10.1117/2.1201101.003443 Electrowetting: a flexible electronic-paper technology Andrew Steckl, Duk-Young Kim, and Han You A new approach to color displays promises to combine the look and feel of paper with video speed for e-paper applications on flat and pliable substrates. Electronic readers, with their attractive functionality, are a rapidly growing market despite shortcomings of the most commonly used e-paper technology, electrophoretic displays (EPDs). 1 The limitations include a slow response time not suit- able for video operation, relatively low contrast, and an absence of color. However, since EPD e-readers operate using ambient light (in reflective mode), they have very low power consump- tion, resulting in long battery operation. The current main competition comes from liquid crystal display-based tablets, which use backlit transmissive displays with full color and video. Tablets have many additional capabilities beyond being e-readers. However, they consume more power and are generally bulkier and heavier than EPDs. Another approach (see Figure 1) being pursued uses electro- wetting 2 (EW) to form a light valve by moving two immiscible liquids (one clear and one colored) in and out of the light path by applying an electric field. Figure 2(a) shows a schematic illustra- tion of the EW effect in a pixel containing oil and water for zero and applied bias along with photographs of a portion of an ar- ray under corresponding conditions. EW technology has many applications, including flat-panel displays, electronic-focus lenses, and microfluidic devices. The EW light-valve display ap- proach is quite versatile, allowing for reflective, 3 transmissive, 4, 5 and even emissive 6 operation. Most important, its switching speed is in the millisecond time range, 3, 6 which enables video operation. An important consideration in all display technologies is power consumption. In EW displays, several approaches for minimizing power use have been reported, including bistable, 7 multi-value stable, 8 and complementary operation. 9 Figure 2(b) shows complementary operation of EW devices under applied voltage. We have achieved a reversal of the normal two- fluid competitive (water vs. oil) EW on a dielectric by plasma Figure 1. Flexible electronic-paper technology. irradiation of the normally hydrophobic fluoropolymer fol- lowed by thermal annealing. Similar to the reduction in power dissipation obtained when nMOS and pMOS transistors are combined in complementary CMOS, this method can lead to low-power operation of EW devices. The ability to reverse the polarity effect of EW and to operate in bistable modes is another indication of the flexibility of this technology. Multicolored EW displays using side-by-side subpixels and thin-film filters are now commercially available. 10 However, the side-by-side approach limits the ultimate resolution because a full-color pixel requires three subpixels. We have demonstrated 11 three-color EW pixels by stacking three color levels vertically, thus keeping the overall pixel size the same as that of each individual level. The levels can be operated inde- pendently, allowing for many color combinations (see Figure 3). We are currently developing flexible displays on paper substrates. 12 Figure 4 shows EW action on paper bent to form a small-diameter tube. We have obtained EW switching times that are nearly as fast as those on conventional glass substrates, indicating the possibility of video-rate display operation. Continued on next page