A bioinspired snap-through metastructure for manipulating micro-objects.

Micro-objects stick tenaciously to each other-a well-known show-stopper in microtechnology and in handling micro-objects. Inspired by the trigger plant, we explore a mechanical metastructure for overcoming adhesion involving a snap-action mechanism. We analyze the nonlinear mechanical response of curved beam architectures clamped by a tunable spring, incorporating mono- and bistable states. As a result, reversible miniaturized snap-through devices are successfully realized by micron-scale direct printing, and successful pick-and-place handling of a micro-object is demonstrated. The technique is applicable to universal scenarios, including dry and wet environment, or smooth and rough counter surfaces. With an unprecedented switching ratio (between high and low adhesion) exceeding 104, this concept proposes an efficient paradigm for handling and placing superlight objects.

Film-Terminated Fibrillar Microstructures with Improved Adhesion on Skin-like Surfaces.

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.

Attachment of bioinspired microfibrils in fluids: transition from a hydrodynamic to hydrostatic mechanism.

Reversible and switchable adhesion of elastomeric microstructures has attracted significant interest in the development of grippers for object manipulation. Their applications, however, have often been limited to dry conditions and adhesion of such deformable microfibrils in the fluid environment is less understood. In the present study, we performed adhesion tests in silicone oil using single cylindrical microfibrils of a flat-punch shape with a radius of 80 µm. Stiff fibrils were created using three-dimensional printing of an elastomeric resin with an elastic modulus of 500 MPa, and soft fibrils, with a modulus of 3.3 MPa, were moulded in polyurethane. Our results suggest that adhesion is dominated by hydrodynamic forces, which can be maximized by stiff materials and high retraction velocities, in line with theoretical predictions. The maximum pull-off stress of stiff cylindrical fibrils is 0.6 MPa, limited by cavitation and viscous fingering, occurring at retraction velocities greater than 2 µm s-1. Next, we add a mushroom cap to the microfibrils, which, in the case of the softer material, deforms upon retraction and leads to a transition to a hydrostatic suction regime with higher pull-off stresses ranging from 0.7 to 0.9 MPa. The effects of elastic modulus, fibril size and viscosity for underwater applications are illustrated in a mechanism map to provide guidance for design optimization.

Water as a "glue": Elasticity-enhanced wet attachment of biomimetic microcup structures.

Octopus, clingfish, and larva use soft cups to attach to surfaces under water. Recently, various bioinspired cups have been engineered. However, the mechanisms of their attachment and detachment remain elusive. Using a novel microcup, fabricated by two-photon lithography, coupled with in situ pressure sensor and observation cameras, we reveal the detailed nature of its attachment/detachment under water. It involves elasticity-enhanced hydrodynamics generating "self-sealing" and high suction at the cup-substrate interface, converting water into "glue." Detachment is mediated by seal breaking. Three distinct mechanisms of breaking are identified, including elastic buckling of the cup rim. A mathematical model describes the interplay between the attachment/detachment process, geometry, elasto-hydrodynamics, and cup retraction speed. If the speed is too slow, then the octopus cannot attach; if the tide is too gentle for the larva, then water cannot serve as a glue. The concept of "water glue" can innovate underwater transport and manufacturing strategies.

Preventing Catastrophic Failure of Microfibrillar Adhesives in Compliant Systems Based on Statistical Analysis of Adhesive Strength.

Adhesives based on fibrillar surface microstructures have shown great potential for handling applications requiring strong, reversible, and switchable adhesion. Recently, the importance of the statistical distribution of adhesive strength of individual fibrils in controlling the overall performance was revealed. Strength variations physically correspond to different interfacial defect sizes, which, among other factors, are related to surface roughness. For analysis of the strength distribution, Weibull's statistical theory of fracture was introduced. In this study, the importance of the statistical properties in controlling the stability of attachment is explored. Considering the compliance of the loading system, we develop a stability criterion based on the Weibull statistical parameters. It is shown that when the distribution in fibril adhesive strength is narrow, the global strength is higher but unstable detachment is more likely. Experimental variation of the loading system compliance for a specimen of differing statistical properties shows a transition to unstable detachment at low system stiffness, in good agreement with the theoretical stability map. This map serves to inform the design of gripper compliance, when coupled with statistical analysis of strength on the target surface of interest. Such a treatment could prevent catastrophic failure by spontaneous detachment of an object from an adhesive gripper.

Elastohydrodynamic Dewetting of Thin Liquid Films: Elucidating Underwater Adhesion of Topographically Patterned Surfaces.

In underwater adhesion of a topographically patterned surface with a very soft material such as human skin, the elastic deformation can be large enough to achieve solid-on-solid contact not only on top of the hills but also in the valleys of the substrate topography. In this context, we have studied the dynamics of dewetting of a thin liquid film confined between a rigid, periodic micropillar array and a soft, elastic sphere. In our experiments, we observed two very distinct dewetting morphologies. For large ratios of array period to micropillar height and width, the dewetted areas tend to have a diamond-like shape and expand with a rate similar to a flat, unpatterned substrate. When the array period is reduced, the morphology of the dry spot becomes irregular and its expansion rate is significantly reduced. We developed a fully coupled numerical model of the dewetting process that reproduces the key features observed in experiments. Moreover, we performed contact mechanics simulations to characterize the deformation of the elastomer and the shape of the dewetted area in a unit cell of the micropillar array.

Enhancing Dry Adhesion of Polymeric Micropatterns by Electric Fields.

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.

Nanopillar Diffraction Gratings by Two-Photon Lithography.

Two-dimensional photonic structures such as nanostructured pillar gratings are useful for various applications including wave coupling, diffractive optics, and security features. Two-photon lithography facilitates the generation of such nanostructured surfaces with high precision and reproducibility. In this work, we report on nanopillar diffraction gratings fabricated by two-photon lithography with various laser powers close to the polymerization threshold of the photoresist. As a result, defect-free arrays of pillars with diameters down to 184 nm were fabricated. The structure sizes were analyzed by scanning electron microscopy and compared to theoretical predictions obtained from Monte Carlo simulations. The optical reflectivities of the nanopillar gratings were analyzed by optical microscopy and verified by rigorous coupled-wave simulations.

Statistical properties of defect-dependent detachment strength in bioinspired dry adhesives.

Dry adhesives using surface microstructures inspired by climbing animals have been recognized for their potentially novel capabilities, with relevance to a range of applications including pick-and-place handling. Past work has suggested that performance may be strongly dependent on variability in the critical defect size among fibrillar sub-contacts. However, it has not been directly verified that the resulting adhesive strength distribution is well described by the statistical theory of fracture used. Using in situ contact visualization, we characterize adhesive strength on a fibril-by-fibril basis for a synthetic fibrillar adhesive. Two distinct detachment mechanisms are observed. The fundamental, design-dependent mechanism involves defect propagation from within the contact. The secondary mechanism involves defect propagation from fabrication imperfections at the perimeter. The existence of two defect populations complicates characterization of the statistical properties. This is addressed by using the mean order ranking method to isolate the fundamental mechanism. The statistical properties obtained are subsequently used within a bimodal framework, allowing description of the secondary mechanism. Implications for performance are discussed, including the improvement of strength associated with elimination of fabrication imperfections. This statistical analysis of defect-dependent detachment represents a more complete approach to the characterization of fibrillar adhesives, offering new insight for design and fabrication.

Strong Wet and Dry Adhesion by Cupped Microstructures.

Recent advances in bio-inspired microfibrillar adhesives have resulted in technologies that allow reliable attachment to a variety of surfaces. Because capillary and van der Waals forces are considerably weakened underwater, fibrillar adhesives are however far less effective in wet environments. Although various strategies have been proposed to achieve strong reversible underwater adhesion, strong adhesives that work both in air and underwater without additional surface treatments have yet to be developed. In this study, we report a novel design-cupped microstructures (CM)-that generates strong controllable adhesion in air and underwater. We measured the adhesive performance of cupped polyurethane microstructures with three different cup angles (15, 30, and 45°) and the same cup diameter of 100 µm in dry and wet conditions in comparison to standard mushroom-shaped microstructures (MSMs) of the same dimensions. In air, 15°CM performed comparably to the flat MSM of the same size with an adhesion strength (force per real contact area) of up to 1.3 MPa, but underwater, 15°CM achieved 20 times stronger adhesion than MSM (∼1 MPa versus ∼0.05 MPa). Furthermore, the cupped microstructures exhibit self-sealing properties, whereby stronger pulls lead to longer stable attachment and much higher adhesion through the formation of a better seal.

A Self-Adhesive Elastomeric Wound Scaffold for Sensitive Adhesion to Tissue.

Pressure sensitive adhesives based on silicone materials are used particularly for skin adhesion, e.g., the fixation of electrocardiogram (ECG) electrodes or wound dressings. However, adhesion to sensitive tissue structures is not sufficiently addressed due to the risk of damage or rupture. We propose an approach in which a poly-(dimethylsiloxane) (PDMS)-based soft skin adhesive (SSA) acts as cellular scaffold for wound healing. Due to the intrinsically low surface free energy of silicone elastomers, functionalization strategies are needed to promote the attachment and spreading of eukaryotic cells. In the present work, the effect of physical adsorption of three different proteins on the adhesive properties of the soft skin adhesive was investigated. Fibronectin adsorption slightly affects adhesion but significantly improves the cellular interaction of L929 murine fibroblasts with the polymeric surface. Composite films were successfully attached to explanted tympanic membranes. This demonstrates the potential of protein functionalized SSA to act as an adhesive scaffold in delicate biomedical applications.

Adhesion and relaxation of a soft elastomer on surfaces with skin like roughness.

For designing new skin adhesives, the complex mechanical interaction of soft elastomers with surfaces of various roughnesses needs to be better understood. We systematically studied the effects of a wide set of roughness characteristics, film thickness, hold time and material relaxation on the adhesive behaviour of the silicone elastomer SSA 7-9800 (Dow Corning). As model surfaces, we used epoxy replicas obtained from substrates with roughness ranging from very smooth to skin-like. Our results demonstrate that films of thin and intermediate thickness (60 and 160 µm) adhered best to a sub-micron rough surface, with a pull-off stress of about 50 kPa. Significant variations in pull-off stress and detachment mechanism with roughness and hold time were found. In contrast, 320 µm thick films adhered with lower pull-off stress of about 17 kPa, but were less sensitive to roughness and hold time. It is demonstrated that the adhesion performance of the silicone films to rough surfaces can be tuned by tailoring the film thickness and contact time.

Roll-to-Roll Manufacturing of Micropatterned Adhesives by Template Compression.

For the next generation of handling systems, reversible adhesion enabled by micropatterned dry adhesives exhibits high potential. The versatility of polymeric micropatterns in handling objects made from various materials has been demonstrated by several groups. However, specimens reported in most studies have been restricted to the laboratory scale. Upscaling the size and quantity of micropatterned adhesives is the next step to enable successful technology transfer. Towards this aim, we introduce a continuous roll-to-roll replication process for fabrication of high-performance, mushroom-shaped micropatterned dry adhesives. The micropatterns were made from UV-curable polyurethane acrylates. To ensure the integrity of the complex structure during the fabrication process, flexible templates were used. The compression between the template and the wet prepolymer coating was investigated to optimize replication results without structural failures, and hence, to improve adhesion. As a result, we obtained micropatterned adhesive tapes, 10 cm in width and several meters in length, with adhesion strength about 250 kPa to glass, suitable for a wide range of applications.

Elevated temperature adhesion of bioinspired polymeric micropatterns to glass.

Micropatterned polymer surfaces that operate at various temperatures are required for emerging technical applications such as handling of objects or space debris. As the mechanical properties of polymers can vary significantly with temperature, adhesion performance can exhibit large variability. In the present paper, we experimentally study temperature effects on the adhesion of micropatterned adhesives (pillar length 20µm, aspect ratios 0.4 and 2) made from three different polymers, i.e., polydimethylsiloxane (PDMS), perfluoropolyether dimethacrylate (PFPEdma), and polyurethane (PU-ht). PU specimens showed the highest pull-off stresses of about 57kPa at 60°C, i.e., more than twice the value of unpatterned control samples. The work of separation similarly showed a maximum at that temperature, which was identified as the glass transition temperature, Tg. PDMS and PFPEdma specimens were tested above their Tg. As a result, the adhesion properties decreased monotonically (about 50% for both materials) for temperature elevation from 20 to 120°C. Overall, the results obtained in our study indicate that the operating temperature related to the glass transition temperature should be considered as a significant parameter for assessing the adhesion performance of micropatterned adhesives and in the technical design of adhesion devices.

Composite Pillars with a Tunable Interface for Adhesion to Rough Substrates.

The benefits of synthetic fibrillar dry adhesives for temporary and reversible attachment to hard objects with smooth surfaces have been successfully demonstrated in previous studies. However, surface roughness induces a dramatic reduction in pull-off stresses and necessarily requires revised design concepts. Toward this aim, we introduce cylindrical two-phase single pillars, which are composed of a mechanically stiff stalk and a soft tip layer. Adhesion to smooth and rough substrates is shown to exceed that of conventional pillar structures. The adhesion characteristics can be tuned by varying the thickness of the soft tip layer, the ratio of the Young's moduli and the curvature of the interface between the two phases. For rough substrates, adhesion values similar to those obtained on smooth substrates were achieved. Our concept of composite pillars overcomes current practical limitations caused by surface roughness and opens up fields of application where roughness is omnipresent.

Bioinspired polydimethylsiloxane-based composites with high shear resistance against wet tissue.

Patterned microstructures represent a potential approach for improving current wound closure strategies. Microstructures can be fabricated by multiple techniques including replica molding of soft polymer-based materials. However, polymeric microstructures often lack the required shear resistance with tissue needed for wound closure. In this work, scalable microstructures made from composites based on polydimethylsiloxane (PDMS) were explored to enhance the shear resistance with wet tissue. To achieve suitable mechanical properties, PDMS was reinforced by incorporation of polyethylene (PE) particles into the pre-polymer and by coating PE particle reinforced substrates with parylene. The reinforced microstructures showed a 6-fold enhancement, the coated structures even a 13-fold enhancement in Young׳s modulus over pure PDMS. Shear tests of mushroom-shaped microstructures (diameter 450µm, length 1mm) against chicken muscle tissue demonstrate first correlations that will be useful for future design of wound closure or stabilization implants.

The springtail cuticle as a blueprint for omniphobic surfaces.

Omniphobic surfaces found in nature have great potential for enabling novel and emerging products and technologies to facilitate the daily life of human societies. One example is the water and even oil-repellent cuticle of springtails (Collembola). The wingless arthropods evolved a highly textured, hierarchically arranged surface pattern that affords mechanical robustness and wetting resistance even at elevated hydrostatic pressures. Springtail cuticle-derived surfaces therefore promise to overcome limitations of lotus-inspired surfaces (low durability, insufficient repellence of low surface tension liquids). In this review, we report on the liquid-repellent natural surfaces of arthropods living in aqueous or temporarily flooded habitats including water-walking insects or water spiders. In particular, we focus on springtails presenting an overview on the cuticular morphology and chemistry and their biological relevance. Based on the obtained liquid repellence of a variety of liquids with remarkable efficiency, the review provides general design criteria for robust omniphobic surfaces. In particular, the resistance against complete wetting and the mechanical stability strongly both depend on the topographical features of the nano- and micropatterned surface. The current understanding of the underlying principles and approaches to their technological implementation are summarized and discussed.

In situ experiments to reveal the role of surface feature sidewalls in the Cassie-Wenzel transition.

Waterproof and self-cleaning surfaces continue to attract much attention as they can be instrumental in various different technologies. Such surfaces are typically rough, allowing liquids to contact only the outermost tops of their asperities, with air being entrapped underneath. The formed solid-liquid-air interface is metastable and, hence, can be forced into a completely wetted solid surface. A detailed understanding of the wetting barrier and the dynamics of this transition is critically important for the practical use of the related surfaces. Toward this aim, wetting transitions were studied in situ at a set of patterned perfluoropolyether dimethacrylate (PFPEdma) polymer surfaces exhibiting surface features with different types of sidewall profiles. PFPEdma is intrinsically hydrophobic and exhibits a refractive index very similar to water. Upon immersion of the patterned surfaces into water, incident light was differently scattered at the solid-liquid-air and solid-liquid interface, which allows for distinguishing between both wetting states by dark-field microscopy. The wetting transition observed with this methodology was found to be determined by the sidewall profiles of the patterned structures. Partial recovery of the wetting was demonstrated to be induced by abrupt and continuous pressure reductions. A theoretical model based on Laplace's law was developed and applied, allowing for the analytical calculation of the transition barrier and the potential to revert the wetting upon pressure reduction.

The multi-layered protective cuticle of Collembola: a chemical analysis.

Collembola, also known as springtails, are soil-dwelling arthropods that typically respire through the cuticle. To avoid suffocating in wet conditions, Collembola have evolved a complex, hierarchically nanostructured, cuticle surface that repels water with remarkable efficiency. In order to gain a more profound understanding of the cuticle characteristics, the chemical composition and architecture of the cuticle of Tetrodontophora bielanensis was studied. A stepwise removal of the different cuticle layers enabled controlled access to each layer that could be analysed separately by chemical spectrometry methods and electron microscopy. We found a cuticle composition that consisted of three characteristic layers, namely, a chitin-rich lamellar base structure overlaid by protein-rich nanostructures, and a lipid-rich envelope. The specific functions, composition and biological characteristics of each cuticle layer are discussed with respect to adaptations of Collembola to their soil habitat. It was found that the non-wetting characteristics base on a rather typical arthropod cuticle surface chemistry which confirms the decisive role of the cuticle topography.

Biologically inspired omniphobic surfaces by reverse imprint lithography.

Springtail skin morphology is translated into robust omniphobic polymer membranes by reverse imprint lithography. The combination of overhanging cross-sections and their arrangement in a self-supporting comblike pattern are crucial for mechanically stable coatings that can be even applied to curved surfaces.