Accuracy of Tissue Oxygen Saturation Measurements of a Textile-Based NIRS Sensor

Pressure injuries (PI) are dangerous tissue lesions that heal very slowly and pose a high risk of serious infections. They are caused by pressure applied to the tissue, which stops blood circulation and therefore induces hypoxia, i.e., low tissue oxygen saturation (StO₂). PI cause severe suffering and are expensive to treat. Hence it is essential to prevent them with a device that detects a dangerous situation, e.g., by measuring StO₂ using near-infrared spectroscopy (NIRS). For such a device to be wearable without causing PI, it must not introduce pressure points itself. This can be achieved by integrating optical fibers into a textile to transport light to and from the tissue.The aim of this paper is to investigate the accuracy of StO₂ measurements using a NIRS device based only on textile-integrated optical fibers.Bundles of fibers were stitched into a textile in such a way that loops of <1 mm diameters were formed at the stitching locations. Detection points (DPs) on the fabric consisted of 8 fibers with 3 loops each. Emission points (EPs) were made from 4 fibers with 3 loops each. All fiber ends of a DP were connected to an avalanche photodiode. One end of each fiber belonging to an EP was connected to an LED (740 nm, 810 nm, or 880 nm; 290, 560, or 610 mW).To verify the accuracy of this textile-based sensor, we placed it on a subject's forearm and compared the derived StO₂ during arterial occlusion to the values of a gold-standard NIRS device (ISS Imagent), which was placed on the forearm too.We found that our textile-based sensor repeatedly measured StO₂ values over a range of 40% with a deviation of <10% from the reference device.By showing the ability to measure StO₂ using textile-integrated optical fibers accurately, we have reached a significant milestone on our way to building a wearable device to monitor tissue health and prevent PI.