies by many different researchers. Therefore, different
studies have often yielded inconsistent results. However, the right
judgment must be based on a comprehensive assessment of all such results.
To this end, this Review has comprehensively reviewed the relevant
published literature and described as many of the latest findings
as possible. We conclude that the number of studies reporting the
biological safety of CNTs as biomaterials is increasing and that most
of the recently published reviews have concluded that CNTs are very
safe.
91,99,191,326,492
Taken together, the
findings suggest that CNTs will find clinical application as biomaterials
through a stepwise process involving appropriate methods and sites
of use.
6.1.1
In Vivo Studies
As stated above,
no reported in vivo studies have found that CNTs are associated with
life-threatening or otherwise serious toxicities such as carcinogenicity.
Furthermore, recently reported studies for the most part have shown
that inflammation in response to highly pure CNTs implanted in the
body was not intense and resolved quickly.
58,89,91,155,304
The organ(s) sites of CNTs accumulation after transport
through the bloodstream, the response of tissues and cells to the
accumulation, and period of CNTs accumulation are important issues.
No study has reported any problem resulting from intravenous injection
of CNTs.
91,191,310
However,
more accurate techniques must be developed for monitoring the behavior
and disposition of CNTs intravenously injected in large amounts for
use in DDSs and imaging. Bearing in mind that CNTs can enter the pulmonary
circulation when inhaled, the development of such techniques is ongoing
worldwide.
86,144,306,312−315
Hence, the distribution of CNTs after passage through the bloodstream
will be revealed in the near future. At present, researchers should
refrain from clinically applying CNTs to sites with abundant blood
supply until adequate data are available to verify its safety.
On the other hand, it is necessary to determine whether CNTs topically
used as biomaterials enter the bloodstream. Because CNTs are particles,
they should be limited by size from entering the bloodstream. MWCNTs
above a certain size are thought to rarely enter the bloodstream.
310
Even if they enter from a local site, the concentration
is expected to be too low to have a major impact on the body.
91,139,191,310
In conclusion, on the basis of available in vivo data, it
can be
concluded that MWCNTs are likely to be useful topical biomaterials.
152
Of course, sites of intensive inflammatory
reaction to MWCNTs and likely sites of MWCNTs entry into the bloodstream
appear to depend on the organ and tissue where the biomaterials are
implanted.
304
For this reason, investigations
should be conducted for each site separately to determine the hazard
to each organ and each tissue.
6.1.2
In
Vitro Studies
The results from
in vitro toxicity studies are difficult to interpret. Scientific international
standards, typically ISO standards, have been established to assess
the toxicity of bulk biomaterials,
467
but
not nanoparticles like CNTs, which have distinct properties. In most
recent studies, CNTs (adequately dispersed in solution) are regarded
as chemical substances. This assumption seems to be reasonable for
purposes of assessing the safety of particulate substances. In 2012,
the Organization for Economic Co-operation and Development (OECD)
announced, “Although most testing/assessing methods for conventional
chemical substances are also suitable for nanomaterials, corrections
according to the characteristics of nanomaterials may be needed in
some cases.”
493
It should
be noted that it is difficult to determine which of the many factors
relevant to CNTs is being assessed in an assessment of in vitro CNTs
toxicity. These factors include thickness, length, shape, surface
reactivity, and aggregability as well as the influence of residual
metals, dispersants, and assay reagents.
117,263,334
Because these factors are inter-related,
their influences are difficult to determine separately.
494,495
Variation in the thickness and length of CNTs may be associated
with their cytotoxicity. SWCNTs and MWCNTs differ in toxicity profiles;
for example, SWCNTs are more likely than MWCNTs to induce oxidative
stress. Length is also important; long CNTs are likely to induce oxidative
stress because they are not completely phagocytosed by macrophages.
The toxicity of inhaled CNTs increases with increase in length above
10–20 μm, although this observation has not been confirmed.
412
While the maximum acceptable length of CNT
particles used as biomaterials is unknown, long CNTs are not considered
to pose a problem (such as an inhalation problem), as stated in section 6.2. Surface
reactivity also differs depending on
the type of CNTs; furthermore, the toxicity of chemically modified
CNTs should be thoroughly examined for each modification. Although
the influence of residual metals is not negligible, many studies have
found that CNTs with a quite high purity pose no major problem.
72,365,400−402,407,408,496
On the other hand, the influence
of aggregability of CNTs and the choice of dispersant on toxicity
do pose problems.
483
Many published studies
are thought to have established the total toxicity of CNTs and dispersant.
371,373,385,497
Furthermore, because the aggregability of CNTs and the choice of
dispersant vary widely among different studies, the range of CNT concentrations
used in respective toxicity studies is wide (from 1 ng/mL to 10 μg/mL),
making assessment more difficult.
309,447
From now
on, in vitro studies should be conducted under a standard set of conditions
(dispersant selected to not affect the test cells, and range of concentrations
selected to avoid CNTs aggregation) whenever possible.
6.1.3
Correlations between in Vivo and in Vitro
Data
For CNTs, unlike drugs and other chemical substances,
little is known about correlations between in vivo and in vitro data.
498
One reason is that no in vivo data are available;
nanoparticles have not yet been used as biomaterials, and this issue
cannot be resolved until clinical application is realized. Another
reason is that there are no standard ways for dealing with a wide
variety of factors that can influence the results of correlation studies,
such as the animal species, method of administration used in the in
vivo studies, and the choice of cell and culture broth used in the
in vitro studies.
91
In the future, it will
be necessary to ensure these factors are consistent for all studies,
to collect and analyze data from international sources, and to determine
the correlations between in vivo and in vitro toxicity assessments
of CNTs, at least in small animals such as mice.
191
6.2
Appropriate References
for Safety Evaluation
of CNTs
6.2.1
Requirements for References
Only
a few articles on the safety of CNTs biomaterials were published before
2010, and these reported many conflicting results. However, as the
number of articles increased, the number of safety evaluations increased,
and the conclusion drawn from these results is that CNTs are safe
to use as biomaterials.
89,91,99,191
Nevertheless, clinical application
has been frustrated because researchers have been unable to rule out
CNTs toxicity. This situation is due primarily to the lack of a best
reference material for the evaluation of CNTs safety. The finding
of such a reference would facilitate evaluation of CNTs safety.
68
CNTs biomaterials, like all other biomaterials,
are foreign to living organisms irrespective of their biocompatibility.
At concentrations exceeding a certain level, CNTs exhibit toxicity
in vitro and in vivo. The absence of a reference with established
safety makes it difficult to determine the in vivo safety of CNTs.
For example, when test cells lose activity in the presence of CNTs
at concentrations exceeding a certain level, it is wrong to conclude
that the CNTs are cytotoxic. However, it is right to conclude that
CNTs are not cytotoxic when test cells lose activity in the presence
of a reference with established safety. Biological safety cannot be
assessed without comparison to a reference that has been proven to
be safe in living organisms as described above. However, unfortunately,
no such reference has been found to evaluate the safety of CNTs biomaterials.
Inevitably, CNT toxicity studies have yielded inconsistent results
so that no safety evaluation has been regarded as reliable. The lack
of a reference with confirmed biological safety is attributed to the
traditional view that nanoparticulates are not biomaterials.
6.2.2
Carbon Black
We found a reference
material (carbon black) and proposed its use as a reference in our
research articles published in 2011 and 2012.
97,98
Carbon black is the primary component of black tattoo ink, and tattooing
of the human body has a long history dating back before ancient times,
and is currently commonly performed.
475
Some researchers may dispute the use of carbon black as a reference
for CNTs because CNTs particles are fibrous and carbon black particles
are spherical. It is reasonable to attach importance to this difference
if the research focus is on inhalation toxicity. Cells are unable
to completely absorb long fibrous nanoparticles. It has been hypothesized
that oxidative stress “frustrates” phagocytosis, and
prolongs inflammation and other events.
281,409,411,412
However, this merely accounts for prolonged inflammatory reactions
to CNTs in the thoracic cavity, where inhalation exposure to CNTs
occurs, and in the abdominal cavity (a surrogate for the thoracic
cavity), where exposure to CNTs is experimentally mimicked. Many researchers
have found that even when a considerable amount of CNTs is implanted
subcutaneously and elsewhere, only transient, very mild inflammation
develops and resolves quickly.
58,307−309
This fact suggests that no frustrated phagocytosis occurs at least
in subcutaneous tissue. Hence, because no in vivo implantation study
found that CNTs cause frustrated phagocytosis at sites of transient
inflammation, it can be concluded that fibrous nanoparticles pose
no risk. Generally, animal experiments have shown the improbability
that sites of prolonged CNT-induced inflammation contain CNTs particles.
Therefore, provided that appropriately designed in vivo implantation
studies produce no evidence of prolonged inflammation, then fibrous
CNTs can be used as biomaterials without safety concerns.
Essentially,
sharing all of the characteristics of the test material is not the
only requirement for a substance to be used as a reference. This is
also true for bulk biomaterials; even with different characteristics,
they have been used as references with satisfactory results for desired
effects.
467
Importantly, both CNTs and
carbon black belong to a new category of biomaterials known as nanoparticulate
substances. Logically, as with bulk biomaterials, no problem arises
from the use of carbon black as a reference material for CNTs. Carbon
black (like CNTs) is a nanosized particulate substance, even though
other characteristics may be different. Another advantage of carbon
black as reference is that mass can be used as an index because both
CNTs and carbon black are pure carbon particulates.
97,98
Mass is by far an easier index to use in toxicity studies than particle
count and volume. In view of these facts, we propose the use of highly
pure carbon black as a good reference material for CNTs.
Many
articles are available on the use of carbon black as a reference
for assessment of biological reactions to CNTs.
292,365,381,499−506
However, no clear evidence has been presented supporting the claim
that carbon black is safe to use in living organisms. According to
many researchers who have used carbon black as a reference for CNTs,
carbon black is intuitively the best reference. We have verified the
scientific intuition of many researchers by providing them with a
rationale (i.e., carbon black is safe because it is a component of
black tattoo ink). Because there are a large number of such articles,
we believe that many researchers will agree with the conclusion of
this Review that carbon black is suitable as a reference material
for safety evaluation of CNTs.
6.2.3
International
Standards
What should
happen soon after a consensus is reached that carbon black is suitable
for use as a reference material for CNTs? The safety of CNTs should
be evaluated both in vivo and in vitro using the new reference material.
As stated above, it is easy to say, “Risk may exist”,
but it is difficult to say, “No risk exists”. It is
necessary for as many researchers as possible to conduct as many studies
as possible. All studies then need to use standardized carbon black
as a reference to allow the results to be assessed collectively and
comprehensively compared.
There are many types of carbon black
with somewhat variable biological safety. Above all, highly pure carbon
black (a suspension of nanoparticles, which is equivalent to the carbon
black used in tattoo ink) can be used as a reference for CNTs.
475
At present, we think that carbon black particles
(diameter of about 50 nm and a purity of 99.5% or more) are suitable,
but we would like to suggest here that many experts discuss extensively,
choose, and designate the best carbon black powder as the international
standard.
Special attention should be paid to the carbon black
dispersant
(usually a surfactant) because CNTs particles in the test solution
are less dispersible than carbon black particles.
366−371
The same dispersant should be used in both the CNTs and the reference
solutions, provided the dispersant (when used at concentrations that
fully disperse the CNTs and reference particles) has no major impact
on living organisms and cells. In vitro, in particular, particles
precipitate over time, which can alter the cellular reactions depending
on the precipitation rate. To ensure a valid comparison with the reference,
it is desirable to use a dispersant that minimizes precipitation of
particles. At present, we think polyvinyl alcohol is the best dispersant.
97
However, there may be better dispersants with
higher dispersion efficacy, lower toxicity, greater ease of handling,
and other superior characteristics; therefore, an internationally
acceptable dispersant should be chosen after much discussion on the
basis of a consensus of expert opinions.
6.2.4
Method
of Safety Evaluation
CNTs
with equal or less toxicity than that of the reference carbon black
should be considered “safe”. This judgment can be made
without further research. If the toxicity of CNTs is found to be greater
than that of carbon black, the decision should be deferred. Strictly
speaking, further assessment is impossible because carbon black is
the only currently available reference. Particular attention should
be paid if toxicity is far greater than that of carbon black (e.g.,
toxic concentrations one-tenth of the reference concentration). On
the other hand, if neither CNTs nor carbon black is toxic, CNTs should
be considered nontoxic, or the study conditions should be considered
inappropriate. We propose to collect safety data through various evaluations
of CNTs using carbon black as a common reference, while paying attention
to these facts.
Unfortunately, carbon black cannot be used as
a reference material in the assessment of in vivo kinetics because
particle shape affects localized nanoparticle accumulation and nanoparticle
migration from tissue to bloodstream.
327
As compared to CNTs particles (that are fibrous), carbon black particles
(that are spherical) migrate more readily between tissues and the
bloodstream. However, a reference is not needed to track the in vivo
migration of CNTs. If CNTs accumulate in a certain organ, a study
of CNT implantation may be conducted to assess the biological reactions
at the site using carbon black as a reference.
6.3
Decision To Start Clinical Application of
CNT-Based Biomaterials
As stated above, many researchers
have shown that pristine (very pure) MWCNTs with few failures as biomaterials
are very safe to use as biomaterials. MWCNTs are safe to use topically
but not at special sites such as the lung and abdominal cavity.
91,191,305,306
The safety of using MWCNTs as DDSs or the like and involving access
to the bloodstream has not yet been verified. Furthermore, using tattoo
carbon black as a reference, we showed that pristine MWCNTs are at
least as safe as carbon black.
97,98
Because the above-described
remarkable advances in research into the application of CNTs as biomaterials
have led to the judgment that CNTs biomaterials are probably very
safe (provided the method and site of use are appropriate), now is
a time to start using CNTs clinically. We are planning to clinically
apply MWCNTs (carbon purity, of 99.5% or more; mean diameter, about
60 nm [40–90 nm]; mean length, about 10 μm; and specific
surface area, 25–30 m2/g; produced using the chemical
vapor deposition technique [MWNT-7, Hodogaya Chemical, Tokyo, Japan]).
Of course, a composite material containing 5 wt % or less of MWCNTs
(the safest form of CNTs) will be used.
6.4
Path
to Clinical Application of CNT-Based
Biomaterials
Importantly, we will begin with the safest clinical
application of CNTs and proceed in steps according to the magnitude
of risk involved. We divided the time course into four stages differing
in degree of risk estimated on the basis of the nature of the biomaterial
(composite versus particulate), the site of use (topical versus systemic),
and the degree of in vivo exposure to the particles (high versus low)
(Table 4).
Table 4
Stages of Clinical
Application of
CNT-Based Biomaterialsa
atage
nature of the biomaterial
site of use
degree
of in vivo exposure
risk
example of use
stage 1
composite
topical
none/low
none/low
artificial joints and
interbody fusion materials
stage 2
particulate
topical
intermediate
low/intermediate
DDSs and imaging for cancer
treatment
stage 3
particulate
topical
intermediate
low/intermediate
regenerative medicine scaffolds and DDS for topical treatments
stage 4b
particulate
systemic
high
high
DDSs and imaging that circulate via bloodstream
a
Clinical application of CNTs to
biomaterials should progress demonstrating the safety at each stage.
b
The decision of proceeding
to stage
4 requires extremely careful consideration.
The first stage is characterized by the use of a CNTs
composite
material for implantation (stage 1). Generally, the CNT content in
a composite material is not more than 10 wt %, and the likelihood
of in vivo exposure to CNTs particles is zero or minimal. Therefore,
problems due to CNTs in the human body are unlikely to occur. As the
first biological application of CNTs, we are planning to use composites
of MWCNTs with existing biomaterials in artificial joints or spine
interbody fusion materials.
In application to artificial joints,
we are developing an MWCNT/polyethylene
composite material and an MWCNT/ceramics composite material. Although
the polyethylene used in sliding parts of artificial joints is ultrahigh
molecular weight polyethylene (UHMWPE), it wears during long-term
use and can necessitate resurgery.
242−245,507
For this reason, cross-linked UHMWPE has become commonly used, although
its excessive hardness and easy breakability are problematic.
508−512
Having favorable characteristics that are absent in conventional
materials, that is, high wear resistance and low breakability, MWCNT-conjugated
UHMWPE is suitable as a sliding parts material for artificial joints
(Figure 12). On the other hand, ceramics are
also used in the sliding parts of artificial joints. Although ceramics
wear very slightly, they are breakable so that resurgery is sometimes
needed.
513−518
Combining CNTs with ceramics increases fracture toughness and can
transform ceramics into an ideal, wear-free, antifracture, sliding
parts material for artificial joints.
Figure 12
For application to sliding
parts of artificial joints, an ultrahigh
molecular weight polyethylene (UHMWPE) conjugated with MWCNTs has
been developed. (a) A UHMWPE socket (left panel) and an MWCNT-conjugated
UHMWP socket (right panel) for use in sliding parts of artificial
joints. (b) A prototype artificial joint with a socket made of CNTs.
Having favorable characteristics that have not been achieved with
conventional materials, that is, high wear resistance and low breakability,
MWCNT-conjugated UHMWPE is suitable as a sliding parts material for
artificial joints.
To improve the quality
of interbody fusion material, we are now
engaged in developing an MWCNT/PEEK composite. PEEK is a highly biocompatible
material possessing excellent biological safety and mechanical characteristics.
519,520
Because of its low compatibility for bone tissue, however, PEEK
has been associated with the problem of insufficient bone union when
used in implants that are directly exposed to bone, such as interbody
fusion cages.
521−524
MWCNTs have been reported by many research teams, including ours,
to possess bone induction potential.
58,62,63,67,213,215,216,218,525,526
If conjugation with MWCNTs further
improves the mechanical characteristics of PEEK and also induces osteogenesis,
then MWCNT/PEEK composite will become an ideal interbody fusion material
(Figure 13).
Figure 13
For application to spine interbody fusion
material, a polyetheretherketone
(PEEK) composite with MWCNTs has been developed. (a) A conceptual
diagram showing that PEEK, when conjugated with MWCNTs, will become
an innovative spine interbody fusion material possessing excellent
mechanical characteristics and bone compatibility. (A) The MWCNTs
on the surface confer bone compatibility. (B) The internally conjugated
MWCNTs control the elastic modulus. (b) (A) A PEEK spine interbody
fusion cage (left panel) and an MWCNT-conjugated PEEK cage (right
panel). (B) A prototype interbody fusion cage made of CNTs.
In 2012, the European Commission
(EC) announced a draft regulation
as amended to oblige manufacturers of medical equipment used to make
nanomaterial-containing products, to properly label medical devices
containing nanomaterials categorized under Class III (most dangerous
substances). This rule shall apply only in cases where such medical
devices are used for the intended purposes and in the absence of measures
(such as encapsulation and coupling) to prevent nanomaterials from
entering the patient’s body and the user’s body.
527
Hence, use of CNTs composites as biomaterials
may not be subject to legal regulations because they are bound to
the base material. For this reason, we believe that stage 1 poses
only a minimal safety risk and can be safely implemented, provided
the appropriate legal procedures of each country are followed.
In stage 2, CNTs particles are used within the body. This stage
represents the first high barrier to clinical application of CNTs
because nanoparticulate substances come into direct contact with the
body. This usage is subject to legal regulations according to the
definition of the EC, and is thought to require international approval
from an ethical viewpoint as well. Hence, research activities cannot
proceed to this stage until an extensive assessment is performed following
the establishment of international standards for evaluation of biosafety.
Initially, the use of CNTs must be limited to localized sites. Furthermore,
top priority should be given to the use of CNTs in situations where
the benefits from their use by far outweigh the risks involved. Specifically,
the most likely field appears to be cancer treatment, where no other
treatment is available or treatment with CNTs is highly advantageous
over other treatments. This is currently the most hopeful field of
clinical application of CNTs. It is evident that if CNTs become applicable
to DDSs and imaging for cancer treatment, dramatic advances in the
treatment and diagnosis of cancer will be achieved, which is expected
to contribute substantially to the health and welfare of many patients.
Stage 3 also concerns the topical use of CNTs particles as in stage
2, but the coverage is expanded to include the treatment of diseases
requiring higher safety than in stage 2. CNTs are used clinically
in topical treatments (including regenerative medicine scaffolds)
and for the treatment and diagnosis of diseases that are less life-threatening
than cancer, such as diabetes mellitus. In this stage, coverage of
target diseases and use sites is much wider, and application of CNTs
biomaterials more common. Stages 2 and 3 involve the same level of
risk but have different benefits.
Finally, we will proceed to
stage 4 aimed at the treatment of diseases
involving the injection of CNTs and their systemic circulation via
the bloodstream for the purpose of drug delivery and whole-body imaging.
However, this decision requires extremely careful consideration.
327
As of 2012, the EC had approved 20 nanopharmaceuticals
(of course, other than CNTs).
301
Although
drug delivery and whole-body imaging using CNTs are highly effective
procedures, major risk arises from their systemic circulation via
the bloodstream. No clinical application should be started until the
disposition of CNT particles and their effects on the heart, lung,
liver, spleen, kidney, and other organs are extensively investigated
and sufficient data are available to obtain an international consensus.
At present, it remains unknown whether research activities will advance
to stage 4.
It is important to make steady progress through
these stages of
clinical application and exercise discretion to demonstrate the safety
of CNTs at each stage. This biological application and technical improvements
in the biological application of CNTs would help accelerate the development
of groundbreaking new therapeutic methods.
6.5
Establishing
International Standards for Biological
Safety Evaluation
To date, studies evaluating CNTs biomaterials
safety have been conducted all over the world; however, interpretation
of the collective results has been problematic because different methods
of assessment were used by different researchers. Hence, it has been
impossible to build a centralized toxicity database, which is essential
for the assessment of CNTs safety and efficiency in biological systems.
91
International standards for biological safety
evaluation need to be established as soon as possible, to conduct
toxicity studies using one method of assessment and one set of standards,
and to provide access to all results internationally. By doing so,
many reliable results from all over the world can be analyzed by many
experts, allowing them to make the right consensus decision. There
are a great many types of CNTs and numerous derivatives produced by
chemical modification. To achieve safe clinical application of these
CNTs as soon as possible, there is an urgent need to establish international
standards for the evaluation of biosafety.
70,191
In the biological application of CNTs, it is critical to evaluate
the safety of functionalized CNTs (f-CNTs), which are likely to find
application as DDSs, for in vivo imaging, and in regenerative medicine
scaffolds. Chemical modification is also important to increase the
dispersion efficacy of CNTs, a key to successful biological application.
331
Of course, f-CNTs must be examined for safety
individually. Furthermore, some researchers are working to functionalize
CNTs to make them safer to living organisms.
257,334,528
To facilitate the application
of numerous f-CNTs as biomaterials, it is of paramount importance
to establish international standards for safety evaluation.
Provided that criteria are logically formulated on the basis of
the published results from studies evaluating the safety of CNTs biomaterials,
international standardization of the CNTs safety evaluation methodology
would not be difficult. The first task is to establish standards for
the topical use of CNTs. Specifically, in vivo and in vitro studies
should first be conducted in the same manner as with ISO-standardized
ordinary bulk biomaterials to assess the toxicity resulting from the
dissolution of impurities contained in CNTs and some or all of the
molecules bound to the CNTs. In vivo studies then should be conducted
to assess the CNTs toxicity intrinsic to their identity as nanoparticles.
This involves implantation of CNTs at the sites of their potential
use to determine biocompatibility with a particular organ or tissue.
The in vitro studies involve the dispersion of CNTs with a standard
dispersant and use of ISO-compliant test methods similar to those
used for ordinary chemical substances.
467
The in vivo and in vitro studies for determination of the intrinsic
toxicity of CNTs involve comparison with a nanoparticulate reference
material, carbon black as described above. With a standard reference,
international standards for the evaluation of the biological safety
of topically used CNTs particles can be established without delay.
Subsequently, efforts will be made to establish international standards
for the evaluation of CNTs safety in applications involving passage
through the bloodstream. Basically, in vivo studies on CNTs well dispersed
in solution will be conducted using the same criteria as those used
for ordinary chemical substances. However, it is unknown which substance
(possibly an existing nanoparticulate material already used clinically
in DDSs and possibly transported through the bloodstream, with confirmed
safety and properties similar to those of CNTs) will be the appropriate
reference material. Selection of a reference for this application
of CNTs, which circulate in the bloodstream, is a major challenge
to be tackled in the future.
In all cases, international standards
for the evaluation of CNTs
biosafety need to be established as soon as possible because ultimately
CNTs will revolutionize cancer treatment and regenerative medicine,
which are top priorities in today’s medicine. Now is the time
to translate research on safe CNT composite implants into clinical
applications. International standards for evaluation of CNTs biosafety
must be established to enable the topical use of CNTs particles. Research
into any important medical issue should always proceed without interruption.
7
Conclusion
The study of the application of
CNTs as biomaterials has been increasing
dramatically because CNTs have been shown to be extremely effective
and very safe biomaterials. Biomaterials that have doubtful biosafety
are unlikely to find clinical application in the future. Although
it is logically impossible to say that CNTs are completely safe to
use in living organisms, CNTs can be judged to be extremely safe if
no evidence of biological risk has been obtained by a vast number
of studies investigating their biological application. Most researchers
in this field think CNTs are safe to use in living organisms, provided
that the appropriate method and site of delivery are used.
CNTs
biomaterials if fully utilized could lead to many revolutionary
and important medical technologies. Because of the extremely advantageous
characteristics unique to CNTs, the biological safety evaluation issue
making us reluctant to start their clinical application must be solved
as soon as possible.
Thanks to the painstaking efforts of a
great many researchers,
much evidence supports the claim that CNTs are generally safe as biomaterials.
Accordingly, now is the time to start clinical application of CNT
composite implants, the biologically safest form of CNTs, because
there is little possibility that CNTs will be directly exposed to
the living organism. To quickly proceed topical use of CNTs particles,
it is necessary for researchers to establish international standards
for biosafety evaluation as soon as possible. In this process, the
carbon black reference will play an important role. When taking the
next and most risky step toward clinical application (that involves
the entry of CNTs into the circulation), the utmost caution must be
exercised to ensure safe use.
Because many researchers can now
evaluate the biosafety of CNTs
using the power of the latest science and technology, we should now
embark on a journey toward the clinical use of CNT-based biomaterials
in an ethical and courageous manner.