Enhanced mechanical properties of photo-clickable thiol–ene PEG hydrogels through repeated photopolymerization of in-swollen macromer†
Current hydrogels used for tissue engineering are limited to a single range of mechanical properties within the replicated tissue construct. We show that repeated in-swelling by a single hydrogel pre-cursor solution into an existing polymerized hydrogel followed by photo-exposure increases hydrogel mechanical properties. The process is demonstrated with a photo-clickable thiol–ene hydrogel using a biocompatible precursor solution of poly(ethylene glycol) dithiol and 8-arm poly(ethylene glycol) functionalized with norbornene. The polymer fraction in the precursor solution was varied by 5, 10, and 20 percent by weight and an off-stoichiometric ratio of thiol : ene was used, leaving free enes available for subsequent reaction. Multiple swelling and exposure cycles for the same precursor solution were performed. The compressive modulus increased by a factor between three and ten (formulation dependent), while volume swelling ratio decreased by a factor of two, consistent with increased crosslink density. The modified hydrogels also demonstrate increased toughness by fracturing at compressive forces five times greater than the initial hydrogel. We attribute the increased toughness to subsequent increases in crosslink density created by the repeated photopolymerization of in-swollen macromer. This technique demonstrates the ability to significantly modify hydrogel network properties by exploiting swelling and polymerization processes that can be applied to traditional three-dimensional printing systems to spatially control local mechanical properties.