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

Abstract

AbstractCovalent 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. Furthermore, 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, a high molecular weight (>60 kDa) HA alkyl‐hydrazone crosslinked hydrogel is modified 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 increases stability. The slow gelation kinetics of the SPAAC reaction in the 12% SPAAC hydrogel enables transient self‐healing properties and a similar extrusion force as the 0% SPAAC hydrogel. Methyl‐PEG4‐hydrazide is then introduced to further slowdown network evolution, which temporarily lowers the extrusion force, improves printability, and increases 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.