Skin-Inspired Capacitive Stress Sensor with Large Dynamic Range via Bilayer Liquid Metal Elastomers

Soft devices that sense touch are important for prosthetics, soft robotics, and electronic skins. One way to sense touch is to use a capacitor consisting of a soft dielectric layer sandwiched between two electrodes. Compressing the capacitor brings the electrodes closer together and thereby increases capacitance. Ideally, sensors of touch should have both large sensitivity and the ability to measure a wide range of stress (dynamic range). Although skin has such capabilities, it remains difficult to achieve both sensitivity and dynamic range in a single manmade sensor. Inspired by skin, this work reports a soft capacitive pressure sensor based on a bilayer of liquid metal elastomer foam (B-LMEF). The B-LMEF consists of an elastomer slab (elastic modulus: ≈655 kPa) laminated with a soft liquid metal elastomer foam (LMEF, elastic modulus: ≈7 kPa). The LMEF deforms at small stresses (<10 kPa), and both layers deform at large stresses (>10 kPa). The B-LMEF has high sensitivity (0.073 kPa^–1) at small stress and can operate over a large range of stress (200 kPa), which leads to a large dynamic range (≈4.1 × 10⁵). Additionally, the elastomer slab has a large energy dissipation coefficient; the skin uses this property to cushion the human body from external stress and strain.