  We investigate the force generation by polymerizing bundles of filaments,
which form because of short-range attractive filament interactions. We show
that bundles can generate forces by a zipping mechanism, which is not limited
by buckling and operates in the fully buckled state. The critical zipping
force, i.e. the maximal force that a bundle can generate, is given by the
adhesive energy gained during bundle formation. For opposing forces larger than
the critical zipping force, bundles undergo a force-induced unbinding
transition. For larger bundles, the critical zipping force depends on the
initial configuration of the bundles. Our results are corroborated by Monte
Carlo simulations.
