  We suggest a simple model for reversible cross-links, binding and unbinding
to/from a network of semiflexible polymers. The resulting frequency dependent
response of the network to an applied shear is calculated via Brownian dynamics
simulations. It is shown to be rather complex with the timescale of the linkers
competing with the excitations of the network. If the lifetime of the linkers
is the longest timescale, as is indeed the case in most biological networks,
then a distinct low frequency peak of the loss modulus develops. The storage
modulus shows a corresponding decay from its plateau value, which for
irreversible cross-linkers extends all the way to the static limit. This
additional relaxation mechanism can be controlled by the relative weight of
reversible and irreversible linkers.
