Abstract
We report a method on quantitative and simultaneous non-contact in-vivo hydration and sebum measurements of the skin using an infrared optical spectroscopic set-up. The method utilizes differential detection with three wavelengths 1720, 1750, and 1770 nm, corresponding to the lipid vibrational bands that lay "in between" the prominent water absorption bands. We have used an emulsifier containing hydro- and lipophilic components to mix water and sebum in various volume fractions which was applied to the skin to mimic different oily-dry skin conditions. We also measured the skin sebum and hydration values on the forehead under natural conditions and its variations to external stimuli. Good agreement was found between our experimental results and reference values measured using conventional biophysical methods such as Corneometer and Sebumeter.
Highlights
Skin hydration and sebum are considered to be important factors in skin health; a right balance between these components is an indication of healthy skin and plays a central role in protecting and preserving skin integrity [1]
Even though the other absorbers such as collagen [30], blood [31] and proteins [32] can influence our experimental results in this spectral region around 1720 nm the absolute values of absorption coefficient and the amount of these chromophores present in the measurement volume of our set-up is relatively lower compared to water and sebum [11]
The spectral band around 1700 nm is expected to be less sensitive for other chromophores when the sampling depth [33] of the optical set-up is chosen to be optimized for measuring skin barrier function, which in turn depends on the properties of stratum corneum
Summary
Skin hydration (moisture) and sebum (skin surface lipids) are considered to be important factors in skin health; a right balance between these components is an indication of healthy skin and plays a central role in protecting and preserving skin integrity [1]. Optimal balance between hydration and sebum levels provides the skin with a radiant, smooth texture and a natural pigmentation appearance, which is important from a cosmetic perspective. Hydration and sebum retaining ability of the skin is primarily related to the stratum corneum (SC). The SC plays the role of the barrier to water loss and is composed of the corneocytes and an intercellular lipid bilayer matrix. The presence of natural hygroscopic agents collectively referred to as natural moisturizing factor (NMF) and the SC intercellular lipids arrange orderly to form a barrier to prevent transepidermal water loss.
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