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.

Internet of Things (IoT) technology is rapidly emerging in medical applications as it offers the possibility of lower-cost personalized healthcare monitoring. At the present time, the 2.45 GHz band is in widespread use for these applications but in this paper, the authors investigate the potential of the 915 MHz ISM band in implementing future, wearable IoT devices. The target sensor is a wrist-worn wireless heart rate and arterial oxygen saturation (SpO2) monitor with the goal of providing efficient wireless functionality and long battery lifetime using a commercial Sub-GHz low-power radio transceiver. A detailed analysis of current consumption for various wireless protocols is also presented and analyzed. A novel 915 MHz antenna design of compact size is reported that has good resilience to detuning by the human body. The antenna also incorporates a matching network to meet the challenging bandwidth requirements and is fabricated using standard, low-cost FR-4 material. Full-Wave EM simulations are presented for the antenna placed in both free-space and on-body cases. A prototype antenna is demonstrated and has dimensions of 44 mm × 28 mm × 1.6 mm. The measured results at 915 MHz show a 10 dB return loss bandwidth of 55 MHz, a peak realized gain of $-$ 2.37 dBi in free-space and $-$ 6.1 dBi on-body. The paper concludes by highlighting the potential benefits of 915 MHz operation for future IoT devices.

In this letter, the design and modeling of a bandwidth-enhanced 915 MHz wristwatch antenna is presented. This letter extends previous work by the authors, with a key focus on significantly improving the impedance bandwidth of the antenna. The antenna proposed here achieves a measured 107% bandwidth improvement compared to the previously reported work. This is achieved by optimizing the feed structure to excite two closely located resonant modes that can be independently tuned to maximize the impedance bandwidth. Conceptual equivalent circuit models for the previous and proposed antennas are derived to explain the bandwidth enhancement. In addition, an equivalent transmission line model is, then, presented in order to accurately explain and analyze the impedance characteristics of the antenna. On-body measurements of the prototype antenna show a –10 dB impedance bandwidth of 62 MHz, a peak realized gain of –4.6 dBi, and a simulated radiation efficiency of 17.8% at 915 MHz. This antenna exhibits a low specific absorption rate value of 0.02 W/kg with 11 dBm input power. The previous performance makes the antenna a suitable candidate for 915 MHz wristwatch-based wireless applications.