The <solid> tag encloses a description of the solid matrix constitutive relation and associated material properties, as may be selected from the list provided in Section 4.1.3↑. The <permeability> tag encloses a description of the permeability constitutive relation and associated material properties, as may be selected from the list presented in Section 4.8.2↓. The parameter <phi0> must be greater than 0 (no solid) and less than 1 (no porosity). The volume fraction of fluid (also known as the porosity) in the reference configuration is given by $1-\varphi_{r}^{s}$ . The fluid density $\rho_{T}^{w}$ specified in <fluid_density> and the solid density $\rho_{T}^{s}$ specified in <density> within the <solid> tag are needed only when body forces are prescribed.

Example:

<material id="1" name="Biphasic tissue" type="biphasic">
  <solid name="Elasticity" type="neo-Hookean">
    <E>1.0</E>
    <v>0.3</v>
  </solid>
  <phi0>0.2</phi0>
  <permeability name="Permeability" type="perm-const-iso">
    ... (description of permeability material)
  </permeability>
</material>