Liquid metals as ultra-stretchable, soft, and shape reconfigurable conductors

Abstract

Conventional, rigid materials remain the key building blocks of most modern electronic devices, but they are limited in their ability to conform to curvilinear surfaces. It is possible to make electronic components that are flexible and in some cases stretchable by utilizing thin films, engineered geometries, or inherently soft and stretchable materials that maintain their function during deformation. Here, we describe the properties and applications of a micromoldable liquid metal that can form conductive components that are ultra-stretchable, soft, and shapereconfigurable. This liquid metal is a gallium-based alloy with low viscosity and high conductivity. The metal develops spontaneously a thin, passivating oxide layer on the surface that allows the metal to be molded into nonspherical shapes, including films and wires, and patterned by direct-write techniques or microfluidic injection. Furthermore, unlike mercury, the liquid metal has low toxicity and negligible vapor pressure. This paper discusses the mechanical and electrical properties of the metal in the context of electronics, and discusses how the properties of the oxide layer have been exploited for new patterning techniques that enable soft, stretchable and reconfigurable devices.

Interfacial Tension Modulation of Liquid Metal via Electrochemical Oxidation

“…Thus, controlling 𝛾 is a powerful way to generate forces on small volumes of fluid. Using such forces and combining with key properties of liquid metals such as their low viscosity [11], high thermal and electrical conductivity [12], it is possible manipulate liquid metals into different positions and shapes [13]- [16], and even create devices such as pumps [17], valves [18], and RF devices [19] by manipulating liquid metals inside microfluidic channels [11], [20]- [23].…”

“…The first known observation of this decrease in effective interfacial tension attributed its cause to electrocapillarity [37], and that mistaken association still arises today. Electrocapillarity lowers the effective interfacial tension via an applied electric potential that draws counter ions from the surrounding solution to the surface of a metal [13], [38], [39]. These ions enhance the electrical double layer that forms at the interface between a metal and electrolyte, and act as a capacitor.…”

Finding Common Ground

“…Recent advances in material science and chemical engineering have resulted in conductive polymers for the creation of flexible and stretchable electronics [128], conductive yarns [152], foils [137], liquid metals [44,128,125], and non-rigid micro and nano-structures [83,193]. These new materials are well suited for interfacing with the organic and dynamic contours of the human body.…”

Recent progress on liquid metals and their applications

“…There are now several excellent reviews covering applications of liquid metal on different specific properties [1,8,11,15,16]. Here, we will cover the current progress on the main advantaged properties of Ga-based liquid metal and the applications derived from these advantages.…”

“…The surfaces of pure Ga or eutectic Ga-based alloys are easily oxidised and form an amorphous Ga oxide layer in the ambient environment that decreases the surface tension of the liquid metal [11][12][13]. The thickness of the intrinsically formed Ga oxide surface layer is 0.5-3 nm [14][15][16]. Nevertheless, the thickness of the oxide layer can be modulated by using an electrochemical method.…”