Nanostructures help naturally inspired solutions

Nanotechnology is making the science of copying nature ever more effective, with the result that biomimetic design principles are increasingly being used to solve industrial problems. Here DTI International Technology Promoter Martin Kemp reports on some of the technologies being developed.



Scientists at Chalmers University of Technology in Gothenburg, Sweden, are developing nanostructured surface technologies with enormous commercial potential. One project has seen them take materials modelled on a well-known biomimetic target – shark skin – to the next level by using their expertise in engineering at the nanoscale.



Materials that mimic the structure of shark skin, which is covered with tiny tooth-like, or ‘riblet’, structures that reduce drag, have already been used on racing yachts and swimsuits. Until now, though, few researchers have thought of applying highly engineered structures with riblets to the inside of pipes to reduce frictional resistance – and the energy requirements of air and fluid systems.



Professor Bengt Kasemo and his team have manufactured simulated shark skin structures using computer modelling combined with processing techniques such as photo- and electron beam lithography. They are also using biomimetics to attempt to solve another problem within pipes – molecular and organic debris. ‘We are using a microstructure based on lotus leaf surface extract to promote self-cleaning,’ says Professor Kasemo. ‘Together, the two techniques could improve the energy efficiency and performance of ventilation systems and natural gas and hydrogen pipelines.’



Professor Kasemo has also developed a technique to understand how nanostructured surfaces can improve the performance of catalysts. His approach involves manufacturing a catalytic nanostructured surface on a planar surface using colloidal and electron beam lithographies, and analysing and optimising catalytic properties of the system through a number of techniques including the use of spectroscopy and microreactors.



The potential of this work in the automotive emission cleaning and fuel cell sectors has encouraged Saab, Volvo, GM and Ford to work with Chalmers at its Catalysis Centre. ‘The same type of nanofabricated catalysts can also be used in photocatalysis, where light-induced excitation rather than heat is used to promote reactions,’ says Professor Kasemo. ‘We have another programme exploring light as an energy source for use in solar cells for hydrogen production. I would be extremely keen to hear from UK organisations interested in this work.’



Low density materials



Nature also specialises in developing lightweight, high-performance materials with low density or cellular structure. For example, birds need bones which are extremely lightweight but stiff in order to achieve flight, and their internal structure resembles a closed-cell foam. This structure is being replicated by several laboratories in Germany which are investigating the manufacture of foamed magnesium and foamed aluminium structures within a solid skin. Such materials show extremely good energy absorption and are being used in crash elements for cars.



Taking cellular structures to the next level of optimisation, researchers at the Forschungszentrum Karlsruhe in Germany have studied the growth mechanisms in bones and trees. They have developed software which can design from the ‘bottom up’ highly optimised structures with webbed or variable pore internal geometry. The program identifies under-loaded regions and makes them ‘softer’, and if they remain under-loaded, they are removed. Such an approach mimics the mechanism which selectively promotes growth of material in the more highly stressed regions of bones and trees.



Another example using cellular materials is the structure of a plant stem or skull. This design principle is reflected in composite sandwich construction, which comprises a core layer bonded between upper and lower skin sheets.



The traditional core material has been a structure made from folded and bonded paper which mimics a honeycomb. These materials and, more recently, advanced foams, metal sheet or thermoplastic honeycombs are also used to make flat panels for floors and internal walls, for example in aircraft, boats and trains.



Manufacture of complex curved sandwich structures is more difficult, and one solution is offered by a new foam technology from Switzerland. This foam is soft and pliable for forming, but when treated with an electron beam, becomes rigid, allowing the application of woven cloth or prepreg (pre-impregnated) skins to produce a final structure. At JEC Paris in April, the company exhibited an eye-catching two-wheeled ‘Blue Cocoon’, a near-nine-metre-long sculpture by Swiss artist Marco Ganz, to show the potential of the material.



An alternative method to produce an in situ fine-pore foam core is a Swedish-developed polymer powder filled with foaming agent. On heating, the particles expand from 10µm to microspheres 40µm in diameter to produce a form-fit foam insert. This technology has found wide applications, including thermal barriers on paper coffee cups, filling a tennis ball to maintain high bounce, and impact protection in helmets.


This article has been reprinted from Global Watch, the monthly magazine of the DTI Global Watch Service. For further information about the activities of the Service, please click here.