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

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.