Three-Dimensional Bioprinting of Decellularized Extracellular Matrix-Based Bioinks for Tissue Engineering

Three-dimensional (3D) bioprinting is one of the most promising additive manufacturing technologies for fabricating various biomimetic architectures of tissues and organs. In this context, the bioink, a critical element for biofabrication, is a mixture of biomaterials and living cells used in 3D printing to create cell-laden structures. Recently, decellularized extracellular matrix (dECM)-based bioinks derived from natural tissues have garnered enormous attention from researchers due to their unique and complex biochemical properties. This review initially presents the details of the natural ECM and its role in cell growth and metabolism. Further, we briefly emphasize the commonly used decellularization treatment procedures and subsequent evaluations for the quality control of the dECM. In addition, we summarize some of the common bioink preparation strategies, the 3D bioprinting approaches, and the applicability of 3D-printed dECM bioinks to tissue engineering. Finally, we present some of the challenges in this field and the prospects for future development.     

A universal self‐eroding sacrificial bioink that enables bioprinting at room temperature

Natural polymer‐based hydrogel bioinks are widely used in bioprinting due to their suitability for recapitulation of in vivo cellular activities. However, preservation of the target geometry in a cell‐laden hydrogel is difficult to achieve. The aim of this study was to develop a universal sacrificial bioink that allows high cell viability and a better shape fidelity in the cell‐laden construct. A polysaccharide‐based universal sacrificial bioink was developed for microextrusion‐based bioprinting and was optimized to erode in 48 hours in the cell culture medium without formation of any undesired by‐products. The sacrificial hydrogel was prepared from alginate and agarose via a microwave oven assisted method and bioprinted at room temperature to generate microchannels in the cell‐laden hydrogel or to support a tubular structure and its biocompatibility determined by live/dead assay. Bioprinting time was significantly reduced, down to a few minutes for a large‐scale tissue model (1 minute 52 seconds for a 2 cm tubular structure), by means of a high bioprinting speed up to 25 mm/s. After 48 hours in the cell culture, the sacrificial bioink completely detached from the cell‐laden construct without causing any changes in its printed shape. Cell viability in the cell‐laden construct was observed to be more than 95% at the end of 3‐day culture. This novel sacrificial bioink enables bioprinting at room temperature without affecting oxygen and nutrient penetration into the cell‐laden hydrogel and allows retention of high cell viability and shape fidelity.     

A General Strategy for Extrusion Bioprinting of Bio‐Macromolecular Bioinks through Alginate‐Templated Dual‐Stage Crosslinking

The recently developed 3D bioprinting technology has greatly improved the ability to generate biomimetic tissues that are structurally and functionally relevant to their human counterparts. The selection of proper biomaterials as the bioinks is a key step toward successful bioprinting. For example, viscosity of a bioink is an important rheological parameter to determine the flexibility in deposition of free‐standing structures and the maintenance of architectural integrity following bioprinting. This requirement, however, has greatly limited the selection of bioinks, especially for those naturally derived due to their commonly low mechanical properties. Here the generalization of a mechanism for extrusion bioprinting of bio‐macromolecular components, mainly focusing on collagen and its derivatives including gelatin and gelatin methacryloyl, is reported. Specifically, a templating strategy is adopted using a composite bioink containing both the desired bio‐macromolecular component and a polysaccharide alginate. The physically crosslinkable alginate component serves as the temporal structural support to stabilize the shape of the construct during bioprinting; upon subsequent chemical or physical crosslinking of the bio‐macromolecular component, alginate can be selectively removed to leave only the desired bio‐macromolecule. It is anticipated that this strategy is general, and can be readily expanded for use of a wide variety of other bio‐macromolecular bioinks.     

Designing Gelatin Methacryloyl (GelMA)‐Based Bioinks for Visible Light Stereolithographic 3D Biofabrication

Bioinks play a key role in determining the capability of the biofabricatoin processes and the resolution of the printed constructs. Excellent biocompatibility, tunable physical properties, and ease of chemical or biological modifications of gelatin methacryloyl (GelMA) have made it an attractive choice as bioinks for biomanufacturing of various tissues or organs. However, the current preparation methods for GelMA‐based bioinks lack the ability to tailor their physical properties for desired bioprinting methods. Inherently, GelMA prepolymer solution exhibits a fast sol–gel transition at room temperature, which is a hurdle for its use in stereolithography (SLA) bioprinting. Here, synthesis parameters are optimized such as solvents, pH, and reaction time to develop GelMA bioinks which have a slow sol–gel transition at room temperature and visible light crosslinkable functions. A total of eight GelMA combinations are identified as suitable for digital light processing (DLP)‐based SLA (DLP‐SLA) bioprinting through systematic characterizations of their physical and rheological properties. Out of various types of GelMA, those synthesized in reverse osmosis (RO) purified water (referred to as RO‐GelMA) are regarded as most suitable to achieve high DLP‐SLA printing resolution. RO‐GelMA‐based bioinks are also found to be biocompatible showing high survival rates of encapsulated cells in the photocrosslinked gels. Additionally, the astrocytes and fibroblasts are observed to grow and integrate well within the bioprinted constructs. The bioink's superior physical and photocrosslinking properties offer pathways of tuning the scaffold microenvironment and highlight the applicability of developed GelMA bioinks in various tissue engineering and regenerative medicine applications.