Man-made materials can be either tough or strong, but so far efforts to marry both attributes have struggled. As one of the researchers in the MIT team Francisco Martin-Martinez tells, "graphene oxide (GO) may be easier to fabricate than graphene but it is weaker." Assembling 2D GO sheets into films that can cover centimetres instead of just micrometres—"GO paper"—leaves the strength further compromised still.

"This is where polydopamine (PDA) comes into the game," says Martin-Martinez, describing how the polymer was inspired by the adhesion properties of mussels threads. Considering how stubbornly mussels bind to rocks and other coastal structures under the onslaught of tidal turbulence, it is easy to see why the adhesive strength of these molluscs might have stimulated ideas for the team.

"Additionally, we also got inspired by the structure of nacre [mother of pearl], which resembles a brick and mortar configuration and has an outstanding mechanical performance," adds Martin-Martinez. "Thus, with a lot of inspiration from natural systems, we designed a nacre-inspired GO-(mussel-inspired) PDA nanocomposite, which indeed has better mechanical performance than pristine GO films."

While other groups have studied PDA composites, previous work has been restricted to experiments. As pointed out by Chun-Teh Chen, the first author on the report of this work, ambiguities over the polymerization mechanism for PDA and the chemical reactions between PDA and GO had floored prior attempts to model the system. The MIT researchers were able to get around this to create molecular models and then feed back into the model from experiments and material characterization.

"This represents an advanced predictive tool that makes the production of materials more efficient, reducing the number of experiments required by incorporating atomistic design into the fabrication process," says Martin-Martinez.

From model to adhesive mechanisms

The researchers identified the most reactive positions of 5,6-dihydroxyindole (DHI), which forms covalent linkages as PDA self-polymerizes from solutions of GO and dopamine, as well as reacting with the GO. They then simplified models of this polymerization process using basic building blocks of PDA.

By introducing water at varying concentrations into their model, the researchers could determine the effects of humidity on the hydrogen bonding interactions between GO layers and the interlayer spacing. They also identified a humidity-driven shrinking mechanism of GO–PDA nanocomposites due to non-uniform stresses on the sheets. Using these models they investigated the strength of the nanocomposites when adjacent layers were pulled in opposite directions, revealing an increase in strength with humidity, and superior strength in the GO–PDA nanocomposite compared with GO.

The results contrast with previous assumptions that suggested the strength of these composites stems primarily from covalent crosslinks. The model used by Buehler and his team did not include these covalent crosslinks and yet the adhesive strength of GO–PDA in the simulations still exceeded GO and compared well to experiments, suggesting that covalent crosslinks contribute negligibly to the material’s mechanical strength.

"Bottom-up design" future potential

The researchers acknowledge certain limitations in the model but are still confident in the wider applications of the "bottom-up design" approach based on simulations and experiments.. "From the simulations, we can tell which material design yields higher mechanical performance even though we cannot get its actual strength and toughness in the simulations," says Chen. There may also be many other materials where humidity plays a crucial role in the mechanical properties as it does in GO–PDA.

"The authors report achieving higher mechanical strength and toughness than other nacre mimics and their modelling points towards future improvements being yet possible," says Rodney Ruoff, who was not involved with the MIT team’s recent work, but whose research at the University of Texas in Austin has also focused on graphene composites. "It will be interesting to see if self-folding structures with folding controlled by humidity can be achieved in the future."

Future work will focus on improving the model for PDA, exploring other bio-inspired materials, continuing work on more realistic models, and additional synthetic strategies to improve the experimental aspects.

Full details are reported in Nano Futures.