Functional Microstructures
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The theoretical framework of conventional contact mechanics is based on idealized as- sumptions that have shaped the field for more than 140 years. Unfortunately, these assumptions do not lend themselves to the modelling of thin films, viscoelastic materials and frictional interfaces. Therefore, the present thesis is concerned with the system- atic generalization of these assumptions and their GFMD implementation to simulate a variety of previously inaccessible, realistic contact problems. First, finite material thickness is considered in the design of film-terminated fibril struc- tures for skin adhesion. An elastic film resting on a hard foundation is effectively more stiff than its bulk counterpart, which reduces its ability to conform to counter-faces and therefore reduces the adhesion to roughness. Second, the velocity-dependence of soft, adhesive multi-asperity contacts is studied, revealing the importance of topographical saddle points and the initial configuration, from which detachment is initiated. Further- more, we identify a scaling relation describing how short-ranged microscopic interactions slow down the macroscopic relaxation of a contact. Finally, we explore the influence of interfacial friction, showing that it increases local stress concentrations and impedes the fluid flow through the interface. The reported results provide new insight into commonly neglected phenomena, whose practical significance is reinforced by direct comparisons to experiments.
As ubiquitous defense mechanisms in Nature, stinger-like structures cover a size range over six orders of magnitude. While their composition varies, we uncovered a common geometric trait: a non-linear relationship between diameter and distance from the tip, following a power law with an exponent universally between 2 and 3. Through a combination of theoretical mechanics and experiments, we interpret this universal shape to be the result of a competition between penetration and buckling, motivated by the limitations of the mechanical properties of the stinger material. Our study not only resolves a mystery underlying the structural optimization of convergently evolved natural stingers, but also can offer inspiration for efficient needles in technology or biomedicine, made from sustainable non-metallic materials.
The dragonfish is a voracious predator of the deep sea with an arsenal of tools to hunt prey and remain concealed. In contrast to its dark pigmented skin, the dragonfish is equipped with transparent teeth. Here, we establish the structure, composition, and mechanical properties of the transparent teeth for the first time. We find the enamel-like layer to consist of nanocrystalline hydroxyapatite domains (∼20 nm grain size) embedded in an amorphous matrix, whereas in the dentin layer the nanocrystalline hydroxyapatite coats nanoscale collagen fibrils forming nanorods. This nanoscale structure is responsible for the much-reduced Rayleigh light scattering, which is further ensured by the sufficiently thin walls. Here, we suggest that the nanostructured design of the transparent dragonfish teeth enables predatory success as it makes its wide-open mouth armed with saber-like teeth effectively disappear, showing no contrast to the surrounding blackness of the fish nor the background darkness of the deep sea.
Micropatterned dry adhesives rely mainly on van der Waals interactions. In this paper, we explore the adhesion strength increase that can be achieved by superimposing an electrostatic field through interdigitated subsurface electrodes. Micropatterns were produced by replica molding in silicone. The adhesion forces were characterized systematically by means of experiments and numerical modeling. The force increased with the square of the applied voltage for electric fields up to 800 V. For larger fields, a less-than-quadratic scaling was observed, which is likely due to the small, field-dependent electrical conductivity of the materials involved. The additional adhesion force was found to be up to twice of the field-free adhesion. The results suggest an alternative method for the controlled handling of fragile or miniaturized objects.
The remarkable properties of bio-inspired microstructures make them extensively accessible for various applications, including industrial, medical, and space applications. However, their implementation especially as grippers for pick-and-place robotics can be compromised by multiple factors. The most common ones are alignment imperfections with the target object, unbalanced stress distribution, contamination, defects, and roughness at the gripping interface. In the present work, three different approaches to assess the contact phenomena between patterned structures and the target object are presented. First, in-situ observation and machine learning are combined to realize accurate real-time predictions of adhesion performance. The trained supervised learning models successfully predict the adhesion performance from the contact signature. Second, two newly developed optical systems are compared to observe the correct grasping of various target objects (rough or transparent) by looking through the microstructures. And last, model experiments are provided for a direct comparison with simulation efforts aiming at a prediction of the contact signature and an analysis of the rate and preload-dependency of the adhesion strength of a soft polymer film in contact with roughness-like surface topography. The results of this thesis open new perspectives for improving the reliability of handling systems using bioinspired microstructures.
Adhesives for interaction with human skin and tissues are needed for multiple applications, from wearable electronics to medical devices for diagnostics and therapy. Bioinspired fibrillar structures, initially developed for robotics, were upgraded for adhesion to biological surfaces to solve problems in medicine. Using a fibrillar array topped by a soft skin adhesive (SSA) layer, the film-terminated design exhibits effective adhesion to skin-like rough surfaces compared to unstructured samples. The glue-free, reliable adhesion to skin opens a large spectrum of possibilities for applications in biomedicine. Moreover, we investigated the adhesion of the microstructure to explanted mouse eardrums for application as wound dressing for eardrum perforations. The subsurface microstructure was also found to dampen any impact, protecting the sensitive membrane during application. Animal tests showed promising results to replace current surgical approaches with a less invasive and more effective treatment with microstructured adhesives.
We study how the commonly neglected coupling of normal and in-plane elastic response affects tribological properties when Hertzian or randomly rough indenters slide past an elastic body. Compressibility-induced coupling is found to substantially increase maximum tensile stresses, which cause materials to fail, and to decrease friction such that Amontons law is violated macroscopically even when it holds microscopically. Confinement-induced coupling increases friction and enlarges domains of high tension. Moreover, both types of coupling affect the gap topography and thereby leakage. Thus, coupling can be much more than a minor perturbation of a mechanical contact.
Purpose: A powerful principle in nature is the presence of surface patterns to improve specific characteristics or to enable completely new functions. Here, we present two case studies where bioinspired surface patterns based on the adhesive system of geckos may be applied for biomedical applications: residue-free adhesion to skin and gecko-inspired suture threads for knot-free wound closure. Methods: Gecko-inspired skin adhesives were fabricated by soft lithography of polydimethylsiloxane with successive inking and dipping steps. Their adhesion was measured using a home built adhesion tester designed for patterned surfaces. Preliminary lap shear tests on the back of a human hand were also performed. Commercial suture threads from different materials were patterned in the group of A. del Campo at the Max-Planck-Institute for Polymer Research (Mainz, Germany) using oxygen plasma. The treated threads were pulled through artificial skin in both directions measuring the peak force and the pull through force. Results and Conclusions: Unpatterned reference samples of the skin adhesive did not stick to human skin, while the patterned samples all showed notable adhesion up to 1.2 Newton for a sample size of approximately 3 cm². First results with the patterned suture threads indicated that the surface patterning of the thread has only a minor effect on the pull-through forces. To achieve knot-free sewing the surface geometry of the suture threads needs to be optimized and more realistic testing procedures, e.g. testing on human skin, are necessary.
Bioinspired fibrillar adhesives have been proposed for novel gripping systems with enhanced scalability and resource efficiency. Here, we propose an in-situ optical monitoring system of the contact signatures, coupled with image processing and machine learning. Visual features were extracted from the contact signature images recorded at maximum compressive preload and after lifting a glass object. The algorithm was trained to cope with several degrees of misalignment and with unbalanced weight distributions by off-center gripping. The system allowed an assessment of the picking process for objects of various mass (200, 300, and 400 g). Several classifiers showed a high accuracy of about 90 % for successful prediction of attachment, depending on the mass of the object. The results promise improved reliability of handling objects, even in difficult situations.
