Matt Wilkinson

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 cm2 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 μm and 50 μm 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.

Frictional and elastic energy in gecko adhesive detachment

Geckos use millions of adhesive setae on their toes to climb vertical surfaces at speeds of over 1 m s−1. Climbing presents a significant challenge for an adhesive since it requires both strong attachment and easy, rapid removal. Conventional pressure-sensitive adhesives are either strong and difficult to remove (e.g. duct tape) or weak and easy to remove (e.g. sticky notes). We discovered that the energy required to detach adhering tokay gecko setae (Wd) is modulated by the angle (θ) of a linear path of detachment. Gecko setae resist detachment when dragged towards the animal during detachment (θ=30°) requiring Wd=5.0±0.86 (s.e.) J m−2 to detach, largely due to frictional losses. This external frictional loss is analogous to viscous internal frictional losses during detachment of pressure-sensitive adhesives. We found that, remarkably, setae possess a built-in release mechanism. Setae acted as springs when loaded in tension during attachment and returned elastic energy when detached along the optimal path (θ=130°), resulting in Wd=−0.8±0.12 J m−2. The release of elastic energy from the setal shaft probably causes spontaneous release, suggesting that curved shafts may enable easy detachment in natural, and synthetic, gecko adhesives.

Changes in materials properties explain the effects of humidity on gecko adhesion

SUMMARY Geckos owe their remarkable stickiness to millions of dry setae on their toes, and the mechanism of adhesion in gecko setae has been the topic of scientific scrutiny for over two centuries. Previously, we demonstrated that van der Waals forces are sufficient for strong adhesion and friction in gecko setae, and that water-based capillary adhesion is not required. However, recent studies demonstrated that adhesion increases with relative humidity (RH) and proposed that surface hydration and capillary water bridge formation is important or even necessary. In this study, we confirmed a significant effect of RH on gecko adhesion, but rejected the capillary adhesion hypothesis. While contact forces of isolated tokay gecko setal arrays increased with humidity, the increase was similar on hydrophobic and hydrophilic surfaces, inconsistent with a capillary mechanism. Contact forces increased with RH even at high shear rates, where capillary bridge formation is too slow to affect adhesion. How then can a humidity-related increase in adhesion and friction be explained? The effect of RH on the mechanical properties of setal β-keratin has escaped consideration until now. We discovered that an increase in RH softens setae and increases viscoelastic damping, which increases adhesion. Changes in setal materials properties, not capillary forces, fully explain humidity-enhanced adhesion, and van der Waals forces remain the only empirically supported mechanism of adhesion in geckos.

Rate-dependent frictional adhesion in natural and synthetic gecko setae

Geckos owe their remarkable stickiness to millions of dry, hard setae on their toes. In this study, we discovered that gecko setae stick more strongly the faster they slide, and do not wear out after 30 000 cycles. This is surprising because friction between dry, hard, macroscopic materials typically decreases at the onset of sliding, and as velocity increases, friction continues to decrease because of a reduction in the number of interfacial contacts, due in part to wear. Gecko setae did not exhibit the decrease in adhesion or friction characteristic of a transition from static to kinetic contact mechanics. Instead, friction and adhesion forces increased at the onset of sliding and continued to increase with shear speed from 500 nm s−1 to 158 mm s−1. To explain how apparently fluid-like, wear-free dynamic friction and adhesion occur macroscopically in a dry, hard solid, we proposed a model based on a population of nanoscopic stick–slip events. In the model, contact elements are either in static contact or in the process of slipping to a new static contact. If stick–slip events are uncorrelated, the model further predicted that contact forces should increase to a critical velocity (V*) and then decrease at velocities greater than V*. We hypothesized that, like natural gecko setae, but unlike any conventional adhesive, gecko-like synthetic adhesives (GSAs) could adhere while sliding. To test the generality of our results and the validity of our model, we fabricated a GSA using a hard silicone polymer. While sliding, the GSA exhibited steady-state adhesion and velocity dependence similar to that of gecko setae. Observations at the interface indicated that macroscopically smooth sliding of the GSA emerged from randomly occurring stick–slip events in the population of flexible fibrils, confirming our model predictions.

Effects of humidity on the mechanical properties of gecko setae

We tested the hypothesis that an increase in relative humidity (RH) causes changes in the mechanical properties of the keratin of adhesive gecko foot hairs (setae). We measured the effect of RH on the tensile deformation properties, fracture, and dynamic mechanical response of single isolated tokay gecko setae and strips of the smooth lamellar epidermal layer. The mechanical properties of gecko setae were strongly affected by RH. The complex elastic modulus (measured at 5 Hz) of a single seta at 80% RH was 1.2 GPa, only 39% of the value when dry. An increase in RH reduced the stiffness and increased the strain to failure. The loss tangent increased significantly with humidity, suggesting that water absorption produces a transition to a more viscous type of deformation. The influence of RH on the properties of the smooth epidermal layer was comparable with that of isolated seta, with the exception of stress at rupture. These values were two to four times greater for the setae than for the smooth layer. The changes in mechanical properties of setal keratin were consistent with previously reported increases in contact forces, supporting the hypothesis that an increase in RH softens setal keratin, which increases adhesion and friction.

Dynamic friction in natural and synthetic gecko setal arrays

Geckos can cling to almost any surface using dense arrays of microscopic, hierarchical setae. The flat, terminal branches of the setae adhere by the van der Waals dispersion force, and the mechanics of the gecko attachment system are a current topic among biologists and researchers of smart materials for adhesion. We studied the interaction between shear velocity (v = 0.0005 mm s−1 to 158 mm s−1) and materials properties on dynamic friction of isolated natural gecko setal arrays. We varied the materials properties (complex modulus) of the setal β-keratin by adjusting atmospheric humidity (RH). Alongside the natural material, we performed similar experiments on synthetic arrays of polyurethane micropillars. Our experiments demonstrate the presence of two regimes in the friction force (F) vs. velocity behavior of the natural adhesives: a materials/RH-dependent domain exists at low v (<1 mm s−1) and a materials/RH-independent domain at higher v. At intermediate velocities, F(v) curves at different RH converge to an RH-independent value. From the dynamic experiments on natural arrays, we calculated a high-v activation volume (V*) of (90.1 ± 0.3) nm3. V* gives an indication of the strength of coupling between sliding elements. Velocity strengthening occurred in synthetic arrays. However, in contrast to the natural material, strengthening of adhesion and friction of synthetic gecko setae occurred at low v and weakened at high v. Activation volumes calculated for the synthetic arrays indicate weaker coupling. These results indicate (i) that the theory of state-rate friction (SRF) can adequately describe the behavior of sliding fibrillar adhesives and (ii) that the macroscopic performance of natural and synthetic setal arrays, when interpreted with an SRF model, provides some insight into the microscopic dynamics of frictional sliding.

Macroscale adhesion of gecko setae reflects nanoscale differences in subsurface composition

Surface energies are commonly used to determine the adhesion forces between materials. However, the component of surface energy derived from long-range forces, such as van der Waals forces, depends on the materials structure below the outermost atomic layers. Previous theoretical results and indirect experimental evidence suggest that the van der Waals energies of subsurface layers will influence interfacial adhesion forces. We discovered that nanometre-scale differences in the oxide layer thickness of silicon wafers result in significant macroscale differences in the adhesion of isolated gecko setal arrays. Si/SiO2 bilayer materials exhibited stronger adhesion when the SiO2 layer is thin (approx. 2 nm). To further explore how layered materials influence adhesion, we functionalized similar substrates with an octadecyltrichlorosilane monolayer and again identified a significant influence of the SiO2 layer thickness on adhesion. Our theoretical calculations describe how variation in the SiO2 layer thickness produces differences in the van der Waals interaction potential, and these differences are reflected in the adhesion mechanics. Setal arrays used as tribological probes provide the first empirical evidence that the ‘subsurface energy’ of inhomogeneous materials influences the macroscopic surface forces.

The Crowding Model as a Tool to Understand and Fabricate Gecko-Inspired Dry Adhesives

A model based on geometrical considerations of pillars in a square lattice is analyzed to predict its compression behavior under an applied normal load. Specifically, the “crowding model” analyzes the point at which tilting pillars become crowded onto neighboring pillars, which limits the achievable tilt angle under an applied normal load, which in turn limits their adhesion and friction forces. The crowding model is applied to the setal arrays of the tokay gecko. Good agreement is found between the predictions of the crowding model (a critical tilt angle of θc = 12.6° to the substrate corresponding to a vertical compression of Δz =49 μm of the setae within the setal array) and experimental data for the compression of tokay gecko setal arrays. The model is also used as a criterion to predict the number density of setae in a tokay gecko setal array based on the lateral inter-pillar spacing distance, s, between tetrads of setae and the effective diameter, d, of the tetrad. The model predicts a packing density of ∼14,200 setae/mm2, which is again in good agreement with the measured value of ∼14,400 setae/mm2. The crowding model can be used as a tool to determine the optimum geometrical parameters, including the diameter and the spacing distance between pillars, to fabricate dry adhesives inspired by the gecko.