The protein matrix and hard tissue of insects, worms, ants and spiders naturally contains metals such as zinc, manganese and copper. These metals are responsible for the mechanical strength of teeth, jaws and mandibles, and also for the toughness of silk. Being able to artificially incorporate metals, or perhaps even semiconducting materials, into these protein structures would be good news indeed and should help reinforce them even further.

The researchers, led by Nicola Maria Pugno, who is head of the Laboratory of Bio-inspired and Graphene Nanomechanics at the University of Trento, achieved their results by simply spraying Pholcidae spiders with water containing different types of single-walled carbon nanotubes (which are rolled up sheets of graphite) or graphene (which is a sheet of carbon just one atom thick). They then collected the silk spun by the creatures.

Getting inside the protein matrix

The team measured a fracture strength for the silk of as high as 5.4 GPa, a Young’s modulus of up to 47.8 GPa and a toughness modulus of up to 2.1 GPa. These values are the highest ever reported to date – even when compared to the toughest knotted artificial fibres known – also studied by Pugno and colleagues. The toughness modulus of the new “bionicomposites”, as they have been dubbed, is also higher than that of Kevlar, which is a synthetic polymeric fibre (routinely used in bullet-proof vests).

This is the first time that researchers have succeeded in actually introducing materials into the protein matrix of silk. Pugno’s team reckons that the spiders have probably ingested the nanosolutions and incorporated the carbon nanostructures into their bodies in this way.

Wide range of potential applications

Thanks to its outstanding mechanical, conducting and electrical properties and the fact that it is biocompatible, spider silk is a promising material for many applications in a wide range of fields. These include textiles, electronics, sensors, actuating devices and in biomedicine – for example, as suture threads, biomimetic muscles, nerve regeneration and tissue repair.

Spiders have perfected their silk over 400 million years of evolution and the fibre they spin is a semi-crystalline composite biopolymer made up of several amino acids organized into semi-amorphous helical-elastic alpha-chains and beta-pleated nanocrystals. The beta-sheets are connected to each other by hydrogen bonds, which are among the weakest types of chemical bond – far weaker, for example, than the covalent bonds found in most organic molecules. However, by stacking large numbers of these sheets, spider silk fails “gracefully”, with the hydrogen bonds breaking one by one under an external force.

Highest toughness modulus

Spider silk is already among the best spun polymer fibres when to comes to its tensile strength (actually higher than that of high grade steel) and strain, and especially toughness modulus. The new bionicomposite is even better. There is a problem, however, in that it is difficult to harness this silk. Attempts at spider-silk farming have failed – for one because spiders are cannibalistic and secondly because forcing the creatures to produce silk on demand results in silk with lower strength and toughness. This is not the case for silkworms, which have been successfully exploited by man for thousands of years.

The Italy researchers say that they will now be looking into overcoming this problem, and trying to produce the fibres in larger quantities. “We also hope to improve on the properties of this new silk-based bionicomposite and are working on extending the technique to other animals (like silkworms) and plants,” adds Pugno, who is also professor at the School of Engineering and Materials Science at Queen Mary University in London and the Bruno Kessler Foundation in Trento. “’Bionicomposites’ – a term that I have put forward – are extremely high-performance materials and as such will be ideal for extreme applications,” he tells nanotechweb.org.

The researchers detail their work in a paper published on arxiv.org/abs/1504.06751.