Robin boundary conditions

Hi all,

I wanted to apply Robin boundary conditions, in the nearly-incompressible hyperelastic model (Mooney Rivlin), using the 3D cylinder mesh. The Robin boundary condition should be like:

S·n + a·u = 0,

where S is the stress, n is the the normal vector to the surface, u is the displacement, and a is a parameter representing the elastic response due to the presence of surrounding tissue (for example 1·10^-2),. Robin BC are imposed on Gamma_R = Gamma, the surface of the cylinder.

How can I implement this?

Thanks a lot!