Measurement of the time of flight of photons into the skin: influence of site, age and gender, correlation with other skin parameters.

Background/purpose: The speed of light (time of flight) into the skin is obviously relied to its structure, and might appear as a tool for non-invasive investigation of skin physico-chemical properties, among them aging is of primary importance. Though already published, such time of flight measurements have never been extensively correlated with other well-documented skin parameters such as localization, the influence of gender and age, the elasticity and roughness, and the water trans-epidermal diffusion (TEWL).

Methods: A specific practical device was designed to routinely measure the time of flight (TOF) of the light into the human skin 'in vivo', in a totally non-invasive process. This system was tested on volunteers, to relate the TOF parameter to the widely investigated skin properties already mentioned. An Infra-red laser at 1064 nm delivered powerful pulses of less than 1 ns duration, sent to the skin surface through a lossless fibre. The light backscattered at a given distance was collected and led onto an Avalanche Photodiode, and the mean TOF was measured on a fast sampling scope. A resolution and a reproducibility of a few picoseconds has been achieved. Experiments were carried out on 100 volunteers of both gender, aged from 18 to 65, on 12 different locations.

Results: No matter age and gender, important variations of TOF according to the localization were observed: On the inner forearm, an increase from wrist to elbow, and much higher values on the forehead and neck, whether orientation parallel or perpendicular to Langer lines did not appear significant. Ageing appeared to increase the TOF on forehead and neck, while this effect could not be confirmed on the forearm. Usual skin parameters such as elasticity, roughness and TEWL have been compared to TOF on the same location for each volunteer: TOF and skin roughness were significantly anti-correlated, i.e. the TOF got shorter when the Roughness increased, while a striking correlation was observed between TEWL and TOF.

Conclusion: These results assert the dependence of TOF on the nature of the skin upper layers (roughness, water diffusion) and on the dermis layer (ageing), and show the potential capabilities offered by TOF to investigate deeply into the skin structure. They have to be confirmed through further experiments, involving measurements at shorter wavelengths, at which the light path into the skin is much smaller, to get a distribution of the TOF inside the tissue.