Investigation of Electromechanical Reliability and Radio Frequency Performance of a Highly Stretchable Liquid Metal Conductor for Wearable Electronics

Abstract

Liquid metal-based gallium conductors exhibit unique physical and electromechanical properties, which make them excellent candidates for the next generation of wearable electronics. In this study, a novel fluid phase-based gallium conductor was stencil printed on thermoplastic polyurethane (TPU) to fabricate a stretchable conductor as well as a stretchable radio frequency (RF) transmission line. The electromechanical reliability of the conductor during high elongation as well as cyclic tension and bend fatigue was evaluated and compared with commercially available stretchable silver-filled polymer paste. The microstructure of the liquid metal conductor and the silver paste was investigated via scanning electron microscopy (SEM) before and after the samples were subjected to high elongation (>100%). Unlike the silver paste, the liquid metal conductor maintained its microstructural integrity while its resistance showed a linear response to changes in length. A cyclic tension fatigue test confirmed the fatigue-free performance of the liquid metal conductor during 8000 stretching cycles at a strain amplitude of 30%. The electromagnetic structure of the RF transmission line was simulated and then compared to the measured data. The measurements for insertion loss showed that U-bending, 90 deg twisting, and 1000 stretching cycles at a strain amplitude of 100% did not have a significant impact on the RF performance. Details of the DC tests and RF measurements, including the microstructural analysis and simulation results, will be discussed in this article.