Progress of flexible strain sensors for physiological signal monitoring

Flexible strain sensors, as a key component of the cutting-edge wearable and implantable electronics, have facilitated applications pertaining to human health monitoring and diagnosis. To fulfill the increasing requirements of sensing performance and broadening the application scope, novel materials and device design strategies have been continuously developed over the past decade. Herein, the recent progresses of flexible strain sensors developed for monitoring the physiological signals are selectively reviewed, from the perspective of the possible correlation between the device microstructure and their corresponding applications. Firstly, representative strain sensors developed based on four fundamental working mechanisms: piezoresistance, capacitance, piezoelectricity and triboelectricity are respectively introduced, subclassified by the type of active material or the similarity in microstructure. Next, a number of biomedical applications of flexible strain sensors are highlighted, including the detections of different types of physiological signals using specific microstructured strain sensors. Lastly, the role of the transduction mechanism and the device microstructure in the sensing characteristic are comprehensively discussed, and prospective forms of flexible strain sensors to meet the existing and future challenges in wearable/implantable electronics are summarized.