Fracture toughness of hydrogel laminates: Experiments, theory and modeling

Possessing enhanced mechanical durability and multiple novel functions, hydrogel laminates have found wide application in diverse areas including stretchable and bio-integrated electronics, soft robotics, tissue engineering and biomedical devices. In the above scenarios, hydrogels are often required to sustain large deformation without mechanical failure over a long time. Compared to the fast movement in functions design, the failure mechanism of hydrogel laminates has been much less explored and researched, as well as laminates' fracture toughness – a key parameter characterizing their fracture behavior. To address this largely unexplored issue, this paper further studies the fracture toughness of hydrogel laminates both experimentally and theoretically. A kind of modified pure-shear test suitable for measuring the fracture toughness of hydrogel laminates is proposed, which is then applied to testing a PAAm-PAA laminate's toughness. Through theoretical analysis and numerical modeling, the experimentally observed enhancement in the fracture toughness of PAAm-PAA laminates is explained – the fracture toughness of the laminates covers the energy required for both the crack and concomitant interfacial delamination to propagate, and the theoretical predictions agree well with the experimental results. The results from this study provide quantitative guidance for understanding the fracture behavior of hydrogel laminates.