cell move in a microchannel

Hi all,

I am doing simulation about a cell moving in a microchannel. I want to model the cell as a layer of elastic material with water in the closed membrane. Attached please find the geometry. I use fluid-structure interaction to model the cell in the channel but I am confused about the boundary condition of the interface of the water in the membrane and the inner membrane layer, can anyone help me about this problem?

In the attached picture, I choose domain 1&2 in the node of fluid -structure interaction, do I need to choose domain 3 also?

Thank you!