Liquid metal particles and polymers: a soft-soft system with exciting properties

Conspectus Gallium-based liquid metal alloys are a special type of material that is in the liquid state at (or near) room temperature. They are particularly attractive due to their unique combination of a fluidic and metallic body, together with a chemically reactive and functionable surface. As a fluid, liquid metals provide the best union of stretchability, deformability, and electrical conductivity among all soft materials. Such an advantage in combination with their low toxicity and relatively good biocompatibility have imparted liquid metals with unique features that can be harnessed for versatile applications in fields such as electronics, energy, chemistry, and biomedical research. More importantly, the fluidic nature of liquid metals allows them to be readily processed using shear for making particles with variable sizes (from nm to mm), which is not possible with solid materials. These particles have a liquid metal core-solid metal oxide shell (conductor-semiconductor) structure, allowing them to merge, transform shape, change phase, respond to stimuli, and self-heal.Despite these unique features, limited surface stability and functionality, unpredictable reactivity, and uncontrollable hydrophilicity of liquid metal particles niche their wider applications in biomedical fields. To bestow liquid metal particles with desirable surface properties while taking the benefits offered by soft features, another important soft material-polymers-can be synthesized and engineered on an on-demand basis to coat or embed liquid metal particles. This leads to the formation of liquid metal-polymer soft composites with versatile surface properties. More specifically, polymer segments with corresponding functions for surface anchoring, tuning solubility, enhancing biocompatibility, providing stimuli-responsive properties, and further bioconjugation can be linked together, thereby forming macromolecules to graft liquid metal particles for yielding soft-soft systems with exciting properties.Herein, we provide a concise review of our contributions to the production, investigation, characterization, and application of liquid metal particle-polymer composites. First, we summarize various top-down techniques developed for producing micro- to nanosized liquid metal particles. We highlight two platforms we developed for tackling long-existing problems encountered by sonication-the most widely adopted method for producing liquid metal particles. Second, we discuss the design of polymers for surface modification of particles. Various grafting strategies for polymers synthesized using different approaches are elaborated. We also discuss factors that affect the colloidal and chemical stability of the composite in biological buffers. Methods for further surface functionalization of the composite are presented, followed by providing examples of biomedical and sensing applications for the system. Next, we introduce the fabrication, unique properties, and applications of elastomeric hybrid composites incorporating liquid metal particle fillers. Finally, we offer a perspective on the opportunities and challenges for the future development of this exciting soft-soft system for realizing synergistic outcomes.