Ultra-thin and Skin-conformable Strain Sensors Fabricated by Inkjet Printing for Soft Wearable Electronics

Ultra-thin strain sensors have received vast attention due to their ultra-thin and ultra-soft skin-conformable nature with numerous applications in wearable electronics for soft robotics, health monitoring, and human-machine interfaces. With the most recent developments in printing technologies, printing electronics directly on ultra-thin substrates is now more beneficial comparing with the conventional lithographic based electronic fabrication techniques, as printing offers several unique benefits in terms of wide-ranging material processability, process simplification, rapidness, and lower costs. Here, we report an ultra-thin and high performance strain sensor based on metal/polymer composite films, fully fabricated by inkjet-printing on a biocompatible decal transfer substrate (thickness $\approx 1 \mu {\mathrm {m}}$). The sensor patches are consist of two inkjet printed layers i.e. a highly conductive metal bottom-layer made of silver nanoparticles and a polymer top-layer made of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). The sensor patches exhibit the average gauge factor (GF) of around 12, and stretchability of up to 10%, with excellent durability (stretch/release tests up to 500 cycles). As application demonstrations, the strain sensors are employed to monitor the subtle human muscle movements, demonstrating excellent performance. The results show that our ultra-thin strain sensors have broad applications in next-generation smart wearable electronics.