Wearable Upper Arm SpO₂ Sensor for Wellness Monitoring

This paper describes the full development of a sensor for measuring optical heart rate (OHR) and blood oxygen saturation (SpO₂).

A wearable sensor with a new type of skin compatible dispensed lens was designed and manufactured. All critical optical components, light emitting diodes (LEDs) and photodiode (PD) were close to skin and gave maximum light intensity due to minimal loss in the lens structure. Lens and optical components formed a thin monolithic structure.

Suppressed crosstalk between LED and PD was achieved by using two types of dispensed material: light blocking and transparent. High signal to noise ratio (SNR) and amplitude in the alternating current (AC) part of the photoplethysmography (PPG) signal were achieved. User comfort was achieved by having a small sensor located on the upper arm. When re-training the algorithm from our first iteration, the multiwavelength PPG sensor showed an SpO₂ RMSE of 2.61% with a 7-second average analysis for 25 participants. The average RMSE of heart rate over all 25 participants was 1.6 ± 1.1%.

This study demonstrates a sensor with a clinical grade SpO₂ measurement and a highly accurate OHR measurement that is also comfortable and easy to wear.

A dispensing method provides a new way of manufacturing sensor elements for wearable sensors with increased performance with reduced crosstalk.