The ability to monitor, in real time, the mechanical forces on tendons after surgical repair could allow personalised rehabilitation programs to be developed for recovering patients. However, the development of devices capable of such measurements has been hindered by the strict requirements of biocompatible materials and the need for sensors with satisfactory performance. Here we report an implantable pressure and strain sensor made entirely of biodegradable materials. The sensor is designed to degrade after its useful lifetime, eliminating the need for a second surgery to remove the device. It can measure strain and pressure independently using two vertically isolated sensors capable of discriminating strain as small as 0.4% and the pressure exerted by a grain of salt (12 Pa) without interfering with one another. The device has minimal hysteresis, a response time in the millisecond range, and an excellent cycling stability for strain and pressure sensing, respectively. We have incorporated a biodegradable elastomer which was optimized to improve the strain cycling performances by 54%. An in vivo study shows that the sensor exhibits excellent biocompatibility and function in a rat model, illustrating the potential applicability of the device to the real-time monitoring of tendon healing. Text body In the U.S. alone, around 14 million people per year suffer from tendon, ligament, and joint injuries 1. After injury, tissues in the body undergo changes in their native biomechanical properties in order to repairs themselves. This is true for both hard tissues (bones) and soft tissues (tendons, skin, muscles). The objective of surgery and rehabilitation is to restore the tissues to their pre-injury function, with biomechanical properties as close as possible to native properties 2. A diagnostic tool that measures the biomechanical properties of the repair site in real-time would represent a significant step towards improved assessment of healing and the development of personalized rehabilitation strategies 3. Current clinical practice for monitoring tissue rehabilitation includes magnetic resonance imaging (MRI) or ultrasound, which provide a snapshot of tissue density and inflammation 4. Implantable sensors could give continuous information about tissue strain during rehabilitation protocols, as well as during the patient's daily activities, allowing activities to be tailored based on what the tissue can tolerate. Previously described implantable sensors have limited biocompatibility or have been designed for laboratory biomechanics studies rather than clinical practice 4,5 .

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