Chemoresistive LIG Sensors on Colorless Polyimide with Monolithic VIA for Enhanced Chemical Stability

Skin-interfaced wearable electronics are gaining significant attention in areas of reliable health monitoring and nondestructive therapeutics with growing interest in on-demand healthcare services that can offer precise real-time data and personalized medical descriptions against the aging society. Since those devices are directly attached to the human body, polyimide (PI) film is selected as a promising substrate with exceptional mechanical, chemical, and electrical properties in addition to its biocompatible characteristics when applied on the skin. Among the diverse utilization strategies of PI-oriented electronic devices, laser processing emerges as a one-step transformative approach to address tedious manufacturing procedures by enabling a facile fabrication of laser-induced graphene (LIG) using laser pyrolysis reaction. Recent notable results of the multifunctional uses of LIG, produced by the laser-induced pyrolytic method, include applications in electrochemical fields such as sensors, energy devices, and microheaters.<br/>In this connection, a transparent colorless polyimide (CPI) has been newly spotlighted for the potential to comprise imperceptible skin-interface devices while retaining all the existing advantages of native PI. The superior optical transmittance of CPI is accomplished by introducing the fluorine which effectively impedes the formation of charge-transfer complexes and loosens the polymer chain density. This optical transparency of the substrate is a pivotal aspect when integrated with any other functional systems since the clear vision of the target is crucial to assess the status when employed in remote monitoring of wound healing for instance. However, transparent materials present inherent challenges in laser-driven manufacturing due to their poor absorbance of light energy. To resolve this drawback, a breakthrough technique of successive laser pyrolysis (SLP) has emerged which strategically places an initiating point to trigger a sequential reaction through an opaque layer produced after the first pyrolysis reaction. Moreover, the SLP of transparent materials not only enables in-plane patterning but also facilitates out-of-plane modification as the beam reaches the target by passing through the transparent medium.<br/>Lately, the efforts of embedding conductive interconnection channels, representatively through-silicon via (TSV) and through-glass via (TGV), between multilayered electronic circuits are of great interest for their capability in the miniaturization of the device. This vertical integration allows solid connection of electrical components without wires exposed and reduces the length of wiring resulting in mechanical and electrical advantages. This technology is essential for skin-interfaced devices where size reduction and robust reliability are vital under harsh operating environments. In this study, we fabricated a monolithic structure of LIG sensor integrated with seamless vertical interconnect access (VIA) electrode channels between the opposite CPI film surfaces by utilizing the innate optical properties of the substrate and the pyrolytic product during laser processing. The proposed LIG VIA (LIG-V) simultaneously guarantees the physical and electrical connection of electrodes while protected from external threats of disconnection as firmly embedded in the substrate. As a result, the monolithic LIG-V sensor, with all the elements composed of LIG, is successfully applied in human-machine interface applications such as responsive sensing of surrounding chemicals and mutual input/output of heat stimulations based on its chemoresistive performance.