Fabrication of wearable pressure and strain sensors

Wearable sensors attract significant attention due to their potential applications especially in personal health monitoring, health data collection and wearable electronics integrated in smart clothing. Breathable, biocompatible, stretchable sensors are easily skin-mountable and together with sensors embedded in fabrics, open up possibilities for real-time measurements of health and wellness without hindering comfort of the users. Thus, it comes as no surprise that within the next four years, the global wearable sensors market size is predicted to be worth $2.9 billion. Commonly recognized main classes of pressure sensors are capacitive, piezoelectric and piezoresistive sensors. It has to be acknowledged that among these categories, capacitive pressure sensors exhibit smaller temperature drift, higher accuracy, lower consumption, usually a more robust structure, and less sensitivity to environmental effects like humidity. What is more, capacitive sensors provide a promising solution towards highly sensitive pressure sensors. In this report, introduction, fabrication, and characterization of three classes of wearable capacitive pressure sensors and two classes of wearable capacitive strain sensors are present. The following categories of the sensors were distinguished: yarn-based, textile-sandwiched, and silver nanowire (AgNW)-based sandwich-type. As a dielectric layer, polydimethylsiloxane (PDMS) was utilized and as conductive component silver-coated nylon yarns or AgNWs were used. In the first approach, 8 iterations before the final pressure sensor prototype were completed. The leading prototype exhibits overall change in capacitance of 18% and sensitivity of 0.02 kPa-1 upon applying pressure in the range from 0.2 kPa to 9.5 kPa. In the second approach, 14 pressure sensing devices iterations were assembled, and the top-notch prototype shows 37% of capacitance change and 0.12 kPa-1 of sensitivity. Lastly, two pressure sensors were created in the third category and the surpassing iteration demonstrates increase of capacitive response of 22% and sensitivity of 0.04 kPa-1. Consequently, two proof of concept (POC) strain sensors complementary to the two first classes were fabricated. Together with aforementioned pressure sensing devices, realization of multifunctional sensors becomes achievable. Obstacles and insights from all experiments were collected, and finally recommendation and future vision were suggested. Further research focusing mostly on scaling up the samples, selection and impact of other dielectric materials, like wool, and their porosity caused by template choice, e.g., PDMS foam based on a nickel template, should be carried out