Smartphone on your skin, imagination to reality

Professor Yongtaek Hong from SNU

Recently, the flexible display has been evolving. If we think of the conventional display, a square and hardboard come up to mind. It is because they are made out of glass that has no flexibility. Rollable or flexible display, recognized as the next-generation display, is made out of plastic instead of glass making it possible to be deformed by being rolled or bent. Samsung Electronics is about to release its foldable phone and LG Electronics is planning to showcase a rollable TV.

The transformation of the display does not end here. Beyond folding or bending displays, we are close to seeing the ‘stretchable display’ known as the very end destination for displays.

Professor Yongtaek Hong, whom we met at the Inter-University Semiconductor Research center, SNU, Gwan-ak gu, Seoul, on June 20th, is working on implementing an electronic system that can be applied on round surfaces such as the human body to make rubber band-like stretchable display.

If stretchable displays are commercialized, we may no longer need TVs that are taking up a huge space in our house. While IoT is integrated into recent electronic devices, these displays can also be integrated into electronics with a curved surface. We can also imagine a smartphone that can be applied on our wrist as if it is a pain-relieving patch. With a certain amount of flexibility, the display can be able to handle the stress coming from distortions such as folding and rolling, which can compensate for the negative aspects of current displays.

Professor Hong revealed, “If the display can be extended by just about 5~10%, it will be possible to implement them to be applied to curved surfaces such as the car surface. We can create high-end value-added services with technical maturity.”

- What is ‘stretchable display’?

“It is a type of display that can be stretched like rubber bands having elasticity and comes back to its original shape when released.”

- It is hard to imagine such type of display.

“Recollect the scene in the movie ‘In Time’. You may have seen the LED beaming on the main character’s wrist. This is an example of a display that can be applied like a sticker or planted on the skin. You’ve seen the green board in electronic devices like USIM, right? It is called the Printed Circuit Board(PCB), which has many semiconductor chips. It is like putting the chips on this square hardboard to a flexible rubber board. An easier example would be to compare to pain relieving patches. If the plastic film that protects the pain-relieving patch is the conventional flexible display, the stretchable display is a pain relieving patch that can be stretched. The latter can be easily deformed and can be applied to various curved surfaces.”

- What do you mean by ‘stretchable hybrid electronic system implementation’?

“It is a method that connects fixed-size devices like IC chips, which are high-performance semiconductor devices made out of silicon and LEDs to an electrode that has a spring structure showing consistent performance regardless of deformation so that the complete electronic system can be stretchable. This is a hybrid technology in the sense that it integrates hard devices and soft electrodes or sensors.”

- Foldable and rollable devices are evolving more and more these days.

“That’s right. The platform is changing from displays on glasses to plastic displays and on to stretchable displays. Flexible displays are becoming a trend.”

-Why do we need stretchable displays?

“Recently, stretchable display technology, which has elasticity as well as flexibility, is gaining attention. Flexible display using plastic can be deformed only along a single axis such as bending and folding. Therefore, it cannot adhere to a curved surface. However, display on flexible materials can be deformed in many different directions. If the display can be stretched by just 5~10%, it can be integrated on curved surfaces like the surface of a car. Based on this technology, there has been active research on implementing an electronic system that can be put on curved and flexible surfaces like human skin.”

- What is the order in terms of implementation difficulty?

“Flexible displays are developed as bendable, foldable, or rollable displays. The difficulty of implementation has a direct relationship with the shape and amount of stress the display receives from the deformation. For example, when the display is bent, the inner part is contracted while the outer part is stretched. Depending on the number of film layers in the deformed area, the amount of energy it receives differs. When the display is rolled, it receives energy throughout a larger area compared to when it is simply bent. If the rolling radius is smaller, the receiving energy is bigger. If the display is stretched, larger energy is applied which leads to bigger stress. Hence, the implementation is hard because for each deformation that gives bigger stress to the display needs a method that can handle it for stable performance. Normally, the difficulty increases in the order: bendable<foldable<rollable

The evolution of PCB. Conventional PCB (left) and Skin attachable PCB

- Won’t the resolution drop when the display is stretched?

“If the display is stretched, the distance between pixels might increase which leads to a resolution drop. However, the high-resolution display can have numerous applications if it can be stretched by just 5~10%. Stretching it by 5~10 % should have minimal effect on the resolution.”

-What are the applications?

“It can be used for wearable devices that can be attached to clothing or human body for measuring body signal or other displays. In the far future, we can also imagine that it can be integrated into pieces of furniture with various curved surfaces. Hence, since the displays are made out of fixed-size glasses and PCBs currently, it can only be designed as flat squares. With stretchable display and PCB, it will be possible to apply displays on bent parts of the furniture like its edges. Then, we might not need a TV in our house which takes up a large space. The rollable TV introduced by LG recently, received a lot of attention in terms of spatial efficiency since it can be rolled at put away when not used. Wouldn’t it be possible to take a TV out from the closet to be watched and get rid of it when it is not in use? Furthermore, new high-end value-added services will be possible.”

-What type of display is used on current wearable devices?

“Various types are used. Some VR devices borrow smartphone displays, some use micro-displays, and other wearable devices use lasers or other methods.”

-What are the advantages when stretchable technology is applied to wearable devices?

“We believe that wearable devices will be able to evolve towards being more comfortable by enhancing adhesiveness through changing it to be attachable than wearable since it can be made out of soft materials instead of hard devices.”

-What is the main field of your research?

“When electronic systems are stretched, the performance of the semiconductor component normally decreases and reduces its lifetime. Ultimately, a stretchable device that has no change in performance and lifespan regardless of deformation will be developed, but it still seems far away. Therefore, if a stretchable hybrid platform is developed that can prevent the actual device to be stretched even when the total electronic system is, the performance and lifespan of the device will not degrade. This can accelerate the development of the high-performance stretchable electronic device. In our lab, we are working on the stretchable platform and printed electronics for this. In the long term, our research aims to develop a device that has no problems with stretches. To explain more about the stretchable platform, the semiconductor chips and devices should be electrically connected to each other for the display to work. However, if we pull one side of the board, the area which receives a lot of energy can be broken. Therefore, a platform that can protect this area is needed. Hence, we are trying to develop the core platform solutions that allows the electronic system to be implemented on a stretchable board Also, we are working on applying printing technology to implement an electrode that has a spring structure so that the semiconductor chips and devices are not cut off.”

-How far are we with the technology?

“I believe that the stretchable hybrid electronic system is close to commercialization. Attachable display that can detect human body signals and show simple information can be developed currently. It can be commercialized soon enough if the production technology is verified and adequate application can be discovered.”

-What materials are used for stretchable display?

“The most well-known material is PDMS. It is a silicon-based rubber. To be used for stretchable display, not only it needs to be stretchable, but it should also be able to endure high temperature and have high resistance to chemical reactions. How well the PDMS satisfies these criteria will be known by further verifications.”

- What are the remaining works for the completion of the technology?

“We need production optimization for implementing string structure electrodes and verification for protection layer production after bonding solid chips that can endure various deformation. To apply this to wearables, we need sufficient battery and power supply technology. Furthermore, to develop stretchable electronic devices, in other words, high-resolution stretchable display, there will be further research on applying key component technology of the stretchable hybrid electronic system into current high-resolution flexible display used in smartphones and TV.”

- How long will it take for the commercialization of high-resolution stretchable displays?

“We look forward to seeing stretchable display products with new ideas in next 3~5 years.”

-Recently, the Galaxy Fold, a foldable smartphone by Samsung Electronics, is suffering from fatal flaws that are postponing the product release even after intense tests. Will it be possible to overcome the issues of conventional foldable and rollable displays if stretchable displays are developed?

“For foldable and rollable displays that have a very small radius of curvature, additional elasticity will be very helpful for the stability of the products. Therefore, there is a possibility that current problems can be overcome. It can be easily understood by comparing when the deformation area is made out of plastic or rubber. The part that is stressed by the deformation when implementing foldable or rollable displays can be handled if it can return to its original shape even after slight stretches.”

-What are some of the upcoming ripple effects as one of the 100 future technology?

“Stretchable display technology will bring innovation in the shape of mobile devices and will further play a key role in discovering new markets with the development of wearables and IoT. In particular, implementing a display on a stretchable platform rather than plastic will create new additional values in the field of display and semiconductor, which Korea stands in a leading position in the world, and will make huge contributions in widening the technology gap from late-movers like China. In addition, since new materials and core technology in production has bigger ripple effects on other fields, it will accelerate the introduction of attachable flexible electronic devices which will be the key component of the fourth industrial revolution including the field of sensor, bio, robot, environment, and energy.”

Who is professor Yongtaek Hong?

1994 BS in Electrical Engineering, SNU

1994 MS in Electrical Engineering SNU

2003 PhD in University of Michigan Ann Arbour Electrical Engineering

2003 ~ 2006 Senior researcher at Eastman Kodak

2006~ Professor at Department of Electrical and Computer Engineering, SNU

2012~2013 Visiting professor at Department of Chemical Engineering, Stanford University

2012~2014 2017~2018 Advisory professor of Samsung DMS

2016 ~ 2017 Advisory professor at Samsung Electronics

2018~ Advisory professor at Samsung Display, Samsung Electronics Wireless Division

Source: http://ee.snu.ac.kr/community/news?bm=v&bbsidx=49025

Translated by Kyungjin Lee, English Editor of Department of Electrical and Computer Engineering, jin11542@snu.ac.kr