Implantable thin NIRS probe design and sensitivity distribution analysis

An ultra-thin optical probe based on spatially resolved near infrared spectroscopy (NIRS) is developed and the measurement sensitivity of cerebral tissue using minimally invasive implantation of the optical probe is examined. The optical sensor head consists of bare chips of light-emitting diodes and photodiodes, which were mounted on a polyimide-based flexible substrate. The minimum and maximum thicknesses of the sensor head were 80 and 300 μm, respectively. The light propagation of the NIRS measurement with implanted optical sensor was analysed using the Monte Carlo simulation based on transport theory. The optical path lengths for brain and scalp were 2.3 times and 1/20th, respectively, as compared with generally available NIRS probes, which were attached on the body surface. The influences of the optical block on measurement sensitivity were revealed, and the volume of the sensor head was minimised. Findings also show that the sensitivity distribution is adjustable by changing the medium between sources and detectors.