  We revisit the problem of pinning a reaction-diffusion front by a defect, in
particular by a reaction-free region. Using collective variables for the front
and numerical simulations, we compare the behaviors of a bistable and
monostable front. A bistable front can be pinned as confirmed by a pinning
criterion, the analysis of the time independant problem and simulations.
Conversely, a monostable front can never be pinned, it gives rise to a
secondary pulse past the defect and we calculate the time this pulse takes to
appear. These radically different behaviors of bistable and monostable fronts
raise issues for modelers in particular areas of biology, as for example, the
study of tumor growth in the presence of different tissues.
