• near infrared (NIR);
  • skin;
  • hydration;
  • relative humidity

Background: Near-infrared (NIR) spectroscopy was used to document the skin water content as a function of product usage and changes in the % relative humidity (%RH) in vitro and in vivo. The objective of the investigation was to determine if the NIR could provide comparable skin water content information as that obtained using gravimetric, conductivity, or visual assessment methods without having to invoke complex chemometric calculations.

Methods: NIR data were obtained using an NIR5000 spectrophotometer with a fiber optic probe (Smartprobe™) attachment to complete the clinical studies and a Direct Contact™ Analyzer module to carry out the in vitro experiments. Conductivity measurements were completed using the Skicon® 200, which measures conductance at a fixed frequency of 3.5 MHz. Three moisturization-based clinical studies were carried out assessing the NIR's ability to detect skin hydration changes. In Clinical Study A, NIR and Skicon data were collected for panelists who had only washed their outer calf with water over a 4-week period. During this time, the daily average %RH was recorded. In Clinical Study B, 10% solutions of glycerin, choline chloride, and the sodium salt of pyrrolidone carboxylic acid were applied to the panelist's outer calf and biophysical measurements were completed to assess the hydration and desorption properties of these humectants. In Clinical Study C, a 10% solution of choline chloride was applied to panelist's outer calf and the cumulative effect of using this product was evaluated over a 3-week period. For all in vitro studies, porcine skin was used as the substrate.

Results: Comparable NIR, Skicon, and visual dryness results were obtained for most of the product usage-based clinical experiments completed. However, the NIR was particularly more effective at detecting skin water content differences as a function of %RH changes. In the absence of abrupt (>50%) relative humidity variations, there was a direct correlation between the NIR readings and the %RH (R2=0.83) unlike what was observed for the Skicon measurements (R2=0.22).

Conclusions: NIR spectroscopy demonstrated the changes in the skin water content as a function of product usage; the results were consistent with those obtained using the Skicon conductivity meter and visual dryness assessment scores. More importantly, the differences detected were obtained without having to use chemometric manipulations in the data analysis as is the common practice. Of all the methods used, the NIR gave the best linear regression for %RH-induced skin water content changes.