Stretch of Imagination

by Ruiqing (Ray) Ma

I don’t know about you, but every day before I sit down at work, I reach into my pocket, take out my phone, and put it on my desk. These devices are so big and rigid nowadays, they no longer fit comfortably in a pocket when you’re sitting down. It is frustrating that we still don’t have a truly flexible and conformal device, even though displays with these capabilities were developed a long time ago in the lab. In fact, the display community has already moved on to the next frontier of flexible displays – stretchable. In this special issue of Information Display, we have prepared three excellent articles to report the latest developments in the field of stretchable and wearable display technology.

To electronically address a stretchable display, either a stretchable electrode or a stretchable interconnect must be developed. Professor Hong and his colleagues provide an overview of the status of stretchable displays in their article, “Stretchable Displays: From Concept Toward Reality.” To make a display itself stretchable, one method is to make every layer of the display stretchable, including the electrodes. Another is to make the display so thin that it can be pre-wrinkled into a much smaller area, making stretching possible later. For a high-information-content display with high-density pixels, one practical approach is to make a hybrid structure – leaving the active display pixel alone and having the interconnect take on the large strain during stretching. One can imagine making such a hybrid system is extremely challenging. You will be impressed by what the researchers have achieved – a fully printed, soft, platform-based passive-matrix LED display capable of withstanding 30 percent biaxial tensile strain.

In the article, “Stretchable Oxide TFT for Wearable Electronics,” Dr. Li and Professor Jin Jang report their latest results on stretchable oxide TFTs. First, they developed an ultra-thin oxide TFT between two 1.5-μm polyimide (PI) substrates. Because they are placed in a neutral plane, these TFTs are super flexible – operational even after bending 20,000 times to a radius of 0.25 mm. These TFTs were then transferred onto pre-treated islands of polydimethylsiloxane (PDMS) stretchable substrate. Due to UV/O₃ treatment, these islands are relatively rigid, providing necessary protection for the TFT during stretching. With a mobility of ~14 cm²/Vs, these TFTs showed less than 8 percent change during repeated stretching up to 50 percent strain, which is a remarkable achievement. With further optimization and improvement, this technology could offer exciting opportunities in wearable and stretchable electronics.

The last article takes a novel approach from the liquid-crystal level. We all know why we don’t have a truly flexible device yet – the display doesn’t exist by itself. There are so many other components in the device that are rigid and that also need to be made flexible. However, there may be a simple solution for this – getting rid of those components. In the article, “Developing Liquid-Crystal Functionalized Fabrics for Wearable Sensors,” Dr. Junren Wang and his colleagues describe the development of a simple device – a passive sensor using liquid crystal (LC). The flexibility and stretchability are elegantly addressed by the structural arrangement of fibers – the basic component in fabrics and textiles. In addition, liquid crystals, such as short-pitch cholesteric, smectic C, and blue phase, are perfect visual sensing materials, as they respond through color change to a wide variety of stimuli such as temperature, pressure, and other types of mechanical stress, as well as electrical, magnetic, and optical fields. Of course, integrating liquid crystals with fibers for wearable application is not easy. Many factors need to be considered: temperature range, adhesion to the fibers, and visual appearance, just to name a few. Overcoming many challenges, the team successfully developed thermochromic fabrics by bonding microcapsules of LC to fiber surfaces, or by fabricating coaxial fibers consisting of an LC core surrounded by a polymer sheath.

As shown in these three articles, significant progress has been made in stretchable displays in the past several years. Stretchable electrodes, interconnects, TFTs, and AMOLED displays have all been demonstrated. However, we are still facing the same challenge – how can we integrate this great technology into a device to make, dare to say, a stretchable device? The wearable sensor is a great idea – color-changing sensing for a color-dominant clothing application that requires no other components (not even electrodes). Alternatively, we need to develop applications that don’t use many rigid components, or applications for which the rigid components can be hidden.

We can already stretch our displays; now let’s try stretching our imaginations.  •