We investigated the use of attenuated total reflection-Fourier transform infrared spectroscopy as a method to study differences in the molecular components of human stratum corneum in vivo. These variations as a function of the anatomic site and of the depth into its layered structure are important to understand the biology and physiology of the tissue. In this preliminary study we have investigated spectroscopic changes in 18 healthy individuals. Total reflection-Fourier transform infrared spectroscopy represents a potentially powerful tool to study biophysical properties of surfaces. We observed that, in vivo, biophysical parameters of the stratum corneum (such as hydration, lipid composition, and conformation of the aliphatic chains) are indeed dependent on the anatomic site. As in total reflection-Fourier transform infrared spectroscopy experiments the penetration depth of the evanescent field into the stratum corneum is comparable with the thickness of a layer of corneocytes, this technique can be used to follow the distribution of lipids, water, and proteins as a function of depth into the tissue. We found that, in vivo, these molecular components are non-uniformly distributed, in agreement with the presence of water and lipid reservoirs as observed with ex vivo ultrastructural investigations. Composition and conformational order of lipids are also a function of depth into the stratum corneum. Finally we compared the in vivo superficial hydration measured using the infrared absorption of the OH stretch of water, with the hydration measured using the Skicon hygrometer. Our results indicate that total reflection-Fourier transform infrared spectroscopy might be useful to measure important chemical and biophysical parameters of stratum corneum in vivo.

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The pigment responses of human skin to broadband UVA radiation (320-400 nm) occur in three distinct phases. The first phase includes immediate pigment darkening (IPD), the pigment that appears immediately after irradiation. The second phase involves an intermediate step, termed persistent pigment darkening (PPD), which leads to the third phase of neomelanogenesis or delayed tanning (DT). Since DT results from synthesis of new melanin, it persists beyond 5-7 days. We conducted studies on human subjects to investigate the dynamic responses of the IPD and PPD reactions to broadband UVA radiation at threshold and superthreshold doses. The threshold doses for IPD, PPD, and DT were found to be approximately 1, 11, and 18 J/cm 2 , respectively. The colorimetry ΔL* value corresponding to minimal clinically perceptible pigmentation was found to be 0.8 ± 0.1. IPD appeared immediately and had an associated decay constant of approximately 1.4 minutes. At doses greater than PPD threshold, IPD reaction decayed while PPD developed indicating toward IPD being used as a substrate in the formation of PPD. K E Y W O R D Scolor measurements, immediate pigment darkening, persistent pigment darkening, skin reactions, threshold doses, UV radiation