Design and implementation of a wearable patch antenna that serves as a longitudinal strain sensor

Abstract

This paper describes the design and simulation of a rectangular wearable patch antenna. A parametric study of the antenna was conducted to determine the effect of subjecting it to longitudinal mechanical strain (along the x-axis) on its resonant frequency. The antenna-sensor was based on a cotton fabric dielectric and conductors made of flexible copper sheets that operated at a central frequency of 2.4 GHz, which is in one of the Industrial, Scientific, and Medical application bands. The Deming’s cycle, or Plan, Do, Check, and Act, was the adopted methodology in this study to address this research problem. The resonant cavity technique was implemented to find the relative permittivity and loss tangent of the textile substrate, and a universal testing machine was used to establish its mechanical properties (i.e., Young’s modulus and Poisson’s ratio). The mechanical properties of the dielectric materials, the elastic modulus in tensile loading (69.34 MPa), and the experimental value of the Poisson's ratio (0.342) were extracted from the literature. Based on the CST (Computer Simulation Technology) datasheet of flexible copper, its elastic modulus in tensile loading is 240 MPa and its Poisson's ratio is 0.39. A Computer Numerical Control machine was employed to cut the flexible copper, and the cotton fabric was cut by hand based on the dimensions of the ground plane. The patch was sewn with strong textile thread at the center of the ground plane and the cotton fabric. Such sewing ensured the physical resistance of the antenna to withstand the conditions of the multiple strains it was subjected to. The antenna implemented here resonated at a frequency of 2.3968 GHz (with a 0.13% error rate) and was well coupled with the transmission line with a Standing Wave Ratio of 1.04. CST Microwave Studio® software was used to simulate the antenna frequency response to mechanical strains based on the reaction of its return losses ( $S_{11}$ in dB) to be compared with experimental rigs that bend at a different level of stresses. In line with the theory in the literature, the resonant frequency of the antenna was linearly and inversely proportional to the applied stress, which enabled us to calculate the transduction ratio of the sensor in terms of strain versus frequency. In the experimental setup, the range of variation of the resonant frequency of the sensor was 143.6 MHz, with a very good sensitivity of 2.38 $\left(\frac{\mathrm{MHz}}{\mu \phi }\right)$ . These results pave the way for future studies in which this sensor is used as part of a biomedical system to monitor the vital signs of patients (such as respiratory rate, lung capacity, and performance under different types of physical effort; for example, in high-performance athletes) and diagnose diseases or other kinds of disorders associated with respiratory problems.