Fakuma 2018

•••2••• Innovationen Boosting the adhesive effect of silicone Scientists structured the surface on the micro scale based on the example of beetle feet G eckos, spiders and beetles have shown us how to do it: thanks to special adhesive ele- ments on their feet, they can eas- ily run along ceilings or walls. The science of bionics tries to imitate and control such biological func- tions, for technological applica- tions and the creation of artificial materials. A research team from Kiel University (CAU) has now suc- ceeded in boosting the adhesive effect of a silicone material sig- nificantly. To do so they combined two methods: First, they structured the surface on the micro scale based on the example of bee- tle feet, and thereafter treated it with plasma. In addition, they found out that the adhesiveness of the structured material chang- es drastically, if it is bent to vary- ing degrees. Among other areas of application, their results could be interesting for the develop- ment of tiny robots and gripping devices. They have been pub- lished in the latest editions of the scientific journals Advanced Mate- rials and ACS Applied Materials & Interfaces. Elastic synthetic materials such as silicone elastomers are very pop- ular in industry. They are flexible, re-usable, cheap and easy to pro- duce. They are therefore used, for example, as seals, for insulation or as corrosion protection. How- ever, due to their low surface en- ergy, they are hardly adhesive at all. This makes it difficult, for ex- ample, to paint silicone surfaces. Mushroom-like microstructure Professor Stanislav N. Gorb and Emre Kizilkan from the Function- al Morphology and Biomechan- ics working group are research- ing how to improve the adhesive properties of silicone elastomers. Their example to mimic is the sur- face structure of certain male leaf beetles (Chrysomelidae), looking like mushrooms. In two recent studies, they discovered that silicone elastomers adhere best if their surface is modified into mushroom-like structures and thereafter specifically treated with plasma. The electrically-charged gas is the fourth state of matter, alongside solids, liquids and gases. Thus, the researchers combined a geomet- rical and a chemical method, to imitate biology. In addition, they showed that the degree of curva- ture of the materials affects their adhesion. “Animals and plants provide us with a wealth of ex- perience about some incredible features. We want to transfer the mechanisms behind them to arti- ficial materials, to be able to con- trol their behaviour in a targeted manner,” said the zoologist Gorb. Their goal of a reversible adhesion in the micro range without tradi- tional glue could make completely new application possibilities con- ceivable – for example in micro- electronics. In a first step, the research team compared silicone elastomers of three different surfaces: one unstructured, one with pillar- shaped elements and a third with a mushroom-like structure. Using a micro-manipulator, they stuck a glass ball onto the surfaces and then removed it again. They tested how the adhesion changes when the materials with micro- structured surfaces are bent con- vex (inwards) and concave (out- wards). “In this way, we were able to dem- onstrate that silicone materials with a mushroom-like structure and curved concave have the dou- ble range of adhesive strength,” said doctoral researcher Emre Kizilkan, first author of the study. “With this surface structure, we can vary and control the adhesion of materials the most.” In a second step, the scientists treated the silicone elastomers with plasmas. This method is normally used to functionalise plastic materials, in order to in- crease their surface energy and to improve their adhesive prop- erties. In comparison with other methods using liquids, plasma treatments can promise greater longevity – however, they often damage the surfaces of materi- als. Surprising findings To find out how plasma treat- ments can significantly improve the adhesion of a material with- out damaging it, the scientists varied different parameters, such as the duration or the pressure. They found that the adhesion of unstructured surfaces on a glass substrate increased by approxi- mately 30 per cent after plasma treatment. On the mushroom-like struc- tured surface the adhesion even increased by up to 91 per cent. “These findings particularly sur- prised us, because the structured surface is only half as large as the unstructured, but adhesion en- hancement was even three times better after the plasma treat- ment,” explained Kizilkan. Silicone surfaces treated with plasma (bottom) have stronger adhesiveness than untreated surfaces (top). Photo: Emre Kizilkan Different configurations change the adhesive effect of the silicone material, whose surface has been given a mushroom-like structure. Photo: Emre Kizilkan