Equation 1.

Fick’s second law of diffusion to model one-dimensional radial diffusion, with Dex concentration (C), constant Dex source concentration (C_o), position (x), diffusivity (D), and time (t).

$$\frac{C}{C_o}=\mathit{\text{erfc}}\left(\frac{x}{\sqrt{4Dt}}\right)$$