Pearl shells, cobwebs, and gecko feet are revealing nature’s secrets, and the results are extremely attractive.
From gecko-inspired superglue that could hold the weight of a car on a patch no bigger than a centimetre squared, to strong-but-lightweight materials based on shells and teeth, it’s all thanks to technology that allows us to see what’s going on at the scale of molecules.
In recent decades, nanoscale observation techniques, such as X-ray scattering and high-resolution microscopes, have started to reveal the ground rules of how nature brings ingredients together.
‘The challenge now is to understand how the molecules pack harmoniously into the basic building blocks of biological materials,’ said Professor Olli Ikkala, at Aalto University in Finland.
It’s something that Prof. Ikkala has received funding to investigate from the EU’s European Research Council (ERC). His project, MIMEFUN, is learning to mimic some of nature’s materials in the lab, using technologically viable techniques and incorporating functionalities better suited to human applications.
One objective is to crack the secret of lightweight biomaterials such as pearl shells that can be sturdier than many metals.
‘Silk, nacre, bone, and teeth are all made of nanoscopic crystals embedded in a soft, oily glue,’ he said. Because this glue can deform and dissipate energy in deformation, biomaterials can be strong without being as heavy or brittle as man-made surrogates.
The problem is that no one yet knows how to make these complex nanostructures in a test tube. ‘Nature’s tricks are too complicated for us to just copy,’ said Prof. Ikkala. ‘Instead, we try to grasp their essence and do something similar with raw materials that are better suited to our goals.’
The best way to steal nature’s technology is to study animals themselves. A prime target is the wall-climbing ability of geckos.
According to Professor Eduard Arzt, at the INM Leibniz-Institute for New Materials in Saarbrücken, Germany, the ability of geckos to scale walls was only really understood in the year 2000, when scientists discovered that thin hairs on the lizards’ feet gave rise to massive van der Waals forces – which rely on the attraction between molecules.
‘If we could spin cellulose the way that nature does, our polymers would require 1 000 times less energy to produce.’
Professor Fritz Vollrath, University of Oxford, UK
‘When two materials are brought in contact, interactions between their surface electrons can cause them to stick together,’ he explained. ‘Over a perfectly smooth square-centimetre interface, these forces would be strong enough to lift an entire car.’
As part of the ERC SWITCH2STICK project, Prof. Arzt is engineering adhesive surfaces inspired from gecko feet using fibrils - fine fibres - of silicone and other polymer materials.
His patented Gecomer technology can lift objects as large as electronic tablets and release them on command. The biomaterial sticks and releases its grip when gentle pressure reorients the pattern on its surface.
Prof. Arzt tours conferences with a little robot that demonstrates the technology. Audiences from all over the world have flagged up potential applications, from factory production lines to healing patches, satellite debris collectors, and feel-responsive smart phones.
With over 50 companies in talks with the INM, van der Waals-based adhesives could become the first bio-inspired nanomaterial to ever reach the market.
Spun, not grown
Another group has turned for inspiration to spiders, who produce light-but-strong silk by spinning.
‘Silk is the only material in the natural world that is spun, not grown,’ said Professor Fritz Vollrath at the University of Oxford, UK. ‘Humans do not yet really understand how to grow complex biomaterials, but we do know how to spin them.’
This ability results from decades of experience in turning petrochemicals into plastic filaments. Prof. Vollrath said that transferring this know-how to fibres extracted from biomass such as cellulose could provide low-energy ways to make plastic-like polymers.
‘Spiders have learnt to produce silk fibres at ambient temperature and pressure, using water as a solvent and food as a raw material,’ said Prof. Vollrath. ‘If we could spin cellulose the way that nature does, our polymers would require 1 000 times less energy to produce than current methods.’
In a research project called SABIP, ERC-funded Prof. Vollrath has combined tools from genetics, chemistry and computer modelling to explain how spiders keep the silk molecules that they secrete liquid until the spinning process begins. This innovative model is now used by researchers to understand the production process of other biomaterials than silk.
‘The trick lies in a chemical configuration we termed an aquamelt,’ he said. ‘Silk proteins and H2O combine to form a complex that can easily shed the water to solidify.’
In addition to applications in the textile industry, silk’s great strength and biocompatibility are finding applications as diverse as lightweight car panels and artificial cartilage.
Interaction with industry is an integral part of Prof. Vollrath’s research and several small- and medium-sized enterprises (SMEs) have already spun out of his group.
‘Big companies are good at rolling out good ideas on a large scale,’ he said, considering the future development of biomaterials. ‘But SMEs are generally key to getting them off the ground in the first place.’
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