TLC-Embedded PDMS-Based Wearable Skin Patch for Vein Visualization Applications

A flexible skin patch based on thermochromic liquid crystals (TLCs) embedded within a polydimethylsiloxane (PDMS) elastomer is presented for passive visualization of skin-vein thermal contrasts. Five ternary TLC formulations composed of cholesteryl oleyl carbonate (COC), cholesteryl nonanoate (CN), and cholesteryl benzoate (CB) were systematically engineered to tune mesophase stability and helical pitch sensitivity within the physiological temperature range (32-37 °C). Small compositional adjustments (≤1 wt.%) produced pronounced and predictable shifts in color-play bandwidth and optical sensitivity, governed by thermally induced contraction of the cholesteric helix. Patterned TLC microdomains were integrated onto optically absorptive PDMS substrates and encapsulated within a transparent PDMS overlayer, yielding thin, mechanically compliant, and breathable films with uniform thermal transmission. Optical spectroscopy, optical microscopy, and RGB analysis revealed linear wavelength-temperature relationships (R² > 0.9) and red-to-green sensitivities of up to ~87 nm °C^-1, demonstrating high thermochromic sensitivity within the physiological skin temperature range. Mechanical characterization confirmed skin-matched elasticity, while preliminary sterilization studies indicated that the films retained their mechanical properties following ethanol immersion and UV exposure. As a feasibility study, the results demonstrate the potential of a compositionally tunable thermochromic elastomer platform for visualizing physiological skin temperature variations associated with superficial veins, providing a basis for further development toward wearable vein visualization for self-cannulation in home hemodialysis and related thermal sensing applications.