We are introducing a wireless and passive strain sensing scheme that utilizes ultrasound imaging of a highly stretchable hydrogel embedded with zinc oxide (ZnO) nanoparticles, named "ZnO-gel". The incorporation of ZnO nanoparticles into a polymer network of the hydrogel improves both its elasticity and strength. It also serves as an ideal biocompatible ultrasound contrast agent that allows remote interrogation of the changes in volume or dimensions of the hydrogel in response to mechanical strains through simple ultrasound imaging. A systematic study of various ratios of ZnO nanoparticle fillers (ranging from 0 to 40% w/w), cross-linked within the poly (DMA-co-MAA) hydrogel, was performed to identify the appropriate ZnO-to-gel ratio that provided the optimal mechanical and ultrasound imaging properties. The results of these investigations showed that 10% w/w of ZnO nanoparticles provided the highest stretchability of 260% with the effective amount of contrast agents to achieve clear visibility of the hydrogel dimension during ultrasound imaging. In general, the applied strain deforms the ZnO-gel specimens by reducing the cross-sectional area at a linear rate of 0.24% area change per % of applied strain for strain levels of up to 250%. Biocompatibility tests with stromal cells (fibroblasts) did not show any acute toxicity of the hydrogel and the ZnO nanoparticles used in this technology. It is anticipated that this technology can be applied to a broad range of wireless and passive monitoring of physiological functions for which microenvironmental strain matters throughout the body, simply by tuning both the mechanical properties of the hydrogel and ZnO nanoparticle concentration.
Keywords: implant device; strain sensor; stretchable hydrogel; ultrasound imaging; zinc oxide nanoparticles.