Self-calibrated single-wavelength biosensor for measuring SpO2

Arterial oxygen saturation (SpO2), a key indicator of respiratory health, reflects the proportion of oxygenated hemoglobin in the blood and is essential for monitoring conditions such as hypoxia. Traditional pulse oximetry methods use multiple wavelengths to calculate SpO2, which cause errors due to differences in optical pathlengths, affected by different scattering coefficients. This study presents an optical biosensor for non-invasive measurement of SpO2, utilizing the iso-pathlength (IPL) point concept. Our biosensor overcomes the inherent limitations of the classic method by using a single light source and detecting reemitted light at the IPL point, where light intensity is invariant to scattering. This enables accurate SpO2 measurements without the need for external calibration. The biosensor operates with a red LED at 655 nm and five photodetectors, one positioned at the IPL point, which allows the extraction of absorption coefficients from which a PPG signal can be derived. Previous work showed an error of 0.4%, demonstrating the biosensor’s accuracy. In this work we tested 43 subjects by placing the biosensor on the inner side of their upper arms, a location commonly used for blood pressure measurement. The SpO2 values ranged from 95.2% to 100.3%, with an average of 99.3%.