3D Printing of Liquid Metal Embedded Elastomers for Soft Thermal and Electrical Materials

Abstract

Liquid metal embedded elastomers (LMEEs) are composed of a soft polymer matrix embedded with droplets of metal alloys that are liquid at room temperature. These soft matter composites exhibit exceptional combinations of elastic, electrical, and thermal properties that make them uniquely suited for applications in flexible electronics, soft robotics, and thermal management. However, the fabrication of LMEE structures has primarily relied on rudimentary techniques that limit patterning to simple planar geometries. Here, we introduce an approach for direct ink write (DIW) printing of a printable LMEE ink to create three-dimensional shapes with various designs. We use eutectic gallium–indium (EGaIn) as the liquid metal, which reacts with oxygen to form an electrically insulating oxide skin that acts as a surfactant and stabilizes the droplets for 3D printing. To rupture the oxide skin and achieve electrical conductivity, we encase the LMEE in a viscoelastic polymer and apply acoustic shock. For printed composites with a 80% LM volume fraction, this activation method allows for a volumetric electrical conductivity of 5 × 104 S cm–1 (80% LM volume)significantly higher than what had been previously reported with mechanically sintered EGaIn–silicone composites. Moreover, we demonstrate the ability to print 3D LMEE interfaces that provide enhanced charge transfer for a triboelectric nanogenerator (TENG) and improved thermal conductivity within a thermoelectric device (TED). The 3D printed LMEE can be integrated with a highly soft TED that is wearable and capable of providing cooling/heating to the skin through electrical stimulation.

Transparent Electronics for Wearable Electronics Application

“…Recently, liquid metal-based composites are developed to resolve these limitations. To fabricate soft composite, liquid metals are normally smashed into small particles through ultrasonication and blended with mechanical backbones such as polymer matrices ,, or metal nanowires. , For example, the surface of liquid metal nanoparticles was functionalized for uniform blending with polymer matrix and surface-initiated atom transfer radical polymerization (Figure A) . Through surface modification, a uniform blending of nanoparticles was possible for various polymer matrices.…”

“…Together with composite techniques, high-resolution grid patterning is an alternative approach to utilizing liquid metal as transparent conductors. Photolithography, laser sintering, , and 3D printing are recently reported to fabricate micropatterned liquid metal composites. Liquid metal composite fabricated by lithography showed over 500% of strain without loss of its electrical conductivity.…”

Recent advances in the material design for intelligent wearable devices

“…Recently, liquid metals have received widespread attention in the sensing field due to their unique mechanical, electrical, thermal and optical properties. [118][119][120][121][122] Compared to other soft or conductive materials, liquid metals can make electronics more accessible to the tissues, human body and clothing while maintaining mechanical flexibility. Kim et al reported a freestanding patterned liquid metal thin-film conductor (FS-GaIn).…”

Rational Design of Conductive Pathways in Flexible Tactile Sensors via Indirect 3D-Printing of Liquid Metal for High-Precision Monitoring and Recognition

“…45 Yet, all the prepared microchannels are quadrilateral cross-sections, whose deformation rate is also restrained. Moreover, the depth-width ratio of microchannels prepared by photolithography is limited by the thickness of cured photoresist, 46 whereas the microchannels by laser engraving are unsmooth and irregular because of the uneven laser focusing. 33 Also, when casting microchannels with large depth-width ratios by using the molding method, the demolding process often results in the tearing of materials.…”