An Electronics-Free Ultrasonic Sensor for Continuous and Wireless Monitoring of Deep Tissue Strains

Implantable devices that enable continuous, wireless monitoring of internal tissue strains are vital for personalized health care. However, conventional electronic sensors, despite being small and flexible, face challenges related to biomechanical compatibility, biodegradability, biocompatibility, and wireless communication. In this work, we present an electronics-free implantable strain sensor based on an ultrasonic metagel, designed for ongoing wireless monitoring of tissue strains. The metagel, a hydrogel-based phononic crystal, is composed of periodic air columns within a soft hydrogel matrix. When the sensor is deformed, its ultrasonic bandgap shifts, which can be detected using an external ultrasound probe. The metagel sensor has successfully measured strains on porcine tendons, injured tissues, and hearts in ex-vivo experiments. Additionally, it monitored tendon stretching, respiration, and heartbeats in live pigs. It maintained reliable function over a 30-day implantation period and degraded almost completely within 12 weeks in live pigs. The sensor’s biodegradability and high performance underscore its potential for future clinical applications.