Micro-encapsulation is based on the same technology as scratch-and-sniff perfume ads in magazines. Both applications enclose a substance within a tiny capsule. In the ads, the substance is a sample of the perfume inside capsules that break under the pressure of a fingernail and release the aroma. In the ink, the capsules contain tiny chips of a positively charged pigment floating in a dye. The capsules are sandwiched between two electrodes that can apply positive or negative charges to groups of capsules. A negative charge on the top electrode attracts the pigment chips to the tops of a group of capsules. We see the capsules turn the color of the pigment chips. A positive charge on the top electrode repels the chips, pushing them to the bottoms of a group of capsules; we see the capsules turn the color of the dye. The different patterns of colors create the text on the digital paper.

For example, with white chips in a black dye, a negative charge attracts the chips to the tops of the capsules and that area turns white. A positive charge repels the chips to the bottoms of the capsules and that area turns black. The result is white text on a black background, or vice versa. The pattern of electrical charges is controlled by a microprocessor.

This technology was first developed by MIT's Media Lab and is now being commercialized by E Ink Corp. of Cambridge, MA, with a commercial release planned for late 1999. (Signs using the technology were being tested in the sporting goods department of a J.C. Penney's in the Solomon Pond Mall in Marlboro, MA in April 1999.) In addition, other electronic ink technologies are under investigation by PARC in Palo Alto, CA and by an IBM research group. (The IBM group's design study for an electronic "newspaper" based on electronic ink won a design award from Business Week - Business Week, June 7, 1999, page 88.)

The biological approach is based on a protein called Bacteriorhodopsin. This protein, found in a bacterium that lives in salt marshes, switches color between yellow and blue when hit by light or an electrical charge. Sandwiching a layer of the protein between two plates with addressable arrays of electrodes could produce fast, high-contrast displays. However, the biological approach is still a laboratory curiosity due to the need for a charge of several thousand volts to produce the effect.

• Electronic "pages" mean that a book's contents are no longer fixed. A book is simply a display device, like a screen, that can show anything. One sheet of digital paper might form a book, packaged inside a hard cover solely to simulate the look of a book. (If electronic ink vendors reach their goal of reducing the cost of digital paper to that of real paper, the paper cost of documentation will fall to near zero.)
  
  
• The distinction between online and hardcopy becomes blurry. One complaint about PDF documents is that they're not quite paper, but not quite online. However, what if the screen is the same size as the printed page and feels the same? Is this hardcopy, online, or something new - ePaper? Imagine dynamic "Read Me First" instruction sheets or errata sheets that connect to a vendor's web site and update themselves.

  (For years, there's been talk about creating personalized newspapers to avoid printing and distributing sections, like Autos, that most people don't read. The software exists but the result doesn't offer the tactile experience of a real newspaper. Reading a "newspaper" on a screen doesn't cut it. Printing the personalized newspaper is better, but it's the wrong size. A sheet of digital paper could be the same size as your regular newspaper, providing a very similar experience.)

• The cost model changes. Production costs are near zero since there's no assembly. Storage costs drop to zero since the "warehouse" is some space on a hard disk. Scrap costs are zero since there are no out-of-date printed manuals; you just overwrite an old file with a new one. Mail costs are zero since you're no longer sending pounds of material through the interoffice or US Mail system; you're just shooting it over a network. None of these benefits are unique to electronic ink, but they are benefits.
  
  
• Business travelers should love digital paper because of the weight savings it promises. Memory chips in the "spine" of a book might hold entire civil engineering standards books or field service specs, and a novel for reading back in the hotel --- thousands of pages of material in a package the size of one hard-cover book but a fraction of the weight. (eBooks are conceptually similar and available today at prices ranging from $199 to $1,500. They're convenient, but more expensive and fragile than digital paper books should eventually turn out to be.) If you create documentation used in the field, start watching the progress of electronic ink.

And, consider the off-the-wall applications:

• Video greeting cards. Remember your first musical Christmas card? Once cutting-edge, they're sold today at discount stores. As the cost of digital paper falls and the interfacing issues are settled, we'll be able to videotape the family singing Happy Birthday and mail the result bound into a card.
  
  
• Electronic clothing. If vendors can create digital paper that resists damage (since breaking the capsules spills the dye), it could be sewn into clothing to create dynamic designer labels.
  
  
• Real flat screens. Today's flat screens are expensive and still bulky. Flat screens made of digital paper could be pinned to the wall during working hours and rolled up and put away at the end of the day.

EInk Corp., www.electronic-ink.com

Electronic ink from modified bacteriorhodopsin, Bulletin of the American Physical Society, March 1997 (hardcopy)

Intro to microencapsulation, www.swri.org/3pubs/ttoday/summer/microeng.html