Hybrid Hydrogels with Extremely High Stiffness and Toughness

Abstract

The development of hydrogels for cartilage replacement and soft robotics has highlighted a challenge: load-bearing hydrogels need to be both stiff and tough. Several approaches have been reported to improve the toughness of hydrogels, but simultaneously achieving high stiffness and toughness remains difficult. Here we report that alginate-polyacrylamide hydrogels can simultaneously achieve high stiffness and toughness. We combine short-and long-chain alginates to reduce the viscosity of pregel solutions and synthesize homogeneous hydrogels of high ionic cross-link density. The resulting hydrogels can have elastic moduli of ∼1 MPa and fracture energies of ∼4 kJ m −2 . Furthermore, this approach breaks the inverse relation between stiffness and toughness: while maintaining constant elastic moduli, these hydrogels can achieve fracture energies up to ∼16 kJ m −2 . These stiff and tough hydrogels hold promise for further development as load-bearing materials.

Flaw sensitivity of highly stretchable materials

“…The stretchable materials in the current work are compared with other materials, e.g., natural rubbers [47,66], polyacrylamide hydrogels [26], alginate hydrogels [26,67], and tough hydrogels, [26,68], as well as steels, aluminum, bone, human skin, acrylic glass, epoxy, aluminum oxide, and silica glass. [69] For brittle hard solids (e.g., a silica glass), measuring σ * and W * in the small-flaw limit is a difficult experimental task, and is rarely done in practice, because the small-flaw limit is reached when the flaws approach the atomic scale. In practice, brittle hard solids nearly always operate in the large-flaw limit, where the Griffith fracture mechanics applies.…”

Recent developments in dynamic fracture: some perspectives

“…Lately, new classes of materials are becoming available that join natural rubber as elastic, yet brittle, materials that can sustain large background strains prior to fracture. The fracture resistance of some of these synthetic materials, such as double-network hydrogels made of ionically and covalently cross-linked networks [214][215][216][217][218][219], can be made to be enormous and new applications are expected to abound [220,221]. In addition to such "chemical gels", there is also clear experimental evidence that elastic nonlinearities are important to fracture processes in "physical gels" [108], i.e.…”

Time-dependent mechanical properties of tough ionic-covalent hybrid hydrogels

“…When a hybrid gel is subject to external force the crosslinks are "unzipped", dissipating dissociation energy of the ionic bonds and entropic energy of the loaded network strands. High toughness of the hybrid gel is obtained when the alginate network is highly-crosslinked and the PAAm network is loosely-crosslinked [9,[14][15][16][17][18], which mirrors the topologies known to maximize toughness in DN gels [19]. The ionic bonds that are pulled apart during the hybrid gel deformation are re-formed at zero-stress at a temperature-induced network recovery process to largely retrieve the mechanical properties of the virgin gel [9].…”