Signals that reproduce the four well-known major taste components – salt, sweet, sour, bitter – are transmitted through a silver electrode touching the tip of the tongue. The taste receptors are fooled by a varying alternating current and slight changes in temperature controlled by semiconductor elements that heat and cool very rapidly.

http://www.newscientist.com/article/mg22029444.500-electrode-recreates-all-four-tastes-on-your-tongue.html?cmpid=RSS%7CNSNS%7C2012-GLOBAL%7Conline-news#.UpRIJX3TXMK