Membrane boundary condition
  Using a microscopic phase-space model of the membrane system, the boundary condition at a membrane is derived. According to the condition, the substance flow across the membrane is proportional to the difference of the substance concentrations at the opposite membrane surfaces. The Green's function of the diffusion equation is found for the derived boundary condition and the exact solution of the equation is given. 
Tadeusz Kosztolowicz and Stanislaw Mrowczynski