Liquid Metal Incorporated Devices Toward Multifunctional Soft and Stretchable Electronics

In the past decade, liquid metals, especially gallium and its alloys, have surfaced as rapidly emerging cutting-edge functional materials with a view to revolutionize the field of soft and stretchable devices. This special class of materials possesses a handful of fascinating properties. As "liquid" they have low viscosities almost similar to water, along with higher surface tension, negligible vapor pressure and low toxicity; and as "metal" they possess high electrical and thermal conductivity, and transition from their liquid phase to solid phase and vice versa is easily attainable through nucleation and low temperature melting. Effective tuning of these properties and their inclusions in elastomeric materials in various forms via several unconventional techniques can lead to elastomeric metamaterials displaying interesting properties. Herein, we present a compilation of our research works which focuses on liquid metal inclusion in several forms in elastomeric substrates. We shed light on the use of liquid metal in three forms: as bulk injectable electrode material into elastomeric fibers and films, as rheologically modified ink for electrode printing and as micro fillers in elastomeric substrates to amplify electrical properties. Whereas presence of bulk liquid metal as core in 1D fibers and 2D films can render shape memory properties through phase change along with electrical conductivity, solidified gallium wires can be assembled into 3D structures through "room temperature welding" which can be subsequently included in elastomers to realize a 3D fluidic network. The rheology of liquid metal can be tuned to obtain an ink suitable for electrode printing on polymeric films in order to fabricate flexible, ultrasoft and ultrastretchable electronics systems with unhindered electrical and mechanical functionalities. The fluidic nature and low viscosity of liquid metals facilitates their inclusion in elastomeric matrix in the form of micro particles via high shear mixing in order to tune electrical and thermal properties through liquid metal particle inclusions. In short, our compilation provides an insight into conjugating liquid metals in various forms into elastomeric substrates of various physical shapes and mechanical properties via which we were able to design several soft and stretchable systems with specific properties: stretchable 1D fibers and 2D films with shape memory and electrical conductivity, elastomers with 3D fluidic electrode inclusions, flexible, ultrasoft and ultrastretchable sensos with fluidic electrodes and soft devices capable of functioning as healable/reconfigurable waste energy harvesters.