Develop an pair of equations that can be used to calculate the fraction released of drug F, from a spherical device of radius a into a perfect sink, based on the Higuchi assumptions, with diffusion coefficient D for the drug molecule, drug solubility C, and initial drug "concentration" (solid+dissolved) c*, uniform throughout the sphere. Use the following steps: a) At time t, a spherical moving front at R(t) separates the inner core containing both dissolved and solid drug, from the outer shell layer containing only dissolved drug. Determine the pseudo-steady state concentration profile of drug in the outer shell layer. From this, derive an equation for F, in terms of R(1). b) Using the pseudo steady state mass balance techniques derived in class, derive and integrate the differential equation describing the motion of R(t). You will end up with an implicit algebraic equation, which cannot be solved directly.