The role of water desorption on optical clearing of biotissue: studied with near infrared reflectance spectroscopy.

In order to understand the role of water desorption, i.e., the opposite of water adsorption, in optical clearing of biological tissue created by the application of hyperosmotic agents, dynamics of water loss in fresh porcine muscle tissue administrated with glycerol or ethylene glycol was investigated with the near-infrared reflectance spectroscopy. It is found that there exist three stages of water desorption with the elapse of time within the tissue after the application of agents: i.e., the rapid stage during the first minute; at the second stage (1-10 min) water loss increases linearly at the rate slower than at the first stage; at the third stage (10-30 min) water content decreases exponentially at the slowest rate and tends to equilibrate. The progress of optical clearing measured by the optical coherence tomography system and spectrophotometer, respectively, corresponds very well with the three stages of water desorption. It indicates that optical clearing induced by hyperosmotic agents is strongly correlated with dehydration. The overall water loss in fresh porcine muscle tissue at 30 min after the application of 80% glycerol and ethylene glycol are approximately 39% and 23% respectively. The second derivative spectra show that glycerol is able to make bulk and bound water desorb. As a consequence, due to its high refractive index and high dehydration capability, glycerol is a more effective agent in optical clearing than ethylene glycol.