A cyclic freezing-thawing approach to layered Janus hydrogel tapes with single-sided adhesiveness for wearable strain sensors

Hydrogel strain sensors, which have potential applications in wearable devices, soft robots and human–machine interfaces, etc., are increasingly investigated. But it remains challenging to fabricate hydrogel strain sensors with integrated wearability and high sensing performances. To address this issue, we herein propose a cyclic freezing-thawing approach to layered Janus hydrogel tapes for wearable strain sensors. The hydrogel tapes are composed of an adhesive poly(vinyl alcohol)/phytic acid (PVA/PA) layer and a non-adhesive poly(vinyl alcohol)/polyaniline (PVA/PANI) layer and exhibit controllable flexibility and approximate elastic modulus to soft tissues. A robust interface forms between the two layers and the interfacial peeling force reaches 114.6 N/m. Strain sensors assembled from the Janus hydrogel tapes achieve low response time (loading: 60 ms, unloading: 62 ms), reversible response and high sensitivity (gauge factor: 3.4). Interestingly, such strain sensors can be further integrated with a Bluetooth system to fabricate wireless wearable sensors for convenient monitoring of physiological activities such as walking, squatting, stair climbing, etc. Overall, this work will provide not only a new approach to Janus hydrogels, but also a new clue for developing functional flexible devices.