Fig. 1. HELP bioinks are engineered, dynamic hydrogels whose printability can be tuned through the addition of small molecules.

(A) Schematic of the engineered HELP bioink, which consists of HA modified with either an ALD or BZA group and an ELP modified with HYD groups. (B) When the HA and ELP components are mixed, they form hydrazone bonds. The printability of these hydrazone-based inks is modulated by introducing two small molecules: a competitor and a catalyst. The competitor binds to free aldehydes present in the bioink, reducing the number of cross-links present in the print syringe. The catalyst increases the rate of bond exchange. Catalyst 1 was used for all acellular experiments and catalyst 2 for all cellular experiments. (C) The two polymer components, the competitor, and the catalyst are mixed with cells to form a bioink. The bioink is loaded into the print cartridge and printed into a gel support bath. Following printing, the small molecule competitor and catalyst can diffuse away from the printed structure into the support bath while the ink remains in place, stiffening and stabilizing the bioink.