Liquid metal/CNTs hydrogel-based transparent strain sensor for wireless health monitoring of aquatic animals

Strain sensors are intriguing in advanced electronics and are typically composed of active sensing materials. Good elastic recovery, self-healing, good adhesion, high transparency, and great mechano-response are essential but can be rarely met in one material. Here, we report a wireless strain sensor that exhibits tissue-like mechanical properties (softness), is conformable to soft tissues (heart and tail) and has the lowest limit of detection (0.25 % stretch). After introducing multiwalled carbon nanotubes (CNTs) bridged liquid metal (LM) composites (LM/CNTs), the strain sensor simultaneously achieves excellent transparency and conductivity. This LM/CNTs hydrogel overcomes the limitations of the traditional hydrogel and achieves high conductivity (up to 94 S/m), good stretchability (2,200 %), and a high degree of transparency (93 %), in addition to enough adhesion (20 kPa) and rapid self-healing (20 min). The strain sensor device is composed of intrinsically soft LM/CNTs hydrogel as sensing material, a microcontroller, signal-processing circuits, and Bluetooth transceiver. This device is applied to recording high-quality heartbeat signals and clear in-site observation of cardiac bleeding, which has helped a Chinese sturgeon survive after Caesarean. This approach provides an ideal strategy for protecting and saving endangered animals and broadens the applications of strain sensors.