Soft Liquid Metal Infused Conductive Sponges

Abstract

202101500is known for its near room temperature melting point of 29.76 °C and presents much lower toxicity comparing to liquid metal mercury. In recent decades, Ga has attracted growing research interests due to its fascinating properties including fluidity, metallic electrical and thermal conductivities. [1] Gallium and its alloys have shown applications in microfluidics, [2] sensing, [3] catalysis, [4] self-healing materials, [5] soft composites for biomedical devices, wearable electronics, [6] and electromagnetic wave shielding materials. [7] Gallium and its alloys are attractive materials for establishing soft composites for flexible and malleable electronics and sensors due to their variable electrical resistance under the modulation by a mechanical load. Especially, Ga and Ga-based alloy droplets, embedded into polymeric matrices, can deform alongside the polymeric materials, unlike solid particles. [8] In other words, by combining fluidic behaviors and metallic nature, Ga and its alloys offer distinct advantages that no other traditional fillers can offer. However, these Ga-based liquid metals also present a native thin passivating oxide layer on their surfaces, which drastically reduces the electrical conductivity of the composites. The insulating properties of Ga-based droplets can be overcome by mechanical rupturing of the oxide layer via pressing, [9] stretching, [10] expansion, [11] and twisting. [12] Therefore, Ga and its alloys have been commonly investigated as electrically conductive fillers for the formation of pressure and motion sensing elastomer composites.A variety of polymers, such as silicone (including polydimethylsiloxane (PDMS)), [13] polyvinyl alcohol (PVA), [6a,14] and poly(methyl methacrylate) (PMMA), [15] have been utilized as matrices for the inclusion of Ga-based fillers. However, a large volume fraction of liquid metal additives and mechanical activation are commonly required to achieve high sensitivity for the elastomer composites. [5a,6a,16] Additionally, such elastomers have intrinsic stretchability limitations. As compared to elastomers, sponge materials allow for much higher degrees of compressibility and reversibility, which present new opportunities for the synthesis of highly sensitive pressure-sensing devices and other electrical components.The distribution of the liquid metal additives into polymeric matrices was shown to affect the resulting thermal and electrical properties. [16][17] Typical strategies to disperse liquid Liquid metal droplets of gallium (Ga) and Ga-based alloys are traditionally incorporated as deformable additives into soft elastomers to make them conductive. However, such a strategy has not been implemented to develop conductive sponges with real sponge-like characteristics. Herein, polyurethanebased sponges with Ga microdroplets embedded inside the polyurethane walls are developed. The liquid phase (at 45 °C) and solid phase (at room temperature) transition of the Ga fillers shows the temperature-dependent functional variations in the mec...

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