Liquid metal elastomer (LME)-that is, liquid metal particles dispersed in elastomer-is a soft material that has useful electric, dielectric, and thermal properties. Two issues with LME are sought to be addressed: 1) the dense liquid metal (LM) particles can settle before curing of the elastomer, and 2) the LM particles are separated by a thin layer of insulating elastomer and therefore require some "mechanical sintering" to break this layer to create conductive paths. These issues are addressed using an LME containing elastic particles (LMEP). Elastic polydimethylsiloxane particles (PPs) and LM particles jam to prevent particle settling. Meanwhile, the PPs reduce the loading necessary to create conductive paths, thus decreasing the density and cost relative to LME. Surprisingly, the particles percolate into conductive paths prior to curing the LMEP but not in LME. The dielectric constant, electrical conductivity, and thermal conductivity of LMEPs are investigated by changing the volume fraction of LM particles, polydimethylsiloxane prepolymer and PPs, and propose an LMEP with the optimal ratio. In addition, LMEP-based sensors and circuits are demonstrated for wearable electronics.

soft electronics based on intrinsically stiff materials (graphene, carbon nanotube, MXene, etc.), soft electronics based on LMs have better deformability, stretchability, and durability. [2] Among LMs-based electronics, LMs-based soft sensors have attracted extensive research attention due to the high consistency and low hysteresis.LMs-based soft stress sensors are typically based on one of two mechanisms: piezoresistive [3] or piezocapacitive. [4] The piezoresistive LMs-based soft stress sensors can be fabricated by three methods: 1) injecting LMs into an elastomer (polydimethylsiloxane, PDMS) channel. [3b,5] 2) mixing LM particles with elastomers (PDMS, Ecoflex, Dragon skin). [6] 3) depositing LMs on a subtract (elastomers or textiles). [7] Applying stress deforms the geometry of the LM conductive paths and changes the resistance, which provides a route to measure stress. The piezocapacitive LMs-based soft stress sensors consist of LM particles dispersed in elastomer to form a dielectric layer prepared by mixing LM and elastomers. [4a,8] The piezocapacitive soft stress sensors achieve high initial capacitance and capacitance variation benefiting from the equipotential characteristic of LM particles. In summary, LMs-based soft stress sensors exhibit the advantages Liquid metal (LM)-based soft and wearable sensors have great potential for applications in human motionmonitoring and human-machine interfaces. To avoid smearing of exposed LM, it is necessary to encapsulate LM with elastomers, which decreases the breathability and results in discomfort and even skin irritation. To solve this problem, this work proposes a breathable encapsulated LM foam (BELMF)-based soft stress sensor. By using two polydimethylsiloxane foam layers, the BELMF-based sensor achieves non-smearing and breathability. Meanwhile, a three-dimensional electrode is designed to implement the electric connection without decreasing breathability. Appling stress to the BELMF-based sensor makes the LM foam collapse and forms new conductive paths, which reduces the resistance of the sensor. The sensitivity of the BELMF-based sensor can reach 1.4 Pa −1 . The mechanisms of the breathable encapsulation and the sensor are investigated through experiments and simulations. In addition, the potential applications of the BELMFbased sensor for monitoring human motion are demonstrated.

Three-dimensional graphene foam (GF)-based flexible piezoresistive sensors have the advantages of high conductivity, large deformable range and light weight, which has been utilized in human health measurement, electronic skin, and wearable devices. However, the encapsulation and electrode connections of such sensors reduce breathability, resulting in discomfort and even skin irritation. In this study, a self-encapsulated breathable GF (SEB–rGO–MF) and an SEB–rGO–MF-based piezoresistive sensor (SEB–rGO–MF–PS) are proposed. The SEB–rGO–MF–PS consists of a SEB–rGO–MF and two electrodes, ensuring encapsulation and electrical connection without compromising breathability and stability. We performed material characterization and breathability tests of the SEB–rGO–MF and investigated the sensing mechanism of the SEB–rGO–MF–PS. The SEB–rGO–MF–PS exhibits a sensitivity of 0.108 Pa–1, a measurement range of 0.013–15 kPa, rapid response/recovery times (83 and 166 ms), and excellent stability (2000 cycles at 15 kPa). In addition, this study demonstrates the application of the SEB–rGO–MF–PS for human motion monitoring.

Flexible piezoresistive sensors (FPSs) have the advantages of compact structure, convenient signal acquisition and fast dynamic response; they are widely used in motion detection, wearable electronic devices and electronic skins. FPSs accomplish the measurement of stresses through piezoresistive material (PM). However, FPSs based on a single PM cannot achieve high sensitivity and wide measurement range simultaneously. To solve this problem, a heterogeneous multi-material flexible piezoresistive sensor (HMFPS) with high sensitivity and a wide measurement range is proposed. The HMFPS consists of a graphene foam (GF), a PDMS layer and an interdigital electrode. Among them, the GF serves as a sensing layer, providing high sensitivity, and the PDMS serves as a supporting layer, providing a large measurement range. The influence and principle of the heterogeneous multi-material (HM) on the piezoresistivity were investigated by comparing the three HMFPS with different sizes. The HM proved to be an effective way to produce flexible sensors with high sensitivity and a wide measurement range. The HMFPS-10 has a sensitivity of 0.695 kPa−1, a measurement range of 0–14,122 kPa, fast response/recovery (83 ms and 166 ms) and excellent stability (2000 cycles). In addition, the potential application of the HMFPS-10 in human motion monitoring was demonstrated.

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