Thermal Reflow Transfer Printing of Ultra-Thin Metal Conductive Layer for Flexible Sensors on Fabric Substrate

Fabric substrates offer mechanical flexibility, air permeability, and textile compatibility, making them ideal for wearable sensors. However, their porous and irregular surfaces pose challenges for integrating ultra-thin, highly conductive metal layers, which are essential for electrical conductivity and signal transmission in flexible electronics. Conventional techniques struggle with precise deposition and uniform coverage on such substrates. Here, we present a thermal reflow transfer printing technique using a caramel-corn syrup mixture as a reflowable transfer medium. Under mild heating, this sugar-based stamp transitions into a rubbery state, enabling the metal layer to conform seamlessly to the fabric surface. Using this method, we successfully printed large-area 30 nm-thick ultra-thin gold electrodes onto fabric substrates, achieving low resistivity of $6.0\times 10^{-{8}}~\Omega \cdot $ m and outstanding mechanical flexibility under bending. As application demonstrations, we fabricated and tested humidity and pressure sensors using the transferred interdigitated gold electrodes on fabric substrates. The measurements confirm the good sensitivity, reliability, and stability of these sensors. The versatility of the transfer printing method, combined with the outstanding properties of the transferred metal layers, makes it a promising solution for the development of next-generation wearable sensing technologies.