Effect of water content on physical adhesion of polyacrylamide hydrogels

Hydrogels are widely used in flexible electronics and biomedical areas owning to their superior biocompatibility and stretchability. Previous studies have demonstrated that water content has significant effect on the elastic and fracture behaviors of hydrogels. However, the effect of water content on hydrogel adhesion has not been systematically explored. Herein, we carry out a series of experiments on polyacrylamide hydrogels with a wide range of water content and reveal that the physical adhesion properties of hydrogels are related to the water content. We find that with a variation of water content, the adhesion energy-water content relationship can be divided into four regions. The surface chain density and bulk energy dissipation of the polyacrylamide hydrogel and the water content and effective contact area over the interface between the hydrogel and the substrate synergistically contribute to this relationship. Specifically, at extremely low water content of ∼ 30%, polyacrylamide hydrogels exhibit one order of magnitude higher adhesion energy than that at relatively high water content. It is also found that with the decrease of water content, the maximum peeling force presents a similar trend to that of adhesion energy. According to our findings, we may control the adhesion performance and even achieve reversible adhesion of a hydrogel by simply adjusting the water content.