Nonlinear diffusivity of analytes in tissues

Noninvasive assessments of optical clearing and permeability coefficients of tissues pose great possibilities in advanced diagnostics and medical applications. In order for both of these to become utilized in common practice, a greater understanding of molecular diffusivity in multi-layered tissues is required. In biological tissues, the different layers are comprised of differentiated cells and/or collagen fibrils which come together to form that specific layer. Therefore, a patchwork of layers is created each with its own set of properties. In our current study we analyze and describe the dynamics of matter diffusion and its underlying non-linear character in various epithelial tissues. For instance, the permeability coefficient (PC) of 20% concentrated mannitol in the rabbit eye sclera showed an increasing trend as it was measured deeper into the tissue. The PC was found to be 2.18 × 10^-6 cm/sec at 50 μm away from the epithelial layer. It increased to about 7.33 × 10^-6 cm/sec when it was computed at 210 μm from the epithelial layer. Different layers in the sclera showed different clearing response to glucose solution as well. The first 100 μm region from the epithelial layer cleared about 10% whereas the next 100 μm cleared about 17-22%. The importance of this study is that it may offer a novel explanation to how a layer's composition affects optical clearing and the permeability coefficient of analytes and solutions.