Mixed electronic-ionic conductors with high flexibility and stretchability can serve as key materials in rapidly emerging applications such as soft electronics and soft robotics. The combined use of two functional liquid materials, ionic liquids (ILs) and liquid metals (LMs), is of significant interest in the development of highly deformable mixed conductors. In this study, composite gels embedded with both IL and LM are prepared for the first time using a Ga-In eutectic (eGaIn), 1-ethyl-3-methyl imidazolium bis(trifluoromethanesulfonyl)amide, and poly(vinylidene fluoride-co-hexafluoropropylene) as the LM, IL, and polymer matrix, respectively. The composite gels exhibit low electronic conductivities of the order of 10 −3 S cm −1 . The addition of an optimal amount of Ni particles (NiPs) improve the electronic conductivity to ≈25 S cm −1 while retaining mechanical flexibility. The electronic conductivity is further enhanced upon elongation due to the reversible alignment and elongation of the LM in the gel matrix along the stretching direction. Owing to a bicontinuous structure composed of the IL-based gel and metal component phases, the composite gels exhibit high ionic conductivity of the order of 10 −3 S cm −1 . This study demonstrates a rational approach for designing stretchable mixed conductors using ILs and Ga-based LMs.

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Introduction Eutectic gallium-indium (Ga-In) has excellent properties, such as low melting point of 15.3 °C, high thermal and electronic conductivity, and metallic luster. Ga-In has promise as a liquid electron-conducting material owing to its low viscosity, negligible vapor pressure and low toxicity.1, 2) On the other hand, ionic liquids are ambient temperature molten salts that have attracted considerable attention because of unique properties such as high ionic conductivity, non-volatility and thermal stability. We proposed that ion gels, composed of macromolecular networks swollen with ionic liquids, exhibit self-standing film-forming ability in addition to the unique properties of ionic liquids. In this study, we prepared composite gel materials containing ionic liquid and Ga-In. This composite gel (metal gel) might have high electronic conductivity based on Ga-In and high ionic conductivity originated from the ionic liquid, as well as good mechanical properties based on the polymer, such as flexibility and strechability. These new materials are applicable to flexible or stretchable devices in wearable and flexible electronics applications. Experimental We chose hydrogen bonding copolymers of N,N-dimethylacrylamide (DMAAm) and acrylic acid (AAc) (P(DMAAm-r-AAc)) as the matrix polymers. This copolymer was combined with a hydrophobic ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([C2mim][NTf2]) to form an ion gel.3) In order to improve dispersibility of Ga-In in the composite gel, bulk Ga-In was ultra-sonicated in ethanol and the suspension of Ga-In microdroplets was mixed with P(DMAAm-r-AAc) and [C2mim][NTf2]. The composite gels were prepared by solution casting method either in the air where thin oxide layer is formed on the Ga-In particles4) or under inert atmosphere to examine the effects of preparation conditions on their properties. Results and Discussion In tensile tests, Young’s modulus increased with increasing volume fraction of Ga-In in the composite gels. In rheological measurements, storage modulus was higher than loss modulus, confirming soft solid-like behavior of the composite gels. In both measurements, modulus of composite gels was higher than that of ion gels. We found difference in the temperature dependent rheological properties between the composite gels prepared in air and under inert atmosphere. The presence/absence of the surface oxide layer on the Ga-In particles was likely responsible for the difference in the rheological responses. Electronic conductivity was improved by a factor of 106 for the composite gels prepared under inert atmosphere compared to that of the composite gels prepared in the air. It was found that the oxide layers on the Ga-In particles had a significant impact on the rheological and electronic properties. However, electronic conductivity of the composite gels prepared under inert atmosphere was still low compared to that of bulk Ga-In. To achieve high electronic conductivity comparable to the bulk value, volume fraction of Ga-In microdroplets needs to be increased in the composite gels. In order to improve dispersibility of high-loading Ga-In in the composite gel, Ga-In microdroplets were prepared with dispersants. The results suggested that there is a trade-off between dispersibility of the Ga-In microdroplets and the electronic conductivity: better dispersibility of Ga-In microdroplets resulted in lower electronic conductivity. Acknowledgement This study was supported in part by Core Research for Evolutionary Science and Technology (CREST) of the Japan Science and Technology Agency (JST). References 1) Kazem, N. et al, Adv. Mater., 2017, 29, 1-14. 2) Anderson, T. J. et al, Phase Equilibria, 1991, 12, 64-72. 3) Tamate, R. et al, Adv. Mater, 2018, 30, 1802792 4) Ren, L. et al, Adv. Funct. Mater., 2016, 26, 8111-8118. Figure 1

Ga-based liquid metals (LMs) are expected to be suitable for wiring highly deformable devices because of their high electrical conductivity and stable resistance to extreme deformation. Injection and printed wiring, and wiring using LM–polymer composites are the most popular LM wiring approaches. However, additional processing is required to package the wiring after LM patterning, branch and interrupt wiring shape, and ensure adequate conductivity, which results in unnecessary wiring shape changes and increased complexity of the wiring methods. In this study, we propose an LM–polymer composite comprising LM particles and ion gel as a flexible matrix material with low viscosity and specific gravity before curing. Moreover, the casting method is used for wire patterning, and the material is cured at room temperature to ensure that the upper insulative layer of the ion gel self-assembles simultaneously with the formation of LM wiring in the lower layer. High conductivity and low resistance change rate of the formed wiring during deformation are achieved without an activation process. This ion gel–LM bilayer wiring can be used for three-dimensional wiring by stacking. Furthermore, circuits fabricated using ion gel–LM bilayer wiring exhibit stable operation. Therefore, the proposed method can significantly promote the development of flexible electronic devices.