Shaoting Lin1 Xuanhe Zhao1

1, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

In recent decades, unprecedented amounts of electronics are being integrated with human body as wearable, implantable and edible devices for monitoring, sensing and responding. One main challenge relates to the high stiffness of the typical electrical components used in these devices when compared with the surrounding compliant soft biological tissues. To accommodate the modulus mismatch between electrical components and biological tissues, synthetic soft materials are widely adopted to achieve flexibility and stretchability. Particularly, hydrogels (i.e. polymer networks infiltrated with water) not only mechanically soft and flexible but also accommodating transportation and reaction of species for versatile functional responses are one of the most ideal soft materials for bioelectronics. However, existing hydrogel-based electronics mostly suffer from the limitation of mechanical robustness owing to the weak nature of common synthetic hydrogels and poor interfacial performance between hydrogels and other surrounding materials. Therefore, the understandings of the fracture process in soft materials and relevant interfaces are imperative to the design of mechanically robust bioelectronics and devices, which enables long-term biocompatibility and functionality (e.g. electrical stability).

Although various soft tough materials have been developed in recent decades, it is still not well understood how the intrinsic fracture energy of soft materials (i.e. the energy required to rupture a layer of polymer chains in front of the crack) and the mechanical dissipation in process zones around crack cooperate synergistically to give rise to high toughness of soft materials. Here, we report a theoretical scaling and continuum model that quantitatively account for the synergistic contributions of intrinsic fracture energies and dissipations to the total fracture energies of soft materials. Based on the model, we further calculate a toughening diagram that can guide the design of new soft materials.

In addition, we systematically study the formation, transition and interaction of mechanical instabilities in soft elastic interfacial layers under tension. Through combined experimental, numerical and theoretical analysis, we find that the mode of instability is determined by both geometry and mechanical properties of the layer through two non-dimensional parameters: layer thickness over its lateral dimension and elastocapillary length over the defect size. A phase diagram is calculated to quantitatively predict the occurrence of any mode of instability. Systematically understanding of the formation and interactions of various mechanical instabilities in elastic layers under tension can provide a guideline for the design of robust adhesives by rationally harnessing the desired mode of instabilities while suppressing the other modes.