In vivo simultaneous measurement of urea and water in the human stratum corneum by diffuse-reflectance near-infrared spectroscopy

Abstract

Background/purpose: As near-infrared spectroscopy is a powerful non-invasive technique, information about several functional groups such as OH, NH, and CH can be monitored simultaneously. This study aimed to coincidentally measure urea and water contents in the human stratum corneum after the treatment of urea-containing cream, in vivo, by a Fourier transform near-infrared spectrometer with a fiber-optic probe.

Method: Diffuse reflectance spectra of heels of five healthy Japanese volunteers in the 1250-2500 nm region were measured in vivo before and after the 2-h treatment of urea-containing cream. The relative urea content of the heel was calculated from the peak height ratio at 1976 and 2175 nm in the second derivative spectra and that of water was obtained from the 1900 nm water band and that at 2175 nm. Water mobility was calculated from the peak top wavelength of the 1900 nm water band.

Results: The estimated water content tended to increase 1 and 2 h after the treatment compared with before the treatment and subsequently gradually decreased with time. The estimated urea content significantly increased 1 h after the treatment. Significant changes in peak wavelength to shorter wavelengths 1, 2, and 4 h after the treatment was observed, suggesting that the water mobility increased.

Conclusion: The estimated urea and water contents in heels before and after the treatment with urea-containing cream were monitored separately in vivo by diffuse reflectance near-infrared spectroscopy. We demonstrated the potential of NIR spectroscopy for evaluating the efficacy of topical application of a urea-containing cream to the skin in vivo.