Printing and Recycling of Liquid Metal Composites for 3D Architected Thermoelectric Wearables

Abstract

Wearable thermoelectric devices (TEDs) enable the conversion between thermal and electrical energy as on-skin electronics by harvesting electricity from body heat or actively modulating skin temperature. However, their deployment has been challenged by insufficient energy conversion, limited conformability, impermeable interfaces, and a lack of recyclability. To address these bottlenecks, four studies are presented here to address the following research objectives: (1) Tailoring composites’ material properties in each device layer through filler types and sizes and controlling device geometry via 3D printing enhance thermoelectric performance; (2) Engineering 3D device architecture improves thermal management while maintaining stretchability in wearable TEDs; (3) Electrohydrodynamic printing enables fabrication of permeable and thermally conductive skin–device interfaces; and (4) Thermally stable and recyclable liquid metal composites can be achieved by utilizing thermoset polymer matrix with dynamic covalent bonds. The studies presented in this dissertation collectively investigate materials, manufacturing processes, and device design to overcome the challenges associated with wearable TEDs.In the first study, material properties and printability of multifunctional elastomer composites that comprise flexible TEDs are characterized. Liquid metal elastomer composites (LMECs) with high thermal conductivity function as compliant thermal interfaces or stretchable conductors depending on the size of liquid metal (LM) inclusions. Different synthesis strategies enable control of LM particle size, producing particles below 8 m or above 100 m for use in thermal interfaces or interconnects, respectively. Due to the passivating gallium oxide layer on the LM surface, the thermal interface LMEC is electrically insulating to prevent short circuits. In contrast, LMEC interconnects can be electrically activated through mechanical sintering as the oxide shell on larger LM particles has a reduced effect. In the device core layer, hollow microsphere elastomer composite (HMEC) serves as a lightweight thermal insulator that minimizes thermal bypasses. These composite inks show shear-thinning behavior, allowing direct-ink-writing (DIW) of every layer with precisely controlled device geometry in 3D printed TEDs. The devices with functional layers successfully guide heat flux onto embedded thermoelectric (TE) pellets and deliver a great power density of 650 μW·cm-2 at a temperature gradient of 60 °C. Additionally, their tensile specimens withstand more than 15,000 stretching cycles at 30% strain due to soft composite encapsulations. Furthermore, the benefits of DIW are demonstrated via seamless integration on textiles and direct printing of stretchable heatsinks on TEDs, which also allowed further understanding of the importance of heat transfer in thermoelectric performance. The printable composites are utilized in the second study to build 3D thermoelectric architectures, tailored for thermoelectric energy conversion. The resulting TEDs, featuring a novel air pocket structure in the core layer and a stretchable LMEC heatsink, achieved a power density of 115.4 μW·cm⁻² at thermal equilibrium with an initial temperature gradient of 10 °C. Especially, the 3D printed HMEC air pocket separates the top and bottom thermal interfaces and allows TE pellets to be surrounded by air to maintain the temperature gradient across the thermoelectric semiconductors. This 3D structure results in higher output voltages and improved heating and cooling performance. In addition, the 2D lattice core layer structure with 12% infill density pattern also offers low stiffness and structural support to prevent early failure from deformation. The 3D-architectured device exhibits exceptional stretchability, minimal resistance variation, mechanical resilience, and electrical self-healing, which are favorable in active working scenarios. This series of work overcomes key limitations in wearable TEDs, demonstrated through powering wearable sensors, charging batteries, and illuminating LEDs by scavenging body heat at room temperature. The third study addresses long-term wearing comfort and breathability at skin-device interfaces. Electrohydrodynamic (EHD) printing is employed to deposit a liquid metal-boron nitride-thermoplastic polyurethane (LM–BN–TPU) hybrid filler composite as a high-resolution fiber mat with lattice patterns designed to avoid occluding sweat pores. Despite the presence of air and moisture transport pathways, the EHD-printed interface achieves a through-plane thermal conductivity of 0.37 W·m-1·K-1. This layer also functions as a micropatterned heatsink when integrated on the cold side of the devices. Skin compatibility is validated through water vapor transmission rates and a week-long attachment test on a forearm. The breathable TED generates 5.32 W at equilibrium under an initial temperature gradient of 10 °C. Furthermore, a comprehensive recycling process recovers LM and BN fillers from the permeable interfaces, and LM fillers and TE pellets from the core layer, respectively. These components are reused to fabricate new TEDs, which exhibit thermoelectric performance comparable to the originals. The fourth study develops recyclable liquid metal–vitrimer composites to address sustainability in flexible electronics. Vitrimer, a thermoset polymer with dynamic covalent bonds, combines the mechanical robustness of thermosets with the reprocessability of thermoplastics. At 50% LM volume fraction, the composite achieves a 6.53-fold increase in thermal conductivity while maintaining 137% failure strain, thermal stability above 360 °C, and electrical activation through mechanical sintering. Two distinct recycling strategies are demonstrated: (1) thermomechanical reprocessing, which enables circuit reconfiguration through consecutive fragmentation and reassembly; (2) chemical recycling, which recovers 94% of the LM filler to reuse it in new composites. Change in material properties during consecutive reprocessing and chemical interactions between the metallic filler and the polymer matrix are investigated to better understand the composites under repeated use. Collectively, by progressing from composite formulation and 3D device architecture to skin-compatible interfaces and sustainable end-of-life strategies, these four studies establish a body of knowledge for liquid metal composites and wearable thermoelectric devices.