Wearable Antennas for Sensor Networks and IoT Applications: Evaluation of SAR and Biological Effects

Abstract

In recent years, there has been a rapid development in the wearable industry. The growing number of wearables has led to the demand for new lightweight, flexible wearable antennas. In order to be applicable in IoT wearable devices, the antennas must meet certain electrical, mechanical, manufacturing, and safety requirements (e.g., specific absorption rate (SAR) below worldwide limits). However, the assessment of SAR does not provide information on the mechanisms of interaction between low-intensity electromagnetic fields emitted by wearable antennas and the human body. In this paper, we presented a detailed investigation of the SAR induced in erythrocyte suspensions from a fully textile wearable antenna at realistic (net input power 6.3 mW) and conservative (net input power 450 mW) conditions at 2.41 GHz, as well as results from in vitro experiments on the stability of human erythrocyte membranes at both exposure conditions. The detailed investigation showed that the 1 g average SARs were 0.5758 W/kg and 41.13 W/kg, respectively. Results from the in vitro experiments demonstrated that the short-term (20 min) irradiation of erythrocyte membranes in the reactive near-field of the wearable antenna at 6.3 mW input power had a stabilizing effect. Long-term exposure (120 min) had a destabilizing effect on the erythrocyte membrane.

Keywords: EMF; SAR; antenna for sensor applications; biological effects; erythrocyte membrane; fully textile antenna; reactive near-field; wearable antennas.

Figure 1

Overview of the exposure setups for investigation of the biological responses at: (a) realistic exposure conditions (net input power 6.3 mW) relevant to actual human exposure conditions when the antenna is placed on the human body; (b) conservative exposure conditions (net input power 450 mW) at sufficiently high exposure levels.

Figure 2

Numerical models of (a) wearable antenna, cuvettes, and erythrocyte suspension, and (b) flat homogeneous phantom and SAR distribution on the surface of the phantom induced from the wearable textile antenna, as incorporated in the FDTD model.

Figure 3

Thermographic image of the wearable antenna and the two cuvettes with erythrocyte suspensions immediately after EMF exposure.

Figure 4

Antenna performance: (a) VSWR and (b) 2D radiation patterns in xz- and yz-plane.

Figure 5

Histograms and descriptive statistics of SAR distribution for the erythrocyte suspensions at net input power of 450 mW in: (a) left and (b) right cuvette.

Figure 6

Histograms and descriptive statistics of SAR distribution for the erythrocyte suspensions in the layer with the highest SAR values at net input power of 450 mW: (a) left cuvette; (b) right cuvette; (c) left cuvette color-scale visualization of the SAR distribution; (d) right cuvette color-scale visualization of the SAR distribution.

Figure 6

Histograms and descriptive statistics of SAR distribution for the erythrocyte suspensions in the layer with the highest SAR values at net input power of 450 mW: (a) left cuvette; (b) right cuvette; (c) left cuvette color-scale visualization of the SAR distribution; (d) right cuvette color-scale visualization of the SAR distribution.

Figure 7

Histograms and descriptive statistics of SAR distribution at net input power of 450 mW for the erythrocyte suspensions in the layer at the bottom of (a) left cuvette; (b) right cuvette; (c) left cuvette color-scale visualization of the SAR distribution; (d) right cuvette color-scale visualization of the SAR distribution.

Figure 8

Histograms and descriptive statistics of SAR distribution at net input power of 450 mW for the erythrocyte suspensions in the cross-section of (a) left cuvette; (b) right cuvette; (c) left cuvette color-scale visualization of the SAR distribution; (d) right cuvette color-scale visualization of the SAR distribution.

Figure 8

Histograms and descriptive statistics of SAR distribution at net input power of 450 mW for the erythrocyte suspensions in the cross-section of (a) left cuvette; (b) right cuvette; (c) left cuvette color-scale visualization of the SAR distribution; (d) right cuvette color-scale visualization of the SAR distribution.

Figure 9

Normalized levels of the hemoglobin released by the control (sham-exposed) and exposed erythrocyte suspensions for 120 min to 2.41 GHz (WSA SAR = 0.4149 W/kg, psSAR1g = 0.5758 W/kg, wearable antenna input power 6.3 mW, Zigbee-like signal). The mean of 7 experiments ± SEM is shown.

Figure 10

Normalized levels of the hemoglobin released by the control (sham-exposed) and exposed erythrocyte suspensions for 20 min to 2.41 GHz (WSA SAR = 0.4149 W/kg, psSAR1g = 0.5758 W/kg, wearable antenna input power 6.3 mW, Zigbee-like signal). The mean of 10 experiments ± SEM is shown.

Figure 11

Normalized levels of the hemoglobin released by the control (sham-exposed) and exposed erythrocyte suspensions for 20 min to 2.41 GHz (WSA SAR = 29.6325 W/kg, psSAR1g = 41.13 W/kg, wearable antenna input power 450 mW, Zigbee-like signal). The mean of 4 experiments ± SEM is shown.

Figure 12

Normalized levels of the hemoglobin released by the control (sham-exposed) and exposed erythrocyte suspensions for 120 min to 2.41 GHz (WSA SAR = 29.6325 W/kg, psSAR1g = 41.13 W/kg, wearable antenna input power 450 mW, Zigbee-like signal). The mean of 4 experiments ± SEM is shown.

Figure 13

Normalized levels of the hemoglobin released by the control (sham-exposed), conventional heat exposed (38 °C) and EMF-exposed erythrocyte suspensions for 60, 120, 180, and 240 min to 2.41 GHz (WSA SAR = 29.6325 W/kg, psSAR1g = 41.13 W/kg, wearable antenna input power 450 mW, Zigbee-like signal). The mean of 4 experiments ± SEM is shown.