Histological Assessment of an Implantable Optical Sensor and Spinal Cord Tissue Interface

Background: We developed an implantable optical sensor based on near-infrared spectroscopy (NIRS) to continuously monitor spinal cord oxygenation and hemodynamics in patients with acute spinal cord injury (SCI). As a safety assessment measure, we aimed to study the effect of near-infrared (NIR) light emission and contact compression of the NIRS sensor on spinal cord tissue structure. Our previous in-vitro heat tests indicated no heat generation by the NIRS sensor. This study evaluated whether the NIRS sensor resulted in any potential compression damage to the spinal cord using histological analysis. Methods: Six Yucatan mini-pigs received a T10 SCI. A custom implantable NIRS sensor (version 2) was placed extradurally on the spinal cord and fixed with magnets and cross-connectors. After seven days of continuous data collection at 100Hz, the sensor was removed to allow for histological examination of the spinal cord tissue. Cellular damage was observed in the spinal cord at the NIRS sensor placement site in two animals. The design, shape, and material of the NIRS sensor were significantly revised to reduce the sensor footprint, minimize the compression on the cord, increase the sensor flexibility, and improve its clinical application. An in-vivo pilot experiment was performed on a Yucatan miniature pig with a T10 SCI to evaluate potential compression damage of the spinal cord tissue from placement and direct contact of the refined NIRS sensor (version 5). A fibrin sealant, TISSEEL, was utilized to fix the version 5 NIRS sensor on the spinal cord. Result: There were no signs of cellular damage, indentation, and significant flattening on the dorsal surface of the spinal cord where the version 5 NIRS sensor was placed for up to 4.5 hours. Conclusion: The refined NIRS sensor did not cause any compression damage to the porcine spinal cord after implantation for 4.5 hours. Implanting this sensor on the spinal cord of SCI patients requires further in-vivo examinations to ensure the sensor is safe to use for up to 14 days.