The emergence of responsive polymers is of fundamental interest, and their ability
to reversibly reply to a given stimulus, such as heat, electric voltage, or light,
has practical applications in several fields, including soft robotics, active sensing,
and actuation.[1] Notably, conjugated polymers (CP), such as polypyrrole (PPy), polyaniline,
and poly(3,4-ethylenedioxythiophene) (PEDOT), have shown great potential with respect
to actuation and significant efforts have been focused in particular on the realization
of “dry” CP actuators that can function in air.[2] Recently, Okuzaki et al.3 proposed
a new class of CP actuators that function in ambient air based on the cooperation
between the electrical conductivity and hygroscopic nature of conductive polymers.
These authors discovered that electrochemically synthesized PPy films exhibit a significant
reversible volume expansion in air resulting from the absorption and desorption of
water vapor present in ambient air.4 Furthermore, a volume contraction was observed
under the application of an electric current, which was attributed to the desorption
of water vapor as a result of Joule heating.5 A similar electrically induced isotropic
dimensional change was subsequently observed in ≈20 μm thick poly(3,4-ethylenedioxythiophene):polystyrene
sulfonate (PEDOT:PSS) films prepared using the solution-casting method.6 The ubiquitous
presence of humidity in ambient air and its variation makes the development of humidity-responsive
actuators both appealing and of importance. By chance, the capability to convert simple
environmental stimuli, such as humidity, into mechanical reversible motion is regularly
observed in living systems, particularly plants.7 These systems are capable of converting
the sorption and desorption of water into driving forces for movement. A well-known
example is the release of ripe seeds from pine cones, which open due to a bending
movement of their scales during drying in ambient air and close in wet conditions.8
Similarly, seeds from wild wheat are propelled into soil after being released, which
is solely due to the daily change in humidity that induces a curvature of the awns
depending on the moisture level.9 Interestingly, many actuation systems in plants
have a common structural feature: the humidity-responsive actuation results from the
coupling of two differently structured tissue layers with different elongations along
a specific direction. These layers are interfacially bound to each other to form a
laminated, double-layered composite structure.10 Although the aforementioned actuators
developed by Okuzaki's group undergo isotropic dimensional changes, anisotropic motions
have not been thoroughly explored. Inspired by the differential swelling or shrinking
of natural double-layered structures, one approach for achieving anisotropic motion
in artificial actuators is to use a bilayer system in which the active layer is composed
of a humidity-responsive material and the other layer is a passive material that is
inert to humidity, that provides mechanical strength and that converts the isotropic
volume change into a bending motion. In this work, we present a double-layered, anisotropic
humidity-driven actuator based on an ultrathin active layer of PEDOT:PSS and a passive
layer composed of poly(dimethylsiloxane) (PDMS), with intrinsic sensing capability,
and able to be controlled both by joule effect or directly by environmental humidity
variations.
PDMS elastomer was selected as the passive substrate because of its elasticity, good
flexibility, chemical and humidity inertness, remarkable durability against repeated
deformation, and resistance to high temperature. Because of these properties, in our
system, PDMS should easily deform as a result of the contraction/expansion of the
active layer and should withstand repeated and reversible deformation. PEDOT:PSS was
selected as the active material due to the good combination of several features that
influence actuator performance, such as its suitable electrical conductivity, humidity
responsiveness, mechanical properties, and chemical and thermal stability.11 Moreover,
PEDOT:PSS is commercially available in the form of a ready-to-use waterborne dispersion;
therefore, the active conducting polymer layer can be deposited by spin-coating, which
results in a well-controlled homogeneous and reproducible thickness in a range between
a few tens to several hundreds of nanometers, as we reported in some recent works.[12]
The proposed actuator has the potential to simultaneously combine active/passive actuation
and sensing within a single-composite material. Movement can be controlled through
the application of an electric current or by changes in environmental humidity (Figure
1
a and Video S1, Supporting Information). In addition to actuation, the hygroscopic
properties of the PEDOT:PSS layer impart sensing capabilities because a variation
in the environmental humidity induces a reversible, reproducible variation in its
electrical resistance, typically on the order of 1.25%/10% relative humidity (RH).[12c]
Furthermore, because of the piezoresistive behavior of PEDOT:PSS, where its gauge
factor has been reported to be between 5.2 and 17.8, this material has been proposed
for use as a sensitive layer in strain and touch sensors.[13]
Figure 1
a) Schematic representation of the working principle behind the actuators based on
the sorption/desorption of environmental moisture: beginning from the original position
(central), in which the actuator is in equilibrium with its environment, the application
of an electric current drives a contraction of the PEDOT:PSS layer due to Joule-heating-induced
water desorption, which subsequently induces a bending motion toward the PEDOT:PSS
layer (right); in the reverse process, as the environmental moisture content increases,
the actuator bends from its original position toward the PDMS layer due to the sorption
of water until a new equilibrium is established (left). b) Overview of the actuator
fabrication process: (i) silanization of the silicon wafer; (ii) deposition of the
PDMS layer by spin coating; (iii) air plasma treatment and deposition of PEDOT:PSS
(12 layers) by spin coating; (iv) laser cutting and patterning; (v) deposition of
gold electrodes by DC sputtering; and (vi) peeling of the bilayer. c) Actuation movement
of an electrically driven flower-shaped actuator and corresponding thermal images.
d) When a voltage is applied between the electrodes, the pattern drives the flow of
current along each petal, which allows the structure to fold (see also Video S3, Supporting
Information). e) Scanning electron microscopy (SEM) images (40° tilted view) of a
cross section of the PEDOT:PSS/PDMS bilayer, which was cut using a razor blade. The
magnification of the left image is 800× (scale bar represents 50 μm), and the magnification
of the right image is 80 000× (scale bar represents 500 nm). The image was postcolorized:
the PDMS layer is green, and the PEDOT:PSS layer is blue.
All of the aforementioned unique features along with the tailorability of the fabrication
process provide possibilities for a series of interesting applications for these structures
as active bioinspired elements with intrinsic sensing and actuation capabilities.
A schematic representation of the developed fabrication procedure (see also Experimental
Section), is shown in Figure 1b. We followed a mask-free, time- and material-saving
approach based on direct machining of the PEDOT:PSS/PDMS structures using a commercial
CO2 laser cutting system, which allowed actuators to be fabricated with different
shapes and circuit patterns (see Video S2, Supporting Information). By varying the
laser cutting parameters, we were able to simultaneously cut and pattern the surface
of the actuators, thereby avoiding misalignment problems and reducing the processing
time and manipulation. The patterning of the active surface in the form of electric
circuits was performed by engraving only a thin PEDOT:PSS layer. This allowed the
current flow between the electrodes to be driven along specific paths, thereby inducing
local bending of selected portions of the structure (Figure 1c–d). Due to the silanization
of the silicon wafers, the PEDOT:PSS/PDMS structures could easily be peeled off from
the substrate after laser machining and used as free-standing actuators. The electrical
connection was easily established using small flat alligator clips.
Notably, the ratio of the active-to-passive layer thickness is extremely small, as
clearly evidenced in the scanning electron microscopy (SEM) images of an actuator
cross section (Figure 1e), in which the thicknesses of the PEDOT:PSS and PDMS layers
were 600 ± 29 nm and 120 ± 10 μm, respectively. The maximum moisture desorption-related
contraction in PEDOT:PSS was estimated to be on the order of 2%.6 Despite the thinness
of the active layer, the properties of the selected materials and the actuator design
are such that this small contraction is capable of causing a large bending displacement
of the relatively thick bilayer. However, the use of a thin active layer allows for
a rapid sorption/desorption of water, which results in a rapid actuation time. Moreover,
the PEDOT:PSS film is characterized by a multilayered structure due to the multiple
spin-coating deposition steps (magnified part of Figure 1e). We believe that this
stacked layer microstructure has a beneficial role in relaxing the stress induced
by bending and in reducing the effect of crack propagation within the active layer.
For characterization, the electro-thermo-mechanical responses of our actuators were
tested on beam-shaped structures. To evaluate how the surface patterns affect actuator
performance, three beam structures were fabricated (namely s1, s2, s3) that had the
same dimensions (3 mm wide, 10 mm long) but different pattern lengths (h = 2, 5, and
8 mm, respectively) above the base of the beam (Figure
2
a). The resistances of the resulting conductive paths were 1.13 ± 0.03, 1.63 ± 0.03,
and 2.50 ± 0.18 kΩ for samples s1, s2, and s3, respectively. The electro-thermomechanical
characterization was performed on cantilever beams in which one end was fixed and
the other end was free to move. A rectangular area was provided at one end of the
beam for clamping and for the electrical connection. Step input voltages of 5, 10,
15, 20, 25, and 30 V were applied to the samples, and the resulting bending displacements
were recorded using a digital camera. Depending on the pattern length, different portions
of the beam were heated by the current, which induced different bending behaviors
(Figure 2a). Indeed, s3 was bent along almost its entire length, i.e., almost to the
tip, whereas s1 and s2 exhibited shorter bending regions, which corresponded to the
heated portion, with the rest of beam remaining essentially straight. For each input
voltage, the surface temperature, curvature, and blocking force were determined, and
the measurement results are summarized in Figure 2b–d. Regarding the surface temperature,
the actuators were tested up to a maximum temperature of ≈200 °C, which was considered
to be the safe limit for reliable performance of the materials. The three samples
exhibited a linear temperature increase as the power increased and an increased curve
slope as the pattern length decreased (Figure 2b). This trend was expected because
a longer line pattern (s3) resulted in heating throughout a wider portion of the actuator
compared with that of a smaller portion (s1, s2); hence, a lower temperature was reached
under the same applied power. The curvatures of samples s1–3 were estimated by image
processing, as described in the Experimental Section, by considering the heated beam
portions. Examples of the processed images are presented in Figure S1 (Supporting
Information), in which a straight portion closer to the tip is clearly visible, particularly
for higher curvatures. As expected, at the same temperature, the curvature of the
bending segment was found to be independent of the pattern length, which is highlighted
by the similar trends of curvature versus temperature shown in Figure 2c, where the
maximum curvature was ≈0.4 mm−1.
Figure 2
a) The superpositions of images taken at different input voltages for s1, s2, and
s3 highlight the difference between the bending behaviors of the samples. Actuator
surface temperature versus applied power. b) and curvature versus temperature c) for
the three samples. d) Measured blocking force at different temperatures for sample
s3 compared with the theoretical force (s3mod). e) On/Off curvature of sample s3 powered
with a square wave voltage with an amplitude of 20 V at different frequencies. Time
profiles of surface temperature, curvature. f), and corresponding temperature dependence
of curvature g) of sample s3 in response to a step input voltage of 20 V. h) Superposition
of images taken for sample s3 before and after 1000 cycles of actuation with a square
wave input voltage with a frequency of 0.05 Hz and amplitude of 20 V. All scale bars
represent 5 mm. i) Actuator preliminary design chart (example): for a given thickness
h
s of the PDMS layer, each curve shows the maximum actuator length L
max that should not be exceeded to ensure F
b > σW, where σ is a targeted value. This chart shows that actuation is more effective
for sample lengths that are less than ≈1 cm.
The blocking force was measured on sample s3 by contacting the tip of the sample with
the force sensor. The maximum force detected here (0.32 mN, 32 mg) was already 12
times of the actuator weight (2.7 mg) (Figure 2d).
The actuation dynamics was investigated by evaluating the time profiles of the surface
temperature and corresponding curvature of sample s3 in response to a step input voltage
of 20 V (Figure 2f). Such a voltage caused a temperature increase of ΔT = 80 °C, and
a stable maximum temperature of 112 °C was reached in ≈10 s. The corresponding curvature
profile was characterized by three different trends: rise (0–10 s, red), bent state
(10–60 s, green), and recovery (60–85 s, blue). The rise time was relatively short,
with the curvature rapidly increasing from the initial (off) state, which exhibited
a negative curvature, as the input voltage was applied. Once the bent state (on) was
reached, with a maximum curvature of nearly 0.3 mm−1, it remained approximately stable
as long as current was supplied. When the voltage was switched off, the actuator recovered
its initial curvature within 25 s, which was the time required to reset at room temperature
(see also Video S4, Supporting Information). The forward and backward curvature paths
exhibited a remarkable overlap with no evidence of hysteresis (Figure 2g). To determine
the operating frequency range of the actuators, we applied square wave input voltages
with different frequencies to the samples. As the actuation frequency increased, we
observed a reduced curvature change between the on and off states because the step
voltage duration was insufficient for both the bent and relaxed states to reach stability
(Figure 2e). Nevertheless, the actuator bending movement was appreciable up to 5 Hz.
The long-term reliability of the actuator performance was evaluated by repetitive
cycling of the actuator, up to 1000 cycles, and negligible changes in the curvature
were observed (Figure 2h).
The static performance of the actuator was modeled using relevant expressions from
linear beam theory, which was extended to a bilayer14 (see the Supporting Information
for details). In particular, we firstly obtained from the curvature (k) data the mismatch
strain (α, in agreement with previous studies,6 and we then predicted the corresponding
blocking force (F
b), which is reported in Figure 2 d. (labeled s3mod) against the experimental measurements.
Based on the above result, we used the model to further characterize the application
range of the actuator. In more detail, denoted by W the actuator weight, we derived
the maximum length (
) beyond which the ratio
falls below a desired threshold σ, so that actuation might cease to be effective.
The obtained expression reads:
(1)
where: h
s,
, and Es
, respectively, indicate the height, the density, and Young's modulus of PDMS layer
(
= 960 kg m−3;
= 2.28 MPa, see Experimental section); g is gravity acceleration;
, with
and
, where h
f and E
f indicate the height and the Young's modulus of PEDOT:PSS layer (h
f = 600 nm, E
f = 1 GPa[12]). Illustrative trends for Equation 1 are presented in Figure 2i for
selected values of σ; it is clear that the developed actuator is more effective for
lengths that are less than ≈1 cm, consistently with our experimental observations.
For a given length, the actuator force can be enhanced by increasing the beam width.
Despite its inherent simplifications, the considered model can be suitably used for
the preliminary actuator design.
As previously mentioned, in addition to electrical actuation, the bilayer is able
to macroscopically react to variations in environmental humidity through passive movements
without requiring any electrical or thermal stimulus. The humidity-responsive passive
actuation behavior of the PEDOT:PSS/PDMS films was demonstrated by placing the beam-shaped
samples inside a humidity chamber and exposing them to variations in RH at a constant
temperature (30 °C). The bilayer responded to the variations in humidity with a bending
movement in a fully reversible manner and in the opposite direction with respect to
that for electrical stimulus (Figure
3
a). Because the humidity chamber is slow to reach a preset humidity level, it is not
possible to appreciate the speed of the response from these results. The speed of
the response is better evidenced, although only qualitatively, in the second part
of Video S3 (Supporting Information), which shows a leaf-shaped actuator bending when
a finger comes in close proximity to the PEDOT:PSS surface. A rapid response to the
moisture evaporating from the skin is clear, where the leaves move away and then quickly
reset back to the resting position when the finger is removed. The features of this
passive behavior make this material interesting for the development of hygromorphic
actuators. Because humidity is ubiquitous in real-world applications, it is important
to assess how variations in RH levels influence electrically induced actuation. Indeed,
different equilibrium states are established between the water content of the PEDOT:PSS
film and the surrounding environmental humidity, which affects the bending radii of
electrically activated actuators, with each input voltage changing with the RH level
(Figure S2, Supporting Information).
Figure 3
a) Passive bending motion of a beam caused by variations in the RH level in a humidity-controlled
chamber at T = 30 °C (side view, PEDOT:PSS layer on the right). b) Hand-shaped actuator
with individually addressable fingers. c) A 6-finger gripper prototype used to demonstrate
the ability to lift a lightweight object (a piece of polystyrene foam). d) and to
stand up on a plane. e) A leaf-shaped actuator (left) as a multifunctional system
able to respond to touch stimuli (central) by closing its leaves (right). All scale
bars represent 1 cm.
The proposed bilayer actuators have applications as soft grippers, manipulators, or
as active elements in millimeter-scale walking robots. Due to the mask-free and rapid
fabrication process, it is easy to change the design of the actuators to meet the
requirements of the target application in terms of geometry, bending behavior, and
force. Site-specific electrical actuation was demonstrated with a patterned bilayer
film cut in the shape of a small hand with individually addressable fingers. Five
insulated and independent circuits were used to control the bending of each finger,
as shown in Figure 3b (see also Video S5, Supporting Information).
As a first proof-of-concept manipulator, we created a 2 cm long, 6-finger gripper
that was able to grasp and lift lightweight objects with a weight comparable with
its own (Figure 3c). Moreover, a similarly shaped actuator, lying flat on a plane,
was able to stand up on its legs and lift an object with a weight three times that
of its own (Figure 3d). In this case, the electrical connection was established by
attaching two thin copper wires to the gold electrode with silver paint.
One of the most interesting features of the proposed bilayered composites is the possibility
of functioning simultaneously as a structural and functional (actuation, sensing)
material. This capability indeed has long been pursued by engineers and robot designers
in the search for multifunctional materials and structures for the construction of
smarter and simpler tools and robots.15 Recently, significant advances have been reported
in the development of artificial muscles with tactile sensitivity (sensing muscles)
which can sense by themselves while working any mechanical perturbation, not requiring
additional sensors.16
Considering this feature, this multifunctional material can be useful for mimicking
the behaviors of materials often encountered in nature. Indeed, several species of
plants are capable of sensing and responding to mechanical stimuli, e.g., closing
their leaves for protection (Mimosa pudica) or even to catch prey (Dionaea muscipula).17
As an example of the potential of our approach, we attempted to imitate the behavior
of M. pudica by exploiting the variation in electrical resistance induced in PEDOT:PSS
layer structures when the bilayer is mechanically deformed. For this purpose, a simple
prototype was constructed with a leaf-shaped form; this “artificial” M. pudica was
able to close its leaves when touched (see Figure 3e and Video S5, Supporting Information).
The sensing of touch is performed by dedicated electronics that monitor the resistance
of the PEDOT:PSS layer with a small current through a suitable microcontroller-based
system. The system triggers the Joule heating driver when a rapid resistance variation
is detected. Suitable detectable variations in resistance were observed even for small
deformations resulting from a gentle touch (more details on the electronics are reported
in Figure S3, Supporting Information).
In conclusion, this investigation provided a straightforward and versatile method
for fabricating bioinspired soft actuators and provided insight into how the relevant
mechanisms of humidity sorption/desorption and Joule heating affect the actuation
properties of the proposed bilayer systems. The role of geometry on the performance
of the beam-shaped actuators was also addressed from experimental and theoretical
perspectives. The adopted processing strategy, which consists of a few steps of spin
coating, direct laser cutting, and patterning, is particularly appealing because it
enables rapid and simple fabrication to test soft actuators with more complex designs.
The proposed smart material structure possesses intrinsic multifunctionality, such
as simultaneous structural properties, sensing, and actuation capabilities that exhibit
similarities with notable examples in plants and that promote the integration of multiple
functions in bioinspired soft devices/robots.
Experimental Section
Fabrication of PEDOT:PSS/PDMS Actuators: Silicon wafers (400 μm thick, p-type, boron
doped, <100>, Si-Mat Silicon Materials) were silanized by placing them in a desiccator
for 30 min along with a vial that contained a few drops of silanizing agent (chlorotrimethylsilane,
Sigma–Aldrich). A film of PDMS ( 10:1 ratio of base elastomer to curing agent, Sylgard
184 silicone elastomer base and curing agent, Dow Corning Corp.) was then spin coated
onto the silanized Si substrates for 60 s at a speed of 500 rpm and then cured at
T = 95 °C for 60 min in an oven. A subsequent air plasma treatment (Harrick PDC-002
Plasma Cleaner, Harrick Plasma) was applied with a power of P = 5 W for 60 s. A commercially
available PEDOT:PSS aqueous dispersion (Clevios PH 1000, 1:2.5 PEDOT:PSS ratio, solid
content 1.0%–1.3%; H.C. Starck Gmb) was filtered (Minisart, average pore size of 1.20
μm, Sartorius) and mixed with 1 wt% of the fluorosurfactant Zonyl-FS300 (Sigma–Aldrich).
12 layers of PEDOT:PSS were deposited by spin coating at a rate of s = 2000 rpm for
60 s onto the plasma-treated PDMS substrate. After each deposition step, the samples
were placed on a hot plate in ambient air for 5 min at 170 °C and air plasma treated
at 5 W for 10 s. Finally, the samples were subjected to a thermal treatment (1 h;
T = 170 °C). A direct-write CO2 laser (VersaLaser, Laser Systems) with tunable laser
power, speed, and resolution was then used to cut and pattern the samples, making
it feasible to fabricate a wide range of soft actuator designs and configurations.
To provide the electrical input voltage to the material, gold electrodes were added
to the surfaces of the actuators. A gold layer (≈100 nm thick) was deposited on the
PEDOT:PSS surface by DC sputtering (Q150T S Sputter Coater, Quorum Technologies Ltd.)
through dedicated shadow masks. The actuators were then peeled off from the Si substrate.
Morphology of the Actuators: Thickness measurements were performed using a P-6 stylus
profilometer (KLA Tencor, USA). For the PDMS layer, measurements were performed on
PDMS strips that had been peeled off from the Si substrate prior to the deposition
of PEDOT:PSS. For the PEDOT:PSS layer, the PEDOT:PSS was gently scratched from the
PDMS substrate with tweezers, and the height of the profile was measured across the
scratch. SEM images (40° tilted view) were obtained with a Helios NanoLab 600i Dual
Beam Focused ion beam/field-emission SEM instrument (FEI, USA). Images were acquired
of the cross section of the bilayer cut using a razor blade.
Electrically Induced Bending: The electro-thermo-mechanical response of the actuators
was investigated by recording the responses of the actuators to different step voltages
with a digital optical microscope. Videos and images of the samples during the experiments
were recorded with a uEye UI-2250-MM CCD camera (Imaging Development Systems, Obersulm,
Germany) equipped with a Zoom 6000 zoom lens (Navitar, Rochester, NY). The temperature
at the film surface was monitored by real-time thermal imaging using an IR thermal
camera (A325sc, FLIR Systems, 60 Wilsonville, OR). The experiments were performed
in ambient air (about 25 °C, 44% RH).
Image Processing: Curvature measurements have been performed processing images (61
px mm–1 resolution) of the activated samples using semiautomatic procedure implemented
in Matlab (The Mathworks, Natick, MA, USA) (see Supporting Information for details).
Blocking Force Measurement: The measurement of the blocking force of the actuators
was performed using silicon-based microforce sensors (FT-S540, FemtoTools). The tip
of the force sensor was placed in contact with sample s3 located 8 mm from the actuator
base (i.e., the clamped beam section). Once current was applied, the bending moment
induced in the actuator was counteracted by the tip contact force, which was then
acquired by the sensor. Samples s1 and s2 were not considered because the relatively
long beam section not subjected to activation resulted in the measurement being strongly
dependent on the specific location of the sensor. In other words, the conditions to
properly measure the blocking moment, and thus the blocking force, were only well
defined for sample s3.
PDMS Modulus Measurement: Because the model for the bending actuator is extremely
sensitive to Young's modulus of the substrate and because the Young's modulus of PDMS
is strongly dependent on the cross-linking process, we measured the modulus of our
PDMS samples rather than using data taken from the literature. Stress/strain measurements
were performed with up to a 10% applied strain on samples with dimensions of 10 mm
× 3 mm × 0.12 mm using a custom dedicated set-up described elsewhere.18
Humidity Response: To investigate the humidity-responsive deformation of the bilayer
PEDOT:PSS/PDMS actuators in a spatially homogeneous humid air environment, the samples
were placed inside a climatic test chamber (CTC 256, Memmert) with controlled humidity
and temperature; this chamber was equipped with a glass window to permit visual inspection
of the samples. The samples were subjected to various humidity cycles at a controlled
temperature (30 °C) from 30% RH up to 80% RH while simultaneously video recording
the bending of the samples. The RH inside the chamber was monitored using a standard
precalibrated humidity meter.