Wearable Sensor System for Diver Vital Sign Monitoring.

INTRODUCTION: Military divers operate at the limits of human physiology, where inadequate oxygen delivery to the brain-caused by hypoxemia or reduced cardiac output-can rapidly lead to loss of consciousness and death. Existing dive safety systems provide limited means to monitor vital signs underwater. Pulse oximetry (PO) and photoplethysmography (PPG) can yield critical indicators for anticipating diving casualties, but conventional devices fail under motion, immersion, and communication constraints. To address this gap, we developed the HYbrid Diver Remote Observation System (HYDROS), a wearable network enabling continuous underwater monitoring and transmission of physiological data. MATERIALS AND METHODS: HYDROS integrates a wearable optical sensor with a topside monitoring interface. The wearable unit combines a multi-wavelength PPG array, inertial sensors, embedded processing, and an ultrasonic transducer for underwater communication. Heart rate (HR) and oxygen saturation (SpO2) are estimated using motion-tolerant Bayesian algorithms, and frequency-multiplexed acoustic signaling enables simultaneous data transmission from multiple divers to a single receiver. Validation included: (1) a clinical hypoxia study (N = 11) comparing HYDROS SpO2 with arterial blood gas (ABG) SaO2; (2) a functional activity study (N = 5) assessing motion robustness; (3) acoustic communication tests in pool and open-water settings; and (4) open-water dives to 130 ft assessing full system function. RESULTS: HYDROS SpO2 closely matched ABG SaO2 (2.71% RMSE). HR and SpO2 remained accurate during movement and simulated hypoxia (4.86 bpm and 4.56% relative RMS error vs. reference). Acoustic transmission achieved >90% packet fidelity up to 300 ft and supported multi-diver communication with up to 20 devices. The system maintained complete functionality during repeated 130-ft dives without mechanical or data degradation. CONCLUSIONS: HYDROS is the first fully integrated, noninvasive underwater physiological monitoring system combining motion-tolerant PPG sensing with acoustic data transmission. It enables real-time diver health monitoring and early hypoxia detection, enhancing operational safety and medical readiness in military diving.