Since its discovery in 2014, laser-induced graphene (LIG) has shown great prospects in the field of wearable devices and systems ^[1]. This graphene material has a flexible three-dimensional (3D), porous structure and possesses excellent electrical and mechanical properties. The maskless direct laser writing process makes the material suitable for large-scale, low-cost device fabrication [2,3]^[2][3]. Moreover, because LIG has few constraints regarding its substrate material and growing environment, it is a promising candidate to meet the massive demand for high-performance wearable devices for applications such as health care and HCI [4,5,6]^[4][5][6].

The first trend encompassed the multifunctionality of the devices. The reason for their development was to meet the requirements of long-term daily usage when applied to health care and HCI. Such requirements can generally not be met using a single signal or a single function, so the development of devices has gradually shifted from a device level to a system level [10,11,12]^[7][8][9]. Wearable devices can now not only perform individual functions such as sensing or actuating, but they are also becoming integrated systems that can realize multi-mode sensing [13^[10][11],14], and are even endowed with a power supply [15]^[12] and signal acquisition–processing–transmission–feedback [16]^[13]. The second main trend treats intelligent systems. Wearable devices are becoming smarter systems that can not only realize basic signal processing functions but can also use intelligent algorithms for disease diagnosis or intelligent human–computer interaction (HCI) [17,18]^[14][15]. These two development trends are not independent of each other. At present, quite a number of research reports on wearable systems for health care and HCI comprehensively consider endowing devices with multifunctional and intelligent characteristics to achieve systems more in line with the requirements of the applications [19,20,21]^[16][17][18].

2. LIG-Based Wearable Systems for Multifunctional and Intelligent Health Care

When applied to sensors, LIG is effective for monitoring a range of physiological signals such as electrophysiological signals, temperature, electrochemical signals, and mechanical signals. Recently, there has been a growing interest in researching multifunctionality in LIG-based devices due to the increasing demand for systematization. This is primarily due to the following reasons. First, many diseases and health conditions are evaluated based on a combination of physiological signals, so multi-mode physiological signal monitoring is crucial for LIG-based wearable systems. Second, LIG-based wearable systems are becoming “portable doctors” to meet the needs of long-term health monitoring in daily life. This means that LIG-based wearable systems can not only collect physiological information, but also process or transmit physiological information in situ, and even realize the diagnosis and treatment of diseases. This kind of application demand puts forward the requirements of integrated, all-in-one, self-powered, and intelligent LIG-based wearable systems [22,23,24,25]^{[19][20][21][22]}.

2.1. Gas-Permeable and Multifunctional On-Skin System

The LIG-based wearable system must be designed to accommodate the daily wear and tear it will face without hindering the wearer’s daily activities while also minimizing the impact of the wearer’s activities on the functionality of the system. If the on-skin wearable system is gas-permeable, it can avoid interfering with the perspiration of the wearer, which may cause redness or inflammation, and can improve comfort during daily long-term wearing. To make the LIG-based wearable system gas-permeable, Sun et al. proposed a gas-permeable and multifunctional on-skin wearable system based on a sugar-templated porous substrate [10]^[7]. A variety of LIG-based devices including electrophysiological signal electrodes, hydration sensors, temperature sensors, and joule-heating elements were realized by CO₂ laser writing on polyimide (PI). By designing open-mesh patterns and serpentine layouts, the sensors adapt to the stretching of the human skin during wearing. To achieve the gas-permeability, they used a soft silicone elastomer (Young’s modulus ~170 kPa) and cane sugar powders to make the porous elastomer substrate. The silicone elastomer was mixed with the sugar powder at a 1:2 weight ratio. When the mixture was partially cured, the LIG-based devices on the PI substrate were transferred onto the elastomer–sugar composite substrate through direct transfer printing with added pressure. 

2.2. Electronically Enhanced Paper-Based Green Wearable System

The research of wearable systems has recently focused on the development of degradable green electronic devices. Some studies have attempted to replace non-degradable materials like PI with degradable ones to mitigate the environmental impacts of LIG-based wearable systems [3,4]^[3][4]. Although precursors like paper sheets or cardboard, which consist of aliphatic-rich cellulose, can serve as templates for graphitization, they do not possess the intrinsic aromatic carbon structure that can help the graphitization process. As a result, LIG made from cellulose-rich precursor materials does not perform electrical and chemical properties as well as LIG using common polymer precursors. Faced with this challenge, Pinheiro et al. increased the conductive properties of paper-based LIG by introducing external aromatic moieties and controlling the laser fluence meticulously [11]^[8]. The manufacturing processes of the paper-based wearable system are as follows. First, the wax-treated paper substrate is irradiated using a laser with different spot-size focus profiles to depolymerize, deoxygenize, and dehydrate the aliphatic carbon rings within the cellulose and the hydrocarbon and aromatic structures in the wax. This pyrolysis process leads to the graphitization of the paper-based substrate and produces LIG. Second, once the LIG-based devices are made through laser treatment, they are separated from the paper substrate through deionized water as the cellulose fibers and fibrils have a higher affinity for water due to the hydrogen bonds they contain. The LIG-based devices are then transferred onto a medical-grade polyurethane tape with a polyacrylate glue surface, resulting in the creation of a multi-functional LIG-based wearable system.

2.3. Self-Alarm Health-Monitoring System

Every year, numerous deaths are caused by sudden illnesses such as cardiovascular diseases and sleep apnea, which can occur unexpectedly in daily life. While wearable health monitoring systems can track important physiological signals of the user in real-time, they lack the ability to provide feedback or warnings. This issue may result in patients missing vital opportunities for rescue. When used as strain sensors, these LIG-based devices are capable of detecting pulse signals on the human wrist, revealing clear percussion (P), tidal (T), and diastolic (D) waves. Additionally, when used as a sensor on a facial mask, it can continuously monitor respiration signals during sleep. With its high sensitivity, the strain sensor is able to accurately detect muscle movements and voices with high resolution. When attached to the human throat, the sensor can distinguish different words spoken by the wearer while recoding laryngeal movements such as coughing and swallowing. When used as an acoustic device, the LIG-based device can emit sound with a wide frequency range (200~20 kHz) under the excitation of alternating voltage based on the thermoacoustic effect of graphene, and the sound pressure level (SPL) can reach over 60 dB between approximately 15 and 20 kHz.

2.4. Self-Powered Sensing System

Wearable systems have consistently been the focus of research due to the need for a stable energy supply. For a LIG-based wearable system to be used for extended periods of daily life, it must possess a dependable self-powering capability. While some studies have shown that combining a stretchable energy harvester, power management circuit, and energy storage unit can lead to self-charging capability [26,27]^[23][24], many systems still struggle with low and inconsistent energy output, particularly during body movements that cause deformation. To realize a simple and stable self-powered health monitoring system, Zhang et al. proposed a LIG-based self-powered sensing system driven by human motion [15]^[12]. The 3D porous structure of LIG with high specific surface area and excellent charge transport ability provide effective flows of triboelectric electrons in the triboelectric nanogenerator (TENG). To enhance energy density, the LIG foam can be surface coated or doped to form MSC arrays. For instance, the addition of Au nanoparticles in LIG results in a TENG with an output voltage of 320 V, and the output power was as high as 0.022 W, even when subjected to a strain of 40%, which exceeds the stretching limit of human skin. The LIG-based MSC arrays have a PVA/H2_SO₄ gel electrolyte and the LIG foam acts as the active material, the current collector, and the conductive substrate. The MSC arrays can deliver a maximum energy density of 0.569 μWh cm⁻² at a power density of 0.014 mW cm⁻². To achieve a stable energy supply module, a power management circuit was used to integrate the LIG-based TENG and the LIG-based MSC arrays.

2.5. Machine Learning-Based Multi-Modal Electrochemical Analytical System

The detection of multiple biomolecules is crucial in various fields including disease diagnosis, environmental monitoring, and food safety. However, it is challenging to use a single material such as LIG to accurately detect multiple biomolecules in a mixture. To address this challenge, Kammarchedu et al. proposed an electrochemical analytical wearable system that integrates LIG-based devices with machine learning algorithms. This system enables the multiplexed detection of tyrosine and uric acid in both sweat and saliva. The LIG-based electrochemical sensor was realized by irradiating a commercial CO₂ laser system. The three-electrode sensor consisted of a working electrode (eMoS_x-LIG), a counter electrode (LIG), and a reference electrode (LIG). The working electrode was made functional through the addition of MoS_x through electrodeposition, which led to a significant improvement in the sensor’s performance with a 3-fold increase in the electrochemically active specific surface area (ECSA) and a 1.5-fold increase in the heterogeneous electron transfer rate when compared to the bare LIG-based electrode. In order to fulfill the need for extensive electrochemical data in machine learning, a specialized sensor bracket was created to simultaneously gather signals from eight LIG-based sensors.

2.6. Electrothermally Controlled Soft Actuators for On-Demand Health Care

A comprehensive wearable health care system should not only track physiological signals, but also provide “physician-like intervention” to allow for real-time physiological signal monitoring, feedback, and treatment capabilities, similar to a medical professional. To address this requirement, Ling et al., proposed an electrothermally controlled, mechanically guided, 3D assembly actuator based on LIG [19]^[16]. The actuator consisted of three layers including the PI, LIG, and polydimethylsiloxane (PDMS). The LIG acts as a flexible heater to heat the PI layer and PDMS layer on demand. Due to the difference in the thermal expansion coefficient of PI and PDMS, the temperature change of LIG led to the deformation of the PI layer and PDMS layer in different degrees, thereby enabling the electrothermally controlled actuator to function. Inspired by kirigami and origami, the actuator can be customized to different shapes through precise structural design and modeling and forming a variety of actions and 3D structures under different control voltages. The LIG-based actuator could realize an excellent bending angle of 1080° and a bending curvature of 3.3 cm⁻¹. Furthermore, the actuator could bend from 0° to 180° in about 3 s and recover in about 4 s.

2.7. Multifunctional Sensing System with an Anti-Interference Design

Multifunctional sensing systems are of great significance in health monitoring. In order to accurately measure multiple physiological signals, it is crucial to eliminate any interference between them. Wang et al. invented a double-sided wearable system that both enables multi-modal sensing and avoids the interference between various signals [20]^[17]. The system allows for multi-modal sensing of the electrophysiological signals (ECG, EMG, etc.), mechanical stimuli/movements (strain, pressure, proximity), concentrations of sodium ions (Na⁺), hydrogen ions (H⁺), acetone gas, and nitrogen dioxide (NO₂), and has an energy collection and storage unit. The structure of the double-sided system enables the placement of skin-interacting sensors such as body temperature, electrophysiological, and ion sensors on the inside, while motion, room temperature, distance, and gas sensors, which interact with limbs and the external environment, are placed on the outside.

2.8. Integrated Multi-Modal Wireless System with Feedback Functions

For those who have limited ability to care for themselves such as the elderly, disabled, and infants, health monitoring systems that not only provide long-term daily use but also feedback and alarm functions are especially crucial. Considering the high rate of sudden infant death during sleep, Xu et al. developed a wearable system suitable for real-time health monitoring and the alerting of abnormal physical conditions of infants during sleep or in the intensive care unit [21]^[18]. The system can monitor the sleeping postures, respiration rate, and diaper moisture wirelessly, and immediately provide an alarm if there are any abnormal situations. To continuously track the baby’s sleep status with high comfort, the whole flexible system (in addition to mobile signal analysis software) was integrated into the diaper using a customized layout. The system consisted of three components: flexible sensor sheets, a flexible processing circuit with a Bluetooth module, and a mobile phone with a signal analysis and alarm module. The flexible sensor sheets contained three kinds of sensors including a tilt sensor, a respiration sensor, and a humidity sensor. The tilt sensor was used to record sleeping postures. This encapsulates a non-toxic gallium-based liquid metal droplet in a laser etching hollow space with PDMS and bare LIG electrodes and uses its slide to feel the baby’s postures. To sense the sliding posture of the liquid metal droplet, eight pairs of LIG-based electrodes were realized using a two-step laser direct writing process, where the rough surface of LIG allows the liquid metal droplet to slide freely on it. The attachment and separation between the electrode and the liquid metal droplet caused by the different tilt directions of the sensor belonged to the “on” and “off” states, respectively. Eight pairs of electrodes could distinguish the eighteen tilt angles.

3. LIG-Based Wearable Systems for Multifunctional and Intelligent HCI

Human–computer interfaces, which serve as the foundation for the interaction between humans and machines, have been extensively utilized in various fields such as robot control, virtual reality (VR), augmented reality (AR), and autonomous driving. The growing interest in the metaverse in 2021 spotlighted the crucial role of HCI systems. Similar to the health care system above-mentioned, HCI systems are also developing toward a more wearable, multifunctional, and intelligent design due to several factors. (1) The miniaturization of the HCI system, which is necessary for long-term usage in daily life, can be achieved through advancements in device-integrated technology. Especially with the development of flexible sensors, actuators, and circuits, more and more HCI systems are becoming flexible and even stretchable. On one hand, this enhances the comfort of human objects in wearing the device, while on the other hand, it makes machine objects more adaptable, particularly on irregular surfaces that are challenging for conventional HCI devices [28,29]^[25][26]. (2) HCI systems are becoming increasingly multifunctional and equipped with a variety of sensors, actuators, and other interactive modules. This trend enables HCI systems to perform a wider range of tasks and cater to a diverse array of application objects including humans, animals, plants, and robots as well as various application environments such as land and sea [30,31,32,33]^{[27][28][29][30]}. (3) The integration of system functions and the improvement in system integration has led to increased information processing demands for HCI systems. In response to this trend, HCI systems have become more intelligent, utilizing intelligent algorithms to enhance the collection, transmission, and analysis of information. Additionally, there has been an emphasis on improving the information processing efficiency through the integration of sensing, storage, and computing capabilities in HCI systems [12,13,16,34]^{[9][10][13][31]}.

3.1. LIG and Liquid Metal Integrated Multifunctional Wearable HCI System

The development of high-sensitivity motion detection is crucial for monitoring human movement, robot activity, and human–machine interaction. In particular, the development of HCI systems that can achieve highly sensitive motion interaction is of great significance for robot control, which can realize high-precision remote surgery, remote robot control, and so on. Babatain et al. developed an integrated multifunctional wearable HCI system for motion monitoring and human–machine interfacing through the clever combination of LIG and liquid metal [12]^[9]. The work also determined motion by detecting the contact between a sliding liquid metal droplet and the LIG electrode. However, the work improved the sensitivity of motion monitoring by using the principle of inertial sensing and improving the sliding ability of the liquid metal droplet on the PI-based flexible substrate. The LIG particles provided a 3D interpenetrated shell around the liquid metal droplet to prevent infiltration to laser-treated PDMS, PI, and LIG surfaces, which allowed the liquid metal droplet to slide in the sensing channel freely. Moreover, by designing the arc structure composed of the PI substrate, PDMS package, and LIG interfinger electrode, the liquid metal droplet could slide freely under the influence of acceleration and quickly return to the initial point thanks to gravity. This ensures accurate real-time acceleration monitoring and direct reading.

3.2. Electrooculography and Tactile Perception Collaborated 3D HCI System

Traditional HCI systems have limitations in their ability to perform well in multi-scene applications or 3D interactive experiences as they rely on a single type of device or signal. To address this issue, Xu et al. proposed an electrooculography (EOG) and tactile perception collaborated wearable HCI system that could realize accurate and convenient interaction in three dimensions [13]^[10]. The system consisted of two parts: the EOG interaction module and the tactile array module. Both LIG-based modules were made by transferring a laser-induced sensor pattern from the PI substrate onto a flexible and stretchable medical PU (polyurethane) adhesive film. The difference was that the EOG module used the LIG layer as the electrode and was in direct contact with the skin. The system, therefore, adopted a single-sided structure that was not encapsulated, while the capacitive 4 × 4 tactile array module consisted of two LIG-PU films stacked together in the same direction: one layer of PU film served as the encapsulation while the other was used as a dielectric layer. The resulting ultra-thin and soft PU substrate (45 μm) could be closely attached to the human skin for high-quality signal acquisition and comfortable wear.

3.3. The Machine Learning-Assisted Dual-Function Acoustic HCI System

Vocalization and hearing are important ways for people to communicate with the outside world; the ear and throat are natural “HCI systems” of humans. Due to its excellent acoustic properties, LIG has been increasingly used in the research of acoustic sensors and actuators [35,36,37]^[32][33][34]. However, it is a challenge to combine the function of “hearing” and “speaking” and apply it to wearable HCI systems. Sun et al. proposed a LIG-based dual-function transducer for human–robot interaction [16]^[13]. The whole system was composed of a LIG artificial ear based on a triboelectric sensor and a LIG artificial mouth based on a thermoacoustic effect actuator. The artificial ear and the artificial mouth were all realized by the same device structure including a PI film with single-sided LIG, a polyethylene (PET) ring spacer, and a PI film with double-sided LIG. The LIG has excellent mechanical, electrical, and thermal properties, making it suitable for multifunctional integration. Furthermore, the whole device has a low-cost advantage (~0.0036 USD) that is suitable for large-scale applications.

3.4. Intelligent Speech Recognition and Motion Control System

The LIG, being thin, flexible, and inexpensive, presents benefits when utilized in popular wearable and HCI systems. Based on the excellent electrical, thermal, and mechanical properties of LIG, Zhang et al. leveraged its advantages to produce cost-effective yet high-performing HCI systems, and developed an intelligent speech recognition and motion control system [18]^[15]. The fabrication process is as follows: a LIG-based sound detector was prepared on a 25 μm PI substrate with the help of a low-power laser machine (1.3 W). The porous structure of LIG is highly sensitive to vibrations caused by sound waves, making it an ideal material for sound detection. Furthermore, because of its good flexibility and electrical properties, LIG maintains good acoustic performance when it is attached to the curved surface of the human body or robot. Due to the principle difference between the LIG-based microphone and the traditional microphone, the research team designed an intelligent speech recognition algorithm based on one-dimensional CNN to efficiently and accurately recognize the language information received by the LIG-based microphone. To train the intelligent algorithm, the research team used the LIG-based microphone to record datasets of 10 numbers (0~9) and 20 commonly used sentences. By recording each word and sentence 50 times by the male and female, the team obtained 1500 speech samples. By using an end-to-end one-dimensional CNN, the important information in speech commands can be inferred. The intelligent algorithm successfully classified the sounds gathered using the LIG microphone with 98% accuracy.

3.5. Intelligent Artificial Throat

Most traditional sound sources and acoustic detectors are separated. However, to enhance integration and achieve a multifunctional acoustic HCI system, it is beneficial to incorporate both sound collection and generation functions into a single device. Facing this need, Tao et al. invented a LIG-based intelligent artificial throat that could achieve functions of emitting and detecting sound by using a single LIG film, and its sound-detecting range and sound-emitting range could cover the range of human hearing [28]^[25]. The artificial throat consisted of the LIG of the PI substrate made by the laser direct writing technique (450 nm laser). When the artificial throat was used as a sound source, it worked by using the thermoacoustic effects of the graphene. By applying periodic AC voltages to it, the LIG could produce a controllable sound in a wide frequency range (100 Hz~40 kHz), covering most of the frequency range of the human voice, and had good high-frequency characteristics. Additionally, research has revealed that the thinner the LIG, the greater the sound pressure level it can produce. By working as a strain sensor, the LIG device can be used as a sound receptor. It can distinguish vocalizations, hums, coughs, etc. by detecting the laryngeal vibration and laryngeal muscle movements with high sensitivity as well as identify different words and sentences spoken by different wearers.

3.6. Bioinspired Dual-Channel Speech Recognition System

Acoustic HCI systems based on microphones are often susceptible to interference from noisy environments, resulting in poor signal quality and further affecting the accuracy of HCI. To address this issue, Tian et al. designed a dual-channel speech recognition system using LIG that combined both EMG and mechanical signals [25]^[22]. When an expression is about to be produced, an action potential is generated in the motor cortex, and the electrical signal is transmitted to the epidermal muscles through the neurons and stimulates muscle fibers to contract. Ion channels in the muscle are opened to release the ions, and EMG electrodes attached to the skin can convert the ion current into the electrical current to acquire the EMG signal, while the deformation of the skin caused by the muscle contraction can be picked up by mechanical sensors. Therefore, both the EMG signal and the muscle movement signal of the larynx contain speech information, and combining the two signals for speech recognition can greatly improve the accuracy of speech analysis. These layers include a hydrogel layer that reduces skin-electrode contact impedance and eliminates noise interference on EMG signals, a LIG-based EMG electrode, a PI separation layer that separates the EMG electrode and the mechanical sensor to prevent mutual interference, a LIG-based mechanical sensor, a PI substrate layer, and a thin-film PU tape to encapsulate and attach the system to the skin. Each patch contains a pair of EMG electrodes and two mechanical sensors that monitor one-channel EMG signals and two-channel strain signals.

3.7. Humidity-Based HCI System for Health Care and Non-Contact Interaction

Currently, HCI systems based on LIG are mainly based on piezoresistive sensors, self-powered sensors, visual sensors, and acoustic sensors. While these sensors are effective in various HCI applications, they are not as reliable for monitoring respiration [38,39,40,41,42,43]^{[35][36][37][38][39][40]}. On the other hand, humidity sensors have a high level of reliability for respiratory monitoring, but there is limited research on HCI systems using humidity sensors. To address this, Zou et al. proposed a humidity-based HCI system that consisted of a LIG-based humidity sensor and LIG-based acoustic device [29]^[26]. The humidity sensor and the acoustic device were made on two sides of a Nomex fire-resistive paper. The acoustic device was made of LIG obtained by laser writing directly on the surface of the Nomex fire resistive paper, while the capacitive humidity sensor was made by a two-step process. The first step was to obtain a LIG-based interdigital electrode by laser writing directly on the paper. The second step was to apply a graphene oxide (GO) solution to electrodes to form a GO film. The flexible humidity-based HCI system can be attached to a medical mask and operates in the following manner: the humidity sensor side of the system is aimed toward the wearer for continuous monitoring of respiratory signals, and the acoustic device side of the system is the outer side to ensure the sound emitting effect. In the case of abnormal breathing patterns such as respiratory arrest or rapid breathing, the system will trigger an alarm through the acoustic device.

3.8. Soft and Stretchable Gesture HCI System

To realize a closed-loop HCI function, the wearable HCI system not only needs sensitive and multi-mode sensing ability, but also execution ability. A device that can act as both a multi-mode motion sensor and a motion actuator is highly valuable for achieving a low-cost, multi-functional, and highly efficient motion HCI system. Wang et al. proposed LIG-based soft and stretchable electronics with a PI/PDMS substrate for multi-mode sensing and human–machine interaction [30]^[27]. Even though the system has three main applications (voice and motion sensors, gesture recognition gloves, and remotely controlled soft robots), all of the core components of the system were fabricated using the following method. This involved incorporating PI particles with a specific concentration into a PDMS solution, curing the mixture, and exposing the PI/PDMS substrate to an infrared laser to produce the porous LIG layer, which then resulted in the creation of the LIG-based multifunctional device. Moreover, the electromechanical properties of the device could be adjusted flexibly by adjusting the laser flux. Based on the softness and stretchability of the PI/PDMS substrate, the LIG-based wearable system could sustain a mechanical tension of over 15%, which was more suitable for wearable motion monitoring than the system with the PI substrate.