Highly Permeable Skin Patch with Conductive Hierarchical Architectures Inspired by Amphibians and Octopi for Omnidirectionally Enhanced Wet Adhesion

Amphibian adhesion systems can enhance adhesion forces on wet or rough surfaces via hexagonal architectures, enabling omnidirectional peel resistance and drainage against wet and rough surfaces, often under flowing water. In addition, an octopus has versatile suction cups with convex cup structures located inside the suction chambers for strong adhesion in various dry and wet conditions. Highly air‐permeable, water‐drainable, and reusable skin patches with enhanced pulling adhesion and omnidirectional peel resistance, inspired by the microchannel network in the toe pads of tree frogs and convex cups in the suckers of octopi, are presented. By investigating various geometric parameters of microchannels on the adhesive surface, a simple model to maximize peeling strength via a time‐dependent zig‐zag profile and an arresting effect against crack propagation is first developed. Octopus‐like convex cups are employed on the top surfaces of the hexagonal structures to improve adhesion on skin in sweaty and even flowing water conditions. The amount of reduced graphene oxide nanoplatelets coated on the frog and octopus‐inspired hierarchical architectures is controlled to utilize the patches as flexible electrodes which can monitor electrocardiography signals without delamination from wet skin under motion.     

Gecko‐Inspired Dry Adhesive for Robotic Applications

Most geckos can rapidly attach and detach from almost any kind of surface. This ability is attributed to the hierarchical structure of their feet (involving toe pads, setal arrays, and spatulae), and how they are moved (articulated) to generate strong adhesion and friction forces on gripping that rapidly relax on releasing. Inspired by the gecko's bioadhesive system, various structured surfaces have been fabricated suitable for robotic applications. In this study, x–y–z asymmetric, micrometer‐sized rectangular flaps composed of polydimethylsiloxane (PDMS) were fabricated using massively parallel micro‐electromechanical systems (MEMS) techniques with the intention of creating directionally responsive, high‐to‐low frictional‐adhesion toe pads exhibiting properties similar to those found in geckos. Using a surface forces apparatus (SFA), the friction and adhesion forces of both vertical (symmetric) and angled/tilted (x–y–z asymmetric) microflaps under various loading, unloading and shearing conditIons were investigated. It was found that the anisotropic structure of tilted microflaps gives very different adhesion and tribological forces when articulated along different x–y–z directions: high friction and adhesion forces when articulated in the y–z plane along the tilt (+y) direction, which is also the direction of motion, and weak friction and adhesion forces when articulated against the tilt (–y) direction. These results demonstrate that asymmetric angled structures, as occur in geckos, are required to enable the gecko to optimize the requirements of high friction and adhesion on gripping, and low frictional‐adhesion on releasing. These properties are intimately coupled to a (also optimum) articulation mechanism. We discuss how both of these features can be simultaneously optimized in the design of robotic systems that can mimic the gecko adhesive system.     

Normally Oriented Adhesion versus Friction Forces in Bacterial Adhesion to Polymer‐Brush Functionalized Surfaces Under Fluid Flow

Bacterial adhesion is problematic in many diverse applications. Coatings of hydrophilic polymer chains in a brush configuration reduce bacterial adhesion by orders of magnitude, but not to zero. Here, the mechanism by which polymer‐brush functionalized surfaces reduce bacterial adhesion from a flowing carrier fluid by relating bacterial adhesion with normally oriented adhesion and friction forces on polymer (PEG)‐brush coatings of different softness is studied. Softer brush coatings deform more than rigid ones, which yields extensive bond‐maturation and strong, normally oriented adhesion forces, accompanied by irreversible adhesion of bacteria. On rigid brushes, normally oriented adhesion forces remain small, allowing desorption and accordingly lower numbers of adhering bacteria result. Friction forces, generated by fluid flow and normally oriented adhesion forces, are required to oppose fluid shear forces and cause immobile adhesion. Summarizing, inclusion of friction forces and substratum softness provides a more complete mechanism of bacterial adhesion from flowing carrier fluids than available hitherto.     

Torrent Frog‐Inspired Adhesives: Attachment to Flooded Surfaces

Anatomic differences on the toe pad epithelial cells of torrent and tree frogs (elongated versus regular geometry) are believed to account for superior ability of torrent frogs to attach to surfaces in the presence of running water. Here, the friction properties of artificial hexagonal arrays of polydimethylsiloxane (PDMS) pillars (elongated and regular) in the presence of water are compared. Elongated pillar patterns show significantly higher friction in a direction perpendicular to the long axis. A low bending stiffness of the pillars and a high edge density of the pattern in the sliding direction are the key design criteria for the enhanced friction. The elongated patterns also favor orientation‐dependent friction. These findings have important implications for the development of new reversible adhesives for wet conditions.     

Microtribology of Aqueous Carbon Nanotube Dispersions

The tribological behavior of carbon nanotubes (CNTs) in aqueous humic acid (HA) solutions was studied using a surface forces apparatus (SFA) and shows promising lubricant additive properties. Adding CNTs to the solution changes the friction forces between two mica surfaces from “adhesion controlled” to “load controlled” friction. The coefficient of friction with either single‐walled (SW) or multi‐walled (MW) CNT dispersions is in the range 0.30–0.55 and is independent of the load and sliding velocity. More importantly, lateral sliding promotes a redistribution or accumulation, rather than squeezing out, of nanotubes between the surfaces. This accumulation reduced the adhesion between the surfaces (which generally causes wear/damage of the surfaces), and no wear or damage was observed during continuous shearing experiments that lasted several hours even under high loads (pressures ～10 MPa). The frictional properties can be understood in terms of the Cobblestone Model where the friction force is related to the fraction of the adhesion energy dissipated during impacts of the nanoparticles. We also develop a simple generic model based on the van der Waals interactions between particles and surfaces to determine the relation between the dimensions of nanoparticles and their tribological properties when used as additives in oil‐ or water‐based lubricants.     

Supramolecular β‐Sheet Suckerin–Based Underwater Adhesives

Nature has evolved several molecular strategies to ensure adhesion in aqueous environments, where artificial adhesives typically fail. One recently‐unveiled molecular design for wet‐resistant adhesion is the cohesive cross‐β structure characteristic of amyloids, complementing the well‐established surface‐binding strategy of mussel adhesive proteins based on 3,4‐l ‐dihydroxyphenylalanine (Dopa). Structural proteins that self‐assemble into cross β‐sheet networks are the suckerins discovered in the sucker ring teeth of squids. Here, light is shed on the wet adhesion of cross‐β motifs by producing recombinant suckerin‐12, naturally lacking Dopa, and investigating its wet adhesion properties. Surprisingly, the adhesion forces measured on mica reach 70 mN m^?1, exceeding those measured for all mussel adhesive proteins to date. The pressure‐sensitive adhesion of artificial suckerins is largely governed by their cross‐β motif, as evidenced using control experiments with disrupted cross‐β domains that result in complete loss of adhesion. Dopa is also incorporated in suckerin‐12 using a residue‐specific incorporation strategy that replaces tyrosine with Dopa during expression in Escherichia coli. Although the replacement does not increase the long‐term adhesion, it contributes to the initial rapid contact and enhances the adsorption onto model oxide substrates. The findings suggest that suckerins with supramolecular cross‐β motifs are promising biopolymers for wet‐resistant adhesion.     

Bioinspired Multiscale Wet Adhesive Surfaces: Structures and Controlled Adhesion

In nature, many organisms are able to accommodate a complex living environment by developing biological wet adhesive surfaces with unique functions such as fixation and predation. Significantly, most of these outstanding functions originate from the specialized micro/nanostructures and/or chemical components of these natural organisms. To design artificial surfaces with remarkable wet adhesive properties, the underlying mechanisms of the fascinating adhesion phenomena are further explored and summarized to provide continuous inspiration. Herein, a systematic overview of biological wet adhesive surfaces and the corresponding artificial counterparts from the perspective of surface micro/nanostructures is provided. First, the research progress of the typical biological wet adhesive surfaces such as the octopus, tree frogs, and mayfly larvae is introduced. Then, the fundamental models of surface adhesion in natural organisms and the commonly used instruments for measuring adhesion force are discussed. Later, the corresponding artificial wet adhesive surfaces inspired by these representative organisms are highlighted. After that, the typical methods for fabricating these surfaces are briefly introduced. Finally, future challenges and opportunities to develop bioinspired multiscaled wet adhesive surfaces with controlled adhesion are presented.     

Microtemplated Electrowetting for Fabrication of Shape-Controllable Microdomes in Extruded Microsucker Arrays toward Octopus-Inspired Dry/Wet Adhesion

Inspired by the prominent adhesion ability of octopus suckers, many dry/wet adhesives with specific micro-structure have been fabricated for applications in smart robots, manipulators, and medical treatments. However, the reported octopus-inspired adhesive patches are either suction-assistant without tight-sealing, or suction-sealed but inefficient under both dry/wet environments. Here, a microtemplated electrowetting method is developed for the fabrication of reversible dry/wet adhesive pads consisting of extruded microsuckers with suction-enhanced microdomes and sealing-ring tips. The mechanism toward the morphology regulation of microdomes illustrates the uneven electrohydrodynamic force on the liquid–air interface that changes the liquid meniscus and achieves the precise regulation of the microdomes curvature ratio (from 0.45 to 0.74). The tip spacing can be controlled (from 0 to 50 µm) by using different templates. Theoretical and experimental insights into the mechanism of the microdomes morphology and the tip spacing on adhesion are discussed. With optimized microdomes and maximized sealing-tips, this adhesive patch generates strong and repeatable adhesion on a silicon wafer in both air (≈ 86 kPa) and underwater (≈ 61 kPa) environments. Besides, considerable adhesion to the rough surfaces are also revealed. Its adhesion ability is demonstrated with stable transportation of various objects under air/underwater environments, providing a potential application in cross-media operation.     

Forces between Surfactant‐Coated ZnS Nanoparticles in Dodecane: Effect of Water

The forces between mica surfaces confining solutions of spherical and rod‐shaped ZnS nanoparticles (diameter ca. 5 nm) coated with hexadecylamine or octadecylamine surfactant in dodecane have been measured in the absence and after the introduction of trace amounts of water. Initially, or at very low water content, the water molecules cause the nanoparticles to aggregate and adsorb on the hydrophilic mica surfaces, resulting in a long‐range exponentially decaying repulsive force between the surfaces. After longer times (> 20 h), water bridges nucleate and grow between the nanoparticles and mica surfaces, and attractive capillary forces then cause a long‐range attraction and a strong (short‐range) adhesion. It is found, as has previously been observed in nonaqueous bulk colloidal systems, that even trace amounts of water have a profound effect on the interactions and structure of nanoparticle assemblies in thin films, which in turn affect their physical properties. These effects should be considered in the design of thin‐film processing methodologies.     

Biomimetic Bidirectional Switchable Adhesive Inspired by the Gecko

The gecko adhesive system has attracted significant attention since the discovery that van der Waals interactions, which are always present between surfaces, are predominantly responsible for their adhesion. The unique anisotropic frictional–adhesive capabilities of the gecko adhesive system originate from complex hierarchical structures and just as importantly, the anisotropic articulation of the structures. Here, by cleverly engineering asymmetric polymeric microstructures, a reusable switchable gecko‐like adhesive can be fabricated yielding steady high adhesion ( ≈ 1.25 N/cm²) and friction ( ≈ 2.8 N/cm²) forces when actuated for “gripping”, yet release easily with minimal adhesion ( ≈ 0.34 N/cm²) and friction (≈ 0.38 N/cm²) forces during detachment or “releasing”, over multiple attachment/detachment cycles, with a relatively small normal preload of 0.16 N/cm² to initiate the adhesion. These adhesives can also be used to reversibly suspend weights from vertical (e.g., walls), and horizontal (e.g., ceilings) surfaces by simultaneously and judiciously activating anisotropic friction and adhesion forces. This design opens the way for new gecko‐like adhesive surfaces and articulation mechanisms that do not rely on intensive nanofabrication in order to recover the anisotropic tribological property of gecko adhesive pads, albeit with lower adhesive forces compared to geckos.     

Peptide Length and Dopa Determine Iron‐Mediated Cohesion of Mussel Foot Proteins

Mussel adhesion to mineral surfaces is widely attributed to 3,4‐dihydroxyphenylalanine (Dopa) functionalities in the mussel foot proteins (mfps). Several mfps, however, show a broad range (30%–100%) of tyrosine (Tyr) to Dopa conversion suggesting that Dopa is not the only desirable outcome for adhesion. Here, a partial recombinant construct of mussel foot protein‐1 (rmfp‐1) and short decapeptide dimers with and without Dopa are used and both their cohesive and adhesive properties on mica are assessed using a surface forces apparatus. Our results demonstrate that at low pH, both the unmodified and Dopa‐containing rmfp‐1s show similar energies for adhesion to mica and self–self‐interaction. Cohesion between two Dopa‐containing rmfp‐1 surfaces can be doubled by Fe³⁺ chelation, but remains unchanged with unmodified rmfp‐1. At the same low pH, the Dopa‐modified short decapeptide dimer did not show any change in cohesive interactions even with Fe³⁺. The results suggest that the most probable intermolecular interactions are those arising from electrostatic (i.e., cation–π) and hydrophobic interactions. It is also shown that Dopa in a peptide sequence does not by itself mediate Fe³⁺ bridging interactions between peptide films: peptide length is a crucial enabling factor.     

Reduction of a micro-object's adhesion using chemical functionalisation

The adhesion and interaction properties of functionalised surfaces (substrate or cantilever) were investigated by means of atomic force microscope (AFM)-related force measurements. The surfaces were functionalised with a polyelectrolyte - poly(allylamine hydrochloride) (PAH) - or with silanes - 3-(ethoxydimethylsilyl) propyl amine (APTES) or (3-aminopropyl) triethoxysilane (APDMES). Measurements of forces acting between a bare glass sphere (functionalised or not) and a functionalised surface indicated repulsive or attractive forces, depending on functionalisation and medium (wet or dry). Adhesion forces (pull-off) can be observed in dry medium, whereas in wet medium this phenomenon can be cancelled. Now, the pull-off forces represent an important problem in the automation of micro-object manipulations. The cancellation of this force by chemical functionalisation is thus a promising way of improving micro-assembly in the future.     

Treefrog Toe Pad‐Inspired Micropatterning for High‐Power Triboelectric Nanogenerator

Inspired by treefrog's toe pads that show superior frictional properties, herein, an industrially compatible approach is reported to make an efficient dielectric tribosurface design using customizable nonclose‐packed microbead arrays, mimicking the friction pads of treefrogs, in order to significantly enhance electrification performance and reliability of triboelectric nanogenerator (TENG). The approach involves using an engineering polymer to prepare a highly ordered large‐area concave film, and subsequently the molding of a convex patterned triboreplica in which the concave film is exploited as a reusable master mold. A nature‐inspired TENG based on the patterned polydimethylsiloxane (PDMS) paired with flat aluminum (Al) can generate a relatively high power density of 8.1 W m^?2 even if a very small force of ≈6.5 N is applied. Moreover, the convex patterned PDMS‐based TENG possesses exceptional durability and reliability over 25 000 cycles of contact–separation. Considering the significant improvements in power generation of TENG; particularly at very small force, together with cost‐effectiveness and possibility of mass production, the present methodology may pave the way for large‐scale blue energy harvesting and commercialization of TENGs for many practical applications.     

Controllable Anisotropic Dry Adhesion in Vacuum: Gecko Inspired Wedged Surface Fabricated with Ultraprecision Diamond Cutting

Gecko adhesion has inspired the fabrication of various dry adhesive surfaces, most of which are developed to be used under atmospheric conditions. However, applications of gecko‐inspired surfaces can be expanded to vacuum and even space environment due to the characteristics of van der Waals interactions, which are always present between materials regardless of the surrounding environment. In this paper, a controllable, anisotropic dry adhesion in vacuum is demonstrated with gecko‐inspired wedged dry adhesive surfaces fabricated using an ultraprecision diamond cutting mold. The adhesion and friction properties of the wedge‐structured surfaces are systematically characterized in loading–pulling mode and loading–dragging–pulling mode. The surfaces show significant anisotropic adhesion (P_ad ≈ 10.5 kPa vs P_ad ≈ 0.7 kPa) and friction (P_f ≈ 50 kPa vs P_f ≈ 30 kPa) when actuated in gripping and releasing direction, respectively. The wedge‐structured surfaces in vacuum show comparable properties as exposed in atmosphere. A three‐legged gripper is designed to pick up, hold, and release a patterned silicon wafer in vacuum. The study demonstrates a green, high‐yield, and low‐cost method to fabricate a reliable and durable mold for gecko inspired anisotropic dry adhesive surfaces and the potential application of dry adhesive surface in vacuum.     

喷雾清洗中微颗粒受到流体作用力的研究

对喷雾清洗过程中微颗粒所受到的流体力进行了研究.由于液滴撞击在平面上产生的不稳定流,无法用现有的层流作用力公式来预测颗粒所受到的作用力,本文采用了计算流体力学模拟的方法对流场分布进行了模拟,并且计算颗粒上相应受到的作用力.通过计算结果,讨论了影响颗粒清除效果的关键因素.研究表明,在微米尺度内,平面的可湿性质对液滴展开的初始阶段流场分布有显著影响,从而也大大影响了平面上颗粒所受到的作用力.此外,撞击在干燥表面时,颗粒受到的拖拽力会比撞击到湿表面上时大三个数量级以上.而在湿表面上时,提高撞击速度会比增大液滴大小来得更有效,主导颗粒去除的力为拖拽力.     

喷雾清洗中微颗粒受到流体作用力的研究

对喷雾清洗过程中微颗粒所受到的流体力进行了研究.由于液滴撞击在平面上产生的不稳定流, 无法用现有的层流作用力公式来预测颗粒所受到的作用力,本文采用了计算流体力学模拟的方法对流场分布进行了模拟,并且汁算颗粒上相应受到的作用力.通过计算结果,讨论了影响颗粒清除效果的关键因素.研究表明,在微米尺度内,平面的可湿性质对液滴展开的初始阶段流场分布有显著影响,从而也大大影响了平面上颗粒所受到的作用力.此外,撞击在下燥表面时,颗粒受到的拖拽力会比撞击到湿表面上时大三个数量级以上.而在湿表面上时,提高撞击速度会比增大液滴大小来得更有效,主导颗粒去除的力为拖拽力.     

The digital pads of rhacophorid tree-frogs

The digital pads of rhacophorid tree-frogs were studied by light and electron microscopy using a tannic acid-containing fixative. The digital pads are concave mucous epithelial structures surrounded by a soft raised epithelial border. The cells in the first epithelial layer are separated by deep intercellular fissures and the epithelial cell surface is densely covered with thousands of setaceous keratinized microvilli. These are estimated to be 0.1-0.5 microm in width and have flattened tips. A longitudinal section view of the pad's first epithelial cell layer shows a rugose pattern. Deep intercellular fissures in between the cells are formed by the enzymatic activity of invading mononuclear leukocytes in the interepithelial cell junctions. The 'rugose' surface epithelial cell layer is peeled off from the underlying second epithelial layer by the epithelial metabolism that occurs when the leukocytes invade the second interepithelial cell spaces. Thus, the second epithelial cell layer becomes the new 'rugose' epithelial cell layer. The ultrastructures of the frog digital pads are compared with those of other biological suction cups, such as those of octopuses and geckos. Further discussed are their interatomic or intermolecular mechanofunctional aspects, such as hanging upside down and moving easily over smooth surfaces with the aid of interatomic or intermolecular forces, the so-called 'van der Waals forces', without any energy expenditure.     

Microtribological and Nanomechanical Properties of Switchable Y‐Shaped Amphiphilic Polymer Brushes

We have characterized the morphology and nanomechanical properties of surface‐grafted nanoscale layers consisting of Y‐shaped binary molecules with one polystyrene (PS) arm and one poly(acrylic acid) (PAA) arm. We examined these amphiphilic brushes in fluids (in‐situ visualization), and measured their microtribological characteristics as a function of chemical composition. Atomic force microscopy (AFM)‐based nanomechanical testing has shown that nanoscale reorganization greatly influences the adhesion and elastic properties of the nanoscale brush layer. In water, a bimodal distribution of the elastic modulus, arising from the mixed chemical composition of the topmost layer, is observed. In contrast, the top layer is completely dominated by PS in toluene. As a result of this reorganization, the Y‐shaped‐brush layer exhibits a dramatic variation in the friction and wear properties after exposure to different solvents. Unexpectedly, the tribological properties are enhanced for the hydrophilic and polar, PAA‐dominated, surface, which shows a lower friction coefficient and higher wear stability, despite higher adhesion and heterogeneous surface composition. We suggest that this unusual behavior is caused by the combination of the presence of a thicker water layer on the PAA‐enriched surface that acts as a boundary lubricant and the glassy state of the PAA chains.     

Adhesion Selectivity Using Rippled Surfaces

Highly selective adhesion can be achieved between surfaces by patterning them with ripples. Materials with such surfaces are fabricated by successive molding of an elastomer, poly(dimethylsiloxane) (PDMS), against a master with a surface rippled by instability of a residually stressed surface thin film. Adhesion of interfaces between both complementary and non‐complementary rippled surfaces was measured. Complementary surfaces showed significantly enhanced interfacial adhesion with increasing ripple amplitude. In contrast, interfaces with mismatched amplitudes had nearly negligible adhesion. Rate‐dependence of adhesion in these surfaces was also studied. For complementary surfaces with low amplitudes we found a multiplicative coupling between the structure and rate enhancement of adhesion. A quantitative model developed for adhesion between complementary surfaces explains these observations.     

Theoretical and experimental study of the forces between different SNOM probes and chemically treated AFM cantilevers

A shear-force mechanism between a chemically etched scanning near-field optical microscope tip and different chemically treated atomic force microscope cantilevers has been experimentally and theoretically investigated as a function of the tip-to-sample distance for different amplitudes of the tip oscillation. The experimental results show, in agreement with the theoretical predictions, that as the tip approaches the cantilever, the electrostatic force is the most influential in the shear-force mechanism, independently of the nature of the tip or the sample. As the tip-to-sample distance decreases, other forces come into play, and the type of interaction depends on the chemical nature of tip and sample surfaces. Thus, for hydrophobic cantilevers, the decrease in the vibration amplitude is mostly due to the solid friction forces resulting from electrostatic interactions. However, if the sample surface is hydrophilic, there is a decrease in the electrostatic force, a water meniscus is formed, and the decrease in the tip amplitude is mostly due to dynamic friction related to capillarity.