http://www.gizmag.com/ge-develops-battery-...fid-tags/10194/
"GE Global Research has announced a new type of radio-frequency identification (RFID) sensor that by-passes the need for on-board batteries by drawing power wirelessly from a hand-held reading device. The significance of the design is that it facilitates the manufacture of much smaller sensors at low cost, opening up a new range of potential applications from measuring the freshness of packaged food goods to more effective detection of biological threats."