Textile-Integrated Wearable Radio Frequency (RF) Wireless Power Transfer and Harvesting for Battery-Free Medical Sensing

Abstract

Body-worn sensing is becoming increasingly important for patient monitoring, particularly the elderly. The vital signs can be directly transferred to their physicians, personal caregivers or used for self-diagnosis. To achieve this, wireless, wearable, and self-powered sensors are required. Wireless RF powering/charging, implemented into clothing, upholstery, and smart dressing presents itself as a convenient and practical solution for powering wearable sensors. In this dissertation, the implementation of such wireless charging platforms that are integrated into clothing, and the integration of medical sensor electronics is shown. Fabric-based wireless charging allows for lightweight, scalable, flexible, battery- less, and comfortable wearing. It is also important to address misalignment issues as users may not always be aligned with the RF transmitters used for charging. In this work, we develop wireless power transfer modalities with the receiving antennas integrated into clothing. The proposed antenna and circuits are realized by embroidering conductive threads onto fabric substrates. The dissertation begins with a characterization of the conductive textiles and microwave circuits used to optimize the substrate materials. Best design guidelines suggested the use of organza fabrics and the conductive textiles embroidered in the direction of the RF currents for up to 10 GHz. These design guidelines are used to develop textile-based rectifying circuits exhibiting RF-to-DC efficiency of 77.23% with 22.5 dBm illumination and RF-to-DC efficiency of 70% when illuminated with 8 dBm. Three clothing-integrated wireless charging platforms are presented. The first configuration used a near-_eld power transfer for ergonomic charging application. For this specific case, wireless charging was achieved by integrating the receiving antennas and rectifiers into clothing. The transmitters into items such as chairs and bedsheet. This system employed anchor-shaped antennas connected to a rectifying circuit resonant at 360 MHz to collect DC power up to 10 mW across an area of 4 ft X 4 ft with the near-field transmitting power at 1 W. A second configuration used a far-field transmitter illuminating a 2 X 3 textile-based patch rectenna array (antenna + rectifier) resonating at 2.45 GHz. This setup exhibited a DC power collection of 0.6 mW from a boosted Wi-Fi signal. In this case, the transmitter was within 10 cm of the receiver. In the third configuration, an electrochemical sensor, data-modulation and transmitter antenna blocks were added. This sensor system was illuminated by an interrogator transmitting RF power to the bandaid at 2.45 GHz. The bandaid featured a rectifying circuit to convert the external RF power into DC to power a textile-based electrochemical sensor and a voltage-controlled oscillator (VCO). The subject electrochemical sensor was uric acid detector whose uric acid concentration was translated into a frequency shift in the response of the VCO. Uric acid concentrations from 0.2 mM to 1 mM were detected and can be used to assess the health status of chronic wounds. The VCO outputted an RF signal whose frequencies are modulated by the uric acid sensor, and was found to be between 1089 MHz and 1120 MHz. The average sensitivity of the system was 44.67 MHz/mM and the maximum power used was 0.38 mW.