Geckos, spiders, and beetles have special adhesive elements on their feet, enabling them to easily run along ceilings or walls. The science of bionics tries to imitate and control such biological functions for technological applications and the creation of artificial materials.
Elastic synthetic materials such as silicone elastomers are very popular in industry. They are flexible, reusable, inexpensive, and easy to produce. They are used, for example, as seals, for insulation, or as corrosion protection; however, due to their low surface energy, they are hardly adhesive at all. This makes it difficult to paint silicone surfaces.
Researchers have succeeded in boosting the adhesive effect of a silicone material significantly by combining two methods. First, they structured the surface on the micro scale based on the example of beetle feet, and then treated it with plasma. In addition, they found that the adhesiveness of the structured material changes drastically if it is bent to varying degrees.
The researchers have improved the adhesive properties of silicone elastomers by mimicking the surface structure of certain male leaf beetles (Chrysomelidae). The team discovered that silicone elastomers adhere best if their surface is modified into mushroom-like structures and then is specifically treated with plasma — the electrically charged gas that is the fourth state of matter alongside solids, liquids, and gases. A geometrical and a chemical method were combined to imitate biology. It was also shown that the degree of curvature of the materials affects their adhesion.
In a first step, silicone elastomers of three different surfaces were compared: one unstructured, one with pillar-shaped elements, and a third with a mushroomlike structure. Using a micro-manipulator, a glass ball was stuck onto the surfaces and then removed again to test how the adhesion changes when the materials with microstructured surfaces are bent convex (inwards) and concave (outwards). In this way, the team demonstrated that silicone materials with a mushroom-like structure and curved concave have double the range of adhesive strength.
In a second step, the silicone elastomers were treated with plasmas, a method that is used to functionalize plastic materials in order to increase their surface energy and improve their adhesive properties. In comparison with other methods using liquids, plasma treatments can promise greater longevity; however, they often damage the surfaces of materials.
Silicone surfaces treated with plasma have stronger adhesiveness than untreated surfaces. The material only separates from the glass surface after 50 seconds, while the untreated material already separates after 32.8 seconds.