Development of e-textile based RFID enabled moisture sensor for wearable technologies

Abstract

The applications of e-textiles are progressively expanding with the advancements in technology on a daily basis. Particularly versatile, wearable, flexible, light, thin and small electronic structures can be seamlessly employed across diverse domains, owing to the incorporation of textile materials. It is noteworthy that the sensors used in heart rate, respiratory rate monitoring, sweat and wetness detection used in health monitoring areas have recently consisted of textile-based electronic structures. Although there are many sensor structures used for wetness detection, the disadvantages of their usage are that these structures are generally rigid, hard, large and uncomfortable. Instead of these sensors, thin, flexible, light, textile-based sensors that are compatible with the user's movement are needed. On the other hand, studies indicate that the world population is gradually aging and urinary incontinence is becoming more common health problem in aging people. For these reasons, structures that detect urinary incontinence on time are needed for sick elderly individuals in terms of both personal comfort and health of users, and to alleviate the workload of their caregivers. This thesis aims to develop a textile-based, wearable, flexible, lightweight and comfortable RFID enabled moisture sensor structures that detects moisture and wetness while transmitting RF signals. Furthermore, the objective lies on printing techniques and ink formulations for crafting these sensors. This thesis consists of three articles, arranged by paying attention to the integrity of the scope. All of the articles given within the scope of the thesis are original and add innovation to the literature. In the articles, important and unique results of the thesis topic are discussed. In the first article, the design of a humidity sensor for wetness detection with respect to design criteria found in the literature research, was presented. The designed humidity sensor was printed on a polyamide-based taffeta label fabric using PEDOT:PSS conductive polymer by inkjet printing method. Ink characterizations were carried out to make PEDOT:PSS polymer suitable for inkjet printing. Sensor performance was measured in a closed humidity chamber where the relative humidity is controlled by an automatic humidifier against change in electrical resistance values of the sensor. Response and recovery times, sensitivity and repeatability of the sensor sensor were also measured. The humidity sensor was integrated onto the diaper to detect urinary incontinence. The variations in electrical resistance values of the sensor were analyzed by exposing it to varying quantities of an urine-like water solution, applied to the sensor integrated onto the diaper. The second article examines the UHF-RFID antenna performance of the proposed sensor structure. The sensor structure designed in this article was printed on polyamide-based taffeta label fabric using silver nanoparticle ink by pad printing method with different print passes. The importance of the sintering process was investigated and its effect on the results presented in the article was discussed. While the sintering process was applied to one group of sensor samples, the sintering process was not applied to the other group for comparison. UHF-RFID antenna performance of all samples was examined using a vector network analyzer. In addition to impedance measurements, gain and bending measurements of the textile antennas were carried out in order to show the antennas' flexibility. It has been determined that the sensor samples exposed to the sintering process operate as antennas. Finally, the third article encompassed the execution of an RFID sensor design capable of serving dual roles as both a sensor and an antenna. The proposed structure was designed using the CST Studio program, aiming to operate at 867 MHz according to European band. The RFID sensor was printed on polyamide-based taffeta label fabric by pad printing method using silver nanoparticle ink. A chip was integrated onto the structure so that the RFID sensor operates at the targeted frequency. The RFID sensor was integrated onto the diaper considering the real usage environment. RSSI measurements were made using both near-field RSSI measurement setup and highly antenna arrays. Additionally, the strength of the proposed structure was investigated by applying bending simulation and deformation tests. RSSI changes of the structure were examined by dropping distilled water, salt water and artificial urine solutions to simulate urinary incontinence. In addition, the maximum reliable reading range was determined by changing the distances between the RFID sensor and the reader antenna. In summary, all three articles presented within the scope of the thesis contribute to the literature separately. The articles reveal that the thesis topic and the results obtained as a result of the studies are original and innovative. The fact that the thesis topic is up-to-date and coupled with its relevance to personal health monitoring makes the results of the study important for everyone. The outcomes delineated in the articles and the devised sensors carry profound significance in addressing the contemporary issue of urinary incontinence-wetness detection. Moreover, it is anticipated that the findings of this thesis will serve as a guiding source for researchers in this field.