Tough Hydrogel [Nature 489, pp (2012)] Hydrogels: scaffolds for tissue engineering, drug delivery, actuators, ECM model system Improved mechanical property stretchability and toughness Notch stretch: Crack bridging by covalent crosslinks and hysteresis by unzipping of ionic crosslink network Healing (deformation removal upon unloading) Mechanisms of deformation and energy dissipation Tough and notch-insensitive gel by introducing energy-dissipating mechanisms double-network gel covalent short chain and long chain (X) covalent and noncovalent bonds (O) ---- recoverable energy dissipation (ionic interaction, hydrophobic association)

Alginate(a) and Polyacrylamide(b) double-network gel [ionic and covalent networks] Water content (86%) / Curing with UV / humid box stabilization /water evaporation with N2 High stretch > 20 times (a, b) and notch-insensitive (c, d)

(a) elastic modulus: synergistic increase at rupture for the hybrid gel (b) hysteresis of recovery (energy dissipation): alginate: hysteresis with permanent deformation PAG: no hysteresis with full recovery hybrid gel: pronounced hysteresis with small deformation (c) hybrid gel w/ varying max. stretch (d) healing after one loading/unloading cycle (e, f) recovery at 80C

Composition affects the hybrid gel behavior Elastic modulus decreases as acrylamide increases 89wt% acrylamide: critical stretch at rupture & fracture energy Deformation and energy dissipation Two network between alginate and polyacrylamide Stretch: alginate is unzipping while PAAm network remains intact [pronounced hysteresis and little permanent deformation] --- healing of the internal damage as the ionic crosslinks re-form Notch to crack: alginate chains crossing the crack plane unzip, leaving the network elsewhere intact Crack bridging (PAAm) and background hysteresis (alginate) Fracture energy of hydrogels increase by combining weak and strong crosslinks Ex) cell encapsulation, contact lenses