CROSS REFERENCE TO RELATED
This application is a non-provisional of Provisional Application Ser. No. 60/419,019, filed Oct. 16, 2002. This application is also a continuation-in-part of application Ser. No. 08/983,404, filed Mar. 26, 1999 now U.S. Pat. No. 7,106,296, which is the United States National Phase of 10 International Application No. PCT/US96/12000, filed Jul. 19, 1996, which is itself a continuation-in-part of application Ser. No. 08/504,896, filed Jul. 20, 1995 (now U.S. Pat. No. 6,124,851). The entire disclosure of all the aforementioned applications, and of all U.S. patents and published applica- 15 tions mentioned below, is herein incorporated by reference.
BACKGROUND OF INVENTION
This invention relates to dielectrophoretic displays, and 20 methods for operating such displays. More specifically, this invention relates to dielectrophoretic displays in which solid particles and a suspending fluid are held within a cavity.
Electrophoretic displays have been the subject of intense research and development for a number of years. Such 25 displays use a display medium comprising a plurality of electrically charged particles suspended in a fluid. Electrodes are provided adjacent the display medium so that the charged particles can be moved through the fluid by applying an electric field to the medium. In one type of such 30 electrophoretic display, the medium comprises a single type of particle having one optical characteristic in a fluid which has a different optical characteristic. In a second type of such electrophoretic display, the medium contains two different types of particles differing in at least one optical character- 35 istic and in electrophoretic mobility; the particles may or may not bear charges of opposite polarity. The optical characteristic which is varied is typically color visible to the human eye, but may, alternatively or in addition, be any one of more of reflectivity, retroreflectivity, luminescence, fluo- 40 rescence, phosphorescence, or (in the case of displays intended for machine reading) color in the broader sense of meaning a difference in absorption or reflectance at nonvisible wavelengths.
Electrophoretic displays can be divided into two main 45 types, namely unencapsulated and encapsulated displays. In an unencapsulated electrophoretic display, the electrophoretic medium is present as a bulk liquid, typically in the form of a flat film of the liquid present between two parallel, spaced electrodes. Such unencapsulated displays typically 50 have problems with their long-term image quality which have prevented their widespread usage. For example, particles that make up such electrophoretic displays tend to cluster and settle, resulting in inadequate service-life for these displays. 55
An encapsulated, electrophoretic display differs from an unencapsulated display in that the particle-containing fluid is not present as a bulk liquid but instead is confined within the walls of a large number of small capsules. Encapsulated displays typically do not suffer from the clustering and 60 settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates.
For further details regarding encapsulated electrophoretic 65 displays, the reader is referred to U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588;
Patent Applications Publication Nos. 2002/0019081; 2002/ 0021270; 2002/0053900; 2002/0060321; 2002/0063661; 2002/0063677; 2002/0090980; 2002/0106847; 2002/ 0113770; 2002/0130832; 2002/0131147; 2002/0145792; 2002/0171910; 2002/0180687; 2002/0180688; 2002/ 0185378; 2003/0011560; 2003/0011867; 2003/0011868; 2003/0020844; 2003/0025855; 2003/0034949; 2003/ 0038755; 2003/0053189; 2003/0076573; 2003/0096113; 2003/0102858; 2003/0132908; 2003/0137521; 2003/ 0137717; and 2003/0151702; and International Applications Publication Nos. WO 99/67678; WO 00/05704; WO 00/38000; WO 00/38001; WO 00/36560; WO 00/67110; WO 00/67327; WO 01/07961; and WO 01/08241. All of these patents and applications are in the name of, or assigned to, the Massachusetts Institute of Technology (MIT) or E Ink Corporation.
Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called "polymer-dispersed" electrophoretic display in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, U.S. Pat. No. 6,392,786, at column 6, lines 44-63. See also the aforementioned U.S. Patent Application Publication No. 2002/0131147, and the corresponding International Application PCT/US02/06393 (Publication No. WO 02/075443. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.
A related type of electrophoretic display is a so-called "microcell electrophoretic display", sometimes also called a "microcup" electrophoretic display. In a microcell electrophoretic display, the charged particles and the suspending fluid are not encapsulated within microcapsules but instead are retained within a plurality of cavities formed within a carrier medium (or substrate), typically a polymeric film. See, for example, International Applications Publication No. WO 02/01281, and published U.S. Application No. 2002/ 0075556, both assigned to Sipix Imaging, Inc.
Hereinafter, the term "microcavity electrophoretic display" will be used to cover both encapsulated and microcell electrophoretic displays.
One of the problems with electrophoretic displays is the limited range of colors which each pixel of the display can produce. The prior art describes two main types of electrophoretic media. One type comprises a single type of electrically charged particle in a colored medium. This type of medium is only capable of producing two colors at each pixel; either the color of the particles or the color of the medium is seen. The second type of medium comprises two different types of electrically charged particles in an essentially uncolored medium; the two types of particles may differ in polarity of electric charge or have charges of the