# Direct Fabrication of Ultrahydrophobic Laser-Induced Graphene for Strain Sensors
## 1 Introduction  
Fiber-reinforced polymer composites are progressively replacing conventional materials such as metals, alloys, and ceramics in aerospace, infrastructure, and maritime industries [1; 2; 3]. This is due to their mechanical properties, such as superior strength, high elastic modulus, thermal insulation, lightweight footprint, and corrosion resistance [4; 5; 6]. However, due to their complex structure, composite materials are prone to various structural failures in their matrix phase, reinforcing phase, and debonding [7; 8]. Additionally, for composites, failure accumulation occurs over some time. Therefore, performing structural health monitoring of the composite material is crucial to schedule timely maintenance and prevent any catastrophic failure while in application [9]. Commercially available strain gauges are generally based on metallic and semiconductor materials. Although these strain gauges are cost-effective and represent the commercial grade, they suffer from critical limitations such as poor mechanical flexibility, low sensitivity, and inability to function in extreme conditions such as those underwater [10; 11] As an alternative, piezoresistive materials such as carbon nanotubes (CNT) and Graphene/Graphene-oxide based materials are currently being investigated to fabricate strain sensors [12; 13; 14]. [PERSON] et al. fabricated a waterproof, flexible, and wearable strain sensor using CNT/Carbon-black [15]. In comparison to the chemical-based sensor fabrication approach, laser material processing for surface texturing/chemical modification has attracted tremendous interest recently as a cost-effective approach [16; 17]. Rapid fabrication along with the possibility for selective surface patterning with laser represents a more scalable approach [18; 19; 20]. In this frame of reference, laser-induced graphene (LIG) from polymer precursors such as polyimide (PI) are being increasingly studied as an alternative means to fabricate strain sensors [21; 22; 23; 24]. As strain sensors also involve outdoor applications, the LIG material must be functionally stable under extreme conditions such as high humidity or water immersion [25; 26]. Towards this consideration, the natural water repellency of the superhydrophobic surface could prevent the gradual functional degradation of the sensor under humid conditions. [PERSON] et al. fabricated by laser processing a porous graphene surface coated with fluorosilane to obtain a superhydrophobic and efficient anti-icing and de-icing surface [27]. [PERSON] et al. directly fabricated a superhydrophobic LIG by performing laser ablation in an argon environment [28]. Although the above-mentioned methods achieve excellent water-repellent properties, the scalability and simplicity in fabrication, at the core of LIG-based sensors and functional surfaces, remains a challenge. In certain applications, such as electrochemical sensing, there is a need for the fabrication of miniaturized electrodes [29]. In such cases, the comparatively large-size features fabricated by CO\({}_{2}\) lasers could be disadvantageous, whereas UV lasers are relatively expensive. In this context, NIR industrial fiber laser systems with high tunability in laser parameters such as pulsewidth, pulse repetition rate, average power, and scan speed offer greater flexibility to the fabrication process. These systems enhance precision surface modification due to their relatively short wavelength, thanks to their tunable low-pulsewidth in the nanosecond regime. Few researchers have taken advantage of the tunability of laser writing parameters to minimize the surface oxidation of exposed LIG to maintain its hydrophobic property [30; 31; 32]. The hydrophobicity provided by the surface carbon content along with the rough and hierarchical surface texture of LIG could result in surface superhydrophobicity. These studies successfully demonstrate a one-step fabrication approach of superhydrophobic LIG.