Bioinspired Dry Adhesive Materials and Their Application in Robotics: A Review

Journal of Bionic Engineering 13 (2016) 181–199

Bioinspired Dry Adhesive Materials and Their Application in Robotics: A Review Yasong Li, Jeffrey Krahn, Carlo Menon MENRVA Research Group, School of Engineering Science, Simon Fraser University, 8888 University Drive, Burnaby, BC, V5A 1S6, Canada

Abstract Dry adhesives inspired from climbing animals, such as geckos and spiders, rely on van der Waals forces to attach to the opposing surface. Biological fibrillar dry adhesives have a hierarchical structure closely resembling a tree: the surface of the skin on the animal’s feet is covered in arrays of slender micro-fibrils, each of which supports arrays of fibrils in submicron dimensions. These nano-meter size fibrils can conform closely to the opposing surfaces to induce van der Waals interaction. Bioinspired dry adhesives have been developed in research laboratories for more than a decade. To mimic the biological fibrillar adhesives, fibrillar structures have been prepared using a variety of materials and geometrical arrangements. In this review article, the mechanism and selected fabrication methods of fibrillar adhesives are summarized for future reference in adhesive development. Robotic applications of these bioinspired adhesives are also introduced in this article. Various successful applications of bioinspired fibrillar adhesives can shed light on developing smart adhesives for use in automation. Keywords: bioinspired gecko adhesive, van der Waals forces, biomimetic, climbing robot, bioinspired tape Copyright © 2016, Jilin University. Published by Elsevier Limited and Science Press. All rights reserved. doi: 10.1016/S1672-6529(16)60293-7

1 Introduction Geckos, a type of creatures that can climb rapidly everywhere, have been amazing humans for centuries[1]. The secret of their extraordinary climbing ability, however, was revealed just a decade ago[2–4], thanks to continuously improving electron microscopy techniques and micro-fabricated sensors. What appeared to be soft and smooth skin on the gecko’s climbing feet has been shown to be hierarchical fibrillar structures that provide compliancy with the climbing surface[5–7]. The hierarchical fibrillar structures on climbing animals possess fibrils in different length scales, which are arranged in a pattern closely resembling a tree[8]. The open ends of these tree-like structures consist of nano-meter size arrays of fibrils, which can conform to both smooth and rough surfaces with the help of the hierarchical structures. Millions of nano-fibrils, in close proximity to the contacting surface, each induces van der Waals force and collectively provide enough attraction forces to support the weight of the climbing animal[2,3]. The discovery of hierarchical fibrillar structures on the feet of animals has Corresponding author: Carlo Menon E-mail: [email protected]

inspired not only the adhesives mimicking the mechanism of fibril-surface interactions, but also robotic applications using these fibrillar adhesives. In this article, the development and applications of bioinspired fibrillar adhesives are reviewed in the following sections: The mechanism of biological and artificial fibrillar adhesives is introduced in section 2; fabrication of bioinspired fibrillar adhesives using different materials and methods is introduced in section 3; recent advances in functionalized dry adhesion as a new research direction are introduced in section 4; robotic applications making use of the bioinspired fibrillar adhesives are introduced in section 5; and finally conclusions of this review study are drawn in section 6.

2 Mechanism of the bioinspired fibrillar adhesive Geckos[2,4] and spiders[9–12] are two of the animals that have been found to have nano-scale fibrillar adhesives on their climbing feet. These fibrillar adhesives are different from the structures that are found on beetles[13,14], which rely on chemical secretion to create “wet

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contact” with the climbing surfaces. Therefore the fibrillar adhesives are referred to as “dry adhesives”[4] because they don’t rely on wet contact. There are seven properties that summarize the gecko’s dry adhesive[15,16]: 1) anisotropic attachment (adhesion is different when the adhesive is pulled from different directions); 2) high pull-off to preload ratio (high pull-off adhesion measured with low compression preload at interfaces engagement); 3) low detachment peeling force (easily being removed by peeling rather than pull-off); 4) material independence (the adhesive is able to provide similar adhesion to surfaces with different materials); 5) self-cleaning (adhesive is reusable with no need to be cleaned); 6) anti-self matting (the fibrillar adhesive will not degrade due to possible fibril collapsing); and 7) non-sticky default state (the adhesive is not activated when no load is applied on it). These seven properties can be summarized into three key aspects to investigate gecko adhesives: geometry, material and climbing gait; these aspects are reviewed in this section. Biological dry adhesives contain fibrils in different length scales. For example, on a gecko’s dry adhesives, there are four length scales of structures which effect the level of adhesion[8]: the soft skin on the flexible skeleton of the geckos form the macro-scale of structure (see Fig. 1a); the millimetre-scale “lamellar” structures are arranged as thin blades (see Fig. 1b); on the surface of the ends of the lamellar structures, micrometre-scale of “setae” are arranged as arrays (see Fig. 1c); at the end of each seta (single form of setae), arrays of nanometre-scale “spatulae” are the “end-effectors” to create intimate contact with the climbing surfaces (see Fig. 1d). Millions of spatulae induce van der Waals forces with the contacting surfaces, collectively forming a firm grip to support the gecko’s own weight. The hierarchical fibrillar structures on geckos are proven to provide compliancy to conform to rough surfaces[5,17–19]. The material that makes up the skin on geckos is called “beta-keratin”, which has a Young’s modulus of ~1.4 GPa[20,21]. However, the effective elastic modulus of the dry adhesive structure, when taking the four layers making up the structures into consideration, is calculated to be ~100 kPa, which is between the definition of “tacky” and “non-tacky” according to the Dahlquist’s criterion[8,21–23]. The softness provided by the hierarchical structures helps the gecko feet to conform to the climbing surface. A model based on spider adhesives[24]

is given in another work, which compares the adhesion performance of spiders and geckos. Adhesion in spiders is comparable in strength to the one in gecko. Claws assist the spider’s adhesive to climb on rougher surfaces. The fibrils in the tree-like hierarchical structures are closely packed in order to create a large contact area. The slender and close-packed fibrils can collapse together if they are formed of hydrophilic materials. Due to the low surface energy of beta-keratin[20], the hierarchical fibrillar structures do not clump together and instead provide flexibility for each fibril to extend into contact with the contacting surface. In addition to the low surface energy property, the hierarchical structures are self-cleaning and reduce the adhesion of dust and contamination picked up from the climbing surfaces[25–27]. The gecko is an efficient climber with step intervals of ~ 15 milliseconds[15]. The gecko’s adhesives can hold approximately three orders of magnitude more than their own weight[3]. The gecko’s fibrillar adhesives are often referred to as “smart” adhesives[15] for their “programmable” activation and de-activation: the curvature of the fibrils allows the adhesives to engage climbing surfaces at an angle of ~ 30˚, and detach from the surfaces at an angle of slightly higher than 30˚[2]. The attaching and detaching angles affect the movements that geckos use to climb - they use a “Load-Drag-Pull” series of movement in order to interact with the climbing surfaces during each step[28–30]. During climbing, the gecko brings its foot into contact with the climbing surface and

Fig. 1 Hierarchical surface structures forming the gecko’s dry adhesives. Images are adapted from Ref. [8]. a) Flexible body and skin of the gecko forms the marco-scale structures; b) lamellar structures on the toes form the meso-scale; c) arrays of setae form the micro-scale structures; d) magnified view of a single seta (~130 μm long); the inset provides a magnified view of the tip of a seta which is formed of arrays of nano-scale spatulae.

Li et al.: Bioinspired Dry Adhesive Materials and Their Application in Robotics: A Review

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then drags its foot towards its body in order to engage the adhesive surface by dragging it against the direction of the natural curvature of the fibrils[28,30]. When detaching its foot from a surface, the gecko peels its toes from the surface first, followed by the remainder of its foot from front to back[28,30]. Fig. 2 illustrates the results of an experiment that drags the gecko seta arrays in opposite directions after loading[29]. Both the vertical and lateral forces are larger in magnitude when the drag direction is along the curvature of the seta, compared to the forces when the drag direction is against the curvature of the seta. The vertical forces represent the adhesion force (pointing out from the climbing surface) when the seta was dragged along its curvature, as compared to the compression force (pointing into the climbing surface) when the seta was dragged against its curvature. In other words, the foot movements that geckos use during climbing result in the dry adhesives achieving rapid and reliable attachment and detachment.

3 Fabrication of bioinspired adhesives Studies of the biological fibrillar dry adhesives such as those found on geckos have inspired the engineering and fabrication of artificial adhesives in many different ways. Artificial fibrillar adhesives have been made in a wide range of different materials and geome-

try with friction and adhesion forces being measured in order to quantify the properties of the adhesives[31–33]. In this section, bioinspired artificial adhesives are presented and categorized by their structural geometry. From this point forward in our discussion, adhesives that contain only one length scale of fibrils are referred to as single layer fibrillar adhesives while adhesives that contain two or more length scales of fibrils (e.g. millimetre scale fibrils supporting micrometer scale fibrils) are referred to as hierarchical fibrillar adhesives. 3.1 Single layer fibrillar adhesives In order to mimic the biological fibrillar adhesives which utilize van der Waals interactions, the arrays of fibrils with a diameter in the sub-micrometer scale were first prepared by molding polymers using nano-indentations and nano-porous filters[34–36]. A lack of control in the fibril aspect ratio resulted in the fibril collapsing or stubby fibrillar arrays, which did not arguably demonstrate any advantageous properties of the fibrillar adhesives[34–36]. Preparation of single level adhesives has since diverged from these early trials: micrometer sizes of polymeric fibrillar arrays have demonstrated extraordinary friction and adhesion properties[37–42], which outperform nano-fibrillar adhesives. Although nano-scale fibrils continue to be investigated and

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Fig. 3 Examples of micro-scale mushroom shape fibrillar adhesives. (a) Polyurethane (PU) micro-fibrillar arrays with fibril diameter of 4.5 μm, tip diameter of 9 μm and length of 20 μm[41]; (b) Polyvinylsiloxane (PVS) micro-fibrillar arrays with fibril diameter of 35 μm (middle shaft) and length of 100 μm[42]; (c) Polydimethylsiloxane (PDMS) micro-fibrillar arrays with fibril diameter of 10 μm and length of 20 μm[38].

improved as compared to early designs[43–46], various micro-fibrils have demonstrated excellent adhesion and friction properties towards flat substrates, such as silicon wafers[38] and glass substrates[40]. In the research of artificial micro-scale fibrillar adhesives, polyurethane fibrils with spatula tips (Fig. 3a) have demonstrated over 7 times higher adhesion forces than the flat substrate made of the same polymer[41]. The adhesion pressure was measured to be 18 N·cm−2, which was much higher than the Tokay Gecko’s (10 N·cm−2)[41]. In this research, adhesion measurements were performed using a load cell with a 6 mm diameter glass hemisphere. The glass hemisphere was used as an interfacial substrate interacting with the fibrillar arrays. The hemisphere was lowered vertically toward the dry adhesive surface until in contact with the fibrillar arrays. The glass hemisphere and attached load cell was then pulled vertically away from the dry adhesive surface in order to measure the detachment force. The use of the hemisphere is believed to avoid non-uniform contact formed between two planar substrates, i.e. “alignment error”. It is also believed that the spatula tips, sometimes referred to as the mushroom cap shape of the fibrils, have drastically improved adhesion performances. However, it is suspected that these micro-mushrooms mainly rely on suction cup or vacuum effects rather than making use of van der Waals forces[47,48]. Other researches (Figs. 3b and 3c) have demonstrated similar shapes of micro-fibrils with compatible adhesion force measured[42,49,50]. High yield methods[51] have also been proposed for applications which require scale-up adhesion using large sheets of fibrillar adhesives.

Advanced technologies redirected the spot light of single level adhesives research back to preparing nano-size fibrillar arrays, which have a closer shape resemblance to the gecko’s adhesive[44–46]. Polyurethane acrylate (PUA) nano-fibrillar arrays (Fig. 4a) were prepared and tested for adhesion and frictional forces[43]. By using electron beam irradiation, the polymeric hairs could be bent in certain directions for use as anisotropic directional adhesives. Further tuning of their customized polymeric material allowed the fibrillar arrays to realize a variety of effective modulus. Although the adhesion of these fibrils did not show improvements as measured by electron beam bending, the measured frictional forces demonstrate the usefulness of anisotropic fibrillar arrays. An adhesive patch with a 1 × 1 cm2 area was able to bear a shear load of more than 1 kg on an indium tin oxide (ITO) coated glass substrate. Other research (Fig. 4b) demonstrated the shear-induced adhesion of polypropylene nano-fibrils (~300 nm diameter) with improved adhesion and friction proportional to the sliding distance[44,52]. The adhesion force measurements of these nano-fibrils were performed by combining load cells and LDP procedures. However, it was observed that polypropylene nano-structures deformed under repeated dragging measurements, which resulted in both adhesion and friction reductions after ~100 measurement cycles. Fibril tip modification on polystyrene (PS) nano-fibrils (~270 nm diameter) showed increased adhesion by increasing the size of the fibril tip[45] (original round tip fibrils - see Figs. 4c and 4d). Although it was reported that the shape and adhesion of the PS fibrils passed a durability test[45], the test procedure only involved

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Fig. 4 Examples of nano-fibrillar adhesives. (a) Polyurethane acrylate (PUA) nano-fibrillar arrays[43]. Half of the fibrils on the left were exposed to electron beam irradiation, resulting in tilted fibrils. (b) Polypropylene (PP) nano-fibrillar arrays with a 600 nm diameter fibril and 18 μm length[44]. (c) and (d) Top-view and side-view of Polystyrene (PS) nano-fibrillar adhesive, with a ~330 nm diameter fibril and ~1.5 μm length[45].

10 measurement cycles. Also, the tip shaping method involved the cold pressing of PS fibrils in room temperature, which might lead to the conclusion that the fibrils can be deformed during extensive usage. Another research proposed a replica molding method to avoid using thermal plastics as the structural material[53]. This replica molding method also allows potentially low-cost and large scale manufacturing of adhesives. The disadvantages of polymeric materials have driven researchers to explore other possible construction materials such as carbon nano-tubes (CNT)[46,54]. CNT arrays were prepared with smaller dimensions than that of polymeric fibrils[37–40,42–44] and bundled into blocks to mimic the setae structure of the gecko’s adhesive[54]. Although improvements over polymeric fibrils were observed with these CNT adhesives, the interaction between the contact substrate and the CNT forests was not reversible. This type of adhesive is generally not reusable and fabrication of CNT structures can be expensive. Another type of CNT adhesive was fabricated by transferring vertically aligned carbon nanotube (VACNT) onto a flexible substrate (polyethylene terephthalate film) to enhance the compliancy[55]. One research[56] showed the feasibility of fabricating large area (centimetre sizes) of VACNT growing on both flexible and rigid substrates. The shear adhesion is enhanced by this flexible substrate, provided an example for direct proportional relationship between adhesion and compliancy. Packing density of the CNT also influences the adhesion forces[57]. Generally higher packing density gives higher surface area ratio and both higher shear and normal adhesion strengths. Other in-

organic materials explored include silicon carbide[58] and hybrid germanium-parylene nanowires[59]. Nanowire ‘forests’ demonstrate excellent shear loading resistance but the problem of irreversible damage persists. Exploration of single level fibrillar adhesives has demonstrated the superior adhesion and friction properties when adhesives are interacting with flat surfaces. However, these adhesives have not yet surpassed adhesion strength of gecko fibrillar adhesives on rough surfaces[60]. Hierarchical fibrillar structures in the gecko’s adhesive provide a possible solution for this problem. 3.2 Hierarchical fibrillar adhesives Geckos rely on a hierarchical fibrillar structure to adapt to surfaces of different roughness[6,15,19,61]. Taking this bio-inspiration, one of the earliest hierarchical fibrillar structures simply combined two length-scales of stubby fibrils with different aspect ratios on the bottom supporting fibrils[62] (Fig. 5a). Adhesion forces measured were much lower than the single level adhesive, since the space between fibrils reduced the potential contact area between the adhesive and the contacting substrate. Adhesion measurements with varying preload forces should have provided more information on structural compliancy, but a trend of enhanced compliancy was not obvious from the results. Variations of hierarchical structures have flourished ever since, trying to reproduce the hierarchy of the gecko’s fibrillar structure[5,7,63–73]. The hierarchical mushroom shaped fibrils (Fig. 5b) were fabricated and then tested with indentation (or push-pull) procedures, showing an improvement in adhesion when the preload

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Fig. 5 SEM images of some hierarchical fibrillar adhesives. (a) Simple PDMS hierarchical fibrillar arrays molded from an SU-8 mold[62]. (b) PU hierarchical fibrillar arrays prepared by a molding method[63]. Inset is a magnified view of the top of a large fibril. (c) PUA hierarchical fibrillar adhesive with nano-size fibrils tilted in an angle[72]. (d) PS hierarchical nano-fibrillar arrays prepared by hot-pulling from an aluminum oxide template[64]. (e) Polymeric hierarchical fibrillar arrays prepared by direct laser writing on a photoresist layer[65].

force was larger than 100 nN[63]. Adhesion measurements were performed with an indentation setup composed of a load cell attached to a 12 mm diameter glass hemisphere tip acting as the interaction substrate. Another hierarchical adhesive (Fig. 5c) with spatula tips and a tilted fibril arrangement[72] were prepared which demonstrated enhanced frictional properties for use as a pick and place device. The test substrates in this case

were with nano-patterned glass and silicon surfaces and only friction was investigated. This research showed an increase in the frictional response when the scale of the test substrate roughness increased to the dimension of the fibril’s diameter which indicates that mechanical interlocking might also contribute to adhesion alongside van der Waals interactions. Some of the hierarchical fibrillar structures demonstrated a close resemblance to

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Fig. 6 SEM images demonstrating the structure damage of hierarchical fibrillar adhesives after shear measurements. (a) Polycarbonate (PC) hierarchical fibrils which were pulled and bent (noted by red ovals) after shear force measurements[67]. (b) CNT hierarchical structure which deformed after a single shear force measurement (top layer material: CNT and bottom layer material: SU-8)[75].

the shapes of the gecko’s adhesive (Figs. 5d and 5e), however the improvement on adhesion was limited[64–66], or was only focused on testing the frictional response[67]. A hierarchical structure containing both micro- and nano-scale fibrils was fabricated using a combination of replica molding and dip-and-transfer method[74]. This hierarchical dry adhesive showed an enhanced adhesion force compared to the single level nano-fibrillar adhesive. A correlation between the enhanced adhesion force and structural compliancy was also demonstrated from the large quantity of adhesion force measurements. Fibrillar structural damage remained a problem for both the polymeric materials (Fig. 6a)[67] and CNT (Fig. 6b)[75]. Characterization methods in recent studies have moved on to using a hemisphere and making use of scanning probe microscopy to automatically collect large amounts of data points[64–66,76–78]. Another type of hierarchical structure is formed by a continuous thin film with smaller scale fibrils on the top and the larger scale fibrils sandwiched between the thin film and the substrate (Figs. 7a and 7b)[68–71]. This type of hierarchical fibrillar structure does not have a topographical resemblance to the gecko’s adhesive structures. However, this design can provide compliancy of the fibrillar structure while avoiding contact area reduction. The performance of these hierarchical structures demonstrates some improvement in both adhesion and friction although the linked film limits the movement of the larger fibrils and resulted in lower compliancy with shear loading and shear-induced adhe-

sion[5,7,73]. A variation of the thin film type hierarchical structure is a lamellar like structure (Figs. 7d–7f), with nano-fibrils on the thin flap of polymer[79] and the results of this research focused on the enhanced frictional performance.

4 New direction: Functionalized dry adhesives In the last few years, steps have been taken to enhance adhesion through functionalization. There have been a few different approaches to dry adhesive functionalization including the inclusion of conductive or magnetic nanoparticles to enhance dry adhesion with additional properties such as electrostatic adhesion, force sensing or adhesion switching. Combining the benefits of the electrostatic chuck and the surface roughness adaptability of the gecko foot-hair was proposed in 2006[80] although the testing of the proposed theory was performed using magnets instead of electrostatics. On the other hand, enhancing dry adhesion, which mainly relies on van der Waals forces, was the motivation for the inclusion of conductive nanoparticles into what have been referred to as electro-dry-adhesives[81,82]. By doping either the dry adhesive surface features and/or the backing layer with carbon black, Krahn and Menon[81] showed that a large increase (~4×) in shear adhesion was possible by the application of a sufficiently high voltage (up to 4 kV approximately) on non-conducting surfaces. Improvements to the design enabled the electro-dry-adhesives to

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Fig. 7 Hierarchical fibrillar structure with thin film forming the interaction layer. (a) SEM image of PDMS hierarchical fibrillar structures with a continuous film on top of fibrils as contact interface with other substrate[68]. (b) SEM image of PDMS hierarchical structure with a continuous film on top of the larger fibrils being used as the contact interface[70]. (c) High Density Polyethylene (HDPE) hierarchical structures with lamellar shaped of flaps with micro-fibrils on the surface of the continuous thin film layer. (d) SEM side view image of a single lamellar showing the micro-fibrillar layer. (e) SEM side view image of arrays of micro-fibrils. (c)-(e) are reproduced from Ref. [79].

be used on conducting surfaces while including both positive and negative electrodes on a single sample[82]. The electro-dry-adhesives were also capable of self-preloading by using the electrostatic forces to attract the adhesives to the contact surface. More recent electrostatic dry adhesives were fabricated by embedding conductive mesh within a silicone rubber with angled fibre microstructures as a dry adhesive contact surface[83]. A further advantage of the addition of conductive particles was the ability to measure a change in the re-

sistance of the dry adhesive material as the material was compressed or stretched. The change in the resistance of the dry adhesives as they were compressed or stretched was found to be related to the force applied to the dry adhesives, which allowed force-sensing functional dry adhesives to be made[84]. The addition of carefully arranged millimetre-scaled posts allowed the detection and measurement of torques applied to the dry adhesives as well. Other methods for increasing the strength of dry adhesives through functionalization include adhesion

Li et al.: Bioinspired Dry Adhesive Materials and Their Application in Robotics: A Review

enhanced by coating functional molecules[85,86], adhesive switching enabled by changing the stiffness of the backing layer which was done by embedding a phase-change wax layer into a PDMS backing layer[87] or shape memory alloy material[88], embedding iron oxide nanoparticles[89] or carbonyl iron micro-particles[90], in situ synthesizing gold nanoparticles on the PDMS fibrillar structure[91], and doping PDMS microstructures with photoresponsive chromophores[92]. Sitti et al.[85] showed the adhesion enhanced by grafting poly(n-butyl acrylate) (PBA) into the PDMS mushroom shape fibrillar adhesives. Different molecular weights of the PBA grafted resulted in different surface roughness of the mushroom tips. All grafted surfaces showed higher adhesion than the ones with no extra molecule grafting. On the other hand, doping of a thin layer of poly(dopamine methacrylamideco-methoxyethyl acrylate) (p(DMA-co-MEA), inspired from mussel adhesive, on the gecko fibrillar adhesive enabled the PDMS adhesive to achieve improved adhesion performance in both dry and wet environments[86]. By embedding a temperature-controlled phasechange material into the backing layer[87], Krahn et al. showed that when the backing layer was in the liquid state, the adhesion significantly decreased while a large increase in adhesion was observed when contact was made with an object with the backing layer in the liquid phase which was then allowed to cool to a solid phase before adhesion measurements were made. The increase in adhesion was largely due to an increased contact area and a resistance to peeling. Except for embedding a phase-change material into the adhesive, shape memory alloy type of materials can also serve as actuation mechanism in controlling the adhesion of the fibrillar structure. Frensemeirer et al.[88] proposed an adhesive structure which has a trained layer of nickel-titanium surfaces attached as a backing layer. The adhesive can be totally shut off (provide almost no adhesion) by heating the structure to 80 ˚C. With iron-oxide nanoparticles embedded into the backing layer of the dry adhesives[89], adhesion switching was observed depending on the strength and direction of an externally applied magnetic field. In the presence of the magnetic field, the iron oxide particles tried to orient themselves to the magnetic field, which resulted in the stiffening of the backing layer. An in-

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crease in adhesion was observed when the magnetic field was applied after adhesive contact had been made. Another research[90] showed the micro-ridges actuated by rotating magnetic fields. With micron size carbonyl iron powder embedded in the PDMS ridges, visible deformations were observed by different magnetic fields applied, enabling the adhesive to pick up and drop down glass spheres from 500 μm to 1 mm. To avoid nanoparticle aggregation during polymer crosslinking, an in situ method of embedding gold nanoparticles on the micro-fibrillar adhesive was introduced, which showed an improved adhesion on such structure[91]. Four samples were prepared using different combinations of gold salt solution concentration and incubation time. All samples appeared to have rougher surfaces than the flat PDMS. Three of the samples showed at least one time higher adhesion pressure compared to the control which contained only pure PDMS in the fibrillar structure. The only sample having lower adhesion pressure appeared to be due to stiffness change of the adhesive sample resulting from excessive amounts of gold nanoparticles in the material. Doping of a photoresponsive material, spiropyrans, changed the surface adhesion of a micro-fibrillar adhesive[92]. Shining UV or visible light on the photochromic material can alter the molecules (close or open the molecular ring) on top of the polymeric fibrils. The opening and closing of the molecular ring appears to be responsible for a change in the surface energy of the PDMS fibrillar adhesive which results in different (‘switched’) adhesion forces. Switching of the surface properties are reversible and appear to have similar trend in each cycle.

5 Robotic applications of bioinspired adhesives Intensive development of artificial bioinspired adhesives has led to the development of applications where dry adhesives may excel. Dry adhesives, because of their reusability, long-term adhesion capability, ability to adhere without leaving a residue and inspiration from the gecko, are suited for robotic applications such as climbing robots and robotic grippers. Also, some versions of the adhesives are designed to replace medical tapes[60,93] and potentially medical tools such as mechanical clamps for temporary fixtures. In this section, robotic applications making use of artificial dry adhe-

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sives are reviewed. 5.1 Climbing robots Dry adhesives, inspired from climbing animals, are naturally suitable for use in climbing robots[94]. However, the preloading and peeling mechanisms are essential for climbing robots to use the fibrillar adhesives in both attachment and detachment phases. Various methods have been developed to accomplish both preloading and peeling mechanisms. Some of the first climbing robots to incorporate both peeling and preloading mechanisms were simple tread-based locomotion platforms and wheel-leg (wheg) designs[95]. In both cases, a tail provided the preloading mechanism and in the tread-based design, adhesive peeling naturally occurred at the rear of the tread as the robot moved forward. Stickybot[96] took the inspiration of the entire gecko, which was designed to climb on vertical surfaces (see Fig. 8a). The robot has compliant structures from toes to the entire body, providing flexibility to conform to different surface topography. The dry adhesive used in this robot is made of a soft elastomer; the micro-fibrils are made in the shape of wedges. The wedges are arranged to utilize the robots own weight to preload the adhesives on the climbing surfaces. An upgraded version of dry adhesive with micro-wedges made in hierarchical structure (Fig. 7b) empowered the Stickybot to climb on rougher surfaces, e.g. wooden walls[29,96]. The flexible body of the robot, however, does not provide the ability for the robot to transition from parallel to vertical surfaces. The successive version of the robot, Stickybot III (Fig. 8b), used a tendon mechanism inspired from the gecko’s foot and a directional adhesive to climb on vertical surfaces[97,98]. This tendon mechanism, also used in a heavy legged robot, RISE[98], significantly increased the payload of a climbing robot, which reaches a maximum adhesion pressure of ~ 10.5 kPa[97], a 7.5 times larger normal pressure compared with a robot without the tendon mechanism. A similar robot (Fig. 8c) was developed by another research group[99,100], but the focus of the research is to use force feedback for enhanced gait planning. Methods adapted from discontinuous-constraint metamorphic mechanism were used for gecko robot gait planning[99–102]. Simulation results[102] indicated the requirement of adhesion force between the climbing feet and the surfaces when climbing on an up-side-down position. The group also

developed a 3D force measurement system[103] that can be used to measure force exerted by each leg, on both gecko and climbing robot on a vertical climbing position. Mini-whegs[13] are a series of robots which use whegs for locomotion. In this case, each wheel has four spokes (see Fig. 9a). The spokes are attached to adhesive strips which form the feet and the flexibility of the feet acts as a hinge between the feet and the spokes. As the wheels turn, the feet contact the substrate, bend as the wheel turns and slowly peel off from the substrate before springing back into position for the next contact. The wheg design was further improved by Waal[104,105] bot which used whegs with three feet and was the first climbing robot, relying on dry adhesives, to realize transition from horizontal to vertical surfaces (see Fig. 9b) although sometimes Waalbot failed to transition due to the non-perpendicular angle at which the forward foot may contact a wall. Rotating the foot brings the next adhesive pad into contact with a surface as the robot moves forward. A tail was used to ensure that the adhesive pads maintained adhesive contact with a surface by providing a preloading force for the fibrillar adhesive during both locomotion and transition from horizontal surfaces to vertical walls. Originally, Waalbot did not use fibrillar adhesives but rather a flat tacky pad on each foot pad of the “leg”[105]. The upgraded version Waalbot II (Fig. 9c), which used 50 μm diameter micro-fibrils in the dry adhesives, was able to climb on wooden walls with enhanced agility over that of the original version: Waalbot II was able to transition between horizontal and vertical surfaces freely and can even climb up-side-down[106]. More recently, a wheg-like smart mechanism was developed as a “self-loading” mechanism[107] where rigid flat adhesive pads were attached to asymmetrical rectangular wheels. The flat rigid adhesive pads were attached to two opposing corners of the rectangular wheels using a hanging mechanism that enables the flat rigid adhesive pad to remain in contact with a surface through a 180 degree rotation of the wheel. Various forms of tread-based or ‘tank’ robots have been developed and often incorporate some sort of tail or preloading mechanism into their design. Tank robots are based on a relatively simple mechanism that relies on rotating adhesive belts for locomotion. As the adhesive belts rotate, the body of the robot moves forward

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Fig. 8 Gecko-like climbing robots. (a) Stickybot on a smooth surface[96]; (b) Stickbot III[97] with tendon mechanism showed in the inset; (c) gecko-like robot having force-feedback sensors for gait planning research[100].

Fig. 9 Wheel-leg robots. (a) Mini-wheg with micro-fibrillar adhesive (green tapes) and 25 cm long tail (not shown)[13]; (b) Waalbot design, no fibrillar adhesive used[105]; (c) Waalbot II with micro-fibrils as adhesive and improved tails[106].

bringing a new portion of the adhesive belt into contact with the substrate. At the same time, at the rear of the robot, the adhesive belt is peeled from the substrate. The

size of the wheels upon which the adhesive belts rotate can be optimized to provide the optimum peeling angle. The first tank robots (Fig. 10a) were simple designs

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with fixed tails and pre-tension belts driven by a single motor[95]. The simple design proved to be robust and allowed the robots to climb vertical or near vertical surfaces, however, with a fixed tail, robots had difficulty in transitioning from horizontal to vertical surfaces and vice-versa. The addition of an active tail[108] allowed tankbot to transfer from vertical to horizontal surfaces. Tailless tank robots were first proposed with TBCP-I[109] which utilized two separate tank-like modules joined together by an active joint (Fig. 10b). Vertical climbing and horizontal to vertical transitioning was only achieved with TBCP-II[110] however. TBCP-II (Fig. 10c) improved the design of TBCP-I by integrating an active joint which allowed the front and rear modules to be preloaded as the robot was moving forward and utilized dry adhesive belts with micro-scale adhesive structures with mushroom caps. The active joint could be used to control the distance separating the modules and the individual preloads applied to each module, and allowed the robot to transition from vertical to horizontal (c)

(a)

surfaces and vice-versa while ensuring maximal adhesive contact. Infrared distance measuring sensors were used to measure the distance between the robot and the substrate it was climbing and to detect when detachment conditions were beginning to occur. TBCP-II was then able to use the active joint to apply a preloading force to either the front or rear joint as required to ensure adhesion of the dry adhesive belts. A further improvement to the tank-like robot design was the development of Multitank (Fig. 10d) – a tank-like module-based climbing robot using passive compliant joints[111]. In this case however, the adhesive were flat dry unstructured elastomers. Another approach to design climbing robots has been through spider-like robots. Unlike other types of robots, such as the tank-like robots, spider-like robots have a higher mobility and are capable of manoeuvring on surfaces with different gradients. They also have the ability to cross trenches, ledges and rotate to different angles. One of the main drawbacks with the spider-like (d)

A

hCG

B

C

D

(b)

Fig. 10 Tank-like climbing robots. (a) First tank robot with one passive tail and miniaturized body[95]; (b) timing belt based climbing platform (TBCP-I) with four modules[109]; (c) TBCP-II on a smooth surface for testing maximum preload capability[110]; (d) multitank design schematics[111].

Li et al.: Bioinspired Dry Adhesive Materials and Their Application in Robotics: A Review

(a)

193

(c)

(b)

Fig. 11 Spider-like climbing robots. (a) Abigaille I sitting on a flat surface[112]; (b) Abigaille-II sitting on a flat surface with dual-level adhesive attached[113]; (c) Abigaille III posture on a vertical smooth surface[114].

robots is the complexity, as each leg requires one or more motors, and a complex control/electronic system is required to synchronize the movements of each leg. Abigaille-I[112] was a 6-legged robot with each leg having 6 Degrees-of-Freedom (DOF) and requiring 18 miniaturized revolute motors. Abigaille-I (Fig. 11a) was capable of climbing sloped but not vertical surfaces. Abigaille-II (Fig. 11b) increased the number of DOF from 18 to 36 with 18 DOF being actively controlled[113]. Abigaille-II was capable of short vertical climbing and transitioning from horizontal to vertical surfaces and relied upon duel-level dry adhesives for adhesion. The most recent spider-like climbing robot was Abigaille-III[114], a 6-legged robot using 24 miniature gear motors to control movements with up to four motors per leg (Fig. 11c). A single motor was dedicated to peeling each adhesive foot pad from the substrate using a cam which reduced the undesired vibrations encountered when forcibly peeling an adhesive foot pad from the climbing substrate as observed with both Abigaille I & II. While their multi-legged design provided the ability for the Abigaille series robots to climb over multiple surfaces, the spider-like robots tend to move slower than the other types of robots. A few recent climbing robots have been designed to

use an inchworm style of climbing[115,116]. The ACROBOT robot utilizes directional adhesives that can be controlled to be in either an ON or OFF state by orienting the adhesive pads in opposite directions. This enables the ACROBOT to climb a surface regardless of the orientation of the robot or regardless of significant gravitational forces. Two much smaller, 9 g and 20 mg robots with an inchworm gait were recently shown to be capable of hoisting weights of more than 1 kg and 500 mg respectively and were shown to be robust to missed steps[116]. 5.2 Robotic grippers Perhaps due to their inspiration from geckos, dry adhesives have most often been designed to work with climbing robots. Another interesting use for dry adhesives is as a gripping interface. Grippers that utilize dry adhesives are reusable and create both firm grip and limited damage to the objects being grasped. This type of grippers is potentially useful in robotic manufacturing industries which involve handling large LCD panels, advertising board and also integrating circuitry chips. Zhou et al.[117] designed a tripod-like gripper (Fig. 12a) for transferring light objects such as silicon wafers. Their gripper utilized a dry adhesive attached to the base of tri-pod like posts. The positioning of the posts was

(a)

(c)

The leg springs

T+

T−

The gecko-inspired surface The guiding part

Threads

h Ad

esi

al t rmnen 2T y No po om T x on C

Ty T

Co n ten stant s in ion f Lin ilm dec ea r r e te as ly in nsioning film

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Disengaged adhesive Engaged adhesive, Uniform shear load Y

X mg = 2Ty

Gripper prototype (b)

(i)

(ii) Gripper body

Gripper body

Object

Air

Adhesive membrane 10 mm

Membrane Steel ball

Fig. 12 Grippers which make use of fibrillar adhesives. (a) Tripod-like grippers with dry adhesives on each leg[117]; (b) soft, inflatable gripper (Left: schematic; middle and right: Prototype.) with micro-fibrillar adhesive membrane[118]; (c) shear adhesion gripper with fibrillar thin film (Left: Prototype griping a basketball, right: Schematics of working mechanism)[119].

controlled using a servo motor which either brought the feet closer together for gripping or forced them apart for detaching the substrate from the adhesives. Jeong et al.[72] performed a test using their adhesives for transferring glass LCD panels. During the test, the dry adhesive was attached to the LCD panel and the panel was lifted, moved to a new position, and released. Inspections using an Atomic Force Microscope (AFM) showed that there was virtually no contamination on the surface of the glass after the pick and place operation. More recently, a soft, inflatable robotic gripper (Fig. 12b) was developed[118] that uses a dry adhesive coated deformable membrane to seal a chamber. The pressure within the chamber can be controlled by a pump that is used to either inflate or deflate the membrane and allows an object to be grasped or released by forcing it to conform or peel from the surface of the object respectively. The dry adhesive contact layer enables the gripper to grasp a wide range of objects and to ensure good contact with the object being grasped. A thin-film dry adhesive gripper (Fig. 12c) was recently able to grasp various objects ranging in size from a roll of tape to a basketball relying on sheer adhesion without squeezing[119]. The gripper relied entirely on directional dry adhesives to engage the object under

shear loads while the detachment of the adhesive was performed by peeling the adhesives in the opposite direction of the loading. Using a similar design to some of the inchworm style climbing robots mentioned previously, a gripper that utilized opposed controllable adhesives with an approximately 1000:1 attachment to detachment force ratio was recently described by Hawkes et al.[120]. In this case, the gripper contained no moving parts but instead relied on tethers to connect two adhesive plates. As the weight of the object was transferred to the tethers, the tethers pulled the adhesive plates in opposing directions and provided a shear loading to the adhesives which ensured that the adhesive plates maintained a good grasping strength. A similar scaled up design was recently proposed for grasping space debris[121]. Using PDMS micro-wedges attached to arrays of plates which provided controllable directional adhesion, four ‘grippers’ were used to support the weight of a 70 kg climber as he ascended a 3.7 m tall vertical glass surface in about 90 seconds[122]. The PDMS micro-wedges were described as being ideal for climbing because of their directionality which enabled strong adhesive contact when loaded but were easily detached when pulled free from the surface in the opposite direction.

Li et al.: Bioinspired Dry Adhesive Materials and Their Application in Robotics: A Review

6 Conclusion In this article, the mechanisms of biological fibrillar adhesives are reviewed. The key aspects of biological fibrillar adhesives, which are geometry, material and climbing gaits, are discussed. From a geometrical viewpoint, the single level adhesives and multi-level adhesives are reviewed. These fibrillar adhesives are made in different materials. Some of the fibrils are tilted with respect to the backing layer which demonstrate the directional properties as the gecko’s feet. Using these fibrillar adhesives, climbing robots with different climbing mechanisms are developed, including gecko-like robots, wheel-leg (wheg) robot, tank robots and spider-like robots. Another popular application for dry adhesives is in the field of robotic grippers. A new direction of adhesive fabrication, functionalized dry adhesives, is also discussed. Changing the materials by embedding another type of material, modifying the polymer composition and surface treatment on the fibrillar adhesives open a new gate of functionalized adhesives. This research into dry adhesives sheds light on the future development of bioinspired dry adhesives based on the dry adhesives which are already being explored in various aspects.

Acknowledgment This work was supported in part by the Natural Sciences and Engineering Research Council (NSERC) of Canada.

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