Television screens and computer monitors are so ubiquitous in our daily lives that the notion of a “display” is almost inevitably linked to something rigid and rectangular. However, even as technology moves toward smaller and more portable devices, there is also a strong and growing interest in the physical, tangible reality of the things we interact with.
Here, we explore emerging display technologies, emphasizing their application in flexible and deformable devices that potentially take on any shape or form. In each case, the most important factor is how the technology is (or might be) adapted to integrate with an existing flexible substrate (such as a solid sheet of plastic or woven fabric). In this sense, the field of flexible displays is as much a matter of process innovation and materials research as it is a question of electrical or computer engineering.
Sometimes hailed as a successor to LCD technology, organic light-emitting diode (OLED) displays are based on organic polymer molecules that compose emissive and conductive layers of the display structure composited together through a form of printing. The layers are deposited in rows and columns that result in a matrix of pixels that emit light. Emissive OLED displays do not require backlighting and are viewable at oblique angles. They are also transparent; red, green, and blue layers can be stacked such that a full-color (RGB) pixel is a fully color-mixed single pixel with depth, rather than a closely spaced planar cluster of pixels as in traditional CRT and LCD displays.
OLEDs are commercially available today in a mass market of smaller displays (such as in car stereos, MP3 players, and cell phones). More innovative use of the technology awaits the streamlining of manufacturing methods; for example, flexible display screens are being developed using plastic substrates (such as thin polyester films and bendable metallic foils). Technology demonstrations by a number of companies, including Polymer Vision (polymervision.com), feature devices with rollable displays (see Figure 1). Taking the concept a step further, we anticipate development of large, flexible display interfaces that bend, flex, and conform to many surfaces.
OLED technology is also a focus of interest as a path to energy-efficient solid-state lighting. Since organic polymer layers can be manufactured as large-area active elements, it is possible to combine area color, shape, and flexibility to create novel interactive objects and interfaces. For instance, researchers at General Electric Global Research (www.ge.com/research/) anticipate light-emissive curtains and wallpapers.
Electrophoretic displays (EPDs), often associated with the brand name E-Ink (www.eink.com/), are marketed as alternatives to traditional flexible paper. One type of particle display, “electronic ink,” consists of thousands of microcapsules deposited onto a substrate. Each microcapsule contains positively charged white particles and negatively charged black particles suspended in a clear fluid. When a negative electric field is applied, the white particles move to the top of the microcapsule, causing that “pixel” to appear white and vice versa (see Figure 1). The microcapsules are bi-stable, meaning that once configured as black or white, no further energy is required to maintain their state. As with OLED, flexibility is achieved through the use of flexible substrates (plastic) and conductors (metal foil or printed conductive traces).
Applications of E-Ink are primarily as a paper substitute in traditionally monochrome paper-based media (such as signage and e-books) but also include irregularly shaped and flexible displays. For example, the Seiko Watch Corporation (www.seikowatches.com) has produced a limited run of unique watches based on a small, flexible E-Ink display. E-Ink also prototyped various color displays and demonstrated multicolor EPDs using color filters [2].
In addition to even newer technologies still being developed and refined, existing technologies offer some of their benefits in the form of displays or light-emitting materials. For example, electroluminescent lighting (EL) is a technology through which thin, flexible lamps are produced via an industrial printing process. A layer of phosphor is sandwiched between two conductive layers, illuminating when an alternating electrical current is applied across the layers. EL is widely used in backlighting applications for portable electronics, as well as for large-surface-area applications. Manufactured EL panels can be cut into irregular shapes, as well as printed onto, lending themselves to advertising and signage.
At www.aeolab.com, we have applied the materials and processes used in the manufacturing of commercial electroluminescent panels to hand-print custom-designed light panels on paper and fabric within a studio environment. The Puddlejumper raincoat (www.mintymonkey.com/puddlejumper_p1.html) developed by Elise Co features EL panels silk-screened onto flexible fabric and activated via water-droplet sensors printed with conductive inks (see Figure 2).
In addition to flat panels, EL lamps are also manufactured as wire elements and packaged in clear plastic tubing of varying diameters. In this form, the material is well-suited to creative manipulation as a fiber, combined with other materials or integrated directly into textiles with either a woven or knitted structure. Artists and designers have used EL wire to make light-emitting artifacts ranging from garments to lamps to spatial installations (such as Loop’s Sonumbra (www.loop.ph), a net-like illuminated canopy.
Optical fiber is another product that can be used creatively as a material for lighting and display. Specially treated “side-emitting” fibers (with outer coatings that diffuse light along the length of the strand rather than reflecting it perfectly within the interior core) are produced in thicknesses of up to a quarter of an inch. Strands of such fiber are woven into fabric and embedded into other materials, then coupled with light sources at the fiber ends to create unique textile and flexible-display surfaces. These integration techniques are also applicable to standard (end-emitting) fibers, resulting in small points of light rather than glowing lines or strands.
Discrete light-emitting diode (LED) lights are also used to create active surface topologies. Although LEDs are not inherently flexible, their small manufacturable size and simple circuitry means they can be dispersed over a flexible substrate. ColorKinetics, Inc. (www.colorkinetics.com) and Element Labs (www.elementlabs.com) offer products that incorporate LEDs into collapsible bendable matrix configurations and flexible strands. Others embed matrices of LEDs in flexible substrates that can be curved, formed to a surface, and even used as a wearable material. Lumalive technology from Philips Research (www.lumalive.com/business/) features fabrics and clothes embedded with LED matrix displays constructed in this fashion.
Innovative display interfaces are not limited to light-emitting sources. Alternative active materials (such as thermochromic inks that change color with temperature) can be used to construct novel display surfaces. A number of artists and designers, including International Fashion Machines (ifmachines.com) and XS Labs (www.xslabs.net), have used these inks as overprints on top of textiles that incorporate conductive spun threads. When a current is applied to the textile, resistive heating activates the printed ink and initiates a color change. Heating and cooling the metal filament manipulates the color of the textile-display over time [1]. As with the other examples we’ve outlined here, even the relatively limited behavior of such a system can be parlayed into a sophisticated multichannel output device through creativity of process and craft.
Conclusion
The practical application of flexible displays to particular user scenarios appears to be strongest in the fields of product, military, and fashion design. Flexibility or perhaps elasticity is inherently desirable for anything worn on the body. The tactile properties of soft and malleable surfaces also make sense in myriad design and interactive environments. What is interesting about the general domain of nonrigid displays is that so many aspects of design and engineering converge to generate displays that are also materials. From them we can imagine displays that curve to fit any space or form, flex to accommodate motion, and deform in response to physical interaction. Rollable or foldable displays for portable devices, large-scale interactive surfaces, and textiles with integrated displays in turn permit the design of user interfaces that are physically, as well as conceptually, flexible.
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