Scientists further strengthen a biomaterial already tougher than most man-made fibres
Spider silk, already one of the strongest fibres known, can be made even stronger by infusing metals into its protein structure, scientists in Germany say. The team hope that their findings will lead to lighter weight and tougher new materials.
Seung-Mo Lee and Mato Knez at the Max Planck Institute of Microstructure Physics in Halle and their team have embedded the protein fibres of natural silk from an Araneus spider with zinc, aluminium and titanium and found it increases their strength.
Protein structures embedded with metals are already known in nature - organisms such as leaf-cutter ants and locusts are known to have tiny amounts of metals incorporated into the protein structures of body parts such as claws and jaws, making them incredibly strong. ’As far as I know nobody has artificially added metals into this type of biological material before,’ says Knez. ’People normally use either natural materials or artificial materials, but we have mixed these together and produced a completely novel type of material,’ he explains. ’We chose to start with spider’s silk because it is already known to be very tough.’
The team obtain their silk from a spider they captured in the Institute’s garden. ’Most of the experiments - in order to compare them - were done using this one spider,’ says Knez. To embed metal into the silk the team use widely used technology called atomic layer deposition - a process normally used to coat one material with a thin film of another. But the team found that the silk was not only coated with the metal but the metal ions had penetrated the fibres and reacted with the protein structure.
The team first tested zinc - by exposing the silk to a vapour of diethylzinc and water - and compared its strength with untreated silk, finding a considerable improvement in toughness. They then used aluminium (Al2O3) and titanium (TiO2) with similar results. The team were also able to show that the outer metal coating of the silk was of minor importance in the improvement of strength, and therefore that the phenomena was caused by the metals imbedded in the protein fibres.
Knez attributes the strengthening effect to the metal’s displacement of hydrogen bonds within the silk’s protein structure. The water vapour disrupts the hydrogen bonding network of the silk proteins, and these bonds are replaced by stronger metal-coordinated or covalent metal-protein bonds as the metal ions slowly infiltrate the silk.
Fritz Vollrath, an expert in the physical and chemical properties of silks from the University of Oxford, UK, say that metal deposition is an interesting approach to strengthening spider silks. ’This process could be commercially of interest - perhaps not so much for spider silks, which are after all rather impracticable as mass produced fibres, but for silks of the mulberry worms, which are an important commercial fibre already,’ he adds.
’The spider silk itself is just a model system,’ adds Knez. ’Once we have learnt to control the deposition system, and really understand what is happening, we hope to apply the method to make better, lighter weight and tougher new materials which are of more interest for technology or medicine,’ he adds.
S-M Lee et al, Science, 2009, 324, 488 (DOI: 10.1126/science.1168162)