Abstract:

Objective This study aims to address the limitations of conventional rigid temperature sensors, such as poor flexibility, complex fabrication, and unsuitability for wearable healthcare monitoring. By utilizing laser-induced graphene (LIG) technology on polyimide (PI) substrates, a new low-cost, scalable, and flexible temperature sensor is developed. The primary goal is to enhance sensing performance, mechanical flexibility, and long-term stability, thereby enabling real-time monitoring of human physiological signals in intelligent healthcare and wearable electronics.

Method PI films were pretreated in an alkaline solution to introduce oxygen-containing functional groups to facilitate the subsequent LIG formation. Using a CO₂ laser under optimized conditions for power and scanning speed, porous LIG patterns were fabricated on both PI and modified PI membranes. The prepared LIG was characterized by SEM, XPS, Raman spectroscopy, and sheet resistance tests. Finally, copper electrodes were attached with a conductive silver paste, and the device was encapsulated in polydimethylsiloxane (PDMS) to yield a flexible LIG-based temperature sensor.

Results Alkaline treatment significantly reduced PI surface roughness from 1.91 nm to 0.269 nm and enhanced hydrophilicity, facilitating more uniform LIG formation. SEM images revealed a porous 3D graphene structure with improved uniformity in modified PI-LIG. XPS and Raman analyses confirmed higher graphitization and reduced oxygen content in modified samples, with I_D/I_G ratio decreasing from 1.83 to 0.83. The optimal LIG exhibited a sheet resistance of 18 Ω/□ at 40% laser power and 550 mm/s scan speed. The sensor demonstrated a linear resistance-temperature relationship from 25-75 ℃, with a temperature coefficient of resistance 0.134%/℃ and excellent linearity (R²=0.997 3). It showed rapid response and recovery times, high repeatability over 10 cycles, and stable performance over 10 d. Applications included real-time monitoring of breathing patterns (slow, normal, and rapid breathing) and skin temperature at various body sites (forehead, wrist, and knee), with accurate and consistent readings matching physiological ranges.

Conclusion This research demonstrates a facile and efficient method to fabricate high-performance flexible temperature sensors using LIG technology on alkali-modified PI substrates. The developed device combines excellent linear sensitivity, fast response, repeatability, and long-term stability with low-cost, scalable manufacturing. Its proven ability to monitor both body temperature and respiratory behaviors indicates strong potential for integration into wearable electronics, smart healthcare systems, and personalized medical monitoring. In future work, sensor miniaturization, multi-signal integration, and wireless data transmission may further expand its application prospects, paving the way for advanced intelligent healthcare platforms.

Key words: laser-induced graphene, polyimide, temperature sensor, health monitoring, sensing performance, electrical conductivity, flexible sensor