Pattern analysis of 532- and 1064-nm microlens array-type, picosecond-domain laser-induced tissue reactions in ex vivo human skin

Optical pulses from picosecond lasers can be delivered to the skin using microlens array (MLA) optics or a diffractive beam splitter to generate multiple, focused, high-intensity, micro-injury zones in the epidermis and dermis. The aim of our study was to histopathologically and immunohistochemically evaluate the patterns of 532- and 1064-nm MLA-type, picosecond laser-induced tissue reactions in human skin immediately after treatment. Picosecond neodymium:yttrium–aluminum–garnet (Nd:YAG) laser treatment using an MLA-type beam at the wavelengths of 532 nm and 1064 nm was delivered ex vivo to human skin. Irradiated skin specimens were then microscopically analyzed after hematoxylin and eosin staining and CD31 and Melan-A immunostaining. A single pulse of 532-nm MLA-type, picosecond laser treatment elicited cystic cavitation lesions at sizes of 83.4 ± 16.5 μm × 70.2 ± 17.3 μm (31-mm distance step) and 91.0 ± 44.7 μm × 81.2 ± 36.3 μm (48-mm distance step) in the epidermis and papillary dermis. Meanwhile, a single pulse of 1064-nm laser treatment generated cystic cavitation lesions at sizes of 107.0 ± 18.1 μm × 83.3 ± 37.4 μm (single-pulse mode) and 100.8 ± 40.4 μm × 83.1 ± 29.4 μm (dual-pulse mode) throughout the lower epidermis and upper papillary dermis. Lining epithelial cells in cystic cavitation lesions in the epidermis showed Melan-A-positive immunoreactivity, while cystic cavitation lesions in the dermis exhibited CD31-positive or CD31-negative/Melan-A-negative immunoreactivity. The present data can be used to predict 532- and 1064-nm MLA-type, picosecond-domain laser-induced tissue reactions in human skin.