Opportunities and Challenges in Flexible and Stretchable Electronics: A Panel Discussion at ISFSE2016

The 2016 International Symposium of Flexible and Stretchable Electronics (ISFSE2016), co-sponsored by the Flexible Electronics Research Center, Huazhong University of Science and Technology (HUST) & State Key Laboratory of Digital Manufacturing and Equipment Technology, National Natural Science and Engineering (NSFC), was successfully held in Wuhan, China, 29–30 June 2016.[...]

high-performance flexible and stretchable electronic devices using high-quality single crystal inorganic materials and novel biocompatible materials that enable a new generation of implantable biomedical systems with novel capabilities and increased performance. design soft materials that possess unprecedented properties and functions. Dr. Zhao is a recipient of the NSF CAREER Award, the ONR Young Investigator Award, and the Early Career Researchers Award from AVS Biomaterial Interfaces Division.
Dr. Haixia Zhang is a professor in the Institute of Microelectronics, Peking University. Her research fields include micro/nano energy harvesting technologies, selfpowering systems and active sensors. She has published more than 100 papers in prominent journals, six books/book chapters and 30 patents on micro/nanotechnology. 1. What are the key challenges in flexible electronics? What is the niche market and the killer application for stretchable electronics?
Huang: One killer application of flexible or stretchable electronics is in medicine. Dae-Hyeong, you have done a lot of work in this area, can you make some comments?
Kim: I totally agree with Prof. Huang. The limitation of flexible and stretchable electronics is that the device performance may be lower than conventional rigid electronics. Flexible and stretchable electronics may not be able to compete with rigid electronics in device performances. When we change the substrate from the rigid silicon wafer to plastics, the device performance would be decreased significantly. Therefore, what we need to do is to find out new markets and applications, such as novel medical systems based on flexible electronics. In medical applications, the device should be human-friendly. Individual organs and/or tissues are quite soft, and their shape is curvilinear. So, the device should also be deformable to conform to these biological systems, which is a key property of flexible and stretchable electronics.
Zhao: I fully agree with Yonggang and Dae-Hyeong's points. In the health care industry, I think flexible and stretchable electronics indeed has a niche market. I also want to add some additional points. Flexible and stretchable electronics may address some critical issues in this aging society, in 1.
What are the key challenges in flexible electronics? What is the niche market and the killer application for stretchable electronics?
Huang: One killer application of flexible or stretchable electronics is in medicine. Dae-Hyeong, you have done a lot of work in this area, can you make some comments?
Kim: I totally agree with Prof. Huang. The limitation of flexible and stretchable electronics is that the device performance may be lower than conventional rigid electronics. Flexible and stretchable electronics may not be able to compete with rigid electronics in device performances. When we change the substrate from the rigid silicon wafer to plastics, the device performance would be decreased significantly. Therefore, what we need to do is to find out new markets and applications, such as novel medical systems based on flexible electronics. In medical applications, the device should be human-friendly. Individual organs and/or tissues are quite soft, and their shape is curvilinear. So, the device should also be deformable to conform to these biological systems, which is a key property of flexible and stretchable electronics.
Zhao: I fully agree with Yonggang and Dae-Hyeong's points. In the health care industry, I think flexible and stretchable electronics indeed has a niche market. I also want to add some additional points. Flexible and stretchable electronics may address some critical issues in this aging society, in addition to health care; for example, monitoring the well-being of senior people. We will probably not pay a thousand dollars or more to buy a flexible cell phone. But if the cell phone were able to be used as a very comfortable device for senior people and were able to monitor many of their vital signals, we might buy it. Another potential application is in education. We now learn many new things through cell phones. But it would be better if we could have a more conformal and natural way to receive different types of information.
Huang: I totally agree with your points on health care. But I do not really follow your comment on education. I am not sure I understand it yet.
Zhao: Books and rigid devices such as tablet computers have allowed us to learn new information. Flexible electronics, for example, flexible goggles, and different types of virtual reality devices that are more flexible and more conformable, may even do better in this regard. Learning is no longer limited to the classroom; learning is everywhere. I think this kind of conformal devices that can be very comfortably attached to your body may lead to innovative ways of learning and education.
Huang: I fully agree with what you have just said. Zhang: I want to add some comments here again. So, regarding health care, I agree, we should certainly pay more attention to the wearable market. Another hot topic is artificial intelligence (AI). Recently, AI has become very popular everywhere-such as in famous human-like robots that resemble the most beautiful ladies-and draws a lot of attention. So, if we see this as the future of AI, the future of robots, then soft electronics and conformal electronics are exactly what is needed. If we can put all these on top of our skin, they can detect not only temperature but also many other parameters. Then, these robots will be much smaller in many conditions. I think we should pay attention to these advanced technologies. Is polydimethylsiloxane (PDMS) the best carrier? Or hydrogel? What is the area of greatest potential for hydrogel?
Huang: I do not think there is one best carrier for everything. There are so many substrates that we have used, such as PDMS, Ecoflex ® , and Silbione ® , depending on the applications. I do not think we need to identify one single material that suits all the purposes. For example, the elastic modulus of Ecoflex matches well with that of the skin, and is therefore useful for epidermal applications. For some other applications, PDMS may be a good choice.
Zhao: I agree with Yonggang. The choice of material is application-dependent, especially when considering integrating devices with the human body. Different parts of human bodies have different properties, so this is a material design and system design issue. PDMS has been widely used in flexible electronics, while hydrogels have broad applications in biomedicine. But I do not think there is a best material; it will depend on the application.
Huang: Dae-Hyeong uses one special type of substrate, silk, for bio-integrated electronics, which can dissolve inside the human body. This cannot be achieved by PDMS. Therefore, different applications require different substrates.

3.
Shall we focus more on the reliability or new functions?
Cheng: I think it very much depends on what your interest is. If you are interested in applications, I think the reliability is probably more important for consideration. Many excellent research works about different flexible or stretchable electronics have already been reported, but most of these works have not been accepted by the market as commercial products, because most of them failed in reliability or in functionality. For example, I just talked about perovskite solar cells, showing 20% efficiency. But their long-term performance is poor, currently lasting for two weeks or a month. So, from that point of view, I think it would be a huge contribution to the whole field if we can improve the reliability of one or two research outcomes and make them really accepted by the market. However, if you are a young researcher, just coming into this field, and you want to do something exciting or achieve good publications, then focusing on reliability may not be your best choice. This is because the study of reliability is time-consuming and may be difficult to produce many journal papers with very high impact factor. So, if you really want to publish in Nature or Science, you would probably prefer to work on something that is new and more exciting.
Zhang: From my experience in my field, MEMS (Micro-Electro-Mechanical System) is probably not a good field for publication. In most MEMS journals, the impact factor is pretty low, for example, below 2. So, that presents many issues for students, because they want to graduate with very good records. Especially in China, a lot of university academics now asking for high impact publications. So, if you are a student, I strongly suggest that you work on the functions and innovations, and offer some new innovations for good publications. That is not only to make your resume look impressive, it is also very good educational training. At a very young age, you should find something really exciting, not invest your time in something reliable. Reliability research is not for the young students in the lab. So I suggest, if you are working in the lab, and still want to continue academia career as a professor, you should try your best to make something new, try to make something really exciting. After many years, you may move into a big production market, and then you will be able to hire a bunch of people to work on the reliability. I think that is the strategy.
Zhao: For a wide range of applications of flexible and stretchable electronics, reliability is a very import issue. The first light bulb invented by Thomas Edison only lasted a few hours, which probably would not be widely adopted in the society. Edison and his team further improved the design so that it could last over a thousand hours, and then the widespread applications of light bulks made a major impact on our society. For the field of flexible and stretchable electronics, we do need to invent new functions and applications. At the same time, especially for translational and industrial applications, we also need to pay attention to the reliability.
Kim: For students, I think that innovation should be emphasized. It is true that reliability is always important. For example, all the medical devices that we are working on always need to be highly reliable. But I personally think that new ideas and innovations are more important, as we are conducting research at the university. Of course, if you are working at a company, reliability is very important. However, we are working on new ideas, and creating new frontiers and new innovations. So, students should be working more on new, innovative functions.

4.
Is it of interest to actuating technology? For what kind of applications?
Kim: Ok, usually I work on soft medical devices, and it is related to sensors and actuators. For the sensing purpose, sometimes we need actuators. If you just want to measure the electrical features, an amplifier might be good enough. But if you want to measure some mechanical properties of specific tissues or organs, then appropriate actuation gives us better sensing results. We need to combine sensors and actuators for a better quality diagnosis. Meanwhile, I think that in the future soft robotics will be a hot field. In the past, robots were rigid. But in the future, we imagine soft, human-like robots. And in that case, we will probably need soft actuators.
Zhang: Actually, it is very important for smart systems. Now, it is very popular to design mini robots to put inside the body, or inject into the body to make detections and perform some surgeries.
For that purpose, robots should have internal actuators. I have not worked on these actuators, but I do think they are a very important field and research direction.
Kim: I have something to add. Actuators are important in medical systems. For example, the brain and heart are organs operated by electrical signals. And electrical impulses and stimulations can be used to treat many diseases related to the brain and heart. Also, drug delivery should be controlled by appropriate actuators for controlled drug release.

5.
What are the unsolved problems in flexible electronics manufacturing?
Huang: Flexible electronics manufacturing is in its infancy, and there are numerous unsolved problems in this area. In fact, the United States government investigated $70M to form a center on the manufacturing of flexible electronics.
Zhao: Manufacturing of flexible electronics requires different techniques. Is 3D printing a possible technique for the fabrication of flexible and stretchable electronics?
Huang: I will answer this. 3D printing can print polymers and some metals, but it can never print single crystal silicon or other important materials for electronics.
Zhao: How about 3D printing plus transferring? Huang: This is an interesting idea. Would you like to explain a bit more? Zhang: 3D printing is good but it is not suitable for mass production. So, if we want to make electronics that are flexible, we must use this kind of traditional technology and basic tools. Therefore, I agree with Yonggang's comments; it is important that fabrication technology tries to use all these existing successful technologies and makes something based on them. This will be easy for mass production and for the actual industry.
Huang: It is important to take advantage of the existing, mature semiconductor fab to develop inorganic, flexible and stretchable electronics.
Cheng: I think printing technologies could be used for flexible electronics manufacturing. 3D printing has potential in the flexible electronics field, such as for health-related products. Many health-related flexible products are associated with individual people. So, for these kinds of products, 3D printing is probably quite suitable. This is a quite new field and we do not need to restrict our minds and initiatives. I guess the point is that while recognizing the existing technologies, such as silicon technology, we should not be afraid of trying and integrating new things.