in which the solid phase is not saturated by fluid.

Figure \[satswelfig\] schematically depicts two possible scenarios. If the tissue is unsaturated in its current configuration, as in A, then, on a microscopic scale, it contains unfilled voids. It is thus capable of allowing an influx of fluid, which tends to increase its degree of saturation until fully saturated, as in B. This increase does not cause swelling of the tissue in the local stress-free state, as there is free volume for incoming fluid to occupy. However, once the tissue is saturated in the current configuration, an increase in the fluid content causes swelling in the stress-free state, as depicted in C, since there is no free volume for the entering fluid to occupy. It is this second case that is modelled by (\[isotropicgrowth\]). It is worth emphasizing that this argument holds for $\bF^{\mathrm{g}^\mathrm{f}}$, which is the local stress-free state of deformation of the fluid-containing pores at a point. The actual deformation gradient, $\bF =
\bF^{\mathrm{e}^\mathrm{f}}\bF^{\mathrm{g}^\mathrm{f}}$, also depends on the the elastic part, $\bF^{\mathrm{e}^\mathrm{f}}$, which is determined by the constitutive response of the fluid. Under stress, an incompressible fluid will have $\mathrm{det}\bF^{\mathrm{e}^\mathrm{f}} = 1$ and therefore a fluid-saturated tissue will 