Science of Skin

Photoprotection

Photoprotection describes the act of protecting against light and in particular ultraviolet radiation (UVR). Ultraviolet radiation is emitted by light sources, most commonly from the sun and sun beds, and can be broken up into three regions based on their wavelength: UVA, UVB, and UVC. UVC is absorbed by the atmosphere before it reaches the Earth's surface and poses no risk to life. UVA and UVB both reach the Earth's surface and may cause skin damage, such as accelerated skin ageing (photoaging) and, more importantly, may induce DNA mutations leading to skin cancers. Skin cancer affects animals and humans. To survive in our mostly sun drenched planet, organisms have adapted natural defense mechanisms against ultraviolet radiation.

Natural photoprotection

Skin cross sections illustrating photoprotection
Skin cross sections illustrating photoprotection

Humans have developed a natural photoprotective system that protects cellular DNA from the harmful UV radiation emitted from the sun. Photoprotection is mostly offered by biological chromophores, chemical groups of molecules that are capable of selective light absorption. Some of the organic compounds found in man are DNA, urocanic acid, amino acids, melanin precursors and eumelanin.

Skin biometrics offers the opportunity to assess spectroscopic measurements. Various factors, such as solubility of the chromophores determine the ability to assess the individual absorption spectra and scattering properties of human tissue.

Hemoglobin and melanin are the main chromophores in the visible range of the electromagnetic spectrum (460-560 nm).

Photoprotection in humans: chromophores

The absorption of light by an object is directly correlated to the incident radiation and the absorption coefficient (AC) of the object, and in skin characterized by its chromophore concentration. Chromophores are also referred to as quenchers of photoexcited states (QPES), by quenching or dissipating the energy transferred from sunlight to the dermis. The main chromophores of human skin are hemoglobin, melanin and water. Hemoglobin shows a high molar extinction coefficient in the “blue” region of the spectrum (405-450 nm) in human skin.  Eumelanin sees a drop in its absorption curve from 200 to 70 nm, making its absorption most important in the “blue” band around 400 nm. Water has a very low AC below 800 nm.
Traditionally, reflectance spectroscopy has been used to determining chromophore concentrations and absorbing molecules in human skin at various wavelengths. It has been found that the wavelength dependence of the scattering coefficient of skin remains relatively constant among individuals.

By using the entire visible spectrum to assess chromophore concentration of e.g hemoglobin and melanin, the method becomes less sensitive to minor reflectance fluctuations. Furthermore, this method can be applied to any added chromophore with known absorption properties which is present in sufficiently high concentrations to perturb the reflectance spectrum.

Advanced techniques to map individual chromophores are referred to as cutaneous optics; mathematical models to explain the interaction of sunlight with skin using spectrophotometric analysis. Another recent technique makes use of spectrophotometric intracutaneous analysis (SIA) measure in vivo the concentration and distribution of eumelanin, oxyhemoglobin, and dermal collagen to produce concentration maps of these chromophores (SIAscopy).

References

  • Lim, H W & Soter, N A (2001). Clinical Photomedicine. CRC Publishing.
  • Matts, P J (2008). "New Insights into Skin Appearance and Measurement." Journal of Investigative Dermatology Symposium Proceedings. Vol 13, pp 6-9.
  • Shimada, M et al (2001). "Melanin and blood concentration in a human skin model studied by multiple regression analysis: assessment by Monte Carlo simulation." Phys. Med. Biol. Vol 46, pp2397-2406.
  • Stamatas, G N, Zmuzdka, B, Kollias, N & Beer, J (2004). "Non-invasive measurement of skin pigmentation in situ." Pigment Cell Research. Vol 17 (6), pp618-626.
  • Walker, S L & Young, A R (2007). "An action spectrum (290-320 nm) for TNFα protein in human skin in vivo suggests that basal-layer epidermal DNA is the chromophore." Proc Natl Acad Sci. Vol 104 (48).
  • Zonios, G, Bykowski, J & Kollias, N (2001). "Skin Melanin, Hemoglobin, and Light Scattering Properties can be Quantitatively Assessed In Vivo Using Diffuse Reflectance Spectroscopy." Journal of Investigative Dermatology. Vol 117, pp1452-1457.

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