Gecko-Inspired Dry Adhesive Based on MicroâNanoscale Hierarchical

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Gecko-inspired Dry Adhesive Based on Micro-Nanoscale Hierarchical Arrays for Application in Climbing Devices Hemant Kumar Raut, Avinash Baji, Hassan Hussein Hariri, Hashina Parveen, Gim Song Soh, Hong Yee Low, and Kristin L. Wood ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09526 • Publication Date (Web): 07 Dec 2017 Downloaded from http://pubs.acs.org on December 8, 2017

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ACS Applied Materials & Interfaces

Gecko-inspired Dry Adhesive Based on Micro-Nanoscale Hierarchical Arrays for Application in Climbing Devices Hemant Kumar Raut,* Avinash Baji, Hassan Hussein Hariri, Hashina Parveen, Gim Song Soh, Hong Yee Low,* Kristin L. Wood ∗

Division of Engineering Product Development, Singapore University of Technology and Design, 8 Somapah Rd, Singapore 487372, Republic of Singapore

ABSTRACT The unusual ability of geckos to climb vertical walls underlies a unique combination of hierarchical structural design and a stiffer material composition. While a dense array of microscopic hierarchical structures enable the gecko toe pads to adhere to various surfaces, a stiffer material (β-keratin) composition enables them to maintain reliable adhesion over innumerable cycles. This unique strategy has been seldom implemented in engineered dryadhesives because fabrication of high aspect ratio hierarchical structures using a stiffer polymer is challenging. Herein, we report the fabrication of high aspect-ratio hierarchical arrays on flexible polycarbonate sheets (stiffness comparable to that of β-keratin) by sacrificial layer mediated nanoimprinting technique. Dry-adhesive films comprising the hierarchical arrays showed a formidable shear adhesion of 11.91±0.43 N/cm2. Cyclic adhesion tests also showed that the shear adhesion of the adhesive films reduced by only about 20% after 50 cycles and remained nearly constant until about 200 cycles. Most importantly, the high aspect-ratio hierarchical arrays were integrated onto the feet of a miniature robot and the locomotion on a 30° inclined surface was demonstrated.

Keywords:

Bioinspired,

Dry-adhesive,

Gecko,

Hierarchical

Array,

Shear

Nanoimprinting. ∗

* Corresponding authors’ email addresses: [email protected], [email protected]

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Adhesion,

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1. Introduction The ability of geckos to climb vertical walls with extraordinary dexterity has been an area of enduring scientific interest. Incredibly fast attachment and easy detachment to various surfaces, fouling resistance and life-time durability are some of the key features that make the gecko adhesive pads unrivalled by any man-made pressure-sensitive adhesives (PSAs) manufactured thus far.1-3 Yet, what is most surprising is that, while the PSAs have ubiquitously relied on the use of softer viscoelastic polymers to readily conform to various surface topographies, the gecko’s toe pads are composed of a material that is orders of magnitude stiffer than any viscoelastic polymer.4-6 This seemingly counterintuitive strategy makes perfect sense by analyzing the hierarchical structural design of the gecko’s toe pads. Their toe pads are covered with parallel rows of densely packed microscopic pillars called setae.2 Each seta has a diameter of 5-10 µm and an aspect-ratio as high as 25.7,9 The seta further branches out at the tips into numerous fibrillar structures called spatula, that have diameters ranging between 200–500 nm and aspect ratio between 10 and 25. This multi-level branching into increasingly finer features results in a progressive lowering of effective stiffness at each level of hierarchy.5 This helps in inducing cumulative van der Waals forces between the fibrils and the contacting surface, resulting in a formidable adhesion of ~ 10 N/cm2.10,11 Thus, the hierarchical design combined with superior mechanical properties of the constituent β-keratin, simultaneously fulfils the crucial requirements of topographical compliance and structural robustness. Successful implementation of this bio-inspired strategy has tremendous potential in critical application areas such as robotics, medical diagnostics and devices, energy efficient handling of industrial products and space applications.12-16 This has led to a sustained effort towards fabrication of synthetic dry-adhesives. While a significant body of work exists on nonhierarchical microstructural arrays employing mostly viscoelastic polymers, there has also been an emerging interest in fabrication of hierarchical arrays in recent years. Photolithographic pathways have typically involved two-step exposure techniques to produce a template with hierarchical holes which is then used for soft-molding.17 However, diffraction effects have limited the dimensions as well as the density of structures, so much so that the resulting hierarchical arrays had orders of magnitude lower adhesion than its non-hierarchical counterparts. An alternative two-step UV-assisted capillary molding technique was used for 2 ACS Paragon Plus Environment

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fabrication of hierarchical arrays comprising polyurethane acrylate (E ~ 20 MPa). The resulting hierarchical arrays were covered with slanted pillar-like tips and achieved a shear adhesion of ~ 9 N/cm2.18 Most of these reports involve the use of polymers with elastic modulus (E) ranging between kPa and MPa. Recently though, efforts have been made to incorporate relatively stiffer polymers for the fabrication of hierarchical arrays. A notable approach in this regard, involves the use of hierarchical porous alumina templates.19,20 Generally, porous anodic aluminum oxide (AAO), comprising a hexagonal array of uniformly sized parallel channels are perfect templates for fabrication of fibrillar arrays.20,21 Anodization technique also enables fabrication of multitiered AAO templates. These templates have been employed for fabrication of hierarchical arrays based on polycarbonate (PC) (E ~ 2 GPa).20 However, the shear adhesion strength of the PCbased dry-adhesive films are less than the shear adhesion of the gecko toepads. This is possibly because of the limitation encountered in fabrication of hierarchical arrays with desirable aspectratios or density because the largest pore diameters achievable by the anodization pathway remains within the nanoscale regime.22 Incorporation of stiffer polymers for fabrication of hierarchical arrays requires careful optimization of the aspect-ratio of the arrays to modulate their effective stiffness. This is a crucial design aspect for gecko-inspired dry-adhesives. Furthermore, use of hierarchical molds for fabrication of tall hierarchical arrays might pose risk of deforming the arrays during mold extraction. Other techniques for fabrication of hierarchical arrays such as dip-transfer method, 3D direct laser writing, nanoyielding etc. also encounter challenges in scaling up to larger areas thereby limiting their use in fabrication of dry-adhesives for practical applications. 23-27 Thus, there exists a need for a simpler fabrication technique that can be easily scaled up and makes it possible to use a stiffer polymeric material for producing synthetic dry-adhesives. Nanoimprinting is a simple pattern transfer technique that involves filling of a negative template with a thermoplastic polymer at an optimum temperature (above the polymer’s glass transition) and pressure, followed by release of the template.28-31 Although nanoimprinting is a costeffective technique that can be easily scaled up, fabrication of hierarchical arrays using this technique has been particularly challenging.29-31 Fabrication of secondary structures on top of primary imprinted structures is difficult as the interplay of higher temperature and pressure during imprinting can damage the primary structures. In this paper, we demonstrate the fabrication of gecko inspired hierarchical arrays by the nanoimprinting based technique called 3 ACS Paragon Plus Environment


sacrificial layer mediated nanoimprinting (SLAN). In this technique the primary structures are protected from deformation by a sacrificial polymer layer, during the imprinting of the secondary patterns.28 Finally, the sacrificial layer is removed by selective dissolution in a suitable solvent. The above technique enabled high fidelity and large-area fabrication of hierarchical arrays. The resulting dry-adhesive films exhibited shear adhesion characteristics comparable to their biological counterpart. A stiffer material composition also enabled the hierarchical arrays to exhibit consistent adhesion behaviour over repeated attachment-detachment cycles making them potentially suitable for application in wall-climbing robots that have traditionally relied on softer viscoelastic adhesive pads.12,13 2. Results, Discussion and Application 2.1 Fabrication of hierarchical arrays The sacrificial-layer mediated nanoimprinting (SLAN) technique for the fabrication of geckoinspired hierarchical arrays is illustrated in Figure 1. In the first step, micropillar arrays (primary structures) were fabricated by imprinting the corresponding mold on a polycarbonate (PC) sheet (Figure S2(a), supplementary information). The micropillar arrays are arranged in a hexagonal close packed (hcp) pattern to maximize the density of the arrays which is essential for inducing van der Walls forces.4 These micropillar arrays were then covered in a matrix of a preferentially soluble polymer layer comprising, e.g., poly(sodium 4-styrenesulfonate) (PSS), such that only the pillar tips are exposed (Figure S2(b), supplementary information). This was achieved by controlling the speed (1000 rpm) and coating duration (30 s) during the spin-coating process.32 Fabrication of the fibrillar arrays (secondary structures) on top of the micropillar arrays was subsequently obtained by a second imprinting using the AAO templates (at 170 °C and a pressure of 30 bar for 1000 s). During the second nanoimprinting step, the underlying micropillars remained upright as they were laterally encaged in the PSS matrix. Since the PSS has a higher melting temperature (Tm) of 450 °C than that of the PC (Tm = 155 °C), the temperature during the second nanoimprinting did not soften the PSS matrix and no appreciable thermal deformation of the micropillars occurred. After demolding the AAO template, the imprinted sheet was immersed in water to completely dissolve the PSS layer and expose the hierarchical arrays. This technique relies on the preferential solubility of PSS and PC in polar 4 ACS Paragon Plus Environment

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and non-polar solvents respectively.28 It enables selective dissolution of only the PSS matrix in water, while keeping the underlying PC structures chemically unaffected. However, the introduction of water for dissolution of PSS also caused elastocapillary coalescence of the fibril tips during drying (Figure S3(a), supplementary information). Such clumping or agglomeration of fibrillar arrays can drastically reduce the adhesion strength because agglomeration reduces the effective contact area of the sample.4 Furthermore, agglomerated pillars have a higher effective stiffness that reduces their compliance to a surface which in turn deteriorates their adhesion charecteristics.33 Freeze-drying technique was used to overcome this issue. Dipping in liquid nitrogen instantaneously freezes the free-standing fibrillar features present on the hierarchical arrays thereby preserving them in their upright and free-standing state. This step was followed by freeze drying to convert the water directly from solid phase to gaseous phase. Finally, freestanding fibrillar arrays on top of micropillar structures were obtained as shown in Figure 2(b) and (c). The SEM images shown in Figure 2(b) and (c) demonstrate that this approach led to successful fabrication of high aspect-ratio hierarchical arrays. 2.2 Structural characterization of hierarchical arrays The gecko-inspired hierarchical arrays consist of a two-level structural design – primary micropillar structures covered with a secondary array of high aspect-ratio fibrillar structures (here after referred to as fibrillar arrays). It is known that fabrication of upright and free-standing structures depend on the aspect ratio of the structures and the mechanical properties of the material (e.g., polymer’s elastic modulus, E).34 Beyond a critical aspect ratio, the structures laterally collapse resulting in fibrillar agglomeration that can severely reduce their adhesion.4 For PC (E = 2 GPa), we estimated that the critical aspect-ratio for the fibrillar structures to fulfil the non-collapse criteria is ≈ 13 (for fibril diameter d = 200 nm). Details on the calculation can be found in the Section S1 in the supplementary information. In the present study, the aspect aspect-ratio (λF) of the fibrils was kept constant at ~ 10. Stable upright fibrillar arrays of λF = 10 were obtained on a flexible PC sheet with high yield > 90%. These single-level fibrillar arrays served as a control sample to the two-level hierarchical arrays. Figure 2(a) shows the SEM images of the densely packed fibrillar arrays with d ~ 200 nm. The height of the fibrillar arrays is ~ 2 µm as inferred from the cross-sectional SEM image shown in 5 ACS Paragon Plus Environment


the inset in Figure 2(a). The fibril diameter (200 nm) is based on the following contact mechanics analysis: an uneven stress usually develops at the edge of two contacting bodies when they are subjected to externally applied load. This uneven stress drives crack to propagate through the contact thereby ultimately breaking the adhesion. On the other hand, fibrillar features can ensure a uniform stress distribution across the entire contact region. Secondly, adhesion becomes increasingly insensitive to fibril-tip geometry below a certain critical fibril diameter. For gecko spatula, this critical fibril diameter has been estimated to be 225 nm.35,36 In fact, this estimation is very close to the actual dimensions of the gecko spatula which has possibly evolved to maximize adhesion despite contact flaws. The fabrication of hierarchical arrays is also successfully demonstrated. As evident from the low magnification SEM image in Figure 2(b) hierarchical arrays were fabricated with a high yield by the SLAN technique. It should be noted that imprinting performed in the absence of the sacrificial layer, even if carried out at relatively lower temperature, resulted in complete obliteration of the micropillars as evident in Figure S3(b) (refer to supplementary information). In contrast, it is evident from the SEM image in Figure 2(c), that the hierarchical arrays are upright and covered with a dense array of fibrillar structures (diameter 200 nm and λF = 10). The aspect-ratio of the micropillar structures (λM) of these hierarchical arrays is ~ 2 (diameter 3 µm). Since our hierarchical arrays are based on a stiffer polymer (E ~ 2 GPa), fabrication of hierarchical arrays with taller micropillar structures or higher λM is also desirable. By increasing the λM of the hierarchical arrays, the effective stiffness at the interface of the dry-adhesive film can be lowered. This reduction in effective stiffness can enhance the compliance of the dryadhesive to various surface topographies which is an important design criterion for geckoinspired dry-adhesives.4-6 In order to investigate this phenomenon, two different dry-adhesive films comprising hierarchical arrays with λM = 2 and 4 respectively were fabricated. The diameter of the micropillars were kept constant i.e., D = 3 µm and only the height of the micropillar structures were varied between these two samples. The SEM images of these two types of hierarchical arrays with λM = 2 and 4 are shown in Figure 2 (c) and (d) respectively. The choice of λM is currently limited by the aspect-ratios of the micropillar master molds.37 Secondly, although fabrication of hierarchical arrays have been demonstrated in various previous reports, the current nanoimprinting based fabrication process is readily scalable and well suited for 6 ACS Paragon Plus Environment

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transforming flexible polymeric sheets into dry-adhesive films.15-20 Direct patterning on flexible polymeric sheets is advantageous as the resulting dry-adhesive film can be integrated into a wallclimbing robot in a variety of ways.12,13 Hierarchical arrays on flexible PC sheet has also been previously fabricated using specialized multi-tiered branched porous AAO templates.20 However, the shear adhesion of the resulting hierarchical arrays was less than that of the gecko’s toepads. A detail qualitative and quantitative summary of gecko inspired hierarchical arrays reported in literature is given in Table 1 (supplementary information). 2.3 Adhesion characteristics of the hierarchical arrays The shear adhesion of both the hierarchical arrays and the control samples (single-level fibrillar arrays) were measured by a customized set up based on the one described in ref. 20 (Figure S1, supplementary information). Shear force of the samples were measured by sliding the samples in the shear direction against a polished Si surface at a uniform speed. The shear adhesion is expressed as the maximum shear force recorded per unit area (cm2) of the adhesive sample. A small dead weight of ~ 50 mN load was constantly applied on the sample during the shear adhesion measurement. This set-up is based on the experimental design adopted in previous reports on gecko-inspired dry-adhesives.20,38-40 In fact, the geckos have been observed to apply a perpendicular preloading force in combination with a few micrometers of sliding of the seta to generate a high shear adhesion. The preload alone has been observed to be insufficient in developing effective adhesive states (adhesion strength is almost 10 times less).10,11 All the samples tested here were of the same size ~ 2 × 0.5 cm2 to match the size of the overlying dead weight to ensure an even distribution of load. Five specimen were tested for each sample type i.e., non-patterned, fibrillar and hierarchical. Figure 3(a) shows the graph of shear force versus displacement for both the control samples as well as the hierarchical arrays. Two distinct phases can be identified in the graphs – an initial transient phase where the shear force is seen to increase with the sliding distance followed by a steady phase where the shear force reaches saturation. For the control samples the shear force saturates at a maximum value of ~ 4.59 N. Thus, the maximum shear adhesion of the fibrillar arrays is determined to be 4.45±0.42 N/cm2 (based on all the fibrillar samples tested). On the other hand, the shear adhesion for the hierarchical arrays with micropillar aspect ratio (λM) = 2, is determined to be 8.04±0.85 N/cm2. The shear adhesion of the hierarchical arrays is almost 100% higher than that of the control 7 ACS Paragon Plus Environment


sample. Secondly, the shear force versus displacement curves for the hierarchical arrays is notably different from that of the control sample. The slope of the curve in case of the hierarchical arrays is steeper than that of the control sample (Figure 3(a)). This indicates that the shear force for hierarchical arrays reaches higher values in comparison to that of the control sample for the same sliding distance. This is especially useful when the hierarchical arrays are used for climbing applications in miniature robots. A higher shear adhesion could be realized by minimal sliding of the robot’s adhesive footpads. The dead weight (~ 50 mN) placed on the samples during the adhesion measurements is also found to be important. The shear adhesion reduced to ~ 0.5 N in the absence of the dead weight. A constant vertical load on dry-adhesives has been applied during adhesion measurement in previous studies as well.20 Superior shear adhesion of the hierarchical arrays is due in part to the lowering of effective stiffness around the micropillar region of the hierarchical arrays. This lowering of effective stiffness enhances the overall compliance of the hierarchical arrays to the adherent and enables the finer fibrillar features present in the hierarchical arrays to penetrate closer to the microscopically rough topography of the adherent. The effective stiffness (kM) of micropillar arrays of diameter D and aspect-ratio λM can be expressed as:41 =

3 64 λ

where E is the elastic modulus of the material (PC). Based on the above relationship, an increase in λM can further lower the effective stiffness of the hierarchical arrays. The change in effective stiffness (kM) by increasing the λM from 2 to 4 was experimentally determined by nanoindentation on micropillar arrays of the respective λM. Similar approach to measure the stiffness of individual/arrays of metallic as well as polymeric pillars has been used in other studies.42-46 Nearly 3-4 micropillars remain under the indenter tip (diameter = 10 µm) during each indentation. No residual plastic deformation was noticed for the micropillars after indentation, which indicates full recovery of the micropillar arrays. From the indentation loaddisplacement curves (Figure S4, supplementary information), the effective stiffness of the taller micropillar structures (λM = 4) is estimated to be nearly 8 times lower than that of micropillar structures with λM = 2 (refer to section S2 of the supplementary information section for


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calculations). This corroborates with the theoretical predictions by Bhushan et al.41 As the aspect ratio increases, the pillars have a higher tendency to bend and buckle rather than undergo linear compression. Wang et al. observed the indentation stiffness of CNT forest arrays to scale inversely with the square of the height of the CNT arrays.46 Since micropillar arrays of λM = 4 have a lower effective stiffness, hierarchical arrays with λM = 4 were fabricated by the SLAN technique. Analysis of their shear force versus displacement curve [in Figure 3(b)] showed that the shear adhesion performance of hierarchical arrays with λM = 4 is superior to that of hierarchical arrays with λM = 2. The shear adhesion for hierarchical arrays with λM = 4 is determined to be around 11.91±0.43 N/cm2. Additionally, the rate of attainment of maximum shear force for hierarchical arrays (λM = 4) is also higher than that of hierarchical arrays with λM = 2. This indicates that aspect-ratio of hierarchical arrays influences the rate at which maximum shear adhesion is attained. The shear adhesion obtained in case of hierarchical arrays (λM = 4) is impressive as it higher than the shear adhesion reported for PC-based hierarchical arrays (ref. 20) and comparable to the shear adhesion exhibited by the gecko toepads. The high shear adhesion is also due in part to the high fibril density on top of the hierarchical arrays (D = 3 µm) which was possible because of the SLAN based fabrication of the arrays. We also investigated the shear adhesion performance of the samples against a glass surface which is commonly employed as a test surface in climbing demonstrations for robots.12,13 The shear adhesion strength (N/cm2) of the samples tested against glass surface are summarized along with those tested on polished Si surface [as shown in Figure 4(a)]. The shear adhesion strength of all the dry-adhesive samples are slightly higher when measured on polished Si than on glass. It has been reported that a dry adhesive’s performance is affected by the roughness of the contacting surface.18,47 Shear adhesion strength of the dry-adhesive reduces marginally on glass surface, possibly due to slight variation in roughness. However, with an increase in the λM (≈ 4) of the hierarchical arrays a relatively high shear adhesion strength measuring ~ 9.95±0.41 N/cm2 is obtained. This value is also comparable to the shear adhesion strength of the biological counterpart. We anticipate that with further increase in the λM of the hierarchical arrays, adhesion strength of the hierarchical arrays on glass could be further enhanced. Reduction in backing layer thickness (especially in case of a stiffer polymeric material) can also influence the shear adhesion strength of the samples on different surfaces.48,49 9 ACS Paragon Plus Environment


To understand the adhesion mechanism of the hierarchical arrays, SEM image of the hierarchical arrays and that of the control sample were obtained immediately after the shear adhesion experiments. As shown in the inset in Figure 4(b) (and Figure S5(a) in the supplementary information section), the initially upright fibrils of the hierarchical arrays (before the adhesion test) are observed to bend along the shear direction (after the adhesion test). This unidirectional bending of the fibrils helps enhance the shear adhesion as the side wall of the fibrils come in contact with the substrate. The shear force (SFibril) generated by fibril tips can be estimated by the expression SFibril = τAt, where At is the true tip contact area and τ is the interfacial shear strength.50,51 The sidewall contact area of the fibrils (At) is higher than tip contact area. Consequently, shear adhesion increases. This is also noticed for the control samples (single-level fibrillar arrays) as shown in Figure S5(b) (in supplementary information). Previous studies on single-level polypropylene (E ~ 1 GPa), polyethylene microfiber arrays, CNT arrays etc. have also attributed to the enhanced shear adhesion to fibril deformation in the shear direction.20,38,39,52 The capability to generate reliable and steady adhesion over numerous attachment-detachment cycles is also crucial for a gecko-inspired dry-adhesive. This characteristic is important for the adhesive’s application in wall-climbing robots.53-56 We investigated the shear adhesion behavior of the hierarchical arrays (λM = 4) by subjecting them to cyclic load–drag–pull tests based on the set-up recommended in ref. 39 and 40. As shown in Figure 4(b), in the very first few cycles a large increase in shear adhesion from ~ 7.5 N to 12.3 N was observed. We believe that during these initial cycles (10-20 cycles), the fibrils present in the hierarchical arrays undergo slidinginduced directional bending (discussed previously) which improves their shear adhesion. This phenomenon has been previously observed for various fibrillar structures (comprising polypropylene/polyethylene) and is termed as a “training effect” in literature.38,39 Over the course of next 30-40 cycles, the shear adhesion of the hierarchical arrays gradually decreased by about 20-25% from the peak value before plateauing. The same trend has also been observed in ref. 23 and 39. Possible reasons for this marginal reduction in adhesion could be local variations in fibril height (due to bending), shear-induced plastic creep of fibrillar tips (due to high shear adhesion) or adhesive wear. However, the exact cause for the reduction is currently being investigated. Nevertheless, the fact that the shear adhesion eventually plateaus shows that the hierarchical arrays based on PC (with modulus comparable to that of β–keratin) are able to maintain their


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shear adhesion over repeated cycles of engagement with the contacting surface. Additionally, no polymeric residue on the adherent’s (Si) surface after the cyclic adhesion experiments is noticed. In contrast, PSAs have been reported to leave residue on the adherent’s surface after cyclic shear adhesion tests which may indicate viscous deformation of the softer polymeric structures.38 Finally, macroscale wrinkling/deformation of dry-adhesives is often observed which reportedly affects their adhesion performance.38 A recent work incorporates stiff carbon fiber mat on elastomeric adhesive film to eliminate this issue.57 We did not observe any macroscale deformation of the PC adhesive sheet which can be attributed to the relatively high stiffness of PC. 2.4 Application of PC hierarchical arrays Wall climbing robots that can be deployed for disaster relief, surveillance and maintenance activities in inaccessible location remains a challenging endeavor. In addition to the complex mechanical functionalities, wall climbing robots demand a glue-free adhesive much like insects such as geckos.12,13 A variety of dry adhesives such as PSAs comprising softer viscoelastic or elastomeric materials, single-level microscopic pillars arrays and microscopic wedge-shaped hierarchical features have been incorporated in robots for climbing purposes.53-56 However, the adhesion strength of PSAs have been observed to decline during attachment-detachment cycles.38 In contrast, the hierarchical arrays described in this study maintain their adhesion strength for more than 200 cycles which is potentially advantageous for wall climbing robots. Thus, a dryadhesive film comprising the hierarchical arrays is integrated onto a legged piezoelectric miniature robot (LPMR) and its locomotion on an inclined plane is recorded. The LPMR used in this study is a miniaturized piezoelectric-driven robot.58,59 Its propelled by a standing wave generated by a piezoelectric actuator integrated into the body of the robot (as shown in Figure S7, supplementary information). The standing wave results in a bi-directional motion of the legs of the robot that enables it to move. The weight of the LPMR is ~ 6.27 g. Further details about the LPMR can be found elsewhere.58,59 Figure 5 shows the time-lapse images of the LPMR climbing on a glass substrate. A detailed video (Movie M1) is provided in the electronic supplementary information section, which shows the climbing behavior of LPMR equipped with the hierarchical arrays versus that with a nonpatterned film (control samples) versus the LPMR with bare legs (without any polymer film). 11 ACS Paragon Plus Environment


The LPMR equipped with the hierarchical arrays is able to climb the inclined surface while the LPMR with bare legs was observed to slide down and topple (Figure S8, supplementary information) on the same surface. Additionally, as shown in Figure 5(a), the LPMR equipped with the hierarchical arrays, commenced climbing from a small angle of inclination and continued climbing even when the angle of inclination of the glass reached > 30°. However, LPMR provided with non-patterned film was only able to climb up to a small angle of inclination (5-10°) and began sliding down when the angle of inclination was further increased. Similar behavior is also seen for the LPMR with only bare legs (Movie M1), where the LPMR slides down the inclined glass surface. 3. Conclusion In this work, large-area flexible dry adhesive films comprising gecko-inspired hierarchical arrays were fabricated by a sacrificial-layer mediated nanoimprinting technique. The fabricated hierarchical arrays were based on a material (polycarbonate) that has elastic modulus comparable to that of the β-keratin present in the gecko’s foot pads. The hierarchical arrays owing to their high aspect-ratio, exhibited a high shear adhesion strength of 11.91±0.43 N/cm2. Due to a stiffer material composition, the hierarchical arrays showed minimal variation in shear adhesion strength for over 200 attachment-detachment cycles, indicating their reusability. The fibrils present in the hierarchical arrays were observed to bend in the shear direction during the adhesion tests thereby maximizing the contact area of the fibrils with the adherent. An increase in contact areas results in an increase in shear adhesion. Dry adhesive films comprising the hierarchical arrays were integrated with a miniature robot and climbing was demonstrated on an inclined surface (inclination slightly above 30°). 4. Experimental section 4.1 Materials Flexible polycarbonate (PC) sheets of 1 mm thickness were obtained from Goodfellow Cambridge Limited, UK. Thermoplastic PC is chosen as its elastic modulus (2.19 ± 0.09 GPa) is comparable to the elastic modulus of the β-keratin found in the biological adhesive structures.


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Poly(sodium 4-styrenesulfonate) solution (20 wt.% in water) was purchased from Sigma Aldrich. Ultrapure Millipore water was used in all the experiments. 4.2 Nanoimprint lithography Si nanoimprinting molds containing hexagonally packed micro-hole patterns of diameter 3 µm and two different heights, ~ 8 µm and 14 µm respectively, were used to imprint the micropillar arrays. Anodized alumina templates with uniformly cylindrical pores of diameter 200 nm and nearly 2 µm pore depth were fabricated on 0.14 mm thick high purity (99.999%) aluminum foil by the Masuda’s two-step anodization process.21 The anodization was carried out at 40 V d.c. at 15-17 ºC and the second anodization step was performed for 10 mins. All the molds were thoroughly cleaned by treating with oxygen plasma. A self-assembled monolayer (SAM) of 1H,1H,2H,2H-perfluorodecyltrichlorosilane was formed on the AAO molds to reduce their surface energy to ensure easy and defect free de-molding of the high aspect-ratio structures. A SOLVES® thermal imprinter (SOLVES, Singapore) was used to perform the imprinting. Freeze drying was performed in a FreeZoneTM freeze dryer (LabconcoTM, USA). 4.3 Characterization of hierarchical arrays The morphology of the hierarchical and fibrillar arrays was investigated using a JEOL JSM7600F field-emission scanning electron microscope (FE-SEM) operating at an accelerating voltage of 5 kV. Shear adhesion measurements were performed at room temperature (∼22 °C) and ∼70% humidity. The set up for shear adhesion measurement is based on the design adopted in ref. 20, 38 and 39. An Instron® 5940 single-column universal testing machine (UTM) equipped with a 100 N load cell was used for characterizing the shear adhesion of the samples. A specialized UTM accessory called friction fixture equipped with a ball-bearing Teflon pulley was mounted on the UTM as shown in Figure S1 (supplementary information). This set up converted the vertical movement of the clamps into lateral movement of the adhesive pads against the contacting substrate (e.g., Si or glass) fixed to the stage. A thin copper wire was connected between the load cell and the adhesive pad and passed over the pulley to complete the set-up. A dead weight (~ 50 mN) was applied on the samples during the test. The shear adhesion was recorded at the onset of sliding of the adhesive sample on the Si surface, as the crosshead extended upward. The samples were pulled in shear at a constant speed of 10 mm/min. All the 13 ACS Paragon Plus Environment


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samples used for shear adhesion measurements had a size of ~ 2 × 0.5 cm2. This sample size was chosen because it matches the size of the dead weight to ensure an even distribution of normal load. Five pristine specimen were tested for each sample type i.e., non-patterned, fibrillar and hierarchical. Shear adhesion tests were performed on both polished Si surface and clean glass surface. 4.4 Stiffness measurement of micropillar arrays To estimate the reduction in effective stiffness by an increase in aspect-ratio (from λM = 2 to λM = 4) of the hierarchical arrays, micropillar arrays of corresponding dimensions were fabricated on PC sheets. Both types of micropillar arrays (λM = 2 and 4) have the same diameter of 3 µm and a bulk backing layer thickness of ∼150 µm. The effective stiffness of the micropillar arrays was measured using a TI 950 nanoindenter (Hysitron Inc., Minneapolis, MN, USA). Indentation was performed using a calibrated 60° conical, flat diamond indenter (tip diameter 10 µm). A flat indenter tip is used because the deformation induced by the flat tip remains within a relatively homogeneous stress field. Quasi-static displacement controlled compression tests were carried out in ambient conditions at a displacement rate of 30 nm-1 to a maximum depth of 1µm. The latter ensured that the indentation depth was less than one-tenth of sample's thickness to eliminate any substrate influences. Load vs. tip displacement curves were recorded during the indentation tests. The indentation was performed in a continuous stiffness measurement (CSM) mode as it enables continuous measurement of the instantaneous stiffness of the pillars. The instantaneous stiffness measurements of the pillars also include the compliance associated with the pillars acting like a flat punch indenter into the bulk PC. This is known as the Sneddon correction (CSneddon), given by the expression: =

√ )

where ν and E are the

Poisson’s ratio and elastic modulus of the polymer (PC) respectively, and Ap is the crosssectional area of the pillar.60 The true stiffness (Spillar) can be calculated by subtracting the Sneddon correction from the inverse of the stiffness measured by the nanoindenter (SI), that is, !!"# =

$& '()*+,,-* %

.

In this study, the pillar height distribution and the small initial misalignment between the indenter and the sample might affect the loading part of the curve. Thus, the stiffness was 14 ACS Paragon Plus Environment

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measured using the slope of the initial 30% of the unloading load-displacement curve. A minimum of 5 tests were conducted for each micropillar specimen. The indentation locations for subsequent tests were chosen such that they were at least 50µm (> two times the contact radius) away from the previous test location. 4.4 Cyclic adhesion experiments Cyclic adhesion experiments involving load-drag-pull step cycles were performed on a two-axis force-displacement apparatus that was built based on the design described in ref. 38, 39 and 40. As shown in Figure S6 (in supplementary information), this apparatus consists of two main components. The first is an adhesive sample holder attached to a force sensor (ATI AI Nano 43 F/T sensor) for measurement of shear force. Two perpendicular linear actuators (OpenBuilds aluminum rails) control the x and y axis movements of the sample holder during the cyclic experiments. The actuators are equipped with micrometers for finer adjustments and are controlled by stepper motors. The second component is a vertical glass frame onto which a Si wafer is rigidly mounted facing the sample holder. The load-drag-pull step cycles for the hierarchical PC adhesive samples were performed under displacement control. An adhesive sample was first gently pressed onto the Si wafer until a normal force of 50 mN is reached. The sample was then dragged by ~ 2 mm at a speed of 10 mm/min on the Si surface and released.38 Shear force was recorded at every engagement cycle of the sample with the Si surface. The cycles were repeated until the shear force showed a significant and consistent drop. All the adhesive sample used in this experiment had a surface area of 1 cm2. All the adhesive samples were dragged in the same direction in each cycle. Supporting Information The Supporting Information includes experimental set-up for adhesion measurement, SEM images of sacrificial-layer encapsulating micro-pillar arrays, load-displacement curves (from nanoindentation) measuring the effective stiffness of micro-pillar arrays, experimental set-up of cyclic load–drag–pull tests, schematic of the LPMR (miniature robot) and additional images demonstrating climbing of the LPMR on inclined surface, calculation of critical aspect ratio of the fibrillar arrays and calculations of the effective stiffness of micro-pillar, and a summary of hierarchical array based dry-adhesives reported in literature. 15 ACS Paragon Plus Environment


Acknowledgement H.K.R would like to thank Professor Ramakrishnan Ganesan of the Birla Institute of Technology and Science (BITS) Pilani, Hyderabad Campus (India) for his enthusiastic discussion and helpful comments. H.K.R would also like to thank Dr. Jaslyn Bee Khuan Law and Mr. Abram Tan Jia Han of the Ngee Ann polytechnic, Singapore for their assistance in assembling the cyclic adhesion test apparatus. This study is based on STARS (Systems Technology for Autonomous Reconnaissance and Surveillance) project funded by Temasek Laboratories@SUTD and with complementary support by the SUTD-MIT International Design Centre (IDC, idc.sutd.edu.sg). References [1] Maderson, P. F. A. Keratinized Epidermal Derivatives as an Aid to Climbing in Gekkonid Lizards. Nature 1964, 203, 780–781. [2] Russell, A. P. A. A Contribution to the Functional Analysis of the Foot of the Tokay, Gekko Gecko (Reptilia: Gekkonidae). J. Zool. 1975, 176, 437–476. [3] Boesel, L. F.; Greiner, C.; Arzt, E.; Del Campo, A. Gecko-Inspired Surfaces: A Path to Strong and Reversible Dry Adhesives. Adv. Mater. 2010, 22, 2125-2137. [4] Autumn, K.; Majidi, C.; Groff, R. E.; Dittmore, A.; Fearing, R. Effective Elastic Modulus of Isolated Gecko Setal Arrays. J. Exp. Biol. 2006, 209, 3558-3568. [5] Heinzmann, C.; Weder, C.; Montero de Espinosa, L. Supramolecular Polymer Adhesives: Advanced Materials Inspired by Nature. Chem. Soc. Rev. 2016, 45, 342-358. [6] Dahlquist, C. A. In Treatise on Adhesion and Adhesives; Patrick, R. L. Eds.; Marcel Dekker, NY, 1969; Chapter 2, pp. 219 -260. [7] Ruibal, R.; Ernst, V.; The Structure of the Digital Setae of Lizards. J. Morphol. 1965, 117, 271 -294. [8] Williams, E. E.; Peterson, J. A. Convergent and Alternative Designs in the Digital Adhesive Pads of Scincid Lizards. Science 1982, 215, 1509 -1511. [9] Rizzo, N. W.; Gardner, K. H.; Walls, D. E. A.; Keiper-Hrynko, N. M.; Ganzke, T. S.; Hallahan, D. L. Characterization of the Structure and Composition of Gecko Adhesive Setae. J. R. Soc. Interface. 2006, 3, 441-451. [10] Autumn, K.; Liang, Y. A.; Hsieh, S. T.; Zesch, W.; Chan, W. P.; Kenny, T. W.; Fearing, R.; Full, R. J. Adhesive Force of a Single Gecko Foot-Hair. Nature 2000, 405, 681–684. 16 ACS Paragon Plus Environment

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[11] Autumn, K.; Sitti, M.; Liang, Y. A.; Peattie, A. M.; Hansen, W. R.; Sponberg, S.; Kenny, T. W.; Fearing, R.; Israelachvili, J. N.; Full, R. J. Evidence for Van der Waals Adhesion in Gecko Setae. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12252-12256. [12] Yu, J.; Chary, S.; Das, S.; Tamelier, J.; Pesika, N. S.; Turner, K. L.; Israelachvili, J. N. Gecko‐Inspired Dry Adhesive for Robotic Applications. Adv. Funct. Mater. 2011, 21, 30103018. [13] Li, Y.; Krahn, J.; Menon, C. Bioinspired Dry Adhesive Materials and Their Application in Robotics: A Review. J. Bionic. Eng. 2016, 13, 181-199. [14] Gecko Feet Inspire Climbing Space Robots, www.space.com/30258-nasa-gecko-spacerobot-sticky-feet.html, (accessed November 2017). [15] Kwak, M. K.; Jeong, H. E.; Suh, K. Y. Rational Design and Enhanced Biocompatibility of a Dry Adhesive Medical Skin Patch. Adv. Mater. 2011, 23, 3949-3953. [16] Song, S.; Sitti, M. Soft Grippers Using Micro-fibrillar Adhesives for Transfer Printing. Adv. Mater. 2014, 26, 4901-4906. [17] Greiner, C.; Arzt, E.; del Campo, A. Hierarchical Gecko-Like Adhesives. Adv. Mater. 2009, 21, 479-482. [18] Jeong, H. E.; Lee, J.-K.; Kim, H. N.; Moon, S. H.; Suh, K. Y. A Nontransferring Dry Adhesive with Hierarchical Polymer Nanohairs. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 5639– 5644. [19] Kustandi, T. S.; Samper, V. D.; Ng, W. S.; Chong, A. S.; Gao, H. Fabrication of a GeckoLike Hierarchical Fibril Array Using a Bonded Porous Alumina Template. J. Micromech. Microeng. 2007, 17, N75. [20] Ho, A. Y. Y.; Yeo, L. P.; Lam, Y. C.; Rodríguez, I. Fabrication and Analysis of GeckoInspired Hierarchical Polymer Nanosetae. ACS Nano 2011, 5, 1897-1906. [21] Masuda, H.; Yada, K.; Osaka, A. Long-Range-Ordered Anodic Porous Alumina with LessThan-30 nm Hole Interval. Jpn. J. Appl. Phys. 1998, 37, L1340. [22] Ono, S.; Saito, M.; Asoh, H. Self-ordering of Anodic Porous Alumina Formed in Organic Acid Electrolytes. Electrochim. Acta. 2005, 51, 827-833. [23] Murphy, M. P.; Kim, S.; Sitti, M.; Enhanced Adhesion by Gecko-Inspired Hierarchical Fibrillar Adhesives. ACS Appl. Mater. Interfaces 2009, 1, 849-855.



[24] Röhrig, M.; Thiel, M.; Worgull, M.; Hölscher, H. 3D Direct Laser Writing of Nano- and Microstructured Hierarchical Gecko-Mimicking Surfaces. Small 2012, 8, 3009-3015. [25] Lee, D. Y.; Lee, D. H.; Lee, S. G.; Cho, K. Hierarchical Gecko-Inspired Nanohairs with a High Aspect Ratio Induced by Nanoyielding. Soft Matter 2012, 8, 4905-4910. [26] Lee, H.; Bhushan, B. Effect of Hair Morphology and Elastic Stiffness on the Wetting Properties of Hairy Surfaces. J. Colloid Interface Sci. 2012, 372, 231-238. [27] Zhang, Y.; Lin, C.T.; Yang, S. Fabrication of hierarchical pillar arrays from thermoplastic and photosensitive SU‐8. Small 2010, 6, 768-775. [28] Raut, H. K.; Dinachali, S. S.; Loke, Y. C.; Ganesan, R.; Ansah-Antwi, K. K.; Gora, A.; Khoo, E. H.; Ganesh, V. A.; Saifullah, M. S.; Ramakrishna, S. Multiscale Ommatidial Arrays with Broadband and Omnidirectional Antireflection and Antifogging Properties by Sacrificial Layer Mediated Nanoimprinting. ACS Nano 2015, 9, 1305-1314. [29] Guo, L. J.; Nanoimprint Lithography: Methods and Material Requirements. Adv. Mater. 2007, 19, 495-513. [30] Raut, H. K.; Dinachali, S. S.; He, A. Y.; Ganesh, V. A.; Saifullah, M. S.; Law, J.; Ramakrishna, S. Robust and Durable Polyhedral Oligomeric Silsesquioxane-Based Antireflective Nanostructures with Broadband Quasi-Omnidirectional Properties. Energy Environ. Sci. 2013, 6, 1929-1937. [31] Raut, H. K.; Ganesh, V. A.; Nair, A. S.; Ramakrishna, S. Anti-Reflective Coatings: A Critical, In-Depth Review. Energy Environ. Sci. 2011, 4, 3779-3804. [32] Yuan, B.; Li, Y.; Wang, D.; Xie, Y.; Liu, Y.; Cui, L.; Jiang, X. A General Approach for Patterning Multiple Types of Cells Using Holey PDMS Membranes and Microfluidic Channels. Adv. Func. Mater. 2010, 20, 3715-3720. [33] Sitti, M.; Fearing, R. S. Synthetic Gecko Foot-Hair Micro/Nano-structures as Dry Adhesives. J. Adhes. Sci. Technol. 2003, 17, 1055-1073. [34] Glassmaker, N. J.; Jagota, A.; Hui, C. Y.; Kim, J. Design of Biomimetic Fibrillar Interfaces: 1. Making Contact. J. R. Soc. Interface. 2004, 1, 23-33. [35] Gao, H.; Yao, H. Shape Insensitive Optimal Adhesion of Nanoscale Fibrillar Structures. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 7851-7856. [36] Xia, Z. In Biomimetic Principles and Design of Advanced Engineering Materials; John Wiley & Sons: Chichester, UK, 2016; Chapter 5, pp. 101-109. 18 ACS Paragon Plus Environment

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[37] del Campo, A.; Greiner, C. SU-8: A Photoresist for High-Aspect-Ratio and 3D Submicron Lithography. J. Micromech. Microeng. 2007, 17, R81. [38] Lee, J.; Majidi, C.; Schubert, B.; Fearing, R. S. Sliding-Induced Adhesion of Stiff Polymer Microfibre Arrays. I. Macroscale Behaviour. J. R. Soc. Interface 2008, 5, 835-844. [39] Gillies, A. G.; Fearing, R. S. Shear Adhesion Strength of Thermoplastic Gecko-Inspired Synthetic Adhesive Exceeds Material Limits. Langmuir 2011, 27, 11278-11281. [40] Hansen, W. R.; Autumn, K. Evidence for Self-Cleaning in Gecko Setae. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 385-389. [41] Bhushan, B.; Peressadko, A.G.; Kim, T.W. Adhesion Analysis of Two-Level Hierarchical Morphology in Natural Attachment Systems for 'Smart Adhesion'. J. Adhes. Sci. Technol. 2006, 20, 1475-1491. [42] Lee, W.L.; Low, H.Y.; Ortiz, C. Geometry-Dependent Compressive Responses in Nanoimprinted Submicron-Structured Shape Memory Polyurethane. Soft Matter, 2017, 13, 33143327. [43] Han, L.; Wang, L.; Chia, K.K.; Cohen, R.E.; Rubner, M.F.; Boyce, M.C.; Ortiz, C. Geometrically Controlled Mechanically Responsive Polyelectrolyte Tube Arrays. Adv. Mater., 2011, 23, 4667-4673. [44] Guruprasad, T.S.; Bhattacharya, S.; Basu, S. Size Effect in Microcompression of Polystyrene Micropillars. Polymer, 2016, 98, 118-128. [46] Wang, L.; Ortiz, C.; Boyce, M.C. Mechanics of Indentation into Micro-and Nanoscale Forests of Tubes, Rods or Pillars. J. Eng. Mater. Technol., 2011, 133, p.011014. [47] Rodriguez, I.; Lim, C. T.; Natarajan, S.; Ho, A. Y. Y.; Van, E. L.; Elmouelhi, N.; Low, H. Y.; Vyakarnam, M.; Cooper, K. Shear Adhesion Strength of Gecko Inspired Tapes on Surfaces with Variable Roughness. J. Adhes. 2013, 89, 921-936. [48] Kim, S.; Sitti, M.; Hui, C.Y.; Long, R.; Jagota, A. Effect of Backing Layer Thickness on Adhesion of Single-Level Elastomer Fiber Arrays. Appl. Phys. Lett., 2007, 91, 161905. [49] Long, R.; Hui, C.Y.; Kim, S.; Sitti, M. Modeling the Soft Backing Layer Thickness Effect on Adhesion of Elastic Microfiber Arrays. J. Appl. Phys., 2008, 104, 044301. [50] Pooley, C.M.; Tabor, D. Friction and Molecular Structure: The Behaviour of Some Thermoplastics. Proc. Royal Soc. A, 1972, 329,251-274.



[51] Johnson, K. L. Adhesion and Friction Between a Smooth Elastic Spherical Asperity and a Plane Surface. Proc. R. Soc. A. 1997, 453, 163–179. [52] Rong, Z.; Zhou, Y.; Chen, B.; Robertson, J.; Federle, W.; Hofmann, S.; Steiner, U.; Goldberg‐Oppenheimer, P. Bio‐Inspired Hierarchical Polymer Fiber–Carbon Nanotube Adhesives. Adv. Mater. 2014, 26, 1456-1461. [53] Kim, S.; Spenko, M.; Trujillo, S.; Heyneman, B.; Mattoli, V.; Cutkosky, M.R. Smooth Vertical Surface Climbing with Directional Adhesion. In IEEE Transactions on Robotics and Automation, Roma, Italy, February 2008; IEEE Robotics and Automation Society, 2008; Vol. 24, pp 1268-1273. [54] Unver, O.; Uneri, A.; Aydemir, A.; Sitti, M. Geckobot: A Gecko Inspired Climbing Robot Using Elastomer Adhesives. In Proceedings of the 2006 IEEE International Conference on Robotics and Automation, Orlando, USA, June 2006; pp 2329-2335. [55] Aksak, B.; Murphy, M. P.; Sitti, M. Gecko Inspired Micro-Fibrillar Adhesives for Wall Climbing Robots on Micro/Nanoscale Rough Surfaces. Proceedings of the 2008 IEEE International Conference on Robotics and Biomimetics, Pasadena, USA, June 2008; pp 30583063. [56] Asbeck, A.; Dastoor, S.; Parness, A.; Fullerton, L.; Esparza, N.; Soto, D.; Heyneman, B.; Cutkosky, M. Climbing Rough Vertical Surfaces with Hierarchical Directional Adhesion. Proceedings of the 2009 IEEE international conference on Robotics and Automation, Kobe, Japan, July 2009; pp 2675-2680. [57] Bartlett, M.D.; Croll, A.B.; King, D.R.; Paret, B.M.; Irschick, D.J.; Crosby, A.J. Looking Beyond Fibrillar Features to Scale Gecko‐Like Adhesion. Adv. Mater. 2012, 24, 1078-1083. [58] Hariri, H. H.; Prasetya, L. A.; Foong, S.; Soh, G. S.; Otto, K. N.; Wood, K. L. A TetherLess Legged Piezoelectric Miniature Robot Using Bounding Gait Locomotion for Bidirectional Motion. In Proceedings of the 2016 IEEE International Conference on Robotics and Automation, Stockholm, Sweden, May 2016; pp. 4743-4749. [59] Hariri, H. H.; Soh, G. S.; Foong, G. S.; Wood, K. L.; Otto, K. Miniature Piezoelectric Mobile Robot Driven by Standing Wave. In Proceedings of the 14th IFToMM World Congress, Taipei, Taiwan, October 2015; pp. 325-330. [60] Sneddon, I. N. The Relation Between Load and Penetration in the Axisymmetric Boussinesq Problem for a Punch of Arbitrary Profile. Int. J. Eng. Sci. 1965, 3, 47-57. 20 ACS Paragon Plus Environment

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FIGURES


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Figure 1: Fabrication of gecko-inspired hierarchical arrays by sacrificial-layer mediated nanoimprinting (SLAN). Process step shown in the schematic are: (a) Imprinting of micropillar arrays on a flexible polycarbonate sheet. (b) Formation of a sacrificial layer of poly(sodium 4styrenesulfonate) (PSS) encapsulating the micropillar arrays. (c) Imprinting of fibrillar arrays over the micropillar structures secured within the sacrificial layer, using a porous anodized alumina template. The sacrificial layer protects the micropillar arrays from deforming during the second imprinting stage. (illustrated in the inset) (d) Dissolution of the sacrificial layer by immersion in water, thereby exposing the hierarchical arrays.



Figure 2: SEM images of the fibrillar and hierarchical arrays. (a) Densely packed fibrillar arrays of diameter ~ 200 nm and aspect-ratio (λ) = 10 fabricated on polycarbonate sheets. The inset depicts a cross-section view of the fibrillar arrays showing free-standing and upright fibrils. The scale bar for the inset is the same as that of (a). (b) Low magnification SEM showing hierarchical arrays with very high yield fabricated over a larger area by the sacrificial layer mediated nanoimprinting (SLAN) technique. Cross-section view of hierarchical arrays with the same micropillar diameter (3 µm) and two different aspect-ratios (of micro-pillars): (c) λM = 2 and (d) λ M = 4, respectively.


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Figure 3: Shear adhesion performance of hierarchical arrays versus fibrillar arrays. (a) Shear force versus sliding displacement curves recorded for fibrillar arrays and non-patterned polycarbonate sheets. (b) Shear force versus sliding displacement curves of hierarchical arrays with aspect ratios, λM = 2 and 4, respectively. It is evident that the maximum shear force



measured for hierarchical arrays with λM = 4 is higher than that of the hierarchical arrays with λM = 2.

Figure 4: Adhesion of hierarchical arrays on different surfaces. (a) Comparison of shear adhesion exhibited by fibrillar arrays and hierarchical arrays of different aspect-ratios, on polished Si and glass. The error bars indicate the variation in shear adhesion observed for 5 specimens of each type on the respective adherents. (b) Shear force recorded for the hierarchical arrays (λM = 4) during each cycle of the cyclic load–drag–pull experiments. The inset in the graph shows the directional tilting of the fibril present in the hierarchical arrays during the shear adhesion experiment.


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Figure 5: Demonstration of robotic climbing on an inclined surface. (a) Time-lapse photographs of legged piezoelectric miniature robot (LPMR) climbing on a glass surface, with successively increasing angles of inclination (θ). The LPMR equipped with the hierarchical arrays can be seen to climb the surface until θ reached slightly above 30°. (b) LPMR equipped with non-patterned planar films, can be seen to climb only up to a shallow θ (b2) and with further increase in θ, the LPMR can be seen to slide down (b3-b4).



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Gecko-Inspired Dry Adhesive Based on MicroâNanoscale Hierarchical

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