Dry Adhesives Research Articles

Combined dry and wet adhesion between a particle and an elastic substrate

We theoretically model the combined dry and wet adhesion between a rigid sphere and an elastic substrate, where the dry contact area is surrounded by a liquid meniscus. The influence of the liquid on the interfacial adhesion is twofold: inducing the Laplace pressure around the dry contact area and altering the adhesion energy between solid surfaces. The behavior of such combined dry and wet adhesion shows a smooth transition between the JKR and DMT models for hydrophilic solids, governed by the prescribed liquid volume or environmental humidity. The JKR-DMT transition vanishes when the solids become hydrophobic. An inverse scaling law of adhesive strength indicates that size reduction helps to enhance the adhesive strength until a theoretical limit is reached. This study also demonstrates the jumping-on and jumping-off hysteresis between the combined dry-wet adhesion and pure liquid bridge in a complete separation and approach cycle.

AnyClimb: A New Wall-Climbing Robotic Platform for Various Curvatures

Climbing robots are being developed to clean, paint, or inspect high areas that are hard to reach for human beings. To widen their application area, robotic platforms have been suggested using bio-inspired attachment methods, such as dry and wet adhesion for smooth and rough surfaces. Generally, robots can climb either walls or poles, but there has been no single robotic platform that can climb both. We developed a new robotic platform to vertically climb a surface with various curvatures in the horizontal direction. Flat dry adhesive was used to make eight footpads. Walking locomotion of the footpads was achieved by a four-bar mechanism with a single motor. To adapt to curved walls, a compliant mechanism is suggested with an asymmetric four-bar mechanism. The kinematics and compliance design parameters were determined by analysis, and experiments were performed on walls with different curvatures. We plan to apply the platform to clean the surfaces of solar panels with different curvatures after modifying the design for commercialization.

Adhesion of gecko on vertical surfaces with different roughness

Locomotion is the basis of predation, escape, reproduction for animals. Geckos, spiders and insects run and climb with exceptional speed, strength and agility for their size, representing in many respects an ideal model system for the study of terrestrial locomotion. Biologists make comparative biology study with animals to develop a deeper understanding of the foundmental biomechanical design rules common to all legged organisms. Engineers look to animals’ locomotion for design principles to improve the performance of legged robots including wall-climbing robots and other complex systems. Claws and setae are two kinds of adhesive devices in geckos’ toe pads. These two devices can adhere on surfaces depending on different attachment mechanism. Synergistic action between these two devices makes geckos freely move on vertical walls and ceilings with arbitrary roughness. The interaction of claws with a substrate is determined by the roughness of the substrate, the friction coefficient between the claw and the substrate, and the relative dimension between the claw and the substrate. The stability of the interaction depends on the mechanism of mechanical inter-locking. The adhesion of geckos’ setae is thought to be due to dry adhesion, which is generated by van der Waals force. The key parameters for the setae adhesion here is the distance between the terminal part of the setae and the substrate, which is in the range of a few nanometers in order to generate van der Waals forces. In addition that the microstructure of gecko setae affects to the adhesion performance, the morphology of substrate surface also plays an important role on the adhesion performance. Geckos with claws amputation were placed on 14 grades of vertical sandpapers to study setae’ adhesive ability. The ratio of particles’ diameter to distance on the surface of sandpapers was calculated. According to the ratios, 14 grades of sandpapers were classified into three new grades by the Q cluster analysis method which principle is that the difference between two neighbouring grades is the smallest among all grades. On the surface of sandpaper which the ratio is relatively smaller, geckos will slip down and try to make transverse adduction, even adjust its posture to keep adhering on the substrate. The maximum friction force between gecko spatula and sandpaper was calculated by the contact area. Then the relationship between the maximum friction force and adhesive force was also deduced. The results show that slipping velocity, adduction speed of geckos on all grades of sandpapers surface are different. Slipping velocity decreases with the ratio of particles’ diameter to their distance on the surface of sandpaper increasing. The result from calculation is consistent with the conclusion from the experiments. The adhesive ability of geckos is determined by the contact area between the surface of the setae on the distal tip and sandpaper, and surface roughness is not a key factor. The study of adhesive performance of geckos setae can contribute to the design of wall climbing robot based on the adhesion of gecko-inspired setae.

Fibrillar Elastomeric Micropatterns Create Tunable Adhesion Even to Rough Surfaces

Biologically inspired, fibrillar dry adhesives continue to attract much attention as they are instrumental for emerging applications and technologies. To date, the adhesion of micropatterned gecko‐inspired surfaces has predominantly been tested on stiff, smooth substrates. However, all natural and almost all artificial surfaces have roughnesses on one or more different length scales. In the present approach, micropillar‐patterned PDMS surfaces with superior adhesion to glass substrates with different roughnesses are designed and analyzed. The results reveal for the first time adhesive and nonadhesive states depending on the micropillar geometry relative to the surface roughness profile. The data obtained further demonstrate that, in the adhesive regime, fibrillar gecko‐inspired adhesive structures can be used with advantage on rough surfaces; this finding may open up new applications in the fields of robotics, biomedicine, and space exploration.

Switchable Dry Adhesion with Step-like Micropillars and Controllable Interfacial Contact.

Dry adhesives have attracted much attention because of their repeatable and reversible attachment. Many research groups have made fruitful achievements in fabricating and designing various dry adhesives. However, most of these studies focus on imitating bioinspired geometry to achieve this smart adhesion, neglecting the contact interface control through their peeling motion. Here, we present an alternative design to achieve this switchable adhesion on the basis of controlling contact areas. This unique design includes micropillars array with large overhanging caps and a "step" located at the center line of the cap. When dragging the pillars in the direction of the upper surface of the step, the lower surface is brought into contact, rapidly yielding stronger adhesion (switched-on state). Alternatively, when dragging the pillars in the direction of the lower surface of the step, the contact areas decrease sharply, leading to weak adhesion (switched-off state). Such switchable property under strong adhesion force is exactly what many practical applications need, and the ability to achieve this property by controlling the adhesion area size presented here opens a new way to dry adhesives design.

The microstructure morphology on ant footpads and its effect on ant adhesion

Ants show a bionic application prospect due to their special climbing ability. However, different ant species living in different environments exhibit different adhesion abilities. In order to reveal their mechanical mechanisms, footpads of four representative ant species are investigated experimentally. Regular microstructures on the ventral side of ant footpads are clearly observed, which possess different shapes and sizes for different ant species. The normal adhesion force for each kind of ant is further measured with a spinning technique, which is significantly affected by the microstructure on the ventral surface of ant footpads. Theoretical models are established in order to disclose the mechanical mechanism, in which both the capillary force and the van der Waals interaction are considered. It is found that the real contact area depends significantly on the microstructures on the ventral side, which further affects the total adhesion force. The coexistence mechanism of wet and dry adhesion for ant footpads is proposed. The finding in the present paper should be useful for deep understanding of the adhesion mechanism of different ant species and helpful for the design of bio-inspired intelligent adhesion surfaces.

Bioinspired design and fabrication of green-environmental dry adhesive with robust wide-tip shape

In this communication, we have introduced bioinspired polymeric dry adhesive structures made by using low-expertise yet robust over-etching fabrication process and chemical treatment method. The structure of gecko-foot in nature has been adopted into various applications for its remarkable smart adhesion performance. Although a lot of methods have been proposed to fabricate the gecko-inspired dry adhesion systems, they have faced on problem of structural failure of wide-tip on the top of the micro structures. To solve the problem, we have focused on developing proper wide-tip shape of the polymeric dry adhesive structures. Through the controlled over-etching process and C4F8 deposition method, we have made wide-tip microstructures without any structural failure. As a result, we develop the robust dry adhesive structures with highly reproducible and accurate wide-tip arrays for strong potential applications such as in green-environmental industries.

Electrophoretic Deposition of Nanoparticles into Three-Dimensional Felt As an Electrocataylst for the Solar Sulfur Ammonia Thermochemical Cycle

The penetration and coating of nanoparticles by electrophoretic deposition (EPD) into three-dimensional substrates was studied. Specifically, EPD of cobalt ferrite nanoparticles (20 nm) was investigated as a possible electrocatalyst to improve the kinetically slow anodic reaction of the oxidation of ammonium sulfite to ammonium sulfate in the hydrogen production sub-cycle of the solar sulfur-ammonia thermochemical water-splitting process. The primary goals were to uniformly penetrate and coat nanoparticles into 3D substrates by electrophoretic deposition and then to test their electrocatalytic activity by linear sweep voltammetry in 2 M ammonium sulfite. Electrophoretic deposition of cobalt ferrite nanoparticles from five bath compositions was performed on the following substrates: aluminum foil, graphite paper, 3 mm carbon felt, and 6 mm graphite felt. The bath compositions were 2 g/L of cobalt ferrite nanoparticles in 90/10 vol. % water/isopropanol with 1 mM (CHigh) or 0.05 mM (CLow) of hexadecyltrimethlyammonium bromide (CTAB), in 100% ethanol (E), in 100% acetylacetone (AA), and 100% acetylacetone with 0.2 wt. % polyethylenimine (AA-PEI). For electrophoretic deposition, a constant voltage was applied from 20 V to 128 V for a deposition time from 1 to 13 minutes. The zeta potential was measured to be 23 1 mV at pH of 6 for CHigh bath and 13 2 mV at pH 5 for E bath. Dry and wet adhesion of EPD deposits on aluminum was performed to determine adherence of nanoparticles to the substrate both after deposition and during the electrochemical process. Nanoparticles had good adherence to the substrate. To determine if nanoparticles could penetrate into 3D felt substrates by EPD, the ratio of Peclet number to Damköhler number (ratio of electrophoretic velocity to local deposition rate) was calculated to be greater than 1000, which indicated full penetration. Scanning electron microscopy (SEM) of the top and middle of the EPD deposits was used to examine the nanoparticle penetration and deposit morphology. It was confirmed by SEM images that there was full penetration and coating of nanoparticles into the 3 mm carbon felt and the 6 mm graphite felt from the 100% ethanol bath. The electrocatalytic activity of the EPD deposits on aluminum foil, graphite paper, 3 mm carbon felt, and 6 mm graphite felt substrates were analyzed by linear sweep voltammetry at 50 mV/s for the oxidation of 2 M ammonium sulfite. For the 3 mm carbon felt substrates, the highest electrochemical activity occurred on EPD deposits from the AA-PEI bath and then E bath. The highest electrochemical activity of the EPD deposits on 6 mm graphite felt occurred from the bath chemistries in decreasing order: CLow, AA-PEI, and E. The EPD deposit on 6 mm graphite felt from CLow bath obtained the highest electrochemical activity of all the deposits. For the 3 mm carbon felt, the ratio of current density of deposit compared to blank substrate ranged from 4 to 8 from E bath. The ratio of current density of EPD deposit on graphite paper and 6 mm graphite felt from E bath compared to blank substrate was about 1.6. The 3 mm carbon felt EPD deposits from AA-PEI bath ranged from 9 to 14, for the ratio of current density of the deposit compared to the blank substrate. EPD deposits from AA-PEI bath had a ratio of current density of deposit to blank substrate of 1.9 for deposits on aluminum foil, graphite paper, and 6 mm graphite felt. Overall, EPD was successful as a method to penetrate and coat cobalt ferrite nanoparticles up to 6 mm thick 3D felt. EPD deposits from AA-PEI bath improved the electrochemical activity compared to the blank substrate more than EPD deposits from E bath. But the EPD deposits from E bath were thin and uniform as well as deposited at much lower voltages compared to deposits from AA-PEI bath. EPD deposits from 3 mm carbon felt resulted in the largest improvement in electrochemical activity compared to the blank substrate from both E and AA-PEI bath. However, EPD deposits into 6 mm graphite felt had the highest electrochemical activity from all of the deposits. The EPD deposits with the highest electrochemical activity into 6 mm graphite felt in decreasing order were CLow, AA-PEI and E. The EPD conditions for these deposits were 40 V for 1 min from CLow resulting in current density of 75 mA/cm2, 127 V for 4 min from AA-PEI resulting in current density of 60 mA/cm2, and 40 V for 1 min from E resulting in current density of 38 mA/cm2.

Bioinspired, Highly Stretchable, and Conductive Dry Adhesives Based on 1D-2D Hybrid Carbon Nanocomposites for All-in-One ECG Electrodes.

Here we propose a concept of conductive dry adhesives (CDA) combining a gecko-inspired hierarchical structure and an elastomeric carbon nanocomposite. To complement the poor electrical percolation of 1D carbon nanotube (CNT) networks in an elastomeric matrix at a low filler content (∼1 wt %), a higher dimensional carbon material (i.e., carbon black, nanographite, and graphene nanopowder) is added into the mixture as an aid filler. The co-doped graphene and CNT in the composite show the lowest volume resistance (∼100 ohm·cm) at an optimized filler ratio (1:9, total filler content: 1 wt %) through a synergetic effect in electrical percolation. With an optimized conductive elastomer, gecko-inspired high-aspect-ratio (>3) microstructures over a large area (∼4 in.(2)) are successfully replicated from intaglio-patterned molds without collapse. The resultant CDA pad shows a high normal adhesion force (∼1.3 N/cm(2)) even on rough human skin and an excellent cycling property for repeatable use over 30 times without degradation of adhesion force, which cannot be achieved by commercial wet adhesives. The body-attachable CDA can be used as a metal-free, all-in-one component for measuring biosignals under daily activity conditions (i.e., underwater, movements) because of its superior conformality and water-repellent characteristic.

Morphology and adhesion mechanisms of newt <italic>Cynops orientalis</italic>’ foot pads

Amphibian species such as tree frogs, torrent frogs, and arboreal salamanders possessing adhesive organs on their feet show superior wet adhesion abilities. In recent years, many research efforts have been carried out to understand the wet adhesion mechanisms. The surface tension (namely capillary force) from a thin film of liquid that is secreted into the contact zone between the pad and the substrate by organisms is considered to be the primary mechanism involved in wet adhesion. Meanwhile, other mechanisms such as close contact, dry adhesion, boundary friction, and suction effect may have synergic effect on wet adhesion. In this study, the newts Cynops orientalis were chosen to investigate their wet adhesion mechanisms, because they are able to adhere to and move over wet environments without falling. A tilting platform with controlled wetting conditions and dismountable substrates was built to test the adhesion ability of newts Cynops orientalis under different conditions. The platform can be manually rotated from 0° to 360° and was used to carry out the adhesion experiments by measuring the falling angle of the newts. And then a variety of microscopical techniques including scanning electron microscope (SEM), transmission electron microscope (TEM), and atomic force microscope (AFM) were used to investigate the morphology of newt foot pads. The experimental results show that the newts were able to adhere to the smooth surface of perspex with a tilting angle of more than 90° under dry and wet conditions, indicating strong adhesion and resistance against shear. Moreover, the young newts showed relatively stronger adhesive property of foot pads as compared with that of the sub-adult ones. The microscopic pictures of the foot pads show that the polygonal epidermal cells of foot pads of the sub-adult newts are mainly separated by narrow clefts and lots of mucous pores were observed at the boundaries between epidermal cells, while the epidermal cells of foot pads of the young newts are tightly packed and surrounded by clearly raised boundaries (337±82 nm in width and 180±55 nm in height) with middle narrow channels, and few mucous pores were observed on the foot pad surface. The epidermal cell surface of the sub-adult and young newts is covered by a layer of dense array of columnar nanopillars with density of about 60 μ m - 2. The nanopillars surrounded by small channels are semi-spherical at the apex and approximately as tall as their width (about 160 nm), which corresponds to an aspect ratio of about 1.0. The adhesive foot pad in the newts is very soft with an effective elastic modulus of 564.8±239.1 kPa, as measured by the atomic force microscope in nanoindentation mode. Based on the adhesive property of newt foot pads, the function of the characteristic morphology of foot pads and the adaptive adhesion mechanisms are discussed. The research indicates that the newts Cynops orientalis primarily utilize the capillary force generated by the fluid-filled joint between the pad and the substrate to achieve strong adhesion and resistance against shear. The young newts may also utilize the suction effect on smooth substrate surface as a result of the raised boundaries on their pads surface. The dense nanopillar arrays on the epidermal cells surface are compliant enough to possibly combine close contact, friction and dry adhesion together to enhance the effect of wet adhesion. This study will provide a useful suggestion for the design and development of new bio-inspired underwater adhesives.

Application and artificial weathering performance of translucent coatings on resin-treated and dye-stained beech-wood

Abstract Beech wood boards ( Fagus sylvatica L.) modified with thermosetting N -methylol melamine (NMM) and phenol-formaldehyde (PF), partly with a dye added (NMM-dye, PF-dye), were coated solely with a translucent water-borne acrylic binder and a formulation containing the same binder and UV-protective agents (UV-PA: UV absorbers, HALS). The aim was to improve the weathering performance and to maintain the colour stability of the modified wood. The applicability of the coating on the modified boards was similar to that of untreated control boards, but the drying times were prolonged. Capillary water uptake of modified, uncoated and coated boards was clearly lower than that of respective controls. Dry and wet adhesion of the coating film and the proportion of wood fracture were higher on the modified substrates than on the controls, but no clear difference was observed among the modified boards. The colour stability during weathering depended in the modification and coating. Untreated and NMM modified samples became lighter, while PF-modified boards turned darker. The weathered coating on the modified boards, particularly with PF resin, showed less blistering, flaking and cracking than that on the controls. The coating on all resin-modified boards maintained higher adhesion during weathering than the controls. UV-PA stabilised the colour and adhesion on all boards compared to the single binder formulation. The study shows that wood modification with NMM and PF improves the weathering performance of wood coated with acrylic coatings. Combined modification with staining can diversify the optical appearance of wood coated with translucent finishes.

Fabrication and Characterization of Gecko-inspired Dry Adhesion, Superhydrophobicity and Wet Self-cleaning Surfaces

In this study, gecko-inspired polydimethylsiloxane (PDMS) microfiber surfaces were fabricated by combining Inductively Coupled Plasma (ICP) and micro-mold casting. The effect of roughness and surface energy of counterface on the adhesion of gecko-inspired microfiber surfaces and its superhydrophobicity and wet self-cleaning were studied. The adhesion of gecko-inspired microfiber surfaces depended on the roughness of the counterfaces due to the influences of contact area and interlocking mechanism. SEM images of interfaces between counterfaces with different roughness and gecko-inspired microfiber surfaces revealed the matched and dis-matched contact directly. The gecko-inspired microfiber surface got the larger adhesive force from the higher surface energy counterface, which is consisted with Johnson-Kendall-Roberts (JKR) theory. The smaller dimension and lower duty ratio of microfibers on PDMS resulted in the increasing of Water Contact Angle (WCA) and the decreasing of Sliding Angle (SA) compared to those of smooth PDMS. Particularly, sample P-8-28-20 had the biggest WCA (155°) and SA (7°), which displayed the superhydrophobicity and the best wet self-cleaning efficiency in all samples. The present studies showed that the roughness and surface energy of counterface both affected the adhesion of gecko-inspired microfiber surfaces. The smaller dimension and lower duty ratio of microfibers on PDMS endowed it with the superhydrophobicity and the wet self-cleaning abilities.

English

Based on the foot structure of the climbing biology and multivariate coupling bionic technology, the bionic flexible convex surface was designed and a 3D model was created using the digital modeling software. Finite Element Analysis software was used for contacting analysis to the bionic flexible convex foot structure in the state of dry friction and wet adhesion, and then studied frictional contact performance. The results of Finite Element Analysis shows that the contact stress of the convex is much larger than the stress of the area around it in the dry friction state and the deformation is mainly concentrated in the convex’s top. The friction between the hemispherical convex surface and the contact surface is the maximum and the cylindrical convex surface is the minimum. The friction between the bionic flexible convex structure and the solid contact surface in wet adhesion state is larger than dry state. Key words: Bionic, flexible, contact, finite element, wet adhesion.

Effect on temporary protection and adhesion promoter of silane nanofilms applied on electro-galvanized steel

In this paper, the effectiveness of γ-mercaptopropyltrimethoxysilane (MTMO) films as temporary corrosion protection and adhesion promoter on electro-galvanized steel was studied. The films were synthesized from hydrolyzed MTMO with ethanol or methanol, applied by immersion on electro-galvanized steel and cured under different conditions. The porosity of the coating was evaluated by cyclic voltammetry, the corrosion behavior by polarization curves and the protection degree by exposure in the humidity and prohesion chambers. The films were characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). From the obtained experimental results, it was concluded that this protection is markedly affected by the coating curing conditions and the alcohol used in the hydrolysis. MTMO is a good temporary protector against electro-galvanized steel corrosion in high humidity environment but it is less effective in environments containing aggressive ions such as Cl− and SO4−2.In a second stage, pretreated samples were coated with an alkyd anticorrosive paint and exposed to the salt spray chamber. Dry adhesion tests as well as SEM and EDS studies were also done in order to know the nature of the disbonding fails (adhesion or cohesion).

Correlation between Physical Properties and Shear Adhesion Strength of Enzymatically Modified Soy Protein‐Based Adhesives

AbstractThis work was to correlate physical properties with adhesion properties of soy protein‐based adhesives. By building such a correlation, the adhesion properties can be predicted by measuring physical properties of soy protein‐based adhesives. In this context, three important physical properties, viscosity, tacky force, and water resistance, were selected to correlate with adhesion strength of enzymatically modified soy protein‐based adhesives (ESP). Response surface methodology, specifically central composite design, was used with three independent variables to prepare ESP: trypsin concentration (X1), incubation time (X2), and glutaraldehyde (GA) concentration (X3). The three physical properties measured were all greatly affected by our three independent variables with significance at the 95 % confidence level. The responses were then correlated with the adhesion properties of ESP. In conclusion, viscosity can be used to predict the dry adhesion strength of ESP based on the coefficient of determination (R2) of 0.8558. In addition, tacky force and water resistance can be used to represent wet adhesion strength of ESP based on R2 of 0.7082 and 0.6930, respectively (P < 0.05). This work preliminarily identified the significant physical properties that can predict the adhesion strength of the ESP system crosslinked with GA, but the results need to be further confirmed by another protein modification system to give a generic conclusion.

Orthogonal Control of Stability and Tunable Dry Adhesion by Tailoring the Shape of Tapered Nanopillar Arrays.

Tapered nanopillar structures of different cross-sectional geometries including cone-, pencil-like, and stepwise are prepared from anodized aluminum oxide templates. The shape effect on the adhesion strength is investigated in experiments and simulation. Cone-shaped nanopillars are highly bendable under load and can recover after unloading, thus, warranting high adhesion strength, 34 N cm(-2) . The pencil-like and stepwise nano-pillars are, however, easily fractured and are not recoverable under the same conditions.

롤투롤 임프린트 공정 중 균일한 레진 코팅을 위한 닥터블레이드형 디스펜서 설계 및 제작

This study reports on a uniform resin coating method by using a doctor blade type dispenser. For high productivity, continuous imprint-lithography has been studied, and developed fabrication systems are used in several applications such as anti-reflection films, dry adhesives, and water collecting surfaces. In the continuous fabrication field, researchers have typically focused on patterning and demolding procedures. During the roll-to-roll fabrication process, however, the uniform resin coating process is also important in order to obtain a high quality product, which can be evaluated by uniform thickness, precise geometric expressions, and a thin residual layer. To achieve these, a doctor blade type dispenser was designed and fabricated. As a result, thickness of coated resin was well controlled by modulating the flow rate of the resin and blading gap. In addition, a very thin layer coating process () was achieved by softly contacting the blade on the substrate.

From hydration repulsion to dry adhesion between asymmetric hydrophilic and hydrophobic surfaces

Using all-atom molecular dynamics (MD) simulations at constant water chemical potential in combination with basic theoretical arguments, we study hydration-induced interactions between two overall charge-neutral yet polar planar surfaces with different wetting properties. Whether the water film between the two surfaces becomes unstable below a threshold separation and cavitation gives rise to long-range attraction, depends on the sum of the two individual surface contact angles. Consequently, cavitation-induced attraction also occurs for a mildly hydrophilic surface interacting with a very hydrophobic surface. If both surfaces are very hydrophilic, hydration repulsion dominates at small separations and direct attractive force contribution can-if strong enough-give rise to wet adhesion in this case. In between the regimes of cavitation-induced attraction and hydration repulsion we find a narrow range of contact angle combinations where the surfaces adhere at contact in the absence of cavitation. This dry adhesion regime is driven by direct surface-surface interactions. We derive simple laws for the cavitation transition as well as for the transition between hydration repulsion and dry adhesion, which favorably compare with simulation results in a generic adhesion state diagram as a function of the two surface contact angles.

Geckos as Springs: Mechanics Explain Across-Species Scaling of Adhesion.

One of the central controversies regarding the evolution of adhesion concerns how adhesive force scales as animals change in size, either among or within species. A widely held view is that as animals become larger, the primary mechanism that enables them to climb is increasing pad area. However, prior studies show that much of the variation in maximum adhesive force remains unexplained, even when area is accounted for. We tested the hypothesis that maximum adhesive force among pad-bearing gecko species is not solely dictated by toepad area, but also depends on the ratio of toepad area to gecko adhesive system compliance in the loading direction, where compliance (C) is the change in extension (Δ) relative to a change in force (F) while loading a gecko’s adhesive system (C = dΔ/dF). Geckos are well-known for their ability to climb on a range of vertical and overhanging surfaces, and range in mass from several grams to over 300 grams, yet little is understood of the factors that enable adhesion to scale with body size. We examined the maximum adhesive force of six gecko species that vary in body size (~2–100 g). We also examined changes between juveniles and adults within a single species (Phelsuma grandis). We found that maximum adhesive force and toepad area increased with increasing gecko size, and that as gecko species become larger, their adhesive systems become significantly less compliant. Additionally, our hypothesis was supported, as the best predictor of maximum adhesive force was not toepad area or compliance alone, but the ratio of toepad area to compliance. We verified this result using a synthetic “model gecko” system comprised of synthetic adhesive pads attached to a glass substrate and a synthetic tendon (mechanical spring) of finite stiffness. Our data indicate that increases in toepad area as geckos become larger cannot fully account for increased adhesive abilities, and decreased compliance must be included to explain the scaling of adhesion in animals with dry adhesion systems.

Recent innovations in adhesive technology

Purpose – This paper aims to provide an insight into recent innovations in adhesive technology by considering a selection of commercial developments and academic research activities. Design/methodology/approach – Following an introduction, this paper first discusses a selection of commercially developed adhesives used in the healthcare, photovoltaics and aerospace industries. It then considers biomimetic adhesive research, specifically dry adhesives which mimic the principles of gecko adhesion and wet adhesives based on the chemistry which underpins mussel adhesion. Finally, brief concluding comments are drawn. Findings – This shows that new adhesives continue to be developed to meet a growing range of industrial requirements, and a major research effort into biologically inspired adhesion mechanisms is poised to yield new families of high-performance adhesives. Originality/value – This provides details of recent commercial and academic developments in adhesive technology.