Recent Strategies in Extrusion-Based Three-Dimensional Cell Printing toward Organ Biofabrication

Three-dimensional bioprinting utilizing sacrificial material support and longitudinal printability evaluation through volumetric imaging

In three-dimensional (3D) bioprinting, the internal channel network is vital for nutrient and oxygen transport, crucial for cell survival and tissue construction. However, bioinks' poor mechanical properties hinder precise control over these networks. Advancements in 3D printing strategies, structure characterization, and deformation monitoring can improve hydrogel scaffolds with interconnected channels. Using label-free, non-invasive, in-situ optical coherence tomography (OCT) imaging, we monitored the dynamic deformation of 3D bioprinted hydrogel scaffolds. We validated sacrificial materials' role in enhancing internal channels and introduced deformation characteristics, lateral pore ratio and pore-specific surface area as new parameters. Results from cell-laden hydrogels show that 3D bioprinting with sacrificial materials achieves high fidelity, minimizing collapse, inter-filament fusion, and enhancing lateral porosity. Furthermore, these promote cell proliferation and cell viability.

Measurement of Covalent Bond Formation in Light-Curing Hydrogels Predicts Physical Stability under Flow

Photo-crosslinking hydrogels are promising for tissue engineering and regenerative medicine, but challenges in reaction monitoring often leave their optimization subject to trial and error. The stability of crosslinked gels under fluid flow, as in the case of a microfluidic device, is particularly challenging to predict, both because of obstacles inherent to solid-state macromolecular analysis that prevent accurate chemical monitoring and because stability is dependent on size of the patterned features. To solve both problems, we obtained 1H NMR spectra of cured hydrogels which were enzymatically degraded. This allowed us to take advantage of the high-resolution that solution NMR provides. This unique approach enabled the measurement of degree of cross-linking (DoC) and prediction of material stability under physiological fluid flow. We showed that NMR spectra of enzyme-digested gels successfully reported on DoC as a function of light exposure and wavelength within two classes of photo-cross-linkable hydrogels: methacryloyl-modified gelatin and a composite of thiol-modified gelatin and norbornene-terminated polyethylene glycol. This approach revealed that a threshold DoC was required for patterned features in each material to become stable and that smaller features required a higher DoC for stability. Finally, we demonstrated that DoC was predictive of the stability of architecturally complex features when photopatterning, underscoring the value of monitoring DoC when using light-reactive gels. We anticipate that the ability to quantify chemical cross-links will accelerate the design of advanced hydrogel materials for structurally demanding applications such as photopatterning and bioprinting.

Hybrid biofabricated blood vessel for medical devices testing

Current in vitro and in vivo tests applied to assess the safety of medical devices retain several limitations, such as an incomplete ability to faithfully recapitulate human features, and to predict the response of human tissues together with non-trivial ethical aspects. We here challenged a new hybrid biofabrication technique that combines bioprinting and Fast Diffusion-induced Gelation strategy to generate a vessel-like structure with the attempt to spatially organize fibroblasts, smooth-muscle cells, and endothelial cells. The introduction of Fast Diffusion-induced Gelation minimizes the endothelial cell mortality during biofabrication and produce a thin endothelial layer with tunable thickness. Cell viability, Von Willebrand factor, and CD31 expression were evaluated on biofabricated tissues, showing how bioprinting and Fast Diffusion-induced Gelation can replicate human vessels architecture and complexity. We then applied biofabricated tissue to study the cytotoxicity of a carbothane catheter under static condition, and to better recapitulate the effect of blood flow, a novel bioreactor named CuBiBox (Customized Biological Box) was developed and introduced in a dynamic modality. Collectively, we propose a novel bioprinted platform for human in vitro biocompatibility testing, predicting the impact of medical devices and their materials on vascular systems, reducing animal experimentation and, ultimately, accelerating time to market.

Coaxial Bioprinting of Enzymatically Crosslinkable Hyaluronic Acid-Tyramine Bioinks for Tissue Regeneration

Three-dimensional (3D) bioprinting has emerged as an important technique for fabricating tissue constructs with precise structural and compositional control. However, developing suitable bioinks with biocompatible crosslinking mechanisms remains a significant challenge. This study investigates extrusion-based bioprinting (EBB) using uniaxial or coaxial nozzles with enzymatic crosslinking (EC) to produce 3D tissue constructs in vitro. Initially, low-molecular-weight dextran-tyramine and hyaluronic acid-tyramine (LMW Dex-TA/HA-TA) bioink prepolymers were evaluated. Enzymatically pre-crosslinking these prepolymers, achieved by the addition of horseradish peroxidase and hydrogen peroxide, produced viscous polymer solutions. However, this approach resulted in inconsistent bioprinting outcomes (uniaxial) due to inhomogeneous crosslinking, leading to irreproducible properties and suboptimal shear recovery behavior of the hydrogel inks. To address these challenges, we explored a one-step coaxial bioprinting system consisting of enzymatically crosslinkable high-molecular-weight hyaluronic acid-tyramine conjugates (HMW HA-TA) mixed with horseradish peroxidase (HRP) in the inner core and a mixture of Pluronic F127 and hydrogen peroxide in the outer shell. This configuration resulted in nearly instantaneous gelation by diffusion of the hydrogen peroxide into the core. Stable hydrogel fibers with desirable properties, including appropriate swelling ratios and controlled degradation rates, were obtained. The optimized bioink and printing parameters included 1.3% w/v HMW HA-TA and 5.5 U/mL HRP (bioink, inner core), and 27.5% w/v Pluronic F127 and 0.1% H2O2 (sacrificial ink, outer shell). Additionally, optimal pressures for the inner core and outer shell were 45 and 80 kPa, combined with a printing speed of 300 mm/min and a bed temperature of 30 °C. The extruded HMW HA-TA core filaments, containing bovine primary chondrocytes (BPCs) or 3T3 fibroblasts (3T3 Fs), exhibited good cell viabilities and were successfully cultured for up to seven days. This study serves as a proof-of-concept for the one-step generation of core filaments using a rapidly gelling bioink with an enzymatic crosslinking mechanism, and a coaxial bioprinter nozzle system. The results demonstrate significant potential for developing designed, printed, and organized 3D tissue fiber constructs.

Optimizing Printhead Design for Enhanced Temperature Control in Extrusion-Based Bioprinting

This study addresses critical challenges in the field of tissue engineering, specifically in the optimization of bioprinting technologies for the construction of complex, multicellular tissues. By utilizing a homemade piston-driven extrusion-based bioprinting (EBB) printhead, we performed detailed thermal and flow analyses to investigate the effects of temperature variations on the extrusion process of temperature-sensitive gelatin-alginate bioink. Through finite element method (FEM) simulations, we explored the temperature distribution within the printhead and its impact on bioink properties, such as viscosity, pressure, and shear stress. Key findings reveal significant temperature gradients from the printhead barrel to the nozzle tip, influencing bioink extrusion and filament morphology. This study further introduces an innovative hardware optimization with thermal insulators, designed to mitigate heat loss at the nozzle tip and ensure uniform temperature distribution. Both simulation and empirical printing experiments confirm the efficacy of thermal insulators in enhancing bioprinting fidelity and efficiency. This research contributes to the advancement of bioprinting technology by optimizing printhead design, with implications for improving the quality of bioprinted tissues and organs.

Measurement of covalent bond formation in light-curing hydrogels predicts physical stability under flow

Photocrosslinking hydrogels are promising for tissue engineering and regenerative medicine, but challenges in reaction monitoring often leave their optimization subject to trial and error. The stability of crosslinked gels under fluid flow, as in the case of a microfluidic device, is particularly challenging to predict, both because of obstacles inherent to solid-state macromolecular analysis that prevent accurate chemical monitoring, and because stability is dependent on size of the patterned features. To solve both problems, we obtained 1H NMR spectra of cured hydrogels which were enzymatically degraded. This allowed us to take advantage of the high-resolution that solution NMR provides. This unique approach enabled the measurement of degree of crosslinking (DoC) and prediction of material stability under physiological fluid flow. We showed that NMR spectra of enzyme-digested gels successfully reported on DoC as a function of light exposure and wavelength within two classes of photocrosslinkable hydrogels: methacryloyl-modified gelatin and a composite of thiol-modified gelatin and norbornene-terminated polyethylene glycol. This approach revealed that a threshold DoC was required for patterned features in each material to become stable, and that smaller features required a higher DoC for stability. Finally, we demonstrated that DoC was predictive of the stability of architecturally complex features when photopatterning, underscoring the value of monitoring DoC when using light-reactive gels. We anticipate that the ability to quantify chemical crosslinks will accelerate the design of advanced hydrogel materials for structurally demanding applications such as photopatterning and bioprinting.

Silk fibroin-based inks for in situ 3D printing using a double crosslinking process

The shortage of tissues and organs for transplantation is an urgent clinical concern. In situ 3D printing is an advanced 3D printing technique aimed at printing the new tissue or organ directly in the patient. The ink for this process is central to the outcomes, and must meet specific requirements such as rapid gelation, shape integrity, stability over time, and adhesion to surrounding healthy tissues. Among natural materials, silk fibroin exhibits fascinating properties that have made it widely studied in tissue engineering and regenerative medicine. However, further improvements in silk fibroin inks are needed to match the requirements for in situ 3D printing. In the present study, silk fibroin-based inks were developed for in situ applications by exploiting covalent crosslinking process consisting of a pre-photo-crosslinking prior to printing and in situ enzymatic crosslinking. Two different silk fibroin molecular weights were characterized and the synergistic effect of the covalent bonds with shear forces enhanced the shift in silk secondary structure toward β-sheets, thus, rapid stabilization. These hydrogels exhibited good mechanical properties, stability over time, and resistance to enzymatic degradation over 14 days, with no significant changes over time in their secondary structure and swelling behavior. Additionally, adhesion to tissues in vitro was demonstrated.

Photocuring 3D Printing of Hydrogels: Techniques, Materials, and Applications in Tissue Engineering and Flexible Devices

Photocuring 3D printing of hydrogels, with sophisticated, delicate structures and biocompatibility, attracts significant attention by researchers and possesses promising application in the fields of tissue engineering and flexible devices. After years of development, photocuring 3D printing technologies and hydrogel inks make great progress. Herein, the techniques of photocuring 3D printing of hydrogels, including direct ink writing (DIW), stereolithography (SLA), digital light processing (DLP), continuous liquid interface production (CLIP), volumetric additive manufacturing (VAM), and two photon polymerization (TPP) are reviewed. Further, the raw materials for hydrogel inks (photocurable polymers, monomers, photoinitiators, and additives) and applications in tissue engineering and flexible devices are also reviewed. At last, the current challenges and future perspectives of photocuring 3D printing of hydrogels are discussed.

3D bioprinting of microorganisms: principles and applications

In recent years, the ability to create intricate, live tissues and organs has been made possible thanks to three-dimensional (3D) bioprinting. Although tissue engineering has received a lot of attention, there is growing interest in the use of 3D bioprinting for microorganisms. Microorganisms like bacteria, fungi, and algae, are essential to many industrial bioprocesses, such as bioremediation as well as the manufacture of chemicals, biomaterials, and pharmaceuticals. This review covers current developments in 3D bioprinting methods for microorganisms. We go over the bioink compositions designed to promote microbial viability and growth, taking into account factors like nutrient delivery, oxygen supply, and waste elimination. Additionally, we investigate the most important bioprinting techniques, including extrusion-based, inkjet, and laser-assisted approaches, as well as their suitability with various kinds of microorganisms. We also investigate the possible applications of 3D bioprinted microbes. These range from constructing synthetic microbial consortia for improved metabolic pathway combinations to designing spatially patterned microbial communities for enhanced bioremediation and bioprocessing. We also look at the potential for 3D bioprinting to advance microbial research, including the creation of defined microenvironments to observe microbial behavior. In conclusion, the 3D bioprinting of microorganisms marks a paradigm leap in microbial bioprocess engineering and has the potential to transform many application areas. The ability to design the spatial arrangement of various microorganisms in functional structures offers unprecedented possibilities and ultimately will drive innovation.

Acoustic Cell Patterning for Structured Cell-Laden Hydrogel Fibers/Tubules

Cell-laden hydrogel fibers/tubules are one of the fundamentals of tissue engineering. They have been proven as a promising method for constructing biomimetic tissues, such as muscle fibers, nerve conduits, tendon and vessels, etc. However, current hydrogel fiber/tubule production methods have limitations in ordered cell arrangements, thus impeding the biomimetic configurations. Acoustic cell patterning is a cell manipulation method that has good biocompatibility, wide tunability, and is contact-free. However, there are few studies on acoustic cell patterning for fiber production, especially on the radial figure cell arrangements, which mimic many native tissue-like cell arrangements. Here, an acoustic cell patterning system that can be used to produce hydrogel fibers/tubules with tunable cell patterns is shown. Cells can be pre-patterned in the liquid hydrogel before being extruded as cross-linked hydrogel fibers/tubules. The radial patterns can be tuned with different complexities based on the acoustic resonances. Cell viability assays after 72 h confirm good cell viability and proliferation. Considering the biocompatibility and reliability, the present method can be further used for a variety of biomimetic fabrications.

The 3-dimensional printing for dental tissue regeneration: the state of the art and future challenges

Tooth loss or damage poses great threaten to oral and general health. While contemporary clinical treatments have enabled tooth restoration to a certain extent, achieving functional tooth regeneration remains a challenging task due to the intricate and hierarchically organized architecture of teeth. The past few decades have seen a rapid development of three-dimensional (3D) printing technology, which has provided new breakthroughs in the field of tissue engineering and regenerative dentistry. This review outlined the bioactive materials and stem/progenitor cells used in dental regeneration, summarized recent advancements in the application of 3D printing technology for tooth and tooth-supporting tissue regeneration, including dental pulp, dentin, periodontal ligament, alveolar bone and so on. It also discussed current obstacles and potential future directions, aiming to inspire innovative ideas and encourage further development in regenerative medicine.

3D Bioprinting: An Important Tool for Tumor Microenvironment Research

The tumor microenvironment plays a crucial role in cancer development and treatment. Traditional 2D cell cultures fail to fully replicate the complete tumor microenvironment, while mouse tumor models suffer from time-consuming procedures and complex operations. However, in recent years, 3D bioprinting technology has emerged as a vital tool in studying the tumor microenvironment. 3D bioprinting is a revolutionary biomanufacturing technique that involves layer-by-layer stacking of biological materials, such as cells and biomaterial scaffolds, to create highly precise 3D biostructures. This technology enables the construction of intricate tissue and organ models in the laboratory, which are utilized for biomedical research, drug development, and personalized medicine. The application of 3D bioprinting has brought unprecedented opportunities to fields such as cancer research, tissue engineering, and organ transplantation. It has opened new possibilities for addressing real-world biological challenges and improving medical treatment outcomes. This review summarizes the applications of 3D bioprinting technology in the context of the tumor microenvironment, aiming to explore its potential impact on cancer research and treatment. The use of this cutting-edge technology promises significant advancements in understanding cancer biology and enhancing medical interventions.

Current Status of Auricular Reconstruction Strategy Development

Microtia has severe physical and psychological impacts on patients, and auricular reconstruction offers improved esthetics and function, alleviating psychological issues. Microtia is a congenital disease caused by a multifactorial interaction of environmental and genetic factors, with complex clinical manifestations. Classification assessment aids in determining treatment strategies. Auricular reconstruction is the primary treatment for severe microtia, focusing on the selection of auricular scaffold materials, the construction of auricular morphology, and skin and soft tissue scaffold coverage. Autologous rib cartilage and synthetic materials are both used as scaffold materials for auricular reconstruction, each with advantages and disadvantages. Methods for achieving skin and soft tissue scaffold coverage have been developed to include nonexpansion and expansion techniques. In recent years, the application of digital auxiliary technology such as finite element analysis has helped optimize surgical outcomes and reduce complications. Tissue-engineered cartilage scaffolds and 3-dimensional bioprinting technology have rapidly advanced in the field of ear reconstruction. This article discusses the prevalence and classification of microtia, the selection of auricular scaffolds, the evolution of surgical methods, and the current applications of digital auxiliary technology in ear reconstruction, with the aim of providing clinical physicians with a reference for individualized ear reconstruction surgery. The focus of this work is on the current applications and challenges of tissue engineering and 3-dimensional bioprinting technology in the field of ear reconstruction, as well as future prospects.

3D/4D printing of cellulose nanocrystals-based biomaterials: Additives for sustainable applications

Cellulose nanocrystals (CNCs) have gained significant attraction from both industrial and academic sectors, thanks to their biodegradability, non-toxicity, and renewability with remarkable mechanical characteristics. Desirable mechanical characteristics of CNCs include high stiffness, high strength, excellent flexibility, and large surface-to-volume ratio. Additionally, the mechanical properties of CNCs can be tailored through chemical modifications for high-end applications including tissue engineering, actuating, and biomedical. Modern manufacturing methods including 3D/4D printing are highly advantageous for developing sophisticated and intricate geometries. This review highlights the major developments of additive manufactured CNCs, which promote sustainable solutions across a wide range of applications. Additionally, this contribution also presents current challenges and future research directions of CNC-based composites developed through 3D/4D printing techniques for myriad engineering sectors including tissue engineering, wound healing, wearable electronics, robotics, and anti-counterfeiting applications. Overall, this review will greatly help research scientists from chemistry, materials, biomedicine, and other disciplines to comprehend the underlying principles, mechanical properties, and applications of additively manufactured CNC-based structures.

Diverse Applications of Three-Dimensional Printing in Biomedical Engineering: A Review

A three-dimensional (3D) printing is a robotically controlled state-of-the-art technology that is promising for all branches of engineering with a meritorious emphasis to biomedical engineering. The purpose of 3D printing (3DP) is to create exact superstructures without any framework in a brief period with high reproducibility to create intricate and complex patient-tailored structures for organ regeneration, drug delivery, imaging processes, designing personalized dose-specific tablets, developing 3D models of organs to plan surgery and to understand the pathology of disease, manufacturing cost-effective surgical tools, and fabricating implants and organ substitute devices for prolonging the lives of patients, etc. The formulation of bioinks and programmed G codes help to obtain precise 3D structures, which determines the stability and functioning of the 3D-printed structures. Three-dimensional printing for medical applications is ambitious and challenging but made possible with the culmination of research expertise from various fields. Exploring and expanding 3DP for biomedical and clinical applications can be life-saving solutions. The 3D printers are cost-effective and eco-friendly, as they do not release any toxic pollutants or waste materials that pollute the environment. The sampling requirements and processing parameters are amenable, which further eases the production. This review highlights the role of 3D printers in the health care sector, focusing on their roles in tablet development, imaging techniques, disease model development, and tissue regeneration.

Triaxial bioprinting large-size vascularized constructs with nutrient channels

Bioprinting has demonstrated great advantages in tissue and organ regeneration. However, constructing large-scale tissue and organsin vitrois still a huge challenge due to the lack of some strategies for loading multiple types of cells precisely while maintaining nutrient channels. Here, a new 3D bioprinting strategy was proposed to construct large-scale vascularized tissue. A mixture of gelatin methacrylate (GelMA) and sodium alginate (Alg) was used as a bioink, serving as the outer and middle layers of a single filament in the triaxial printing process, and loaded with human bone marrow mesenchymal stem cells and human umbilical vein endothelial cells, respectively, while a calcium chloride (CaCl2) solution was used as the inner layer. The CaCl2solution crosslinked with the middle layer bioink during the printing process to form and maintain hollow nutrient channels, then a stable large-scale construct was obtained through photopolymerization and ion crosslinking after printing. The feasibility of this strategy was verified by investigating the properties of the bioink and construct, and the biological performance of the vascularized construct. The results showed that a mixture of 5% (w/v) GelMA and 1% (w/v) Alg bioink could be printed at room temperature with good printability and perfusion capacity. Then, the construct with and without channels was fabricated and characterized, and the results revealed that the construct with channels had a similar degradation profile to that without channels, but lower compressive modulus and higher swelling rate. Biological investigation showed that the construct with channels was more favorable for cell survival, proliferation, diffusion, migration, and vascular network formation. In summary, it was demonstrated that constructing large-scale vascularized tissue by triaxial printing that can precisely encapsulate multiple types of cells and form nutrient channels simultaneously was feasible, and this technology could be used to prepare large-scale vascularized constructs.

Modification, 3D printing process and application of sodium alginate based hydrogels in soft tissue engineering: A review

Sodium alginate (SA) is an inexpensive and biocompatible biomaterial with fast and gentle crosslinking that has been widely used in biological soft tissue repair/regeneration. Especially with the advent of 3D bioprinting technology, SA hydrogels have been applied more deeply in tissue engineering due to their excellent printability. Currently, the research on material modification, molding process and application of SA-based composite hydrogels has become a hot topic in tissue engineering, and a lot of fruitful results have been achieved. To better help readers have a comprehensive understanding of the development status of SA based hydrogels and their molding process in tissue engineering, in this review, we summarized SA modification methods, and provided a comparative analysis of the characteristics of various SA based hydrogels. Secondly, various molding methods of SA based hydrogels were introduced, the processing characteristics and the applications of different molding methods were analyzed and compared. Finally, the applications of SA based hydrogels in tissue engineering were reviewed, the challenges in their applications were also analyzed, and the future research directions were prospected. We believe this review is of great helpful for the researchers working in biomedical and tissue engineering.

A vertical additive-lathe printing system for the fabrication of tubular constructs using gelatin methacryloyl hydrogel

Reproducing both the mechanical and biological performance of native blood vessels remains an ongoing challenge in vascular tissue engineering. Additive-lathe printing offers an attractive method of fabricating long tubular constructs as a potential vascular graft for the treatment of cardiovascular diseases. Printing hydrogels onto rotating horizontal mandrels often leads to sagging, resulting in poor and variable mechanical properties. In this study, an additive-lathe printing system with a vertical mandrel to fabricate tubular constructs is presented. Various concentrations of gelatin methacryloyl (gelMA) hydrogel were used to print grafts on the rotating mandrel in a helical pattern. The printing parameters were selected to achieve the bonding of consecutive gelMA filaments to improve the quality of the printed graft. The hydrogel filaments were fused properly under the action of gravity on the vertical mandrel. Thus, the vertical additive-lathe printing system was used to print uniform wall thickness grafts, eliminating the hydrogel sagging problem. Tensile testing performed in both circumferential and longitudinal direction revealed that the anisotropic properties of printed gelMA constructs were similar to those observed in the native blood vessels. In addition, no leakage was detected through the walls of the gelMA grafts during burst pressure measurement. Therefore, the current printing setup could be utilized to print vascular grafts for the treatment of cardiovascular diseases.

New Frontiers in Three-Dimensional Culture Platforms to Improve Diabetes Research

Diabetes mellitus is associated with defects in islet β-cell functioning and consequent hyperglycemia resulting in multi-organ damage. Physiologically relevant models that mimic human diabetic progression are urgently needed to identify new drug targets. Three-dimensional (3D) cell-culture systems are gaining a considerable interest in diabetic disease modelling and are being utilized as platforms for diabetic drug discovery and pancreatic tissue engineering. Three-dimensional models offer a marked advantage in obtaining physiologically relevant information and improve drug selectivity over conventional 2D (two-dimensional) cultures and rodent models. Indeed, recent evidence persuasively supports the adoption of appropriate 3D cell technology in β-cell cultivation. This review article provides a considerably updated view of the benefits of employing 3D models in the experimental workflow compared to conventional animal and 2D models. We compile the latest innovations in this field and discuss the various strategies used to generate 3D culture models in diabetic research. We also critically review the advantages and the limitations of each 3D technology, with particular attention to the maintenance of β-cell morphology, functionality, and intercellular crosstalk. Furthermore, we emphasize the scope of improvement needed in the 3D culture systems employed in diabetes research and the promises they hold as excellent research platforms in managing diabetes.

Biomaterial-based 3D bioprinting strategy for orthopedic tissue engineering

The advent of three-dimensional (3D) bioprinting has enabled impressive progress in the development of 3D cellular constructs to mimic the structural and functional characteristics of natural tissues. Bioprinting has considerable translational potential in tissue engineering and regenerative medicine. This review highlights the rational design and biofabrication strategies of diverse 3D bioprinted tissue constructs for orthopedic tissue engineering applications. First, we elucidate the fundamentals of 3D bioprinting techniques and biomaterial inks and discuss the basic design principles of bioprinted tissue constructs. Next, we describe the rationale and key considerations in 3D bioprinting of tissues in many different aspects. Thereafter, we outline the recent advances in 3D bioprinting technology for orthopedic tissue engineering applications, along with detailed strategies of the engineering methods and materials used, and discuss the possibilities and limitations of different 3D bioprinted tissue products. Finally, we summarize the current challenges and future directions of 3D bioprinting technology in orthopedic tissue engineering and regenerative medicine. This review not only delineates the representative 3D bioprinting strategies and their tissue engineering applications, but also provides new insights for the clinical translation of 3D bioprinted tissues to aid in prompting the future development of orthopedic implants. STATEMENT OF SIGNIFICANCE: 3D bioprinting has driven major innovations in the field of tissue engineering and regenerative medicine; aiming to develop a functional viable tissue construct that provides an alternative regenerative therapy for musculoskeletal tissue regeneration. 3D bioprinting-based biofabrication strategies could open new clinical possibilities for creating equivalent tissue substitutes with the ability to customize them to meet patient demands. In this review, we summarize the significance and recent advances in 3D bioprinting technology and advanced bioinks. We highlight the rationale for biofabrication strategies using 3D bioprinting for orthopedic tissue engineering applications. Furthermore, we offer ample perspective and new insights into the current challenges and future direction of orthopedic bioprinting translation research.

Emerging 3D bioprinting applications in plastic surgery

Plastic surgery is a discipline that uses surgical methods or tissue transplantation to repair, reconstruct and beautify the defects and deformities of human tissues and organs. Three-dimensional (3D) bioprinting has gained widespread attention because it enables fine customization of the implants in the patient's surgical area preoperatively while avoiding some of the adverse reactions and complications of traditional surgical approaches. In this paper, we review the recent research advances in the application of 3D bioprinting in plastic surgery. We first introduce the printing process and basic principles of 3D bioprinting technology, revealing the advantages and disadvantages of different bioprinting technologies. Then, we describe the currently available bioprinting materials, and dissect the rationale for special dynamic 3D bioprinting (4D bioprinting) that is achieved by varying the combination strategy of bioprinting materials. Later, we focus on the viable clinical applications and effects of 3D bioprinting in plastic surgery. Finally, we summarize and discuss the challenges and prospects for the application of 3D bioprinting in plastic surgery. We believe that this review can contribute to further development of 3D bioprinting in plastic surgery and provide lessons for related research.

Tailoring micro/nano-fibers for biomedical applications

Nano/micro fibers have evoked much attention of scientists and have been researched as cutting edge and hotspot in the area of fiber science in recent years due to the rapid development of various advanced manufacturing technologies, and the appearance of fascinating and special functions and properties, such as the enhanced mechanical strength, high surface area to volume ratio and special functionalities shown in the surface, triggered by the nano or micro-scale dimensions. In addition, these outstanding and special characteristics of the nano/micro fibers impart fiber-based materials with wide applications, such as environmental engineering, electronic and biomedical fields. This review mainly focuses on the recent development in the various nano/micro fibers fabrication strategies and corresponding applications in the biomedical fields, including tissue engineering scaffolds, drug delivery, wound healing, and biosensors. Moreover, the challenges for the fabrications and applications and future perspectives are presented.

3D bioprinted mesenchymal stromal cells in skin wound repair

Skin tissue regeneration and repair is a complex process involving multiple cell types, and current therapies are limited to promoting skin wound healing. Mesenchymal stromal cells (MSCs) have been proven to enhance skin tissue repair through their multidifferentiation and paracrine effects. However, there are still difficulties, such as the limited proliferative potential and the biological processes that need to be strengthened for MSCs in wound healing. Recently, three-dimensional (3D) bioprinting has been applied as a promising technology for tissue regeneration. 3D-bioprinted MSCs could maintain a better cell ability for proliferation and expression of biological factors to promote skin wound healing. It has been reported that 3D-bioprinted MSCs could enhance skin tissue repair through anti-inflammatory, cell proliferation and migration, angiogenesis, and extracellular matrix remodeling. In this review, we will discuss the progress on the effect of MSCs and 3D bioprinting on the treatment of skin tissue regeneration, as well as the perspective and limitations of current research.

Tissues and organ printing: An evolution of technology and materials

Since its beginnings, three-dimensional printing (3DP) technology has been successful because of ongoing advances in operating principles, the range of materials and cost-saving measures. However, the 3DP technological progressions in the biomedical sector have majorly taken place in the last decade after the evolution of novel 3DP systems, generally categorised as bioprinters and biomaterials to provide a replacement, transplantation or regeneration of the damaged organs and tissue constructs of the human body. There is now substantial scientific literature accessible to support the benefits of digital healthcare procedures with the help of bioprinters. It is of the highest significance to know the fundamental principles of the available printers and the compatibility of biomaterials as their feedstock, notwithstanding the huge potential of bioprinting systems to manufacture organs and other human body components. This paper provides a precise and helpful reading of the different categories of bioprinters, suitable biomaterials, numerical simulations and modelling and examples of much acknowledged clinical practices. The paper will also cite the prominent issues that still have not received desired solutions. Overall, the article will be of great use for all the professionals, scholars and engineers concerned with the 3DP, bioprinting and biomaterials.

Three-Dimensional (3D) Printing in Cancer Therapy and Diagnostics: Current Status and Future Perspectives

Three-dimensional (3D) printing is a technique where the products are printed layer-by-layer via a series of cross-sectional slices with the exact deposition of different cell types and biomaterials based on computer-aided design software. Three-dimensional printing can be divided into several approaches, such as extrusion-based printing, laser-induced forward transfer-based printing systems, and so on. Bio-ink is a crucial tool necessary for the fabrication of the 3D construct of living tissue in order to mimic the native tissue/cells using 3D printing technology. The formation of 3D software helps in the development of novel drug delivery systems with drug screening potential, as well as 3D constructs of tumor models. Additionally, several complex structures of inner tissues like stroma and channels of different sizes are printed through 3D printing techniques. Three-dimensional printing technology could also be used to develop therapy training simulators for educational purposes so that learners can practice complex surgical procedures. The fabrication of implantable medical devices using 3D printing technology with less risk of infections is receiving increased attention recently. A Cancer-on-a-chip is a microfluidic device that recreates tumor physiology and allows for a continuous supply of nutrients or therapeutic compounds. In this review, based on the recent literature, we have discussed various printing methods for 3D printing and types of bio-inks, and provided information on how 3D printing plays a crucial role in cancer management.

Investigation on the Temperature Control Accuracy of a Print Head for Extrusion 3D Printing and Its Improved Design

For the extrusion 3D printing process, the printing temperature has a significant impact on the filament formation process because the rheological properties of the printed materials are extremely thermal sensitive, which requires a high temperature control accuracy of the print head. This paper presents a numerical and experimental investigation on the temperature field of a homemade print head. A finite element simulation model for analyzing the temperature field of the print head was established, by which the temperature distribution inside the print head can be acquired. Moreover, to improve the temperature control accuracy, an improved configuration was proposed, and two schemes were compared. The temperature control error dropped from 28% to 6.2% with the improved print head, which was verified experimentally. Furthermore, printing trials were conducted by the optimized print head. The filament diameter could be regulated by changing the temperature of the print head, which validates the feasibility to control the filament diameter during the extrusion process via temperature regulation.

A Simple and Efficient Strategy for Preparing a Cell-Spheroid-Based Bioink

Cell spheroids are a promising bioprinting building block that can mimic several physiological conditions in embryonic development. However, it remains challenging to efficiently prepare cell-spheroid-based bioink (Sph-bioink) with favorable printability and spheroid fusion ability. In this work, a poly(N-isopropylacrylamide) (PNIPAAm)-based porous hydrogel is developed as an "all-in-one" platform for Sph-bioink preparation. On the one hand, the nonadhesive porous structure in hydrogels is an effective tool for fabricating adipose-derived stem cell (ASC) spheroids in high yield, and the hydrogel itself also serves as a "carrier" for conveniently transferring cell spheroids to the bioprinter. On the other hand, the integration of redox/thermo-responsiveness allows the hydrogel to shift from a solid spheroid-making tool to an extrudable bioprinting medium that is sensitive to temperature. These features enabled a simple procedure for preparing Sph-bioink, in which the cell spheroids were densely packed to retain fusion capability. The present study also demonstrates that ASC spheroids formed in hydrogels have good biological preservation and superior chondrogenic differentiation, and verified the feasibility of using Sph-bioink to build custom-shaped mature cartilage. In conclusion, this strategy provides a simple, efficient, and standardized approach for Sph-bioink preparation, making it possible to produce tissue-engineered constructs with accelerated maturation and functionalization.

Manufacturing of self-standing multi-layered 3D-bioprinted alginate-hyaluronate constructs by controlling the cross-linking mechanisms for tissue engineering applications

3D bioprinting of self-supporting stable tissue and organ structure is critically important in extrusion-based bioprinting system, especially for tissue engineering and regenerative medicine applications. However, the development of self-standing bioinks with desired crosslinking density, biocompatibility, tunable mechanical strength and other properties like self-healing, in situ gelation, drug or protein incorporation is still a challenge. In this study, we report a hydrogel bioink prepared from alginate (Alg) and hyaluronic acid (HA) crosslinked through multiple crosslinking mechanisms, i.e., acyl-hydrazone, hydrazide interactions and calcium ions. These Alg-HA gels were highly dynamic and shear-thinning with exceptional biocompatibility and tunable mechanical properties. The increased dynamic nature of the gels is mainly chemically attributed to the presence of acyl-hydrazone bonds formed between the amine groups of the acyl-hydrazide of alginate and the monoaldehyde of the hyaluronic acid. Among the different combinations of Alg-HA gel compositions prepared, the A5H5 (Alginate-acyl-hydrazide: HA-monoaldehyde, ratio 50:50) one showed a gelation time of ~60 s, viscosity of ~400 Pa.s (at zero shear rate), high stability in various pH solutions and increased degradation time (>50 days) than the other samples. The A5H5 gels showed high printability with increased post-printing stability as observed from the 3D printed structures (e.g., hollow tube (~100 layers), porous cube (~50 layers), star, heart-in, meniscus and lattice). The scanning electron microscopy analysis of the 3D constructs and hydrogels showed the interconnected pores (~181 µm) and crosslinked networks. Further, the gels showed sustained release of 5-amino salicylic acid and bovine serum albumin. Also, the mechanical properties were tuned by secondary crosslinking via different calcium concentrations. In vitro assays confirmed the cytocompatibility of these gels, where the 3D bioprinted lattice and tubular (~70 layers) constructs demonstrated high cell viability under fluorescence analysis. In in vivo studies, Alg-HA gel showed high biocompatibility (>90%) and increased angiogenesis (3 folds) and reduced macrophage infiltration (2-fold decrease), demonstrating the promising potential of these hydrogels in 3D bioprinting applications for tissue engineering and regenerative medicine with tunable properties.

Review on Multicomponent Hydrogel Bioinks Based on Natural Biomaterials for Bioprinting 3D Liver Tissues

Three-dimensional (3D)-printed in vitro tissue models have been used in various biomedical fields owing to numerous advantages such as enhancements in cell response and functionality. In liver tissue engineering, several studies have been reported using 3D-printed liver tissue models with improved cellular responses and functions in drug screening, liver disease, and liver regenerative medicine. However, the application of conventional single-component bioinks for the printing of 3D in vitro liver constructs remains problematic because of the complex structural and physiological characteristics of the liver. The use of multicomponent bioinks has become an attractive strategy for bioprinting 3D functional in vitro liver tissue models because of the various advantages of multicomponent bioinks, such as improved mechanical properties of the printed tissue construct and cell functionality. Therefore, it is essential to review various 3D bioprinting techniques and multicomponent hydrogel bioinks proposed for liver tissue engineering to suggest future directions for liver tissue engineering. Accordingly, we herein review multicomponent bioinks for 3D-bioprinted liver tissues. We first describe the fabrication methods capable of printing multicomponent bioinks and introduce considerations for bioprinting. We subsequently categorize and evaluate the materials typically utilized for multicomponent bioinks based on their characteristics. In addition, we also review recent studies for the application of multicomponent bioinks to fabricate in vitro liver tissue models. Finally, we discuss the limitations of current studies and emphasize aspects that must be resolved to enhance the future applicability of such bioinks.

Advances in Single-Cell Printing

Single-cell analysis is becoming an indispensable tool in modern biological and medical research. Single-cell isolation is the key step for single-cell analysis. Single-cell printing shows several distinct advantages among the single-cell isolation techniques, such as precise deposition, high encapsulation efficiency, and easy recovery. Therefore, recent developments in single-cell printing have attracted extensive attention. We review herein the recently developed bioprinting strategies with single-cell resolution, with a special focus on inkjet-like single-cell printing. First, we discuss the common cell printing strategies and introduce several typical and advanced printing strategies. Then, we introduce several typical applications based on single-cell printing, from single-cell array screening and mass spectrometry-based single-cell analysis to three-dimensional tissue formation. In the last part, we discuss the pros and cons of the single-cell strategies and provide a brief outlook for single-cell printing.

Recent approaches towards bone tissue engineering

Bone tissue engineering approaches have evolved towards addressing the challenges of tissue mimetic requirements over the years. Different strategies have been combining scaffolds, cells, and biologically active cues using a wide range of fabrication techniques, envisioning the mimicry of bone tissue. On the one hand, biomimetic scaffold-based strategies have been pursuing different biomaterials to produce scaffolds, combining with diverse and innovative fabrication strategies to mimic bone tissue better, surpassing bone grafts. On the other hand, biomimetic scaffold-free approaches mainly foresee replicating endochondral ossification, replacing hyaline cartilage with new bone. Finally, since bone tissue is highly vascularized, new strategies focused on developing pre-vascularized scaffolds or pre-vascularized cellular aggregates have been a motif of study. The recent biomimetic scaffold-based and scaffold-free approaches in bone tissue engineering, focusing on materials and fabrication methods used, are overviewed herein. The biomimetic vascularized approaches are also discussed, namely the development of pre-vascularized scaffolds and pre-vascularized cellular aggregates.

Advanced Strategies for 3D Bioprinting of Tissue and Organ Analogs Using Alginate Hydrogel Bioinks

Alginate is a natural polysaccharide that typically originates from various species of algae. Due to its low cost, good biocompatibility, and rapid ionic gelation, the alginate hydrogel has become a good option of bioink source for 3D bioprinting. However, the lack of cell adhesive moieties, erratic biodegradability, and poor printability are the critical limitations of alginate hydrogel bioink. This review discusses the pivotal properties of alginate hydrogel as a bioink for 3D bioprinting technologies. Afterward, a variety of advanced material formulations and biofabrication strategies that have recently been developed to overcome the drawbacks of alginate hydrogel bioink will be focused on. In addition, the applications of these advanced solutions for 3D bioprinting of tissue/organ mimicries such as regenerative implants and in vitro tissue models using alginate-based bioink will be systematically summarized.

3D bioprinting: current status and trends—a guide to the literature and industrial practice

The multidisciplinary research field of bioprinting combines additive manufacturing, biology and material sciences to create bioconstructs with three-dimensional architectures mimicking natural living tissues. The high interest in the possibility of reproducing biological tissues and organs is further boosted by the ever-increasing need for personalized medicine, thus allowing bioprinting to establish itself in the field of biomedical research, and attracting extensive research efforts from companies, universities, and research institutes alike. In this context, this paper proposes a scientometric analysis and critical review of the current literature and the industrial landscape of bioprinting to provide a clear overview of its fast-changing and complex position. The scientific literature and patenting results for 2000–2020 are reviewed and critically analyzed by retrieving 9314 scientific papers and 309 international patents in order to draw a picture of the scientific and industrial landscape in terms of top research countries, institutions, journals, authors and topics, and identifying the technology hubs worldwide. This review paper thus offers a guide to researchers interested in this field or to those who simply want to understand the emerging trends in additive manufacturing and 3D bioprinting.

Graphic abstract

Construction of 3D hierarchical tissue platforms for modeling diabetes

Diabetes mellitus (DM) is one of the most serious systemic diseases worldwide, and the majority of DM patients face severe complications. However, many of underlying disease mechanisms related to these complications are difficult to understand with the use of currently available animal models. With the urgent need to fundamentally understand DM pathology, a variety of 3D biomimetic platforms have been generated by the convergence of biofabrication and tissue engineering strategies for the potent drug screening platform of pre-clinical research. Here, we suggest key requirements for the fabrication of physiomimetic tissue models in terms of recapitulating the cellular organization, creating native 3D microenvironmental niches for targeted tissue using biomaterials, and applying biofabrication technologies to implement tissue-specific geometries. We also provide an overview of various in vitro DM models, from a cellular level to complex living systems, which have been developed using various bioengineering approaches. Moreover, we aim to discuss the roadblocks facing in vitro tissue models and end with an outlook for future DM research.

Marine origin materials on biomaterials and advanced therapies to cartilage tissue engineering and regenerative medicine

The body's self-repair capacity is limited, including injuries on articular cartilage zones. Over the past few decades, tissue engineering and regenerative medicine (TERM) has focused its studies on the development of natural biomaterials for clinical applications aiming to overcome this self-therapeutic bottleneck. This review focuses on the development of these biomaterials using compounds and materials from marine sources that are able to be produced in a sustainable way, as an alternative to mammal sources (e.g., collagens) and benefiting from their biological properties, such as biocompatibility, low antigenicity, biodegradability, among others. The structure and composition of the new biomaterials require mimicking the native extracellular matrix (ECM) of articular cartilage tissue. To design an ideal temporary tissue-scaffold, it needs to provide a suitable environment for cell growth (cell attachment, proliferation, and differentiation), towards the regeneration of the damaged tissues. Overall, the purpose of this review is to summarize various marine sources to be used in the development of different tissue-scaffolds with the capability to sustain cells envisaging cartilage tissue engineering, analysing the systems displaying more promising performance, while pointing out current limitations and steps to be given in the near future.

Injectable chitosan/collagen hydrogels nano-engineered with functionalized single wall carbon nanotubes for minimally invasive applications in bone

Mechanical robustness is an essential consideration in the development of hydrogel platforms for bone regeneration, and despite significant advances in the field of injectable hydrogels, many fail in this regard. Inspired by the mechanical properties of carboxylated single wall carbon nanotubes (COOH-SWCNTs) and the biological advantages of natural polymers, COOH-SWCNTs were integrated into chitosan and collagen to formulate mechanically robust, injectable and thermoresponsive hydrogels with interconnected molecular structure for load-bearing applications. This study presents a complete characterisation of the structural and biological properties, and mechanism of gelation of these novel formulated hydrogels. Results demonstrate that β-glycerophosphate (β-GP) and temperature play important roles in attaining gelation at physiological conditions, and the integration with COOH-SWCNTs significantly changed the structural morphology of the hydrogels to a more porous and aligned network. This led to a crystalline structure and significantly increased the mechanical strength of the hydrogels from kPa to MPa, which is closer to the mechanical strength of the bone. Moreover, increased osteoblast proliferation and rapid adsorption of hydroxyapatite on the surface of the hydrogels indicates increased bioactivity with addition of COOH-SWCNTs. Therefore, these nano-engineered hydrogels are expected to have wide utility in the area of bone tissue engineering and regenerative medicine.

Employing Extracellular Matrix-Based Tissue Engineering Strategies for Age-Dependent Tissue Degenerations

Tissues and organs are not composed of solely cellular components; instead, they converge with an extracellular matrix (ECM). The composition and function of the ECM differ depending on tissue types. The ECM provides a microenvironment that is essential for cellular functionality and regulation. However, during aging, the ECM undergoes significant changes along with the cellular components. The ECM constituents are over- or down-expressed, degraded, and deformed in senescence cells. ECM aging contributes to tissue dysfunction and failure of stem cell maintenance. Aging is the primary risk factor for prevalent diseases, and ECM aging is directly or indirectly correlated to it. Hence, rejuvenation strategies are necessitated to treat various age-associated symptoms. Recent rejuvenation strategies focus on the ECM as the basic biomaterial for regenerative therapies, such as tissue engineering. Modified and decellularized ECMs can be used to substitute aged ECMs and cell niches for culturing engineered tissues. Various tissue engineering approaches, including three-dimensional bioprinting, enable cell delivery and the fabrication of transplantable engineered tissues by employing ECM-based biomaterials.

Tissue-Specific Decellularized Extracellular Matrix Bioinks for Musculoskeletal Tissue Regeneration and Modeling Using 3D Bioprinting Technology

The musculoskeletal system is a vital body system that protects internal organs, supports locomotion, and maintains homeostatic function. Unfortunately, musculoskeletal disorders are the leading cause of disability worldwide. Although implant surgeries using autografts, allografts, and xenografts have been conducted, several adverse effects, including donor site morbidity and immunoreaction, exist. To overcome these limitations, various biomedical engineering approaches have been proposed based on an understanding of the complexity of human musculoskeletal tissue. In this review, the leading edge of musculoskeletal tissue engineering using 3D bioprinting technology and musculoskeletal tissue-derived decellularized extracellular matrix bioink is described. In particular, studies on in vivo regeneration and in vitro modeling of musculoskeletal tissue have been focused on. Lastly, the current breakthroughs, limitations, and future perspectives are described.

In Situ 3D Printing: Opportunities with Silk Inks

In situ 3D printing is an emerging technique designed for patient-specific needs and performed directly in the patient's tissues in the operating room. While this technology has progressed rapidly, several improvements are needed to push it forward for widespread utility, including ink formulations and optimization for in situ context. Silk fibroin inks emerge as a viable option due to the diverse range of formulations, aqueous processability, robust and tunable mechanical properties, and self-assembly via biophysical adsorption to avoid exogenous chemical or photochemical sensitizer additives, among other features. In this review, we focus on this new frontier of 3D in situ printing for tissue regeneration, where silk is proposed as candidate biomaterial ink due to the unique and useful properties of this protein polymer.

Construction of Tissue-Level Cancer-Vascular Model with High-Precision Position Control via In Situ 3D Cell Printing

During tumor progression, the size and location of the tumor are important factors closely associated with the metastatic potential of the cancer as they largely govern tumor hypoxia and angiogenesis. However, despite the achievements of previous studies, these critical factors are poorly studied, mainly due to the lack of a flexible technique that can readily control 3D tumor mimicking constructs and their spatial relations with vasculature. Here, a novel tissue-level platform consisting of a metastatic cancer unit (MCU) and a perfusable vascular endothelium system (VES) is presented using in situ 3D cell printing. Size-tunable and position-controllable 3D cancer spheroids (500-1000 µm) are directly printed within the established bath bioink with a self-driven perfusable vascular channel. The cancer-vascular interactions are generated through controlling the distance between MCU and VES to investigate metastasis-associated changes at adjacent and distal regions. The result shows that MCU in 600 µm diameter includes hypoxia, invasion, and angiogenetic signaling. The further observations demonstrate that the proximity of MCU to VES augments the epithelial-mesenchymal transition (EMT) in MCU and vascular dysfunction/inflammation in VES, corroborating the positional significance in tumor metastasis. The platform with the precise-positioning control enables the recapitulation of patient's detailed metastatic progression, opening the chance for precision cancer medicine.

Recent advancements in the bioprinting of vascular grafts

Recent advancements in the bioinks and three-dimensional (3D) bioprinting methods used to fabricate vascular constructs are summarized herein. Critical biomechanical properties required to fabricate an ideal vascular graft are highlighted, as well as various testing methods have been outlined to evaluate the bio-fabricated grafts as per the Food and Drug Administration (FDA) and International Organization for Standardization (ISO) guidelines. Occlusive artery disease and cardiovascular disease are the major causes of death globally. These diseases are caused by the blockage in the arteries, which results in a decreased blood flow to the tissues of major organs in the body, such as the heart. Bypass surgery is often performed using a vascular graft to re-route the blood flow. Autologous grafts represent a gold standard for such bypass surgeries; however, these grafts may be unavailable due to the previous harvesting or possess a poor quality. Synthetic grafts serve well for medium to large-sized vessels, but they fail when used to replace small-diameter vessels, generally smaller than 6 mm. Various tissue engineering approaches have been used to address the urgent need for vascular graft that can withstand hemodynamic blood pressure and has the ability to grow and remodel. Among these approaches, 3D bioprinting offers an attractive solution to construct patient-specific vessel grafts with layered biomimetic structures.

Maturation and Protection Effect of Retinal Tissue-Derived Bioink for 3D Cell Printing Technology

Retinal degeneration is a leading cause of incurable vision loss and blindness. The increasing incidence of retinal degeneration has triggered research into the development of in vitro retinal models for drug development and retinal alternatives for transplantation. However, the complex retinal structure and the retinal microenvironment pose serious challenges. Although 3D cell printing technology has been widely used in tissue engineering, including in vitro model development and regeneration medicine, currently available bioinks are insufficient to recapitulate the complex extracellular matrix environment of the retina. Therefore, in this study, we developed a retinal decellularized extracellular matrix (RdECM) from the porcine retina and evaluated its characteristics. The RdECM conserved the ECM components from the native retina without cellular components. Then, we mixed the RdECM with collagen to form a bioink and confirmed its suitability for 3D cell printing. We further studied the effect of the RdECM bioink on the differentiation of Muller cells. The retinal protective effect of the RdECM bioink was confirmed through a retinal degeneration animal model. Thus, we believe that the RdECM bioink is a promising candidate for retinal tissue engineering.

Engineering of tissue constructs using coaxial bioprinting

Bioprinting is a rapidly developing technology for the precise design and manufacture of tissues in various biological systems or organs. Coaxial extrusion bioprinting, an emergent branch, has demonstrated a strong potential to enhance bioprinting's engineering versatility. Coaxial bioprinting assists in the fabrication of complex tissue constructs, by enabling concentric deposition of biomaterials. The fabricated tissue constructs started with simple, tubular vasculature but have been substantially developed to integrate complex cell composition and self-assembly, ECM patterning, controlled release, and multi-material gradient profiles. This review article begins with a brief overview of coaxial printing history, followed by an introduction of crucial engineering components. Afterward, we review the recent progress and untapped potential in each specific organ or biological system, and demonstrate how coaxial bioprinting facilitates the creation of tissue constructs. Ultimately, we conclude that this growing technology will contribute significantly to capabilities in the fields of in vitro modeling, pharmaceutical development, and clinical regenerative medicine.

Suturable elastomeric tubular grafts with patterned porosity for rapid vascularization of 3D constructs

Vascularization is considered to be one of the key challenges in engineering functional 3D tissues. Engineering suturable vascular grafts containing pores with diameter of several tens of microns in tissue engineered constructs may provide an instantaneous blood perfusion through the grafts improving cell infiltration and thus, allowing rapid vascularization and vascular branching. The aim of this work was to develop suturable tubular scaffolds to be integrated in biofabricated constructs, enabling the direct connection of the biofabricated construct with the host blood stream, providing an immediate blood flow inside the construct. Here, tubular grafts with customizable shapes (tubes, Y-shape capillaries) and controlled diameter ranging from several hundreds of microns to few mm are fabricated based on poly(glycerol sebacate) (PGS) / poly(vinyl alcohol) (PVA) electrospun scaffolds. Furthermore, a network of pore channels of diameter in the order of 100 µm was machined by laser femtosecond ablation in the tube wall. Both non-machined and laser machined tubular scaffolds elongated more than 100% of their original size have shown suture retention, being 5.85 and 3.96 N/mm2 respectively. To demonstrate the potential of application, the laser machined porous grafts were embedded in gelatin methacryloyl (GelMA) hydrogels, resulting in elastomeric porous tubular graft/GelMA 3D constructs. These constructs were then co-seeded with osteoblast-like cells (MG-63) at the external side of the graft and endothelial cells (HUVEC) inside, forming a bone osteon model. The laser machined pore network allowed an immediate endothelial cell flow towards the osteoblasts enabling the osteoblasts and endothelial cells to interact and form 3D structures. This rapid vascularization approach could be applied, not only for bone tissue regeneration, but also for a variety of tissues and organs.

3D Composite Bioprinting for Fabrication of Artificial Biological Tissues

Three-dimensional (3D) bioprinting is an important technology for fabricating artificial tissue. To effectively reconstruct the multiscale structure and multi-material gradient of natural tissues and organs, 3D bioprinting has been increasingly developed into multi-process composite mode. The current 3D composite bioprinting is a combination of two or more printing processes, and oftentimes, physical field regulation that can regulate filaments or cells during or after printing may be involved. Correspondingly, both path planning strategy and process control all become more complex. Hence, the computer-aided design and computer-aided manufacturing (CAD/CAM) system that is traditionally used in 3D printing system is now facing challenges. Thus, the scale information that cannot be modeled in the CAD process should be considered in the design of CAM by adding a process management module in the traditional CAD/CAM system and add more information reflecting component gradient in the path planning strategy.

3D bioprinting of bicellular liver lobule-mimetic structures via microextrusion of cellulose nanocrystal-incorporated shear-thinning bioink

3D bioprinting of living cellular constructs with heterogeneity in cell types and extra cellular matrices (ECMs) matching those of biological tissues remains challenging. Here, we demonstrate that, through bioink material design, microextrusion-based (ME) bioprinting techniques have the potential to address this challenge. A new bioink employing alginate (1%), cellulose nanocrystal (CNC) (3%), and gelatin methacryloyl (GelMA) (5%) (namely 135ACG hybrid ink) was formulated for the direct printing of cell-laden and acellular architectures. The 135ACG ink displayed excellent shear-thinning behavior and solid-like properties, leading to high printability without cell damage. After crosslinking, the ACG gel can also provide a stiff ECM ideal for stromal cell growth. By controlling the degree of substitution and polymer concentration, a GelMA (4%) bioink was designed to encapsulate hepatoma cells (hepG2), as GelMA gel possesses the desired low mechanical stiffness matching that of human liver tissue. Four different versions of to-scale liver lobule-mimetic constructs were fabricated via ME bioprinting, with precise positioning of two different cell types (NIH/3T3 and hepG2) embedded in matching ECMs (135ACG and GelMA, respectively). The four versions allowed us to exam effects of mechanical cues and intercellular interactions on cell behaviors. Fibroblasts thrived in stiff 135ACG matrix and aligned at the 135ACG/GelMA boundary due to durotaxis, while hepG2 formed spheroids exclusively in the soft GelMA matrix. Elevated albumin production was observed in the bicellular 3D co-culture of hepG2 and NIH/3T3, both with and without direct intercellular contact, indicating that improved hepatic cell function can be attributed to soluble chemical factors. Overall, our results showed that complex constructs with multiple cell types and varying ECMs can be bioprinted and potentially useful for both fundamental biomedical research and translational tissue engineering.

3D Cell Printing of Tissue/Organ-Mimicking Constructs for Therapeutic and Drug Testing Applications

The development of artificial tissue/organs with the functional maturity of their native equivalents is one of the long-awaited panaceas for the medical and pharmaceutical industries. Advanced 3D cell-printing technology and various functional bioinks are promising technologies in the field of tissue engineering that have enabled the fabrication of complex 3D living tissue/organs. Various requirements for these tissues, including a complex and large-volume structure, tissue-specific microenvironments, and functional vasculatures, have been addressed to develop engineered tissue/organs with native relevance. Functional tissue/organ constructs have been developed that satisfy such criteria and may facilitate both in vivo replenishment of damaged tissue and the development of reliable in vitro testing platforms for drug development. This review describes key developments in technologies and materials for engineering 3D cell-printed constructs for therapeutic and drug testing applications.

Self-crosslinking hyaluronic acid-carboxymethylcellulose hydrogel enhances multilayered 3D-printed construct shape integrity and mechanical stability for soft tissue engineering

One of the primary challenges in extrusion-based 3D bioprinting is the ability to print self-supported multilayered constructs with biocompatible hydrogels. The bioinks should have sufficient post-printing mechanical stability for soft tissue and organ regeneration. Here, we report on the synthesis, characterization and 3D printability of hyaluronic acid (HA)-carboxymethylcellulose (CMC) hydrogels cross-linked through N-acyl-hydrazone bonding. The hydrogel's hydrolytic stability was acquired by the effects of both the prevention of the oxidation of the six-membered rings of HA, and the stabilization of acyl-hydrazone bonds. The shear-thinning and self-healing properties of the hydrogel allowed us to print different 3D constructs (lattice, cubic and tube) of up to 50 layers with superior precision and high post-printing stability without support materials or post-processing depending on their compositions (H7:C3, H5:C5 and H3:C7). Morphological analyses of different zones of the 3D-printed constructs were undertaken for verification of the interconnection of pores. Texture profile analysis (TPA) (hardness (strength), elastic recovery, etc) and cyclic compression studies of the 3D-printed constructs demonstrated exceptional elastic properties and fast recovery after 50% strain, respectively, which have been attributed to the addition of CMC into HA. A model drug quercetin was released in a sustained manner from hydrogels and 3D constructs. In vitro cytotoxicity studies confirmed the excellent cyto-compatibility of these gels. In vivo mice studies prove that these biocompatible hydrogels enhance angiogenesis. The results indicate that controlling the key properties (e.g. self-crosslinking capacity, composition) can lead to the generation of multilayered constructs from 3D-bioprintable HA-CMC hydrogels capable of being leveraged for soft tissue engineering applications.