Microstructure-Driven Mechanical and Thermal Behavior of Skin-Like Soft Liquid Metal/Elastomer Composite Materials

Mimicking the tissue-like softness of biological systems for a smooth human-machine interface remains a challenge due to the need for mechanical compliance with conformal integration and functionality. Herein, we report bioinspired liquid metal (LM)/elastomer composite materials with exceptional conformability, consisting of a eutectic gallium-indium alloy physically integrated into low-hardness liquid silicone rubber. The material was engineered via cross-linker ratio tuning and unidirectional vacuum mixing, which led to ultrasoft mechanics (elastic modulus ≤170 kPa, Shore A hardness ≤8) that closely replicates the softness of biological tissues. To understand the microstructure-driven mechanical response of the resulting materials, a modified Eshelby approach was employed, which revealed the matrix-stiffening behavior of the LM inclusions due to their proximity to the elastocapillary limit, resulting in the inclusion surface energy exceeding 1000 mJ/m². Microscopic analysis further showed an LM-loading-dependent microstructural transition: elliptical geometry at ≤20 vol % LM and spherical geometry at >20 vol % LM, further supported by Effective Medium Theory (EMT). Thermal conductivity followed the modified Bruggeman model up to a 20 vol % LM loading and the Bruggeman model beyond, further underscoring the geometry-dependent thermal properties. On the other hand, high resilience under cyclic tensile (hysteresis loss ≤ 2.4 kJ/m³) and Neo-Hookean analysis revealed no significant softening, but strain-dependent stiffening while preserving the material's compliance. The microstructure-driven coupling of the mechanical and thermal behavior of the resulting composite offers a fundamental framework for the design of next-generation ultrasoft functional materials.