Mechanics of regulatable hydrogel adhesion with elastic heterogeneity

Hydrogel adhesion regulation remains a key challenge in applications of stretchable electronics, tissue engineering, and soft robotics. Whereas existing strategies are focused on enhancing adhesion, the underlying mechanics of on-demand mechanical regulation from adhesion promotion to adhesion reduction receives insufficient attention. We propose an effective method to manipulate the energy process zone near the crack front by invoking periodic elastic heterogeneity, whose adhesion toughness exhibits high dependence on the periodic structures and peeling direction. Without modifying surface chemistry, the adhesion of heterogeneous interfaces implemented by 3D printing can enhance peak force by 6-fold or weaken interfacial toughness to 1/30, as compared to the case of a homogeneous interface. Based on a variational principle, analytical formulations and finite element simulations are performed to quantify the heterogeneous behaviors. The analysis captures the instability phenomenon and predicts the maximum peel force. Combined experimental, theoretical, and finite element results demonstrate the effects of geometric parameters and peeling directions on the overall adhesion toughness. This data elucidates an energy storage mechanism, namely that the discrete elastic ligaments free of lateral constraints could enable the interface to accommodate large stretching and store intensified strain energy. The results also indicate the necessity and significance of peeling directions. The work provides a design guideline for on-demand enhancement and detachment in hydrogel interface applications.