Ambilateral convergent directional freeze casting meta-structured foams with a negative Poisson’s ratio for high-performance piezoresistive sensors

Abstract

Flexible piezoresistive sensors show broad application value in wearable electronic skin, health monitoring and motion detection. The practical range of conventional piezoresistive sensors, however, is often severely limited by their sensitivity, measuring span, mechanical properties, and lateral expansion caused by positive Poisson's ratio (PPR) effects. Herein we introduce a generally applicable methodology, ambilateral convergent directional freeze casting technique, for the fabrication of anisotropic porous foams with a negative Poisson's ratio (NPR). Different from traditional freeze casting technique, this new design methodology enables precise control of pore types and pore orientations, thereby endowing artificial materials with special mechanical properties. Under uniaxial compression, synthetic foams exhibit macroscopic lateral shrinkage, with a Poisson's ratio below – 4.2% at the strain of 25%. These conductive meta-structured foams are used to construct piezoresistive sensors. Compared with traditional structures, metastructures with NPR effects significantly increase the number of wall-to-wall contacts and the formation of electrical paths, thereby enhancing the sensitivity and sensing reliability of sensors. In addition, the NPR effect markedly enhances the mechanical elasticity and compression resistance of sensors, enabling it to have excellent signal stability even in the face of reiterative compressions. These meta-structured sensors also exhibited smart resistivity repair behavior in the face of overload damage. Integrated sensors can be used for monitoring of pressure distribution and athlete's gait. Lateral contractions caused by NPR effects can avoid the contact of adjacent sensors and narrow the gap between two sensors, thus improving sensing stability and pressure distribution resolution of distributed sensors.