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A wristwatch-based wireless sensor platform for IoT wearable health monitoring applications is presented. The paper describes the platform in detail, with a particular focus given to the design of a novel and compact wireless sub-system for 868 MHz wristwatch applications. An example application using the developed platform is discussed for arterial oxygen saturation (SpO2) and heart rate measurement using optical photoplethysmography (PPG). A comparison of the wireless performance in the 868 MHz and the 2.45 GHz bands is performed. Another contribution of this work is the development of a highly integrated 868 MHz antenna. The antenna structure is printed on the surface of a wristwatch enclosure using laser direct structuring (LDS) technology. At 868 MHz, a low specific absorption rate (SAR) of less than 0.1% of the maximum permissible limit in the simulation is demonstrated. The measured on-body prototype antenna exhibits a −10 dB impedance bandwidth of 36 MHz, a peak realized gain of −4.86 dBi and a radiation efficiency of 14.53% at 868 MHz. To evaluate the performance of the developed 868 MHz sensor platform, the wireless communication range measurements are performed in an indoor environment and compared with a commercial Bluetooth wristwatch device.

This paper reports a comparative study of sub-THz frequency-selective surface (FSS) filter performance in relation to its method of fabrication. Three techniques are considered: conventional inkjet printing, microprecision inkjet printing, and photolithography. The complete design process is presented highlighting steps from substrate selection through to electromagnetic modeling and finally broadband THz filter characterization. Electromagnetic modeling is performed using the CST full-wave frequency-domain solver. Experimental characterization of substrate material, ink, and final FSS designs is done both by THz timedomain spectrometry and quasi-optically at WR-10 and WR-3 waveguide bands using PNA-X vector network analyzer. The center frequencies for bandpass FSS filters are 100 and 300 GHz, which enables prospective utilization in a quasi-optical multiplier system.