Near infrared spectroscopy offers a promising technological platform for continuous glucose monitoring in the human body. NIR measurements can be performed in vivo with an implantable single-chip based optical NIR sensor. However, the application of NIR spectroscopy for accurate estimation of the analyte concentration in highly scattering biological systems still remains a challenge. For instance, a thin tissue layer may grow in the optical path of the sensor. As most biological tissues allow only a small fraction of the collimated light to pass, this might result in a large reduction of the light throughput. To quantify the effect of presence of a thin tissue layer in the optical path, the bulk optical properties of tissue samples grown on sensor dummies which had been implanted for several months in goats were characterized using Double Integrating Spheres and unscattered transmittance measurements. The measured values of diffuse reflectance, diffuse transmittance and collimated transmittance were used as input to Inverse Adding-Doubling algorithm to estimate the bulk optical properties of the samples. The estimates of absorption and scattering coefficients were then used to calculate the light attenuation through a thin tissue layer. Based on the lower reduction in unscattered transmittance and higher absorptivity of glucose molecules, the measurement in the combination band was found to be the better option for the implantable sensor. As the tissues were found to be highly forward scattering with very low unscattered transmittance, the diffuse transmittance measurement based sensor configuration was recommended for the implantable glucose sensor.
Accurate knowledge of the optical properties of turbid media in the light path is important in NIR absorption
spectroscopy of biological tissues where multiple scattering complexes the collected light signals due to the non-uniform
tissue architecture. Several approaches such as time resolved spectroscopy and spatially resolved spectroscopy have been
proposed to measure the bulk optical properties of turbid media. Among them, double integrating sphere (DIS)
measurements are recognized as the "golden standard" for in vitro optical properties measurement of turbid media
because of its high accuracy and robustness in different conditions. A DIS system is convenient to measure the in vitro
optical properties of turbid media like intralipid solutions and biological tissues, since it measures the diffuse reflectance
and transmittance simultaneously. However, DIS measurements have been mostly limited to the optical window region
(400-1000 nm) or suffered from low signal levels on the detectors due to the absorption by water in the NIR region. In
this study, we developed a DIS system for optical property measurement in the 1300-2350 nm region based on a novel
wavelength tunable spectroscopic setup which incorporates a high power broadband supercontinuum laser and a high
precision monochromator. With this system, optical properties of intralipid solutions were measured in the wavelength
region of 1300-2350nm.
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