Advanced electronics with the combined attributes of multifunctionality, high integrability and flexibility are becoming increasingly important due to the development of wearable and flexible electronics. Tremendous efforts have been devoted toward pursuing the functional materials with an inherent flexible or stretchable property, or optimizing the geometrical configurations to meet the complex target shapes. Additionally, the fabrication of the highly conductive material in a fast, low-cost, efficient, and environmentally friendly method always stands out as the major requirement for flexible electronics. Recently, graphene, a two-dimensional material, has been of growing interest due to its excellent electrical conductivity, mechanical, optical, and thermal properties
[1]. They have found their potentials in many fields ranging from wearable electronics, health monitors, motion captures, to soft robots. The graphene has been successfully fabricated through a variety of methods, including mechanical exfoliation
[2], chemical vapor deposition (CVD)
[3], and chemical reduction of graphene oxide (GO)
[4][5]. Mechanical exfoliation can obtain larger sizes and high-quality graphene. However, this method is only suitable for scientific research, and its production efficiency is too low to be used in large-scale production. In addition, sonication assisted liquid-phase exfoliation can successfully exfoliate graphite in liquid environments by exploiting ultrasound to extract individual layers. This method can obtain high-quality graphene inks with a high production efficiency using the common equipment in labs. However, the production of graphene through this fabrication method produces a large amount of waste and requires long lasting sonication treatments
[6]. CVD is one of the ideal methods for large-scale graphene production and can be used to prepare high-quality single or multi-layer graphene, nevertheless, it is difficult to popularize due to strict experimental conditions and multi manufacturing steps limitations. Chemical reduction of GO is a low-cost and high production efficiency method, but there are toxic gas and explosion hazards in the preparation process. However, the multi manufacturing steps, the sophisticated operation procedure, the cost-effective synthesis and patterning of carbon nanomaterials of these manufacturing as mentioned above techniques impose certain limitations on the practical applications, and it is still challenging to fabricate and pattern the graphene through an in situ, one-step and scalable approach. In contrast to chemical reduction of GO, irradiation of the GO film with an infrared laser inside an inexpensive commercially available LightScribe CD/DVD optical drive, reduces the GO to laser-scribed graphene
[7]. Laser equipment has been popular in factories and laboratories, and it can be used for precisely and rapidly fabricating patterns for various functional applications. Laser direct write technology has been demonstrated as a reliable, mask-free and template-free method
[8][9]. The researchers accidently discovered that the PI could be directly converted to porous graphene using an infrared CO
2 laser while cutting PI films in 2014
[10]. The technology of photothermally converting organic films to continuous 3D porous graphene structures by pulsed laser irradiation under ambient air is known as the formation procedure of laser-induced graphene (LIG). This method has a high production efficiency, but only generates multilayer graphene. The underlying mechanisms mainly involve the carbonization occurring on the surface of the PI substrates under laser scribing
[11][12]. The sharp rise in the localized temperature due to the laser irradiation breaks the C–O, C=O, and N–O bond and leads to the recombination of C and N atoms
[13]. In this method, any desired complex graphene pattern can be directly formed on the carbon sources films, facilitating the fabrication procedures for the individually customized electrical devices. Ascribing to its advantages of the mask-free and visual fabrication process, numerous efforts have been devoted to improving the preparation process and enriching the accessible materials for LIG. Recently, LIG have been successfully prepared by a variety of laser sources according to the properties of the initial carbon precursor. Energy storage, catalysis, sensing and biomedical applications also have been realized by controlling the microstructure, doping amount and type, as well as post-deposition methods
[14][15]. Recently, the visible and ultraviolet (UV) lasers also have been demonstrated in successfully preparing LIG on PI substrate
[16][17][18][19][20]. In addition to pulsed lasers, continuous wave (CW) laser beam can also be used to fabricate LIG
[21]. LIG fabricated by CW lasers exhibit optical anisotropy. The detected anisotropy is due to the specific orientation of the graphene-containing formations relative to the PI/LIG interface during the LIG formation. Furthermore, a variety of natural and synthetic materials, ranging from plants
[22][23][24], textiles
[24][25][26][27], papers
[28][29][30] to other organic films
[17][27][31][32][33][34], are experimentally demonstrated in serving as the carbon source to form LIG. Meanwhile, the conductivity, electrochemical performance
[24][27][35][36], biocompatibility
[37][38], and hydrophobicity
[39][40][41] of LIG also have been systematically studied. A variety of LIG devices have been developed, including sensors
[14][15][16][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42], supercapacitors
[17][43][44][45][46][47][48][49][50][51][52][53][54][55], nanogenerators
[54][55][56][57][58][59][60][61], heaters
[28][29][62][63][64][65][66][67], catalysts
[68][69][70][71][72], actuators
[38][73][74][75] and batteries
[45][68][69][70][72][76][77]. This review mainly focuses on the recent progress of LIG based flexible electronics, including the mechanical sensors monitoring the motion of the human body, the temperature or humidity sensors monitoring the environmental changes, the electrochemical electrodes connecting the electrical components, and the heaters that can control the bio-inspired actuators. The research progress in related fields is summarized through the introduction of these devices. Finally, we provide some perspectives on the remaining challenges and opportunities of LIG in terms of preparations and practical applications.