RF-identification (RFID) technology has the potential to provide information beyond logistics, such as a physical state and its time-evolution. Essentially, the tag may act as a self-sensing device, thanks to the dependence of its input impedance and radar cross section (RCS) on the physical and geometrical features of the tagged object or—in general—the close surrounding environment. Among the advantages to this approach is that it does not require any specific embedded sensor or local power supply. At Italy’s DISP-University of Roma Tor Vergata, a team of researchers has extended this idea to implanted antennas. The team’s goal was to sense the evolution of a physiological and pathological process involving a local change of effective permittivity inside the body.

The researchers—Cecilia Occhiuzzi, Giordano Contri, and Gaetano Marrocco—used an ad-hoc design methodology. With the self-sensing tag, there is no decoupling from the operative and structural point of view between the antenna and sensor. In other words, the antenna functions as the sensor and vice versa. As a result, the system’s sensitivity and dynamic range are strictly connected to the antenna’s features—especially its quality factor and bandwidth. Due to their high water content, however, human tissues are characterized by high permittivity and significant losses. Thus, even with a large bandwidth, the implanted tag will exhibit typically poor sensitivity to the change of the local environment.

The team found that sensing performance could be improved with various degrees of freedom, such as the shape of the antenna. For example, the radio sensor’s sensitivity can be enhanced by the use of high-impedance integrated circuits (ICs). Yet inversion curves, which relate the expected tag response to the change of the physical phenomena, can be shaped by means of impedance matching.

The researchers focused on a realistic medical case in which an endovascular device is modified, thereby achieving a STENTag that can sense the state of the vessel wherein that device has been implanted. The STENTag underwent in vitro experiments by means of equivalent liquid phantoms, which showed the simulations and measurements to be in reasonable agreement. See “Design of Implanted RFID Tags for Passive Sensing of Human Body: The STENTag,” IEEE Transactions On Antennas And Propagation, July 2012, p. 3146.