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For monitoring the blood glucose level noninvasively from the skin spectra, improvement of signal to noise ratio (S/N) of the glucose signal is critical. This cannot be achieved by the reduction of instrumental noise alone. To reduce the interference from undesired optical signals arising from the stratum corneum and the subcutaneous tissue, we designed a novel optical fiber probe for the skin spectra. The probe consisted of one central optical fiber around which several optical fibers were arranged in circle. The separation of the central optical fiber from each of the surrounding fibers was set at less than 1 mm. This probe was attached to the skin surface vertically when spectral measurements were performed. The measuring light was shone onto the skin surface through the circle fibers and scattered light reaching the central detecting fiber was collected and transmitted to the detection system. The true light path is not defined at present, but light passing through a long path could be neglected with this geometry. When we choose an adequate fiber distance, we can measure the dermis spectra selectively. Glucose intake experiments were performed with volunteers, for whom near-infrared (NIR) spectra were measured at the forearm, from which the blood glucose level was calculated. Partial least square regression (PLSR) analysis was carried out and we found good correlation between the optically estimated values of the glucose level and directly measured values of blood samples. The correlation coefficient characteristic had a positive peak at around 1600 nm, a typical of the glucose spectrum. In conclusion, our system using the novel optical fiber probe detected the changes in the glucose in the human skin tissue quantitatively and noninvasively.
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