Comparative Evaluation of Laser-Induced Graphene (LIG) Traces on Polyimide Under Soft and Hard Stress for IoT Applications

Laser-induced graphene (LIG) is an emerging manufacturing technique for engraving graphene patterns on precursor substrates. LIG has great potential applications to flexible eco-friendly and wrapping electronics devices, such as on-skin sensors and antennas for human body physiological monitoring. Moreover, LIG-made electronic labels could be embedded into innovative food packages to reduce contamination and waste. As the so-obtained conducting graphene traces form a non-homogeneous and partly volatile structure, it strongly interacts with the external environment. This article resumes a unitary experimental campaign to quantify the stability of the sheet resistance of LIG traces against several external stimuli, such as temperature and humidity gradients, contact with dry and wet objects, and cyclic bending. The results indicate that their effects depend on the tuning parameters of the laser and, in particular, on the power and beam defocusing. The sheet resistance variation during temperature and humidity stress is relatively modest and generally reversible by resorting to a 9 W beam with a few defocusing that generates a very compact and robust graphene substrate. The cyclic touch with dry and especially with wet objects can instead produce a more remarkable variation of the resistance and even permanent degradation of the surface depending on the specific lasing parameters. A conservative ±30% uncertainty of the nominal sheet resistance, hence, must be included in the electromagnetic simulations of LIG-based devices to account for unpredictable dynamic interactions with real-life objects.