Measurement of water variation within skin under probe pressure by diffuse reflectance spectroscopy

In the measurement of in vivo diffuse reflectance spectrum, the contact method is widely adopted. The external pressure caused by fiber optic probe may influence the accuracy of measurement. According to the solid-liquid biphasic mixture model, the mechanism of tissue deformation and composition change under certain compression is researched. The diffuse reflectance spectra of tissue with the increase of pressure and duration are measured and transformed into second-derivation spectra. The amplitudes of 1160 nm and 1220 nm in the second-derivative spectra are related to the contents of bulk and bound water in the tissue which reflects the process of tissue deformation. Experimental results indicate that the variations of bulk and bound water in tissue have a non-linear relationship with the pressure and duration. It is also affected by the differences of tissue structure and physiological status. When the in vivo skin is measured, the pressure of probe should be limited to less than 200 kPa to prevent the bound water in skin from changing into bulk water. Future research on the selections of measurement site and strategy of diffuse reflectance spectra might benefit from the results of this paper.