nWhat version of ANSYS are you using? Newer versions allow you to make simple geometry in Mechanical. In older versions, open the Geometry editor and make a surface much larger than the size over which the cell might stretch. Bring that surface into Mechanical. Set the behavior of the surface to Rigid. Select an Edge and use Remote Displacement to hold the surface fixed. Insert a Frictionless Contact between the surface and the inner face of the cell.n