Enhancing the crack initiation resistance of hydrogels through crosswise cutting

The achievement of tough adhesion between hydrogels and solid surfaces has significantly expanded the potential applications of hydrogels. This robust adhesion is commonly attained by combining tough hydrogels with strong interfacial bonding and it is reflected in high adhesion toughness. However, the practical use of such strategies is limited since enhanced adhesion diminishes rapidly after cyclic loading. Furthermore, relying solely on adhesion toughness as a measure of adhesion quality may underestimate the adhesive's bearing capacity. To address these issues, here we introduce a macro-structural design strategy called crosswise cutting in hydrogels and propose a new measure of hydrogel adhesion named crack initiation resistance. This strategy can significantly enhance hydrogel adhesion by increasing the peak peel force while constraining the crack initiation length, thus improving the crack initiation resistance. The toughening mechanism behind this strategy is further explored by decomposing the crosswise cutting into longitudinal and transverse cuttings. Through extensive experiments, we demonstrate that longitudinal cutting can increase both the peak peel force and crack initiation length during peeling. Conversely, transverse cutting can constrain the crack initiation length while maintaining a high peak peel force. To quantitatively evaluate the crack initiation resistance of an adhesion system, we define a new parameter, the critical crack initiation energy release rate (G_c^*), which considers both the peak peel force and crack initiation length. Theoretical derivations of G_c^* for hydrogels with longitudinal and transverse cuttings are provided. By varying cutting types, hydrogel thickness, and cut sizes, the value of G_c^* can be significantly increased, with the optimal choice being the crosswise cutting satisfying specific dimensions. The underlying mechanisms pertaining to the increase of G_c^* via crosswise cutting are explained by the stress de-concentration effect, and the value of G_c^* can be predicted by the macroscopic Lake-Thomas model. It is hoped that this study will draw attention to the assessment of the adhesion quality of hydrogels in practical applications and provide some new insights for the design of future soft adhesives.