Three-dimensional (3D) bioprinting uses additive manufacturing techniques to fabricate
3D structures consisting of heterogenous selections of living cells, biomaterials,
and active biomolecules [1,2]. To date, 3D bioprinting technologies have transformed
the fields of tissue engineering and regenerative medicine by enabling fabrication
of highly complex biological constructs. Using the patient’s medical imaging data,
patient- and damage- specific implants can be printed with customized cellular and
physiomechanical functionalities [3,4,5]. The main bioprinting methods include extrusion-based,
droplet-based (inkjet), laser-based, and, more recently, vat photopolymerization-based
bioprinting [6,7]. A variety of biomaterials (i.e., bioinks) have been used for tissue
bioprinting, including ceramics, synthetic and natural polymers, decellularized tissues,
and more frequently, hybrid bioinks consisting of a combination of these materials
[8,9,10,11].
While significant and rapid progresses have been made in tissue bioprinting processes
for various in vitro applications, such as disease modeling [12] and drug screening
[13], there are several challenges to address before bioprinting becomes clinically
relevant [14,15,16]. These constraints include: 1) limited number of available bioink
solutions and lack of thorough characterization of their biological and physiomechanical
properties [10,17]; 2) poor understanding of the correlation between printed architecture
and the ultimate tissue function [18,19]; 3) limitations on the quality of imaging
techniques [20,21] and available bioprinters [22]; 4) complex and rather expensive
processes involved pre, during, and post-bioprinting [22]; 5) suboptimal, non-specialized
printing software and their often incompatibilities [23].
There are eight articles published in this Special Issue composed of four research
papers and four review papers. The research articles focus on the influence of electron
beam (E-beam) sterilization on in vivo degradation of composite filaments [24], enhancing
osteogenic differentiation of stem cells using 3D printed wavy scaffolds [25], the
development of a scaffold-free bioprinter [26], and the fabrication of multilayered
vascular constructs with a curved structure and multi-branches [27]. Kang et al. investigated
the effect of E-beam sterilization on the degradation of β-tricalcium phosphate/polycaprolactone
(β-TCP/PCL) composite filaments in a rat subcutaneous model for 24 weeks [24]. Although
they reported that the E-beam sterilization accelerated the degradation rate of the
composite filaments, due to the decreased crystallinity and decreased molecular weight
of PCL after the E-beam irradiation, they concluded that the chemistry of samples
plays a bigger role than the sterilization method in biodegradation. Ji and Guvendiren
investigated the effect of wavy scaffold architecture on human mesenchymal stem cell
(hMSC) osteogenesis by 3D printing as compared to orthogonal scaffold design [25].
They found that when cultured on wavy scaffolds, hMSCs became elongated, formed mature
focal adhesions, and showed significantly enhanced osteogenesis. LaBarge et al. developed
a custom device enabling the printing of an entire layer of spheroids at once to reduce
printing time [26]. They demonstrated the feasibility of this device first using zirconia
and alginate beads, which mimic spheroids, and human-induced pluripotent stem cell-derived
spheroids. This scaffold-free bioprinter could potentially advance the growing field
of scaffold-free 3D bioprinting. Liu et al. developed a combined approached to fabricate
multilayered biodegradable vascular constructs for cardiovascular research [27]. In
their approach, 3D printing was used to fabricate a mold system which was then used
to cast a hydrogel and a sacrificial material. They investigated the channel wall
displacement during blood flow using fluid-structure interaction simulations. They
also demonstrated the feasibility of their devices using human umbilical vein endothelial
cells. Their approach shows a great potential for constructing integrated vasculature
for tissue engineering.
The four review articles focused on advanced polymers for 3D organ printing [28],
chitosan for tissue and organ bioprinting [29], applications of 3D printing for craniofacial
tissue engineering [30], and in vivo tracking of 3D printed tissue-engineered constructs
[31]. Wang reviewed advanced polymers exhibiting excellent biocompatibility, biodegradability,
3D printability and structural stability [28]. The author also summarized the challenges
of polymers for 3D bioprinting of complex organs. Li et al. reviewed the use of chitosan
in tissue repair, including skin, bone, cartilage, and liver tissue, and 3D bioprinting
of organs [29]. Tao et al. focused on the applications of 3D printing for craniofacial
tissue engineering, including periodontal complex, dental pulp, alveolar bone, and
cartilage [30]. Gil et al. reviewed the currently utilized imaging techniques to track
tissue engineering scaffolds in vivo, with particular focus on the in vivo tracking
of 3D bioprinted tissue constructs [31].
We would like to take this opportunity to express our gratitude to all authors who
contributed to this Special Issue. We also wish to thank all the reviewers for dedicating
their time to provide thorough and timely reviews to ensure the quality of this Special
Issue.