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The use of optical clearing agents makes it possible to increase the probing depth of non-invasive optical diagnostic methods. However, the protective epidermal barrier of the skin prevents the penetration of hydrophilic immersion liquids into the deeper layers of the skin. The aim of this study is to improve the efficiency of skin optical clearing by increasing the permeability of the epidermis for polyethylene glycol with MW 300 (PEG-300) using different physical approaches.
We present the results of a comparative analysis of optical immersion clearing of skin in laboratory rats in vivo with and without preliminary ablation of epidermis, fraction microablation and sonophoresis. Laser ablation and microablation has been implemented using a setup based on a pulsed erbium laser (λ= 2940 nm) with different hand-pieces. Sonophoresis has been applied during 2 min with the following parameters: 1 MHz, 2 W/cm2, continuous mode. As an optical clearing agent (OCA) polyethylene glycol (PEG-300) has been used. Basing on optical coherence tomography, we have estimated the attenuation coefficient in the process of optical clearing in two regions at depths of 50–170 µm and 150–400 µm.
The results have shown that both surface ablation of skin in vivo lead to the local edema of the affected region that increases the scattering coefficient. However, the intense evaporation of water from the ablation zone facilitates the optical clearing at the expense of tissue dehydration, particularly in the upper layers. Besides, fractional ablation enhances the both rate and degree of optical clearing in upper dermis layers but not affects the deeper dermis. Sonophoresis provides the most effective penetration of OCA into deeper dermis. Time dependences of attenuation coefficient in two regions of dermis at the separate and combined laser and ultrasound impact on the epidermal permeability for PEG-300 have been obtained.
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