I am trying to simulate a gas sensor model. I have used the surface reaction module to calculate the charge concentration. I want to use this concentration to calculate the change of resistance of the sensing layer. As the charge concentration is a surface charge concentration I can not directly use this in the electric current module otherwise it is giving an error message that concentration is undefined in that domain. So, I want to use this concentration to define a variable and run the electric current module separately. How can I do that? Thanks in advance.