(242b) Extruding Liquid Metal Polymer Composite Elastomeric Filament for 3D Printing

Liquid metal polymer composites (LMPCs) have proven to be impactful dielectric materials for applications ranging from soft robotics to sensors to high voltage electronics. Liquid metals are eutectic alloys, primarily of gallium, that remain liquid and metallically conductive at and below room temperature. Some examples include EGaIn (eutectic gallium-indium) and galinstan (gallium-indium-tin). LMPCs take advantage of the native oxide that forms spontaneously and nearly instantaneously on liquid metal surfaces in atmospheric conditions. While this oxide has proven challenging in many microfluidics or printing applications, as a dielectric composite the oxide enables the stable dispersion of liquid metals into a host polymer without the need for any additional stabilizer. LMPCs consist of discrete liquid metal droplets, i.e., conductive droplets with polymer films in between each, where every droplet is completely encapsulated with the polymer. Electrical properties can be tuned widely based on the liquid metal droplet concentration, size, homogeneity, and shape. Mechanical properties of the composite are also a function of liquid metal parameters as well as the continuous polymer properties, such as elasticity or toughness. For the most part, characterization and application of LMPCs has used molds or thin film casting (either using a doctor blade or a spin coater), which while sufficient for making tensile dog bones or basic pressure sensors limits geometrical complexity and potential 3D structures. To expand LMPCs into applications that require more unique shapes, explore non-standard deformation behaviors, or just reduce the manufacturing burden of LMPCs, 3D printing is a clear next step in material development. However, most LMPCs in literature have been made from thermoset materials and are made in bulk, which does not lend readily to printing. Instead, in the Koh Lab, we have begun making elastomeric LMPC filament made from dispersions of liquid metal (galinstan) in the thermoplastic styrene-ethylene-butylene-styrene (SEBS). Using a custom built micro-single screw extruder, we have created SEBS-LMPC thermoplastic elastomeric filament with tunable dielectric permittivity, modulus, and thermal behavior. In this talk we will demonstrate the design parameters needed to create the desired 3D printable LMPC filament, including both extrusion manufacturing parameters and collection parameters. With the custom-built equipment, we have fine control over each zone temperature as well as screw geometry, and we relate the final filament morphology to bulk thermorheological behavior. The ability to create 3D printable LMPCs dramatically expands the application potential for these materials.