Spider’s silk is already tougher than steel — but physicists in Germany have now found that it can be made even stronger by adding small quantities of metal. The discovery could help researchers understand why some biological structures containing metals — such as jaws and stingers — are so strong. It could also lead to new processes for making tougher natural and artificial materials.

Spider silk is a polymer material made of thin crystalline sheets of proteins bound together by amorphous layers of amino acids. Spiders make many different kinds of silk, but Mato Knez and colleagues at the Max Planck Institute of Microstructure Physics and Martin Luther University (both in Halle) studied “dragline” silk. This is the non-sticky stuff that spiders use to strengthen their webs and hang from.

The team began by harvesting silk from a live spider that they had caught in a nearby garden. Short silk fibres were then attached to a paperclip and dried in a vacuum chamber that the researchers normally use for atomic–layer deposition experiments.

The fibres were then exposed to a metallic vapour followed by water vapour, with the process repeated about 100 times. Between each cycle, the researchers tugged on some of the fibres, measuring the stress and strain until they broke. Stress and strain data were then used to derive the toughness of the fibres.

Eight times tougher

The experiment was repeated using vapours containing three different metals — diethylzinc; trimethlyaluminium; and titanium isopropoxide. The team found that the metal-treated silk could be as much as eight times as tough as the untreated material – with titanium appearing to have the greatest effect.

Knez and colleagues performed X–ray and nuclear magnetic resonance studies of the treated silk to try to understand where the absorbed metals were located and why they made the material tougher. In each case they found that the metals had migrated into the bulk of the fibres.

“We assume that the metal, after infiltrating the silk, binds to or crosslinks the proteins by covalent or metal coordinated bonding”, explains Knez. The team also found that the protein crystallites shrink in size after infiltration, while the amorphous regions get bigger, which Knez believes might also be related to the toughening. However, he says that a more in-depth study is required to understand these processes and tease out more details of the toughening process.

Infiltrating textiles

The team believes their processing technique, which they have dubbed multiple pulsed vapour–phase infiltration (MPI), could be use to toughen other biomaterials such including worm silk used in textiles. “It might even work with manmade materials”, he adds.

The researchers have just shown that collagen found in eggshells can be toughened using MPI — but not nearly as much as spider silk. They are currently doing similar studies of other biomaterials and polymers.

The research is published in Science.