In vivo skin optical clearing by glycerol solutions: mechanism

Abstract

The penetration of light in biological tissues can be enhanced by using optical-clearing techniques. However, researches on the mechanism are limited to in vitro experiments. In this study, rat dorsal skin was used to discuss the mechanism for in vivo optical clearing. Glycerol solutions with different concentrations were applied by dermal injection; then the skin reflectance spectrum, SHG imaging and microstructural changes were monitored. The results showed that with the skin becoming transparent, the corresponding reflectance decreased, and the thickness of dermis and diameter of collagen fibril decreased, but no collagen fiber was dissolved or fractured. Hence, it can be concluded that the thickness decrease of dermis and corresponding more regular packing of tissue fibers plays an important role in the mechanism for glycerol-induced optical clearing of skin in vivo.

Optical clearing of biological tissues: prospects of application in medical diagnostics and phototherapy

“…New original results obtained using the combination of optical clearing with the known optical visualisation methods, such as laser speckle-contrast imaging [34, 71,75,77,79], OCT [20,29,35,[59][60][61]65,69,80,82], microscopic imaging [23,24,83,84], ultra-microscopy [85][86][87], etc., demonstrated high potentiality of their mutual use not only for getting high-resolution structure and functional tissue images in vitro [72,73,[83][84][85][86][87], but also for optical imaging and diagnostics in vivo [25,34,45,50,59,61,65,68,70,71,[75][76][77][79][80][81]. Table 1 shows the increase of the light penetration depth, caused by clearing agents in some diagnostic methods.…”

“…These factors can essentially modify the kinetic characteristics and the magnitude of the clearing effect [25,34,45,59,61,68,70,76,77,79,80,82,97,98,108,113,118,119]. In living tissues the refractive index is a function of the physiological or pathological condition of the tissue.…”

“…Wen et al [76] injected glycerol solutions into the rat skin in vivo, and after 10 minutes the samples were cut for SHG imaging. With the solution having the concentration 75% the dehydration (diameter reduction) of dermis collagen fibres was observed.…”

“…Yeh et al [121] and Wen et al [76] demonstrated that for the collagen-based connective tissues (samples of tendon and model tissues) the optical clearing process using the high-concentration glycerol (13 М) manifested itself in considerable reduction of the backscattered signal intensity and the increase of the transmitted signal.…”

Development of an Optical Tissue Clearing Laser Probe System

“…Rylander et al [18] and Yu et al [19] that skin dehydration is the prominent mechanism of OTC and Yeh et al [20] suggested that the reversible change of both collagen structure and size can affect OTC. In recent in-vivo studies, Ghosn et al monitored and quantified the permeability rate of glucose solutions in in-vivo monkey skin by using OCT [21], and Wen et al reported that the thickness decrease of dermis and regular arrangement of tissue fibers were prominent OTC mechanisms [22]. Tuchin clearly described the clinical diagnostic efficacy of chemical agents [23,24].…”