This paper provides an overview of flexible and wearable respiration sensors with emphasis on their significance in healthcare applications. The paper classifies these sensors based on their operating frequency distinguishing between high-frequency sensors, which operate above 10 MHz, and low-frequency sensors, which operate below this level. The operating principles of breathing sensors as well as the materials and fabrication techniques employed in their design are addressed. The existing research highlights the need for robust and flexible materials to enable the development of reliable and comfortable sensors. Finally, the paper presents potential research directions and proposes research challenges in the field of flexible and wearable respiration sensors. By identifying emerging trends and gaps in knowledge, this review can encourage further advancements and innovation in the rapidly evolving domain of flexible and wearable sensors.

This paper explores the potential of using vias for the miniaturization of frequency selective surfaces (FSSs). A new concept of knitting the loop-type FSS elements in 2.5-D is proposed, where successive segments of the loop are placed alternately on the two surfaces of the substrate and then interconnected through vias. A 2.5-D square-loop FSS (2.5-D SL-FSS) based on the proposed method is designed with the inclusion of ten vias at each side and characterized by a full-wave simulator. The transmission curves indicate a significant size reduction with a figure-of-merit λ ₀ /p = 16, where λ ₀ is the free-space wavelength of resonant frequency and p is the periodicity of unit element. In addition, the frequency response of this miniaturized FSS is also stable for various incident angles and polarizations. Furthermore, a general equivalent circuit model (ECM) is developed for 2.5-D SL-FSS by combining the prevailing electrical models of planar square loop and through-silicon vias. A wide set of parametric simulations for various element sizes, substrate thicknesses, and via counts are carried out with this ECM. Then, its performance is assessed on the basis of root-mean-square error (RMSE) criteria by comparing the results with appropriate electromagnetic simulations. The findings suggest that the ECM has sufficient accuracy for estimating the resonant frequency of 2.5-D SL-FSS with the RMSE values close to 3%. Moreover, the proposed concept of knitting is further validated by measuring two physical prototypes of the 2.5-D SL-FSS and the experimental results show a good consistency with full-wave simulations.

This paper provides an overview of flexible and wearable respiration sensors with emphasis on their significance in healthcare applications. The paper classifies these sensors based on their operating frequency distinguishing between high-frequency sensors which operate above 10 MHz and low-frequency sensors. The operating principles of breathing sensors as well as the materials and fabrication techniques employed in their design are addressed. The research highlights the need for robust and flexible materials to enable the development of reliable and comfortable sensors. Finally, the paper presents potential research directions and proposes research challenges in the field of flexible and wearable respiration sensors. Thus, identifying emerging trends and gaps in knowledge, it encourages further advancements and innovation in the rapidly evolving domain of flexible and wearable sensors.

This paper presents a passive displacement sensor based on the inductive coupling between a printed UHF RFID tag and a metallic strip. The sensor operates by exploiting variations in mutual inductive coupling, which modulate the tag impedance and transmission coefficient, thereby altering the backscattered signal strength and the maximum read range of the RFID tag. The performance of the sensor is validated through simulations and experiments, which demonstrate a sensitivity characterized by an approximately 9 dB reduction in the received signal strength indicator (RSSI) and a 2.3 m decrease in the read range within the first 12 mm of displacement. Furthermore, its potential for wearable applications is showcased through respiratory monitoring, where RSSI variations of approximately 5 dB are observed between the inspiration and expiration phases when positioned on the abdominal region of a volunteer. Thus, the proposed displacement sensing approach offers a cost-effective and battery-free solution for wearable applications with remote monitoring capabilities.

The transmission-line modeling (TLM) numerical method is employed to investigate the high-frequency shielding effectiveness (SE) of a metallic enclosure with unconventional aperture geometries. Analytical methods can predict SE but they have two main limitations: one is they can predict the SE of enclosure with very simple aperture shapes and second is that results cannot be predicted with accuracy after the first resonance frequency of cavity. We have investigated some new geometries for apertures from 0 to 3 GHz frequency band and discovered some important characteristics of these shapes by using the TLM based simulator. A novel concept of complementary apertures is introduced which has pronounced effect on cavity resonance modes and SE of enclosure. Another useful phenomenon was also observed and that is despite of longer aperture lengths perpendicular to electrical field (E-field), some apertures have better SE than others for the same coverage area.