Ultrasoft and Ultrastretchable Wearable Strain Sensors with Anisotropic Conductivity Enabled by Liquid Metal Fillers

Abstract

Herein, ultrasoft and ultrastretchable wearable strain sensors enabled by liquid metal fillers in an elastic polymer are described. The wearable strain sensors that can change the effective resistance upon strains are prepared by mixing silicone elastomer with liquid metal (EGaIn, Eutectic gallium-indium alloy) fillers. While the silicone is mixed with the liquid metal by shear mixing, the liquid metal is rendered into small droplets stabilized by an oxide, resulting in a non-conductive liquid metal elastomer. To attain electrical conductivity, localized mechanical pressure is applied using a stylus onto the thermally cured elastomer, resulting in the formation of a handwritten conductive trace by rupturing the oxide layer of the liquid metal droplets and subsequent percolation. Although this approach has been introduced previously, the liquid metal dispersed elastomers developed here are compelling because of their ultra-stretchable (elongation at break of 4000%) and ultrasoft (Young’s modulus of <0.1 MPa) mechanical properties. The handwritten conductive trace in the elastomers can maintain metallic conductivity when strained; however, remarkably, we observed that the electrical conductivity is anisotropic upon parallel and perpendicular strains to the conductive trace. This anisotropic conductivity of the liquid metal elastomer film can manipulate the locomotion of a robot by routing the power signals between the battery and the driving motor of a robot upon parallel and perpendicular strains to the hand-written circuit. In addition, the liquid metal dispersed elastomers have a high degree of deformation and adhesion; thus, they are suitable for use as a wearable sensor for monitoring various body motions.

Interfacial interaction-induced super-wettability of gallium-based liquid metals: a review

“…The loading of LMs into elastomeric polymers can achieve the super-stretchability of electronic sensors. In Choe et al 's study, 115 the application of an external force made LM particles break on the elastomer. LM penetrated into the elastomer to form conductive traces that maintained good conductivity during strain.…”

Editorial for the Special Issue on Droplet-Based Microfluidics: Design, Fabrication, and Applications

“…Choe et al [ 17 ] present a novel wearable strain sensor that utilizes droplet-based technology. The sensor is made of ultrasoft and ultrastretchable silicone elastomers filled with conductive liquid-metal droplets, exhibiting anisotropic conductivity, and maintaining metallic conductivity when strained.…”

Current Status and Outlook of Low‐Melting‐Point Metals in Biomedical Applications

“…These composite materials are used in sensors, which can effectively improve their performance. [ 140‐145 ] For example, as shown in Figure a, Zhou et al. designed and constructed a PVA‐TA‐EGaIn hydrogel system.…”