OxyLink: A Confidence-Aware Smartphone Photoplethysmography Framework Heart-Rate, Experimental SpO₂ Monitoring, and Temporal Oxygen-Deterioration Assessment

OxyLink is a confidence-aware smartphone photoplethysmography (PPG) research framework for longitudinal heart-rate monitoring, experimental SpO₂ estimation, and temporal oxygen-deterioration assessment. The Android prototype uses the smartphone camera and flash to acquire PPG signals, estimates heart rate and experimental SpO₂, computes measurement-confidence metrics, performs adaptive signal-quality checks, and stores longitudinal measurements for later analysis. This preprint presents the system architecture, signal-processing pipeline, confidence model, individualized temporal deterioration framework, and an initial N=1 longitudinal feasibility analysis based on real-world prototype use. The recovered dataset contains 8 stored measurements collected across 7 days between 21 April and 20 May 2026. Heart-rate feasibility was demonstrated, while the experimental SpO₂ estimator exhibited an upper-ceiling effect at 100%, highlighting the need for further calibration and validation before oxygen-deterioration scoring can be evaluated clinically. OxyLink is intended for research, education, and algorithm development only. It is not a medical device and must not be used as a substitute for clinically validated pulse oximetry or professional medical assessment. The implementation is available as open-source software under the MIT License.