Biodegradable Implant Drug Elution — Radial Diffusion Field

This is the 2D field-based counterpart of the particle-based implant simulator. Rather than following hundreds of discrete drug particles through a 3D random walk, this engine solves Fick's second law directly as a partial differential equation on a 2D finite-difference grid: a disk-shaped cross-section of the polymer matrix sits inside a wider "release medium" region, and a continuous concentration field diffuses outward from the implant; drug counts as "released" once it crosses the implant's own radius into the medium, the same test the 3D model applies to each particle. The matrix's own diffusivity is not constant — it rises over simulated days as first-order polymer hydrolysis erodes the backbone, exactly the same erosion–diffusivity coupling used in the 3D model, so the same biphasic burst-then-erosion release curve emerges from a completely different numerical method (deterministic grid PDE vs. stochastic particle Monte Carlo).