Gecko-inspired Dry Adhesives Research Articles

Democratising dry adhesion development with consumer-grade AM

AbstractThe production of reusable gecko-inspired dry adhesives has traditionally been done with complex nanofabrication methods such as lithography and PDMS casting. This article presents a way of producing and testing dry adhesive samples using consumer-grade AM machines and equipment typically available in a Makerspace. The samples produced exhibit adhesive properties depending on the preload and surface structure, and we conclude that consumer-grade AM is suitable for rapid prototyping and testing of dry adhesion. However, it is limited by the scale and accuracy compared to traditional methods.

Dynamically reprogrammable stiffness in gecko-inspired laminated structures

Adaptive structures are of interest for their ability to dynamically modify mechanical properties post fabrication, enabling structural performance that is responsive to environmental uncertainty and changing loading conditions. Dynamic control of stiffness is of particular importance as a fundamental structural property, impacting both static and dynamic structural performance. However, existing technologies necessitate continuous power to maintain multiple stiffness states or couple stiffness modulation to a large geometric reconfiguration. In this work, reversible lamination of stiff materials using gecko-inspired dry adhesives is leveraged for bending stiffness control. All stiffness states are passively maintained, with electrostatic or magnetic actuation applied for ∼1 s to reprogram stiffness. We demonstrate hinges with up to four passively maintained reprogrammable states decoupled from any shape reconfiguration. Design guidelines are developed for maximizing stiffness modulation. Experimentally, the proposed method achieved a stiffness modulation ratio of up to 14.4, with simulations showing stiffness modulation ratios of at least 73.0. It is anticipated that the stiffness reprogramming method developed in this work will reduce energy requirements and design complexity for adaptation in aerospace and robotics applications.

Examination of gecko-inspired dry adhesives for heritage conservation as an example of iterative design and testing process for new adhesives

AbstractRarely within the conservation of cultural heritage have conservation professionals been lucky enough to have materials custom-designed to meet their requirements. Most of the time the field must adapt solutions developed for other applications. The research presented here was initiated as part of a long-term aim to develop new adhesives for heritage conservation. Gecko-inspired dry adhesives (GDAs) are polymer tapes with micropatterns that are based on the adhesive properties of the pads of gecko lizard feet: they have strong normal and shear adhesion with low peel adhesion. They present potentially versatile and reversible adhesives for heritage conservation applications; which do not require solvents for activation or for removal, and do not migrate or off-gas. In nature, geckos can adhere with their feet to any surface they walk on. This is possible because of micro- and nanostructures on their feet that attach to surface via van der Waals forces. Practical studies aimed at comprehensively assessing their properties, and biomimetic solutions in heritage conservation are still sparse at present. This research has the objective of assessing GDAs properties by mechanical testing of adhesive joints, as well as physical and chemical characterisation of the materials used. The research has also included a museum case study and a two-year natural ageing test. The testing has shown that the GDAs can perform very well as an adhesive patch on the reverse of gelatine-based photographs, achieving shear forces between 0.80 N and 48.10 N on 8 cm2 lap joints (depending on the type of the GDA) and peel forces between 0.20 N and 0.47 N over 2 cm of peel front. This is lower than forces exceeding 1 N recorded in widely available pressure-sensitive tapes. This shows that GDAs may have the potential of a sufficiently strong, yet easily removable adhesive that works well on materials widely present in museum collections.

Gecko-inspired dry adhesives for heritage conservation – tackling the surface roughness with empirical testing and finite element modelling

Gecko-inspired dry adhesives (GDAs) have been developed in an attempt to replicate in polymer material the natural ability of some gecko lizards to attach to nearly any surface. Geckos achieve this with nano-sized structures on their feet that facilitate van der Waals's interactions with the surfaces. The conservation of cultural heritage is an area that could benefit greatly from the introduction of a versatile and easily reversible adhesive. However, the multitude of surface types and various surface textures encountered in this field make the adaptation of GDAs difficult. In this research two types of GDAs, with flat tips and with mushroom-shaped tips have been assessed using pull-off tests on three substrate materials. These are based on real heritage objects’ surfaces (copper and ceramic) with different levels of surface roughness from The Hunterian collection. Adhesive strength varied between different GDAs and as expected adhesive strength reduced with increased substrate roughness. The finite Element Modelling (FEM) of the pull-off tests closely matched empirical results and showed how different behaviours on the microlevel can affect the GDA behaviour on rough surfaces. It helped to understand the microscale behaviour of two different types of GDAs tested. The research has shown the necessary direction for experimental and theoretical research on GDAs which will enable them to be adopted more widely in heritage conservation.

Gecko-Inspired Biomimetic Surfaces with Annular Wedge Structures Fabricated by Ultraprecision Machining and Replica Molding.

The current research on gecko-inspired dry adhesives is focused on micropillar arrays with different terminal shapes, such as flat, spherical, mushroom, and spatula tips. The corresponding processing methods are mostly chemical methods, including lithography, etching, and deposition, which not only are complex, expensive, and environmentally unfriendly, but also cannot completely ensure microstructural integrity or performance stability. The present study demonstrates a high-precision, high-efficiency, and green method for the fabrication of a gecko-inspired surface, which can promote its application in dexterous robot hands and mechanical grippers. Based on the bendable lamellar structures of the gecko, annular wedge adhesive surfaces that stick to the finger surfaces of dexterous robot hands to improve their load capacity are proposed and fabricated via a suitable combined processing method of ultraprecision machining and replica molding. The greater the width, the higher the replication integrity, and when the minimum width is 20 μm, the replication error is less than 5.5% due to the superior processing performance of the nickel–phosphorus (Ni–P) plating of the master mold. The fabricated annular wedge structures with an optimized width of 20 μm not only exhibit a strong friction force of up to 35.48 mN under a preload of 20 mN in the GCr15/poly(dimethylsiloxane) (PDMS) friction pair but also demonstrate an obviously improved anisotropic friction characteristic of up to λ = 1.36, as the molecular force exhibits a stronger increase as compared to the decrease of the mechanical force of the structure with a small width.

Creating Metal Molds for Directional Gecko-Inspired Adhesives

Abstract We describe a process for creating durable metal molds for the fabrication of directional, gecko-inspired dry adhesives. The adhesives require microscopic inclined features with a challenging combination of tapered geometry, high aspect ratio, and smooth surface finish. Wedge-shaped features produced by the new metal mold exhibit the same geometry and surface finish as those cast from single-use wax molds and epoxy molds in previous fabrication methods. They also produce the same levels of adhesion and shear stress. The metal molds, and the adhesives cast from them, show no degradation after repeated molding cycles.

Adhesion Circle: A New Approach To Better Characterize Directional Gecko-Inspired Dry Adhesives.

The number of different designs of directional gecko-inspired adhesives has proliferated over the past 15 years, but some basic characterization tools are still nonstandardized, which can make direct comparisons of different adhesives in the literature difficult. By far the most common type of test for directional adhesives, the load-drag-pull (LDP) test is useful but can miss substantial information on the exact behavior of gecko-inspired adhesives in a variety of loading conditions. Other test techniques, including angled approaches and pull-offs, have been employed by a few groups but they are not as widely adopted; peel tests can be employed but require a larger amount of adhesive material to use in the test, which is not always practical given some current manufacturing constraints. Very few tests have looked at the effect of off-main axis loads on the performance of directional adhesives, however, and this quality of performance may be very important in applications where direct control over displacements or angle of pull-off in pitch and yaw of the peeling interface may not be practical or possible. To address this overlooked area of characterization, we introduce a new test concept for anisotropic adhesives, the adhesion circle, and also compare how the radial normal adhesion performance is altered depending on whether the pull-off comes after a displacement drag or when pulled at a constant angle from vertical after a preload. Testing directional adhesive designs made with different geometries shows that unexpected behaviors at pull-off angles not in the direction of the strong-weak axis can sometimes be seen. The complete adhesion circle tests should help better design directional adhesives for scaled up performance, and can be completed with relatively simple hardware that is typically used in most current directional adhesive tests.

干黏附碳纳米管垂直阵列的转移及其黏附性能评价

Vertically aligned carbon nanotube arrays(VACNTs) exhibit high aspect-ratios and excellent mechanical strength, and have attracted attention in the field of gecko-inspired dry adhesives. However, the poor interfacial bonding between the silicon substrate and the VACNTs, the hard and brittle silicon substrate which is suitable for the growth of VACNTs in chemical vapor deposition(CVD) technique limit the development and application of VACNTs as the gecko-inspired dry adhesive. To overcome these limitations we present a simple and cost-effective heating transfer method without other media-assisted. We combine this with thermal oxidation detachment to transfer the grown VACNTs onto flexible polycarbonate(PC) substrates. The transfer process provides two different VACNTs morphologies; a curly entangled segment in the bottom and an aligning end segment in the top. The bottom transfer maintains the top morphology of the pristine VACNTs, which differs from the flip-over process known as top transfer. This transfer process results in a double transferred VACNTs structure which is a new type of dry adhesive. By optimizing the transfer process, the interfacial bonding force between the VACNTs and PC substrates is nearly three times stronger than that of grown VACNTs on silicon substrates at 250°C. The shear adhesion strengths of the bottom- and top-transferred VACNTs were tested and analyzed. Then, the shear adhesion strength of the transferred VACNTs were tested and analyzed according to the difference between the morphology and the degree of graphitization on the top of VACNTs after the top/bottom transfer methods. The shear adhesion strength of the transferred VACNTs increased with the normal preload. The adhesion strength of the bottom-transferred VACNTs was approximately 5 N/cm2, whereas that of the top-transferred VACNTs was 2.8 N/cm2 under a 16 N/cm2 normal preload force. The bottom-transferred VACNTs showed better adhesion because of the increasing contact area and higher degree of graphitization on the top of the structure according to scanning electron microscope and Raman spectra. Our transfer method is nondestructive, fast and suitable for VACNTs with a height greater than 200 um. Moreover, this method is not restricted by the area of the VACNTs, and thus can improve their integrity and flexibility, and increase their application as gecko-inspired dry adhesives.

Gecko gaskets for self-sealing and high-strength reversible bonding of microfluidics.

We report in this work a novel reversible bonding technique for elastomeric microfluidic devices by integrating gecko-inspired dry adhesives with microfluidic channels which greatly enhances the bonding strength of reversibly sealed channels. The concept is applicable to nearly any elastomer and can be used to bond against any smooth surface which allows for van der Waals interactions. It does not require any solvents or glues or sources for plasma activation or thermal-compressive loading to aid the bonding process and is achievable at zero extra cost. We also demonstrate a quick fabrication technique involving soft master thermo-compressive molding of these microfluidic devices with thermoplastic elastomers. The resultant devices can be used for both pressure driven and non-pressure driven flows. We report the maximum contained pressure of these devices manufactured from two grades of styrene ethylene butylene styrene (SEBS) by conducting a burst pressure test with various substrates.

Human climbing with efficiently scaled gecko-inspired dry adhesives.

Since the discovery of the mechanism of adhesion in geckos, many synthetic dry adhesives have been developed with desirable gecko-like properties such as reusability, directionality, self-cleaning ability, rough surface adhesion and high adhesive stress. However, fully exploiting these adhesives in practical applications at different length scales requires efficient scaling (i.e. with little loss in adhesion as area grows). Just as natural gecko adhesives have been used as a benchmark for synthetic materials, so can gecko adhesion systems provide a baseline for scaling efficiency. In the tokay gecko (Gekko gecko), a scaling power law has been reported relating the maximum shear stress σmax to the area A: σmax ∝ A(-1/4). We present a mechanical concept which improves upon the gecko's non-uniform load-sharing and results in a nearly even load distribution over multiple patches of gecko-inspired adhesive. We created a synthetic adhesion system incorporating this concept which shows efficient scaling across four orders of magnitude of area, yielding an improved scaling power law: σmax ∝ A(-1/50). Furthermore, we found that the synthetic adhesion system does not fail catastrophically when a simulated failure is induced on a portion of the adhesive. In a practical demonstration, the synthetic adhesion system enabled a 70 kg human to climb vertical glass with 140 cm(2) of adhesive per hand.

Fabrication and Characterization of Thermoplastic Elastomer Dry Adhesives with High Strength and Low Contamination

Polydimethylsiloxane (PDMS) and polyurethane elastomers have commonly been used to manufacture mushroom shaped gecko-inspired dry adhesives with high normal adhesion strength. However, the thermosetting nature of these two materials severely limits the commercial viability of their manufacturing due to long curing times and high material costs. In this work, we introduce poly(styrene-ethylene/butylene-styrene) (SEBS) thermoplastic elastomers as an alternative for the manufacture of mushroom shaped dry adhesives with both directional and nondirectional performance. These materials are attractive for their potential to be less contaminating via oligomer transfer than thermoset elastomers, as well as being more suited to mass manufacturing. Low material transfer properties are attractive for adhesives that could potentially be used in cleanroom environments for microscale assembly and handling in which device contamination is a serious concern. We characterized a thermoplastic elastomer in terms of oligomer transfer using X-ray photoelectron spectroscopy and found that the SEBS transfers negligible amounts of its own oligomers, during contact with a gold-coated silicon surface, which may be representative of the metallic bond pads found in micro-electro-mechanical systems devices. We also demonstrate the fabrication of mushroom shaped isotropic and anisotropic adhesive fibers with two different SEBS elastomer grades using thermocompression molding and characterize the adhesives in terms of their shear-enhanced normal adhesion strength. The overall adhesion of one of the thermoplastic elastomer adhesives was found to be stronger or comparable to their polyurethane counterparts with identical dimensions.

Effect of Top Structure on Adhesion of Carbon Nanotubes Based Gecko Inspired Dry Adhesive

Vertically aligned carbon nanotubes (VACNT) arrays were designed to mimic natural foot-hairs of geckos in order to elucidate the adhesion mechanism. We fabricated and systematically investigated adhesive properties of the VACNT arrays with different top structures. Balancing the gas ratio and pressure used for low pressure chemical vapor deposition (CVD) processing controls the top structures of VACNT arrays which determine the strength of adhesion. Their contact behaviors were observed using a scanning electron microscope (SEM). The VACNT arrays attached the target surface by different contact behavior based on the different top structures. The different contact behavior varied adhesive properties of VACNT arrays. VACNT arrays with free top structure present higher adhesion strength due to the side contact with target surface, which indicated that the top structure is an important factor enabling generation of a strong adhesion. These results present clear implications for the role of top structures in VACNT arrays based synthetic gecko systems, which is important for fabrication of gecko-inspired dry adhesives.

Janus-faced micro and nanopillars for geometry and surface chemistry controllable bioinspired dry adhesives

Janus-faced pillar structures are a new class of bioinspired dry adhesives based on the exploitation of unique physical and chemical properties when two different materials are placed on two opposite faces. Using Janus-faced high aspect ratio (HAR) pillars, it is possible to control the geometry and surface chemistry that are key factors in designing gecko-inspired artificial dry adhesives with high performance. In this review, first, we revisit the conventional fabrication techniques for homogeneous, angled HAR structures. Second, we present the fabrication methods for Janus-faced micro and nanopillars via physical and chemical modification of one of the faces. Third, we discuss the geometric features of Janus-faced structures with bending mechanisms of the pillars and the selection of contacting faces. Finally, we cover the theoretical and practical viewpoints of adhesion behavior with a particular focus on the role of work of adhesion, tilt angle, and adhesion hysteresis. We emphasize that Janus-faced structures potentially provide powerful systems to control anisotropic adhesion behavior by taking advantage of both physical and chemical aspects of the angled structures.

3D Direct Laser Writing of Nano‐ and Microstructured Hierarchical Gecko‐Mimicking Surfaces

Applying 3D direct laser writing, artificial hierarchical gecko-type structures are designed and fabricated down to nanometer dimensions. In this way, the elastic modulus and the length scale of the gecko's setae are very closely matched. Direct laser writing is a very flexible rapid prototyping method allowing the fabrication of arbitrary nanostructures. Since the parameters of the structures can be easily changed, this technique is perfect for design studies of dry adhesives. Measuring the adhesional forces by atomic force microscopy, the influence of several design parameters like density, aspect ratio, and tip-shape on dry adhesion performance are systematically examined. In this way, it is revealed that hierarchy is favorable for artificial gecko-inspired dry adhesives made of stiff materials on the nanometer scale.

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.