Kinetic Liquid Metal Synthesis of Flexible 2D Conductive Oxides for Multimodal Wearable Sensing

Transparent conducting oxides (TCOs) are crucial for high-performance displays, solar cells, and wearable sensors. However, their high process temperatures and brittle nature have hindered their use in flexible electronics. We report an approach to overturn these limitations by harnessing the physics Cabrera Mott native oxidation to fabricate large-area, two-dimensional transparent electrodes via liquid metal printing. Our robotic, solvent-free and vacuum-free process deposits ultrathin (2–10 nm thick) 2D indium tin oxide (ITO) with exceptional flexibility, high transparency (> 95%) and superior conductivity (> 1300 S/cm) for wearable bioelectrodes. In a significant advance over previous work, we utilize hypoeutectic In-Sn alloys to print 2D ITO at < 140 ºC on flexible polymers. Our detailed materials characterization and microscopy reveal the efficacy of Sn-doping and high crystallinity with large, platelike grains formed by the liquid metal reaction environment. The ultrathin nature of 2D ITO yields significant enhancement to bending strain tolerance, scratch resistance exceeding durability of traditional PEDOT, and low contact impedance to skin comparable to Ag/AgCl. Finally, we utilize the conductivity and transparency of 2D ITO for synchronous, multimodal measurements via electrocardiography (ECG) and pulse plethysmography (PPG). This order-of-magnitude improvement to printed TCOs could enable new wearable biometrics and display-integrated sensors.