  We present an extensive experimental study and scaling analysis of friction
of gelatin gels on glass. At low driving velocities, sliding occurs via
propagation of periodic self-healing slip pulses whose velocity is limited by
collective diffusion of the gel network. Healing can be attributed to a
frictional instability occurring at the slip velocity $v = V_c$. For $v > V_c$,
sliding is homogeneous and friction is ruled by the shear-thinning rheology of
an interfacial layer of thickness of order the (nanometric) mesh size,
containing a semi-dilute solution of polymer chain ends hanging from the
network. Inspite of its high degree of confinement, the rheology of this system
does not differ qualitatively from known bulk ones. The observed ageing of the
static friction threshold reveals the slow increase of adhesive bonding between
chain ends and glass. Such structural ageing is compatible with the existence
of a velocity-weakening regime at velocities smaller than $V_c$, hence with the
existence of the healing instability.
