Flexible Pressure Sensors Based on Laser-Induced Graphene
Authors: Wang Peng Chi , Kang Tzu-Ling , Lan Yu-Chieh , Tu Wei-Chen 
Published: December 2024

Graphene, with its remarkable attributes such as zero bandgap, broad-spectrum optical absorption, high carrier mobility, mechanical strength, and exceptional electrical conductivity, has gained significant interest across various fields including wearable technology, semiconductor components, and sensor technology. In 2009, Li et al. advanced the use of the chemical vapor deposition (CVD) technique, which is favored for its capability to produce high-quality graphene over large areas. This process involves introducing carbon-containing gases into a controlled environment within a chamber. However, the CVD method is hampered by its complex production scaling and the need for subsequent transfer of graphene to the desired substrate. An alternative approach, the SiC epitaxial growth method developed by Heer et al. in 2006, uses single-crystal silicon carbide as a substrate to synthesize graphene under ultra-high vacuum conditions (10-9torr) and at temperatures around 1300 ℃. Although this method has high potential for producing high-quality graphene, its high cost and significant safety concerns have limited its commercial viability. A more recent innovation is the development of laser-induced graphene (LIG) by Tour et al. in 2014. This technique allows for the rapid production of graphene with high porosity (340 m2/g), excellent thermal stability (up to 900℃), and impressive electrical conductivity (25 S/cm). LIG is created through laser irradiation that induces lattice vibrations and localized heating, causing covalent bonds to break and gases to be released, ultimately forming porous graphene structures. The process is rapid, does not require masks, and is environmentally friendly, making it particularly appealing for applications in flexible electronics and as an eco-friendly alternative. LIG's versatility has been demonstrated in a range of applications, from wearable electronics to supercapacitors, heaters, and biosensors. It can be synthesized on various substrates including polyimide films, plant material, paper, textiles, and other carbon-based materials. This method significantly broadens the range of material choices and facilitates modifications of substrate properties—from hydrophilicity to superhydrophobicity—through doping or control of processing parameters.

Sensitive pressure sensors trends favor compact, highly sensitive sensors that meet the requirements of personal wearable devices, robotic technology, military applications, and sports equipment. Electronic textiles, which provide superior tensile performance, are often combined with small-scale sensors to enhance functionality. Currently, products in this category typically suffer from limited design flexibility and involve complex manufacturing processes. However, textile electronic products created using LIG has great potential due to their ability to be mass-produced and intricately patterned. This patterning is achieved with high precision directly through the computer software controlling the laser machine, offering significant advantages in terms of production efficiency and scalability.

In the study, the fabrication of LIG into a flexible pressure sensor and evaluating its performance is reported. We use a 10.6 µm continuous-wave (CW) CO2 laser to fabricate LIG on carbon fiber substrates, with the aim of developing flexible strain sensors. This fabrication process is designed to not only enhance the mechanical strength of carbon fiber but also to impart electronic functionality. The study primarily focuses on adjusting three specific parameters: pulse per inch (PPI), power, and image density, with the finding that power significantly affects graphene formation more than the other variables. Through the optimization of these parameters, we have successfully produced high-density and high-quality graphene, achieving a reduction in sheet resistance by over 50% post-LIG formation, thereby enhancing the electronic performance of the materials. Additionally, variations in the contact angle among different types of laser-induced graphene samples illustrate differences in hydrophilicity and hydrophobicity, highlighting the functional versatility of LIG. The device also shows prompt response times and high sensitivity, enduring continuous bending without loss of functionality, which underscores its reliability. Experimental results further demonstrate that the sensor can detect cylinders of different radii, proving its potential in applications requiring flexibility and elasticity.