Looked at Your Feet

Kiel research team increases silicone adhesion modeled on beetles

silicone adhesion
, the adhesive effect of the silicone material changes, the surface of which has been structured in a mushroom-like manner. Curved inwards (concave), the adhesive effect is greatest (right). © Emre Kizilkan With varying curvature
silicone adhesion
Professor Stanislav N. Gorb (back) and Emre Kizilkan use the scanning electron microscope to create images of the surface structures, which are only a few micrometers in size. © Julia Siekmann

Geckos, spiders and beetles make it happen: thanks to special adhesive elements on their feet, they can effortlessly run on ceilings or walls. Biological functions like these try to simulate and control the science of bionics for technological applications and artificial materials. A research team of the Christian-Albrechts-Universität Kiel (CAU) has now succeeded in significantly increasing the adhesive effect of a silicone material. To do this, they combined two methods: First, they structured the surface on the microscale, modeled on beetle feet. Then they treated the silicone material with plasma. They also found that the adhesion of the structured material changes dramatically when it is bent. Among other things, their results could be interesting for the development of tiny robots and gripping devices. They now appeared in the journalsAdvanced Materials and ACS Applied Materials & Interfaces.

Resilient plastics such as silicone elastomers are very popular in the industry. They are considered flexible, reusable, cheap and easy to manufacture. They are therefore used, for example, for seals, for insulation or as corrosion protection. Due to their low surface energy, however, they hardly adhere. This makes it difficult, for example, to paint silicone surfaces.

Surfaces with a mushroom-like microstructure adhere better

Professor Stanislav N. Gorb and Emre Kizilkan of the working group Functional Morphology and Biomechanics at the CAU are studying how the adhesive properties of silicone elastomers can be improved on the model of nature. As an example, they use the mushroom-like surface structure of certain male leaf beetles ( Chrysomelidae). In two recent studies, they found that silicone elastomers adhere best when their surface is mushroom-topped and then plasma-targeted. The electrically charged gas forms the fourth state of matter in addition to solid, liquid and gaseous. In order to imitate the biology, the Kiel scientists combine a geometric and a chemical method. In addition, they showed that the adhesion property of the microstructured silicone material is influenced by its degree of curvature.

“Animals and plants offer us a wealth of experience in some incredible properties. We want to transfer the mechanisms behind it to artificial materials in order to be able to control their behavior in a targeted manner, “says zoologist Gorb. For their goal of a reversible adhesion in the micro sector, which works without adhesives, completely new applications are conceivable, for example in microelectronics.

silicone adhesion
In the scanning electron microscope, the surface becomes visible with mushroom-like adhesive elements. They were inspired by the microstructures on the feet of certain leaf beetle species. © Emre Kizilkan

In tests silicone surfaces are bent

In a first step, the research team compared silicone elastomers with three different surfaces, one unstructured, one with columnar elements, and a third with a mushroom-like structure. Using a micromanipulator, they stuck a glass ball to the material and peeled it off again. They tested how the adhesion changes when the microstructured surfaces are bent convexly (outwardly arched) and concave (arched inward). “We were able to show that silicone elastomers with a mushroom-shaped structure in the concavely curved state have a twice as wide range of adhesive strengths,” explains PhD student Emre Kizilkan, first author of the study. “With this surface geometry, we can best vary the adhesion and have the greatest control.”

Exact parameters for a gentle plasma treatment

In a second step, the scientists treated the silicone elastomers with plasma. With this method usually plastic materials are functionalized to increase their surface tension and improve their adhesive properties. Compared to other fluid-based methods, plasma treatments promise greater longevity-but they often damage the surfaces of materials.

silicone adhesion
Plasma-treated silicone surfaces (bottom) show greater adhesion than untreated ones (top). Only after 50.4 seconds, the material dissolves from the attached glass surface (I), the untreated already after 32.8 seconds (D). © Emre Kizilkan

In order to find out how plasma treatments can significantly improve the adhesion of a material without damage, scientists varied various parameters such as duration or pressure in the process. They found that plasma treatment increases the adhesion of unstructured surfaces on a glass slide by about 30%. On the mushroom-like structured surface, the adhesion improves with optimal parameters even up to 91%. “We were particularly surprised by this result, because the structured contact surface is only half as large as the unstructured, but after the plasma treatment has a three times greater adhesion increase,” explains Kizilkan.

What happens in an attempt to peel off the structured surfaces from the glass substrate is shown by the images of a high-speed camera: Due to its higher surface energy, the plasma-treated microstructure remains completely in contact with the glass substrate for 50.6 seconds. The contact surface of the untreated microstructure, however, quickly decreases by about one third – the material dissolves after just 33 seconds from the glass carrier (see picture).

Particularly suitable for applications in microelectronics

“In a very small space we have a strong adhesion that we can vary very widely,” summarizes materials scientist Kizilkan the results together. This makes the results interesting for small-scale applications such as microrobots. The findings of the Kiel working group have already resulted in an extremely strong adhesive tape that works according to the “gecko principle” and can be detached without residue.

Source : Christian-Albrechts-Universität Kiel (CAU)