3D Bioprinting Strategies for Articular Cartilage Tissue Engineering

Articular cartilage is the avascular and aneural tissue which is the primary connective tissue covering the surface of articulating bone. Traumatic damage or degenerative diseases can cause articular cartilage injuries that are common in the population. As a result, the demand for new therapeutic options is continually increasing for older people and traumatic young patients. Many attempts have been made to address these clinical needs to treat articular cartilage injuries, including osteoarthritis (OA); however, regenerating highly qualified cartilage tissue remains a significant obstacle. 3D bioprinting technology combined with tissue engineering principles has been developed to create biological tissue constructs that recapitulate the anatomical, structural, and functional properties of native tissues. In addition, this cutting-edge technology can precisely place multiple cell types in a 3D tissue architecture. Thus, 3D bioprinting has rapidly become the most innovative tool for manufacturing clinically applicable bioengineered tissue constructs. This has led to increased interest in 3D bioprinting in articular cartilage tissue engineering applications. Here, we reviewed current advances in bioprinting for articular cartilage tissue engineering.

Increasing Collagen to Bioink Drives Mesenchymal Stromal Cells-Chondrogenesis from Hyaline to Calcified Layers

The bioextrusion of mesenchymal stromal cells (MSCs) directly seeded in a bioink enables the production of three-dimensional (3D) constructs, promoting their chondrogenic differentiation. Our study aimed to evaluate the effect of different type I collagen concentrations in the bioink on MSCs' chondrogenic differentiation. We printed 3D constructs using an alginate, gelatin, and fibrinogen-based bioink cellularized with MSCs, with four different quantities of type I collagen addition (0.0, 0.5, 1.0, and 5.0 mg per bioink syringe). We assessed the influence of the bioprinting process, the bioink composition, and the growth factor (TGF-ꞵ1) on the MSCs' survival rate. We confirmed the biocompatibility of the process and the bioinks' cytocompatibility. We evaluated the chondrogenic effects of TGF-ꞵ1 and collagen addition on the MSCs' chondrogenic properties through macroscopic observation, shrinking ratio, reverse transcription polymerase chain reaction, glycosaminoglycan synthesis, histology, and type II collagen immunohistochemistry. The bioink containing 0.5 mg of collagen produces the richest hyaline-like extracellular matrix, presenting itself as a promising tool to recreate the superficial layer of hyaline cartilage. The bioink containing 5.0 mg of collagen enhances the synthesis of a calcified matrix, making it a good candidate for mimicking the calcified cartilaginous layer. Type I collagen thus allows the dose-dependent design of specific hyaline cartilage layers.

A review of biomacromolecule-based 3D bioprinting strategies for structure-function integrated repair of skin tissues

When skin is damaged or affected by diseases, it often undergoes irreversible scar formation, leading to aesthetic concerns and psychological distress for patients. In cases of extensive skin defects, the patient's life can be severely compromised. In recent years, 3D printing technology has emerged as a groundbreaking approach to skin tissue engineering, offering promising solutions to various skin-related conditions. 3D bioprinting technology enables the precise fabrication of structures by programming the spatial arrangement of cells within the skin tissue and subsequently printing skin replacements either in a 3D bioprinter or directly at the site of the defect. This study provides a comprehensive overview of various biopolymer-based inks, with a particular emphasis on chitosan (CS), starch, alginate, agarose, cellulose, and fibronectin, all of which are natural polymers belonging to the category of biomacromolecules. Additionally, it summarizes artificially synthesized polymers capable of enhancing the performance of these biomacromolecule-based bioinks, thereby composing hybrid biopolymer inks aimed at better application in skin tissue engineering endeavors. This review paper examines the recent advancements, characteristics, benefits, and limitations of biological 3D bioprinting techniques for skin tissue engineering. By utilizing bioinks containing seed cells, hydrogels with bioactive factors, and biomaterials, complex structures resembling natural skin can be accurately fabricated in a layer-by-layer manner. The importance of biological scaffolds in promoting skin wound healing and the role of 3D bioprinting in skin tissue regeneration processes is discussed. Additionally, this paper addresses the challenges and constraints associated with current 3D bioprinting technologies for skin tissue and presents future perspectives. These include advancements in bioink formulations, full-thickness skin bioprinting, vascularization strategies, and skin appendages bioprinting.

Recent Advances in Alginate-Based Hydrogels for Cell Transplantation Applications

With its exceptional biocompatibility, alginate emerged as a highly promising biomaterial for a large range of applications in regenerative medicine. Whether in the form of microparticles, injectable hydrogels, rigid scaffolds, or bioinks, alginate provides a versatile platform for encapsulating cells and fostering an optimal environment to enhance cell viability. This review aims to highlight recent studies utilizing alginate in diverse formulations for cell transplantation, offering insights into its efficacy in treating various diseases and injuries within the field of regenerative medicine.

Covalent Grafting of Functionalized MEW Fibers to Silk Fibroin Hydrogels to Obtain Reinforced Tissue Engineered Constructs

Hydrogels are ideal materials to encapsulate cells, making them suitable for applications in tissue engineering and regenerative medicine. However, they generally do not possess adequate mechanical strength to functionally replace human tissues, and therefore they often need to be combined with reinforcing structures. While the interaction at the interface between the hydrogel and reinforcing structure is imperative for mechanical function and subsequent biological performance, this interaction is often overlooked. Melt electrowriting enables the production of reinforcing microscale fibers that can be effectively integrated with hydrogels. Yet, studies on the interaction between these micrometer scale fibers and hydrogels are limited. Here, we explored the influence of covalent interfacial interactions between reinforcing structures and silk fibroin methacryloyl hydrogels (silkMA) on the mechanical properties of the construct and cartilage-specific matrix production in vitro. For this, melt electrowritten fibers of a thermoplastic polymer blend (poly(hydroxymethylglycolide-co-ε-caprolactone):poly(ε-caprolactone) (pHMGCL:PCL)) were compared to those of the respective methacrylated polymer blend pMHMGCL:PCL as reinforcing structures. Photopolymerization of the methacrylate groups, present in both silkMA and pMHMGCL, was used to generate hybrid materials. Covalent bonding between the pMHMGCL:PCL blend and silkMA hydrogels resulted in an elastic response to the application of torque. In addition, an improved resistance was observed to compression (∼3-fold) and traction (∼40-55%) by the scaffolds with covalent links at the interface compared to those without these interactions. Biologically, both types of scaffolds (pHMGCL:PCL and pMHMGCL:PCL) showed similar levels of viability and metabolic activity, also compared to frequently used PCL. Moreover, articular cartilage progenitor cells embedded within the reinforced silkMA hydrogel were able to form a cartilage-like matrix after 28 days of in vitro culture. This study shows that hybrid cartilage constructs can be engineered with tunable mechanical properties by grafting silkMA hydrogels covalently to pMHMGCL:PCL blend microfibers at the interface.

Three dimensional (bio)printing of blood vessels: from vascularized tissues to functional arteries

Tissue engineering has emerged as a strategy for producing functional tissues and organs to treat diseases and injuries. Many chronic conditions directly or indirectly affect normal blood vessel functioning, necessary for material exchange and transport through the body and within tissue-engineered constructs. The interest in vascular tissue engineering is due to two reasons: (1) functional grafts can be used to replace diseased blood vessels, and (2) engineering effective vasculature within other engineered tissues enables connection with the host's circulatory system, supporting their survival. Among various practices, (bio)printing has emerged as a powerful tool to engineer biomimetic constructs. This has been made possible with precise control of cell deposition and matrix environment along with the advancements in biomaterials. (Bio)printing has been used for both engineering stand-alone vascular grafts as well as vasculature within engineered tissues for regenerative applications. In this review article, we discuss various conditions associated with blood vessels, the need for artificial blood vessels, the anatomy and physiology of different blood vessels, available 3D (bio)printing techniques to fabricate tissue-engineered vascular grafts and vasculature in scaffolds, and the comparison among the different techniques. We conclude our review with a brief discussion about future opportunities in the area of blood vessel tissue engineering.

Adipose-Derived Stromal Cells Preserve Pancreatic Islet Function in a Transplantable 3D Bioprinted Scaffold

Intra-portal islet transplantation is currently the only clinically approved beta cell replacement therapy, but its outcome is hindered by limited cell survival due to a multifactorial reaction against the allogeneic tissue in liver. Adipose-derived stromal cells (ASCs) can potentially improve the islet micro-environment by their immunomodulatory action. The challenge is to combine both islets and ASCs in a relatively easy and consistent long-term manner in a deliverable scaffold. Manufacturing the 3D bioprinted double-layered scaffolds with primary islets and ASCs using a mix of alginate/nanofibrillated cellulose (NFC) bioink is reported. The diffusion properties of the bioink and the supportive effect of human ASCs on islet viability, glucose sensing, insulin secretion, and reducing the secretion of pro-inflammatory cytokines are demonstrated. Diabetic mice transplanted with islet-ASC scaffolds reach normoglycemia seven days post-transplantation with no significant difference between this group and the group received islets under the kidney capsules. In addition, animals transplanted with islet-ASC scaffolds stay normoglycemic and show elevated levels of C-peptide compared to mice transplanted with islet-only scaffolds. The data present a functional 3D bioprinted scaffold for islets and ASCs transplanted to the extrahepatic site and suggest a possible role of ASCs on improving the islet micro-environment.

Progress in bioprinting technology for tissue regeneration

In recent years, due to the increase in diseases that require organ/tissue transplantation and the limited donor, on the other hand, patients have lost hope of recovery and organ transplantation. Regenerative medicine is one of the new sciences that promises a bright future for these patients by providing solutions to repair, improve function, and replace tissue. One of the technologies used in regenerative medicine is three-dimensional (3D) bioprinters. Bioprinting is a new strategy that is the basis for starting a global revolution in the field of medical sciences and has attracted much attention. 3D bioprinters use a combination of advanced biology and cell science, computer science, and materials science to create complex bio-hybrid structures for various applications. The capacity to use this technology can be demonstrated in regenerative medicine to make various connective tissues, such as skin, cartilage, and bone. One of the essential parts of a 3D bioprinter is the bio-ink. Bio-ink is a combination of biologically active molecules, cells, and biomaterials that make the printed product. In this review, we examine the main bioprinting strategies, such as inkjet printing, laser, and extrusion-based bioprinting, as well as some of their applications.

Technologies for the fabrication of crosslinked polysaccharide-based hydrogels and its role in microbial three-dimensional bioprinting - A review

Three-Dimensional bioprinting has recently gained more attraction among researchers for its wide variety of applicability. This technology involving in developing structures that mimic the natural anatomy, and also aims in developing novel biomaterials, bioinks which have a better printable ability. Different hydrogels (cross-linked polysaccharides) can be used and optimized for good adhesion and cell proliferation. Manufacturing hydrogels with adjustable characteristics allows for fine-tuning of the cellular microenvironment. Different printing technologies can be used to create hydrogels on a micro-scale which will allow regular, patterned integration of cells into hydrogels. Controlling tissue constructions' structural architecture is the important key to ensuring its function as it is designed. The designed tiny hydrogels will be useful in investigating the cellular behaviour within the environments. Three-Dimensional designs can be constructed by modifying their shape and behaviour analogous concerning pressure, heat, electricity, ultraviolet radiation or other environmental elements. Yet, its application in in vitro infection models needs more research and practical study. Microbial bioprinting has become an advancing field with promising potential to develop various biomedical as well as environmental applications. This review elucidates the properties and usage of different hydrogels for Three-Dimensional bioprinting.

Multilayer 3D bioprinting and complex mechanical properties of alginate-gelatin mesostructures

In the biomedical field, extrusion-based 3D bioprinting has emerged as a promising technique to fabricate tissue replacements. However, a main challenge is to find suitable bioinks and reproducible procedures that ensure good printability and generate final printed constructs with high shape fidelity, similarity to the designed model, and controllable mechanical properties. In this study, our main goal is to 3D print multilayered structures from alginate-gelatin (AG) hydrogels and to quantify their complex mechanical properties with particular focus on the effects of the extrusion process and geometrical parameters, i.e. different mesostructures and macroporosities. We first introduce a procedure including a pre-cooling step and optimized printing parameters to control and improve the printability of AG hydrogels based on rheological tests and printability studies. Through this procedure, we significantly improve the printability and flow stability of AG hydrogels and successfully fabricate well-defined constructs similar to our design models. Our subsequent complex mechanical analyses highlight that the extrusion process and the mesostructure, characterized by pore size, layer height and filament diameter, significantly change the complex mechanical response of printed constructs. The presented approach and the corresponding results have important implications for future 3D bioprinting applications when aiming to produce replacements with good structural integrity and defined mechanical properties similar to the native tissue, especially in soft tissue engineering. The approach is also applicable to the printing of gelatin-based hydrogels with different accompanying materials, concentrations, or cells.

Progress in the application of 3D-printed sodium alginate-based hydrogel scaffolds in bone tissue repair

In recent years, hydrogels have been widely used in the biomedical field as materials with excellent bionic structures and biological properties. Among them, the excellent comprehensive properties of natural polymer hydrogels represented by sodium alginate have attracted the great attention of researchers. At the same time, by physically blending sodium alginate with other materials, the problems of poor cell adhesion and mechanical properties of sodium alginate hydrogels were directly improved without chemical modification of sodium alginate. The composite blending of multiple materials can also improve the functionality of sodium alginate hydrogels, and the prepared composite hydrogel also has a larger application field. In addition, based on the adjustable viscosity of sodium alginate-based hydrogels, sodium alginate-based hydrogels can be loaded with cells to prepare biological ink, and the scaffold can be printed out by 3D printing technology for the repair of bone defects. This paper first summarizes the improvement of the properties of sodium alginate and other materials after physical blending. Then, it summarizes the application progress of sodium alginate-based hydrogel scaffolds for bone tissue repair based on 3D printing technology in recent years. Moreover, we provide relevant opinions and comments to provide a theoretical basis for follow-up research.

In-situ ionic crosslinking of 3D bioprinted cell-hydrogel constructs for mechanical reinforcement and improved cell growth

Hydrogels are commonly used in 3D bioprinting technology owing to their ability to encapsulate living cells. However, their inherent delicate properties limit their applicability in the fabrication of mechanically reliable tissue engineering constructs. Herein, we propose a novel reinvented layering integration method for the functional enhancement of 3D cell-hydrogel bioprinting. This was implemented by inserting electrospun microfiber sheets with a crosslinker between the 3D bioprinted layers. When surface-modified microfiber sheets were combined with Ca²⁺ ionic crosslinkers, the as-printed cell-hydrogel strand was immediately crosslinked when it contacted the sheet surface. The in-situ crosslinking in the bioprinting process not only improved the overall structural stability, but also reinforced the compressive strength and elastic modulus. The enhanced structural stability guaranteed the shape fidelity of the 3D architecture, which included the internal channel network, resulting in improved perfusion conditions for cell growth. The growth of NIH3T3 fibroblasts in 3D bioconstructs with in-situ crosslinking increased by up to five times compared to that of normally bioprinted constructs. The strengthened structural integrity was distinctly sustainable during the cell culture period owing to the sustained release of Ca²⁺ ions from the embedded microfiber sheets. The synergistic effect of the reinforced mechanical properties with enhanced cell growth is expected to extend the applicability of the proposed hydrogel-based bioprinting technique for soft tissue engineering.

Classification, processing, and applications of bioink and 3D bioprinting: A detailed review

With the advancement in 3D bioprinting technology, cell culture methods can design 3D environments which are both, complex and physiologically relevant. The main component in 3D bioprinting, bioink, can be split into various categories depending on the criterion of categorization. Although the choice of bioink and bioprinting process will vary greatly depending on the application, general features such as material properties, biological interaction, gelation, and viscosity are always important to consider. The foundation of 3D bioprinting is the exact layer-by-layer implantation of biological elements, biochemicals, and living cells with the spatial control of the implantation of functional elements onto the biofabricated 3D structure. Three basic strategies underlie the 3D bioprinting process: autonomous self-assembly, micro tissue building blocks, and biomimicry or biomimetics. Tissue engineering can benefit from 3D bioprinting in many ways, but there are still numerous obstacles to overcome before functional tissues can be produced and used in clinical settings. A better comprehension of the physiological characteristics of bioink materials and a higher level of ability to reproduce the intricate biologically mimicked and physiologically relevant 3D structures would be a significant improvement for 3D bioprinting to overcome the limitations.

Rheology as a Tool for Fine-Tuning the Properties of Printable Bioinspired Gels

Over the last decade, efforts have been oriented toward the development of suitable gels for 3D printing, with controlled morphology and shear-thinning behavior in well-defined conditions. As a multidisciplinary approach to the fabrication of complex biomaterials, 3D bioprinting combines cells and biocompatible materials, which are subsequently printed in specific shapes to generate 3D structures for regenerative medicine or tissue engineering. A major interest is devoted to the printing of biomimetic materials with structural fidelity after their fabrication. Among some requirements imposed for bioinks, such as biocompatibility, nontoxicity, and the possibility to be sterilized, the nondamaging processability represents a critical issue for the stability and functioning of the 3D constructs. The major challenges in the field of printable gels are to mimic at different length scales the structures existing in nature and to reproduce the functions of the biological systems. Thus, a careful investigation of the rheological characteristics allows a fine-tuning of the material properties that are manufactured for targeted applications. The fluid-like or solid-like behavior of materials in conditions similar to those encountered in additive manufacturing can be monitored through the viscoelastic parameters determined in different shear conditions. The network strength, shear-thinning, yield point, and thixotropy govern bioprintability. An assessment of these rheological features provides significant insights for the design and characterization of printable gels. This review focuses on the rheological properties of printable bioinspired gels as a survey of cutting-edge research toward developing printed materials for additive manufacturing.

Palladium nanoparticle based smart hydrogels for NIR light-triggered photothermal/photodynamic therapy and drug release with wound healing capability

Tumor recurrence and wound repair are two major challenges following cancer surgical resection that can be addressed through precision nanomedicine. Herein, palladium nanoparticles (Pd NPs) with photothermal and photodynamic therapy (PTT/PDT) capacity were successfully synthesized. The Pd NPs were loaded with chemotherapeutic doxorubicin (DOX) to form hydrogels (Pd/DOX@hydrogel) as a smart anti-tumor platform. The hydrogels were composed of clinically approved agarose and chitosan, with excellent biocompatibility and wound healing ability. Pd/DOX@hydrogel can be used for both PTT and PDT with a synergistic effect to kill tumor cells. Additionally, the photothermal effect of Pd/DOX@hydrogel allowed the photo-triggered drug release of DOX. Therefore, Pd/DOX@hydrogel can be used for near-infrared (NIR)-triggered PTT and PDT as well as for photo-induced chemotherapy, efficiently inhibiting tumor growth. Furthermore, Pd/DOX@hydrogel can be used as a temporary biomimetic skin to block the invasion of foreign harmful substances, promote angiogenesis, and accelerate wound repair and new skin formation. Thus, the as-prepared smart Pd/DOX@hydrogel is expected to provide a feasible therapeutic solution following tumor resection.

Recent advances in bioprinting using silk protein-based bioinks

3D printing has experienced swift growth for biological applications in the field of regenerative medicine and tissue engineering. Essential features of bioprinting include determining the appropriate bioink, printing speed mechanics, and print resolution while also maintaining cytocompatibility. However, the scarcity of bioinks that provide printing and print properties and cell support remains a limitation. Silk Fibroin (SF) displays exceptional features and versatility for inks and shows the potential to print complex structures with tunable mechanical properties, degradation rates, and cytocompatibility. Here we summarize recent advances and needs with the use of SF protein from Bombyx mori silkworm as a bioink, including crosslinking methods for extrusion bioprinting using SF and the maintenance of cell viability during and post bioprinting. Additionally, we discuss how encapsulated cells within these SF-based 3D bioprinted constructs are differentiated into various lineages such as skin, cartilage, and bone to expedite tissue regeneration. We then shift the focus towards SF-based 3D printing applications, including magnetically decorated hydrogels, in situ bioprinting, and a next-generation 4D bioprinting approach. Future perspectives on improvements in printing strategies and the use of multicomponent bioinks to improve print fidelity are also discussed.

A sound approach to advancing healthcare systems: the future of biomedical acoustics

Newly developed acoustic technologies are playing a transformational role in life science and biomedical applications ranging from the activation and inactivation of mechanosensitive ion channels for fundamental physiological processes to the development of contact-free, precise biofabrication protocols for tissue engineering and large-scale manufacturing of organoids. Here, we provide our perspective on the development of future acoustic technologies and their promise in addressing critical challenges in biomedicine.

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.

3D Bioprinting for Spinal Cord Injury Repair

Spinal cord injury (SCI) is considered to be one of the most challenging central nervous system injuries. The poor regeneration of nerve cells and the formation of scar tissue after injury make it difficult to recover the function of the nervous system. With the development of tissue engineering, three-dimensional (3D) bioprinting has attracted extensive attention because it can accurately print complex structures. At the same time, the technology of blending and printing cells and related cytokines has gradually been matured. Using this technology, complex biological scaffolds with accurate cell localization can be manufactured. Therefore, this technology has a certain potential in the repair of the nervous system, especially the spinal cord. So far, this review focuses on the progress of tissue engineering of the spinal cord, landmark 3D bioprinting methods, and landmark 3D bioprinting applications of the spinal cord in recent years.

Rheological Analysis of Bio-ink for 3D Bio-printing Processes

3D bio-printing is an emerging technology to fabricate tissue scaffold in-vitro through the controlled allocation of biomaterial and cells, which can mimic the in-vivo counterpart of living tissue. Live cells are often encapsulated into the biomaterials (i.e., bio-ink) and extruded by controlling the printing parameters. The functionality of the bioink depends upon three factors: (a) printability, (b) shape fidelity, and (c) bio-compatibility. Increasing viscosity will improve the printability and the shape fidelity but require higher applied extrusion pressure, which is detrimental to the living cell dwelling in the bio-ink, which is often ignored in the bio-ink optimization process. This paper demonstrates a roadmap to develop and optimize bio-inks, ensuring printability, shape fidelity, and cell survivability. The pressure exerted on the bio-ink during extrusion processes is measured analytically, and the information is incorporated in the bio-ink's rheology design. Cell-laden filaments are fabricated with multiple cell lines, i.e., Human Embryonic Kidney (HEK 293), BxPC3, and prostate cancer cells which are analyzed for cell viability. The cross-sectional live-dead assay of the extruded filament demonstrates a spatial pattern for HEK 293 cell viability, which correlates with our analytical finding of the shear stress at the nozzle tip. All three cell lines were able to sustain a transient shear stress of 3.7 kPa and demonstrate 90% viability with our designed bio-ink after 15 days of incubation. Simultaneously, the shape fidelity and printability matrices show its suitability for 3D bio-printing process.

Coaxial 3D bioprinting of tri-polymer scaffolds to improve the osteogenic and vasculogenic potential of cells in co-culture models

The crosstalk between osteoblasts and endothelial cells is critical for bone vascularization and regeneration. Here, we used a coaxial 3D bioprinting method to directly print an osteon-like structure by depositing angiogenic and osteogenic bioinks from the core and shell regions of the coaxial nozzle, respectively. The bioinks were made up of gelatin, gelatin methacryloyl (GelMA), alginate, and hydroxyapatite (HAp) nanoparticles and were loaded with human umbilical vascular endothelial cells (HUVECs) and osteoblasts (MC3T3) in the core and shell regions, respectively. Conventional monoaxial 3D bioprinting was used as a control method, where the hydrogels, HAp nanoparticles, MC3T3 cells, and HUVECs were all mixed in one bioink and printed from the core nozzle. As a result, the bioprinted scaffolds were composed of cell-laden fibers with either a core-shell or homogenous structure, providing a non-contact (indirect) or contact (direct) co-culture of MC3T3 cells and HUVECs, respectively. Both structures supported the 3D culture of HUVECs and osteoblasts over a long period. The scaffolds also supported the expression of osteogenic and angiogenic factors. However, the gene expression was significantly higher for the core-shell structure than the homogeneous structure due to the well-defined distribution of osteoblasts and endothelial cells and the formation of vessel-like structures in the co-culture system. Our results indicated that the coaxial bioprinting technique, with the ability to create a non-contact co-culture of cells, can provide a more efficient bioprinting strategy for printing highly vascularized and bioactive bone structures.

3D Bioprinting of Cell-Laden Hydrogels for Improved Biological Functionality

The encapsulation of cells within gel-phase materials to form bioinks offers distinct advantages for next-generation 3D bioprinting. 3D bioprinting has emerged as a promising tool for patterning cells, but the technology remains limited in its ability to produce biofunctional, tissue-like constructs due to a dearth of materials suitable for bioinks. While early demonstrations commonly used viscous polymers optimized for printability, these materials often lacked cell compatibility and biological functionality. In response, advanced materials that exist in the gel phase during the entire printing process are being developed, since hydrogels are uniquely positioned to both protect cells during extrusion and provide biological signals to embedded cells as the construct matures during culture. Here, an overview of the design considerations for gel-phase materials as bioinks is presented, with a focus on their mechanical, biochemical, and dynamic gel properties. Current challenges and opportunities that arise due to the fact that bioprinted constructs are active, living hydrogels composed of both acellular and cellular components are also evaluated. Engineering hydrogels with consideration of cells as an intrinsic component of the printed bioink will enable control over the evolution of the living construct after printing to achieve greater biofunctionality.

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.

Long-Fiber Embedded Hydrogel 3D Printing for Structural Reinforcement

Hydrogels are candidate building blocks in a wide range of biomaterial applications including soft and biohybrid robotics, microfluidics, and tissue engineering. Recent advances in embedded 3D printing have broadened the design space accessible with hydrogel additive manufacturing. Specifically, the Freeform Reversible Embedding of Suspended Hydrogels (FRESH) technique has enabled the fabrication of complex 3D structures using extremely soft hydrogels, e.g., alginate and collagen, by assembling hydrogels within a fugitive support bath. However, the low structural rigidity of FRESH printed hydrogels limits their applications, especially those that require operation in nonaqueous environments. In this study, we demonstrated long-fiber embedded hydrogel 3D printing using a multihead printing platform consisting of a custom-built fiber extruder and an open-source FRESH bioprinter with high embedding fidelity. Using this process, fibers were embedded in 3D printed hydrogel components to achieve significant structural reinforcement (e.g., tensile modulus improved from 56.78 ± 8.76 to 382.55 ± 25.29 kPa and tensile strength improved from 9.44 ± 2.28 to 45.05 ± 5.53 kPa). In addition, we demonstrated the versatility of this technique by using fibers of a wide range of sizes and material types and implementing different 2D and 3D embedding patterns, such as embedding a conical helix using electrochemically aligned collagen fiber via nonplanar printing. Moreover, the technique was implemented using low-cost material and is compatible with open-source software and hardware, which facilitates its adoption and modification for new research applications.

Bioprinting Scaffolds for Vascular Tissues and Tissue Vascularization

In recent years, tissue engineering has achieved significant advancements towards the repair of damaged tissues. Until this day, the vascularization of engineered tissues remains a challenge to the development of large-scale artificial tissue. Recent breakthroughs in biomaterials and three-dimensional (3D) printing have made it possible to manipulate two or more biomaterials with complementary mechanical and/or biological properties to create hybrid scaffolds that imitate natural tissues. Hydrogels have become essential biomaterials due to their tissue-like physical properties and their ability to include living cells and/or biological molecules. Furthermore, 3D printing, such as dispensing-based bioprinting, has progressed to the point where it can now be utilized to construct hybrid scaffolds with intricate structures. Current bioprinting approaches are still challenged by the need for the necessary biomimetic nano-resolution in combination with bioactive spatiotemporal signals. Moreover, the intricacies of multi-material bioprinting and hydrogel synthesis also pose a challenge to the construction of hybrid scaffolds. This manuscript presents a brief review of scaffold bioprinting to create vascularized tissues, covering the key features of vascular systems, scaffold-based bioprinting methods, and the materials and cell sources used. We will also present examples and discuss current limitations and potential future directions of the technology.

Extrusion-based 3D (Bio)Printed Tissue Engineering Scaffolds: Process-Structure-Quality Relationships

Biological additive manufacturing (Bio-AM) has emerged as a promising approach for the fabrication of biological scaffolds with nano- to microscale resolutions and biomimetic architectures beneficial to tissue engineering applications. However, Bio-AM processes tend to introduce flaws in the construct during fabrication. These flaws can be traced to material nonhomogeneity, suboptimal processing parameters, changes in the (bio)printing environment (such as nozzle clogs), and poor construct design, all with significant contributions to the alteration of a scaffold's mechanical properties. In addition, the biological response of endogenous and exogenous cells interacting with the defective scaffolds could become unpredictable. In this review, we first described extrusion-based Bio-AM. We highlighted the salient architectural and mechanotransduction parameters affecting the response of cells interfaced with the scaffolds. The process phenomena leading to defect formation and some of the tools for defect detection are reviewed. The limitations of the existing developments and the directions that the field should grow in order to overcome said limitations are discussed.

Engineering Tissue-Specific, Multiscale Microvasculature with a Capillary Network for Prevascularized Tissue

Although there are various pre-existing technologies for engineering vasculatures, multiscale modeling of the architecture of human vasculature at a capillary scale remains a challenge. In this study, a novel technology is developed for the production of a functional, multiscale microvasculature comprising of endothelialized channels and tissue-specific capillary networks. Perfusable, endothelialized channels are bioprinted, after which angiogenic sprouts are grown into user-designed capillary networks. The induction of branched and liver-lobule-like capillary networks confirm that the technology can produce various types of tissue-specific multiscale microvasculatures. Further, the channels and capillaries are deemed to be functional when evaluated in vitro. An ex vivo assay demonstrates that the microvasculature can induce neovessel ingrowth, integrate with host vessels, and facilitate blood flow. Remarkably, blood flows through the implanted capillary network without any change in its morphology. Finally, the technology is applied to produce a vascularized liver tissue; it significantly improves its hepatic function. It is believed that this new technology will create new possibilities in the development of highly vascularized and functional tissues/organs on a clinically relevant scale.

Current Trends on Protein Driven Bioinks for 3D Printing

In the last decade, three-dimensional (3D) extrusion bioprinting has been on the top trend for innovative technologies in the field of biomedical engineering. In particular, protein-based bioinks such as collagen, gelatin, silk fibroin, elastic, fibrin and protein complexes based on decellularized extracellular matrix (dECM) are receiving increasing attention. This current interest is the result of protein's tunable properties, biocompatibility, environmentally friendly nature and possibility to provide cells with the adequate cues, mimicking the extracellular matrix's function. In this review we describe the most relevant stages of the development of a protein-driven bioink. The most popular formulations, molecular weights and extraction methods are covered. The different crosslinking methods used in protein bioinks, the formulation with other polymeric systems or molecules of interest as well as the bioprinting settings are herein highlighted. The cell embedding procedures, the in vitro, in vivo, in situ studies and final applications are also discussed. Finally, we approach the development and optimization of bioinks from a sequential perspective, discussing the relevance of each parameter during the pre-processing, processing, and post-processing stages of technological development. Through this approach the present review expects to provide, in a sequential manner, helpful methodological guidelines for the development of novel bioinks.

Towards the Experimentally-Informed In Silico Nozzle Design Optimization for Extrusion-Based Bioprinting of Shear-Thinning Hydrogels

Research in bioprinting is booming due to its potential in addressing several manufacturing challenges in regenerative medicine. However, there are still many hurdles to overcome to guarantee cell survival and good printability. For the 3D extrusion-based bioprinting, cell viability is amongst one of the lowest of all the bioprinting techniques and is strongly influenced by various factors including the shear stress in the print nozzle. The goal of this study is to quantify, by means of in silico modeling, the mechanical environment experienced by the bioink during the printing process. Two ubiquitous nozzle shapes, conical and blunted, were considered, as well as three common hydrogels with material properties spanning from almost Newtonian to highly shear-thinning materials following the power-law behavior: Alginate-Gelatin, Alginate and PF127. Comprehensive in silico testing of all combinations of nozzle geometry variations and hydrogels was achieved by combining a design of experiments approach (DoE) with a computational fluid dynamics (CFD) of the printing process, analyzed through a machine learning approach named Gaussian Process. Available experimental results were used to validate the CFD model and justify the use of shear stress as a surrogate for cell survival in this study. The lower and middle nozzle radius, lower nozzle length and the material properties, alone and combined, were identified as the major influencing factors affecting shear stress, and therefore cell viability, during printing. These results were successfully compared with those of reported experiments testing viability for different nozzle geometry parameters under constant flow rate or constant pressure. The in silico 3D bioprinting platform developed in this study offers the potential to assist and accelerate further development of 3D bioprinting.

Fabrication techniques involved in developing the composite scaffolds PCL/HA nanoparticles for bone tissue engineering applications

A fine-tuned combination of scaffolds, biomolecules, and mesenchymal stem cells (MSCs) is used in tissue engineering to restore the function of injured bone tissue and overcome the complications associated with its regeneration. For two decades, biomaterials have attracted much interest in mimicking the native extracellular matrix of bone tissue. To this aim, several approaches based on biomaterials combined with MSCs have been amply investigated. Recently, hydroxyapatite (HA) nanoparticles have been incorporated with polycaprolactone (PCL) matrix as a suitable substitute for bone tissue engineering applications. This review article aims at providing a brief overview on PCL/HA composite scaffold fabrication techniques such as sol-gel, rapid prototyping, electro-spinning, particulate leaching, thermally induced phase separation, and freeze-drying, as suitable approaches for tailoring morphological, mechanical, and biodegradability properties of the scaffolds for bone tissues. Among these methods, the 3D plotting method shows improvements in pore architecture (pore size of ≥600 µm and porosity of 92%), mechanical properties (higher than 18.38 MPa), biodegradability, and good bioactivity in bone tissue regeneration.

Vapor-phased fabrication and modulation of cell-laden scaffolding materials

Bottom-up approaches using building blocks of modules to fabricate scaffolds for tissue engineering applications have enabled the fabrication of structurally complex and multifunctional materials allowing for physical and chemical flexibility to better mimic the native extracellular matrix. Here we report a vapor-phased fabrication process for constructing three-dimensional modulated scaffold materials via simple steps based on controlling mass transport of vapor sublimation and deposition. We demonstrate the fabrication of scaffolds comprised of multiple biomolecules and living cells with built-in boundaries separating the distinct compartments containing defined biological configurations and functions. We show that the fabricated scaffolds have mass production potential. We demonstrate overall >80% cell viability of encapsulated cells and that modulated scaffolds exhibit enhanced cell proliferation, osteogenesis, and neurogenesis, which can be assembled into various geometric configurations. We perform cell co-culture experiments to show independent osteogenesis and angiogenesis activities from separate compartments in one scaffold construct.

Visible Light-Curable Chitosan Ink for Extrusion-Based and Vat Polymerization-Based 3D Bioprintings

Three-dimensional bioprinting has attracted much attention for biomedical applications, including wound dressing and tissue regeneration. The development of functional and easy-to-handle inks is expected to expand the applications of this technology. In this study, aqueous solutions of chitosan derivatives containing sodium persulfate (SPS) and Tris(2,2′-bipyridyl) ruthenium(II) chloride (Ru(bpy)₃) were applied as inks for both extrusion-based and vat polymerization-based bioprinting. In both the printing systems, the curation of ink was caused by visible light irradiation. The gelation time of the solution and the mechanical properties of the resultant hydrogels could be altered by changing the concentrations of SPS and Ru(bpy)₃. The 3D hydrogel constructs with a good shape fidelity were obtained from the chitosan inks with a composition that formed gel within 10 s. In addition, we confirmed that the chitosan hydrogels have biodegradability and antimicrobial activity. These results demonstrate the significant potential of using the visible light-curable inks containing a chitosan derivative for extrusion and vat polymerization-based bioprinting toward biomedical applications.

Bionic Organs: Shear Forces Reduce Pancreatic Islet and Mammalian Cell Viability during the Process of 3D Bioprinting

Background: 3D bioprinting is the future of constructing functional organs. Creating a bioactive scaffold with pancreatic islets presents many challenges. The aim of this paper is to assess how the 3D bioprinting process affects islet viability. Methods: The BioX 3D printer (Cellink), 600 μm inner diameter nozzles, and 3% (w/v) alginate cell carrier solution were used with rat, porcine, and human pancreatic islets. Islets were divided into a control group (culture medium) and 6 experimental groups (each subjected to specific pressure between 15 and 100 kPa). FDA/PI staining was performed to assess the viability of islets. Analogous studies were carried out on α-cells, β-cells, fibroblasts, and endothelial cells. Results: Viability of human pancreatic islets was as follows: 92% for alginate-based control and 94%, 90%, 74%, 48%, 61%, and 59% for 15, 25, 30, 50, 75, and 100 kPa, respectively. Statistically significant differences were observed between control and 50, 75, and 100 kPa, respectively. Similar observations were made for porcine and rat islets. Conclusions: Optimal pressure during 3D bioprinting with pancreatic islets by the extrusion method should be lower than 30 kPa while using 3% (w/v) alginate as a carrier.

Printability in extrusion bioprinting

Extrusion bioprinting has been widely used to extrude continuous filaments of bioink (or the mixture of biomaterial and living cells), layer-by-layer, to build three-dimensional (3D) constructs for biomedical applications. In extrusion bioprinting, printability is an important parameter used to measure the difference between the designed construct and the one actually printed. This difference could be caused by the extrudability of printed bioink and/or the structural formability and stability of printed constructs. Although studies have reported in characterizing printability based on the bioink properties and printing process, the concept of printability is often confusingly and, sometimes, conflictingly used in the literature. The objective of this perspective is to define the printability for extrusion bioprinting in terms of extrudability, filament fidelity, and structural integrity, as well as to review the effect of bioink properties, bioprinting process, and construct design on the printability. Challenges related to the printability of extrusion bioprinting are also discussed, along with recommendations for improvements.

Applications of 3D bioprinting in tissue engineering: advantages, deficiencies, improvements, and future perspectives

Over the past decade, 3D bioprinting technology has progressed tremendously in the field of tissue engineering in its ability to fabricate individualized biological constructs with precise geometric designability, which offers us the capability to bridge the divergence between engineered tissue constructs and natural tissues. In this work, we first review the current widely used 3D bioprinting approaches, cells, and materials. Next, the updated applications of this technique in tissue engineering, including bone tissue, cartilage tissue, vascular grafts, skin, neural tissue, heart tissue, liver tissue and lung tissue, are briefly introduced. Then, the prominent advantages of 3D bioprinting in tissue engineering are summarized in detail: rapidly prototyping the customized structure, delivering cell-laden materials with high precision in space, and engineering with a highly controllable microenvironment. The current technical deficiencies of 3D bioprinted constructs in terms of mechanical properties and cell behaviors are afterward illustrated, as well as corresponding improvements. Finally, we conclude with future perspectives about 3D bioprinting in tissue engineering.

Designing Biomaterial Platforms for Cardiac Tissue and Disease Modeling

Heart disease is one of the leading causes of death in the world. There is a growing demand for in vitro cardiac models that can recapitulate the complex physiology of the cardiac tissue. These cardiac models can provide a platform to better understand the underlying mechanisms of cardiac development and disease and aid in developing novel treatment alternatives and platforms towards personalized medicine. In this review, a summary of engineered cardiac platforms is presented. Basic design considerations for replicating the heart's microenvironment are discussed considering the anatomy of the heart. This is followed by a detailed summary of the currently available biomaterial platforms for modeling the heart tissue in vitro. These in vitro models include 2D surface modified structures, 3D molded structures, porous scaffolds, electrospun scaffolds, bioprinted structures, and heart-on-a-chip devices. The challenges faced by current models and the future directions of in vitro cardiac models are also discussed. Engineered in vitro tissue models utilizing patients' own cells could potentially revolutionize the way we develop treatment and diagnostic alternatives.

3D bioprinting for skin tissue engineering: Current status and perspectives

Skin and skin appendages are vulnerable to injury, requiring rapidly reliable regeneration methods. In recent years, 3D bioprinting has shown potential for wound repair and regeneration. 3D bioprinting can be customized for skin shape with cells and other materials distributed precisely, achieving rapid and reliable production of bionic skin substitutes, therefore, meeting clinical and industrial requirements. Additionally, it has excellent performance with high resolution, flexibility, reproducibility, and high throughput, showing great potential for the fabrication of tissue-engineered skin. This review introduces the common techniques of 3D bioprinting and their application in skin tissue engineering, focusing on the latest research progress in skin appendages (hair follicles and sweat glands) and vascularization, and summarizes current challenges and future development of 3D skin printing.

Biomaterials for Bioprinting Microvasculature

Microvasculature functions at the tissue and cell level, regulating local mass exchange of oxygen and nutrient-rich blood. While there has been considerable success in the biofabrication of large- and small-vessel replacements, functional microvasculature has been particularly challenging to engineer due to its size and complexity. Recently, three-dimensional bioprinting has expanded the possibilities of fabricating sophisticated microvascular systems by enabling precise spatiotemporal placement of cells and biomaterials based on computer-aided design. However, there are still significant challenges facing the development of printable biomaterials that promote robust formation and controlled 3D organization of microvascular networks. This review provides a thorough examination and critical evaluation of contemporary biomaterials and their specific roles in bioprinting microvasculature. We first provide an overview of bioprinting methods and techniques that enable the fabrication of microvessels. We then offer an in-depth critical analysis on the use of hydrogel bioinks for printing microvascularized constructs within the framework of current bioprinting modalities. We end with a review of recent applications of bioprinted microvasculature for disease modeling, drug testing, and tissue engineering, and conclude with an outlook on the challenges facing the evolution of biomaterials design for bioprinting microvasculature with physiological complexity.

Safety Considerations in 3D Bioprinting Using Mesenchymal Stromal Cells

Three-dimensional (3D) bioprinting has demonstrated great potential for the fabrication of biomimetic human tissues and complex graft materials. This technology utilizes bioinks composed of cellular elements placed within a biomaterial. Mesenchymal stromal cells (MSCs) are an attractive option for cell selection in 3D bioprinting. MSCs can be isolated from a variety of tissues, can pose vast proliferative capacity and can differentiate to multiple committed cell types. Despite their promising properties, the use of MSCs has been associated with several drawbacks. These concerns are related to the ex vivo manipulation throughout the process of 3D bioprinting. The herein manuscript aims to present the current evidence surrounding these events and propose ways to minimize the risks to the patients following widespread expansion of 3D bioprinting in the medical field.

Effects of Processing Parameters of 3D Bioprinting on the Cellular Activity of Bioinks

In this review, few established cell printing techniques along with their parameters that affect the cell viability during bioprinting are considered. 3D bioprinting is developed on the principle of additive manufacturing using biomaterial inks and bioinks. Different bioprinting methods impose few challenges on cell printing such as shear stress, mechanical impact, heat, laser radiation, etc., which eventually lead to cell death. These factors also cause alteration of cells phenotype, recoverable or irrecoverable damages to the cells. Such challenges are not addressed in detail in the literature and scientific reports. Hence, this review presents a detailed discussion of several cellular bioprinting methods and their process-related impacts on cell viability, followed by probable mitigation techniques. Most of the printable bioinks encompass cells within hydrogel as scaffold material to avoid the direct exposure of the harsh printing environment on cells. However, the advantages of printing with scaffold-free cellular aggregates over cell-laden hydrogels have emerged very recently. Henceforth, optimal and favorable crosslinking mechanisms providing structural rigidity to the cell-laden printed constructs with ideal cell differentiation and proliferation, are discussed for improved understanding of cell printing methods for the future of organ printing and transplantation.

3D bioprinting and craniofacial regeneration

BACKGROUND

Considering the structural and functional complexity of the craniofacial tissues, 3D bioprinting can be a valuable tool to design and create functional 3D tissues or organs in situ for in vivo applications. This review aims to explore the various aspects of this emerging 3D bioprinting technology and its application in the craniofacial bone or cartilage regeneration.

METHOD

Electronic database searches were undertaken on pubmed, google scholar, medline, embase, and science direct for english language literature, published for 3D bioprinting in craniofacial regeneration. The search items used were 'craniofacial regeneration' OR 'jaw regeneration' OR 'maxillofacial regeneration' AND '3D bioprinting' OR 'three dimensional bioprinting' OR 'Additive manufacturing' OR 'rapid prototyping' OR 'patient specific bioprinting'. Reviews and duplicates were excluded.

RESULTS

Search with above described criteria yielded 476 articles, which reduced to 108 after excluding reviews. Further screening of individual articles led to 77 articles to which 9 additional articles were included from references, and 18 duplicate articles were excluded. Finally we were left with 68 articles to be included in the review.

CONCLUSION

Craniofacial tissue and organ regeneration has been reported a success using bioink with different biomaterial and incorporated stem cells in 3D bioprinters. Though several attempts have been made to fabricate craniofacial bone and cartilage, the strive to achieve desired outcome still continues.

Crosslinking Strategies for 3D Bioprinting of Polymeric Hydrogels

Three-dimensional (3D) bioprinting has recently advanced as an important tool to produce viable constructs that can be used for regenerative purposes or as tissue models. To develop biomimetic and sustainable 3D constructs, several important processing aspects need to be considered, among which crosslinking is most important for achieving desirable biomechanical stability of printed structures, which is reflected in subsequent behavior and use of these constructs. In this work, crosslinking methods used in 3D bioprinting studies are reviewed, parameters that affect bioink chemistry are discussed, and the potential toward improving crosslinking outcomes and construct performance is highlighted. Furthermore, current challenges and future prospects are discussed. Due to the direct connection between crosslinking methods and properties of 3D bioprinted structures, this Review can provide a basis for developing necessary modifications to the design and manufacturing process of advanced tissue-like constructs in future.

Photocurable Biopolymers for Coaxial Bioprinting

Thanks to their unique advantages, additive manufacturing technologies are revolutionizing almost all sectors of the industrial and academic worlds, including tissue engineering and regenerative medicine. In particular, 3D bioprinting is rapidly emerging as a first-choice approach for the fabrication-in one step-of advanced cell-laden hydrogel constructs to be used for in vitro and in vivo studies. This technique consists in the precise deposition layer-by-layer of sub-millimetric hydrogel strands in which living cells are embedded. A key factor of this process consists in the proper formulation of the hydrogel precursor solution, the so-called bioink. Ideal bioinks should be able, on the one side, to support cell growth and differentiation and, on the other, to allow the high-resolution deposition of cell-laden hydrogel strands. The latter feature requires the extruded solution to instantaneously undergo a sol-gel transition to avoid its collapse after deposition.To address this challenge, researchers are recently focusing their attention on the synthesis of several derivatives of natural biopolymers to enhance their printability. Here, we present an approach for the synthesis of photocurable derivatives of natural biopolymers-namely, gelatin methacrylate, hyaluronic acid methacrylate, chondroitin sulfate methacrylate, and PEGylated fibrinogen-that can be used to formulate tailored innovative bioinks for coaxial-based 3D bioprinting applications.

Preliminary engineering for in situ in vivo bioprinting: a novel micro bioprinting platform for in situ in vivo bioprinting at a gastric wound site

We first proposed the concept of in situ in vivo bioprinting in order to address the existing deficiencies in conventional bioprinting. Herein we verified this concept taking the case of the treatment for gastric wall injury and presented this work as a preliminary step towards a new method in the field of bioprinting. In this study, a micro bioprinting platform which can be installed to an endoscope was developed to enter the human body and process bioprinting. Printed circuit micro-electro-mechanical-system techniques were used in the design and fabrication of the platform. Control system with high accuracy was developed and performance tests were carried out to verify the feasibility of the platform. The 2-layer tissue scaffolds were printed in a stomach model. Gelatin-alginate hydrogels with human gastric epithelial cells and human gastric smooth muscle cells were used as bioinks to mimic the anatomical structure of a stomach. A 10 d cell culture showed that printed cells remained a high viability and a steady proliferation, which indicated good biological function of cells in printed tissue scaffolds. This work presents an innovative advance not only in the field of bioprinting but also in the clinical sciences.

Hydrogels for Bioprinting: A Systematic Review of Hydrogels Synthesis, Bioprinting Parameters, and Bioprinted Structures Behavior

Nowadays, bioprinting is rapidly evolving and hydrogels are a key component for its success. In this sense, synthesis of hydrogels, as well as bioprinting process, and cross-linking of bioinks represent different challenges for the scientific community. A set of unified criteria and a common framework are missing, so multidisciplinary research teams might not efficiently share the advances and limitations of bioprinting. Although multiple combinations of materials and proportions have been used for several applications, it is still unclear the relationship between good printability of hydrogels and better medical/clinical behavior of bioprinted structures. For this reason, a PRISMA methodology was conducted in this review. Thus, 1,774 papers were retrieved from PUBMED, WOS, and SCOPUS databases. After selection, 118 papers were analyzed to extract information about materials, hydrogel synthesis, bioprinting process, and tests performed on bioprinted structures. The aim of this systematic review is to analyze materials used and their influence on the bioprinting parameters that ultimately generate tridimensional structures. Furthermore, a comparison of mechanical and cellular behavior of those bioprinted structures is presented. Finally, some conclusions and recommendations are exposed to improve reproducibility and facilitate a fair comparison of results.