Engineering Elastomer Dielectric for Low-Cost and Reliable Wearable Health and Motion Monitoring

Abstract

Thin and flexible sensors directly attached on the skin surface or cloths for real-time monitoring of diverse human physiological signals and body motions have attracted wide attention for applications in wearable healthcare and patient rehabilitation [1, 2]. For example, as illustrated in Fig. 1, with the strain and pressure sensors, various physiological signals related to health conditions and body motions can be monitored [3, 4]. To be able to detect these different physical signals in real time, the sensors need to achieve the required limit of detection, detection range, and sensitivity with fast enough response and recovery. For long-term wearable applications, biocompatibility with human skin, low power consumption, and durability of sensing performance are also key prerequisites. Another important feature, which would determine wide adoption of the sensors, is being imperceptible when they are worn on the human body. For that, the sensors need to be ultrathin, light, and of low Young’s modulus. Moreover, in some application cases (e.g., sensors on exposed parts of the skin), transparency or semitransparency is preferred for the sensors to be invisible [5]. Currently, polydimethylsiloxane (PDMS) elastomer is the popularly used base material for constructing various kinds of pressure and strain sensors due to its excellent flexible and elastic properties, biomedical compliance with human tissues, and commercial availability [6–9]. With external pressure or strain being applied, the induced mechanical structure deformation of PDMS (e.g., length or film thickness) results in changes of the resistance or capacitance of the fabricated devices to measure the applied stimuli [10, 11]. Therefore, the sensing performance is determined by the mechanical properties of the elastomer film. Unsatisfactorily, the bulk PDMS film, having a modulus of ∼1 MPa, cannot generate large enough structure deformation to be converted to detectable signal changes under weak pressure or strain forces [12]. Structuring the PDMS films to reduce the modulus for improved sensitivity has become a research hotspot [6, 13–18]. On the other hand, to obtain light and imperceptible sensors, processes and designs of making devices on ultrathin PDMS film are needed [19].