Thin Imptermeable Barrier Boundary Layer in Coefficient Form PDE

Hello everyone,

I am working on a diffusion model but the reaction terms require me to use Coefficient Form PDE rather than transport of dilluted species. I need the thin impermeable barrier equivalent in coefficient form PDE but I am not sure how to set such boundary layer. Does anyone know how such a thing can be done? I imagine I will have to use uflux(u) and dflux(u).

Thank you,