Fig. 2

Focused diode laser stimuli rapidly increase temperature within a small volume of human skin. a Histology of human skin stained for the neuronal marker PGP9.5 to display free epidermal and dermal nerve endings in red color (skin biopsy kindly provided by Claudia Sommer, Würzburg). Dashed lines indicate approximatively borders of epidermis/dermis and dermis to facilitate comparison to agar model in (b). Insert shows intraepidermal fibers at higher resolution. b Spatial distribution of temperature changes with a Gaussian distribution as a consequence of a laser stimulus (100 mW, 200 ms) using agar as a model of human skin (side view with thermocamera Optris PI 640). The heated volume is small and the maximum heating is achieved at about 280 µm depth. c Temperature distribution on the surface of the agar model and d on the skin surface of the human hand dorsum. e Thermoprofile of (b) confirming the maximum temperature at about 280 µm below surface, dotted lines as in (a, b). f–h Time courses of temperature changes due to a single laser stimulus obtained from the experiments shown in (b–d), inset in (f–h) display magnification to visualize threshold temperatures. Scale bars in (a–d) 200 µm