Stretchable, Soft, and Variable Stiffness Elastomer foam with Positive and Negative Piezoresistivity Enabled by Liquid Metal Inclusion

Stretchable and soft piezoresistive composites are appealing for application to tactile sensors, artificial skin, and wearable electronics. The ability of the composites to deform the geometries when they are strained can allow the electrical behavior of the composites to be manipulated. Although rigid metal and semiconductor inclusions have been utilized to create piezoresistive composites, they limit the degree of mechanical deformation. Here, liquid metal (gallium, melting point ≈ 29.7 °C) inclusion into elastomeric foam substrate with 3D open cell morphologies is utilized. Gallium is a fluidic conductor, thus it is possible to infiltrate the liquid metal into the 3D interconnected pore, resulting in soft, stretchable, and shape reconfigurable conductive composites that can change shape and function in response to external stimuli. Applying strain can enable deformation of the liquid metal, generating changes of electrical resistance. Interestingly, it is found that this piezoresistivity of the composite can be positively and negatively manipulated by adjusting the geometries of the liquid metal in the foam. Furthermore, the liquid metal in the elastomeric foam can be reversibly actuated by applying compressive force, resulting in manipulation of the restorative electrical activity of the composites.