Novel thiol-norbornene gelatin–elastin bioink with porogen-enhanced porosity for microvascular self-organization in bioprinted tissues

Bottom-up tissue engineering has gained significant interest for its ability to recreate the complexity of human organs by assembling functional tissue units through techniques such as extrusion-based bioprinting (EBB). To enable the future biofabrication of human-scale organs, new bioinks for EBB must be developed that facilitate the formation of a functional vascular network within the biomaterial. Without a vascular system, high cell densities within the construct struggle to survive due to the diffusion limits of oxygen and nutrients. Additionally, the bioink must exhibit sufficient printability to accurately recreate the 3D CAD model. In the current work, elastin is modified with norbornene groups to enable step-growth polymerization with thiolated gelatin, resulting in a novel hybrid biomaterial. Unmodified gelatin and porogens are incorporated into the elastin-gelatin hydrogel to enhance printability in EBB and increase porosity, respectively. When only unmodified gelatin is added to the elastin-gelatin hydrogel, shape fidelity on a continuous platform is excellent, and the bioink successfully bridges gaps up to 8 mm with a 100% success rate. Upon addition of alginate gel porogen (AGP), quality of printing on a continuous platform is maintained, but the gap-bridging capability becomes limited to gaps smaller than 4 mm. Nonetheless, the elastin-gelatin hydrogel supplemented with both unmodified gelatin and AGP is preferred, as it promotes superior vascular development compared to a wide range of other bioinks, with vasculogenesis-driven self-assembly of embedded endothelial cells reaching a total vascular network length of 26 ± 6 mm mm^-3and angiogenic sprouting from vascularized spheroids reaching a total sprout length of 4 ± 2 mm within the hydrogel by day 7. A bioink that supports this level of vascular development while maintaining sufficient printability represents a valuable addition to the toolkit for bottom-up tissue engineering using EBB.