Strain relief by controlled cracking in highly stretchable multi-layer composites

Stretchable electronics exploit the characteristics of soft biocompatible polymers and utilize microstructural designs to push the boundaries of brittle functional electronic materials. Extreme stretchability is required in novel applications such as in situ health monitors or sensors integrated into, e.g., wound dressing. A novel approach to stretchable electronics aims to increase macroscopic stretchability by introducing controlled cracks in the polymer substrate’s surface (Polywka et al., 2016). The cracks are deliberately induced by a micropattern of soft island crack starters in the hardened surface. Here, we introduce the first numerical model of fracture and deformation behavior of soft islands structures. We study crack evolution and material strain in dependence of the microstructure’s design. The fracture behavior in the polymer is modeled with a cohesive zone model. We examine the microstructured composite’s behavior with regard to its material and geometry properties, e.g. ratio of Young’s moduli, hardened layer thickness, or distance between cracks. The results show that cracks and 3D soft islands accommodate the majority of the applied macroscopic strain while the hardened surface remains almost strain-free. The soft islands microstructure design, thus, demonstrates outstanding strain relief capabilities and can accommodate rigid functional parts while remaining highly stretchable. The simulations reveal critical parameters and allow to identify design principles for large usable surfaces on polymer substrates utilizing controlled cracking.