Coating inkjet-printed traces of silver nanoparticle (AgNP) ink with a thin layer of eutectic gallium indium (EGaIn) increases the electrical conductivity by six-orders of magnitude and significantly improves tolerance to tensile strain. This enhancement is achieved through a room-temperature "sintering" process in which the liquid-phase EGaIn alloy binds the AgNP particles (≈100 nm diameter) to form a continuous conductive trace. Ultrathin and hydrographically transferrable electronics are produced by printing traces with a composition of AgNP-Ga-In on a 5 µm-thick temporary tattoo paper. The printed circuit is flexible enough to remain functional when deformed and can support strains above 80% with modest electromechanical coupling (gauge factor ≈1). These mechanically robust thin-film circuits are well suited for transfer to highly curved and nondevelopable 3D surfaces as well as skin and other soft deformable substrates. In contrast to other stretchable tattoo-like electronics, the low-cost processing steps introduced here eliminate the need for cleanroom fabrication and instead requires only a commercial desktop printer. Most significantly, it enables functionalities like "electronic tattoos" and 3D hydrographic transfer that have not been previously reported with EGaIn or EGaIn-based biphasic electronics.
Stretchable high dielectric materials are crucial for electronic applications in emerging domains such as wearable computing and soft robotics. While previous efforts have shown promising materials architectures in the form of dielectric nano-/micro inclusions embedded in stretchable matrices, the limited mechanical compliance of these materials significantly limits their practical application as soft energy harvesting/storage transducers and actuators.Here, we present a class of liquid metal (LM)-elastomer nanocomposites with elastic and dielectric properties that make them uniquely suited for applications in soft-matter engineering. In particular, we examine the role of droplet size and find that embedding an elastomer with a polydisperse distribution of nanoscale LM inclusions can enhance its electrical permittivity without significantly degrading its elastic compliance, stretchability, or dielectric breakdown strength. In contrast, elastomers embedded with microscale droplets exhibit similar improvements in permittivity but a dramatic reduction in breakdown strength.The unique enabling properties and practicality of LM-elastomer nanocomposites for use in soft machines and electronics is demonstrated through enhancements in performance of a dielectric elastomer actuator and energy harvesting transducer.
Elastomers embedded with droplets of liquid metal (LM) alloy represent an emerging class of soft multifunctional composites that have potentially transformative impact in wearable electronics, biocompatible machines, and soft robotics. However, for these applications it is crucial for LM alloys to remain liquid during the entire service temperature range in order to maintain high mechanical compliance throughout the duration of operation. Here, we introduce LM-based functional composites that do not freeze and remain soft and stretchable at extremely low temperatures. We show that the confinement of LM droplets to micro-/nanometer length scales significantly suppresses their freezing temperature (down to-84.1 C from-5.9 C) and melting point (down to-25.6C from +17.8C) independent of the choice of matrix material and processing conditions. Such a supercooling effect allows the LM inclusions to preserve their fluidic nature at low temperatures and stretch with the surrounding polymer matrix without introducing significant mechanical resistance or inducing internal stress concentrations. These results indicate that LM composites with highly stabilized droplets are capable of operation over a wide temperature range and open up new possibilities for these emerging materials, which we demonstrate with self-powered wearable thermoelectric devices for bio-sensing and personal health monitoring at low temperatures.
Stretchable thermoelectric generators (TEGs) capable of harvesting electrical energy from body heat under cold weather conditions have the potential to make wearable electronic and robotic systems more lightweight and portable by reducing their dependency on on-board batteries. However, progress depends on the integration of soft conductive materials for robust electrical wiring and thermal management. The use of thermally conductive soft elastomers is especially important for conforming to the body, absorbing body heat, and maintaining a temperature gradient between the two sides of the TEGs in order to generate power. Here, we introduce a soft-matter TEG architecture composed of electrically and thermally conductive liquid metal embedded elastomer (LMEE) composites with integrated arrays of n-type and p-type Bi 2 Te 3 semiconductors. The incorporation of a LMEE as a multifunctional encapsulating material allows for the seamless integration of 100 thermoelectric semiconductor elements into a simplified material layup that has a dimension of 41.0 × 47.3 × 3.0 mm. These stretchable thermoelectric devices generate voltages of 59.96 mV at Δ10 °C, 130 mV at Δ30 °C, and 278.6 mV and a power of 86.6 μW/cm 2 at Δ60 °C. Moreover, they do not electrically or mechanically fail when stretched to strains above 50%, making them well-suited for energy harvesting in soft electronics and wearable computing applications.
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