Bichromal balls have two hemispheres, typically one black and one white, each having different electrical properties. Each ball is enclosed within a spherical shell and then a space between the ball and shell is filled with a liquid to form a microsphere so that the ball is free to rotate in response...http://www.google.co.uk/patents/US5604027?utm_source=gb-gplus-sharePatent US5604027 - Some uses of microencapsulation for electric paper
