Smart mud could be the new plastic

Could a mixture of water and clay replace plastics? The desire to wean the world off oil has sparked all manner of research into novel transportation fuels, but manufacturing plastics uses large amounts of oil too. Researchers at the University of Tokyo, Japan, think their material could be up to the task.

Takuzo Aida and his team mixed a few grams of clay with 100 grams of water in the presence of tiny quantities of a thickening agent called sodium polyacrylate and an organic "molecular glue". The thickening agent teases apart the clay into thin sheets, increasing its surface area and allowing the glue to get a better hold on it.

This means that, while the mixture is almost 98 per cent water, it forms a transparent and elastic hydrogel with sufficient mechanical strength to make a 3.5-centimetre-wide self-standing bridge.

Self-repairing hydrogel

The strength of the material depends on the sum of the forces acting between the molecules in the clay nanosheets and the glue, says Aida. These so-called supramolecular forces, such as hydrogen bonds, also help to trap water molecules between the clay sheets.

Some other hydrogels rely on covalent chemical bonds rather than supramolecular forces for their strength. One disadvantage of this is that when the covalent bonds break, the material irreversibly loses its strength, says Aida. Supramolecular forces, on the other hand, can easily reform, so if the material fails under stress it can quickly regain its strength.

The gel takes just 3 minutes to form, and making it requires no understanding of the chemical process involved, Aida says, – a fact that impresses Craig Hawker at the University of California in Santa Barbara, who was not involved with the study. "One of the primary breakthroughs is the overall simplicity of the procedure coupled with the exceptional physical properties of the final assemblies," he says.

New class of materials

"Toughness, self-healing and robustness are just some of the initial physical properties that will be found for this new class of materials," Hawker says. "I predict that this approach will lead to the design of even more impressive materials in the near future."

Polymer scientist Jian Ping Gong at Hokkaido University in Sapporo, Japan, says the work is "beautiful" but points out that the material's mechanical strength falls short of what is possible for plastics and chemically cross-linked gels.

Aida says that strengthening the material is as simple as increasing the quantities of clay, sodium polyacrylate and glue, provided transparency is not important.