Liquid-Metal-Printed Ultraconductive Two-Dimensional Oxides for Multimodal Wearable Bioelectrodes

Conducting oxides are a critical high-performance material for next generation displays, solar cells, and wearable sensors, but their high process temperatures and poor mechanical properties have limited use in printed flexible electronics. We propose to overturn these limitations of inorganic oxides via ultrafast, low-temperature fabrication of durable wearable electrodes via liquid metal printing. We demonstrate solvent-free, vacuum-free printing of highly flexible two-dimensional (2D) conducting oxides for wearable multimodal bioelectrodes. Our robotic liquid metal printing process based on Cabrera Mott surface oxidation deposits ultrathin (2 – 10 nm thick) 2D indium tin oxide (ITO) with excellent transparency (> 95%) and superlative conductivity (> 1300 S/cm) on par with sputtered films. In a significant advance over past printed oxides, we leverage hypoeutectic In-Sn alloys for printing 2D ITO at less than 140 °C, allowing deposition on polymer substrates including PET, PEN, and polyimide. XPS, XRD, AFM, and HRTEM characterization reveals the efficacy of Sn-doping, the highly crystalline nature of 2D oxides, and the large, platelike grains formed by the liquid metal reaction environment. We further demonstrate a significant enhancement to bending strain tolerance for 2D ITO (down to 2 mm bending radius), high scratch resistance exceeding the durability of traditional PEDOT electrodes, and low contact impedance to skin as a wearable bioelectrode compared with control Ag/AgCl gel electrodes. Finally, we utilize the conductivity and transparency of 2D ITO for synchronous, multimodal measurements via electrocardiography (ECG) and pulse plethysmography (PPG). Collectively, these results represent an order of magnitude improvement in the performance of printed metal oxides and introduce a promising material set for display-integrated electrodes and multimodal biometrics.