Electronic ink is ink the pixels of which that can be turned "on" or "off" using an electric field. It's implemented at least two ways, either using electrically charged balls of ink, background colored on one hemisphere and ink colored on the other, or using capsules containing a suspending liquid and ink and background particles, with each type of particle having an opposite electric charge. Application of an electric charge either rotates the ink ball (in the first implementation), or attracts/repels the appropriate type of particles (in the second implementation), to turn the ink on or off. (see: below for explanation of technology, thanks for fixing the ref, Aristotle.)

Using the second implementation, inject the capsules in place of tattoo ink. The tattoed person can use a handheld electric field generator to alter the tattoo to taste.

(In case you wonder, I don't have tattoos. My body's already too distracting.)
-- protean, Aug 08 2001

E-Ink http://www.eink.com/technology/index.htm
An electronic ink company spun off from MIT research. [Aristotle, Aug 08 2001, last modified Oct 04 2004]

Gyricon Media http://www.gyriconm...martpaper/index.asp
As an ex-Xeroid I'm honour bound to mention a competitor spun off from Xerox PARC research. [Aristotle, Aug 08 2001, last modified Oct 04 2004]

US Patent 6,192,890 http://patft.uspto....890&RS=PN/6,192,890
A changeable tattoo system with an electrically modifiable ink that includes biocompatible microspheres or particles encapsulated in a clear microsphere shell of diameter 30-150 microns. [jutta, Aug 08 2001]

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