External Material Model COUPLED with other physics

Hi,

I developed a mechanical constitutive model by using the External material model. I want to use the state variable calculated from the External material model, say damage or plastic strain, into another physics, say, the Darcy's module. I input the variable "extmat1.state.Damage1" into Darcy's module, but the module cannot recgonise the variabe. In addition, how to calculate the time derivative of the state variable? Is it should be done in C code or just in COMSOL, such as d(extmat1.state.Damage1,TIME)?

Does anyone have encountered the same issue? How to solve it?

Thank you for your help and time.