Skin imaged by femtosecond laser irradition: a risk assessment for in vivo applications

During the last couple of years new imaging techniques using femtosecond lasers (fs-lasers) in the near infrared spectral range evolved for a variety of in vitro applications. We wanted to know, whether fs-lasers have a non-invasive imaging potential for in vivo applications for human skin. So far, little is known about possible risks of this irradiation type. To estimate the risk of irradiation damage in human skin we used a "biological dosimeter" in this investigation. We irradiated fresh human skin samples with both an fs-laser and a solar simulator (UV-source) for comparison. DNA damage introduced in the epidermis was evaluated using fluorescent antibodies against cyclobutane-pyrimidin-dimers (CPDs) in combination with immuno-fluorescence image analysis. Various fs-irradiation regimes were evaluated differing in laser power and step width of horizontal irradiation scans. When using 15 mW or 30 mW fs-laser power combined with horizontal irradiation scans applied every 5 pm in depth around the epidermal-dermal junction no induction of CPDs was found. However, induction of CPDs could be seen using 60 mW laser power and 5 mu m step width. Narrowing the step width to 1 mu m and using increasing laser power (up to 35 mW) from the surface of the skin to the epidermis led to CPD formation, too. Quantitative comparison of CPD production at various laser regimes with CPD production using a solar simulator was done. We could show that the number of CPDs formed by the 60 mW laser irradiation regime is comparable to an UV-irradiation giving 0.6 MED (minimal erythemal dose). The smaller step width laser irradiation regime (1 mu m step width and up to 35 mW) was comparable to a UV-irradiation regime resulting in 0.45 MED.