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The finite element formulation for multiphasic materials in FEBio requires that the natural boundary condition for solute $\alpha$ be prescribed as $\tilde{j}_{n}^{\alpha}\equiv j_{n}^{\alpha}+\sum\nolimits _{\beta}z^{\beta}j_{n}^{\beta}$ , where $\tilde{j}_{n}^{\alpha}$ is the effective solute flux. For a mixture containing only neutral solutes ($z^{\beta}=0,\,\forall\beta$ ), it follows that $\tilde{j}_{n}^{\alpha}=j_{n}^{\alpha}$ .