Performance evaluation of textile based passive RFID antennas as wearable sensors

Conformal and stretchable wearable sensors provide real-time information about individual's health conditions. There are a lot of vital signs and parameters of the human body that are supposed to be sensed by the sensors like, body movement, body temperature, Electrocardiogram (ECG), Electroencephalogram (EEG) etc. Hence, there are a lot of health tracking devices available in the market for different purposes. One of the most important sensors are the hydration/moisture/humidity sensors. These sensors are required for the health fitness and for the medical care of the patients. However, as far as the skin sensors are concerned, they are facing one important challenge, which is to have better contact with the body to have better results to analyze as well as providing ease and comfort to the patient/user. In this work, Radio frequency identification technology (RFID) has been used to achieve and overcome the challenge. Since RFID is a prevailing technology in which a microchip in a label used to transmit data when the label is exposed to radio waves. RFID technology can easily be understood by the concept of student cards used in our university where student cards are working as the Tag and the readers planted on the door slots read them. The data/information read by the reader is stored in the database for every specific tag (transponder), to be accessed it later. Passive Ultra-High Frequency (UHF) RFID tags are here used as moisture sensors. The tags for the mentioned challenged used here for different application as stated before, are specifically textile tags. There are two types of textiles (Substrate) that have been used; cotton, which is organic in nature and stretchable synthetic textile, which is a mixture of viscose and polyester. The IC chip containing the information is attached to the antenna that is designed on the substrate which is acting as a Tag (Sensor), one with glue and the other with embroidery. The most specific part is the tag is embroidered with silver thread, which is conductive in nature. Hence embroidery is the fabrication method as well as the vital part of making the tags. The embroidery is accomplished with the help of domestic sewing machine. To get different results, different embroidery designs have been used; single line (less dense), horizontal embroidery and vertical embroidery. Moreover, six tags are fabricated using cotton substrate and two tags are fabricated using stretchable substrate, both substrates have IC antenna attached with sewing as well as glue as mentioned before. When the fabricated sensors (Tags) were tested in the anechoic chamber, all the sensors have different behavior with different read ranges as well as different peak frequencies. The objective was to test the humidity/moisture evaluation on the sensors. Hence, the sensors were very well exposed to the moisture and were tested again. The sensors with less dense embroidery (Single Line) were wetter than the dense embroidery (vertical and horizontal designs), hence, making the frequency more affected in terms of putting the frequency at a lower level in the less dense embroidered sensors than the dense ones. After being dried up, after 48 hours, the sensors were almost back the initial read range values. Therefore, the frequency difference between the initial read ranges and the moist read ranges is of vital importance and all the tags are having different behaviors. As the tags are textile in nature and are embroidered like a simple cloth, they are easy to wear and have very better contact with the body to have better results in terms of moisture evaluation. So further fabrication technique in the prospect of UHF RFID has multiple applications e.g. wounds sensor inside the bandages, soil moisture sensor, moisture/humidity leakage sensor etc. Hence, they have very vital advantages, which include that they are passive, cost-effective, and simple