Contact shape controls adhesion of bioinspired fibrillar surfaces

Effect of nano- and micro-roughness on adhesion of bioinspired micropatterned surfaces

In this work, the adhesion of biomimetic polydimethylsiloxane (PDMS) pillar arrays with mushroom-shaped tips was studied on nano- and micro-rough surfaces and compared to unpatterned controls. The adhesion strength on nano-rough surfaces invariably decreased with increasing roughness, but pillar arrays retained higher adhesion strengths than unpatterned controls in all cases. The results were analyzed with a model that focuses on the effect on adhesion of depressions in a rough surface. The model fits the data very well, suggesting that the pull-off strength for patterned PDMS is controlled by the deepest dimple-like feature on the rough surface. The lower pull-off strength for unpatterned PDMS may be explained by the initiation of the pull-off process at the edge of the probe, where significant stress concentrates. With micro-rough surfaces, pillar arrays showed maximum adhesion with a certain intermediate roughness, while unpatterned controls did not show any measurable adhesion. This effect can be explained by the inability of micropatterned surfaces to conform to very fine and very large surface asperities.

Note: An adhesion measurement setup for bioinspired fibrillar surfaces using flat probes

Current adhesion measurement setups designed for experiments on bioinspired fibrillar surfaces, either commercial or constructed in-house, do not allow adhesion measurements with in situ visualization, high resolution, high force range, and controlled alignment at the same time. In this paper a new adhesion tester is presented, which enables contact experiments with controlled tilt angle (accuracy of ±0.02°). This allows the use of flat probes and thus greatly simplifies the determination of experimental parameters such as pull-off strength or Young's modulus. The deflection of a double-clamped glass beam is measured by laser interferometry with an accuracy of ±60 nm, which yields a precise force measurement over three orders of magnitude force range without changing the glass beam. Contact formation and detachment events can be visualized in situ. The current adhesion tester is designed for force measurements in the range of 1 μN to 1 N and fills the gap between macroscopic tests and atomic force microscopy measurements.

Shear adhesion strength of thermoplastic gecko-inspired synthetic adhesive exceeds material limits

Natural gecko array wearless dynamic friction has recently been reported for 30,000 cycles on a smooth substrate. Following these findings, stiff polymer gecko-inspired synthetic adhesives have been proposed for high-cycle applications such as robot feet. Here we examine the behavior of high-density polyethylene (HDPE) and polypropylene (PP) microfiber arrays during repeated cycles of engagement on a glass surface, with a normal preload of less than 40 kPa. We find that fiber arrays maintained 54% of the original shear stress of 300 kPa after 10,000 cycles, despite showing a marked plastic deformation of fiber tips. This deformation could be due to shear-induced plastic creep of the fiber tips from high adhesion forces, adhesive wear, or thermal effects. We hypothesize that a fundamental material limit has been reached for these fiber arrays and that future gecko synthetic adhesive designs must take into account the high adhesive forces generated to avoid damage. Although the synthetic material and natural gecko arrays have a similar elastic modulus, the synthetic material does not show the same wear-free dynamic friction as the gecko.

Effect of fiber geometry on macroscale friction of ordered low-density polyethylene nanofiber arrays

Ordered low-density polyethylene (LDPE) nanofiber arrays are fabricated from silicon nanowire (SiNW) templates synthesized by a simple wet-chemical process based on metal-assisted electroless etching combined with colloidal lithography. The geometrical effect of nanofibrillar structures on their macroscale friction is investigated over a wide range of diameters and lengths under the same fiber density. The optimum geometry for contacting a smooth glass surface is presented with discussions on the compromise between fiber tip-contact area and fiber compliance. A friction design map is developed, which shows that the theoretical optimum design condition agrees well with the LDPE nanofiber geometries exhibiting high measured friction.

Anisotropic adhesion properties of triangular-tip-shaped micropillars

Directional dry adhesive microstructures consisting of high-density triangular-tip-shaped micropillars are described. The wide-tip structures allow for unique directional shear adhesion properties with respect to the peeling direction, along with relatively high normal adhesion.

Effect of viscoelasticity on adhesion of bioinspired micropatterned epoxy surfaces

The effect of viscoelasticity on adhesion was investigated for micropatterned epoxy surfaces and compared to nonpatterned surfaces. A two-component epoxy system was used to produce epoxy compositions with different viscoelastic properties. Pillar arrays with flat punch tip geometries were fabricated with a two-step soft lithography process. Adhesion properties were measured with a home-built adhesion tester using a spherical sapphire probe as a counter-surface. Compared to flat controls, micropatterned epoxy samples with low viscoelasticity (i.e., low damping factors) showed at least a 20-fold reduction in pull-off force per actual contact area for both low (E' = 2.3 MPa) and high (E' = 2.3 GPa) storage moduli. This antiadhesive behavior may result from poor contact formation and indicates that the adhesion performance of commonly used elastomers for dry adhesives (e.g., polydimethylsiloxane) is governed by the interfacial viscoelasticity. Adhesion significantly increased with increasing viscoelasticity. Micropatterned samples with high viscoelasticity showed a 4-fold reduction in adhesion for aspect ratio (AR) 1.1 patterns but a 2-fold enhancement in adhesion for AR 2.2 patterns. These results indicate that viscoelasticity can enhance the effect of surface patterning on adhesion and should be considered as a significant parameter in the design of artificial patterned adhesives.

Beetle adhesive hairs differ in stiffness and stickiness: in vivo adhesion measurements on individual setae

Leaf beetles are able to climb on smooth and rough surfaces using arrays of micron-sized adhesive hairs (setae) of varying morphology. We report the first in vivo adhesive force measurements of individual setae in the beetle Gastrophysa viridula, using a smooth polystyrene substrate attached to a glass capillary micro-cantilever. The beetles possess three distinct adhesive pads on each leg which differ in function and setal morphology. Visualisation of pull-offs allowed forces to be measured for each tarsal hair type. Male discoidal hairs adhered with the highest forces (919 ± 104 nN, mean ± SE), followed by spatulate (582 ± 59 nN) and pointed (127 ± 19 nN) hairs. Discoidal hairs were stiffer in the normal direction (0.693 ± 0.111 N m(-1)) than spatulate (0.364 ± 0.039 N m(-1)) or pointed (0.192 ± 0.044 N m(-1)) hairs. The greater adhesion on smooth surfaces and the higher stability of discoidal hairs help male beetles to achieve strong adhesion on the elytra of females during copulation. A comparison of pull-off forces measured for single setae and whole pads (arrays) revealed comparable levels of adhesive stress. This suggests that beetles are able to achieve equal load sharing across their adhesive pads so that detachment through peeling is prevented.

The effect of contaminants on the adhesion of the spatulae of a gecko

Many researchers have reported that the robust adhesion that enables geckos to move quickly and securely across a range of vertical and horizontal surfaces is provided by the hierarchical structure of their feet (i.e. lamellae, setae, spatulae, etc.). Maintaining this robust adhesion requires an intimate contact between the terminal tips of the spatulae and the surface. The aim of this study was to investigate the effect on the adhesive properties of the spatulae when a particle becomes trapped at the contact surface. Using the Johnson, Kendall and Roberts (JKR) theory, a model was constructed to assist in the analysis of the interactions between the spatula tip, the particle and the surface. The results showed that the keratin (the natural material of the spatula) provides a robust system for adhesion even when there is a particle in the contact area, and the effective contact area of spatulae will be 80%. When the particle is significantly harder than the surface, the adhesion properties of the contact surface influenced by the particle will be more obvious. The results also reveal that the generated adhesion is considerably higher when the spatula is in contact with a softer surface, such as wood or concrete, rather than a hard surface, such as glass or SiO2.

The effect of aspect ratio on adhesion and stiffness for soft elastic fibres

The effect of aspect ratio on the pull-off stress and stiffness of soft elastic fibres is studied using elasticity and numerical analysis. The adhesive interface between a soft fibre and a smooth rigid surface is modelled using the Dugdale-Barenblatt model. Numerical simulations show that, while pull-off stress increases with decreasing aspect ratio, fibres get stiffer. Also, for sufficiently low aspect ratio fibres, failure occurs via the growth of internal cracks and pull-off stress approaches the intrinsic adhesive strength. Experiments carried out with various aspect ratio polyurethane elastomer fibres are consistent with the numerical simulations.

Numerical study of shearing of a microfibre during friction testing of a microfibre array

Friction testing of microfibre arrays is typically conducted under displacement boundary conditions. For example, after the application of a compressive preload, the translation stage is fixed in the vertical direction while a shear displacement is applied by translating the stage laterally. A nonlinear rod model is used to compute the normal and shear forces acting on a typical microfibre during such a test. The normal load acting on a typical fibre is found to switch from compression to tension as the shear displacement increases. The critical shear force to detach a fibre is found to depend linearly on the compressive preload. The fibre is also found to become more stable as it is sheared, hence it never buckles. On the other hand, instead of fixing the vertical displacement, when a fibre is subjected to constant normal load, it becomes unstable upon shearing. We show that the buckling load (the applied normal load to make a fibre unstable) of a microfibril is reduced by the application of a shear displacement.

Enhanced wet adhesion and shear of elastomeric micro-fiber arrays with mushroom tip geometry and a photopolymerized p(DMA-co-MEA) tip coating

Using principles inspired by the study of naturally occurring sticky systems such as the micro- and nanoscale fibers on the toes of geckos and the adhesive proteins secreted by marine animals such as mussels, this study describes the development and evaluation of a novel patterned and coated elastomeric microfibrillar material for enhanced repeatable adhesion and shear in wet environments. A multistep fabrication process consisting of optical lithography, micromolding, polymer synthesis, dipping, stamping, and photopolymerization is described to produce uniform arrays of polyurethane elastomeric microfibers with mushroom-shaped tips coated with a thin layer of lightly cross-linked p(DMA-co-MEA), an intrinsically adhesive synthetic polymer. Adhesion and shear force characterization of these arrays in contact with a glass hemisphere is demonstrated, and significant pull-off force, overall work of adhesion, and shear force enhancements in submerged aqueous environments are shown when compared to both unpatterned and uncoated samples, as well as previously evaluated patterned and coated arrays with differing geometry. Such materials may have potential value as repeatable adhesives for wet environments, such as for medical devices.

Buckling of sheared and compressed microfibrils

In this paper, we study the stability of an initially straight elastic fibril clamped at one end, while the other end is subjected to a constant normal compressive force and a prescribed shear displacement. We found the buckling load of a sheared fibril to be always less than the Euler buckling load. Furthermore, if the end of the fibril loses adhesion, then the buckling load can be considerably less. Our result suggests that the static friction of microfibre arrays can decrease with increasing normal compressive load and, in some cases, friction force can actually become negative.

Waalbot II: Adhesion Recovery and Improved Performance of a Climbing Robot using Fibrillar Adhesives

This paper presents the design and optimization of a wall-climbing robot along with the incorporation of autonomous adhesion recovery and a motion planning implementation. The result is Waalbot II, an untethered 85 g robot able to climb on smooth vertical surfaces with up to a 100 g payload (117% body mass) or, when unburdened, on planar surfaces of any orientation at speeds up to 5 cm/s. Bio-inspired climbing mechanisms, such as Waalbot II’s gecko-like fibrillar adhesives, passive peeling, and force sensing, improve the overall climbing capabilities compared with initial versions, resulting in the ability to climb on non-smooth surfaces as well as on inverted smooth surfaces. Robot length scale optimization reveals and quantifies trends in the theoretical factor of safety and payload carrying capabilities. Autonomous adhesion recovery behavior provides additional climbing robustness without additional mechanical complexity to mitigate degradation and contamination. An implementation of a motion planner, designed to take into account Waalbot II’s kinematic constraints, results in the ability to navigate to a goal in complex three-dimensional environments while properly planning plane-to-plane transitions and avoiding obstacles. Experiments verified the improved climbing capabilities of Waalbot II as well as its novel semi-autonomous adhesion recovery behavior and motion planning.

Suction component in adhesion of mushroom-shaped microstructure

To shed light on the role of suction in adhesion of microstructure with mushroom-shaped terminal elements, we compared pull-off forces measured at different retraction velocities on structured and smooth surfaces under different pressure conditions. The results obtained allow us to suggest that suction may contribute up to 10 per cent of the pull-off force measured on the structured surfaces at high velocities. We therefore conclude that the attachment ability of this biomimetic adhesive must not be solely based on van der Waals forces. Our experiments also suggest a change in visco-elastic properties of the structured surfaces compared with the bulk material. Based on the results obtained, it is assumed that this adhesive may be suitable in dynamic pick-and-drop processes even under vacuum conditions at which sufficiently high adhesive capability is maintained.

Face selection in one-step bending of Janus nanopillars

We introduce a one-step procedure of bending nanopillars, which simply involves oblique metal deposition at a tilted angle of 45 degrees on the pillars by thermal evaporation. The face selection in the bending procedure was determined by the nature of residual stress generated in the metal film during evaporation. If the stress was tensile as with many metals (sigma(f) > 0), the Janus nanopillars were bent toward the metal face; if the residual stress was compressive as in the case of Al (sigma(f) < 0), they were bent toward the polymer face. It has also been demonstrated that groups of Janus nanopillars could be bent in different directions on the same substrate with the aid of a shadow-mask deposition. The degree of bending increased with the decrease in pillar diameter in the range of 360-800 nm for a fixed height of 1 microm.

Gecko-inspired surfaces: a path to strong and reversible dry adhesives

The amazing adhesion of gecko pads to almost any kind of surfaces has inspired a very active research direction over the last decade: the investigation of how geckos achieve this feat and how this knowledge can be turned into new strategies to reversibly join surfaces. This article reviews the fabrication approaches used so far for the creation of micro- and nanostructured fibrillar surfaces with adhesive properties. In the light of the pertinent contact mechanics, the adhesive properties are presented and discussed. The decisive design parameters are fiber radius and aspect ratio, tilt angle, hierarchical arrangement and the effect of the backing layer. Also first responsive systems that allow thermal switching between nonadhesive and adhesive states are described. These structures show a high potential of application, providing the remaining issues of robustness, reliability, and large-area manufacture can be solved.

Stretchable, adhesion-tunable dry adhesive by surface wrinkling

We introduce a simple yet robust method of fabricating a stretchable, adhesion-tunable dry adhesive by combining replica molding and surface wrinkling. By utilizing a thin, wrinkled polydimethyl siloxane (PDMS) sheet with a thickness of 1 mm with built-in micropillars, active, dynamic control of normal and shear adhesion was achieved. Relatively strong normal (approximately 10.8 N/cm(2)) and shear adhesion (approximately 14.7 N/cm(2)) forces could be obtained for a fully extended (strained) PDMS sheet (prestrain of approximately 3%), whereas the forces could be rapidly reduced to nearly zero once the prestrain was released (prestrain of approximately 0.5%). Moreover, durability tests demonstrated that the adhesion strength in both the normal and shear directions was maintained over more than 100 cycles of attachment and detachment.

Gecko adhesion pad: a smart surface?

Recently, it has been shown that humidity can increase the adhesion of the spatula pads that form the outermost (adhesive) surface of the tokay gecko feet by 50% relative to the main adhesion mechanism (i.e. van der Waals adhesive forces), although the mechanism by which the enhancement is realized is still not well understood. A change in the surface hydrophobicity of a gecko setal array is observed when the array, which supports the spatulae, is exposed to a water drop for more than 20 min, suggesting a change in the hydrophilic-lyophilic balance (HLB), and therefore of the conformation of the surface proteins. A surface force apparatus (SFA) was used to quantify these changes, i.e. in the adhesion and friction forces, while shearing the setal array against a silica surface under (i) dry conditions, (ii) 100% humidity and (iii) when fully immersed in water. The adhesion increased in the humid environment but greatly diminished in water. Although the adhesion forces changed significantly, the friction forces remained unaffected, indicating that the friction between these highly textured surfaces is 'load-controlled' rather than 'adhesion-controlled'. These results demonstrate that the gecko adhesive pads have the ability to exploit environmental conditions to maximize their adhesion and stabilize their friction forces. Future designs of synthetic dry adhesives inspired by the gecko can potentially include similar 'smart' surfaces that adapt to their environment.

Shape-tunable polymer nanofibrillar structures by oblique electron beam irradiation

We introduce a facile method to fabricate geometry-controllable, high-aspect-ratio (AR approximately 10) polymer nanofibrillar structures in the form of stooped or crispated nanohairs by molding and oblique electron beam irradiation. The geometry of polymer nanohairs can be precisely tunable by controlling the tilting angle of the electron beam, the acceleration voltage, and the exposure time. This method provides a simple and versatile route to fabricating various nanofibrillar structures without a multistep process and may have applications in the fields of actuators or biomimetic attachment such as gecko-like adhesive and nano-Velcro.

Frictional adhesion of patterned surfaces and implications for gecko and biomimetic systems

Geckos and smaller animals such as flies, beetles, and spiders have extraordinary climbing abilities: They can firmly attach and rapidly detach from almost any kind of surface. In the case of geckos, this ability is attributed to the surface topography of their attachment pads, which are covered with fine columnar structures (setae). Inspired by this biological system, various kinds of regularly structured or "patterned" surfaces are being fabricated for use as responsive adhesives or in robotic systems. In this study, we theoretically analyze the correlated adhesion and friction (frictional adhesion) of patterned surfaces against smooth (unstructured) surfaces by applying well-established theories of van der Waals forces, together with the classic Johnson-Kendall-Roberts (JKR) theory of contact (or adhesion) mechanics, to recent theories of adhesion-controlled friction. Our results, when considered with recent experiments, suggest criteria for simultaneously optimizing the adhesion and friction of patterned surfaces. We show that both the van der Waals adhesion and the friction forces of flexible, tilted, and optimally spaced setal stalks or (synthetic) pillars are high enough to support not only a large gecko on rough surfaces of ceilings (adhesion) and walls (friction) but also a human being if the foot or toe pads-effectively the area of the hands-have a total area estimated at approximately 230 cm2.

A mechanical model of biomimetic adhesive pads with tilted and hierarchical structures

A 3D model for hierarchical biomimetic adhesive pads is constructed. It is based on the main principles of the adhesive pads of the Tokay gecko and consists of hierarchical layers of vertical or tilted beams, where each layer is constructed in such a way that no cohesion between adjacent beams can occur. The elastic and adhesive properties are calculated analytically and numerically. For the adhesive contact on stochastically rough surfaces, the maximum adhesion force increases with increasing number of hierarchical layers. Additional calculations show that the adhesion force also depends on the height spectrum of the rough surface.

Enhanced adhesion by gecko-inspired hierarchical fibrillar adhesives

The complex structures that allow geckos to repeatably adhere to surfaces consist of multilevel branching fibers with specialized tips. We present a novel technique for fabricating similar multilevel structures from polymer materials and demonstrate the fabrication of arrays of two- and three-level structures, wherein each level terminates in flat mushroom-type tips. Adhesion experiments are conducted on two-level fiber arrays on a 12-mm-diameter glass hemisphere, which exhibit both increased adhesion and interface toughness over one-level fiber samples and unstructured control samples. These adhesion enhancements are the result of increased surface conformation as well as increased extension during detachment.

A microfabricated wedge-shaped adhesive array displaying gecko-like dynamic adhesion, directionality and long lifetime

Gecko adhesion has become a paradigmatic example of bio-inspired engineering, yet among the many gecko-like synthetic adhesives (GSAs), truly gecko-like performance remains elusive. Many GSAs have previously demonstrated one or two features of the gecko adhesive. We present a new wedge-shaped GSA that exhibits several gecko-like properties simultaneously: directional features; zero force at detachment; high ratio of detachment force to preload force; non-adhesive default state; and the ability to maintain performance while sliding, even after thousands of cycles. Individual wedges independently detach and reattach during sliding, resulting in high levels of shear and normal adhesion during drag. This behaviour provides a non-catastrophic failure mechanism that is desirable for applications such as climbing robots where sudden contact failure would result in serious falls. The effects of scaling patch sizes up to tens of square centimetres are also presented and discussed. Patches of 1 cm(2) had an adhesive pressure of 5.1 kPa while simultaneously supporting 17.0 kPa of shear. After 30 000 attachment/detachment cycles, a patch retained 67 per cent of its initial adhesion and 76 per cent of its initial shear without cleaning. Square-based wedges of 20 mum and 50 mum are manufactured in a moulding process where moulds are fabricated using a dual-side, dual-angle lithography process on quartz wafers with SU-8 photoresist as the mould material and polydimethylsiloxane as the cast material.

In vitro adhesion measurements between skin and micropatterned poly(dimethylsiloxane) surfaces

Micropatterned adhesive surfaces may have potential in reconstructive surgery. The adhesion performance of mice ear skin to micropatterned poly(dimethylsiloxane) (PDMS) was investigated, under in vitro conditions, and compared to flat substrates. No significant difference in separation force F was observed between flat substrates and micropatterned surfaces with pillar arrays. However, the energy necessary for separation of the substrate from the skin was sensitive to the topography. Furthermore, our results show that the force-displacement curves depended on the wetness of the skin: Highest force values were obtained for fresh skin while the forces decreased as the skin dried out. The results are encouraging for further studies on the potential of patterned PDMS in biomedical applications.

The effect of preload on the pull-off force in indentation tests of microfibre arrays

We determined how preload and work of adhesion control the force required to pull a circular cylindrical indenter off a microfibre array. Five regimes, with different contact behaviours, are identified for the unloading phase of indentation. These regimes are governed by two dimensionless parameters. Above a critical preload, the pull-off force and the pull-off stress reach a plateau value. The critical preload, as well as the plateau pull-off force (stress), is found to depend on a single dimensionless parameter q , which can be interpreted as a normalized work of adhesion.

The effect of shape on the adhesion of fibrillar surfaces

Experimental data have demonstrated that mushroom-shaped fibrils adhere much better to smooth substrates than punch-shaped fibrils. We present a model that suggests that detachment processes for such fibrils are controlled by defects in the contact area that are confined to its outer edge. Stress analysis of the adhered fibril, carried out for both punch and mushroom shapes with and without friction, suggests that defects near the edge of the adhesion area are much more damaging to the pull-off strength in the case of the punch than for the mushroom. The simulations show that the punch has a higher driving force for extension of small edge defects compared with the mushroom adhesion. The ratio of the pull-off force for the mushroom to that of the punch can be predicted from these simulations to be much greater than 20 in the friction-free case, similar to the experimental value. In the case of sticking friction, a ratio of 14 can be deduced. Our analysis also offers a possible explanation for the evolution of asymmetric mushroom shapes (spatulae) in the adhesion organ of geckos.