Evaluation of the in vivo and ex vivo optical properties in a mouse ear model.

Determination of in vivo optical properties is a challenging problem. Absorption and scattering measured ex vivo are often used for in vivo applications. To investigate the validity of this approach, we have obtained and compared the optical properties of mouse ears in vivo and ex vivo in the spectral range from 370 to 1650 nm. Integrating sphere spectrophotometry in combination with the inverse Monte Carlo technique was employed to determine absorption coefficients, mu(a), scattering coefficients, mu(s), and anisotropy factors, g. Two groups of mice were used for the study. The first group was measured in vivo and ex vivo within 5-10 min post mortem. The second group was measured in vivo and ex vivo every 24 h for up to 72 h after sacrifice. Between the measurements the tissues were kept at 4 degrees C wrapped in a gauze moistened with saline solution. Then the specimens were frozen at -25 degrees C for 40 min, thawed and measured again. The results indicate that the absorption coefficients determined in vivo and ex vivo within 5-10 min post mortem differed considerably only in the spectral range dominated by hemoglobin. These changes can be attributed to rapid deoxygenation of tissue and blood post mortem. Absorption coefficients determined ex vivo up to 72 h post mortem decreased gradually with time in the spectral regions dominated by hemoglobin and water, which can be explained by the continuing loss of blood. Absorption properties of the frozen-thawed ex vivo tissues showed increase in oxygenation, which is likely caused by the release of hemoglobin from hemolyzed erythrocytes. Scattering of the ex vivo tissues decreased gradually with time in the entire spectral range due to the continuing loss of blood and partial cell damage. Anisotropy factors did not change considerably.

Photochemical cross-linking for collagen-based scaffolds: a study on optical properties, mechanical properties, stability, and hematocompatibility.

Collagen presents an attractive biomaterial for tissue engineering because of its excellent biocompatibility and negligible immunogenicity. However, some intrinsic features related to the mechanical stability and thrombogenicity limit its applications in orthopedic and vascular tissue engineering. Photochemical cross-linking is an emerging technique able to stabilize tissue grafts and improve the physicochemical properties of collagen-based structures. However, other important properties of collagen-based structures and the effect of processing parameters on these properties have not been explored. In this study, we aim to investigate the dose dependence of tensile and swelling properties on two parameters, namely, laser energy fluence and rose Bengal photosensitizer concentration. We also study the compression properties using cyclic compression test, long-term stability using subcutaneous implantation, and hematocompatibility using platelets adhesion test, of cross-linked collagen structures. Moreover, because limited optical penetration in turbid media is the major obstacle for light-based techniques, we also characterize the optical properties, which partially determine the effective optical penetration depth in collagen gel samples, during photochemical cross-linking. Laser energy fluence and rose Bengal concentration are important parameters affecting the cross-linking efficiency, which was characterized as the mechanical and the swelling properties, in a dose-dependent manner. Under the experimental conditions in this study, the peak fluence was 12.5 J/cm2 and the minimal rose Bengal concentration for effective cross-linking was >0.00008% (0.786 micromol). Photochemical cross-linking also enhanced the compression strength and long-term stability of collagen structures without compromising the tissue compatibility. Furthermore, photochemical cross-linking reduced platelet adhesion and abolished fibrin mesh formation, thereby improving the hematocompatibility of collagen structures. These results suggest the feasibility of using the photochemically cross-linked collagen structures for orthopedic and vascular tissue engineering. Finally, the effective optical penetration depth in collagen gel samples is wavelength and rose Bengal concentration dependent, and was approximately 12 mm at 514 nm at 0.001% (9.825 micromol), the rose Bengal concentration mostly used in this study.