9.7.1.1 Mechanisms of optical clearing and types o...
OCAs are a kind of chemical reagents with high refractive index，high permeability and good biocompatibility.The effect of these agents on the tissue are reversible.Currently，there are three main proposed mechanisms of optical clearing（OC）.（ⅰ）Refractive matching：OCAs penetrate into the tissue，improve the refractive index of the interstitial fluid，make the refractive index of each tissue components match，reduce the scattering of light in the tissue，and enhance the penetration depth of light.（ⅱ）Dehydration：the entry of the hyperosmolar OCAs into the tissue results in the loss of moisture and the decrease of the refractive index mismatch.（ⅲ）Reversible dissociation of collagen fibers：some OCAs are capable of destabilizing the collagen structure，and reducing light scattering of collagen tissue.Collagen fiber is the main scattering source of dermis.
OCAs are mainly classified as follows：（ⅰ）Alcohols：e.g.，glycerol，poly（ethylene glycol）（PEG），poly（propylene glycol），mannitol，sorbitol and xylitol.（ⅱ）Sugars：e.g.，glucose，dextrose，fructose，ribose and sucrose.（ⅲ）Organic acids：e.g.，oleic and linoleic acids.（ⅳ）Other organic solvents：e.g.，dimethyl sulphoxide（DMSO）or thiazone.（ⅴ）X-ray contrast agents：e.g.，OmnipaqueTM.(https://www.daowen.com)
Although these OCAs are considered non-toxic，long-term use in the body may results in tissue edema and irritation，local hemostasis，shrinkage and even tissue necrosis.In this case，injection of OCAs into the skin may cause necrosis.The penetration of OCAs depends on a number of factors，including the type，refractive index，concentration and tissue exposure time of OCAs，as well as the original turbidity of the biological tissue and permeability to a particular OCA.