Slanted Functional Gradient Micropillars for Optimal Bioinspired Dry

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Slanted Functional Gradient Micropillars for Optimal Bioinspired Dry Adhesion Zhengzhi Wang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b07493 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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Slanted Functional Gradient Micropillars for Optimal Bioinspired Dry Adhesion Zhengzhi Wang* *

Department of Engineering Mechanics, School of Civil Engineering, Wuhan University, Wuhan, Hubei 430072, China E-mail: [email protected] ABSTRACT For biologically-inspired dry adhesives, the fibrillar structure of the surface requires sufficient flexibility to form contacts and meanwhile high rigidity to maintain stability. This fundamental conflict has greatly hindered the advance of the synthetic

adhesives towards mass-scale and practical applications, where adhesion is desired to be simultaneously strong, durable, directional, and roughness-adaptive. In this work, we overcome such a long-term challenge by developing fibrillar structures that combine both slanted geometry and gradient material of micropillars. The termed slanted functional gradient pillars (s-FGPs), fabricated by a magnetically-assisted mold replication technique, exhibit flexible tips for contacts, gradually-stiffened stalks for reinforcement, slanted structure to give rise to anisotropy, and high aspect ratio (AR) to facilitate surface adaptation. We demonstrate that the material and structure of the sFGPs complement each other, synergetic effects of which result in a multifunctional combination of adhesion properties including high strength (~ 9 N/cm2 in shear), ultradurability (over 200 cycles of attachment/detachment without adhesion degradation), super anisotropy (anisotropic ratio of ~ 7), and good adaptability to rough surfaces. The s-FGPs not only step forward the bioinspired adhesion towards optimized designs and performances for practical applications, they may also open up other concepts for various high-AR and structurally-stable fibrillar surfaces with emerging functionalities and applications in the fields of self-cleaning, superhydrophobicity, biosensors, energy harvesting, etc.

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KEYWORDS: bioinspired dry adhesives, micropillars, functional gradient nanocomposite, mechanical compliance, structural stability, ‘gecko’ adhesion

Animals have evolved various adhesion systems to adapt themselves to the complex natural environments.1 One of the most successful system that has been employed widely among species including lizards, spiders and insects features the patterned hairy (or fibrillar) structures.2, 3 Such compliant structures, usually spanning in the micro- to nano-scales and exhibiting extremely high aspect ratios (ARs), facilitate intimate contact between the adhesive pads and the attaching substrates and thus increase the attractive effect of the intermolecular surface forces.4-6 This geometry-dominated adhesion principle (also known as ‘contact splitting’)2,6,7 has inspired numerous attempts aiming at developing biomimetic dry adhesives by replicating the fibrillar micro-nano structures using synthetic materials, e.g. soft polymers and carbon nanotubes (see recent review papers and book chapters).8-14 Despite great successes have been achieved in this field over the past one and half decades,15-25 there are still two major challenges that limit the advancement of bioinspired structured adhesives towards mass-scale and practical applications. The first challenge stems from the fact that the mechanical compliance and the structural stability of hairy structures are mutually exclusive. As a result, hairs demanding flexibility for a high adhesion strength and/or a good surface adaptability usually suffer from a poor structural integrity and thus a low durability.11,13,15 To overcome this trade-off, composite micro/nano-pillar structures based on a combination of stiff and soft materials have been recently proposed and widely spread.26-38 In these structures, the soft material is used as the pillar tips or outside shells for contacts while the stiff material as the pillar stalks or inside cores for reinforcements. Such composite pillars indeed show enhanced adhesion and structural stability compared with the ones made of pure material.26-38 However, these pillars containing sharp transitions between contrasting materials inevitably risk interfacial failures and impair the long-term durability during cyclic deformations.39,40 The second challenge for the structured dry adhesives is the active control over the adhesion anisotropy for not only efficient 2


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attachment but also easy detachment.11,13 Emulating the design principle of the tilting setae and the spatula-shaped tips in gecko’s toe pads,4,41,42 slanted micro/nano hairs43-56 and asymmetric tip shape modifications57-62 have been extensively exploited to generate directional adhesion (i.e. strong adhesion in one direction for gripping and weak adhesion in another for releasing). The slanted structures have proven to be effective for controllable and anisotropic adhesion;63 however, such structures, particularly for high AR ones, are very vulnerable to lateral collapses due to the greatly-reduced effective stiffness64 and the significant effects of surface forces.65 This limit in increasing the structure’s AR further compromises the adaptability of the hairs against “real world” surfaces which usually encompass multiple scales of roughness.6,66-69 These two challenges combined have prevented the current biomimetic dry adhesives from achieving optimal adhesion performances for practical applications, where a combination of all the abovementioned properties (i.e. strong, durable, anisotropic, and roughness-adaptive) is desired.11,42 To this end, here we develop slanted functional gradient pillars (s-FGPs) that simultaneously address both challenges for controllable, directional and robust bioinspired adhesion. Such s-FGPs achieve both geometry (slanted and high AR) and material characteristics (elastic modulus gradually increase from pillar tip to base without introducing interfaces) for the biomimetic microstructures. The material gradient of the s-FGPs mimics the gradient tarsal setae of the ladybird beetle,70 which has been revealed lately as a product of evolutionary optimization.39,40 We experimentally demonstrate that the integrated geometry and material designs of the s-FGPs give rise to multifunctional combination of the critical adhesion properties that truly resemble the natural prototypes. The concept of s-FGPs and the fabrication method reported here can be extended to many other fields for highAR, structurally-stable and directional micropatterned surfaces. RESULTS Fabrication and characterizations of s-FGPs. The s-FGPs were fabricated through a modified mold replication process as shown schematically in Figure 1 and detailed in the Methods. Briefly, a silicon master with patterned angled cavities (leaning angle, 20°; 3


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diameter, 2.5 μm; AR, 8) was first filled with homogeneous, UV-curable polyurethane acrylate (PUA) based nanocomposites. The composites comprised 15 vol.% superparamagnetic iron oxide nanoparticles coated with functionalized silica shells (Fe3O4@SiO2, total diameter of ~ 20 nm) as the reinforcements. The magneticresponsive nano-reinforcements inside the mold cavities were then redistributed by applying a magnetic field gradient along the axial direction of the cavities. Our previous study showed that by properly adjusting the magnetic field intensity following a driftdiffusion theory (here ~ 150 mT, see details in Text S1, Supporting Information),71 a smoothly-gradient distribution of the nano-reinforcements within small volumes could be achieved. The s-FGPs were finally obtained by solidifying the redistributed nanocomposite and peeling them off from the mold. As the controls, slanted pillars made up of non-gradient material (either pure PUA or homogeneous PUA/Fe3O4@SiO2 nanocomposite (15 vol.%)), together with the respective non-slanted (i.e. vertical) counterparts and flat (i.e. unpatterned) PUA film were also fabricated. Figure 2 shows the results of morphological and local mechanical characterizations for the various micropillars fabricated. As can be seen from the scanning electron microscope (SEM) images shown in Figure 2a and 2d, compliant pillars (CPs) made up of pure polymer could not maintain the original straight shapes due mainly to elastocapillarity, self-adhesion and/or self-gravity effects.65 The vertical CPs (v-CPs, Figure 2a) presented significant clustering while the slanted CPs (s-CPs, Figure 2d) showed curved and partially-collapsed structures towards the leaning direction. By contrast, both vertical and slanted stiff pillars (v-SPs and s-SPs, Figure 2b and 2e) made up of uniformly-reinforced nanocomposite exhibited perfectly-aligned structures without any structural failures observed. The FGPs (both v-FGPs and s-FGPs, Figure 2c and 2f), similarly, maintained stably-aligned structures with only slightly-curved tips towards the leaning direction for the s-FGPs. Such curved tips were advantageous to further amplify the adhesion anisotropy of the slanted pillars, which will be demonstrated later. Notably, the structurally-stable FGPs could be fabricated at an even higher AR of ~ 12 without involving pillar collapse or clustering (Figure S1, Supporting 4


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Information), highlighting that the gradient design in material well circumvented the maximum AR limit that homogeneous pillars usually suffered due to the inherent surface mechanics of fibrillar structure (Text S2, Supporting Information).65,72 These super-slender yet still flexible micropillars, which could not be achieved with any fabrication techniques reported previously, were expected to present superior adaptability to rough surfaces.66-69 Material gradient of the FGPs could be visualized by comparing the transmissional optical images shown in the insets of Figure 2a-f, where the pure polymeric CPs exhibited a higher transparency (visually brighter) than the nanocomposite SPs while the FGPs showed a gradual change in the transparency from the bases to the tips. Site-specific nanoindentation tests were performed along individual pillars from the tip to the base to characterize the different distributions of the local elastic modulus. According to the load-depth curves shown in Figure 2g, the CPs and SPs showed consistent results at different positions along the pillars, indicating uniform distributions of the local stiffness. In contrast, for the same indentation depth, both the peak load and the residual indentation depth after unloading gradually increased as the indentation site moved from the pillar tip to the base of the FGPs. This validated a gradient distribution of the local stiffness for the FGPs and manifested that the addition of nano-reinforcements altered the relative elasticity/plasticity behaviors of the polymer composites.73 Elastic modulus extracted from these curves quantified the mechanical gradient of the FGPs (Figure 2h-i), continuously transitioning from ~ 20 MPa for the tip (corresponding to the cross-linked PUA of CPs) to ~ 110 MPa for the base. Comparatively, the SPs exhibited uniform elastic modulus of ~ 60 MPa. The different distributions of the elastic modulus correlated with the different levels of reinforcement inside the pillars. As shown in Figure S2 in the Supporting Information, transmission electron microscope (TEM) observations directly showed that the core-shell nanoparticles uniformly distributed inside the SPs while gradually distributed from the tip (pure PUA) towards the base (highly-filled nanocomposite) inside the FGPs. Directional adhesion of s-FGPs. Adhesion anisotropy of the various microstructures 5


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was evaluated with a typical non-conformal load-drag-pull test. A glass hemispherical indenter (diameter of 4 mm) was first pressed into contact with the sample pillars until a pre-specified load (4 mN) was reached (see Figure 3a for schematic illustrations). Following that, a shear displacement either along (‘gripping direction’) or against (‘releasing direction’) the leaning direction of the slanted pillars was applied to the indenter at a speed of 5 μm/s for 100 μm while the normal displacement kept unchanged. For the vertical control samples, the shear displacement was applied along two random opposite directions (direction 1 and direction 2 in Figure S3a, Supporting Information). The indenter was finally retracted away from the sample until complete detachment. Normal and shear forces were recorded simultaneously during the load-drag processes and shown together in Figure 3b for slanted pillars and Figure S3b for vertical ones. All the vertical pillars showed isotropic behaviors as both the normal and shear forces during dragging were nearly identical along the two opposite directions (Figure S3b). The slanted pillars, on the contrary, presented apparently anisotropic behaviors with the shear forces in the gripping direction greatly higher than those in the releasing direction (Figure 3b). The normal forces in the gripping direction were observed to be adhesive while those in the releasing direction were compressive. These different types of normal forces agreed well with previous studies on slanted pillars and indicated that Coulomb friction contributed partly to the shear forces in the releasing direction while the shear component of adhesion dominated those in the gripping direction.44,45,49,54 Comparing the maximum shear forces of the pillars tested as summarized in Figure 3c, the s-FGPs showed almost the highest shear adhesion forces in the gripping direction (25.1 ± 3.0 mN, comparable to 26.7 ± 3.5 mN of the s-CPs) while the lowest shear forces in the releasing direction (3.5 ± 1.5 mN). As a result, the s-FGPs exhibited an extremely high anisotropy ratio (defined as the ratio of the maximum shear force in the gripping direction to that in the releasing direction) of ~ 7.2 as compared to ~ 2.0 of the s-CPs, ~ 1.9 of the s-SPs and ~ 1.0 (i.e. isotropy) of all the vertical pillars. Such a value of anisotropy was much higher than that reported for the gecko setae (~ 4.5).4,74 The similar adhesion forces (both normal and shear) of the s-FGPs and the s-CPs in the 6


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gripping direction suggested that the flexibility of the pillars, especially the tip regions that determined the actual contact area, played predominant roles for the adhesion provided no structural failures occurred. Therefore, the s-SPs and v-SPs, due to the reduced flexibility in the tips, showed much lower adhesion compared with the respective CP and FGP counterparts; the v-CPs, due to the structural collapse and pillars clustering, possessed significantly reduced adhesion compared with the v-FGPs. During dragging along the releasing direction of the s-FGPs, the gradually-stiffened pillar stalks and the curved pillar tips (Figure 2f) combined provided significant resistances for the pillars to conform with the indenter. As a result, compression forces were required to maintain the normal displacement and the actual contact between the indenter and the s-FGPs were rather limited (see Figure 3a for illustrations). Similar scenarios also applied to the s-SPs. The s-CPs in the releasing direction, due to the flexibility of the entire pillars, could still conform to the indenter to some extent and thus showed a high shear force of 13.2 ± 2.4 mN. When the results came to the pull-off forces (i.e. maximum normal adhesive force during retraction), the differences between the various samples tested were much less significant. As shown in Figure S4 in the Supporting Information, pull-off forces for the various pillars became relatively indistinguishable and all the forces were comparable to that of the unpatterned PUA film (within a range of ~ ±30%). These different bahaviors of the normal vs. shear adhesion may be correlated with a recent study by Varenberg et al.,7 which demonstrates that the ‘contact splitting’ principle only works for side contact (corresponding to the shear adhesion) or thin-film-terminated contact but not the contact in the normal pull-off direction if the true contact area does not change. The pull-off adhesion can be significantly enhanced by modifying the pillar tips to spatula- or mushroom-shape, as has been widely validated in the literature. 14,21,49,54,67,75,76

For the present study, however, we limit our focuses within the shear

adhesion and the multifunctionality of the s-FGPs as will be further demonstrated below. Strong and durable adhesion of s-FGPs. Macroscopic adhesion capacity and durability of the various micropillars were tested using a home-built hanging setup 7


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under a preload of 1 N/cm2, shown schematically in Figure 4a. It should be noted that only shear adhesions were tested here considering the relatively weak pull-off forces shown above. For the slanted pillars, the tests were performed repeatedly along the gripping direction while for the vertical pillars and the flat control sample, the tests were performed consistently along one selected direction. As shown in Figure 4b for the results of 200 repeated tests, shear adhesion of all the CPs and SPs degraded rapidly with the evolving cycle of attachment/detachment. The critical cycle to the adhesion degradation varied from ~ 10 for the s-CPs to ~ 60 for the v-SPs. The FGPs (both vFGPs and s-FGPs), by contrast, showed remarkable durability as the shear adhesion maintained nearly the same level even after 200 cycles of attachment/detachment. The mechanisms for the degraded adhesion of the CPs and SPs and the durable adhesion of the FGPs could be deduced based on the morphology characterizations after repeated tests combined with finite element analyses (FEA) as the results shown together in Figure 4c-h. The CPs (Figure 4c and d), particularly the s-CPs, completely collapsed due to the high flexibility and deformability of the PUA thus lost the ability to conform in later cycles of attachment. The SPs gradually lost their adhesion caused by pillars breaking-off in the basal regions (Figure 4e and f), which should be attributed to both the decreased deformability of the polymer after incorporating with inorganic nanoparticles and the considerable stress concentrations located at the pillar bases upon shear loading. The FGPs (Figure 4g and h), strikingly, kept perfectly-aligned structures with the compliant tips bent towards the shearing direction for conformal contacts and the stiff bases maintained almost undeformed for strong supports. Compared with the SPs, the stress concentrations at the basal regions of FGPs were largely alleviated as the tips sustained most of the strains. Comparing the shear adhesion capacity of the various pillars shown in Figure 4b (before degradation if any), the v-CPs showed very limited capacity of ~ 2.2 N/cm2 (close to 1.9 N/cm2 of the unpatterned PUA film), which could be attributed to the incipient pillars clustering that reduced the actual contact area. The SPs also presented rather low capacities (~ 2.9 N/cm2 for the v-SPs and ~ 3.7 N/cm2 for the s-SPs) despite 8


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the pillar structures were originally orderly-aligned. This was due to the reduction in both the material’s intrinsic adhesion and the structural compliance after the addition of nano-reinforcements. By adapting the homogeneous SPs into FGPs without changing the material compositions and fractions, the shear adhesion drastically increased to ~ 5.8 N/cm2 for the v-FGPs and ~ 8.8 N/cm2 for the s-FGPs. These changes in shear adhesion capacity together with the varied adhesion anisotropy as measured in the loaddrag-pull tests (Figure 3c) should be mainly ascribed to the changes in the true contact area, which can be qualitatively evaluated by comparing the effective elastic modulus (Eeff) of the pillar arrays in the contact direction (i.e. normal to the substrate). Following Autumn et al.’s cantilever model,64 we calculated the Eeff of the homogeneous pillar arrays (i.e. CPs and SPs) by:

Eeff 

3EID sin( ) L 2 cos 2( )[1   tan( )]

(1)

where E is the elastic modulus of the pillar material, I the moment of inertia, D the pillar density, γ the tilting angle of the pillar with respect to the supporting substrate, L the pillar length, and μ the frictional coefficient with the positive/negative sign corresponding respectively to sliding in the gripping/releasing direction. Using a similar theory, we derived a model for the Eeff of the FGP array by assuming a linear transition of the elastic modulus from pillar tip (E1) to base (E2) as (Text S3, Supporting Information):

Eeff ( FGPs) 

where C 

DL sin( ) (2) C cos ( )[1   tan( )] 2

E12 ln( E1 )  E1E2 ln( E2 ) E1L(1  ln( E2 )) L2   is the effective compliance Is3 Is 2 2 Is

of a single FGP with s  ( E2  E1 ) / L being the slope of the modulus transition. By adapting the relevant parameters into Equations (1) and (2), we obtained the Eeff of the various slanted pillar arrays as listed in Table 1. As can be seen, all the slanted pillars exhibited Eeff in the gripping direction much lower than those in the releasing direction. As a result, the slanted pillars presented significant anisotropic behaviors with the 9


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gripping shear adhesion far exceeding the releasing one. The s-FGPs presented significantly lower Eeff than the s-SPs in the gripping direction, which facilitated a larger true contact area and thus a higher shear adhesion strength. It should be noted that a constant tilting angle (i.e. γ = 70°) was adopted in the Eeff calculations for the s-FGPs. According to the SEM image shown in Figure 2f, however, the pillar tips curved towards the tilting direction of the FGPs, which effectively reduced the tilting angle and would further decrease the Eeff in the gripping direction while increase that in the releasing direction. The combined effects led to the extreme adhesion anisotropy of the s-FGPs as measured in the load-drag-pull tests (Figure 3b-c). Adaptive adhesion of s-FGPs. Surface adaptability of the various micropillars were tested using the same hanging setup shown in Figure 4a except that the glass substrate was replaced by a pretreated steel one (nonmagnetic). The steel substrate was roughened with a series of sandpapers and the surface exhibited a root mean square (RMS) roughness of ~ 590 nm (compared to ~ 6.8 nm of the glass used in the durability tests), a mean distance between adjacent asperity peaks of ~ 2.2 μm, and a mean peakto-valley height of ~ 2.8 μm (see inset in Figure 5a for a representative surface profile). The comparable sizes of the surface asperity and the pillar diameter were chosen according to a recent study,69 in order to avoid the nonadhesive regime caused by pillars over-bending and -buckling. As can be seen from the averaged results shown in Figure 5a, all the micropatterned surfaces showed much higher shear adhesion capacity than the unpatterned one, indicating the principle of “contact splitting” that widely prevailed for smooth substrates applied well, even better to rough substrates. Among the various micropillars, the relative ranking in terms of the shear adhesion capacity was similar to that measured against smooth glass (Figure 4b) with the apparent trend that all the slanted structures again outperformed the respective vertical counterparts. These rankings could be generally explained by the variations in the material’s intrinsic adhesion, the structural integrity, and the effective elastic modulus as detailed above. Notably, the s-FGPs presented substantial shear adhesion of 5.2 ± 0.7 N/cm2 against the rough substrate, which retained ~ 59 % of that against the smooth glass. The 10


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superior roughness adaptability of the s-FGPs could be credited to three factors that worked cooperatively. Firstly, the high AR of the structure allowed each pillar to deform independently that facilitated sufficient pillar-asperity adaptations. Secondly, the flexible pillar tips and slanted layout favored intimate tip-asperity contacts with only small elastic deformations of the entire structure. Last but not the least, the gradientlystiffened pillar stalks effectively avoided structural collapse and pillars bending/ buckling that would otherwise store much more energy and reduce the contact area.69 As a demonstration of the macroscale shear adhesion of the s-FGPs, a small sample (~ 1 cm2) supported on a polyethylene terephthalate (PET) backing layer was attached to a smooth compact disk surface (made of polycarbonate) and the combination could support a weight of 500 g in pure shear along the gripping direction (Figure 5b). As a comparison, the same sample attached to a roughened steel surface (the substrate used in the adaptability tests) was able to support 250 g (Figure 5c). For both attachments, the s-FGPs sample could sustain the load for a long time (over 2 h in tests) and a simple lift-up movement (i.e. shear along the releasing direction) gave rise to a fast and effortless detachment. Such attachment/detachment could be repeated for extensive cycles without significant loss of the supporting weight. Controllable slanted microstructures. It is worth emphasizing that although the slanted micropillars presented in this study were fabricated through a well-established casting method using a mold with fixed-angle cavities, the nanocomposite pillars here, due to their magnetic-responsivity, provided an alternative and more controllable way to obtain slanted microstructures without using sophisticated molds. As demonstrated in Figure S5 in the Supporting Information, the vertical pillars made up of homogeneous magneto-nanocomposite (i.e. the v-SPs) could be controlled to different bent configurations by simply applying a magnetic field gradient perpendicular to the axial direction of the pillars (Figure S5a). The bending deformation of the pillars increased with the increase of the magnetic field intensity (Figure S5b-d), which should be ascribed to the increased magnetic forces that drove the pillars to align with the magnetic field. Strikingly, the bending deformation of the pillars was completely 11


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reversible as the pillars returned to their original vertical position once the magnetic field was removed. Quantitative interrelationships between the pillars deformation, the material compositions, and the applied magnetic field were currently underway and the optimized combinations for magnetically-controlled stooped microstructures, expanding on earlier works of Drotlef et al.53 and Evans et al.,77 will be presented in detail in our future studies. DISCUSSION We thereby reveal that the s-FGPs present adhesions that are simultaneously strong (in shear), durable, directional and roughness-adaptive, a combination of which has long been strived for but has been scarcely realized for synthetic fibrillar adhesives. As summarized in Figure 6 for a qualitative comparison of the selected biomimetic dry adhesives reported so far,15-25,27,32,34,36,37,46-57,60-62,69,71,78-88 a fully ‘gecko-like’ adhesion 42

(i.e. within the quadruply-overlapped region) has been previously achieved only in

Jeong et al.’ work21 based on sophisticated hierarchical nanohairs and King et al.’ work based on fibril-less, specific elastomer/fabric-integrated materials.84 By contrast, our sFGPs achieve gecko-comparable adhesions with a single-level, cost-effective, and wellstudied micropillars structure. The optimal performances of the s-FGPs are attributed to the cooperative interplays between the geometry (or structure) and material designs, leading to high compliance normal to the substrate and low compliance in the shear direction following the adhesion scaling theory proposed recently.81 Specifically, the functional gradient material of the micropillars, which is inspired by the material gradient in the tarsal setae of the ladybird beetle found lately,70 represents a mechanically-optimized design for the biomimetic adhesives.66 The flexible tips facilitate contact formation to establish large contact area while the gradually-stiffened stalks provide essential reinforcement to maintain stability for the fibrillar structure without introducing sharp interfaces within the pillars. Only with such functional gradient designs, the micropillars can be made as both slanted and high-AR without involving structural failures even after extensive cycles of loading/unloading. It is just these two geometry factors (i.e. slant and high-AR) that dominate the adhesion 12


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anisotropy and the roughness-adaptability of the s-FGPs, respectively. We note that the material gradient of our s-FGPs is far less significant than that of the beetle setae.70 The local elastic modulus vary 5-6 times within a length of ~ 20 μm from pillar tip to base for the former while they change over 3 orders of magnitude within a length of ~ 40 μm for the latter. The extreme mechanical gradient of the setae allows for extremely high AR (as high as 40) structures that enable effective adaptation of the beetle species to living on various natural substrates.70 In the case of our s-FGPs, the magnitude of the mechanical gradient is limited by the maximum reinforcing fraction of the nanocomposites, which has been optimized for the present PUA/Fe3O4@SiO2 system. Further amplifying the gradient by modifying the material compositions and/or ideally, by generating smooth gradient with matrix materials (e.g. stiffer polymer to softer polymer transitions) would achieve structurally-stable FGPs with even higher ARs and better adaptability to rougher surfaces. We also note that our s-FGPs do not show adhesion capacities, particularly in the normal pull-off direction, as high as some fibrillar adhesives with enlarged pillar tips reported in the literature.21, 25,48,54,60,76,87

Nevertheless, the well-established tip-shape-decoration techniques, such

as the most-developed post-dipping/inking followed by pressed-curing,25,34,49,54,75,89-91 partial curing combined with mechanical yielding,76 using undercut molds obtained by selective etching/lithography,21,22,27,28,32,36,57,59-61,82,87,92-95 etc.96 can be readily applied to our s-FGPs to further enhance both the normal and shear adhesion strengths without interfering with other functionalities.

CONCLUSIONS Our breakthrough in developing slanted functional gradient micropillars (s-FGPs) has resolved the long-term challenge over the conflict between mechanical flexibility and structural stability for bioinspired fibrillar structure. The slanted geometry and the gradient material, realized by an inexpensive magnetically-assisted nanocomposite molding technique, work cooperatively and endow the s-FGPs with a comprehensive combination of adhesion properties that are all crucial for practical applications but are 13


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normally mutually-exclusive. The strong, ultra-durable, super-anisotropic, and roughness-adaptive shear adhesion of the s-FGPs represents thus far the optimal performances that biomimetic dry adhesives have ever achieved. Therefore, the present study not only advances the bioinspired adhesion towards integrated structural and material designs for multifunctional and practical performances, it also provides a generic route to fabricate a wide range of directional and functional gradient surfaces. We envision this concept of FGPs for high-AR, structurally-stable and -controllable fibrillar microstructures will draw wide attentions from various fields including energy harvesting,97 biosensors,98 superhydrophobicity/superwetting,99 self-cleaning,100 etc. for emerging functionalities and applications. METHODS Materials. Dispersed Fe3O4 nanoparticles (average diameter of 10 nm, oleate-stabilized in cyclohexane) were purchased from Nanjing XFNANO Materials Tech Co., Ltd, China. UVcurable polyurethane acrylate (PUA) resins were purchased from Minuta Tech. Co. Ltd., Korea. Tetraethyl orthosilicate (TEOS) for silica coating and 3-trimethoxysilylpropyl methacrylate (γMPS) for surface silanization were purchased from Sigma-Aldrich (Milwaukee, WI, USA). Other related materials and reagents used for the sample preparations and the characterization results can be found in our previous study.71 The superparamagnetic particles were first coated with silica shells (total size of ~ 20 nm) via a combination of ligand exchange and modified Stöber method as detailed elsewhere.101 Our previous study showed that this silica coating step was crucial in enhancing the dispersibility of pure magnetic particles in resin fluid.71 The obtained Fe3O4@SiO2 nanoparticles were then silanized with γ-MPS and mixed with the PUA resin at a ratio of 15 vol.% (29.5 wt.%) to prepare the PUA/Fe3O4@SiO2 resin nanocomposite. The mixing process was accomplished by mechanically stirring the mixture for overnight and subsequently elaborate blending in a centrifugal mixer (DAC 150.1 FVZ-K, Hauschild & Co KG, Hamm, Germany) at 2500 rpm for 30 min. Sample fabrications. The s-FGPs were fabricated by a well-established mold replication method combined with a magnetic actuation process for gradient formation as shown schematically in Figure 1. Silicon master with patterned angle cavities of ~ 2.5 μm diameter, ~ 20 μm length, ~ 6 μm center-to-center distance and 20° leaning angle was prepared by an angled 14


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photo-etching technique as developed by Jeong et al. (step 1).21 Degassed PUA/Fe3O4@SiO2 resin nanocomposite were then drop-dispensed onto the master and covered with a thin PET film (~ 200 μm thick) acting as the backing layer (step 2). The viscous nanocomposite were infiltrated into the high-AR cavities by a vacuum-assisted capillary filling process. The master, the PET backing, together with the infiltrated PUA/Fe3O4@SiO2 nanocomposite were then transferred onto a vibrating stage (AS 200, Retsch, Germany) running at a frequency of 20 Hz and exposed to a static magnetic field (H, intensity of 150 mT) configured along the axial direction of the cavities for 30 min (step 3). The magnetic field was generated by a permanent NdFeB magnet (S-60-05-N, Supermagnete, Germany) mounted right on top of the specimen with the intensity adjusted by varying the space between them and measured by a handheld Gaussmeter (Lakeshore 410, USA). The combined magnetic and mechanical stimuli drifted the nano-reinforcements towards the backing side and form a gradient distribution within the cavities.71 After degassing for 1 h, the patterned resin composite were cured by applying a UV irradiation of 500 mW/cm2 for 5 min (LZ1-10UA00, LED Engin Inc., USA) (step 4). The solidified composite replica backed with the PET film was subsequently peeled off from the master (step 5) and further treated with additional UV irradiation for 2 min to completely crosslink the matrix of the s-FGPs (step 6). The control fibrillar samples were fabricated by similar procedures for the s-FGPs as detailed above. Specifically, the v-FGPs were fabricated by the magnetically-assisted mold replication method using silicon master with vertical cavities. The v-CPs and s-CPs were fabricated by replicating pure PUA using master with vertical and angled cavities, respectively; the v-SPs and s-SPs were fabricated by filling the uniform PUA/Fe3O4@SiO2 nanocomposite (15 vol.%) into the master with vertical and angled cavities, respectively and directly curing them without applying magnetic exposure. The unpatterned PUA film was obtained by spin coating PUA liquid on a PET film (~ 200 μm thick), followed by a 2 min exposure to 500 mW/cm2 UV light. Morphological and mechanical characterizations. The various micropillars fabricated (i.e. v-CPs, v-SPs, v-FGPs, s-CPs, s-SPs and s-FGPs) were sputter-coated with a ~ 5-nm-thick Pt film and visualized via a field-emission SEM (FE-SEM, Sirion 200, FEI). Transmitted optical micrographs of the pillars were also captured using a polarizing microscope (Axio Scope A1, Zeiss, Germany) to qualitatively reveal the different distributions of the fillers inside. As composite with higher filler content would absorb and scatter more lights, the observed 15


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transparency (brightness) was lower for the filler-enriched pillars/regions compared to the filler-depleted ones (insets in Figure 2a-f). The spatial distribution of the nanofillers inside the various micropillars was examined by a TEM (Jeol EM-2100, Japan). Details regarding the sample preparations for the TEM tests can be found in our previous studies.71,102 The spatial variations of the local elastic modulus along individual micropillars were quantitatively evaluated by quasi-static nanoindentation tests.103 The pillars for testing were carefully scraped from the backing layer and placed onto a thin, uncured epoxy resin layer prespread on a glass slide. The resin layer was then cured at 80 oC for 2 h to provide support and clamp for the pillars. Displacement-controlled nanoindentation tests were performed on the exposed pillars from tip to base with an inter-indent spacing of ~ 2 μm (TI 950 TriboIndenter system, Hysitron Inc., USA). The tests were implemented at a maximum indentation depth of 400 nm and a loading/unloading rate of 50 nm/s using a Berkovich tip (tip radius of ~ 100 nm). Preliminary tests showed that varying the loading rate would slightly affect the absolute values of the testing results but did not alter the general trends we reported. For each type of the pillars, at least three replicates were tested. After the tests, the surfaces of the pillars partly embedded in the resin support were scanned by an atomic force microscope (DI Innova, Bruker, USA) to obtain the morphology of the residual indent pits. Adhesion measurements. Adhesion anisotropy of the various micropillars was measured by a commonly-used load-drag-pull test. Detailed descriptions of the setup for the tests can be found in our previous study.54 The movements of the indenter (4 mm-diameter glass hemisphere) in the vertical and horizontal directions for respectively loading/retracting and dragging were controlled by a LabView program (National Instruments, USA) through two microstages (TLSM025A, Zaber Technologies Inc., Canada) assembled in XY configuration. The forces in the two directions during the load-drag-pull processes were recorded by two high-resolution (0.1 mN) load cells (GSO-10, Transducer Techniques, LLC, USA) on each axis. The preload for contact formation (4 mN) was determined by a series of pilot tests, which showed that a 4 mN load on an s-FGPs sample corresponded to an indentation depth of ~ 3 μm and estimated 1045 pillars in contact with the indenter. Preliminary tests of various dragging velocities and distances were also carried out and showed that varying both parameters within certain ranges did not affect the measured forces significantly. For each sample in each direction (i.e. gripping or releasing), 5 tests were repeated at different spots to obtain the standard deviations. 16


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Shear adhesion durability and adaptability were tested using a home-built hanging setup as detailed in our previous studies (Figure 4a).54,71 The testing sample was glued to a glass slide from the backing layer side while the pillar side was brought into contact with either a fixed glass slide (i.e. the smooth substrate in the durability tests) or a pretreated steel sheet (i.e. the rough substrate in the adaptability tests) under a small preload of 1 N/cm2. The surface profiles of the glass and steel substrate were examined by in-situ scanning probe microscope (SPM) equipped with the TriboIndenter system. A commercialized software TriboView (Hysitron Inc., USA) was utilized to analyze the SPM images to obtain the surface roughness information. A weightless wire, supported by a fixed pulley in the middle, was used to connect the glued sample with a hanging weight. The height of the pulley was adjusted to ensure a pure shear force to the sample. Each test was started with a small weight of 20 g to maintain the system steady. Smooth increment was then successively added until detachment occurred. For each sample, 200 repeated trials (along the gripping direction for the slanted pillars and along one selected direction for the vertical pillars and the flat PUA film) were performed to test the durability of the shear adhesion. After the repeated tests, surfaces of the micropillars were observed again using the FE-SEM. For the adaptability tests, 5 measurements were repeated to obtain the standard deviations. Finite element analysis. Finite element analyses were performed (Abaqus/Standard v. 6.13, Dassault Simulia, USA) to compare the stress/strain distributions of the various pillars upon shear loading. For the compliant and stiff pillars (CPs and SPs), a uniform 3D stress model with 8-node linear brick elements (C3D8R in Abaqus) was employed; while for the functional gradient pillars (FGPs), the model was divided into 20 isotropic disks (perfect bonding between adjacent disks was assumed). Elastic constants obtained from the site-specific nanoindentation tests were input into the respective model to assign the pillars with different modulus distributions. Poisson’s ratios were taken using linear rule of mixtures based on the respective values of the matrix (0.48) and the filler (0.17). In order to directly compare the pillars deformation and stress distributions during shear loading, encastre boundary conditions were applied to the pillar bases while a constant shear load of 10 mN was applied to the tip surface of all the pillars. Such an assumed loading was chosen according to the induced lateral collapse of the s-CPs as compared with the experimental observations (Figure 4d).

Supporting Information. Morphology characterizations of v-FGPs with a high AR of 12 17


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(Figure S1), TEM images for the nanoparticle distributions (Figure S2), non-conformal testing data for vertical micropillars (Figure S3), pull-off force data of the various samples tested (Figure S4), demonstration of the micropillars bending controlled by external magnetic field (Figure S5), magnetically-actuated functional gradient nanocomposites (Text S1), critical aspect ratio of micropillars (Text S2), effective elastic modulus of functional gradient pillars array (Text S3). This material is available free of charge via the Internet at http://pubs.acs.org. ORCID: Zhengzhi Wang: 0000-0001-5699-8330. ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (11602177) and the Natural Science Foundation of Hubei Province (2016CFB248). Z.W. acknowledges the support from the starting-up funding of Wuhan University and the Facility Center of Anhui University for assistances with the electron microscope characterizations.

REFERENCES (1) Labonte, D.; Clemente, C. J.; Dittrich, A.; Kuo, C.-Y.; Crosby, A. J.; Irschick, D. J.; Federle, W. Extreme Positive Allometry of Animal Adhesive Pads and the Size Limits of AdhesionBased Climbing. Proc. Natl. Acad. Sci. U.S.A. 2016, 113, 1297-1302. (2) Arzt, E.; Gorb, S; Spolenak, R. From Micro to Nano Contacts in Biological Attachment Devices. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 10603-10606. (3) Gao, H. J.; Yao, H. M. Shape Insensitive Optimal Adhesion of Nanoscale Fibrillary Structures. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 7851-7856. (4) 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, 681-685. (5) 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. (6) Kamperman, M.; Kroner, E.; del Campo, A.; McMeeking, R. M; Arzt, E. Functional Adhesive Surfaces with “Gecko” Effect: the Concept of Contact Splitting. Adv. Eng. Mater. 2010, 12, 335-248. (7) Varenberg, M.; Murarash, B.; Kligerman, Y.; Gorb, S. N. Geometry-Controlled Adhesion: Revisiting the Contact Splitting Hypothesis. Appl. Phys. A 2011, 103: 933-938. (8) Jeong, H. E.; Suh, K. Y. Nanohairs and Naotubes: Efficient Structural Elements for GeckoInspired Artificial Dry Adhesives. Nano Today 2009, 4, 335-346. (9) 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. (10) Jagota, A.; Hui, C.-Y. Adhesion, Friction, and Compliance of Bio-Mimetic and BioInspired Structured Interfaces. Mater. Sci. Eng. R 2011, 72, 253-292. 18


Page 18 of 32

Page 19 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(11) Kwak, M. K.; Pang, C.; Jeong, H.-E.; Kim, H.-N.; Yoon, H.; Jung, H.-S.; Suh, K.-Y. Towards the Next Level of Bioinspired Dry Adhesives: New Designs and Applications. Adv. Funct. Mater. 2011, 21, 3606-3616. (12) Hu, S.; Xia, Z. Rational Design and Nanofabrication of Gecko-Inspired Fibrillary Adhesives. Small 2012, 8, 2464-2468. (13) Mengüç, Y., Sitti, M. Gecko-Inspired Polymer Adhesives. In Polymer Adhesion, Friction, and Lubrication; Zeng, H. B., Ed.; John Wiley & Sons, 2013; pp. 351-389. (14) Bio-inspired Structured Adhesives: Biological Prototypes, Fabrication, Tribological Properties, Contact Mechanics, and Novel Concepts. Heepe, L., Xue, L., Gorb, S. N., Eds.; Springer, 2017; pp. 1-343 (15) Geim, A. K.; Dubonos, S. V.; Grigorieva, I. V.; Novoselov, K. S.; Zhukov, A. A.; Shapoval, S. Y. Microfabricated Adhesive Mimicking Gecko Foot-Hair. Nat. Mater. 2003, 2, 461-463. (16) Lee, H.; Lee, B. P.; Messersmith, P. B. A Reversible Wet/Dry Adhesive Inspired by Mussels and Geckos. Nature 2007, 448, 338-342. (17) Ge, L.; Sethi, S.; Ci, L.; Ajayan, P. M.; Dhinojwala, A. Carbon Nanotube-Based Synthetic Gecko Tapes. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 10792-10795. (18) Glassmaker, N. J.; Jagota, A.; Hui, C. Y.; Noderer, W. L.; Chaudhury, M. K. Biologically Inspired Crack Trapping for Enhanced Adhesion. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 10786-10791. (19) Mahdavi, A; Ferreira, L.; Sundback, C.; Nichol, J. W.; Chan, E. P.; Carter, D. J. D.; Bettinger, C. J.; Patanavanich, S.; Chignozha, L.; Ben-Joseph, E.; Galakatos, A.; Pryor, H.; Pomerantseva, I.; Masiakos, P. T.; Faquin, W.; Zumbuehl, A.; Hong, S.; Borenstein, J.; Vacanti, J.; Langer, R.; Karp, J. M. A Biodegradable and Biocompatible Gecko-Inspired Tissue Adhesive. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 2307-2312. (20) Qu, L.; Dai, L.; Stone, M.; Xia, Z.; Wang, Z. L. Carbon Nanotube Arrays with Strong Shear Binding-on and Easy Normal Lifting-off. Science 2008, 322, 238-242. (21) 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, 56395644. (22) 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. (23) 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. (24) Xu, M.; Du, F.; Ganguli, S.; Roy, A; Dai, L. Carbon Nanotube Dry Adhesives with Temperature-Enhanced Adhesion over a Large Temperature Range. Nat. Commun. 2016, 7, 13450. (25) Song, S.; Drotlef, D.-M.; Majidi, C.; Sitti, M. Controllable Load Sharing for Soft Adhesive Interfaces on Three-Dimensional Surfaces. Proc. Natl. Acad. Sci. U.S.A. 2017, 114, E4344E4353. (26) Yoon, H.; Kwak, M. K.; Kim, S. M.; Sung, S. H.; Lim, J.; Suh, H. S.; Suh, K. Y.; Char, K. Polymeric Nanopillars Reinforced with Metallic Shells in the Lower Stem Region. Small 2011, 7, 3005-2010. (27) Bae, W. G.; Kim, D.; Kwak, M. K.; Ha, L.; Kang, S. M.; Suh, K. Y. Enhanced Skin Adhesive Patch with Modulus-Tunable Composite Micropillars. Adv. Healthcare Mater. 2013, 19


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2, 109-113. (28) Bae, W. G.; Kwak, M. K.; Jeong, H. E.; Pang, C.; Jeong, H.; Suh, K. Y. Fabrication and Analysis of Enforced Dry Adhesives with Core-Shell Micropillars. Soft Matter 2013, 9, 14221427. (29) Shahsavan, H.; Zhao, B. X. Bioinspired Functionally Graded Adhesive Materials: Synergetic Interplay of Top Viscous-Elastic Layers with Base Micropillars. Macromolecules 2014, 47, 353-364. (30) Minsky, H. K.; Turner, K. T. Achieving Enhanced and Tunable Adhesion via Composite Posts. Appl. Phys. Lett. 2015, 106, 201604. (31) Ma, S.; Wang, D.; Liang, Y.; Sun, B.; Gorb, S. N.; Zhou, F. Gecko-Inspired but Chemically Switched Friction and Adhesion on Nanofibrillar Surfaces. Small 2015, 11, 1131-1137. (32) Kim, T.; Park, J.; Sohn, J.; Cho, D.; Jeon, S. Bioinspired, Highly Stretchable, and Conductive Dry Adhesives Based on 1D-2D Hybrid Carbon Nanocomposites for All-in-One ECG Electrodes. ACS Nano 2016, 10, 4770-4778. (33) Balijepalli, R.; Fischer, S. C. L.; Hensel, R.; McMeeking, R. M.; Arzt, E. Numerical Study of Adhesion Enhancement by Composite Fibrils with Soft Tip Layers. J. Mech. Phys. Solids 2017, 99, 357-378. (34) Drotlef, D.-M.; Amjadi, M.; Yunusa, M.; Sitti, M. Bioinspired Composite Microfibers for Skin Adhesion and Signal Amplification of Wearable Sensors. Adv. Mater. 2017, 29, 1701353. (35) Minsky, H. K.; Turner, K. T. Composite Microposts with High Dry Adhesion Strength. ACS Appl. Mater. Interfaces 2017, 9, 18322-18327. (36) Gorumlu, S.; Aksak, B. Sticking to Rough Surfaces Using Functionally Graded BioInspired Microfibers. R. Soc. Open Sci. 2017, 4, 161105. (37) Fischer, S. C. L.; Arzt, E.; Hensel, R. Composite Pillars with a Tunable Interface for Adhesion to Rough Substrates. ACS Appl. Mater. Interface 2017, 9, 1036-1044. (38) Xue, L.; Sanz, B.; Luo, A.; Turner, K. T.; Wang, X.; Tan, D.; Zhang, R.; Du, H.; Steinhart, M.; Mijangos, C.; Guttmann, M.; Kappl, M.; del Campo, A. Hybrid Surface Patterns Mimicking the Design of the Adhesive Toe Pad of Tree Frog. ACS Nano 2017, 11, 9711-9719. (39) Studart, A. R.; Libanori, R.; Erb, R. M. Functional Gradients in Biological Composites. In Bio- and Bioinspired Nanomaterials; Ruiz-Molina, D., Novio, F. & Roscini, C., Eds.; WileyVCH, 2014; pp. 133-146. (40) Liu, Z.; Meyers, M. A.; Zhang, Z.; Ritchie, R. O. Functional Gradients and Heterogeneities in Biological Materials: Design Principles, Functions, and Bioinspired Applications. Prog. Mater. Sci. 2017, 88, 467-498. (41) Tian, Y.; Pesika, N.; Zeng, H.; Rosenberg, K.; Zhao, B.; McGuiggan, P.; Autumn, K.; Israelachvili, J. Adhesion and Friction in Gecko Toe Attachment and Detachment. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 19320-19325. (42) Autumn, K.; Niewiarowski, P. H.; Puthoff, J. B. Gecko Adhesion as a Model System for Integrative Biology, Interdisciplinary Science, and Bioinspired Engineering. Auun. Rev. Ecol. Evol. Syst. 2014, 45, 445-470. (43) Sitti, M.; Fearing R. S. Synthetic Gecko Foot-Hair Micro/Nano-Structures as Dry Adhesives. J. Adhesion Sci. Technol. 2003, 17, 1055-1073. (44) Aksak, B.; Murphy, M. P.; Sitti, M. Adhesion of Biologically Inspired Vertical and Angled Polymer Microfiber Arrays. Langmuir 2007, 23, 3322-3332. (45) Lee, J.; Fearing, R. S.; Komvopoulos, K. Directional Adhesion of Gecko-Inspired Angled 20


Page 20 of 32

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Microfiber Arrays. Appl. Phys. Lett. 2008, 93, 191910. (46) Yoon, H.; Jeong, H. E.; Kim, T.; Kang, T. J.; Tahk, D.; Char, K.; Suh, K. Y. Adhesion Hysteresis of Janus Nanopillars Fabricated by Nanomolding and Oblique Metal Deposition. Nano Today 2009, 4, 385-392. (47) Parness, A.; Soto, D.; Esparze, N.; Gravish, N.; Wilkinson, M.; Autumn, K.; Cutkosky, M. A Microfabricated Wedge-Shaped Adhesive Array Displaying Gecko-Like Dynamic Adhesion, Directionality and Long Lifetime. J. R. Soc. Interface 2009, 6, 1223-1232. (48) Kim, T.; Jeong, H. E.; Suh, K. Y.; Lee, H. H. Stooped Nanohairs: Geometry-Controllable, Unidirectional, Reversible, and Robust Gecko-Like Dry Adhesive. Adv. Mater. 2009, 21, 22762281. (49) Murphy, M. P.; Aksak, B.; Sitti, M. Gecko-Inspired Directional and Controllable Adhesion. Small 2009, 5, 170-175. (50) Rahmawan, Y.; Kim, T.; Kim, S. J.; Lee, K. R.; Moon, M. W.; Suh, K. Y. Surface Energy Tunable Nanohairy Dry Adhesive by Broad Ion Beam Irradiation. Soft Matter 2012, 8, 16731680. (51) Gillies, A. G.; Kwak, J.; Fearing R. S. Controllable Particle Adhesion with a Magnetically Actuated Synthetic Gecko Adhesive. Adv. Funct. Mater. 2013, 23, 3256-3261. (52) Jin, K.; Cremaldi, J. C.; Erickson, J. S.; Tian, Y.; Israelachvili, J. N.; Pesika, N. S. Biomimetic Bidirectional Switchable Adhesive Inspired by the Gecko. Adv. Funct. Mater. 2014, 24, 574-579. (53) Drotlef, D.-M.; Blümler, P.; del Campo, A. Magnetically Actuated Patterns for Bioinspired Reversible Adhesion (Dry and Wet). Adv. Mater. 2014, 26, 775-779. (54) Wang, Z.; Gu, P.; Wu, X. P. Gecko-Inspired Bidirectional Double-Sided Adhesives. Soft Matter 2014, 10, 3301-3310. (55) Im, H. S.; Kwon, K. Y.; Kim, J. U.; Kim, K., S.; Yi, H.; Yoo, P. J.; Pang, C.; Jeong, H. E.; Kim, T. Highly Durable and Unidirectionally Stooped Polymeric Nanohairs for Gecko-Like Dry Adhesive. Nanotechnology 2015, 26, 415301. (56) Tao, D.; Gao, X.; Lu, H.; Liu, Z.; Li, Y.; Tong, H.; Pesika, N.; Meng, Y.; Tian, Y. Controllable Anisotropic Dry Adhesion in Vacuum: Gecko Inspired Wedged Surface Fabricated with Ultraprecision Diamond Cutting. Adv. Funct. Mater. 2017, 27, 1606576. (57) Kwak, M. K.; Jeong, H. E.; Bae, W. G.; Jung, H.-S.; Suh, K. Y. Anisotropic Adhesion Properties of Triangular-Tip-Shaped Micropillars. Small 2011, 7, 2296-2300. (58) Tamelier, J.; Chary, S.; Turner, K. L. Vertical Anisotropic Microfibers for a Gecko-Inspired Adhesive. Langmuir 2012, 28, 8746-8752. (59) Seo, S.; Lee, J.; Kim, K. S.; Ko, K. H.; Lee, J. H.; Lee, J. Anisotropic Adhesion of Micropillars with Spatula Pads. ACS Appl. Mater. Interfaces 2014, 6, 1345-1350. (60) Sameoto, D.; Sharif, H.; Téllez, J. P. D.; Ferguson, B.; Menon, C. Nonangled Anisotropic Elastomeric Dry Adhesives with Tailorable Normal Adhesion Strength and High Directionality. J. Adhesion Sci. Technol. 2014, 28, 354-366. (61) Wang, Y.; Li, X.; Tian, H.; Hu, H.; Tian, Y.; Shao, J.; Ding, Y. Rectangle-Capped and Tilted Micropillar Array for Enhanced Anisotropic Anti-Shearing in Biomimetic Adhesion. J. R. Soc. Interface 2015, 12, 20150090. (62) Wang, Y.; Tian, H.; Shao, J.; Sameoto, D.; Li, X.; Wang, L.; Hu, H.; Ding, Y.; Lu, B. Switchable Dry Adhesion with Step-Like Micropillars and Controllable Interfacial Contact. ACS Appl. Mater. Interface 2016, 8, 10029-10037. 21


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(63) Kwak, M. K.; Jeong, H.-E.; Kim, T.; Yoon, H.; Suh, K. Y. Bio-Inspired Slanted Polymer Nanohairs for Anisotropic Wetting and Directional Dry Adhesion. Soft Matter 2010, 6, 18491857. (64) 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. (65) Chandra, D.; Yang, S. Stability of High-Aspect-Ratio Micropillar Arrays against Adhesive and Capillary Forces. Accounts Chem. Res. 2010, 43, 1080-1091. (66) Spolenak, R.; Gorb, S.; Arzt, E. Adhesion Design Maps for Bio-Inspired Attachment Systems. Acta Biomater. 2005, 1, 5-13. (67) Greiner, C.; Spolenak, R.; Arzt, E. Adhesion Design Maps for Fibrillar Adhesives: the Effect of Shape. Acta Biomater. 2009, 5, 597-606. (68) Greiner, C.; del Campo, A.; Arzt, E. Adhesion of Bioinspired Micropatterned Surfaces: Effects of Pillar Radius, Aspect Ratio, and Preload. Langmuir 2007, 23, 3495-3502. (69) Barreau, V.; Hensel, R.; Guimard, N. K.; Ghatak, A.; McMeeking, R. M.; Arzt, E. Fibrillar Elastomeric Micropatterns Create Tunable Adhesion Even to Rough Surfaces. Adv. Funct. Mater. 2016, 26, 4687-4694. (70) Peisker, H.; Michels, J.; Gorb, S. N. Evidence for a Material Gradient in the Adhesive Tarsal Setae of the Ladybird Beetle Coccinella Septempunctata. Nat. Commun. 2013, 4, 1-7. (71) Wang, Z.; Shi, X.; Huang, H.; Yao, C.; Xie, W.; Huang, C.; Gu, P.; Ma, X.; Zhang, Z.; Chen L.Q. Magnetically Actuated Functional Gradient Nanocomposites for Strong and UltraDurable Biomimetic Interfaces/Surfaces. Mater. Horiz. 2017, 4, 869-877. (72) Glassmaker, N. J.; Jagota, A.; Hui, C.-Y.; Kim, J. Design of Biomimetic Fibrillary Interfaces: 1. Making Contact. J. R. Soc. Interface 2004, 1, 23-33. (73) Díez-Pascual, A. M.; Gómez-Fatou, M. A.; Ania, F.; Flores, A. Nanoindentation in Polymer Nanocomposites. Prog. Mater. Sci. 2015, 67, 1-94. (74) Autumn, K.; Dittmore, A.; Santos, D.; Spenko, M.; Cutkosky, M. Frictional Adhesion: a New Angle on Gecko Attachment. J. Exp. Biol. 2006, 209, 3569-3579. (75) del Campo, A.; Greiner, C.; Arzt, E. Contact Shape Controls Adhesion of Bioinspired Fibrillar Surfaces. Langmuir 2007, 23, 10235-10243. (76) Jeong, H. E.; Suh, K.-Y. Precise Tip Shape Transformation of Nanopillars for Enhanced Dry Adhesion Strength. Soft Matter 2012, 8, 5375-5380. (77) Evans, B. A.; Shields, A. R.; Carroll, R. L.; Washburn, S.; Falvo, M. R.; Superfine, R. Magnetically Actuated Nanorod Arrays as Biomimetic Cilia. Nano Lett. 2007, 7, 1428-1434. (78) Kustandi, T. S.; Samper, V. D.; Yi, D. K.; Ng, W. S.; Neuzil, P.; Sun, W. Self-Assembled Nanoparticles Based Fabrication of Gecko Foot-Hair-Inspired Polymer Nanofibers. Adv. Funct. Mater. 2007, 17, 2211-2218. (79) Northen, M. T.; Greiner, C.; Artz, E.; Turner, K. L. A Gecko-Inspired Reversible Adhesive. Adv. Mater. 2008, 20, 3905-3909. (80) 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. (81) 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. (82) Wang, Y.; Hu, H.; Shao, J.; Ding Y. Fabrication of Well-Defined Mushroom-Shaped Structures for Biomimetic Dry Adhesive by Conventional Photolithography and Molding. ACS Appl. Mater. Interfaces 2014, 6, 2213-2218. 22


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(83) Bartlett, M. D.; Crosby, A. J. High Capacity, Easy Release Adhesives from Renewable Materials. Adv. Mater. 2014, 26, 3405-3409. (84) King, D. R.; Bartlett, M. D.; Gilman, C. A.; Irschick, D. J.; Crosby, A. J. Creating GeckoLike Adhesives for “Real World” Surfaces. Adv. Mater. 2014, 26, 4345-4351. (85) Hawkes, E. W.; Eason, E. V.; Christensen, D. L.; Cutkosky, M. R. Human Climbing with Efficiently Scaled Gecko-Inspired Dry Adhesives. J. R. Soc. Interface 2015, 12, 20140675. (86) Hong, S.; Lundstrom, T.; Ghosh, R.; Adbi, H.; Hao, J.; Jeoung, S. K.; Su, P.; Suhr, J.; Vaziri, A.; Jalili, N.; Jung, Y. H. Highly Anisotropic Adhesive Film Made from Upside-Down, Flat, and Uniform Vertically Aligned CNTs. ACS Appl. Mater. Interfaces 2016, 8, 34061-34067. (87) Yi, H.; Kang, M.; Kwak, M. K.; Jeong, H. E. Simple and Reliable Fabrication of Bioinspired Mushroom-Shaped Micropillars with Precisely Controlled Tip Geometries. ACS Appl. Mater. Interfaces 2016, 8, 22671-22678. (88) Hu, H.; Tian, H.; Shao, J.; Li, X.; Wang, Y.; Wang, Y.; Tian, Y.; Lu, B. Discretely Supported Dry Adhesive Film Inspired by Biological Bending Behavior for Enhanced Performance on a Rough Surface. ACS Appl. Mater. Interface 2017, 9, 7752-7760. (89) del Campo, A.; Greiner, C.; Álvarez, I.; Arzt, E. Patterned Surfaces with Pillars with Controlled 3D Tip Geometry Mimicking Bioattachment Devices. Adv. Mater. 2007, 19, 19731977. (90) Murphy, M. P.; Aksak, B.; Sitti, M. Adhesion and Anisotropic Friction Enhancements of Angled Heterogeneous Micro-Fiber Arrays with Spherical and Spatula Tips. J. Adhesion Sci. Technol. 2007, 21, 1281-1296. (91) Murphy, M. P.; Kim, S.; Sitti, M. Enhanced Adhesion by Gecko-Inspired Hierarchical Fibrillar Adhesives. ACS Appl. Mater. Interface 2009, 1, 849-855. (92) Kim, S.; Sitti, M. Biologically Inspired Polymer Microfibers with Spatulate Tips as Repeatable Fibrillar Adhesives. Appl. Phys. Lett. 2006, 89, 261911. (93) Davies, J.; Haq, S.; Hawke, T.; Sargent, J. P. A Practical Approach to the Development of a Synthetic Gecko Tape. Int. J. Adhes. Adhes. 2009, 29, 380-390. (94) Gorb, S.; Varenberg, M.; Peressadko, A.; Tuma, J. Biomimetic Mushroom-Shaped Fibrillary Adhesive Microstructure. J. R. Soc. Interface 2007, 4, 271-275. (95) Sameoto, D.; Menon, C. A Low-Cost, High Yield Fabrication Method for Producing Optimized Biomimetic Dry Adhesives. J. Micromech. Microeng. 2009, 19, 115002. (96) Heepe, L.; Gorb, S. N. Biologically Inspired Mushroom-Shaped Adhesive Microstructures. Auun. Rev. Mater. Res. 2014, 44, 173-203. (97) Wang, Z. L.; Wu, W. Z. Nanotechnology-Enabled Energy Harvesting for Self-Powered Micro-/Nanosystems. Angew. Chem. Int. Ed. 2012, 51, 2-24. (98) Pang, C.; Lee, G. Y.; Kim, T. I.; Kim, S. M.; Kim, H. N.; Ahn, S. H.; Suh, K. Y. A Flexible and Highly Sensitive Strain-Gauge Sensor using Reversible Interlocking of Nanofibers. Nat. Mater. 2012, 11, 795-801. (99) Su, B.; Tian, Y.; Jiang, L. Bioinspired Interfaces with Superwettability: from Materials to Chemistry. J. Am. Chem. Soc. 2016, 138, 1727-1748. (100) Xu, Q.; Wan, Y.; Hu, T. S.; Liu, T. X.; Tao, D.; Niewiarowski, P. H.; Tian, Y.; Liu, Y.; Dai, L.; Yang, Y.; Xia, Z. Robust Self-Cleaning and Micromanipulation Capabilities of Gecko Spatula and Their Bio-Mimics. Nat. Commun. 2015, 3, 1265. (101) Rho, W.-Y.; Kim, H. M.; Kyeong, S.; Kang, Y. L.; Kim, D. H.; Kang, H.; Jeong, C.; Kim, D. E.; Lee, Y. S.; Jun, B. H. Facile Synthesis of Monodispersed Silica-Coated Magnetic 23


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Nanoparticles. J. Ind. Eng. Chem. 2014, 20, 2624-2649. (102) Wang, Z.; Gu, P.; Wu, X. P.; Zhang, H.; Zhang, Z.; Chiang, M. Y. M. Micro/Nano-Wear Studies on Epoxy/Silica Nanocomposites. Compos. Sci. Technol. 2013, 79, 49-57. (103) Oliver, W. C.; Pharr, G. M. An Improved Technique for Determining Hardness and Elastic Modulus using Load and Displacement Sensing Indentation Experiments. J. Mater. Res. 1992, 7, 1564-1583.

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Table 1. Effective elastic modulus of the slanted pillar arrays for no sliding and sliding along the gripping/releasing direction (unit: kPa). Details of the calculations are given in Text S3, Supporting Information. s-CPs s-SPs s-FGPs no sliding

41.0

123.1

93.6

gripping

24.3

72.9

55.5

releasing

131.0

393.1

299.0

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Figure 1. Schematic of the processing steps for the fabrication of slanted functional gradient pillars (s-FGPs). 1. silicon master with patterned angled cavities, inset showing a cross-sectional scanning electron microscope (SEM) image of the mold; 2. filling the mold cavities with PUA/Fe3O4@SiO2 nanocomposite; 3. redistribution of the nanoreinforcements inside the molded composite by applying a magnetic field gradient along the axial direction; 4. UV curing the redistributed nanocomposite; 5. peeling off the solidified nanocomposite pillars from the mold; 6. s-FGPs obtained with the nanoreinforcements gradually distributed towards the basal regions.

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Figure 2. Morphological and mechanical characterizations of the various micropillars. SEM images of (a) vertical compliant pillars (v-CPs), (b) vertical stiff pillars (v-SPs), (c) vertical functional gradient pillars (v-FGPs), (d) slanted CPs (s-CPs), (e) slanted SPs (s-SPs), and (f) slanted FGPs (s-FGPs) with the top right inset in each figure showing the respective transmission optical micrograph and the bottom right inset showing the respective enlarged SEM image. Scale bars: 10 μm and 5 μm for the images and insets, respectively. (g) Nanoindentation load-depth curves at different positions along individual CPs, SPs, and FGPs, showing uniform stiffness for the CPs and SPs while increasing stiffness from tip to base of the FGPs. Inset showing a representative atomic force microscope (AFM) image of the residual indentation pits on a FGP. (h) Elastic modulus measured by nanoindentation tests along individual micropillars. Dashed lines connecting the data points are provided for visual assistance. Shaded regions in g) and h) indicate the standard deviation of the measurements. (i) Contour illustration of the elastic modulus distribution for the various micropillars corresponding to (a)-(f).

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Figure 3. Directional adhesion of the slanted micropillars measured by non-conformal sliding tests. (a) Illustration of the loading process and pillars deformation for s-CPs, sSPs and s-FGPs when sliding along the gripping and releasing directions. (b) Shear and normal forces during shear displacements after a preload of 4 mN. Positive normal force indicates compression and negative normal force indicates adhesion. Positive shear force denotes force sensed in the releasing direction and negative shear force denotes that in the gripping direction. Shaded regions indicate the standard deviation of the measurements. (c) Maximum shear forces in the gripping and releasing directions and their ratios (i.e. anisotropy ratio) for different slanted pillars together with the vertical control ones. Among all the pillars tested, s-FGPs (highlighted by shading) show almost the highest shear adhesion in the gripping direction and the lowest shear force in the releasing direction and therefore exhibit the highest adhesion anisotropy. All the vertical pillars show nearly isotropic adhesion (i.e. anisotropy ratio of 1, as indicated by the dashed line). 28


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Figure 4. Strong and durable adhesion of the s-FGPs measured by repeated shear adhesion tests against glass substrate. (a) Photograph of an s-FGPs sample supported on a PET backing layer and schematic diagram of the hanging setup for shear adhesion measurement. Slanted pillars are tested along their gripping directions. Scale bar: 1 cm. (b) Shear adhesion capacity of 200 repeated tests for different pillars. Dashed lines are fitted to the data points for visual assistance. The shaded region indicates within the adhesion capacity of unpatterned PUA films. (c-h) SEM images of the different pillars after 200 repeating cycles of shear adhesion tests. Top right inset in each figure showing the respective stress contour under the same shear load obtained by finite element analyses (FEA). The stresses are scaled to the same magnitude as shown in the inset of c). Scale bars: 10 μm.

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Figure 5. Adaptive adhesion of the s-FGPs measured against rough steel substrate. (a) Shear adhesion capacity of the various micropillars against prepared rough substrate. Inset showing a 3D surface profile of the substrate within an area of 60 × 60 μm2 obtained by a scanning probe microscope (SPM). Demonstration of a 1 cm2 s-FGPs sample vertically adhering (b) to a smooth polycarbonate disk supporting 500 g weight and (c) to a coarse steel plate supporting 250 g weight. Insets showing the schematic diagrams of the attachment. Scale bars: 1 cm.

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Figure 6. Scattering map qualitatively comparing the critical properties of bioinspired dry adhesives achieved in this study with those reported in the literature15-25,27,32,34, 36,37,46-57,60-62,69,71,78-88

and those of the natural prototype --- gecko.4,5,41,42,74 Due to the

vast number of the relevant publications in the literature, only selected portion that simultaneously meet at least two of the property criterion as defined below are plotted. Regardless of the specific testing method and conditions, adhesives that present macroscopic adhesion strength comparable to or greater than that of gecko (i.e. ~ 1 N/cm2 normal or ~ 10 N/cm2 shear) are considered as “strong”; those possess effective anisotropic ratio comparable to or higher than that of gecko (~ 4.5) are included as “directional”; those can maintain the adhesion strength for considerable cycles (difficult to quantify) of attachment/detachment are categorized as “durable”; and those show noticeable adhesion against substrates other than glass or other smooth surfaces are classified as roughness-“adaptive”.

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Graphical Table of Contents

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Slanted Functional Gradient Micropillars for Optimal Bioinspired Dry

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