Increased volume responsiveness of macroporous hydrogels

Hydrogels can be fabricated into "smart" materials whose volumes predictably depend on their chemical environment. These "smart" hydrogel materials can be utilized in applications such as sensors, actuators, and for drug delivery materials, for example. The volume response of these hydrogels is well-known to be limited by their crosslink density. Thus, the responsiveness of hydrogels can be increased by decreasing the hydrogel's crosslink density. Unfortunately, this also decreases the hydrogel strength. The hydrogel "effective crosslink density" can be decreased by fabricating macroporous hydrogels where voids are incorporated into the hydrogel. In the work here we demonstrate that this approach increases the volume responsiveness of hydrogels. We fabricated pH responsive macroporous hydrogels by copolymerizing acrylic acid with acrylamide. We compared the pH response of these hydrogels to that of macroporous hydrogels with small water bubbles embedded by vortexing the polymerizing hydrogel in air, or by preparing an inverse opal hydrogel. We then filled these embedded air bubbles with water. The pH responsiveness of these macroporous hydrogels are significantly increased compared to those of non-macroporous hydrogels of similar composition. We find that these macroporous hydrogels appear to be more mechanically robust than are similarly responsive hydrogels without voids. (C) 2017 Elsevier B.V. All rights reserved.