An Adjustable Smart Ring to Monitor Pulse Rate and Peripheral Blood Oxygen Saturation

Purpose: Smart rings are emerging as a promising solution in the field of wearable devices, offering a compact and ergonomic solution for continuous physiological monitoring, yet one of their major limitations is that they are not adjustable, but rather come in different sizes. Another major gap in the literature is the lack of validation data during dynamic activities. This study presents the design, development, and validation under static and controlled-motion conditions of a research-grade adjustable smart ring for tracking pulse rate (PR) and peripheral blood oxygen saturation (SpO₂) using photoplethysmography. Methods: The device features a rigid-flex printed circuit board (PCB) optimized for finger placement, ensuring accurate sensor positioning, and adaptability to different sizes. Data transmission occurs via the ANT protocol, allowing real-time visualization on a mobile application. The smart ring was evaluated on 30 healthy volunteers (13 women and 17 men, mean age 27.5 ± 7.4 years, mean height 172.2 ± 9.0 cm, mean weight 68.1 ± 13.1 kg) through a multi-phase experimental protocol involving spontaneous breathing, apnea, and physical activity, with measurements compared against a gold-standard pulse oximeter. Results: Results demonstrate strong agreement in PR measurements both in static and dynamic conditions (r = 0.91, p < 0.001), while SpO₂ values exhibited an overestimation (mean bias = 1.04%). Conclusion: This work demonstrates the feasibility of an adjustable, research-grade smart ring based on a rigid–flex PCB for finger palmar PR and SpO₂ monitoring and quantifies its agreement with a clinical-grade fingertip oximeter in healthy adults in static and dynamic conditions. The device is conceived primarily as a hardware and measurement site platform that can be reused across ring sizes, while future work is needed to develop open signal-processing algorithms.