October_2021_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 2 1 2 4 screw, or pneumatic pressure pushes a material filament through a nozzle and deposits it onto a print surface [15] . In ink- jet bioprinting, a thermal, piezoelectric, or electromagnetic source produces bubbles that collapse and eject drop- lets of materials [16] . In stereolithography (SLA), a 3D structure is formed out of a vat of resin using polymerization from exposure to light from a laser or lamp. A single beam or 2D pattern of ultraviolet or visible light is directed at the resin. This con- trolled light interacts with the dissolved radical-generating photoinitiators and polymerizes the resin in a specific pat- tern. Then, the printing platform is moved to allow unpolymerized resin to flow over the top of the structure, en- abling repetition of the photopolymer- ization process for the next layer [17] . In laser-assisted bioprinting (LAB) or laser-induced forward transfer (LIFT), a laser is pulsed into an energy-absorb- ing material that is attached to a thin sheet of cell-laden ink. The top layer ab- sorbs the energy from the laser pulse to induce a phase change and deforma- tion of the energy-absorbing material. This process results in the ejection of ink onto the print surface [18] . CELL-LADEN GelMA APPLICATIONS Gelatin methacrylate hydrogels have found a number of uses in tissue engineering and regenerative medicine. In neural cells, GelMA has been fabri- cated as nerve guidance conduits using digital light processing to support cell survival, proliferation, and migration [19] . For spinal cord injuries, 3D biomimet- ic GelMA hydrogels with induced neu- ral stem cells (iNSCs) have been shown to promote cell regeneration. In vitro, the iNSCs encapsulated in GelMA (Gel- MA/iNSCs) survived and differentiated well. As shown in Fig. 1, in vivo mouse spinal cord transection models show that GelMA/iNSCs significantly promote functional recovery. The material also decreases inflammation by reducing activated CD68 + cells [20] . In cardiac tissue engineering, scaf- folds and patches have been made based on GelMA hydrogels. For exam- ple, a hydrogel composite of reduced graphene oxide and GelMA (rGO-GelMA) demonstrates greater cell viability, pro- liferation, and maturation versus pure GelMA. This tissue construct can pro- vide a high-fidelity in-vitro model for drug studies and basic cell biology re- search for understanding cardiac tissue development and other processes [21] . Conductive and adhesive cardiopatches have alsobeenmade by electrospinning GelMA and conjugating a choline-based bio-ionic liquid. These cardiopatches can minimize cardiac remodeling and preserve normal heart function by pro- viding mechanical support and restor- ing electromechanical coupling at myo- cardial infarction sites [22] . In skeletal muscles, acellular GelMA hydrogels have been printed and cross- linked in situ into the defect sites of mice with volumetric muscle loss inju- ry. The GelMA scaffold exhibits prop- er adhesion to the surrounding tissue and promotes growth of remnant skele- tal muscle cells [23] . In vascular networks, GelMA hydrogels have been used as an embedding scaffold for bone mar- row-derived mesenchymal stem cells and human blood-derived endothelial colony-forming cells. In vitro, these 3D constructs generate an extensive capil- lary-like network [24] . In bone tissue, a composite hy- drogel containing GelMA and hy- droxyapatite has been prepared into a biomimetic osteon, a type of bone Fig. 1 — Schematic representation of the hydrogel synthesis and animal experiment. A mixed solution of GelMA and iNSCs crosslinked by a photoinitiator under UV irradiation was developed. After generating the complete transection mouse SCI model, the scaffold was transplanted into the injury site. iNSCs = induced neural stem cells; and SCI = spinal cord injury. Courtesy of L. Fan et al. [20]