Temperature-programmable and enzymatically solidifiable gelatin-based bioinks enable facile extrusion bioprinting

The development of exceptional bioinks with excellent printability, high fidelity, and excellent cell viability maintenance for extrusion bioprinting remains a major challenge. Gelatin is an ideal candidate bioink due to its biocompatibility, biodegradability, and non-immunogenicity. However, its inherently low viscosity and unstable physical gelation under physiological conditions make it unsuitable for direct extrusion bioprinting of tissue-like gelatin constructs with high fidelity. Herein, sequential chemical modification using reversible quadruple-hydrogen-bonded ureido-pyrimidinone (UPy) and enzyme-responsive tyramine moieties (Tyr) were devloped to endow the gelatin with a temperature-programmable viscosity and enzyme-controlled solidification, thus realizing enhanced printability and superior fidelity. As demonstrated in a proof-of-concept study, various cell-laden constructs were built based on our modified gelatin, including two-dimensional human bone marrow mesenchymal stem cell (hBMSC)-laden patterns, three-dimensional interconnected hBMSC-laden scaffolds, a reversible twisting-tension human-scale hBMSC-laden ear, a bicellular tibia-like construct containing hBMSCs and endothelial cells and a hexagonal prism-shaped hepatocyte-laden scaffold. The loaded cells in the construct have high viability of over 90% at 24 h, and show proliferation and protein secretion over one week, suggesting that Gel-UPy-Tyr-based constructs under physiological temperature not only can keep high fidelity, but also can support the growth and functions of the loaded cells.