Tough stuff: Spider silk enhanced with graphene-based materials

Researchers from the Graphene Flagship have demonstrated that graphene-based materials can be used to boost the properties of spider's silk. The silk – produced naturally by the spiders, incorporating graphene and carbon nanotubes (rolled up graphene sheets) introduced in their environment – had enhanced mechanical properties of up to three times the strength and ten times the toughness of the unmodified silks. The work is published in 2D Materials and was a collaboration between the University of Trento, Italy, leading the project, and other research centres and universities in Italy and UK, including the Cambridge Graphene Centre at the University of Cambridge, UK. The research was performed within the Graphene Flagship's Polymer Composites Work Package.

Natural spider silk has excellent mechanical properties, and is of increasing interest. "Humans have used silkworm silks widely for thousands of years, but recently research has focussed on spider silk, as it has promising mechanical properties. It is among the best spun polymer fibres in terms of tensile strength, ultimate strain, and especially toughness, even when compared to synthetic fibres such as Kevlar," said Nicola Pugno (University of Trento, Italy).

Bionic Materials

Artificially modified biological materials are an expanding area of research. Natural materials can have properties that cannot be achieved with lab-produced materials, and taking inspiration from nature is an effective research tool. "We already know that there are biominerals present in the protein matrices and hard tissues of insects, which gives them high strength and hardness in their jaws, mandibles and teeth, for example. So our study looked at whether spider silk's properties could be 'enhanced' by artificially incorporating various different nanomaterials into the silk's biological protein structures," said Pugno.

To enhance the spider's silk, the researchers prepared solutions of graphene and carbon nanotubes (CNTs) which were sprayed within the enclosure the spiders were kept in. After allowing the spiders to ingest the graphene and CNT dispersions from their environment, silk was collected from the spiders and tested for graphene/CNT content and mechanical properties.

The silks showed enhanced mechanical properties compared to reference silks collected from the same spiders, with significant increases in the strength, toughness and elasticity of the biocomposite silk threads. The strongest silk threads had a fracture strength of up to 5.4 GPa, over 3 times as strong as the unmodified silks, as well as a tenfold increase of toughness modulus up to 2.1 GPa.

Tailoring Threads

In graphene composites, the lateral size of the graphene flakes determines the interaction between the flake and the composite material – which influences the strain that can be transferred to the graphene below a threshold size. So, further boosts could be possible using graphene flakes specifically tailored for interaction with the silks.

This study opens up new potentials for tailoring the properties of biological materials to enhance their properties for use in novel applications. For example, these artificially modified silks could find use in high-performance or biodegradable textiles such as parachutes or medical dressings.

"This is the highest fibre toughness reported to date, and a strength comparable to that of the strongest carbon fibres or limpet teeth," said Pugno. "These are still early days, but our results are a proof of concept that paves the way to exploiting the naturally efficient spider spinning process to produce reinforced bionic silk fibres, thus further improving one of the most promising strong materials."

Costas Galiotis, leader of the Graphene Flagship work package on Polymer Composites said that this work is a good example of the Graphene Flagship's activity in biologically-inspired composites.  "While conventional composites, which are now used in so many applications still suffer from inherent weaknesses that prevent a more widespread use, natural or biologically-inspired composites, however, even those encountered in living species, are far superior in many respects because nature itself has worked out ingenious solutions for specific needs," he said. "Indeed, graphene and related materials present an excellent opportunity to design new composites at the nanoscale and hence surpassing the deficiencies of conventional man-made materials."

Andrea Ferrari, director of the Cambridge Graphene Centre, Science and Technology Officer of the Graphene Flagship, and Chair of its management panel, added "The interaction between graphene and related materials and bio-materials is key to broaden their possible applications. This is one of many examples showing potential in this area. This work can help us to design novel composites with enhanced properties, taking inspiration from nature" 

Image courtesy F. Tomasinelli.

Further Reading

Spider silk reinforced by graphene or carbon nanotubes, E. Lepore, F. Bosia, F. Bonaccorso, M. Bruna, S. Taioli, G. Garberoglio, A. C. Ferrari, N. M. Pugno, 2D Materials 4, 031013 (2017).