The Effects of Silicone Enclosure Colour on the Function of Optical Sensors

SIMPLE SUMMARY: Implantable optical sensing is a rapidly growing field that allows for continuous monitoring of internal organs’ physiological states. Near-infrared spectroscopy is an optical sensing technology allowing for low-cost, non-invasive, high-sensitivity measurement of tissue oxygenation and haemodynamic parameters. The colour of an optical sensor’s enclosure affects the sensor’s sensitivity, function, and ability to detect tissue vital signs. This study compared the optical properties of coloured silicone materials and related these properties to the function of silicone enclosed implantable near-infrared spectroscopy sensors. We demonstrated that sensor enclosures highly reflective to red and near-infrared light facilitated light propagation to the photodetector and increased the ability to detect the effects of cardiac pulsation and respiratory rhythm on tissue haemodynamics. In contrast, highly absorptive sensor enclosures resulted in better detection and monitoring of tissue oxygenation. ABSTRACT: The colour of the silicone enclosure of an implantable reflectance-based optical probe plays a critical role in sensor performance. Red-coloured probes that are highly reflective to near-infrared light have been found to increase photodetector power by a factor of 6 for wavelengths between 660 and 950 nm and triple the magnitude of measured cardiac pulsations compared to traditional black probes. The increase in photodetector power and cardiac pulsation magnitude is presumably due to increased spatial range resulting from a higher magnitude of superficial tissue scattering. Conversely, probes with highly absorbent colours such as black and blue result in more stable signals and are expected to have higher spatial resolution and depth of penetration.