Flexible, Implantable, Pulse Oximetry Sensors for Continuous Monitoring of Arterial Blood Oxygen Levels

Pulse oximetry is the most widespread method of monitoring patient heart rate and blood oxygen levels in both clinical and non-clinical settings. These devices tend to be ridged, bulky, and therefore suitable only for short-term use. In addition, they suffer degraded performance due to movement and changes in ambient light levels, and can show significant inaccuracies when used on skin with darker pigmentation as highlighted during the COVID19 pandemic [1]. To facilitate long-term patient monitoring, implanted devices are required. Implantation removes interference from light and sensitivity to skin pigmentation, and allows enhanced performance during movement.<br/>Here we demonstrate a low cost, ultra-flexible pulse oximetry probe. The hybrid flexible devices are fabricated on 5 µm thick Parylene C using laser ablation to define the circuit design, and integrate a small ridged photodetector and LEDs using a flexible adhesive. Finally, contact between the probe and the processing board is achieved using a flat flexible cable and anisotropic conductive film.<br/>Devices are demonstrated <i>in vivo</i> on a sedated porcine subject and calibrated against a standard peripheral SpO₂ meter while the subject’s oxygen intake was varied. In addition to demonstrating the functionality of these implanted devices, we show that the implementation directly on the femoral artery of our devices records a more acute response to the variation in oxygen intake compared to the peripheral measurements.<br/>These devices are low cost and based on biocompatible materials, and can be easily implanted during cardiovascular surgery. This offers a route towards long-term implantation of devices for continuous patient monitoring.<br/><br/>[1] M. W. Sjoding, R. P. Dickson, T. J. Iwashyna, S. E. Gay, and T. S. Valley, “Racial Bias in Pulse Oximetry Measurement,” <i>N. Engl. J. Med.</i>, vol. 383, no. 25, pp. 2477–2478, 2020.