Microscopic Tissue Observation of Porcine Skin and MALDI-MSI of Melanin after Quantum Cascade Laser (6.1 μm) Irradiation

Abstract

Quantum cascade lasers (QCL) are semiconductor lasers that use intersubband transitions in a semiconductor multilayer structure to enable selective wavelength oscillation in the mid-infrared region. Various laser treatments are available for human skin diseases, but there are no laser sources suitable for the mid-infrared range of 6–8 μm. Since absorption by melanin and water occurs in the 6 μm band, a QCL emitting at 6.1 μm would likely have an effect on skin. In addition, in the wavelength region of 5–6 μm, strong characteristic absorption peaks are associated with molecular vibrations of ester bonds (5.75 μm), hydroxyl groups (6 μm) and amide bonds (6 μm) in living organisms. We studied the effects of QCL irradiation on porcine skin tissue excised from the dorsal region of a SPF (specific-pathogen-free) LWD lineage, which is similar to human skin tissue, in order to develop treatments for human skin diseases such as nevi, lentigines, and other melanotic pigmented lesions. Skin samples were irradiated with pulses from a 6.1-μm QCL (0.37–0.45 W, 0.5 s), and the effect on the skin was examined by microscopic tissue observation (hematoxylin and eosin [HE] staining) and matrix-assisted laser desorption ionization-mass spectrometry imaging (MALDI-MSI). HE staining showed that the deepest cells in the basal layer of the epidermis, where melanocytes that produce melanin exist, remained following QCL irradiation. MALDI-MSI revealed that the number of melanin molecules was less than in non-irradiated skin, and that those that remained were scattered in the basal layer of the epidermis. This scattering of melanin molecules indicates the degeneration of melanocytes during QCL irradiation. If the remaining melanocytes were denatured, they would be taken up by surrounding phagocytic cells. Therefore, we concluded that the QCL had an effect on melanin in the skin.