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In this study, a portable near-infrared (NIR) system was newly integrated with a photodiode array detector that has no moving parts, and this system has been successfully applied for the evaluation of human skin moisture. The good correlation between NIR absorbance and the absolute water content of separated hairless mouse skin, in vitro, was showed, depending on the water content (7.4-84.9%) using this portable NIR system. Partial least squares (PLS) regression was used for calibration with the 1150-1650-nm wavelength range. For practical use for the evaluation of human skin moisture, the PLS model for human skin moisture was developed in vivo using the portable NIR system on the basis of the relative water content values of stratum corneum from the conventional capacitance method. The PLS model showed a good correlation. This study indicated that the portable NIR system, as compared to conventional methods, could be a powerful tool for human skin moisture, which may be much more stable to environmental conditions, such as temperature and humidity. Furthermore, to confirm the performance of the newly integrated portable NIR system, a scanning-type conventional NIR spectrometer was used in the same experiments, and the results were compared.

The water content of skin was determined using a FT near infrared (NIR) spectrometer. NIR diffuse reflectance spectra were collected from hairless mouse, in vitro, and from human inner arm, in vivo. It was found that the variation of NIR absorbance band 1450 nm from OH vibration of water and 1940 nm from the combination involving OH stretching and OH deformation, depending on the absolute water content of separated hairless mouse skin, in vitro, using the FT NIR spectrometer. Partial least squares regression (PLSR) was applied to develop a calibration model. The PLS model showed good correlation. For practical use of the evaluation of human skin moisture, the PLS model for human skin moisture was developed in vivo on the basis of the relative water content of stratum corneum from the conventional capacitance method. The PLS model predicted human skin moisture with a standard errors of prediction (SEP) of 3.98 at 1130-1830 nm range. These studies showed the possibility of a rapid and nondestructive skin moisture measurement using FT NIR spectrometer.

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