Strain-stiffening is the effect when a material gets stiffer when it undergoes an elongation. Specifically, in biological tissue, like skin and blood vessels, but also single cells, strain-stiffening is often found as an intrinsic property. In these cases, the stiffening is highly non-linear and force-strain evaluations show a exponential curve progression. Currently, there is often a mismatch between biological tissue and synthetic materials like implants, which can cause problems when the biological tissue and synthetic material are in contact. This is particular true for vascular grafts and train-stiffening materials incorporated in vascular grafts can solve these problems. Conventional strain-stiffening materials are currently limited to being dependent on the applied strain-rate or on a certain stiffness regime and not dynamic. This also applies to commercially available bypass materials like knitted PET. By bioinspiration we were able to mimic the dynamic strain-stiffening mechanism that is found in cells and is based on the cross-linking of cytoskeletal fibers in response to an external stress. Our metamaterial is a minimalistic structure consisting of parallel, slats connected to backbones, they can also be rounded up to form a tube. We demonstrated experimentally that the structures of our strain-stiffening material can be adapted such that the strain required for stiffening, the final stiffness, as well as the degree of stiffening can be controlled, particularly by combining several strain-stiffening elements. The structure is promising for the development of devices that should resemble the mechanical properties of human soft tissues, e.g. skin-integrated flexible electronics and blood vessel grafts.
