Experimental analysis of morphological change in melanin for multiscale modeling of picosecond laser skin treatment

Abstract

In picosecond laser treatments for pigmented skin lesions, selective optical absorption by melanin particles packaged in cutaneous melanosomes results in producing treatment effects. The treatment effects have been numerically analyzed by simulating light propagation and thermal diffusion at a tissue scale. During picosecond laser irradiation, cutaneous melanosomes are disrupted by explosive vaporization through optical absorption by melanin particles. A multiscale modeling of picosecond laser skin treatments is required for the quantitative evaluation of picosecond therapeutic laser devices. By the multiscale modeling, the treatment effects can be evaluated based on the interactions with picosecond laser pulses at each scale of molecule, organelle, cell, and tissue. For the multiscale modeling, the purpose of this study is to investigate a response of melanin particles to picosecond laser pulses at a molecular scale. Homogeneous melanin suspensions were prepared to irradiate 550-picosecond laser pulses at a wavelength of 755 nm. As a result of comparing morphological characteristics between unirradiated melanin particles and melanin particles irradiated by picosecond laser pulses with a scanning electron microscope and a particle size analyzer using dynamic light scattering, almost no significant change was found. The results suggested that melanin particles do not change the structures and are involved as a heat source to cause damage on lipid bilayer membranes within melanosomes in picosecond laser treatments for pigmented skin lesions.