  \textbf{Background} Hydrogels are crosslinked polymer networks that can
absorb and retain a large fraction of liquid. Near a critical sliding velocity,
hydrogels pressed against smooth surfaces exhibit time-dependent frictional
behavior occurring over multiple timescales, yet the origin of these dynamics
is unresolved. \textbf{Objective} Here, we characterize this time-dependent
regime and show that it is consistent with two distinct molecular processes:
sliding-induced relaxation and quiescent recovery. \textbf{Methods} Our
experiments use a custom pin-on-disk tribometer to examine poly(acrylic acid)
hydrogels on smooth poly(methyl methacrylate) surfaces over a variety of
sliding conditions, from minutes to hours. \textbf{Results} We show that at a
fixed sliding velocity, the friction coefficient decays exponentially and
reaches a steady-state value. The time constant associated with this decay
varies exponentially with the sliding velocity, and is sensitive to any
precedent frictional shearing of the interface. This process is reversible;
upon cessation of sliding, the friction coefficient recovers to its original
state. We also show that the initial direction of shear can be imprinted as an
observable "memory", and is visible after 24 hrs of repeated frictional
shearing. \textbf{Conclusions} We attribute this behavior to nanoscale
extension and relaxation dynamics of the near-surface polymer network, leading
to a model of frictional relaxation and recovery with two parallel timescales.
