The wavelength-dependent penetration depth of ultraviolet radiation in human skin is a fundamental parameter for the estimation of the possible photobiological impact of ultraviolet (UV) radiation. We have determined the absorption spectra of human skin in vivo in the wavelength range from 290 to 341 nm in 3-nm steps using laser optoacoustics and calculated the respective penetration depths. Data were analyzed with respect to different skin regions and skin phototype of the 20 subjects in the study (phototype I: n=3; II: n=7; III: n=5; IV: n=5), revealing large variability between individuals. The penetration depth of UV radiation in human skin is highly dependent on wavelength and skin area, but no significant dependence on skin phototype could be found.
Precise determination as well as comparison of optical properties of human skin <i>in vivo</i> and <i>in vitro</i> is of great importance to the understanding of effects of UV exposure. Because of that, the absorption properties of epidermal models without and with elanocytes of skin type IV and VI were examined using optical and optoacoustic spectroscopy. The effect of melanin as an important chromophor in human skin was investigated using a photometer, laser induced fluorescence (LIF) and optoacoustics.
Moreover, an epidemal model irradiated several times with UVA showed similar absorption characteristics as human skin <i>in vivo</i>. Besides, optoacoustic signals are shown to deliver structural characteristics of different epidermal layers that are about 40 μm thick. Using laser optoacoustics and laser induced fluorescence, human skin <i>in vivo</i> can be investigated wavelength-resolved. Therefore, optoacoustics is a promising tool for <i>in vivo</i> determination of different skin types, optimization of phototherapy and testing of protective substances like sunscreens in the future.
The optical properties of human skin in the UV-range are not exactly known. Furthermore, the precise wavelength dependency of important photobiological processes (such as induction of skin cancer) could not be settled yet, either. A better knowledge of the optical properties is necessary in order to achieve a better understanding of UV-induced effects on human skin. Optoacoustics is a new approach to investigate the wavelength dependent optical properties of human skin in the UV-range. This technique allows non-invasive measurements on human skin in vivo, that are indispensable to gain meaningful results concerning the processes induced by UV-radiation in the living tissue. First attempts at measuring UV-induced optoacoustic transients of human skin in vivo and tissue phantoms with a new detector are shown. For analysis, fitting of simulated data onto the experimental data is applied in order to improve the determination of optical properties. First measurements of wavelength dependent optical properties in the UVB-(280-315 nm) and UVA-II-range (290-330 nm) comparing stained artificial layers to human skin in vivo are presented.