Scientific publications

The recently emerging interest in fibrillar biological attachment systems, as those found in the gecko, has led to the fabrication of micropatterned elastomer adhesion surfaces. Reported studies have demonstrated that measurements on micropatterned surfaces are affected by experimental parameters not relevant for flat samples. The present paper investigates the influence on adhesion values of the sample stiffness, the backing layer thickness, the ambient humidity, and of repetitive measurements at the same location. Measurements were performed on PDMS (Sylgard® 184) micropatterned surfaces possessing flat-ended pillars with 10 µm diameter and 10 µm height. We find that adhesion increased with decreasing sample stiffness and decreasing backing layer thickness, whereas it dropped when several tests were carried out at exactly the same location. For ambient humidities between 2 and 90%, no influence on adhesion performance was found.

The biomimetic reproduction of adhesion organs, as found in flies, beetles and geckoes, has become a topic of intense research over the past years. Successes, however, have so far been limited. This is due to the vast range of parameters involved, including fibril size, elastic modulus, contact shape, surface roughness and ambient humidity. In previous studies, design and materials selection charts to determine the optimum materials and design combination for dry adhesive systems have been established. The effect of shape on the adhesive properties of single fibers and fiber arrays has also been a research focus. In this paper both approaches are combined to provide more advanced guidelines for the design of optimal adhesive structures.

Current semiconductor technology demands the use of compliant substrates for flexible integrated circuits. However, the maximum total strain of such devices is often limited by the extensibility of the metallic components. Although cracking in thin films is extensively studied theoretically, little experimental work has been carried out thus far. Here, we present a systematic study of the cracking behavior of 34 to 506 nm thick Cu films on polyimide with 3.5 to 19 nm-thick Ta interlayers. The film systems have been investigated by a synchrotron-based tensile testing technique and in situ tensile tests in a scanning electron microscope. By relating the energy release during cracking obtained from the stress-strain curves to the crack area, the fracture toughness of the Cu films can be obtained. It increases with Cu film thickness and decreases with increasing Ta film thickness. Films thinner than 70 nm exhibit brittle fracture, indicating an increasing inherent brittleness of the Cu films.

Geckos können sich auf nahezu allen Oberflächen bewegen, egal ob glatt oder rau. Dabei entwickeln sie Haftkräfte, die ein Vielfaches ihres Körpergewichts tragen können. Ihr Haftsystem ist selbstreinigend, rückstandsfrei und nachwachsend. Grund genug, sich dieses einmal genauer anzusehen.

Geckos can adhere to nearly every kind of surface, independent of its chemical properties or roughness. The adhesive force generated is high enough to support their own weight multiple times and allows the gecko to walk upside down. Their adhesion system is self-cleaning, renewable (self healing), and can be removed from a surface without residues. It is these unique properties that have kindled steeply rising research interest over the last decade.

During the last few years several research groups have focused on the fabrication of artificial gecko inspired adhesives. For mimicking these structures, different polymers are used as structure material, such as polydimethylsiloxanes (PDMS), polyurethanes (PU), and polypropylene (PP). While these polymers can be structured easily and used for artificial adhesion systems, the effects of repeated adhesion testing have never been investigated closely. In this paper we report on the effect of repeated adhesion measurements on the commercially available poly(dimethylsiloxane) polymer kit Sylgard 184 (Dow Corning). We show that the adhesion force decreases as a function of contact cycles. The rate of change and the final value of adhesion are found to depend on the details of the PDMS synthesis and structuring.

Recently, much work has focused on the size effect in face centered cubic (fcc) structures, however few pillar studies have focused on body centered cubic (bcc) metals. This paper explores the role of bcc crystal structure on the size effect, through compression testing of [001] and [235] Molybdenum (Mo) small-scale pillars manufactured by focused ion beam (FIB). The pillar diameters ranged from 200 nm to 5 μm. Results show that the relationship between yield stress and diameter exhibits an inverse relationship (σy ∝ d−0.22 for [001] Mo and σy ∝ d−0.34 for [235] Mo) weaker than that observed for face centered cubic (fcc) metals (σy ∝ d−0.6 to −1.0). Additional tests at various loading rates revealed that small-scale Mo pillars exhibit a strain rate sensitivity similar to bulk Mo.

Microcompression tests were performed on focused-ion-beam-machined micropillars of several body-centered-cubic metals (W, Mo, Ta, and Nb) at room temperature. The relationship between yield strength and pillar diameter as well as the deformation morphologies were found to correlate with a parameter specific for bcc metals, i.e., the critical temperature Tc. This finding sheds new light on the phenomenon of small-scale plasticity in largely unexplored non-fcc metals.

Molecular architecture affects the properties of surface layers. Photosensitive silanes with branched architectures allow patterning and coupling of proteins and cells on surfaces while maintaining their biofunctional state. Attachment can be directed to the activated regions of irradiated substrates with high selectivity (see image of mouse fibroblasts). Novel photosensitive silanes with a branched molecular architecture combining three end-functionalized oligoethylene glycol (OEG) and alkyl arms are presented. These molecules are synthesized and applied to the modification of silica surfaces. The resulting layers are tested in their ability for the selective, patterned and functional immobilization of proteins and cells. The results demonstrate and accurately quantify the benefits of branched OEG structures against linear analogues for preventing non-specific interactions with the biological material. Linear structures guarantee high selectivity for the attachment of proteins, however, they fail in the case of cells. Branched structures provide good antifouling properties in both cases and allow the formation of protein patterns with higher densities of the target protein, as well as cell patterns. The results demonstrate the careful balance between surface functionality, composition and architecture that is required for maximizing the performance of any surface-based assay in biology.