By Christian Floerkemeier and Rahul Bhattacharyya

Oct. 1, 2010—In the past few years, there has been a lot of research and commercial development of sensor-equipped RFID tags, in particular, to monitor the shipping and handling of perishable foods and pharmaceuticals. The tags that have emerged typically range in price from $10 to $100 each. At the Auto-ID Lab at the Massachusetts Institute of Technology (MIT), our goal is to develop a sensor tag that costs no more than a basic passive tag—inexpensive enough to be placed, for example, on individual milk cartons to monitor temperature violations or in great numbers on concrete slabs to detect cracks.

Christian Floerkemeier (left) and Rahul Bhattacharyya

Our approach is to create a kind of "alarm" sensor, using a passive EPC Gen 2 ultrahigh-frequency tag and employing its antenna as a sensing mechanism. It is a well-known fact that RFID performance tends to degrade in close proximity to metals and water. We turn this shortcoming into an advantage by having a critical change in a physical parameter, such as temperature or strain, trigger a change in the position of a piece of metal or in the state of a dielectric in close proximity to the tag's antenna.

This critical change in the parameter of interest manifests itself in an altered RFID tag signal response, which, in turn, is picked up by the RFID reader. The changes to the antenna's immediate surroundings can be made permanent—even when the tag is not in range of an RFID reader. This means that a critical change in a physical parameter, such as a temperature violation, can be recorded for detection the next time the tag comes within range of a reader.