An electronics-free implant for continuous and wireless monitoring of internal tissue strains

Abstract Implantable devices for continuous and wireless monitoring of internal tissue strains have gained significant attention in personalized health monitoring. Conventional implantable electronic sensors are small, flexible, and can monitor strains or vibrations in situ. However, electronic sensors still face challenges in biomechanical matching, biodegradability, biocompatibility, and wireless monitoring. Here we present an electronics-free strain sensor based on meta-gels for continuous and wireless monitoring of internal tissue strains. The meta-gel is a two-dimensional phononic crystal hydrogel, composed of periodic air columns in soft hydrogels (~ 18 kPa). The deformation of the meta-gel shifts its ultrasonic band gap, which can be wirelessly detected by an ultrasonic probe outside the skin. The meta-gel sensor is verified to monitor tissue strains on the porcine tendon, wounded tissue, and heart in ex vivo demonstration. Implantations in live pigs further demonstrate the meta-gel’s capability in monitoring tendon stretching, respiratory rhythm, and cardiac rhythm. The meta-gel sensor works stably in the pig during 30 days’ implantation and is observed to undergo almost complete degradation in 12 weeks by ultrasonic imaging.