The transfer of phenomena found in nature to the design of technical systems is far from new. Leonardo da Vinci’s attempts to copy the flight of winged birds and design a flying machine is probably the oldest example of nature being used as a source of inspiration. Nature has developed, optimised and tested structures over billions of years. Prof. Dr. Thomas Speck, director of the Botanic Garden at the University of Freiburg, is only too aware that nature has long been used as a source of inspiration for the design of technical systems.

“Nature often gives rise to shapes and structures that are very efficient in terms of energy and material requirements,” Speck said. “Such structures are particularly interesting in times of sustainable technology development.” Speck has observed that although the idea of using nature as a source of inspiration is quite an old one, there are two major changes: on the one hand, the methods used to analyse the biological models have improved considerably; on the other, there are many new ways to apply biological methods and systems to technological ones. “This is why I believe that we are currently in the golden era of bionics; we can analyse biological phenomena, understand them and turn them into technical products if we make a reasonable effort to do so,” the botanist said.

Insects can walk very easily on some plant surfaces, but cannot stick to others at all. This ability can vary within a single plant and even within an organ. The differences are driven by selection pressure. While plants try to keep plant eaters off, they do not mind a visit from pollinating insects. This is why insect legs stick efficiently to flower petals, but cannot find any hold at all on rubber tree (Hevea brasiliensis) leaves. Plants have different hierarchy levels as far as leaf design is concerned. Epidermal cells can be flat, curved or papillose. The epidermis is covered by the cuticle, a thin, non-mineral layer with mechanical properties similar to that of cling film. The cuticle itself can be covered with wax crystals, which gives it different surface structures. The microstructure of the cuticle is the reason why insects find some leaves difficult or impossible to walk on. Thomas Speck, Holger Bohn and Bettina Prüm have studied a variety of different plant surfaces in order to determine what influence cell shape, microstructure and surface chemistry exert on the adhesion behaviour of insects. They found that the insects found it hard to walk on surfaces with cuticular folds of a specific size. “The folds that were particularly difficult for the beetles are only 0.5 micrometres high and wide and have a spacing of between 0.5 and 1.5 micrometers,” said Speck referring to the perfect anti-adhesion surface. The anatomy of the beetles' legs is the reason why the beetles cannot adhere to such medium-sized structures.

Beetles have claws and hairs that enable them to adhere to surfaces, each in its specific way. The claws enable the insects to dig their feet into and adhere to coarsely structured surfaces. A large number of flexible hairs are located right above the claws and are highly suited to adhering to surfaces such as glass. Prof. Speck explains that the beetles’ ability to adhere to surfaces depends only indirectly on the contact surface. Rather, it depends to a greater extent on the contact area between the adhesive hairs on the beetles’ legs and the plant surface. As the hairs are too stiff to dig into the small folds and the claws too big, the contact area is reduced to less than a thousandth of its entire capacity and the beetle slides off.

"The beetles feel quite uncomfortable and do not like to walk on surfaces with medium-sized cuticular folds. Such surfaces are as slippery as ice,” said the bionics expert. If the folds were slightly further apart, the hairs would fit in between; if they were closer, the hairs would have a greater contact zone. In these cases the insects would be able to adhere to the surface and not slide off. “Only cuticular folds that have a particular width, height and spacing are perfect for preventing insects from walking on leaf surfaces.” It was previously known that the cuticular folds had an effect on the refractive index and affected the optical appearance of leaves. Little was known about their mechanical effect.

The researchers conducted adhesion experiments with Colorado potato beetles (a relatively common type of beetle and available all year round). The beetles were connected with a force sensor via a hair that was attached to the beetle with a droplet of wax. Attracted by a light source, the beetles walked across differently structured plant surfaces as well as replicas made of synthetic epoxy resins. As they moved about, the beetles pulled at the hair, which then transmitted the traction forces to the sensor. The researchers found that the beetles had an adhesive power of around 45 mN on glass. They also found that a smooth cell wall covered with a wax layer reduced the adhesive power by around 70 percent. “On rubber trees, the adhesive power was 95 percent lower than on glass. The beetles find it very difficult to walk on rubber tree surfaces,” said Speck.

Working with researchers from the Freiburg Centre for Interactive Materials and Bioinspired Technologies (FIT), Speck has plans to turn nature’s brilliant idea into a technical system. The researchers believe that anti-adhesion surfaces could be used to line cupboards and medicine cabinets in order to prevent harmful insects such as cockroaches and darkling beetles from walking in. Speck’s cooperation partners led by Prof. Dr. Christoph Neinhuis at the Technical University in Dresden have already come up with a polymer spray that confers characteristic anti-adhesive structures on surfaces. “Our ultimate goal is to develop foils,” said Speck who hopes to produce a kind of adhesive tape that could be used to line facades and window frames of granaries and other storage buildings to protect them against cockroaches and other harmful insects. “This is particularly important in the tropics. A foil is relatively cheap to produce. It can be applied to any kind of material and also has the potential to reduce the amount of pesticides that need to be applied,” said Speck.

Further information:
Prof. Dr. Thomas Speck
Plant Biomechanics Group Freiburg
Botanic Garden at the University of Freiburg
Tel.: +49 (0)761 / 203-2875
Fax.: +49 (0)761 / 203-2880
E-mail: thomas.speck(at)biologie.uni-freiburg.de