Hydrogels are used as scaffolds for
tissue engineering, vehicles for drug delivery, actuators for optics and
fluidics, and model extracellular matrices for biological
studies. The scope of hydrogel applications, however, is often severely limited
by their mechanical behaviors. Most hydrogels are brittle, sensitive to
notches, and do not exhibit high stretchability. We report the synthesis of hydrogels
from polymers forming ionically and covalently crosslinked networks.
Although such gels contain 90% water, they can be stretched beyond 20 times
their initial length, and have fracture energies of ~9,000 Jm-2. Even for samples containing notches, a
stretch of 17 is demonstrated. We attribute the gels’ toughness to the synergy
of two mechanisms: crack bridging by the network of covalent crosslinks, and
hysteresis by unzipping the network of ionic cross- links.