**5.2.4 External faces: Insulation/symmetry**

This condition is specified at the perivascular wall and at the up- and down-stream wall boundaries, which should be a sufficient distance away from the stent. It specifies where the domain is well insulated or it can reduce the size of a model by taking advantage of symmetry. Intuitively this condition states that the gradient across the boundary must be zero, therefore it is impermeable to mass transport.
