Near-infrared (NIR) reflectance spectroscopy has been used to differentiate water in different hydrogen-bonding environments of skin in vivo and to determine the effect of moisturizer application on water content. Four types of water have been observed in the second-derivative spectrum of skin. These are assigned as water associated with the lipid bilayers, primary and secondary water of hydration on protein, and bulk (free) water in deeper tissue. The intensities of the lipid-associated water band and the bulk water band depend on anatomical site. The latter, particularly, may be affected by corneocyte size and may have potential in predicting transdermal penetration of drugs. Moisturizer application appears to have little effect on water content, but results in changes in the degree of scattering of radiation, suggesting that moisturizers smooth, rather than hydrate, the skin.
Kathleen Martin, "In Vivo Measurements of Water in Skin by Near-Infrared Reflectance," Appl. Spectrosc. 52, 1001-1007 (1998)