<title>Optical clearing of in vivo human skin with hyperosmotic chemicals investigated by optical coherence tomography and near-infrared reflectance spectroscopy</title>

Although application of clearing agents to skin and other tissues in vitro has been studied, few have led to quantifiable results in vivo human skin. This study was to evaluate the optical clearing of human skin in vivo with topical application of hyperosmotic agents. Optical coherence tomography imaging was used to perform visual assessment of human skin during optical clearing process. Topical application of 80% glycerol and 50% DMSO to palmar skin of volunteers results in an increase in imaging depth and contrast. Imaging depth was increased to 1.1 mm after 15 minute topical treatment. The layers that consist of palmar epidermis, i.e. stratum corneum, stratum granolosum, stratum spinosum and stratum basale were clearly differentiated at 60 minute for 80% glycerol and at 30 minute for 50% DMSO. The difference between glycerol and DMSO is due to their different permeability through skin tissue. DMSO penetrates the membrane and tissue rapidly, and even across the stratum corneum of skin. At the balance point of the decrease in light scattering caused by refractive index matching and the increase of the local reflectance signals caused by dehydration effect, the increase of both imaging contrast and depth were achieved at the optimum time of application. To validate the observation by OCT, diffuse reflectance of human skin in vivo topically treated with the agents was investigated by the use of an integrating sphere in the near infrared wavelengths. Diffuse reflectance was decreased from the skin with treatment time course. The results of optical clearing of in vivo human skin demonstrated by the spectrophotomer measurement is consistent with the observation by OCT imaging. This study provides evident for improved visualization of human skin with topical application of clearing agents.