Ultra-Low Percolation Threshold and High Conductivity in Liquid Metal-Polymer Composites Without Sintering

Liquid metals, represented by gallium (Ga), have garnered significant attention due to their unique combination of flexible material properties and metallic characteristics. Liquid metal-polymer composites also hold great potential for biomedical applications. However, numerous studies have shown that despite very high metal particle volume fractions, liquid metal-polymer composites fail to achieve conductivity. This is due to the insulating nature of polymers and the presence of an oxide layer on the surface of liquid metals. As a result, various sintering methods have been proposed to establish conductive pathways, such as laser sintering, mechanical sintering, and evaporation-induced sintering. These methods aim to break the oxide layer on the liquid metal surface, enabling the connection of the liquid metal cores to form conductive pathways. However, this process irreversibly consumes the liquid cores, and since liquid metals are highly prone to oxidation, it is difficult to achieve stable conductivity under standard temperature and humidity conditions.
In this research, we prepared PVDF (polyvinylidene fluoride) and EGaIn (Gallium 75 wt.% and Indium 25 wt.%) composite. Due to the low surface energy and high dielectric constant of PVDF, the percolation threshold of the composite material is effectively reduced, achieving ultra-high conductivity even at very low volume fractions (less than 20%).
By preparing composites with varying EGaIn content, we identified a linear relationship between EGaIn particle concentration and conductivity. Once the volume fraction reached the percolation threshold, the material transitioned from an insulator to a conductor, and conductivity increased linearly with the volume fraction of conductive filler, without the need for sintering. This phenomenon has not been observed in other polymer-liquid metal composites. Therefore, we hypothesize that this material may exhibit a novel conductive mechanism distinct from sintering. We employed AFM and Cs-TEM analysis to explore the formation of conductive pathways and investigated the resistive behaviour using both AC and DC power sources, examining how impedance and resistance changes with voltage and frequency.
Besides, due to PVDF excellent biocompatibility and the high rupture and stretch resistance of both the matrix and conductive filler, this material demonstrates significant potential for biomedical applications.