The dynamics of laser-induced changes in human skin autofluorescence--experimental measurements and theoretical modeling.

To study the temporal dynamics of human skin autofluorescence photobleaching, we measured the autofluorescence spectral changes of skin in vivo during continuous exposure to 442 nm (He-Cd) laser light. Integral intensities were calculated for various spectral wavelength bands and plotted as a function of time. Mathematical analysis of the time function revealed a double-exponential photobleaching process: I(t) = a exp (-t/tau 1) + b exp(-t/tau 2) + c, in which tau 1 and tau 2 differed by an order of magnitude. A hypothesis for the mechanism of the double-exponential photobleaching dynamics was proposed and evaluated using Monte Carlo modeling of light propagation in the skin and autofluorescence escape from skin. By combining the fluorophore microdistributions, Monte Carlo simulation results and the variation in fluorescence decrease parameters (a, b, c, tau 1, tau 2) with increasing exposure intensities a biophysical explanation for the double-exponential photobleaching function was elucidated. The fast decrease term corresponds to laser-induced photobleaching in the stratum corneum, while the slow decrease term represents fluorophore changes in the dermis. The measured autofluorescence photobleaching dynamics can be used to determine the fractional contributions of different skin layers to the total autofluorescence signal measured in vivo.