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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.
Adhesives for interaction with human skin and tissues are needed for multiple applications. Micropatterned dry adhesives are potential candidates, allowing for a conformal contact and glue-free adhesion based on van der Waals interactions. In this study, we investigate the superior adhesion of film-terminated fibrillar microstructures (fibril diameter, 60 μm; aspect ratio, 3) in contact with surfaces of skin-like roughness (Rz 50 μm). Adhesion decays only moderately with increasing roughness, in contrast to unstructured samples. Sinusoidal model surfaces adhere when their wavelengths exceed about four fibril diameters. The film-terminated microstructure exhibits a saturation of the compressive force during application, implying a pressure safety regime protecting delicate counter surfaces. Applications of this novel adhesive concept are foreseen in the fields of wearable electronics and wound dressing.
The controlled clustering of nanoparticles into defined geometric arrangements has opened a new research area for the design of novel materials with advanced functionalities. This thesis describes a route that exploits liquid droplets as confined templates within which nanoparticles are assembled. Upon removal of the dispersed solvent from the emulsion droplets, the particles formed cluster-like structures. The particles did not arrange into small pieces of dense packings, but resembled clusters predicted for Lennard-Jones interaction potentials. In-situ observation of the assembly process via surface plasmon spectroscopy and small-angle x-ray scattering suggested that assembly occured rapidly, shortly before complete evaporation. The resulting Lennard-Jones geometries represent minimum-energy arrangements of particles. The distribution of the gold nanoparticles in the emulsions was studied for different surfactants. Good surfactants (e.g. Triton X-100) blocked the interface and confined particles in the droplet, whereas others (e.g. Tween 85) formed synergistic mixtures with the nanoparticles at the interfaces. Supraparticles with Lennard-Jones geometries only formed for surfactants that block the interface. The assembly of nanoparticles into minimum-energy clusters was sensitive to interactions between ligands bound to the nanoparticles surfaces, too. The ageing of gold nanoparticles with dodecanethiol ligands was studied for different storage conditions. Surprisingly, fractionation of particles appeared upon ageing. Desorption of ligand was the major process responsible for sedimentation and changes in polarity upon ageing and led to changes in the structure of the supraparticles.
The remarkable adherence of geckos is attributed to the hierarchical structure on their feet pads. Although significant progress has been made, inspired by nature, in fabrication of dry adhesive materials on smooth surfaces, materials with similar adherence against rough surfaces are yet to be found. To better understand the effect of hierarchy on adherence we fabricated macroscopic models made of polydimethylsiloxane with different levels of hierarchy that were brought into contact with glass and with variously rough aluminum substrates. It was shown that adhesion decreases with higher micro- and macroroughness of the substrate. Further no benefits were found for the introduction of hierarchy levels. Another approach to fabricate biomimetic patterns was to exploit polystyrene (PS) µm-particles self-assembled into monolayers on a silicon surface. By treating them with oxygen plasma, nonclose-packed particle arrays with defined spacing were generated. The size and shape evolution of the PS particle layers during etching was analyzed and compared with different etch models. The etch mechanism is more complex than reported in the literature. The resulting patterns were used to fabricate silicon master templates that yield the finest hierarchical level via replica molding. Adhesion measurements were carried out to assess the performance of the softmoldings based on PS particle arrays. The results may help to design new adhesive structures.
Non-covalent adhesion produced by the gecko is attributed to the structured surface of its toes. The synthetic adhesives mimicking this principle have now been around for a decade. However, the characteristic features of reversibility and self-cleaning ability of the gecko adhesive system have not yet been successfully integrated. The present work focuses on developing a switchable adhesive system responsive to an external stimulus. Elastomeric polydimethylsiloxane surfaces are structured with fibrillar arrays. Mechanical instability of the fibrils is recognized and utilized to produce a reversible switch between adhesion and non-adhesion. Normal compression caused the fibrils to buckle inducing a contact transition from their tips to the sides. When the contact transition occurred under moderate compressive loads, tip contact re-formed upon reversal of buckling and adhesion was reversible. However, when reversible buckling occurred under large compressive loads or when fibril side peeled without unbuckling, contact re-formation was impaired. Drastic change incontact area in the re-formed state resulted in a low adhesion state. The role of fibril contact shape, radius, aspect ratio, orientation and the applied compressive load in the adhesion switchability was examined. In situ visualization was employed to study the contact mechanisms. Contact shape, fibril orientation and preload were identified as the key parameters for controlling switchable adhesion.
Since the discovery of the gecko's hairy attachment pads, scientists tried to mimic these surface patterns due to the unique adhesion properties. Lately, scientists succeeded to fabricate artificial adhesives, which show similar complexity in geometry and achieved adhesive forces, exceeding the sticking forces of geckos. Due to the increasing commercial interest, a race has started to fabricate more complex surface patterns. However, due to this race some fundamental scientific aspects have fallen into oblivion, e.g. the distinction between real effects and measurement artefacts. In this work, the adhesion of patterned surfaces was investigated using different probe geometries. It was shown that the adhesion changes with the number of contacts due to material transfer between sample and probe. Adhesion measurements with flat and spherical probes on patterned surfaces were compared and the angle dependent adhesion was determined. Flat tip pillars showed a large tilt angle dependency, while pillars with spherical and mushroom shaped tips exhibited angle independent pull-off forces. Due to the angle dependencies, spherical probes tended to underestimate the adhesion of patterned surfaces compared to well-aligned flat probes. Flat probe measurements allowed a closer investigation of contact phenomena and yielded new information on adhesion and mechanical properties of patterned surfaces. These results may help in a more successful design of bioinspired adhesives.
Small-scale metal structures play a crucial role in a broad range of technological applications. However, knowledge of mechanical properties at this size scale is lacking. Size strengthening effects are generally experienced at the microscale. Compression of non-free defect body centered cubic (BCC) metal micropillars has revealed that the size effect of these metals scales with a temperature ratio that signifies how much the yield strength is governed by screw dislocation mobility. So far, no effort has been made to systematically study the effect of screw dislocation mobility and lattice resistance on the size effect in BCC-based metals. Thus, this work investigated this in BCC tungsten (W) and tantalum (Ta), as well as B2 beta-brass (β-CuZn) and nickel aluminide (NiAl). The influence of temperature on the size effect in W and Ta was studied up to 400 °C, whereas the room-temperature size effect in β-CuZn and NiAl was studied as a function crystal orientation and deformation rate. It was found that the size effect scaled with the magnitude of the lattice resistance, which is strongly related to the screw dislocation mobility. Direct evidence of the mobility of screw dislocations was observed for the first time. The results also showed that plastic anisotropy vanishes with decreasing sample size and that ductility is considerably improved, thus highlighting the importance of dislocation-nucleation controlled deformation and screw dislocation mobility at the sub-micron scale.
Crack propagation in viscoelastic materials has been understood with the use of Barenblatt cohesive models by many authors since the 1970's. In polymers and metal creep, it is customary to assume that the relaxed modulus is zero, so that we have typically a crack speed which depends on some power of the stress intensity factor. Generally, when there is a finite relaxed modulus, it has been shown that the toughness increases between a value at very low speeds at a threshold toughness G0, to a very fast fracture value at Ginf, and that the enhancement factor in infinite systems (where the classical singular fracture mechanics field dominates) simply corresponds to the ratio of instantaneous to relaxed elastic moduli. Here, we apply a cohesive model for the case of a bimaterial interface between an elastic and a viscoelastic material, assuming the crack remains at the interface, and neglect the details of bimaterial singularity. For the case of a Maxwell material at low speeds the crack propagates with a speed which depends only on viscosity, and the fourth power of the stress intensity factor, and not on the elastic moduli of either material. For the Schapery type of power law material with no relaxation modulus, there are more general results. For arbitrary viscoelastic materials with nonzero relaxed modulus, we show that the maximum toughness enhancement will be reduced with respect to that of a classical viscoelastic crack in homogeneous material.
Recent developments in mechanical metamaterials exemplify a new paradigm shift called mechanomaterials, in which mechanical forces and designed geometries are proactively deployed to program material properties at multiple scales. Here, we designed shell-based micro-/nanolattices with I-WP (Schoen’s I-graph–wrapped package) and Neovius minimal surface topologies. Following the designed topologies, polymeric microlattices were fabricated via projection microstereolithography or two-photon lithography, and pyrolytic carbon nanolattices were created through two-photon lithography and subsequent pyrolysis. The shell thickness of created lattice metamaterials varies over three orders of magnitude from a few hundred nanometers to a few hundred micrometers, covering a wider range of relative densities than most plate-based micro-/nanolattices. In situ compression tests showed that the measured modulus and strength of our shell-based micro-/nanolattices with I-WP topology are superior to those of the optimized plate-based lattices with cubic and octet plate unit cells and truss-based lattices. More strikingly, when the density is larger than 0.53 g cm−3, the strength of shell-based pyrolytic carbon nanolattices with I-WP topology was found to achieve its theoretical limit. In addition, our shell-based carbon nanolattices exhibited an ultrahigh strength of 3.52 GPa, an ultralarge fracture strain of 23%, and an ultrahigh specific strength of 4.42 GPa g−1 cm3, surpassing all previous micro-/nanolattices at comparable densities. These unprecedented properties can be attributed to the designed topologies inducing relatively uniform strain energy distributions and avoiding stress concentrations as well as the nanoscale feature size. Our study demonstrates a mechanomaterial route to design and synthesize micro-/nanoarchitected materials.
