Surface Modification of Gallium‐Based Liquid Metals: Mechanisms and Applications in Biomedical Sensors and Soft Actuators

Abstract

This review focuses on surface modifications of liquid metal (LM). Gallium (Ga) and Ga-based LMs show the promising ability to maintain metallic properties under large mechanical strains when encapsulated inside an elastomer matrix. [1-7] This property is useful in a variety of applications, including wearable/deformable devices, sensors, and soft actuators-the focus of this special issue. There are several motives for creating soft and stretchable devices. For example, devices that are soft and flexible can make conformal contact to human skin for continuous and long-term monitoring. [8-11] Likewise, devices assembled on or within soft robots can maintain function during actuation (bending or stretching) while sensing dynamic motion. Conventional conductors in electronic systems are typically rigid (e.g., copper) and thus poorly suited for soft and stretchable devices. To address this issue, efforts have been taken to create stretchable and deformable conductors using thin metal films with special geometries [12-14] (e.g., pop-up and serpentine electrodes) on a deformable substrate or polymer composites filled with conductive nanomaterials in a stretchable matrix. [15,16] These approaches effectively render rigid materials stretchable through clever engineering of their geometry. However, due to the inherently nonstretchable nature of solid metal films or solid conductive particles, these components/inclusions would add to the overall rigidity of the composite and limit their deformability. LMs are interesting as stretchable conductors because they have metallic conductivity and deformability defined primarily by the encasing material (e.g., elastomer).

Rapid meniscus-guided printing of stable semi-solid-state liquid metal microgranular-particle for soft electronics

“…Thus, the annealing of PaLMPs in an acidic condition partially removes of the oxide shell, which extrudes out the gallium. We hypothesize that the removal and reformation of the oxide layer at the interface strengthens the adhesion of PaLMP lm to the substrate 21,33 . We have furthermore observed that only the lm printed with an AA-containing ink on a heated substrate was intrinsically conductive, with a resistivity of 6.67x10 −7 Ωm.…”

“…To overcome the abovementioned drawbacks of bulk LM, LM particle-based approach has been actively studied where the outer oxide layer can restrain its uid-like behavior 20,21 . However, the formation of the native oxide layer deteriorates the electrical conductivity since the oxide is an insulator.…”

Liquid metals: an ideal platform for the synthesis of two-dimensional materials

“…25,26 Interestingly, the oxide layer that forms on the surface of EGaIn is found to be composed of gallium oxide. 27,28 The preferential growth of gallium oxide is due to the more favourable Gibbs free energy of oxide formation for gallium when compared with indium. By exploiting this concept, elements with thermodynamically more favourable oxide formations can be alloyed into gallium-based liquid metals to produce oxides of the dissolved elements.…”

Liquid metal polymer composite: Flexible, conductive, biocompatible, and antimicrobial scaffold

“…This oxidation can affect the apparent surface tension and electrical properties of nanocomposite film. 5,6 SEM images show that the EGaIn is present mainly close to the surface of the film, and the nanocomposite shows a rough surface morphology with cavities (Figures 2B and S2a-c). It is thought that the oxide layer forms quickly on the EGaIn surface, causing the surface to be rough and nonconductive.…”