Enhancement Of Skin Optical Clearing By Photo-irradiation And The Mechanism Investigation

Skin is one of the target organs for optical diagnosis and therapy. But the intrinsic high scattering property limits the penetration depth of light in tissue, which makes skin also the first barrier to be breakthrough for optical imaging in living bodies. Skin optical clearing technique, by introducing hyperosmotic and high refractive agents named optical clearing agents (OCAs) into skin to decrease the scattering and increase the light penetration depth, might help to overcome this limitation. But the natural barrier function of stratum corneum (SC) of in vivo skin restricts the penetration of OCAs into dermis. Many physical and chemical methods have been proposed to enhance the transcutaneous delivery of OCAs, but most of them are invasive.This study was mainly implemented to enhance the permeability of OCA to increase skin optical clearing efficacy and to study the mechanism. The main research contents are as follows:1) Comparative study of enhanced skin optical clearing effect by photo-irradiation. Photo-irradiation with different categories, wavelengths, output modes and doses were attempted to enhance transdermal delivery of glycerol, and reflectance spectroscopy was utilized to assess the in vivo skin optical clearing effect. Results showed that photo-irradiation with appropriate dose could reduce skin reflectance, and the infrared1064-nm Nd:YAG laser irradiation with both Q-switched and long-pulsed modes showed the best enhancement and caused no visualized damage to skin. Further, based on the tissue spectrum measurement of optical property parameters before and after skin optical clearing, it was found that the reduced scattering coefficient and light penetration depth were decreased and increased significantly, respectively.2) Mechanisms of enhanced skin optical clearing effect by1064nm-Nd:YAG lasers irradiation. Infrared thermography, histopathological analysis, two-photon fluorescence microscopy were employed to examine1064nm-Nd:YAG laser-induced changes in temperature distribution of skin surface, micro-structure of the SC and molecular constitution of SC, respectively. It was found that the LP-Nd:YAG laser irradiation increases skin temperature obviously to make corneocytes fragile or exfoliative, resulting in decreased SC auto-fluorescence and interrupted barrier function. Whereas, the QS laser irradiation possessed typical photo-mechanical effect, it can disrupt the keratin or corneocytes completely, perforating some micropores on the SC. Moreover, the change of microstructure of the SC could enhance the optical clearing effect within at least one week.3) Mechanism of skin optical clearing:Effect of temperature raise on skin optical clearing. Thermal effect induced by laser irradiation has previously been found to be effective to improve skin optical clearing efficacy. In this part, the effect of temperature change on skin optical clearing was further studied. Glycerol with different temperature (4,25,32and45C) was used in studies on in vitro and in vivo skin. Results showed that glycerol with temperature lower than that of skin surface (4and25C) induced limited optical clearing efficacy; higher temperature lead to faster and more effect skin optical clearing, and the higher the temperature, the better the effect. It can be concluded that the temperature raise indeed facilitate the skin optical clearing. But for the security consideration of in vivo applications, the temperature of OCA should lower thean the maximum tolerable temperature of skin.4) Pilot study on the effect of skin optical clearing on laser tattoo removal. Tattoo animal model was established and then37℃OCA was applied to the tattoo region. Q-switched Nd:YAG laser (1064nm) irradiation was used to remove the tattoo, and a skin evaluation instrument (Mexameter probe, MPA580) was applied to measure the change of pigment. By comparing the pigment change of the OCA-laser treated area and the single laser treated area, it was found that skin optical clearing could enhance the effect of laser tattoo removal obviously, with an increased pigmentum clearance by1.5-fold, In addition, the Monte Carlo (MC) simulation verified the experimental result. It can be concluded that skin optical clearing effect would improve the targeting property of light transmission in tissue and increase the effective photons reaching deep tattoo regions, which would contribute to the disintegration and elimination of pigment granules.