The main goal of the program division “Functional Surfaces” is the research and development of the “Gecko-Technology”, where the adhesive properties are optimized by fabrication of samples with a specific surface topography. The design of such surfaces often mimics structures found in biology. The core research topics are “bioinspired adhesion systems”, “characterization of microstructured surfaces”, “switchable surfaces”, “patterned surfaces for biomedical applications”, and “adhesion systems for industrial application”. The program division conducts basic research on adhesion mechanisms and, in cooperation with industrial partners, evaluates the potential of application of the “Gecko-Technology”.
Bioinspired Adhesion Systems
Adhesion systems found in nature are often inferior to modern adhesives with regard to reusability, versatility, residue-free detachment and self-cleaning properties. A key principle for controlled adhesion in biological systems is the formation of hair-like structures at the micro and nanoscale. Among many animals which adhere due to their adhesive surface structures, geckos have been in the focus of interest; they are the heaviest animals which can reversibly stick to different surfaces. Their highly complex adhesion system consists of hierarchically structured fibrils. These fibrils branch into smaller rods, ending in tiny angled contact platelets, the so-called spatulae. Despite the growing scientific interest, many details of the contact mechanics and detachment processes are still poorly understood. The program division investigates artificial, bioinspired adhesion systems to gain a deeper understanding of the basic principles linking the structure geometry and their adhesive properties. Hierarchical model structures are fabricated to understand the influence of different parameters, such as the number of contacts and the geometry of the different hierarchy levels, on their adhesion against rough surfaces and on the mechanical properties of these adhesive structures (stiffness, viscoelasticity). (Figure 1)
Characterization of Microstructured Surfaces
A precise determination of surface properties is the key to discover new phenomena or to improve existing systems. For this reason we focus intensively on new measurement methods, which are developed to fulfil the special boundary conditions for the characterization of micropatterned surfaces. Macroscopic measurement methods for the determination of adhesion for micropatterned surfaces are constantly improved. They deliver reliable data, both under controlled laboratory environment and under “real world” conditions. We also perform pioneering work of contact phenomena which are investigated using “in situ” visualization in a scanning electron microscope. Further efforts are taken to develop testing methods for industrial applications. (Figure 2).
The change of a surface property by an external “switch”, for example mechanical pressure, temperature, humidity, and electric or magnetic fields, opens numerous new technical applications. A main goal of the program division is the realization of switchable systems, which can change from an adhesive to a non-adhesive state, and vice versa. This can be successfully realized by combination of two key technologies; First, by micropatterning methods, for example photolithography, reactive ion etching, and soft molding, and second by development of new materials, for example shape-memory polymers, shape memory metals, hybrid materials, and magnetic polymers. In a recently finished PhD project we extensively investigated and characterized the switchability of bioinspired adhesives by external mechanical pressure. The underlying principle is now under investigation for industrial application. Currently we investigate how other properties than adhesion can be switched and used for different fields of application. (Figure 3)
Patterned Surfaces for Biomedical Applications
A promising application of the gecko technology is related to medical engineering. While bioinspired adhesion systems for technical applications often require adhesion to hard and rigid surfaces, adhesives for biomedical application need to stick to soft, humid, living and moving surfaces. We investigate adhesion of bioinspired adhesives both on artificial soft model systems, and “in vitro” on soft tissue samples with complex viscoelastic properties. In cooperation with the Department of Otorhinolaryngology of the University Hospital in Homburg we research the influence of structure geometry, surface chemistry and stiffness of bioinspired adhesives on the adhesion to skin and mucosa (Figure 4). Possible applications can be found in ear surgery, but also in other fields where a selective adhesion to body foreign structures is desired.
Adhesion Systems for Industrial Application
Artificial adhesion systems can only be used for industrial application, if they can be fabricated in large areas and at low cost. For this reason we investigate up-scaling methods together with the program divisions Optical Materials and Nanomers, which allow cost efficient large scale production of micropatterned surfaces. The INM – Leibniz Institute for New Materials with its large scale pilot production facilities is perfectly suited for this task; both for the fabrication of passive and “active” (switchable) surfaces. In cooperation with industrial partners, adhesion systems for special applications are currently developed. Recently we built a robot, which can pick and place small objects without application of vacuum (Figure 5). Possible applications range from chip fabrication tools up to medical applications.