  Motivated by studies suggesting that the patterns exhibited by the
collectively expanding fronts of thin cells during the closing of a wound [Mark
et al., Biophys. J., 98:361-370, 2010] and the shapes of single cells crawling
on surfaces [Callan-Jones et al., Phys. Rev. Lett., 100:258106, 2008] are due
to fingering instabilities, we investigate the stability of actively driven
interfaces under Hele-Shaw confinement. An initially radial interface between a
pair of viscous fluids is driven by active agents. Surface tension and bending
rigidity resist deformation of the interface. A point source at the origin and
a distributed source are also included to model the effects of injection or
suction, and growth or depletion, respectively. Linear stability analysis
reveals that for any given initial radius of the interface, there are two key
dimensionless driving rates that determine interfacial stability. We discuss
stability regimes in a state space of these parameters and their implications
for biological systems. An interesting finding is that an actively mobile
interface is susceptible to fingering instability irrespective of viscosity
contrast.
