LIQUID METALS

Liquid assets for soft electronics

Liquid-metal networks have been developed that can be stretched to extreme deformations with minimal change in electrical resistance, ushering in approaches for breathable and integrated soft and stretchable electronic devices.

Electronics are everywhere. Inside an electronic device, the properties of silicon and other familiar materials seem to enable everything one could want: speed, precision, power. But as we move toward a technologically immersive world where electronics and robots are worn and interact with humans, suddenly we are confronted with a problem: conventional electronics are rigid, inextensible, and flat whereas humans are soft, stretchable, and curved. To overcome this challenge, what if we could replace solid electronic materials like copper with something completely different: liquids. In two independent studies published in Nature Materials, Rebecca Kramer-Bottiglio and colleagues1 and Zijian Zheng and colleagues2 report approaches that utilize liquid-metal networks to achieve highly conductive, ultra-stretchable, and mechanically robust electronics. Taken together, these works show how liquid conductors may pave the way for breathable, integrated devices for next generation, skin-like electronics.

There are three primary approaches in stretchable electronics: geometrically patterned architectures where buckled and serpentine structures endow otherwise solid, rigid materials with stretchability; intrinsically stretchable materials that are conductive and stretchable in their bulk or film form; and conductive soft composites. Each has advantages and disadvantages, which has given rise to an interdisciplinary, global effort to combine materials, mechanics, chemistry, and fabrication approaches for stretchable and soft electronics. However, as we see in these two reports, the boundary between different approaches can be blurred to offer breakthrough performance.

Kramer-Bottiglio and colleagues show that room-temperature liquid metals like eutectic gallium indium (EGaIn) can excel when mixed with solid particles to create a biphasic (liquid–solid) composite mixture1 (Fig. 1a). In contrast to previous works that mixed different metal particles into the liquid metal3, this work forms solid gallium oxide particles in situ. Here, when liquid-metal nanoparticles are heated to 900 °C, a thin solid film forms on the surface due to oxidation and phase segregation; meanwhile, the liquid-metal particles underneath rupture and coalesce into a liquid network. This forms a highly electrically conductive (2.06 × 106 S m–1) film that can then be transferred to soft elastomers. This mixture also wets electronic components, overcoming challenges with liquid metal often being difficult to interface with other surfaces.

Fig. 1: Stretchable electronics with liquid metals.
figure1

Panels reproduced with permission from ref. 1 (a) and ref. 2 (b), Springer Nature Ltd.

a, Biphasic GaIn liquid metal for stretchable circuit board assemblies, before and after stretching to 400% strain. Scale bars, 1 cm. b, Liquid-metal fibre mat composite with high permeability and negligible changes in electrical resistance during stretching. RS, resistance at the stretching state; RS0, resistance at the zero-strain state.

When a solid metal wire is stretched it gets longer and thinner, resulting in an increase in electrical resistance. However, when biphasic liquid metal is formed on a stretchable substrate, it can be stretched to over 1,000% strain (10× the original length) with negligible resistance change. This behaviour is not only fascinating, but also useful. For example, when the biphasic liquid metal is used with rigid electronic components, soft circuits can be stretched with minimal changes in resistance, as if they were not deformed at all. This is demonstrated by creating a multilayer soft circuit on a wearable electronic sleeve that functions just as well when it is bent and unbent.

Zheng and colleagues develop a stretchable conductor consisting of a liquid-metal fibre mat composite2 (Fig. 1b). Here liquid-metal EGaIn is coated on an electrospun elastomeric fibre mat, with the enabling feature emerging upon stretching the film. This creates a self-assembled structure where liquid metal breaks into connected domains amongst an array of pores. This simultaneously achieves two important features: it creates a porous structure that allows air and moisture to pass through the film, and the liquid-metal network becomes a highly conductive (1.80 × 106 S m–1), stretchable conductor. The liquid-metal domains are coated with wrinkled gallium oxide, indicating that the training process fundamentally changes the liquid-metal structure. After this initial training process, the electrical resistance remains constant during stretching even up to extreme strains of >1,500%.

The intrinsic stretchability of liquid metal combined with the unique porous structure has important advantages. Specifically, it overcomes current limitations in many elastomeric-based devices and allows for permeability to gas and liquids while maintaining a high conductivity through liquid metal — both of which are important for biocompatibility for future wearable devices. Relative to controls, the researchers found that the porous liquid-metal mat has good wearability. Cell tests show low cytotoxicity in vitro, in vivo animal experiments on rabbit skin do not cause obvious irritation, and preliminary wearability of volunteer forearms shows that this permeability helps improve wearability over a week-long test.

The room-temperature liquid metal used for both studies is an alloy of EGaIn. The enabling features of this material lie in the low viscosity (2 mPa s, about two times that of water), high conductivity (3.4 × 106 S m–1), and low toxicity (in contrast to liquid metals such as mercury)4. Importantly, the alloy also rapidly oxidizes to form gallium oxide in the presence of oxygen, which results in a thin (~nm) oxide shell on the surface of the material5,6.

In some instances, the oxide creates challenges, but overall, it is a powerful tool that presents several advantages. Both reports utilize the oxide in combination with the liquid nature of the metal conductor as an important design feature to create soft, stretchable electronics. For the biphasic liquid metal, the high-temperature heat treatment forms a dense oxide layer on top of the liquid-metal phase. When the circuits are formed, the oxide breaks up into the liquid metal forming the biphasic mixture. The compatibility between the two phases aids in wetting the substrate and electronic components while maintaining the high electrical conductivity during stretching. Meanwhile, for the liquid-metal fibre mats, the oxide is a thin layer on top of the liquid-metal phase, which becomes structured during the mechanical training process. Upon releasing the large initial strain, the oxide buckles in one direction and breaks up into holes in the other, providing an accordion-like structure for high stretchability and conductivity through the wrinkles, while the holes increase gas and liquid permeability for breathability. Both studies also leverage the ability of the liquid metal to easily deform, which dramatically reconfigures the material structure under mechanical deformation. This has been shown to be advantageous for self-healing liquid-metal electronics7, as well as for creating stretchable conductors that maintain constant resistance during deformation8. The oxide and liquid reconfiguration are distinctive and unique features of gallium-based liquid metal; a hallmark sure to create more and more functions and interesting structures moving forward.

As we have seen in these works, hybrid electronic approaches where soft electronics merge with rigid components is a promising approach for high-performing devices. Challenges remain in this space especially when considering how these devices ultimately get packaged and interact with external hardware. When a soft material interfaces with a rigid, inextensible component, the compliance mismatch creates stresses at the interface during stretching. This can drive delamination, lead to material rupture, and result in electrical connections that fail. The design of these interfaces and adhesion between different components becomes crucial, with more testing needed under real-world scenarios to better understand failure mechanisms.

In a broader sense, both reports show that liquids present a path for creating soft devices and robotics. This trend in soft-matter engineering has emerged using functional fluids such as liquid metals, ionic liquids, and magnetic dispersions9. How completely soft and liquid can we go? The liquid needs to be soft and stretchable, but the resultant soft device must also do what rigid electronics have always done, such as transmit signals, send and store power, and store and compute information. With a growing library of functionality, the possibility of creating entirely soft-matter devices and robotics from soft solids and liquids is emerging10. Moving forward, functional, liquid components will provide appealing opportunities to create imperceptible wearable electronics and robots with skin-like softness.

While the race for functional, stretchable, and highly deformable electronics and robotics rages on, this breakthrough work by Kramer-Bottiglio and colleagues and Zheng and colleagues brings us ever closer to electronics that are truly everywhere and anywhere.

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Correspondence to Michael D. Bartlett.

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Bartlett, M.D. Liquid assets for soft electronics. Nat. Mater. (2021). https://doi.org/10.1038/s41563-021-00939-y

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