Spatially varying wettability and resistivity in laser-induced graphene for flexible microfluidics, programmable heaters, and thermochromic displays

ABSTRACT Flexible microfluidic systems and patterned heaters are critical for applications ranging from point-of-care diagnostics and wearable thermotherapy to de-icing and thermochromic displays, yet most existing platforms rely on multi-step fabrication, metal integration, or external addressing electronics. Here, we present a purely laser-based, single-process strategy to spatially program wettability and resistivity in laser-induced graphene (LIG) on polyimide. By combining controlled beam defocus with multiple laser passes, we define adjacent single-pass (R1) and double-pass (R2) LIG regions with similar porous morphologies and Raman signatures, yet markedly different surface and electrical properties. Specifically, R1 exhibits near-complete wetting (contact angle < 7°), while R2 is comparatively more hydrophobic, and the sheet resistance is reduced to 12 Ω/□ in R2. The contrast is tuned via the raster gap between laser scan lines, with an optimal gap of 356 μm maximizing property differences without compromising continuity. This spatial contrast enables pump-free capillary-driven microfluidic transport, including vertical flow against gravity, as well as programmable electrothermal behavior, with temperature differences exceeding 50 °C between R1 and R2 under a single bias voltage. A three-dimensional finite element model incorporating volumetric Joule heating and convective–radiative losses accurately reproduces the measured infrared temperature distributions. Finally, integration of commercial thermochromic pigments yields a flexible thermochromic display in which discrete pixels are addressed solely by the underlying LIG resistivity pattern, without additional electrodes or control circuitry. This work establishes a scalable, maskless route to all-carbon, self-reporting flexible devices that integrate fluidic, thermal, and visual functionality through spatially programmed laser processing.