“Terminator” liquid metal robot becomes a reality? Pioneer in China’s liquid metal 3D printing research

In 1991, a robot T-1000 appeared in the movie “Terminator 2”. Robots made of liquid metal can not only change shape at will, but can even repair huge damage instantly and automatically. With the development of science and technology, this previous assumption that only existed in science fiction movies may become a reality. The so-called liquid metal usually refers to the metal that is liquid at room temperature or higher, also called low melting point metal, such as gallium-based, bismuth-based metals and their alloys. This type of metal can flow at room temperature, has strong electrical conductivity, excellent thermal properties, and is easy to achieve solid-liquid conversion. It can also be molded at one time, thus eliminating cumbersome processing procedures. The integration of the advantages of functional materials has broken through the application bottleneck of traditional technologies in many fields, thus opening up an extremely broad application space, which will greatly promote the development of many disruptive technologies and equipment. As a new material with “huge potential”, liquid metal is also “promising” in the field of 3D Printing. Research on liquid metals has gradually increased in recent years, especially the research in China has become more prominent and has achieved good results. At the forefront of this field is undoubtedly a research team led by Professor Liu Jing from the Department of Biomedical Engineering of Tsinghua University, a dual-employed researcher at the Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences. In this issue of “3D Printing World”, we have a conversation with Professor Liu Jing to see how he “cross-boundary” has fun with liquid metal 3D printing.

Heat transfer research to 3D printing “cross-border” let it goIn 1987, Professor Liu Jing was admitted to the Department of Thermal Engineering of Tsinghua University as a major in gas turbines. In the second half of his sophomore year, he chose Modern Applied Physics in the Department of Physics as his second degree. In 1992, he obtained both a Bachelor of Engineering degree in Gas Turbine and a Bachelor of Science degree in Modern Applied Physics from the Department of Thermal Engineering of Tsinghua University. In the same year, he obtained a Ph.D. degree and specializes in biological heat transfer under the advice of his supervisor. With multiple academic backgrounds in physics, engineering and biology, Professor Liu Jing said that studying liquid metal 3D printing is a matter of course. As early as 2001, computer chip heat dissipation was still a major problem in the field of engineering thermophysics. Since the surface heat flux density of high-end computer chips is comparable to that of the sun, it is difficult to solve this problem with ordinary water. Professor Liu Jing began to find a way to quickly dissipate heat from computer chips. After many attempts, he found that liquid metal has ultra-high thermal conductivity and a high boiling point. It is still in the liquid phase at temperatures as high as 2000°C. It will not boil or even explode like water, and can be used as an ideal coolant to quickly dissipate the heat in the integrated circuit, thus starting the research in the field of liquid metal. Professor Liu Jing said that the use of liquid metal for computer chip cooling originally belonged to the scope of engineering thermophysics, but it was also closely related to electronics, information and broader fields, so that printed electronics and 3D printing were gradually extended in the subsequent research process. In an early experiment, Professor Liu Jing accidentally splashed liquid metal on the computer screen, and he couldn’t wipe it off with paper and rags. This incident caused him to think about whether liquid metal can be used in the field of electronics. Taking into account the conductivity of liquid metal, and taking advantage of this “inability to wipe off” feature, Professor Liu Jing believes that this material can be used in the manufacture of printed electronic circuits. At first, liquid metal printing of electronic circuits was only on a two-dimensional plane. With the continuous deepening of research, three-dimensional printed circuits and even complex metal objects were gradually formed, and the technology of liquid metal 3D printing came into being.

Since liquid metal was originally used to print electronic circuits, many people have questions about classifying this technology as 3D printing. Faced with such questions, Professor Liu Jing said that this technology is definitely 3D printing for two reasons. One is that liquid metal 3D printing can produce any complex structure that can be made by conventional metal 3D printing, and even 4D printing-because liquid metal is easy to achieve solid-liquid conversion or other deformation under normal temperature environment, it can be controlled by electric or magnetic fields. Metal deformation. Second, liquid metal 3D printing actually surpasses conventional 3D printing. It is a functional 3D printing technology that can directly print electronic components. Conventional metal 3D printing is difficult to produce three-dimensional three-dimensional circuits, but liquid metal 3D printing can not only manufacture, but also print functional components on a variety of different surfaces, even on human skin. Conventional metal 3D printing uses metal consumables with melting points as high as hundreds of degrees or even thousands of degrees, and often requires the use of high-power lasers or energy beams for forming, which is expensive. For example, the use of liquid metal can greatly reduce the cost and increase the manufacturing speed, because the liquid metal is in liquid form during the 3D printing process, but it will solidify when it is stacked. In addition, the melting point of liquid metal is relatively low, and it can be printed together with other materials, such as 3D printing drones, structures containing metal and non-metal, and using liquid metal 3D printing can be done in one go. Liquid metal 3D printing can also be printed into different gels to achieve suspended printing to form flexible electronics that can be stretched and deformed. This is liquid metal that has brought subversive changes to traditional manufacturing and additive manufacturing, but it has been ignored by the world. Cross the river by feeling the stones and finally walk in the forefront of the worldIn the early years, due to the lack of knowledge and technology, the progress of scientific research at home and abroad was slow. The concept of printed electronics was put forward very early, but it took a long time to solve the ink problem. It also took a long time to solve the problem of driving the computer chip to dissipate heat. The process of scientific research is a process of continuous advancement and deepening, and it is gradually becoming known, especially in frontier scientific research. Professor Liu Jing vividly likened cutting-edge scientific research to “crossing the river by feeling the stones”. The research team he led in the field of liquid metal 3D printing has been even more difficult. For a long period of time, Western countries have been in a leading position in many academic studies, but most of China can only follow. But in the field of liquid metal, the situation is just the opposite. The research team led by Professor Liu Jing has gone through a long and dark research period with no one to learn from. Even though most others cannot understand, they still stick to the “front line” of cutting-edge scientific research.

In 2008, Liu Jing’s team was invited to participate in the 2008 Shanghai International Industry Fair. The prototype liquid metal radiator for computer CPU applications they developed won the China International Industry Fair Innovation Award. Since then, Liu Jing led the team’s research on liquid metal more and more deeply, and has repeatedly achieved fruitful results. In 2011, for the research of the first generation of commercial computer CPU liquid metal radiator, it won the annual only best paper award in the journal of the American Society of Mechanical Engineers “Journal of Electronic Packaging”, which attracted international attention in related fields. In 2012, Professor Liu Jing and her students published a 30-page forward-looking paper in English. They originally proposed and systematically explained the technical ideas of direct manufacturing of electronics and even integrated circuits based on liquid metals and their alloys for the first time, marking The beginning of a new field; in 2013, China made major theoretical and technological breakthroughs in the field of printed electronics. Liu Jing’s team reported for the first time in the “Scientific Report” the prototype of the liquid metal desktop 3D printer developed, which can realize conductive ink and packaging materials The results were quickly and widely reported by many international scientific media. In the same year, the team demonstrated for the first time the universally meaningful liquid metal inkjet printing technology that can draw circuits on any material surface. This is praised as a fantastic technology by MIT’s “Technology Review”, which has caused significant repercussions on a global scale. The industry believes: “The race to print circuits on different surfaces can be ended.” In 2014, the team successfully developed the world’s first Taiwan liquid metal desktop electronic circuit printer was published in the “Scientific Report”. This breakthrough has laid a solid foundation for the large-scale application of personalized electronic manufacturing technology. D 100 Award Finalist; in the same year they published landmark work in the field of 3D printing, including: “liquid metal liquid phase environment 3D printing”, “liquid metal hybrid printing technology” and so on. In March 2015, the team published a research paper entitled “Self-Fueled Biomimetic Liquid Metal Mollusk” (2015) on Advanced Materials, which was quickly recognized by hundreds of well-known scientific magazines or professional websites such as New Scientist, Nature Research Highlights, Science News, etc. The special report has aroused major repercussions and heated discussions in the international community, and the results were selected as the top ten scientific and technological progress news in China by the academicians of the two academies in 2015. In 2016, following the development of a self-moving deformable liquid metal machine, they once again discovered that liquid metal has an “endocytosis effect” similar to cells engulfing foreign particles, which makes it possible to create a “terminator” material. In the same year, the “Second Liquid Metal Industry Technology Summit Forum” held in Yunnan concentrated on exhibiting more than 100 new products, and the achievements of scientific and technological progress jumped to the international leading level. In 2017, they published a paper for the first time reporting the concept and method of “liquid metal suspension 3D printing”, which can quickly manufacture three-dimensional flexible metal deformable bodies with arbitrary complex shapes and structures at room temperature and use them to assemble three-dimensional stretchable electronic devices.

At the beginning, there were only three or five laboratories in the world focusing on liquid metal research. After more than ten years of knowledge and technology accumulation, it has now developed into hundreds of laboratories researching liquid metal. China, as a pioneer and pathfinder in the field of liquid metal, began to study research in 2000, and after more than ten years of exploration and advancement, it will finally usher in an explosive period. In recent years, the research and development of liquid metal in China has been advancing rapidly, and there is a broad space for scientific research in the future. The center of gravity of liquid metal scientific research and industrial applications worldwide is gradually being formed in China. China has not only discovered and established the most basic principle knowledge, technical ideas and equipment routes, but many of the earliest underlying core patents have also appeared in China. The speed of research and development in the field of liquid metal has been accompanied by the emergence of related academic achievements and the promotion of applications. Keep accelerating. The hard work of the team led by Professor Liu Jing is indispensable. When the team’s research and development results were shortlisted in the 2014 Chinese Academy of Science and Technology Progress News (20 nationally selected) by the academicians of the Chinese Academy of Engineering, the Chinese Academy of Engineering wrote to praise for this, “The results will play a huge role in promoting the development of engineering technology in this field.” . The sense of mission promotes to show the world that China’s smart manufacturing is a cutting-edge science and technology. Liquid metal 3D printing has only been known in recent years, and there are very few companies that really apply it. Professor Liu Jing said that, like the scientific research field, Western countries that have always been at the forefront of the world invent a brand-new technology, and gradually define the market. After that, the society gradually understands, understands and applies it. Due to historical reasons, most industries in underdeveloped countries are follow-up, gaining markets through continuous replication and partial improvement. This is also the development route followed by many domestic companies in the past. However, the liquid metal field completely overturned the past model, and the company made a new product or application worldwide, which required a lot of time for research and development and promotion. At present, the application of liquid metal 3D printing is still in the pioneering stage. This field is a rigid demand. Although China is at the forefront of the world in this field and has relatively sufficient reserves of knowledge and technology, since there are no related products on the market, people do not know how to apply it. Domestic companies that have the awareness or demand in this area often don’t know where to start. Therefore, it is necessary to conduct market education and market development to encourage Chinese people to carry out large-scale applications. After the technology is applied, the progress in this field can be promoted in the reverse direction.

“The difficulty of the road of Shu, it is difficult to go to the sky!” Isn’t it the promotion and application of liquid metal 3D printing? Professor Liu Jing said that the biggest difficulty in the promotion and application process is the shortage of funds. Although the emergence of desktop liquid metal 3D printing is of great significance to the industry, so far there is no precedent for research in this field to apply for national fund support. Most of these scientific research projects raise funds through self-financing or venture capital. However, since there is no precedent to learn from cutting-edge scientific research projects such as liquid metal 3D printing technology, venture capital may also have an uncertain or even incomprehensible attitude towards such projects. Limited funds directly lead to a lack of manpower, which greatly limits the speed of project advancement. In order to make this technology truly “landing”, the team has mainly delivered some technological achievements to them through cooperation with enterprises in these years, so as to realize the application and promotion of the technology. However, in the process of cooperating with enterprises, Professor Liu Jing found that such cooperation methods were inefficient. On the one hand, he considered that the laboratory not only had to research and develop technology, but also had to develop products. As a result, the efficiency of the connection between the two parties is low. Riding on the “East Wind” of a series of national policies, Professor Liu Jing founded Beijing Mengzhimo Technology Co., Ltd. (hereinafter referred to as “Mengzhimo”) in 2014, covering the entire process from R&D to industry to application. Nowadays, new technologies developed in the laboratory can be directly industrialized internally, and a large number of talents who have the ability to manufacture and promote industrialization of products have been gathered.

Professor Liu Jing talked about the development of the world’s first room temperature liquid metal 3D printer and the original intention of establishing Mengzhimo. First, scientific research will gradually turn to applications at a certain stage, which can be described as taking advantage of the trend; more importantly, the sense of mission in the heart. Drive. As a cutting-edge scientific researcher, Professor Liu Jing hopes to develop technologies that will bring disruptive changes to human life and production methods, and China can also create technologies that can be widely used throughout the world. Although people may not understand the so-called frontier science and may even raise questions about it, Professor Liu Jing said that the emergence of frontier science and technology will eventually change the way of human life and production in the future, and promote the progress of human civilization. This is also the reason why 3D printing is called the “label of the third industrial revolution”, precisely because this technology has changed the original production methods of mankind. After more than ten years of knowledge accumulation and continuous experimentation, liquid metal 3D printing will eventually enter the application industry. In view of the characteristics of liquid metal materials, it can “make a big show” in many fields. In the field of thermal control, liquid metal is used to make thermal paste for chip cooling and computer device heat dissipation. It has a high boiling point and fast heat transfer. Compared with traditional materials, the thermal conductivity has been changed by an order of magnitude; radiators made of liquid metal can Solving the thermal barrier, a worldwide problem that cannot be solved by traditional materials, has formed its unique theory and technical system. In the electronics field, liquid metal can be used as ink for 3D printing to directly generate electronic circuits. Compared with traditional metal, its printing temperature and equipment cost are greatly reduced, and the manufacturing time is shortened, which can be customized. In the medical field, liquid metal has good biocompatibility with the human body, and can connect nerve electrical signals to repair broken nerves. In the mechanical field, compared with traditional polymer materials, liquid metal is self-driving, deformable, capable of running and jumping, providing a path for the development of flexible robots. However, liquid metal 3D printing has not yet been widely used. With sufficient knowledge and technology reserves, it will gradually promote commercialization according to industry needs. Professor Liu Jing also introduced some amazing research work: Liquid metal 3D printing can be applied in the field of biomedicine. Liquid metal can be directly injected into the human body through in-body 3D printing to form implanted electrons. Due to the conductivity of the metal itself, it can be used for cardiac pacing and brain pacing. In addition, it is used in orthopedics and dentistry. The injected liquid material is solidified after entering the human body, and the damaged bone is repaired in situ. It can even be extracted in liquid form. At present, this kind of concept has already begun cutting-edge research, and a series of original papers have been published. I believe it will come true in the near future.

Perhaps many people are skeptical about whether this technology can really benefit ordinary people in the future. But Liu Jing believes that cutting-edge science and technology focus on the future, and the core of future manufacturing will be functional manufacturing. Just like the rise of the Internet and the advancement of science and technology, the effect of “decentralization” is constantly being strengthened. People have more diversified channels for obtaining information and relatively more equal opportunities. The same is true for liquid metal 3D printing technology, which provides an opportunity for functional manufacturing that is not limited by time, space, or cost for future human society. Relying on this technology, the future factory model will be subverted, and people will no longer need to rely on factories to produce electronics in large quantities, and can directly customize them according to their own needs. Moreover, the future of liquid metal is unlimited, and the future application fields are beyond our imagination. When talking about the future development trend of 3D printing, Professor Liu Jing mentioned that “3D printing may develop into a technology that is different from what people understand today. 3D printing in the future is functional manufacturing, and the functions that people need can be printed. ; The future of 3D printing is a large-scale single-piece production, the cost is affordable for everyone, and the functionality meets people’s needs in all aspects; the future of 3D printing is personalized manufacturing, people do not need a large amount of inventory, they can place orders at any time or by themselves according to their own needs manufacture”. The 3D printing technology originated in the United States. At first, it was too expensive to promote the application. Metal 3D printing technology was only used in the industry and aerospace industry. With the advancement of technology and society, the cost of 3D printing will gradually decrease, and the application will become more and more widespread. The most indispensable thing to promote the development of 3D printing technology is the team led by Professor Liu Jing who tirelessly persist in the front line of research and development, and believe that they will eventually realize the dream of “sharing China’s research and development results with the world”.