Nonlinear diffusivity of analytes in tissues

Abstract

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.

Investigation of the permeability and optical clearing ability of different analytes in human normal and cancerous breast tissues by spectral domain OCT

“…This result can be used as the demonstration of the concept that the concentration of hyperosmotic agents inversely affects the permeability coefficient of the compound through the biological tissues, which has been observed in previous studies in different tissues [37,38]. The permeability coefficient of 20% glucose is about 1.46-fold higher than that of 40% glucose in normal breast tissues and 1.76-fold in breast cancer tissues.…”

“…The reverse process is thought to stem from the difference in the concentration gradients inside the tissue, which enforce the net fluid (mainly water) tra-velling from the region of higher concentration to the lower one, induced water re-entering the tissue after diffusing out [20,23,34,35]. The permeability coefficient of 20% glucose in normal breast tissue was found to be 8.55 Â 10 À6 cm/s in Figure 1a, while in breast cancer tissue was found to be 1.85 Â 10 À5 cm/s in Figure 1b.…”

A robust ex vivo method to evaluate the diffusion properties of agents in biological tissues

“…Since this method is based on T c measurements, it can be directly applied to the in vivo tissue and organ examination with some limitations, for example by using specially designed fiber optical probes. However, it can be adapted to the in vivo environment by using different optical modalities based on back reflectance measurements such as OCT , confocal microscopy or two‐photon microscopy . The use of spectral diffuse reflectance ( R d ) measurements is also a way to evaluate agent diffusion in tissues in vivo , but studies must be performed.…”

“…OCA concentration efficiency was also evaluated with tissue depth and improved contrast of OCT or second harmonic generation images at deeper tissue layers . Some mathematical models were developed to describe OCA diffusion in tissues .…”