is the reference concentration value at which the tissue attains saturation in the current configuration.

With this redefinition of $\bF^{\mathrm{g}^\mathrm{f}}$ it is implicit that $\tilde{v}^\mathrm{f} + \tilde{v}^\mathrm{s} > 1$ is non-physical. Saturation holds in the sense that $\tilde{v}^\mathrm{f} +
\tilde{v}^\mathrm{s} = 1$, and it actually allows $\sum_\iota
\tilde{v}^\iota > 1$ if the sum is over all species. It has been common in the soft tissue literature to assume that, under normal physiological conditions, soft tissues are fully saturated by the fluid and is appropriate for $\iota =
\mathrm{f}$. However, this treatment of saturation and swelling induced by the fluid phase is necessary background for Section \[tensionfluid\] where we discuss the response of the fluid phase under tension. This treatment also holds relevance for partial drying, which *ex vivo* or *in vitro* tissue may be subject to under certain laboratory conditions, and is central to the mechanics of drained porous media other than biological tissue, most prominently, soils.

Balance of momenta {#bomom}
------------------

In soft tissues, the species production rate and flux that appear on the right hand-side in Equations (\[massbalance1\]) and (\[massbalcurr\]), are strongly dependent on the local state of stress. To correctly model this coupling, the balance of linear 