MUNICH, Germany - A team of scientists from the Max Planck Institute of Microstructure Physics has succeeded in making spider silk significantly more break resistant and ductile through the addition of metals.
Spiderman would definitely have an easier time of things with this spider silk. For example, if he had to stop a getaway car moving at 100 kilometers per hour, a five-millimeter-thick thread would do the job from a distance of 20 meters. The same task would require a finger-thick thread of untreated spider silk and a steel rod as thick as a forearm. The Max Planck scientists strengthen the natural material by infiltrating it with metal ions. It may also be possible to strengthen other natural and synthetic fibers in this way.
The fact that spider silk treated with metal ions does not break under enormous tension is just one of the advantages it has to offer. "It can be expanded twice as much as natural spider silk," said Mato Knez, who is heading the research at the Max Planck Institute. As the treated material withstands high levels of tension and strain, it absorbs 10 times more energy than the natural material before it breaks. Thus, it is particularly suitable for braking at full speed or free fall.
Materials with such properties could also be used in space technology and aircraft and vehicle construction, or for any application that requires light, strong and flexible materials.
"Our work promises great potential in terms of practical applications, as many other biomaterials can be made more break-resistant and ductile using our method," explains Knez. However, there is one important precondition: the natural materials must contain proteins as their main component.
The strengthening treatment for spider silk and other protein fibers only works when the metal ions can penetrate into the fibers. To achieve this, researchers adapted the atomic layer deposition (ALD) technique. This method is usually used to deposit individual layers of metal oxides on the surface of materials by exposing them to water vapor and a volatile compound comprising metal and organic appendages in rapid succession. Up to a few hundred of such gas pulses stream into the material and coat it with a more-or-less thick layer of oxide. "Because each pulse only lasts fractions of a second, the metal does not penetrate the material," explains Knez. "Therefore, we adapted the equipment so that we could extend an individual pulse to a duration of up to 40 seconds."
In order to make it clear that the new process no longer involves a coating process, as is the case in standard ALD, the researchers refer to the modified technique as Multiple Pulsed Vapor Phase Infiltration, or MPI. By doing this, they avoid any possible confusion in the future. "Actually, it was rather difficult for us to make it clear to colleagues that we are infiltrating materials using a process that previously was only used for coating," said Knez.
Under the transmission electron microscope, researchers were able to detect that metal atoms from the vapor phase could also creep into the interior of the spider silk. For these tests, a scientist from the Martin Luther University Halle-Wittenberg cut 90-nanometer-thin slices of the spider silk. However, the microscopic images did not explain why the metal atoms increase the strength of the protein fibers.
"This would indicate that the aluminum is present in a compound other than a typical aluminum oxide," said Knez. And he has a good idea which compound it is. He assumes that the metal atoms bind the protein molecules to each other. Hydrogen atoms usually form bridges between the molecules, which break far more easily than the strong compounds made using metal atoms. Thus, it becomes plausible for metal-infiltrated spider silk to withstand more weight than the natural version. The better ductility can also be explained in this way: a thread of spider silk can be extended in length because its protein fibers run together like tangled wool in areas referred to as amorphous. In other locations, they arrange themselves in an orderly line like a neat ball of wool. "In these crystalline areas, the hydrogen bridges are probably also replaced by metal ions," said Knez. Consequently, their order dissolves, the amorphous areas increase, and with them the ductility.
Despite its dramatically improved properties, metal-infiltrated spider silk is unlikely to be used to reinforce either fenders or aircraft wings in the future. "It would probably be more or less impossible to obtain large volumes of natural spider silk," said Knez. The insects are very difficult to keep and are not particularly productive when it comes to spinning their silk.
Nonetheless, Knez is convinced of the practical use of this power thrust for materials. "We are pretty certain that we will also be able to improve the properties of synthetic materials that imitate natural ones using our process."
