Recyclable and self-healing stretchable strain sensor based on liquid metal and Diels-Alder polymer for smart wearable applications

The vulnerability of smart wearables necessitates stretchable sensors capable of recovering their functionality after sustaining damage. Recent research on liquid metal-based sensors demonstrates the potential of these highly stretchable, conductive, and reliable sensors. Unlike previous studies using silicone-based substrates, this paper proposes a self-healing, biocompatible strain sensor based on Galinstan embedded in a Diels-Alder polymer encapsulant. The novelty of this sensor lies in its ability to restore sensing and mechanical functionalities through numerous damage-healing cycles. This research outlines the fabrication and quasi-static and dynamic characterization of the strain sensor, enabling analysis of its strength, sensitivity, hysteresis, response time, drift, and healing performance. Healing is investigated by repeatedly rupturing the sensor in half, then healing it at 60°C for 4 hours before recharacterization. On a mechanical level, healing efficiencies of 80% are achieved based on recovered strain, while on a sensor level, the gauge factor is recovered with 105% efficiency. The degree of hysteresis for resistance-strain is less than 1%, and the sensing behavior is independent of strain rate. The sensor has a response time of 220 ms with an acceptable drift of 5% over 800 cycles. This paper demonstrates the feasibility of recycling the sensor by outlining a method to separate the substrate from the liquid metal and reprocess it. Additionally, the sensitivity and biocompatibility of both pristine and healed sensors are validated through case studies, such as tracking finger and knee joint angle bending, highlighting their potential for smart wearable applications. Supplementary video material can be found at https://www.youtube.com/watch?v=SeLYJ6_qT_k.