  The $\pi\pi$ scattering amplitude in the $\sigma$-channel is studied at
finite baryonic density in the framework of a chiral unitary approach which
successfully reproduces the meson meson phase shifts and generates the $f_0$
and $\sigma$ resonances in vacuum. We address here a new variety of mechanisms
recently suggested to modify the $\pi\pi$ interaction in the medium, as well as
the role of the $s-$wave selfenergy, in addition to the $p-$wave, in the
dressing of the pion propagators.
