Engineering a Hydrazone and Triazole Crosslinked Hydrogel for Extrusion-Based Printing and Cell Delivery.

Covalent adaptable crosslinks, such as the alkyl-hydrazone, endow hydrogels with unique viscoelastic properties applicable to cell delivery and bioink systems. However, the alkyl-hydrazone crosslink lacks stability in biologically relevant environments. Further, when formed with biopolymers such as hyaluronic acid (HA), low molecular weight polymers (<60 kDa) or low polymer content (<2 wt%) hydrogels are typically employed as entanglements reduce injectability. Here, we modified a high molecular weight (>60 kDa) HA alkyl-hydrazone crosslinked hydrogel with benzaldehyde-poly(ethylene glycol) 3 -azide to incorporate azide functional groups. By reacting azide-modified HA with a multi-arm poly(ethylene glycol) (PEG) functionalized with bicyclononyne, stabilizing triazole bonds are formed through strain-promoted azide-alkyne cycloaddition (SPAAC). Increasing the fraction of triazole bonds within the hydrogel network from 0% to 12% SPAAC substantially increased stability. The slow gelation kinetics of the SPAAC reaction in the 12% SPAAC hydrogel enabled transient self-healing properties and a similar extrusion force as the 0% SPAAC hydrogel. Methyl-PEG 4 -hydrazide was then introduced to further slowdown network evolution, which temporarily lowered the extrusion force, improved printability, and increased post-extrusion mesenchymal stem cell viability and function in the 12% SPAAC hydrogel. This work demonstrates improved stability and temporal injectability of high molecular weight HA-PEG hydrogels for extrusion-based printing and cell delivery. This article is protected by copyright. All rights reserved.