  Isospin-breaking effects in the baryonic sector are studied in the framework
of a medium-modified Skyrme model. The neutron-proton mass difference in
infinite, asymmetric nuclear matter is discussed. In order to describe the
influence of the nuclear environment on the skyrmions, we include
energy-dependent charged and neutral pion optical potentials in the s- and
p-wave channels. The present approach predicts that the neutron-proton mass
difference is mainly dictated by its strong part and that it strongly decreases
in neutron matter.
