Improving the Strength of Ultrastiff Organic–Inorganic Double-Network Hydrogels

Abstract

Swollen double networks (DNs) are hydrogels with greatly improved stiffness, toughness, and strength compared to classical hydrogels. The highest stiffness is achieved for organic− inorganic DN hydrogels, which show a rather low tensile strength of about 1 MPa so far. It was presumed that this is due to an insufficient reversible bond formation between inorganic and organic phases. Therefore, the influence of the functional groups that form reversible bonds between these two phases was investigated on the example of calcium phosphate-based DN hydrogels. The functional groups were introduced by copolymerization of acrylate-based monomers with acrylamide and N,Ndimethylacrylamide, respectively, to form a hydrogel-containing alkaline phosphatase. After enzyme-induced mineralization, it was found that only acrylic acid (AA) results in improved strength of the formed ultrastiff DN hydrogels. Stress−strain curves with different strain rates revealed that Young's modulus of ∼300 MPa is constant in all cases, while the tensile strength increases from 7 MPa at 5% min −1 to 17 MPa at 750% min −1 . The fracture toughness of these optically transparent DN hydrogels, which are among the stiffest and strongest existing hydrogels, is up to 2000 J m −2 which is also improved by the introduction of AA into the hydrogel.

Enzyme‐Induced Ferrification of Hydrogels for Toughening of Functional Inorganic Compounds

“…Then, the glass slides were carefully separated and the formed polymer film (thickness 300 ± 30 µm) was either ferrified or stored until use at −25 °C. [ 42–47 ]…”

“…Polymer Film Synthesis with Alkaline Phosphatase: Synthesis of polymer networks was carried out according to a previously published protocol. [45,47] First, 0.4 mg of the alkaline phosphatase formulation was dissolved in a mixture of 10 μL of 0.2 m TEA buffer solution and 10 μL of the polyglutaraldehyde solution.…”

“…Polymer Film Synthesis with Urease: Synthesis of polymer networks was carried out according to previously published protocols. [42][43][44][46][47] First, 1 mg of the urease formulation was dissolved in 20 μL of the TEA buffer solution.…”

“…Then, the glass slides were carefully separated and the formed polymer film (thickness 300 ± 30 μm) was either ferrified or stored until use at −25 °C. [42][43][44][45][46][47] Ferrification: 200 mL of the Ferrification solution was poured into a closed Schott bottle containing one enzyme-loaded network (typical polymer to solution ratio of 1 mg 1 mL −1 ). The immersed networks with immobilized alkaline phosphatase were incubated at 20 °C in a climate chamber for 7 d, while networks with immobilized urease were incubated at 20 °C in a climate chamber or at 60 °C using a heated water bath for 24 h. The resulting composite material was taken out after ferrification and thoroughly rinsed with and stored in pure water at 20 °C.…”

“…A recent method to achieve the formation of highly ordered nanostructures within a polymer matrix is the enzyme-induced mineralization (EIM). [42][43][44][45][46][47] This method allows the formation of percolated nanostructures of amorphous and crystalline calcium carbonate and -phosphate within hydrogels. The degree of mineralization of such materials reaches 70-90 wt%, and the fully transparent materials can be tuned in stiffness over several orders of magnitude while also showing a remarkable toughness, with fracture energies of 1000-2000 J m −2 .…”