A promising strategy to design synthetic hydrogels with the ability to self-heal is to substitute the covalently cross-linked polymer chains by supramolecular ones. Although supramolecular hydrogels generally exhibit rapid self-healing without the need for any stimulus, they suffer from low mechanical strength which prevents them from any stress-bearing applications. Here, we describe a novel way for the production of self-healing hydrogels with shape memory behavior of high tensile strength (0.7–1.7 MPa) and stretch at break (800–900%). Hydrophobically modified poly(acrylic acid) (PAAc) chains with cetyltrimethylammonium (CTA) counterions form the physical network of such hydrogels. They were prepared via micellar copolymerization of acrylic acid with 2 mol % stearyl methacrylate (C18) as the hydrophobic comonomer in an aqueous NaBr solution of cetyltrimethylammonium bromide (CTAB). Extraction of free CTAB micelles from the physical gels results in a drastic increase in their Young’s moduli (from 8–30 to 180–600 kPa) and tensile strengths (from 0.1–0.2 to 0.7–1.7 MPa) due to the complex formation between PAAc and CTAB. Loading and unloading cycles conducted on hydrogels both at the state of preparation and at equilibrium in water show a significant hysteresis and good superposition of the successive loading curves, demonstrating damage done during loading is recoverable in nature. The hydrogel samples self-healed via heating and surfactant treatment of the damaged areas withstand up to 1.5 MPa stresses and rupture at a stretch of 600%. Because of the drastic change in the elastic modulus of PAAc hydrogels with a change in temperature, they also exhibit shape memory properties with a recovery ratio of 100%.

pH responsive physical gels were prepared by micellar copolymerization of acrylic acid with 2 mol % 5 stearyl methacrylate (C18) in the solution of worm-like sodium dodecyl sulfate micelles. The physical gels are insoluble in water and exhibit time-dependent dynamic moduli, high Young's modulus (6 -53 kPa), high fracture stress (41 -173 kPa), high elongation at break (1800 -5000 %), and self-healing, as evidenced by rheological and mechanical measurements. Cyclic elongation tests show significant hysteresis due to the existence of reversibly breakable bonds (up to 25 %) within the gel network. As the 10 polymer concentration is increased, both the lifetime of hydrophobic associations and the mechanical strength of gels increase while the elongation ratio at break decreases. As compared to the polyacrylamide gels formed under identical conditions, present hydrogels exhibit a much stronger selfhealing effect, which is attributed to the cooperative hydrogen bonding between the carboxyl groups stabilizing the hydrophobic domains. The efficiency of self-healing significantly increases with 15 increasing temperature due to the decrease of the lifetime of hydrophobic associations. 75 55 † Electronic Supplementary Information (ESI) available: Swelling behavior, temperature dependence of the dynamic moduli, and the effect of urea on the self-healing behavior of PAAc hydrogels. See

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Many natural materials such as intervertebral disk (IVD) are composed of regions with large mismatches in the mechanical properties, yet these regions are integrated through an extremely tough interface. To mimic the mechanical heterogeneity inherent in biological systems, we present here mechanically strong hydrogels consisting of hard and soft components joined together through a strong interface. Stratification of monomer solutions having different densities was used to create two layers of monomer solutions with an interlayer region of a few millimeters in thickness, at which the solutions mix completely. UV-initiated bulk copolymerization of stratified solutions of hydrophilic and hydrophobic monomers leads to the formation of supramolecular, semicrystalline hard/soft hydrogel hybrids with tunable mechanical and thermal properties. By adjusting the comonomer composition in the stratified layers, we were able to create gel/gel interfaces in hybrids that are stronger than their gel components so that they never rupture at the interface region. The hybrids exhibit a high modulus (0.46–74 MPa), tensile strength (0.19–3.9 MPa), and sustain 24–30 MPa stresses at 78–83% compressions, which are comparable to the natural IVD. They also exhibit thermally induced self-healing behavior as well as pseudo triple-shape-memory effect arising from different melting temperatures of crystalline domains belonging to the gel components of hybrids.