  Motivated by the motion of nematode sperm cells, we present a model for the
motion of an adhesive gel on a solid substrate. The gel polymerizes at the
leading edge and depolymerizes at the rear. The motion results from a
competition between a self-generated swelling gradient and the adhesion on the
substrate. The resulting stress provokes the rupture of the adhesion points and
allows for the motion. The model predicts an unusual force-velocity relation
which depends in significant ways on the point of application of the force.
