Interface-Modified Liquid Metal Elastomer Composites for Printing Stretchable Conductors

Liquid metal alloys of gallium, such as eutectic gallium-indium (EGaIn), have garnered attention for their distinctive metallic and fluidic properties. These materials have been utilized in a variety of forms and device architectures, ranging from fluidic microchannels to polymer nanocomposites. Additionally, liquid metal can be embedded in the elastomer matrix to synthesize liquid metal elastomer composites (LMEC) which can augment various functional properties. These composites, with tailored material compositions, can also show high electrical conductivity while having excellent elasticity. However, creating a percolating network of liquid metal (LM) in low modulus elastomers, such as Ecoflex 00-30, is quite challenging. This is because the ultrasoft polymer matrix cannot deliver sufficient mechanical forces to the gallium oxide on the surface of the LM droplets. Since the oxide shell causes separation between the micro-sized LM particles, an innovative strategy for breaking this solid barrier is required to achieve conductive LM pathways in the low elastic modulus LMECs.<br/>In this presentation, we will demonstrate how embedded EGaIn microdroplets in Ecoflex can be “mechanically sintered”, or called “activated”, to become electrically conductive. Surface modification of LM particles is a key for weakening the particle-matrix interface in LMEC, which results in the formation of a percolation network by strain-activation. Furthermore, we will discuss how the formulated composite can be used for printing stretchable conductors. The printed traces can be stretched over 900% tensile strain, and electromechanical testing results indicate minimal changes in electrical resistance under substantial deformations and strain rates. Finally, we will demonstrate the practical application of the formulated printable EGaIn-Ecoflex 00-30 inks in stretchable electronics circuits. Overall, this work highlights the potential of liquid metal elastomer composites for use in functional electronics and provides a promising approach for achieving high electrical conductivity in ultrasoft polymer matrices.