  Active-elastic instabilities are common phenomena in the natural world which
have the aspect of sudden mechanical morphings. Frequently, the driving force
of the instability mechanisms has a chemo-mechanical nature which makes these
kind of instabilities very different from standard elastic instabilities. In
this paper, we describe and study the active-elastic instability occurring in a
swollen spherical closed shell, bounding a water filled cavity, during a
de-hydration process. The description is given through the outcomes of a few
numerical experiments based on a stress-diffusion model which allows to glance
at the phenomenon. The study is carried on from a chemo-mechanical perspective
through a few simplifying assumptions which allow to derive a semi-analytical
model which takes into account both the stress state and the water
concentration into the walls of the shell at the onset of the instability.
Moreover, also the invariance of the cavity volume at the onset of instability,
which is due to the impossibility to instantaneously change the cavity volume
filled with water, is considered. It is shown as the semi-analytic model
matches very well the outcome of the numerical experiments. It is also shown as
a wider range of mechanical instabilities can be produced when inhomogeneous
shells are considered, even when the inhomogeneities can be described by a
small number of parameters.
