Tuning responsiveness and structural integrity of a pH responsive hydrogel using a poly(ethylene glycol) cross-linker

Abstract

Stimulus responsive hydrogels have received considerable attention due to their ability to exhibit reversible volumetric changes in response to external stimuli. The shrinkage or expansion of a hydrogel often results in irreversible structural failure, but limited efforts have been made to resolve the challenge of hydrogel fracture. This study presents that the number (F(M)) of cross-linkers and the molecular weight (M(w)) of the cross-linkers play critical roles determining the responsiveness and the structural integrity of a hydrogel exposed to external stimuli. A pH-responsive hydrogel was prepared by cross-linking poly(acrylamide-co-acrylic acid) with bis-acrylamide (Bis) or poly(ethylene glycol) diacrylate (PEGDA) of varying M(w). The deswelling rate of the hydrogel incubated in an acidic medium was solely controlled by the number of elastically effective cross-links (N) which was increased with F(M) and M(w) of the cross-linker. The structural integrity of the hydrogel during a volumetric change was dependent on the work to fracture and the deswelling rate, which was tuned with the M(w) of the cross-linker. These critical roles of cross-linkers in mediating the elastic response and the structural integrity of the hydrogel were further related to the ability of cross-linkers of different M(w) to self-organize. Ultimately, the results of this study will be highly useful to tuning performance of a wide array of stimulus responsive hydrogels.