Optimal Performance of Epidermal Antennas for UHF Radio Frequency Identification and Sensing

Skin-mounted electronics is the new frontier for unobtrusive body-centric monitoring systems. In designing the wireless devices to be placed in direct contact with the human skin, the presence of the lossy body cannot be ignored because of strong electromagnetic interactions. In this paper, the performance of epidermal antennas, for application to radio frequency identification (RFID) links in the UHF band, was investigated by means of numerical simulations and laboratory tests on fabricated prototypes. The analysis demonstrates the existence of an optimal size of the antennas (from 3 to 6 cm for loops and from 6 to 15 cm for dipoles) and of upper bounds in the achievable radiation gain (less than -10 dB in the case of 0.5 mm thick application substrates) as a consequence of the balance between the two opposing mechanisms of radiation and loss. This behavior, which is controlled by the hosting medium, does not depend on the antenna shape, even if the loop layout permits considerably minimizing the device size. Even the conductivity of the antenna trace plays only a second-order role; low-cost inkjet printable paints with conductivity higher than 104 S/m are suitable to provide radiation performance comparable with the performance of copper-made antennas. Starting from the investigation of the above cited physical phenomena, including the effect of common classes of suitable substrate membranes, guidelines are finally derived for the optimal design of real RFID epidermal antennas.