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Flexible electronics have been researched in the field of medical treatment, health monitoring, flexible robots and flexible skin. It is a key point to prepare flexible circuits with high stretchbility in the application of the flexible electronics. The use of nanocomposites and ionic gels to prepare elastomeric conductors is the main method of flexible circuit fabrication. However, it is still limited to meet the requirements of high flexibility and high conductivity at the same time. Gallium-based alloys, such as Galinstan and EGaIn, become emerging flexible electrode materials, due to its high conductivity and fluid state at room temperature. Different from the conventional fabricated method of the printed circuit board, liquid metal (LM) is printed in the form of conductive ink. High resolution LM pattern can be achieved via the method of selective wetting of LM on the surface of substrates. Therefore, it is important to investigate the method of controlling the wettability of the LM. Femtosecond laser has been widely used in the field of micro-nano fabrication, such as three-dimensional fabrication and wettability control, due to the ability of high-precision fabrication. Prof. Feng Chen and his groups from the International Joint Research Center for Micro/Nano Manufacturing and Measurement Technologies wrote a research paper “Guiding magnetic LM for flexible circuit” on IJEM. In this article, the authors proposed a method to fabricate supermetalphobic surface on the flexible silicone substrate by the femtosecond laser. Magnetic LM (MLM) droplet can be functionally manipulated on the surface of silicone substrate to achieve LM pattern printing and repair by tuning the wettability of the LM. The printed LM circuit has a uniform surface and can be recycled in the Ethanol solution for reuse.
Atomic level deposition methods are reviewed and categorized to extend Moore’s law and beyond in this article:
● A universally applicable method is proposed to control the LM on the surface of the soft materials to print the uniform, high-precision and smooth LM pattern and repair the broken LM pattern.
● The wettability of LM was controlled by a femtosecond laser ablation to achieve the LM pattern printing.
● The printed LM pattern can be used as a flexible circuit. A tensile sensor can be prepared to detect the gesture of the finger based on the circuit.
Gallium-based alloys, such as Galinstan and EGaIn, have been researched as an emerging flexible electrode material. Flexible electronics need maintain high conductivity and elasticity under complex deformation, so the choice of the materials of substrate and electrode is very important. Gallium-based alloys with fluid characteristics has a great electronic and thermal conductivity, and is proved to be non-toxic for human body. Fabricating the flexible circuit by using the gallium-based LM is totally different from the conventional way of PCB, so a new fabricating technology need to be explored. At present, the preparation methods of LM flexible circuits mainly include selective wetting, template printing, spray coating, channel injection and conductive polymer. It is a universal way to pattern the LM by means of the selective wetting behavior of the LM. Gallium-based LMs are easily oxidized in aerobic environment. An oxide layer with high adhesion is wrapped outside, which makes the LM adhere to a smooth surface easily. On the contrary, the rough surface structure can significantly reduce the adhesion of LMs. Printing by using the difference of wettability of LM on two different surfaces is a simple and mask-less patterning method, and high precision and resolution patterns can be obtained via this way. But at present, there are also problems such as poor uniformity of LM circuit, low flexibility of printing method and inability to print and repair circuits simultaneously. In view of the above problems, Professor Chen Feng and his groups introduced a circuit printing method which can achieve high precision and high uniformity by controlling magnetic LM, and can flexibly control the MLM to realize circuit repair and other functions.
3. Recent Advances
The authors realized the high precision, uniform LM patterning via tuning the wettability of the magnetic LM on the silicone surface by femtosecond laser ablation. At the same time, the magnetic LM can also be guided to repair the broken circuit and won’t affect the original electrical properties. Figures 1(a) and (b) show the results of an MLM droplet being guided forward by a magnet on the untreated silicone surface and laser structured silicone surface, respectively. A long LM trace was left on the smooth silicone surface behind the MLM droplet due to the high adhesion between the MLM and the silicone substrate (figure 1(a)). Conversely, there was no LM residue on the path of the MLM droplet on the laser-ablated surface (figure 1(b)). LM pattern can be printed by using the different wettability of the LM (figure 1(c) and (d)). By comparing the two printing results (figure 1(e) and (f)) of guiding MLM on the smooth surface and the laser pre-patterned surface, it can be found that the latter can accurately control the pattern and flexibly control the line width of the pattern in the printing process.
Figure 1. Printing MLM on the laser-patterned surface. (a), (b) Guiding an MLM droplet to move (from left to right) on (a) the smooth silicone surface and (b) the laser-structured silicone surface. (c) Schematic of the process of printing LM on the laser-patterned surface. (d) Mechanism of printing MLM on the smooth domain surrounded by laser-induced microstructures. (e), (f) LM ‘XJTU’ patterns printed on (e) the smooth surface and (f) the laser-patterned surface.
The minimum linewidth of the LM circuit prepared by this method can reach 200 μm and the LM surface is very uniform and smooth (figure 2a-c). The resistance of the prepared LM circuit is completely independent of bending deformation, which also improves the stability of the signal in the application of flexible circuit (figure 2e). When the LM circuit is subjected to tensile deformation, the LM circuit can meet the good resistance variation characteristics and can be applied to the preparation of flexible tension sensor (figure 2f). At the same time, it can be found that the flexible circuit still maintains good circuit continuity performance when it is subjected to various complex deformation (figure 2g).
Figure 2. Electrical and flexible properties of the printed LM wire. (a) Process of preparing an LM wire in a circuit. (b) Laser confocal microscopy image of the surface morphology of the printed LM wire. (c) Image of the printed LM lines with different line widths. (d) Relationship between the resistance and the line width of the printed LM lines at a constant length of 15 mm. (e), (f) Influence of (e) the bending treatment and (f) the stretching treatment on the resistance of the LM wire (the initial resistance value (R0) is 5.3 Ω). (g) Conductivity test of the printed LM circuit under different deformation conditions.
LM can response to magnetic field after mixing the iron particles. After patterned scanning by femtosecond laser, the MLM can be controlled to realize the circuit printing and repair on the surface of flexible silicone substrate. The repair process will not cause short circuit, because the supermetalphobic feature of the laser-ablated surface, and the LM cannot adhere to the non-patterned rough surface. With the assistance of femtosecond laser pre-treatment and the realization of LM functional control under the magnetic field, MLM droplet can be applied in the fields of high-precision flexible circuit printing, circuit repair and LM droplet switch.
Figure 3. Repairing a printed LM circuit by a magnetic field-controlled MLM droplet. (a)–(e) Schematic of the process of repairing a printed LM circuit: (a) laser patterning, (b) printing LM for preparing an original LM circuit, (c) integration of LED lamps, (d) breaking the LM lines in the circuit, and (e) moving an MLM droplet to the damaged position and reprinting LM onto the broken circuit line. (f) Guiding the MLM to print the LM circuit and connecting the LED lamps. (g)–(k) Experimental result of repairing a damaged LM circuit: (g) connected state of the original LM circuit, (h) damaging the LM wires, (i), (j) controlling an MLM droplet to repair the damaged circuit, and (k) moving the MLM droplet away from the circuit.
A method of guiding an MLM droplet by a magnetic field to print and repair a flexible LM circuit on an fs laser-patterned silicone surface was proposed. By adding iron particles into the LM, the researchers endow the LM with the ability of responding to magnetic field. With the assistance of the femtosecond laser pre-patterned ablation, high precision and uniform LM pattern can be obtained for flexible circuit preparation. The laser-patterned method under a magnetic field allows for more flexible and functional LM manipulation, which has great significance for exploring soft circuits in the future.
5. About the Authors
Feng Chen is a full professor of Electronic Engineering at Xi'an Jiaotong University, where he directs the Femtosecond Laser Microfabrication Laboratory and has severed as deputy director of the International Joint Research Center for Micro/Nano Manufacturing and Measurement Technologies. Chen received the B.S. degree in physics from Sichuan University, China in 1991 and received the Ph. D. in Optics from Chinese Avademy of Science in 1997. His current research interests are femtosecond laser microfabrication and Bionic Microfabrication.
Qing Yang is a professor of school of Mechanical Engineering at Xi'an Jiaotong University. Yang received the B.S. degree from Sichuan University in 1992 and received the Ph. D. from Chinese Avademy of Science in 2009. Her current research interests are femtosecond laser microfabrication and microfluidics.