1. Daria K. Tuchina, Alexey N. Bashkatov, Elina A. Genina, Vyacheslav I. Kochubey, Valery V. Tuchin Department of Optics and Biophotonics of Saratov State University, Saratov, Russia

2. For the last two decades the optical method as a tool for clinical functional imaging of physiological conditions, cancer diagnostics and therapies is of great interest due to its unique informative features, simplicity, safety and low cost in contrast to conventional X-ray computed tomography, magnetic resonance imaging and ultrasound. However, the main limitations of the optical imaging techniques are deal with strong light scattering in tissue. Optical clearing is perspective technique for solution of the problem. However, in spite of numerous investigations, optical clearing of skin tissue has not be studied in detail. Goal of the study is to investigate of optical clearing of skin by 40%-glucose solution Saratov State University Department of Optics and Biophotonics

3. Ten samples of intact rat skin were measured to obtain spectra of skin optical properties before and after incubation in 40%-glucose solution. Diffuse reflectance and total and collimated transmittance spectra were measured by LAMBDA 950 (Perkin Elmer, USA) spectrophotometer with an integrating sphere in the spectral range 350 – 2500 nm. All measurements have been performed during 24 hours after obtaining the samples. Then the samples were incubated during 24 hours in 40%-glucose solution and all of the spectra were measured again. The last stage of this study was incubating the samples during 24 hours in physiological solution and measuring spectra after that. Inverse Monte Carlo technique has been used for processing the experimentally measured spectra of the skin samples; and wavelength dependence of absorption and scattering coefficients, and anisotropy factor has been obtained. Saratov State University Department of Optics and Biophotonics

4. LAMBDA 950 (Perkin Elmer, USA) Saratov State University Department of Optics and Biophotonics

5. Wavelength dependence of scattering coefficient for intact skin (■), skin incubated in 40%-glucose solution (■) and skin incubated in physiological solution (■) Saratov State University Department of Optics and Biophotonics

6. Wavelength dependence of absorption coefficient for intact skin (■), skin incubated in 40%-glucose solution (■) and skin incubated in physiological solution (■) Saratov State University Department of Optics and Biophotonics

7. Wavelength dependence of anisotropy factor for intact skin (■), skin incubated in 40%-glucose solution (■) and skin incubated in physiological solution (■) Saratov State University Department of Optics and Biophotonics

8. Wavelength dependence of reduced scattering coefficient for intact skin (■), skin incubated in 40%-glucose solution (■) and skin incubated in physiological solution (■) Saratov State University Department of Optics and Biophotonics

9. Glucose diffusion coefficient was estimated from the measurement of collimated transmittance of ten rat skin samples with USB4000-Vis-NIR spectrometer (Ocean Optics, USA) concurrently with administration of 40%-glucose solution in the spectral range 400-1000 nm using specially developed computer program. The scheme of the experimental setup for measurements of the collimated transmittance Saratov State University Department of Optics and Biophotonics

10. Estimation of glucose diffusion coefficient The one-dimensional diffusion equation of the immersion liquid (the glucose solution) transport has the form: diffusion equation boundary conditions initial conditions C(x,t) is glucose concentration in skin sample, g/ml; D is the diffusion coefficient, cm 2 /sec; t is time of immersion liquid diffusion, sec; x is the spatial coordinate of sample thickness, cm; C 0 is concentration of glucose in the external volume (i.e., in the cuvette), g/ml; l is the thickness of the sample, cm. The average concentration of glucose in the skin sample has a form: Saratov State University Department of Optics and Biophotonics

11. The temporal dependence of the refractive index of the skin interstitial fluid is: Estimation of glucose diffusion coefficient n I0 is a refractive index of the interstitial fluid when t is 0 sec; n osm is a refractive index of glucose solution. The scattering coefficient of the skin sample was estimated as: N is a number of scattering particles in tissue unit of volume; n I is a refractive index of interstitial fluid; λ is a wavelength, nm; a is a radius of scattering particles; n c is a refractive index of scattering particles. Saratov State University Department of Optics and Biophotonics

12. Estimation of glucose diffusion coefficient Collimated transmittance is estimated as: r s is the specular reflection coefficient Estimation of diffusion coefficient of glucose in tissue is based on measuring of time dependence of collimated transmittance of tissue samples placed into glucose solution. The solution of problem is minimization of the target function: N t is the number of time points obtained at registration of time dependence of collimated transmittance; T c (D,t); T c *(t i ) are the calculated and experimental values of the time-dependent collimated transmittance. Saratov State University Department of Optics and Biophotonics

13. The average value of glucose diffusion coefficient was estimated as (1.52±1.62)  10 -6 cm 2 /sec. The presented results can be used for the development of the optical imaging technologies and diagnostics and therapy of diabetes mellitus. Saratov State University Department of Optics and Biophotonics

14. Acknowledgements Grants # 11-02-00560 and 12-02-92610-KO of Russian Foundation of Basis Research Russian Federation governmental contacts 02.740.11.0770, 02.740.11.0879, 11.519.11.2035, and 14.B37.21.0728 Grant #224014 Network of Excellence for Biophotonics (PHOTONICS4LIFE) of the Seventh Framework Programme of Commission of the European Communities Saratov State University Department of Optics and Biophotonics