As the next big trend in the development of the electronic industry, flexible electronics is a brand technology that can revolutionize the future. At present, the application research of flexible electronic devices mainly focuses on human health detection, flexible robot, and human-computer interaction. Flexible electronic devices are realized by combining soft materials and flexible electrode materials to achieve high flexibility. Conventional flexible electrode materials, such as structured metal films, metal nanoparticles/wires, and conductive polymers, cannot meet high stretchability and high conductivity simultaneously. As a new kind of flexible electrode material, gallium-based liquid metal (Ga-LM) with high electrical conductivity and unlimited stretchability has become a research hotspot in recent years. Ga-LM has a melting point below 30 ℃ and the almost negligible vapor pressure. It is non-toxic to the human body and has the excellent conductivity/thermal property, making it an ideal flexible electrode material. Ga-LM based flexible devices are fully flexible compared to traditional electrode materials, which can maintain their electronic performances even under large elastic deformation. This will lead to dramatic improvements in the performance of wearable electronics. The Ga-LM circuits are crucial for the preparation of flexible electronic devices. Although researchers have proposed many methods to prepare the Ga-LM flexible circuits such as screen printing, injection, and spray painting, there still remain problems of limited resolution and integration of Ga-LM circuits. Therefore, to develop a way for the preparation of miniaturized, high-integration, and multifunctional Ga-LM flexible devices is of great interest. Ga-LM patterning is a necessary step in the preparation of Ga-LM based flexible electronic devices. However, there exists a major challenge in Ga-LM patterning due to its fluidity. Hence, the patterning method by tuning the wettability of Ga-LM has been extensively explored in recent years.

As a precision machining method, the laser has good processing advantages in preparing various functional surfaces. Due to its high-power density, the laser can induce micro/nano-structures on the surfaces of various materials and realize the preparation of functional surfaces. Ga-LM is found to show extremely high adhesion on the smooth material surfaces, and show ultra-low adhesion on rough surfaces. Selective adhesion of Ga-LM can be realized by constructing rough structures on the initially smooth material surface, so as to realize the printing of Ga-LM circuits. Laser machining technology has advantages of non-contact, high-precision, and high-controllability processing, which can realize the preparation of high-resolution and high-integration LM circuits. The combination of laser manufacturing technology and newly flexible electrode materials can achieve high-performance flexible electronic devices. This field is growing rapidly, and it is necessary to review and analyze all these efforts to guide its future development more rationally.

The recent efforts in the field of Ga-LM based flexible electronic devices fabricated by a laser are reviewed and the future research directions are indicated. First, this paper introduces the patterning method by tuning the wettability of Ga-LM. The wettability model of Ga-LM is analyzed in-depth (Fig. 2). Then, the characteristics and advantages of laser micromachining are summarized according to previously reported studies. As a precision machining method, the laser is used to prepare various functional surfaces and is one of the main methods to tune the wettability of liquids. Subsequently, recent advances of the Ga-LM-based flexible electronics fabricated by a laser are comprehensively summarized. The research group from the Southern University of Science and Technology has realized the tenability of the wettability of Ga-LM by laser ablation of nanoparticles (Fig. 3). The research group from Xi’an Jiaotong University has reported a method for inducing rough structures directly on the surface of the substrate by a femtosecond laser to change the wettability of Ga-LM on the original smooth surface from the original high adhesion to ultra-low adhesion (Fig. 4). Combining the high precision machining capability of the laser with the excellent electrical properties of Ga-LM, one can fabricate ultra-flexible electronic devices with high-resolution, multi-function, and high-integration. In the end, the applications of Ga-LM based flexible electronics in human health monitoring, human-computer interaction, and soft robots are elaborated.

As a precision machining tool, the laser has good processing advantages in preparing various functional surfaces. Using a laser to tune the wettability of Ga-LM can realize the preparation of Ga-LM circuits with high resolution and high integration, thus greatly improving the performance of Ga-LM based flexible electronic devices. Ga-LM has intrinsic advantages in the field of flexible sensing, and the improvement of sensor performance is closely related to the preparation of microstructures. Using the advantages of laser precision preparation of microstructures to realize the significant improvement of the performance of Ga-LM sensor will become the key factor to promote the application of Ga-LM in the field of flexible sensing. The combination of laser precision machining with Ga-LM is believed to promote the rapid development of flexible electronics.