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 CO₂ 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.

Since the Tour’s group first prepared porous graphene films with three-dimensional networks from commercial polymer films using a CO₂ infrared laser [10]. The PI thin film is still the most common carbonaceous substrate for LIG. The sharp rise of the localized temperature into the PI substrate due to the laser irradiation breaks the C–O, C=O, and N–O bonds and leads to the recombination of the C–C bond. In other words, the sp3-carbon atoms are photo-thermally converted to sp2-carbon atoms by pulsed laser irradiation. The formation of porous graphene morphology is due to rapid outgassing from the PI melt. This laser inducted carbonation process can be divided into four stages: PI sheets, sheet nanostructure breaking into fibers, graphitization occurs in plates and fibers, and transition from fiber to carbonized droplets [13]. Numerous efforts have been devoted toward experimentally analyzing the effects of the manufacturing parameters on the morphology and electrical properties of LIG on PI films. They suggest that laser wavelength, output power, focal length, and pulse distribution all affect the performances of LIG [10,13,24,27,35,36]. The LIG with desired morphology and resistance can be prepared purposefully for its various design parameters. Additionally, the LIG has also been successfully realized on multiple natural and synthetic materials. As shown in Figure 1, diverse substrates such as plants (i.e., woods, leaves, potato skins and coconut shells) [22,23,24], fabrics (i.e., Kevlar and silk) [24,25,26,27], papers (i.e., printing paper and PI paper) [28,29,30], and polymers (i.e., polyethylene terephthalate and phenolic resin) are transformed into graphene directly by laser irradiation [17,27,31,32,33,34]. The exceptional design ability in the electrical performances and the wide selection of carbon precursors demonstrate the potential of LIG in the fields such as flexible, large-scale, and biodegradable electronics.

Since the LIG-based capacitor was first realized in 2014 [10], improved precursor materials and structural designs have been explored to prepare LIG and LIG-based devices with controlled morphologies and tailored properties. This section mainly overviews the LIG fabricated on various carbonaceous substrates and diverse applications of LIG-based devices. By introducing these outstanding research results, the multi-functional applications of LIG ascribing to its unique advantages of a high degree of designing freedom is demonstrated. Figure 2 depicts the applications of LIG-based electrical systems in different fields, ascribing to its free-designability, the multi-substrate adaptation, and excellent electrical properties. Figure 2a illustrates the LIG-based strain gauges with a high sensitive and wide tunable range under the irradiation of the ultraviolet laser [18]. Compared with previously reported LIG formed under the irradiation of the infrared laser, LIG prepared by ultraviolet laser has a smaller minimum line-width down to 50 µm than that achieved by CO₂ infrared lasers (i.e., 100 µm) [78]. The stable and high-quality radial pulse and carotid pulse rate signals were recorded immediately by this LIG-based strain gauges after adhered onto the wrist and neck of a volunteer. Figure 2b gives a novel flexible asymmetric pressure sensor composed of multi-walled carbon nanotubes (MWCNTs) and LIG [79]. Benefiting from the microstructure of 3D porous LIG nanosheets, the pressure sensor exhibits some specific features such as ultra-sensitivity (i.e., minimum to 1.2 Pa), super rapid response recovery (i.e., about 2 ms), and good durability (i.e., >2000 cycles). And the high-performance pressure sensors can detect various subtle human motions (i.e., breath, vocal vibration, and wrist pulse) in real-time. Additionally, the integrated pressure sensor array can simultaneously monitor multiple points with varying degrees of applied pressure. Figure 2c shows an exquisite design, a versatile LIG-based integrated, flexible sensor system, which can wirelessly monitor the sleeping postures, respiration rate, and humidity of the diaper moisture [80]. The unique tilt sensor takes advantage of the super-hydrophobic surface of LIG. Notably, the liquid metal that can roll freely on its surface connects different LIG circuits to identify the tilt direction according to different tilt angles. The tilt sensor confining a liquid metal droplet inside a cavity can track at least 18 slanting orientations. The earliest application of the LIG-PI system as a supercapacitor is realized by Tour’s group [10]. On this basis, as shown in Figure 2d they have improved the micro-supercapacitors (MSC) with interdigitated electrodes using a hybrid composite of LIG [49]. A highly stable and linear LIG-based flexible temperature sensor capable of delivering the real-time monitor of human skin temperature was shown in Figure 2e [81]. Benefiting from the fast electron transfer between the Cu nanoparticles and the porous graphene, a novel LIG-based glucose biosensor provides a new platform for the fabrication of the flexible non-enzymatic glucose sensors (as shown in Figure 2f) [82]. Additionally, the LIG represents a class of triboelectric materials that can be used as a triboelectric generator (TENG) converting mechanical energy into electrical energy. As a passive electronic device that bypasses the limitation of power supply or battery, TENG shows an exceptional promise in the wireless device and the wearable sensing electronic [54,55,57,59]. As shown in Figure 2g, the flexible triboelectric sensing array comprises a touch panel with 9-digital channels based on a LIG-patterned TENG [58,60]. To bypass the constraints from the underlying robustness polymer substrate and the delamination by wear and tear [83,84,85,86,87,88,89,90], LIG was transferred from the substrate by polydimethylsiloxane (PDMS). Notably, the LIG adhered onto the soft and stretchable PDMS film yields flexible, durable, and high-performance electrocardiography monitoring dry electrodes (as shown in Figure 2h) [85]. Figure 2i gives the demonstration (i.e., a LIG-based 3D flower) of the LIG actuator composed of a PI thin film and LIG/PDMS coating. Herein, the LIG serves as a flexible heater to increase the temperate via joule-heating. The thermal strain mismatch between PI and PDMS can induce bending deformations of the actuators toward the PI side [73].