Stiff, strong, and tough hydrogels with good chemical stability

Abstract

Most hydrogels have poor mechanical properties, severely limiting their scope of applications.Here we report on a hybrid hydrogel that combines extremely high stiffness, strength, and toughness, while maintaining physical integrity in electrolyte solutions. The hydrogel consists of a hydrophilic polymer network that is covalently cross-linked and a second polymer network that can crystallize. We show that the crystallites serve as physical cross-links for the second network.The crystallites contribute to the high stiffness and strength of the hydrogel; they unzip and dissipate energy under deformation, and reform due to the incompatibility of the two polymers in the hydrogel. The crystallite-toughened hydrogel can achieve an elastic modulus of 5 MPa, a strength of 2.5 MPa, and a fracture energy of 14,000 Jm -2 . Unlike alginate-based hybrid hydrogels, this hydrogel preserves its mechanical properties in electrolyte solutions and could be considered for further development in a variety of engineering applications. 20 Unfortunately, hydrogels synthesized using this method are compliant and brittle. 22 It is possible to achieve higher stiffness and toughness using a dry-anneal method, but only at the expense of a much lower water content ( Supplementary Fig. S1). 23 Muratoglu and coworkers polymerized acrylamide monomers in the pores of a PVA hydrogel to form uncross-linked chains, and showed that the equilibrium water content of the resulting gels increased with acrylamide content, while the coefficient of friction, tear strength and creep resistance decreased. 21Here we propose that a hybrid network of a crystalline polymer and a covalently crosslinked hydrophilic polymer may form a hydrogel with robust mechanical properties and good chemical stability: the crystalline polymer can generate a large number of crystallites to serve as physical cross-links that are both stable and reversible; the covalently cross-linked hydrophilic polymer maintain the elasticity of the network during deformation and controls the swelling of the hydrogel. We describe one such hybrid hydrogel that combines extremely high stiffness, strength, and toughness. The hydrogel consists of a hydrophilic polyacrylamide (PAAm) polymer network that is covalently cross-linked and a PVA network that forms crystallites. We show that the PVA crystallites result in a high cross-link density, thus producing a gel of remarkable stiffness and strength. The crystallites unzip under deformation, dissipating energy in the process and yielding a hydrogel with exceptional toughness. After deformation, unzipped crystallites recover at room temperature due to the incompatibility of the two polymers in the gel. The 4 crystallite-toughened hydrogel can achieve an elastic modulus of 5 MPa, a strength of 2.5 MPa, and a fracture energy of 14,000 Jm -2 . Moreover, these properties are stable, even in concentrated electrolyte solutions. ResultsWe prepared the hybrid hydrogels in a simple three-step protocol. First, we form a cross- Supplementary Fig. S2); e...

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