Characterization of Electro-textiles Using Wireless Re∞ectometry for Optimization of Wearable UHF RFID Tags

Flexible and maintenance-free wearable ultra-high frequency (UHF) radio fre- quency identiflcation (RFID) tag antennas are desired in wireless sensor systems in security, healthcare, and biomedical applications. Optimization of wearable antennas demands compre- hensive knowledge of the electrical properties of the conductive electro-textiles used in the antenna structure. In this paper, we flrst propose a wireless re∞ectometry technique to accurately extract the sheet resistance of various electro-textiles. The technique relies on the measurement of the resonant peak of a scatterer. Its bandwidth and magnitude are strongly correlated with the con- ductive losses of the strips so that a relationship can be extracted. We then demonstrate that the electro-textiles, including conductive fabrics and embroidery textiles, can be accurately modeled using the sheet resistance as the only conductive material parameter for wearable antenna design. 1. INTRODUCTION Wearable body-centric radio frequency identiflcation (RFID) supports the wellness of people and play a key role in the development toward independent living. Wireless monitoring of people by means of low power and low cost technology is nowadays considered as one of the most promising features of wearable body-centric RFID systems (1,2). These systems include electronic devices, and sensing and signal processing abilities to enable functions such as monitoring of human vital signs (3), human movement and tracking (4), and detection of parameters of the environment, e.g., temperature or toxic gases. Wearable antennas play a key role in establishing a reliable and an e-cient wireless communication link between body-worn electronics and the surrounding environment. These inexpensive antennas should be comfortable to wear and totally maintenance- free. Typically the wearable antenna is using conductive ∞exible materials that are easy to integrate with clothing (2). These conductive materials are known as electro-textiles and include embroidery structures created from conductive threads and commercially available conductive fabrics Wearable passive UHF (860{960MHz) RFID tag antennas are of great interest thanks to their low cost, easy integrability, and large communication ranges. They contribute toward an intelligent environment, where tag antennas will be seamlessly integrated with daily garments to enable wireless body-centric communication everywhere and at anytime (5). As new unconventional materials for wearable antennas are emerging, it is of paramount impor- tance to characterize their applicability in body-centric systems. Compared to conductive fabrics, embroidered structures are inhomogeneous. The conductivity is dependent on the direction of the current ∞ow in the structure and the structure stitching density (2). The thickness of the structure is not unambiguously deflned. This creates challenges in modeling the embroidered structures for wearable antenna design. In this paper we eliminate the uncertainty related to the embroidered structure thickness by using the sheet resistance as the modeling parameter. First, we apply wireless re∞ectometry to characterize the sheet resistance for two difierent embroidered patterns and for three difierent conductive fabrics. We then design and simulate dipole tag antennas using the measured sheet resistance as the only electro-textile modeling parameter. Finally, to verify the validity of the electro-textile model, we fabricate and measure dipole tag antennas in terms of their theoretical read ranges and we compare the measured read ranges with corresponding simulated values.