wileyonlinelibrary.comembedded in the elastomer base. For example, stretchable electrodes utilizing carbon nanotubes (CNTs), [ 10,11 ] metal nanowires, [ 12 ] or metal ions [ 13 ] with an elastomer base have been proposed.As a stretchable electrode, liquid metals, which are in the liquid state at room temperature, are highly attractive materials. [ 14 ] For the liquid metals of a stretchable electrode, a eutectic galliumindium alloy (EGaIn) and GaInSn are often used because of their low toxicity and low melting point. [ 15,16 ] The composition of the constituent metals in EGaIn are 75% Ga and 25% In by weight and the melting point is 15.5 °C. For GaInSn, the composition is 68.5% Ga, 21.5% In, and 10% Sn by weight and the melting point is −19 °C. Because both of these materials are in the liquid state at room temperature, they have attracted attention as a replacement for highly toxic mercury. [ 15,16 ] Table 1 summarizes the melting points and electrical conductivities of EGaIn and GaInSn. [ 17 ] By comparing GaInSn and EGaIn, it is found that the electrical conductivity is almost the same, but GaInSn has a lower melting point; thus, it can be in the liquid state at a lower temperature. The liquid metals have almost limitless deformability compared with solid metals. By using a liquid metal as the electrodes of a stretchable device, its mechanical properties are governed by the mechanical properties of the elastomeric substrate. [ 14 ] Therefore, extremely stretchable electronic devices are possible by using a high failure-to-strain elastomer for sealing liquid metals for application to stretchable wires [ 18 ] and interconnections. [ 19 ] For the high failure-to-strain elastomer, polydimethylsiloxane (PDMS) is often used because PDMS is a biomaterial and is highly stretchable without structural damage over 100% strain, which enables curved surfaces to be followed; therefore, PDMS can be applied to human-body sensing. [ 20,21 ] Moreover, liquid metals are not just for stretchable electrodes or wiring; functional stretchable devices using changes in the electrical properties due to stretching have been developed, such as a stretchable antenna that tunes the resonance frequency, [ 14,17 ] a capacitor and inductor that sense the applied strain, [ 22 ] straincontrolled diffraction of light, [ 23 ] and a device that senses pressure and strain. [ 24,25 ] By using a combination of a low-toxicity and low-meltingpoint liquid metal and a biocompatible PDMS elastomer, various types of wearable electronics have been developed, such as sensor skin for intelligent robots [ 26 ] and a stretchable keyboard For identifying human or fi nger movement, it is necessary to sense subtle movements at multiple points, including the local strain and global deformation simultaneously; however, this has not yet been realized. Therefore, a highly stretchable, global, and distributed local strain sensing electrode made of GaInSn and polydimethylsiloxane is developed for wearable devices. To investigate the electrical properties of multiple se...
