This paper provides a rigorous theoretical formulation to obtain an inductive resonant wireless power transfer (WPT) link with load-independent output voltage or current. This is a crucial working condition for wireless battery recharging, where there is no deterministic knowledge of load variation with respect to the battery charging level. The ideal lossless and the realistic lossy configurations are considered for both voltage- and current-excited WPT links. The link transfer matrix (ABCD matrix) is used to determine the operating frequencies where a load-independent output voltage or current can be obtained. It is shown that in the lossy cases that an almost load-independent behavior can be achieved, provided that the load resistance lies in a suitable range and analytical conditions on the load resistance value have been derived. The analytical relationships obtained by this theory are validated by means of both circuit simulation and experimental data at 13.56 MHz.

In this work, two fully-textile wearable devices, to be used as chipless identification tags in identification and tracking applications are presented. For the fabrication of the fully-textile tags, a layer of fleece was used as a substrate, while an adhesive non-woven conductive fabric was employed for the conductive parts. To allow radio-frequency identification of these chipless tags, two alternative techniques were used. One relies on associating a binary code with the resonance frequency of resonant devices: the presence/absence of the resonance peaks in the transmission scattering parameter, $|S_{21}|$ , of a set of resonators is used to encode a string of bits. The second technique for accomplishing radio-frequency identification of the chipless tags resorts to a frequency-shift coding technique, which is implemented by modifying the configuration of a hairpin resonator. The obtained numerical and experimental results confirm the suitability of the proposed strategies for obtaining entirely-textile, wearable chipless tags for identification and tracking purposes, which can be particularly useful, especially in the industrial sector. In this field, in fact, the proposed solutions would guarantee a seamless integration with clothes and would facilitate the user’s interaction with the IoT infrastructure. In this regard, one of the envisaged application scenarios related to the tracking of hides in the leather industry is also presented.

One of the major goals of Health 4.0 is to offer personalized care to patients, also through real-time, remote monitoring of their biomedical parameters. In this regard, wearable monitoring systems are crucial to deliver continuous appropriate care. For some biomedical parameters, there are a number of well established systems that offer adequate solutions for real-time, continuous patient monitoring. On the other hand, monitoring skin hydration still remains a challenging task. The continuous monitoring of this physiological parameter is extremely important in several contexts, for example for athletes, sick people, workers in hostile environments or for the elderly. State-of-the-art systems, however, exhibit some limitations, especially related with the possibility of continuous, real-time monitoring. Starting from these considerations, in this work, the feasibility of an innovative time-domain reflectometry (TDR)-based wearable, skin hydration sensing system for real-time, continuous monitoring of skin hydration level was investigated. The applicability of the proposed system was demonstrated, first, through experimental tests on reference substances, then, directly on human skin. The obtained results demonstrate the TDR technique and the proposed system holds unexplored potential for the aforementioned purposes.

In this paper, a frequency-signature Radio-Frequency Identification (RFID) chipless tag for wearable applications is presented. The results achieved for a fully-textile solution guaranteeing a seamless integration in clothes are reported and discussed. The proposed tag consists of two planar monopole antennas and a 50 Ω microstrip line loaded with multiple resonators. In order to achieve a compact size, the resonators are slotted on the ground plane of the microstrip line. As for the antennas, the same geometry was exploited for both the TX and the RX tag antenna. In particular, it consists of a proximity fed planar monopole on a ground plane. The selected geometry guarantees easy integration with the multi-resonator structure. Numerical and experimental data referring to a 2-bit implementation are presented and discussed. For fabricating all the prototypes, a layer of pile was used as a substrate, while an adhesive non-woven conductive fabric was exploited for the fabrication of the conductive parts. Experimental tests demonstrate that although the performance of the final device strongly depends on the properties of the used materials and on the imperfections of the fabrication process, the proposed frequency-signature RFID chipless tag is suitable for wearable applications, such as anti-counterfeiting systems and laundry labels.

In spite of the technological advancement of the healthcare system, monitoring skin hydration still remains a challenging task. In particular, the state-of-the-art solutions are inadequate to obtain continuous real-time monitoring, especially in a wearable perspective. Starting from these considerations, this article addresses the implementation of an innovative wearable device that can monitor skin hydration through microwave reflectometry technique. To this purpose, a new flexible wearable sensing element (SE) was designed, manufactured, and validated to sense variation of skin hydration of the body, ensuring simultaneously a better wearability, and surface adherence to the skin. The system was preliminarily validated through a numerical analysis and then experimental tests were carried out through a low-cost portable vector network analyzer (VNA), which is particularly convenient in view of fully-wearable applications and comparable, from the metrological point of view to more expensive VNAs operating in the frequency range of interest. Results showed that the proposed SE configuration and the low-cost VNA hold potential for achieving a fully-wearable system for monitoring skin hydration.