Liquid metal elastomer with flytrap-inspired pillar structure for stress sensing

Flexible and soft capacitive stress sensors are widely used in soft robotics, electronic skins, and human-computer interfaces due to their desirable properties of temperature independence, low energy consumption, and good stability. Increasing the dielectric constant of the dielectric layer of capacitive sensors can effectively improve the sensitivity of the sensor. Mixing materials such as carbon nanotubes and ceramics into the elastomer can increase the dielectric constant, but stiffens the resulting composite. Liquid metals (LMs) are inherently soft and can be mixed with elastomers to produce liquid metal elastomers (LMEs). However, the LME slab is hard to compress, which limits the sensitivity of LME-based stress sensors. The inner surfaces of the Venus flytrap are hair-like soft pillar called trigger hairs, which sense the mechanical stimulation, activating the closure of the Venus flytrap. The trigger hair inspires us to mimic the pillar structure, reducing the effective elastic modulus of the LMEs. Here, this work proposes a capacitive stress sensor based on an LME with flytrap-inspired pillar structure. Mixing LM particles into the elastomer increases the dielectric constant without stiffening the elastomer composite. The mechanism of the sensor is demonstrated through dielectric theory, experiments, and finite element modeling. The sensor has high sensitivity (1.061 kPa−1), large capacitance variation (2.73 pF) and high initial capacitance (2.57 pF). Additionally, the sensor can realize real-time monitoring of human movement and measure the wrist pulse, indicating the potential for various applications.