Hydrogel-based reinforcement of 3D bioprinted constructs

Nanofibrous Material-Reinforced Printable Ink for Enhanced Cell Proliferation and Tissue Regeneration

The three-dimensional (3D) printing of biomaterials, cells, and bioactive components, including growth factors, has gained interest among researchers in the field of tissue engineering (TE) with the aim of developing many scaffolds to sustain size, shape fidelity, and structure and retain viable cells inside a network. The biocompatible hydrogel employed in 3D printing should be soft enough to accommodate cell survival. At the same time, the gel should be mechanically strong to avoid the leakage of cells into the surrounding medium. Considering these basic criteria, researchers have developed nanocomposite-based printable inks with suitable mechanical and electroconductive properties. These nanomaterials, including carbon family nanomaterials, transition metal dichalcogenides, and polymeric nanoparticles, act as nanofillers and dissipate stress across polymeric networks through their electroactive interactions. Nanofiber-reinforced printable ink is one kind of nanocomposite-based ink that comprises dispersed nanofiber components in a hydrogel matrix. In this current review, we compile various TE applications of nanofiber-reinforced printable ink and describe the 3D-printing parameters, classification, and impact of cross-linkage. Furthermore, we discuss the challenges and future perspectives in this field.

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.

Sheet-based extrusion bioprinting: a new multi-material paradigm providing mid-extrusion micropatterning control for microvascular applications

As bioprinting advances into clinical relevance with patient-specific tissue and organ constructs, it must be capable of multi-material fabrication at high resolutions to accurately mimick the complex tissue structures found in the body. One of the most fundamental structures to regenerative medicine is microvasculature. Its continuous hierarchical branching vessel networks bridge surgically manipulatable arteries (∼1-6 mm) to capillary beds (∼10µm). Microvascular perfusion must be established quickly for autologous, allogeneic, or tissue engineered grafts to survive implantation and heal in place. However, traditional syringe-based bioprinting techniques have struggled to produce perfusable constructs with hierarchical branching at the resolution of the arterioles (∼100-10µm) found in microvascular tissues. This study introduces the novel CEVIC bioprinting device (i.e.ContinuouslyExtrudedVariableInternalChanneling), a multi-material technology that breaks the current extrusion-based bioprinting paradigm of pushing cell-laden hydrogels through a nozzle as filaments, instead, in the version explored here, extruding thin, wide cell-laden hydrogel sheets. The CEVIC device adapts the chaotic printing approach to control the width and number of microchannels within the construct as it is extruded (i.e. on-the-fly). Utilizing novel flow valve designs, this strategy can produce continuous gradients varying geometry and materials across the construct and hierarchical branching channels with average widths ranging from 621.5 ± 42.92%µm to 11.67 ± 14.99%µm, respectively, encompassing the resolution range of microvascular vessels. These constructs can also include fugitive/sacrificial ink that vacates to leave demonstrably perfusable channels. In a proof-of-concept experiment, a co-culture of two microvascular cell types, endothelial cells and pericytes, sustained over 90% viability throughout 1 week in microchannels within CEVIC-produced gelatin methacryloyl-sodium alginate hydrogel constructs. These results justify further exploration of generating CEVIC-bioprinted microvasculature, such as pre-culturing and implantation studies.

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.

A New Method for the Production of High-Concentration Collagen Bioinks with Semiautonomic Preparation

It is believed that 3D bioprinting will greatly help the field of tissue engineering and regenerative medicine, as live patient cells are incorporated into the material, which directly creates a 3D structure. Thus, this method has potential in many types of human body tissues. Collagen provides an advantage, as it is the most common extracellular matrix present in all kinds of tissues and is, therefore, very natural for cells and the organism. Hydrogels with highly concentrated collagen make it possible to create 3D structures without additional additives to crosslink the polymer, which could negatively affect cell proliferation and viability. This study established a new method for preparing highly concentrated collagen bioinks, which does not negatively affect cell proliferation and viability. The method is based on two successive neutralizations of the prepared hydrogel using the bicarbonate buffering mechanisms of the 2× enhanced culture medium and pH adjustment by adding NaOH. Collagen hydrogel was used in concentrations of 20 and 30 mg/mL dissolved in acetic acid with a concentration of 0.05 and 0.1 wt.%. The bioink preparation process is automated, including colorimetric pH detection and adjustment. The new method was validated using bioprinting and subsequent cultivation of collagen hydrogels with incorporated stromal cells. After 96 h of cultivation, cell proliferation and viability were not statistically significantly reduced.

Gum Arabic-based three-dimensional printed hydrogel for customizable sensors

Most three-dimensional (3D) printed hydrogel exhibit non-idealized rheological properties in the process of direct ink writing and complicated curing. Therefore, accurate writability and convenient curing for 3D printed hydrogel remain a challenge. In this paper, we developed a typical 3D printed hydrogel which realized direct ink writing (DIW) at temperatures similar to human body. Silicon dioxide (SiO2) and Gum Arabic (GA) formed the Bingham fluid to ensure shape stability. The rapid initiation system of potassium persulfat (KPS) and N,N,N',N' -tetramethylethylenediamine (TMEDA) allowed the 3D printed hydrogel precursor solution to transiently form a hydrophobic conjoined cross-linking network structure of acrylamide (AAM) and lauryl methacrylate (LMA) after printing, resulting in preferable mechanical properties. Hydrogel precursor solution showed better rheological properties with the nature of Bingham fluids, and achieved transient cross-linking at 30 °C for 10 s in the rheological test. A variety of 3D printed hydrogel with individual strain sensing properties are prepared as customizable sensor that could monitor significant strain signals within 0-20 % strain with high sensitivity. Moreover, they were discovered excellent temperature sensitivity over a wide operating range (0-80 °C). The 3D printing hydrogel sensors were expected to have broad application prospects in flexible wearable devices and medical monitoring.

3D printing of chitooligosaccharide-polyethylene glycol diacrylate hydrogel inks for bone tissue regeneration

To date, lack of functional hydrogel inks has limited 3D printing applications in tissue engineering. This study developed a series of photocurable hydrogel inks based on chitooligosaccharide (COS)-polyethylene glycol diacrylate (PEGDA) for extrusion-based 3D printing of bone tissue scaffolds. The scaffolds were prepared by aza-Michael addition of COS and PEGDA followed by photopolymerisation of unreacted PEGDA. The hydrogel inks showed sufficient shear thinning properties required for extrusion 3D printing. The printed scaffolds exhibited excellent shape fidelity and fine microstructure with a resolution of 250 μm. By increasing the COS content, the swelling ratio of the scaffolds decreased, while the compressive strength increased. 3D printed COS-PEGDA scaffolds showed high viability of human bone mesenchymal stem cells in vitro. In addition, scaffolds containing 2 wt% COS showed significantly higher alkaline phosphatase activity, calcium deposition, and bioactivity in simulated body fluid compared to the control (PEGDA). Altogether, 3D printed COS-PEGDA scaffolds represent promising candidates for bone tissue regeneration.

Convergence of melt electrowriting and extrusion-based bioprinting for vascular patterning of a myocardial construct

To progress cardiac tissue engineering strategies closer to the clinic, thicker constructs are required to meet the functional need following a cardiac event. Consequently, pre-vascularization of these constructs needs to be investigated to ensure survival and optimal performance of implantable engineered heart tissue. The aim of this research is to investigate the potential of combining extrusion-based bioprinting (EBB) and melt electrowriting for the fabrication of a myocardial construct with a precisely patterned pre-vascular pathway. Gelatin methacryloyl (GelMA) was investigated as a base hydrogel for the respective myocardial and vascular bioinks with collagen, Matrigel and fibrinogen as interpenetrating polymers to support myocardial functionality. Subsequently, extrusion-based printability and viability were investigated to determine the optimal processing parameters for printing into melt electrowritten meshes. Finally, an anatomically inspired vascular pathway was implemented in a dual EBB set-up into melt electrowritten meshes, creating a patterned pre-vascularized myocardial construct. It was determined that a blend of 5% GelMA and 0.8 mg·ml-1collagen with a low crosslinked density was optimal for myocardial cellular arrangement and alignment within the constructs. For the vascular fraction, the optimized formulation consisted of 5% GelMA, 0.8 mg·ml-1collagen and 1 mg·ml-1fibrinogen with a higher crosslinked density, which led to enhanced vascular cell connectivity. Printability assessment confirmed that the optimized bioinks could effectively fill the microfiber mesh while supporting cell viability (∼70%). Finally, the two bioinks were applied using a dual EBB system for the fabrication of a pre-vascular pathway with the shape of a left anterior descending artery within a myocardial construct, whereby the distinct cell populations could be visualized in their respective patterns up to D14. This research investigated the first step towards developing a thick engineered cardiac tissue construct in which a pre-vascularization pathway is fabricated within a myocardial construct.

Out of Box Thinking to Tangible Science: A Benchmark History of 3D Bio-Printing in Regenerative Medicine and Tissues Engineering

Advancements and developments in the 3D bioprinting have been promising and have met the needs of organ transplantation. Current improvements in tissue engineering constructs have enhanced their applications in regenerative medicines and other medical fields. The synergistic effects of 3D bioprinting have brought technologies such as tissue engineering, microfluidics, integrated tissue organ printing, in vivo bioprinted tissue implants, artificial intelligence and machine learning approaches together. These have greatly impacted interventions in medical fields, such as medical implants, multi-organ-on-chip models, prosthetics, drug testing tissue constructs and much more. This technological leap has offered promising personalized solutions for patients with chronic diseases, and neurodegenerative disorders, and who have been in severe accidents. This review discussed the various standing printing methods, such as inkjet, extrusion, laser-assisted, digital light processing, and stereolithographic 3D bioprinter models, adopted for tissue constructs. Additionally, the properties of natural, synthetic, cell-laden, dECM-based, short peptides, nanocomposite and bioactive bioinks are briefly discussed. Sequels of several tissue-laden constructs such as skin, bone and cartilage, liver, kidney, smooth muscles, cardiac and neural tissues are briefly analyzed. Challenges, future perspectives and the impact of microfluidics in resolving the limitations in the field, along with 3D bioprinting, are discussed. Certainly, a technology gap still exists in the scaling up, industrialization and commercialization of this technology for the benefit of stakeholders.

Advancing Engineered Heart Muscle Tissue Complexity with Hydrogel Composites

A heart attack results in the permanent loss of heart muscle and can lead to heart disease, which kills more than 7 million people worldwide each year. To date, outside of heart transplantation, current clinical treatments cannot regenerate lost heart muscle or restore full function to the damaged heart. There is a critical need to create engineered heart tissues with structural complexity and functional capacity needed to replace damaged heart muscle. The inextricable link between structure and function suggests that hydrogel composites hold tremendous promise as a biomaterial-guided strategy to advance heart muscle tissue engineering. Such composites provide biophysical cues and functionality as a provisional extracellular matrix that hydrogels cannot on their own. This review describes the latest advances in the characterization of these biomaterial systems and using them for heart muscle tissue engineering. The review integrates results across the field to provide new insights on critical features within hydrogel composites and perspectives on the next steps to harnessing these promising biomaterials to faithfully reproduce the complex structure and function of native heart muscle.

Bioink with cartilage-derived extracellular matrix microfibers enables spatial control of vascular capillary formation in bioprinted constructs

Microvasculature is essential for the exchange of gas and nutrient for most tissues in our body. Some tissue structures such as the meniscus presents spatially confined blood vessels adjacent to non-vascularized regions. In biofabrication, mimicking the spatial distribution of such vascular components is paramount, as capillary ingrowth into non-vascularized tissues can lead to tissue matrix alterations and subsequent pathology. Multi-material 3D bioprinting can potentially resolve anisotropic tissue features, although building complex constructs comprising stable vascularized and non-vascularized regions remains a major challenge. Here, we developed endothelial cell(EC)-laden pro- and anti-angiogenic bioinks, supplemented with bioactive matrix-derived microfibers (MFs) that were created from type I collagen sponges (col-1) and cartilage decellularized extracellular matrix (CdECM). EC-driven capillary network formation started two days after bioprinting. Supplementing cartilage-derived MFs to endothelial-cell laden bioinks reduced the total length of neo-microvessels by 29% after 14 days, compared to col-1 MFs-laden bioinks. As a proof of concept, the bioinks were bioprinted into an anatomical meniscus shape with a biomimetic vascularized outer and non-vascularized inner region, using a microgel suspension bath. The constructs were cultured up to 14 days, with in the outer zone the HUVEC-, mural cell-, and col-1 MF-laden pro-angiogenic bioink, and in the inner zone a meniscus progenitor cell (MPC)- and CdECM MF-laden anti-angiogenic bioink, revealing successful spatial confinement of the nascent vascular network only in the outer zone. Further, to co-facilitate both microvessel formation and MPC-derived matrix formation, we formulated cell culture medium conditions with a temporal switch. Overall, this study provides a new strategy that could be applied to develop zonal biomimetic meniscal constructs. Moreover, the use of ECM-derived MFs to promote or inhibit capillary networks opens new possibilities for the biofabrication of tissues with anisotropic microvascular distribution. These have potential for many applications including in vitro models, cancer progression, and testing anti-angiogenic therapies.

Effect of Graphene Addition on Polycaprolactone Scaffolds Fabricated Using Melt-Electrowriting

Melt-electrowriting (MEW) is an emerging method that combines electrospinning and extrusion printing, allowing the fabrication of micron-scale structures suitable for tissue engineering. Compared to other additive fabrication methods, melt-electro written structures can offer more appropriate substrates for cell culture due to filament size and mechanical characteristics of the fabricated scaffolds. In this study, polycaprolactone (PCL)/graphene composites were investigated for fabrication of micron-size scaffolds through MEW. It was demonstrated that the addition of graphene can considerably improve the processability of PCL to fabricate micron-scale scaffolds with enhanced resolution. The tensile strength of the scaffold prepared from PCL/graphene composite (with only 0.5 wt.% graphene) was proved significantly (by more than 270%), better than that of the pristine PCL scaffold. Furthermore, graphene was demonstrated to be a suitable material for tailoring the degradation process to avoid undesirable bulk degradation, rapid mass loss and damage to the internal matrix of the polymer. The findings of this study offer a promising route for the fabrication of high-resolution scaffolds with micron-scale resolution for tissue engineering.

pH Modification of High-Concentrated Collagen Bioinks as a Factor Affecting Cell Viability, Mechanical Properties, and Printability

The 3D bioprinting of cell-incorporated gels is a promising direction in tissue engineering applications. Collagen-based hydrogels, due to their similarity to extracellular matrix tissue, can be a good candidate for bioink and 3D bioprinting applications. However, low hydrogel concentrations of hydrogel (<10 mg/mL) provide insufficient structural support and, in highly concentrated gels, cell proliferation is reduced. In this study, we showed that it is possible to print highly concentrated collagen hydrogels with incorporated cells, where the viability of the cells in the gel remains very good. This can be achieved simply by optimizing the properties of the bioink, particularly the gel composition and pH modification, as well as by optimizing the printing parameters. The bioink composed of porcine collagen hydrogel with a collagen concentration of 20 mg/mL was tested, while the final bioink collagen concentration was 10 mg/mL. This bioink was modified with 0, 5, 9, 13, 17 and 20 μL/mL of 1M NaOH solution, which affected the resulting pH and gelling time. Cylindrical samples based on the given bioink, with the incorporation of porcine adipose-derived stromal cells, were printed with a custom 3D bioprinter. These constructs were cultivated in static conditions for 6 h, and 3 and 5 days. Cell viability and morphology were evaluated. Mechanical properties were evaluated by means of a compression test. Our results showed that optimal composition and the addition of 13 μL NaOH per mL of bioink adjusted the pH of the bioink enough to allow cells to grow and divide. This modification also contributed to a higher elastic modulus, making it possible to print structures up to several millimeters with sufficient mechanical resistance. We optimized the bioprinter parameters for printing low-viscosity bioinks. With this experiment, we showed that a high concentration of collagen gels may not be a limiting factor for cell proliferation.

Bioprinting of Human Liver-Derived Epithelial Organoids for Toxicity Studies

There is a need for long-lived hepatic in vitro models to better predict drug induced liver injury (DILI). Human liver-derived epithelial organoids are a promising cell source for advanced in vitro models. Here, organoid technology is combined with biofabrication techniques, which holds great potential for the design of in vitro models with complex and customizable architectures. Here, porous constructs with human hepatocyte-like cells derived from organoids are generated using extrusion-based printing technology. Cell viability of bioprinted organoids remains stable for up to ten days (88-107% cell viability compared to the day of printing). The expression of hepatic markers, transporters, and phase I enzymes increased compared to undifferentiated controls, and is comparable to non-printed controls. Exposure to acetaminophen, a well-known hepatotoxic compound, decreases cell viability of bioprinted liver organoids to 21-51% (p < 0.05) compared to the start of exposure, and elevated levels of damage marker miR-122 are observed in the culture medium, indicating the potential use of the bioprinted constructs for toxicity testing. In conclusion, human liver-derived epithelial organoids can be combined with a biofabrication approach, thereby paving the way to create perfusable, complex constructs which can be used as toxicology- and disease-models.

A biofabrication method to align cells within bioprinted photocrosslinkable and cell-degradable hydrogel constructs via embedded fibers

The extracellular matrix (ECM) is composed of biochemical and biophysical cues that control cell behaviors and bulk mechanical properties. For example, anisotropy of the ECM and cell alignment are essential in the directional properties of tissues such as myocardium, tendon, and the knee meniscus. Technologies are needed to introduce anisotropic behavior into biomaterial constructs that can be used for the engineering of tissues as models and towards translational therapies. To address this, we developed an approach to align hydrogel fibers within cell-degradable bioink filaments with extrusion printing, where shear stresses during printing align fibers and photocrosslinking stabilizes the fiber orientation. Suspensions of hydrogel fibers were produced through the mechanical fragmentation of electrospun scaffolds of norbornene-modified hyaluronic acid, which were then encapsulated with meniscal fibrochondrocytes, mesenchymal stromal cells, or cardiac fibroblasts within gelatin-methacrylamide bioinks during extrusion printing into agarose suspension baths. Bioprinting parameters such as the needle diameter and the bioink flow rate influenced shear profiles, whereas the suspension bath properties and needle translation speed influenced filament diameters and uniformity. When optimized, filaments were formed with high levels of fiber alignment, which resulted in directional cell spreading during culture over one week. Controls that included bioprinted filaments without fibers or non-printed hydrogels of the same compositions either with or without fibers resulted in random cell spreading during culture. Further, constructs were printed with variable fiber and resulting cell alignment by varying print direction or using multi-material printing with and without fibers. This biofabrication technology advances our ability to fabricate constructs containing aligned cells towards tissue repair and the development of physiological tissue models.

Collagen Bioinks for Bioprinting: A Systematic Review of Hydrogel Properties, Bioprinting Parameters, Protocols, and Bioprinted Structure Characteristics

Bioprinting is a modern tool suitable for creating cell scaffolds and tissue or organ carriers from polymers that mimic tissue properties and create a natural environment for cell development. A wide range of polymers, both natural and synthetic, are used, including extracellular matrix and collagen-based polymers. Bioprinting technologies, based on syringe deposition or laser technologies, are optimal tools for creating precise constructs precisely from the combination of collagen hydrogel and cells. This review describes the different stages of bioprinting, from the extraction of collagen hydrogels and bioink preparation, over the parameters of the printing itself, to the final testing of the constructs. This study mainly focuses on the use of physically crosslinked high-concentrated collagen hydrogels, which represents the optimal way to create a biocompatible 3D construct with sufficient stiffness. The cell viability in these gels is mainly influenced by the composition of the bioink and the parameters of the bioprinting process itself (temperature, pressure, cell density, etc.). In addition, a detailed table is included that lists the bioprinting parameters and composition of custom bioinks from current studies focusing on printing collagen gels without the addition of other polymers. Last but not least, our work also tries to refute the often-mentioned fact that highly concentrated collagen hydrogel is not suitable for 3D bioprinting and cell growth and development.

Calculation of Mass Transfer and Cell-Specific Consumption Rates to Improve Cell Viability in Bioink Tissue Constructs

Biofabrication methods such as extrusion-based bioprinting allow the manufacture of cell-laden structures for cell therapy, but it is important to provide a sufficient number of embedded cells for the replacement of lost functional tissues. To address this issue, we investigated mass transfer rates across a bioink hydrogel for the essential nutrients glucose and glutamine, their metabolites lactate and ammonia, the electron acceptor oxygen, and the model protein bovine serum albumin. Diffusion coefficients were calculated for these substances at two temperatures. We could confirm that diffusion depends on the molecular volume of the substances if the bioink has a high content of polymers. The analysis of pancreatic 1.1B4 β-cells revealed that the nitrogen source glutamine is a limiting nutrient for homeostasis during cultivation. Taking the consumption rates of 1.1B4 β-cells into account during cultivation, we were able to calculate the cell numbers that can be adequately supplied by the cell culture medium and nutrients in the blood using a model tissue construct. For blood-like conditions, a maximum of ~10⁶ cells·mL⁻¹ was suitable for the cell-laden construct, as a function of the diffused substrate and cell consumption rate for a given geometry. We found that oxygen and glutamine were the limiting nutrients in our model.

Pre-shear bioprinting of highly oriented porous hydrogel microfibers to construct anisotropic tissues

Anisotropic tissues in vivo have special structural characteristics and biological functions. Nowadays, bioprinting is widely used in tissue engineering and an effective way to process cell-laden hydrogels. However, the direct bioprinting of oriented cell-laden hydrogel structures to engineer anisotropic tissues is still difficult. Meanwhile, the inherent dense micropore network after the gelation of hydrogel-based bioinks usually limits the normal growth of encapsulated cells due to the inadequate supply of nutrient/oxygen. Herein, we proposed a pre-shear bioprinting strategy of highly oriented porous hydrogel microfibers to construct anisotropic tissues. Firstly, based on the phase separation of viscous high-molecular compound mixtures, we utilized a general viscous porous bioink paradigm, e.g., mixing a polymer thickener (PEO) with a hydrogel precursor (GelMA) with excellent biological properties. Secondly, based on the shear-oriented property of the viscous porous bioink, we designed the pre-shear in situ coaxial bioprinting of highly oriented porous hydrogel microfibers. The viscous porous bioink (GelMA/PEO) was shear-oriented through an injection tube and pumped into the inner needle of a coaxial nozzle. When GelMA/PEO passed through a transparent glass tube connected to the coaxial nozzle, GelMA can be in situ photo-crosslinked to form highly oriented porous microfibers. In addition, we showed the manufacturing of heterogeneous oriented microfibers and the manual assembly of microfibers, and within oriented microfibers, different cells or co-cultured cells exhibited highly oriented growth behaviors similar to that in vivo. As far as we know, the direct bioprinting of anisotropic tissues through high orientation induced by pre-shearing is firstly reported in our study. We believe that the pre-shear bioprinting strategy of anisotropic tissues will open more avenues for further biomedical research.

Biofabrication of a shape-stable auricular structure for the reconstruction of ear deformities

Bioengineering of the human auricle remains a significant challenge, where the complex and unique shape, the generation of high-quality neocartilage, and shape preservation are key factors. Future regenerative medicine–based approaches for auricular cartilage reconstruction will benefit from a smart combination of various strategies. Our approach to fabrication of an ear-shaped construct uses hybrid bioprinting techniques, a recently identified progenitor cell population, previously validated biomaterials, and a smart scaffold design. Specifically, we generated a 3D-printed polycaprolactone (PCL) scaffold via fused deposition modeling, photocrosslinked a human auricular cartilage progenitor cell–laden gelatin methacryloyl (gelMA) hydrogel within the scaffold, and cultured the bioengineered structure in vitro in chondrogenic media for 30 days. Our results show that the fabrication process maintains the viability and chondrogenic phenotype of the cells, that the compressive properties of the combined PCL and gelMA hybrid auricular constructs are similar to native auricular cartilage, and that biofabricated hybrid auricular structures exhibit excellent shape fidelity compared with the 3D digital model along with deposition of cartilage-like matrix in both peripheral and central areas of the auricular structure. Our strategy affords an anatomically enhanced auricular structure with appropriate mechanical properties, ensures adequate preservation of the auricular shape during a dynamic in vitro culture period, and enables chondrogenically potent progenitor cells to produce abundant cartilage-like matrix throughout the auricular construct. The combination of smart scaffold design with 3D bioprinting and cartilage progenitor cells holds promise for the development of clinically translatable regenerative medicine strategies for auricular reconstruction.

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First application of human auricular cartilage progenitor cells for bioprinting.

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Dual-printing of hybrid ear-shaped constructs with excellent shape fidelity over time.

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Strategy and design ensured adequate deposition of cartilage-like matrix throughout large auricular constructs.

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.

Hybrid Printing Using Cellulose Nanocrystals Reinforced GelMA/HAMA Hydrogels for Improved Structural Integration

3D printing of soft-tissue like cytocompatible single material constructs with appropriate mechanical properties remains a challenge. Hybrid printing technology provides an attractive alternative as it combines a cell-free ink for providing mechanical support with a bioink for housing embedded cells. Several hybrid printed structures have been developed, utilizing thermoplastic polymers such as polycaprolactone as structural support. These thermoplastics demonstrated limited structural integration with the cell-laden components, and this may compromise the overall performance. In this work, a hybrid printing platform is presented using two distinct hydrogel inks that share the same photo-crosslinking chemistry to enable simple fabrication and seamless structural integration. A mechanically reinforced hydrogel ink is developed comprising cellulose nanocrystals and gelatin methacryloyl/hyaluronic acid methacrylate (GelMA/HAMA) as the structural component, and GelMA/HAMA as the cytogel containing a mouse chondrogenic cell line, ATDC5. Hybrid printed constructs with encapsulated cells are fabricated using the two optimized inks, and the structural integration of the constructs is evaluated by cyclic mechanical compression. Finally, the cell viability of encapsulated ATDC5 cells in the hybrid printed structures is evaluated.

Printability and Shape Fidelity of Bioinks in 3D Bioprinting

Three-dimensional bioprinting uses additive manufacturing techniques for the automated fabrication of hierarchically organized living constructs. The building blocks are often hydrogel-based bioinks, which need to be printed into structures with high shape fidelity to the intended computer-aided design. For optimal cell performance, relatively soft and printable inks are preferred, although these undergo significant deformation during the printing process, which may impair shape fidelity. While the concept of good or poor printability seems rather intuitive, its quantitative definition lacks consensus and depends on multiple rheological and chemical parameters of the ink. This review discusses qualitative and quantitative methodologies to evaluate printability of bioinks for extrusion- and lithography-based bioprinting. The physicochemical parameters influencing shape fidelity are discussed, together with their importance in establishing new models, predictive tools and printing methods that are deemed instrumental for the design of next-generation bioinks, and for reproducible comparison of their structural performance.

Emulating Human Tissues and Organs: A Bioprinting Perspective Toward Personalized Medicine

The lack of in vitro tissue and organ models capable of mimicking human physiology severely hinders the development and clinical translation of therapies and drugs with higher in vivo efficacy. Bioprinting allow us to fill this gap and generate 3D tissue analogues with complex functional and structural organization through the precise spatial positioning of multiple materials and cells. In this review, we report the latest developments in terms of bioprinting technologies for the manufacturing of cellular constructs with particular emphasis on material extrusion, jetting, and vat photopolymerization. We then describe the different base polymers employed in the formulation of bioinks for bioprinting and examine the strategies used to tailor their properties according to both processability and tissue maturation requirements. By relating function to organization in human development, we examine the potential of pluripotent stem cells in the context of bioprinting toward a new generation of tissue models for personalized medicine. We also highlight the most relevant attempts to engineer artificial models for the study of human organogenesis, disease, and drug screening. Finally, we discuss the most pressing challenges, opportunities, and future prospects in the field of bioprinting for tissue engineering (TE) and regenerative medicine (RM).

A Versatile Open-Source Printhead for Low-Cost 3D Microextrusion-Based Bioprinting

Three-dimensional (3D) bioprinting promises to be essential in tissue engineering for solving the rising demand for organs and tissues. Some bioprinters are commercially available, but their impact on the field of Tissue engineering (TE) is still limited due to their cost or difficulty to tune. Herein, we present a low-cost easy-to-build printhead for microextrusion-based bioprinting (MEBB) that can be installed in many desktop 3D printers to transform them into 3D bioprinters. We can extrude bioinks with precise control of print temperature between 2-60 °C. We validated the versatility of the printhead, by assembling it in three low-cost open-source desktop 3D printers. Multiple units of the printhead can also be easily put together in a single printer carriage for building a multi-material 3D bioprinter. Print resolution was evaluated by creating representative calibration models at different temperatures using natural hydrogels such as gelatin and alginate, and synthetic ones like poloxamer. Using one of the three modified low-cost 3D printers, we successfully printed cell-laden lattice constructs with cell viabilities higher than 90% after 24-h post printing. Controlling temperature and pressure according to the rheological properties of the bioinks was essential in achieving optimal printability and great cell viability. The cost per unit of our device, which can be used with syringes of different volume, is less expensive than any other commercially available product. These data demonstrate an affordable open-source printhead with the potential to become a reliable alternative to commercial bioprinters for any laboratory.

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.

Advances in Extrusion 3D Bioprinting: A Focus on Multicomponent Hydrogel-Based Bioinks

3D bioprinting involves the combination of 3D printing technologies with cells, growth factors and biomaterials, and has been considered as one of the most advanced tools for tissue engineering and regenerative medicine (TERM). However, despite multiple breakthroughs, it is evident that numerous challenges need to be overcome before 3D bioprinting will eventually become a clinical solution for a variety of TERM applications. To produce a 3D structure that is biologically functional, cell-laden bioinks must be optimized to meet certain key characteristics including rheological properties, physico-mechanical properties, and biofunctionality; a difficult task for a single component bioink especially for extrusion based bioprinting. As such, more recent research has been centred on multicomponent bioinks consisting of a combination of two or more biomaterials to improve printability, shape fidelity and biofunctionality. In this article, multicomponent hydrogel-based bioink systems are systemically reviewed based on the inherent nature of the bioink (natural or synthetic hydrogels), including the most current examples demonstrating properties and advances in application of multicomponent bioinks, specifically for extrusion based 3D bioprinting. This review article will assist researchers in the field in identifying the most suitable bioink based on their requirements, as well as pinpointing current unmet challenges in the field.

Orthotopic Bone Regeneration within 3D Printed Bioceramic Scaffolds with Region-Dependent Porosity Gradients in an Equine Model

The clinical translation of three-dimensionally printed bioceramic scaffolds with tailored architectures holds great promise toward the regeneration of bone to heal critical-size defects. Herein, the long-term in vivo performance of printed hydrogel-ceramic composites made of methacrylated-oligocaprolactone-poloxamer and low-temperature self-setting calcium-phosphates is assessed in a large animal model. Scaffolds printed with different internal architectures, displaying either a designed porosity gradient or a constant pore distribution, are implanted in equine tuber coxae critical size defects. Bone ingrowth is challenged and facilitated only from one direction via encasing the bioceramic in a polycaprolactone shell. After 7 months, total new bone volume and scaffold degradation are significantly greater in structures with constant porosity. Interestingly, gradient scaffolds show lower extent of remodeling and regeneration even in areas having the same porosity as the constant scaffolds. Low regeneration in distal regions from the interface with native bone impairs ossification in proximal regions of the construct, suggesting that anisotropic architectures modulate the cross-talk between distant cells within critical-size defects. The study provides key information on how engineered architectural patterns impact osteoregeneration in vivo, and also indicates the equine tuber coxae as promising orthotopic model for studying materials stimulating bone formation.

From Shape to Function: The Next Step in Bioprinting

In 2013, the "biofabrication window" was introduced to reflect the processing challenge for the fields of biofabrication and bioprinting. At that time, the lack of printable materials that could serve as cell-laden bioinks, as well as the limitations of printing and assembly methods, presented a major constraint. However, recent developments have now resulted in the availability of a plethora of bioinks, new printing approaches, and the technological advancement of established techniques. Nevertheless, it remains largely unknown which materials and technical parameters are essential for the fabrication of intrinsically hierarchical cell-material constructs that truly mimic biologically functional tissue. In order to achieve this, it is urged that the field now shift its focus from materials and technologies toward the biological development of the resulting constructs. Therefore, herein, the recent material and technological advances since the introduction of the biofabrication window are briefly summarized, i.e., approaches how to generate shape, to then focus the discussion on how to acquire the biological function within this context. In particular, a vision of how biological function can evolve from the possibility to determine shape is outlined.

Combining multi-scale 3D printing technologies to engineer reinforced hydrogel-ceramic interfaces

Multi-material 3D printing technologies that resolve features at different lengths down to the microscale open new avenues for regenerative medicine, particularly in the engineering of tissue interfaces. Herein, extrusion printing of a bone-biomimetic ceramic ink and melt electrowriting (MEW) of spatially organized polymeric microfibres are integrated for the biofabrication of an osteochondral plug, with a mechanically reinforced bone-to-cartilage interface. A printable physiological temperature-setting bioceramic, based on α-tricalcium phosphate, nanohydroxyapatite and a custom-synthesized biodegradable and crosslinkable poloxamer, was developed as bone support. The mild setting reaction of the bone ink enabled us to print directly within melt electrowritten polycaprolactone meshes, preserving their micro-architecture. Ceramic-integrated MEW meshes protruded into the cartilage region of the composite plug, and were embedded with mechanically soft gelatin-based hydrogels, laden with articular cartilage chondroprogenitor cells. Such interlocking design enhanced the hydrogel-to-ceramic adhesion strength >6.5-fold, compared with non-interlocking fibre architectures, enabling structural stability during handling and surgical implantation in osteochondral defects ex vivo. Furthermore, the MEW meshes endowed the chondral compartment with compressive properties approaching those of native cartilage (20-fold reinforcement versus pristine hydrogel). The osteal and chondral compartment supported osteogenesis and cartilage matrix deposition in vitro, and the neo-synthesized cartilage matrix further contributed to the mechanical reinforcement at the ceramic-hydrogel interface. This multi-material, multi-scale 3D printing approach provides a promising strategy for engineering advanced composite constructs for the regeneration of musculoskeletal and connective tissue interfaces.

Hydrogel Bioink Reinforcement for Additive Manufacturing: A Focused Review of Emerging Strategies

Bioprinting is an emerging approach for fabricating cell-laden 3D scaffolds via robotic deposition of cells and biomaterials into custom shapes and patterns to replicate complex tissue architectures. Bioprinting uses hydrogel solutions called bioinks as both cell carriers and structural components, requiring bioinks to be highly printable while providing a robust and cell-friendly microenvironment. Unfortunately, conventional hydrogel bioinks have not been able to meet these requirements and are mechanically weak due to their heterogeneously crosslinked networks and lack of energy dissipation mechanisms. Advanced bioink designs using various methods of dissipating mechanical energy are aimed at developing next-generation cellularized 3D scaffolds to mimic anatomical size, tissue architecture, and tissue-specific functions. These next-generation bioinks need to have high print fidelity and should provide a biocompatible microenvironment along with improved mechanical properties. To design these advanced bioink formulations, it is important to understand the structure-property-function relationships of hydrogel networks. By specifically leveraging biophysical and biochemical characteristics of hydrogel networks, high performance bioinks can be designed to control and direct cell functions. In this review article, current and emerging approaches in hydrogel design and bioink reinforcement techniques are critically evaluated. This bottom-up perspective provides a materials-centric approach to bioink design for 3D bioprinting.

Volumetric Bioprinting of Complex Living-Tissue Constructs within Seconds

Biofabrication technologies, including stereolithography and extrusion-based printing, are revolutionizing the creation of complex engineered tissues. The current paradigm in bioprinting relies on the additive layer-by-layer deposition and assembly of repetitive building blocks, typically cell-laden hydrogel fibers or voxels, single cells, or cellular aggregates. The scalability of these additive manufacturing technologies is limited by their printing velocity, as lengthy biofabrication processes impair cell functionality. Overcoming such limitations, the volumetric bioprinting of clinically relevant sized, anatomically shaped constructs, in a time frame ranging from seconds to tens of seconds is described. An optical-tomography-inspired printing approach, based on visible light projection, is developed to generate cell-laden tissue constructs with high viability (>85%) from gelatin-based photoresponsive hydrogels. Free-form architectures, difficult to reproduce with conventional printing, are obtained, including anatomically correct trabecular bone models with embedded angiogenic sprouts and meniscal grafts. The latter undergoes maturation in vitro as the bioprinted chondroprogenitor cells synthesize neo-fibrocartilage matrix. Moreover, free-floating structures are generated, as demonstrated by printing functional hydrogel-based ball-and-cage fluidic valves. Volumetric bioprinting permits the creation of geometrically complex, centimeter-scale constructs at an unprecedented printing velocity, opening new avenues for upscaling the production of hydrogel-based constructs and for their application in tissue engineering, regenerative medicine, and soft robotics.

3D Plotted Biphasic Bone Scaffolds for Growth Factor Delivery: Biological Characterization In Vitro and In Vivo

Bioprinting enables the integration of biological components into scaffolds during fabrication that has the advantage of high loading efficiency and better control of release and/or spatial positioning. In this study, a biphasic scaffold fabricated by extrusion-based 3D multichannel plotting of a calcium phosphate cement (CPC) paste and an alginate/gellan gum (AlgGG) hydrogel paste laden with the angiogenic factor VEGF (vascular endothelial growth factor) is investigated with regard to biological response in vitro and in vivo. Rat mesenchymal stromal cells are able to adhere and grow on both CPC and AlgGG strands, and differentiate toward osteoblasts. A sustained VEGF release is observed, which is able to stimulate endothelial cell proliferation as well as angiogenesis in vitro that indicates maintenance of its biological activity. After implantation into a segmental bone defect in the femur diaphysis of rats, a clear reduction of the defect size by newly formed bone tissue occurs from the distal and proximal ends of the host bone within 12 weeks. The CPC component shows excellent osteoconductivity whereas the local VEGF release from the AlgGG hydrogel gives rise to an enhanced vascularization of the defect region. This work contributes to the development of novel therapeutic concepts for improved bone regeneration which are based on 3D bioprinting.

3D bioactive composite scaffolds for bone tissue engineering

Bone is the second most commonly transplanted tissue worldwide, with over four million operations using bone grafts or bone substitute materials annually to treat bone defects. However, significant limitations affect current treatment options and clinical demand for bone grafts continues to rise due to conditions such as trauma, cancer, infection and arthritis. Developing bioactive three-dimensional (3D) scaffolds to support bone regeneration has therefore become a key area of focus within bone tissue engineering (BTE). A variety of materials and manufacturing methods including 3D printing have been used to create novel alternatives to traditional bone grafts. However, individual groups of materials including polymers, ceramics and hydrogels have been unable to fully replicate the properties of bone when used alone. Favourable material properties can be combined and bioactivity improved when groups of materials are used together in composite 3D scaffolds. This review will therefore consider the ideal properties of bioactive composite 3D scaffolds and examine recent use of polymers, hydrogels, metals, ceramics and bio-glasses in BTE. Scaffold fabrication methodology, mechanical performance, biocompatibility, bioactivity, and potential clinical translations will be discussed.

The quest for mechanically and biologically functional soft biomaterials via soft network composites

Developing multifunctional soft biomaterials capable of addressing all the requirements of the complex tissue regeneration process is a multifaceted problem. In order to tackle the current challenges, recent research efforts are increasingly being directed towards biomimetic design concepts that can be translated into soft biomaterials via advanced manufacturing technologies. Among those, soft network composites consisting of a continuous hydrogel matrix and a reinforcing fibrous network closely resemble native soft biological materials in terms of design and composition as well as physicochemical properties. This article reviews soft network composite systems with a particular emphasis on the design, biomaterial and fabrication aspects within the context of soft tissue engineering and drug delivery applications.

[Regeneration - A New Therapeutic Dimension in Otorhinolaryngology]

Regeneration as a therapeutic priniciple and regenerative medicine in general are promising new strategies to add new therapeutic dimensions to our current treatment options. Today, reconstructive surgery, drugs and implants such as the cochlear implant can replace the functions of damaged tissues. In contrast, regenerative therapies aim at the replacement of the damaged tissues themselves while at the same time replacing their lost tissue function. In this review article new technologies such as 3D-bioprinting and the application of decellularised tissues as biomaterials are introduced and explained. A summary of current preclinical and clinical regenerative studies in otorhinolaryngology is complementing these basic aspects.

Osteogenic and angiogenic potentials of the cell-laden hydrogel/mussel-inspired calcium silicate complex hierarchical porous scaffold fabricated by 3D bioprinting

3D printing has been popularly used in the bone tissue engineering, as many of the biomaterials for this field of study can be prepared for and produced from this additive manufacturing technique. In this study, we strategized a solvent-free processing to fabricate the polydopamine-modified calcium silicate (PDACS)/poly-caprolactone (PCL) scaffold with Wharton's jelly mesenchymal stem cells (WJMSCs) incorporated with human umbilical vein endothelial cells (HUVEC)-laden hydrogel. The PDACS/PCL/hydrogel 3D scaffold yielded a Young's modulus of the 3D scaffolds as high as 75 MPa. In addition, the vascular morphogenesis and cellular behaviors regulated by our hybrid scaffolds were also intricately evaluated. Furthermore, the HUVEC in the bioink exhibited higher levels of angiogenic biomarkers and showed potential for the formation of complex vascular networks. Higher levels of bone formation proteins were also observed in our composites. Such a hybrid of synthetic materials with cell constituents not only enhances osteogenesis but also stimulates vessel network development in angiogenesis, presenting the fact that 3D printing can be further applied in improving bone tissue regeneration in numerous aspects. We believe that this method may serve as a useful and effective approach for the regeneration of defective complex hard tissues in deep bone structures.

Multi-length scale bioprinting towards simulating microenvironmental cues

It is envisaged that the creation of cellular environments at multiple length scales, that recapitulate in vivo bioactive and structural roles, may hold the key to creating functional, complex tissues in the laboratory. This review considers recent advances in biofabrication and bioprinting techniques across different length scales. Particular focus is placed on 3D printing of hydrogels and fabrication of biomaterial fibres that could extend the feature resolution and material functionality of soft tissue constructs. The outlook from this review discusses how one might create and simulate microenvironmental cues in vitro. A fabrication platform that integrates the competencies of different biofabrication technologies is proposed. Such a multi-process, multiscale fabrication strategy may ultimately translate engineering capability into an accessible life sciences toolkit, fulfilling its potential to deliver in vitro disease models and engineered tissue implants.

3D Printing of Thermoresponsive Polyisocyanide (PIC) Hydrogels as Bioink and Fugitive Material for Tissue Engineering

Despite the rapid and great developments in the field of 3D hydrogel printing, a major ongoing challenge is represented by the development of new processable materials that can be effectively used for bioink formulation. In this work, we present an approach to 3D deposit, a new class of fully-synthetic, biocompatible PolyIsoCyanide (PIC) hydrogels that exhibit a reverse gelation temperature close to physiological conditions (37 °C). Being fully-synthetic, PIC hydrogels are particularly attractive for tissue engineering, as their properties—such as hydrogel stiffness, polymer solubility, and gelation kinetics—can be precisely tailored according to process requirements. Here, for the first time, we demonstrate the feasibility of both 3D printing PIC hydrogels and of creating dual PIC-Gelatin MethAcrylate (GelMA) hydrogel systems. Furthermore, we propose the use of PIC as fugitive hydrogel to template structures within GelMA hydrogels. The presented approach represents a robust and valid alternative to other commercial thermosensitive systems—such as those based on Pluronic F127—for the fabrication of 3D hydrogels through additive manufacturing technologies to be used as advanced platforms in tissue engineering.

FABRICA: A Bioreactor Platform for Printing, Perfusing, Observing, & Stimulating 3D Tissues

We are introducing the FABRICA, a bioprinter-agnostic 3D-printed bioreactor platform designed for 3D-bioprinted tissue construct culture, perfusion, observation, and analysis. The computer-designed FABRICA was 3D-printed with biocompatible material and used for two studies: (1) Flow Profile Study: perfused 5 different media through a synthetic 3D-bioprinted construct and ultrasonically analyzed the flow profile at increasing volumetric flow rates (VFR); (2) Construct Perfusion Study: perfused a 3D-bioprinted tissue construct for a week and compared histologically with a non-perfused control. For the flow profile study, construct VFR increased with increasing pump VFR. Water and other media increased VFR significantly while human and pig blood showed shallow increases. For the construct perfusion study, we confirmed more viable cells in perfused 3D-bioprinted tissue compared to control. The FABRICA can be used to visualize constructs during 3D-bioprinting, incubation, and to control and ultrasonically analyze perfusion, aseptically in real-time, making the FABRICA tunable for different tissues.

Silk-Based Bioinks for 3D Bioprinting

3D bioprinting field is making remarkable progress; however, the development of critical sized engineered tissue construct is still a farfetched goal. Silk fibroin offers a promising choice for bioink material. Nature has imparted several unique structural features in silk protein to ensure spinnability by silkworms or spider. Researchers have modified the structure-property relationship by reverse engineering to further improve shear thinning behavior, high printability, cytocompatible gelation, and high structural fidelity. In this review, it is attempted to summarize the recent advancements made in the field of 3D bioprinting in context of two major sources of silk fibroin: silkworm silk and spider silk (native and recombinant). The challenges faced by current approaches in processing silk bioinks, cellular signaling pathways modulated by silk chemistry and secondary conformations, gaps in knowledge, and future directions acquired for pushing the field further toward clinic are further elaborated.

Nanoengineered Ionic-Covalent Entanglement (NICE) Bioinks for 3D Bioprinting

We introduce an enhanced nanoengineered ionic-covalent entanglement (NICE) bioink for the fabrication of mechanically stiff and elastomeric 3D biostructures. NICE bioink formulations combine nanocomposite and ionic-covalent entanglement (ICE) strengthening mechanisms to print customizable cell-laden constructs for tissue engineering with high structural fidelity and mechanical stiffness. Nanocomposite and ICE strengthening mechanisms complement each other through synergistic interactions, improving mechanical strength, elasticity, toughness, and flow properties beyond the sum of the effects of either reinforcement technique alone. Herschel-Bulkley flow behavior shields encapsulated cells from excessive shear stresses during extrusion. The encapsulated cells readily proliferate and maintain high cell viability over 120 days within the 3D-printed structure, which is vital for long-term tissue regeneration. A unique aspect of the NICE bioink is its ability to print much taller structures, with higher aspect ratios, than can be achieved with conventional bioinks without requiring secondary supports. We envision that NICE bioinks can be used to bioprint complex, large-scale, cell-laden constructs for tissue engineering with high structural fidelity and mechanical stiffness for applications in custom bioprinted scaffolds and tissue engineered implants.

Assessing bioink shape fidelity to aid material development in 3D bioprinting

During extrusion-based bioprinting, the deposited bioink filaments are subjected to deformations, such as collapse of overhanging filaments, which compromises the ability to stack several layers of bioink, and fusion between adjacent filaments, which compromises the resolution and maintenance of a desired pore structure. When developing new bioinks, approaches to assess their shape fidelity after printing would be beneficial to evaluate the degree of deformation of the deposited filament and to estimate how similar the final printed construct would be to the design. However, shape fidelity has been prevalently assessed qualitatively through visual inspection after printing, hampering the direct comparison of the printability of different bioinks. In this technical note, we propose a quantitative evaluation for shape fidelity of bioinks based on testing the filament collapse on overhanging structures and the filament fusion of parallel printed strands. Both tests were applied on a hydrogel platform based on poloxamer 407 and poly(ethylene glycol) blends, providing a library of hydrogels with different yield stresses. The presented approach is an easy way to assess bioink shape fidelity, applicable to any filament-based bioprinting system and able to quantitatively evaluate this aspect of printability, based on the degree of deformation of the printed filament. In addition, we built a simple theoretical model that relates filament collapse with bioink yield stress. The results of both shape fidelity tests underline the role of yield stress as one of the parameters influencing the printability of a bioink. The presented quantitative evaluation will allow for reproducible comparisons between different bioink platforms.