Evidence for capillarity contributions to gecko adhesion from single spatula nanomechanical measurements

Revisiting the classification of squamate adhesive setae: historical, morphological and functional perspectives

Research on gecko-based adhesion has become a truly interdisciplinary endeavour, encompassing many disciplines within the natural and physical sciences. Gecko adhesion occurs by the induction of van der Waals intermolecular (and possibly other) forces between substrata and integumentary filaments (setae) terminating in at least one spatulate tip. Gecko setae have increasingly been idealized as structures with uniform dimensions and a particular branching pattern. Approaches to developing synthetic simulacra have largely adopted such an idealized form as a foundational template. Observations of entire setal fields of geckos and anoles have, however, revealed extensive, predictable variation in setal form. Some filaments of these fields do not fulfil the morphological criteria that characterize setae and, problematically, recent authors have applied the term 'seta' to structurally simpler and likely non-adhesively competent fibrils. Herein we briefly review the history of the definition of squamate setae and propose a standardized classificatory scheme for epidermal outgrowths based on a combination of whole animal performance and morphology. Our review is by no means comprehensive of the literature regarding the form, function, and development of the adhesive setae of squamates and we do not address significant advances that have been made in many areas (e.g. cell biology of setae) that are largely tangential to their classification and identification. We contend that those who aspire to simulate the form and function of squamate setae will benefit from a fuller appreciation of the diversity of these structures, thereby assisting in the identification of features most relevant to their objectives.

Bioinspired Microsphere-Embedded Adhesive Architectures for an Electrothermally Actuating Transport Device of Dry/Wet Pliable Surfaces

For highly conformable and universal transport devices, bioinspired dry adhesion systems with reversible molecular attractions (e.g., van der Waals forces, capillarity, or suction stress) between the engaged surfaces have recently become favorable for various dry/wet processes in flexible devices and medical applications. In addition, many efforts have been made for switchable attachments of such adhesives by employing costly sophisticated systems such as mechanically deformable chucks, UV-radiating components, or fluidic channels. In this work, we propose a simple electrothermally actuating transport device based on an octopus-inspired microsphere-embedded sucker (OMS). The adhesive with microsphere-embedded suckers offers enhanced adhesion on dry/wet surfaces, in accordance with investigation of the geometric and materials parameters of the novel suction architecture for maximizing adhesion interactions. Inspired by muscle actuation of octopus tentacles, we laminate the electrothermally reactive poly(3,4-ethylenedioxy thiophene):poly(styrenesulfonic acid) (PEDOT:PSS) layer on the backside of the OMS adhesive patch. By controlling inputs of electrical energy, our assembled actuator may actively expand and contract reversibly to induce switchable attachments and detachments. Our bioinspired device can be integrated onto a robotic arm to attach and release against dry/wet flexible thin objects.

Bioinspired photocontrollable microstructured transport device

Geckos, which can walk upside down on vertical and underneath horizontal surfaces, owe this ability to the hierarchical structures under their toes. These structures are responsible for substantial adhesion and, at the same time, for quick detachment by mechanical stimulus through leg movements. Inspired by such stimuli-responsive systems in nature, we developed an artificial, photocontrollable microstructured transport device. Through tunable ultraviolet light illumination, the adhesive ability of this bioinspired transport device is reduced up to a factor of 2.7 in terms of adhesive forces and is quickly recovered when the light stimulus ceases. This bioinspired photocontrollable device has been used in a pick-up and drop-down system for transporting planar and three-dimensional solid objects.

The same but different: setal arrays of anoles and geckos indicate alternative approaches to achieving similar adhesive effectiveness

The functional morphology of squamate fibrillar adhesive systems has been extensively investigated and has indirectly and directly influenced the design of synthetic counterparts. Not surprisingly, the structure and geometry of exemplar fibrils (setae) have been the subject of the bulk of the attention in such research, although variation in setal morphology along the length of subdigital adhesive pads has been implicated to be important in the effective functioning of these systems. Adhesive setal field configuration has been described for several geckos, but that of the convergent Anolis lizards, comprised of morphologically simpler fibrils, remains largely unexplored. Here, we examine setal morphology along the proximodistal axis of the digits of Anolis equestris and compare our findings to those for a model gecko, Gekko gecko. Consistent with previous work, we found that the setae of A. equestris are generally thinner, shorter, and present at higher densities than those of G. gecko and terminate in a single spatulate tip. Contrastingly, the setae of G. gecko are hierarchically branched in structure and carry hundreds of spatulate tips. Although the splitting of contacts into multiple smaller tips is predicted to increase the adhesive performance of a fiber compared to an unbranched one, we posited that the adhesive performance of G. gecko and A. equestris would be relatively similar when the configuration of the setal fields of each was accounted for. We found that, as in geckos, setal morphology of A. equestris follows a predictable pattern along the proximodistal axis of the pad, although there are several critical differences in the configuration of the setal fields of these two groups. Most notably, the pattern of variation in setal length of A. equestris is effectively opposite to that exhibited by G. gecko. This difference in clinal variation mirrors the difference in the direction in which the setal fields of anoles and geckos are peeled from the substrate, consistent with the hypothesis that biomechanical factors are the chief determinants of these patterns of variation. Future empirical work, however, is needed to validate this. Our findings set the stage for future comparative studies investigating the functional morphology of these convergent adhesive apparatuses. Such investigations will lead to an enhanced understanding of the interactions between form, function, and environment of fibril-based biological adhesive systems.

The effect of substrate wettability and modulus on gecko and gecko-inspired synthetic adhesion in variable temperature and humidity

Gecko adhesive performance increases as relative humidity increases. Two primary mechanisms can explain this result: capillary adhesion and increased contact area via material softening. Both hypotheses consider variable relative humidity, but neither fully explains the interactive effects of temperature and relative humidity on live gecko adhesion. In this study, we used live tokay geckos (Gekko gecko) and a gecko-inspired synthetic adhesive to investigate the roles of capillary adhesion and material softening on gecko adhesive performance. The results of our study suggest that both capillary adhesion and material softening contribute to overall gecko adhesion, but the relative contribution of each depends on the environmental context. Specifically, capillary adhesion dominates on hydrophilic substrates, and material softening dominates on hydrophobic substrates. At low temperature (12 °C), both capillary adhesion and material softening likely produce high adhesion across a range of relative humidity values. At high temperature (32 °C), material softening plays a dominant role in adhesive performance at an intermediate relative humidity (i.e., 70% RH).

Comparison of tarsal attachment in two closely related leaf beetle species

Green dock beetles Gastrophysa viridula and Colorado potato beetles Leptinotarsa decemlineata having distinctly different body mass and gait habits were compared with respect to their tarsal morphology and attachment ability. The focus laid on shapes and dimensions of tenent setae related to the peeling line, i.e., the sum of widths of all thin-film elements participating in contact. High-speed rotation of the two leaf beetle species attached to the horizontal and vertical sides of a Plexiglass drum resulted in higher attachment forces of the heavier beetle species that has a larger number of tarsal setae and a larger peeling line length. However, normalizing the measured forces with the corresponding peeling line lengths led to a reversed relationship. This allowed us to assume that the design of adhesive setae in different leaf beetle species matches the requirements imposed by their habitats. In accordance with the theory of thin film peeling, tangential forces were found to be higher than normal forces. The attachment system of females was found to exhibit stronger functional efficiency, which can be correlated to the morphology of their setae.

The Effects of Roughness and Wetness on Salamander Cling Performance

Animals clinging to natural surfaces have to generate attachment across a range of surface roughnesses in both dry and wet conditions. Plethodontid salamanders can be aquatic, semi-aquatic, terrestrial, arboreal, troglodytic, saxicolous, and fossorial and therefore may need to climb on and over rocks, tree trunks, plant leaves, and stems, as well as move through soil and water. Sixteen species of salamanders were tested to determine the effects of substrate roughness and wetness on maximum cling angle. Substrate roughness had a significant effect on maximum cling angle, an effect that varied among species. Substrates of intermediate roughness (asperity size 100-350 µm) resulted in the poorest attachment performance for all species. Small species performed best on smooth substrates, while large species showed significant improvement on the roughest substrates (asperity size 1000-4000 µm), possibly switching from mucus adhesion on a smooth substrate to an interlocking attachment on rough substrates. Water, in the form of a misted substrate coating and a flowing stream, decreased cling performance in salamanders on smooth substrates. However, small salamanders significantly increased maximum cling angle on wetted substrates of intermediate roughness, compared with the dry condition. Study of cling performance and its relationship to surface properties may cast light onto how this group of salamanders has radiated into the most speciose family of salamanders that occupies diverse habitats across an enormous geographical range.

Finite element simulation of friction and adhesion attributed contact of bio-inspired gecko-mimetic PDMS micro-flaps with SiO2spherical surface

The remarkable tribological attributes of the gecko feet have grown much interest in the field of biomimetic tribology over the past two decades. It has been shown that the complexity of friction and adhesion phenomena made it difficult to transfer these exceptional properties into fully functional smart, dry, micro patterned adhesives. The latter, combined with the relative lack of literature on computational oriented studies on these phenomena, is the motive of the current work. Here, a 2D time-dependent finite element model of friction and adhesion attributed contact of polydimethysiloxane (PDMS) micro flaps with a smooth SiO₂spherical surface is presented. The model is tested through simulations concerning changes in the disc curvature, the flap density, as well as different disc mounting heights, representing the effect of preload. Furthermore, the effect of tribological parameters of adhesion and friction coefficient is discussed. Finally, the effect of the use of two hyperelastic material models was examined.

Adhesive pads of gecko and anoline lizards utilize corneous and cytoskeletal proteins for setae development and renewal

The formation of the complex pattern of setae in adhesive pads of geckos and anoline lizards has been analyzed by ultrastructural, autoradiographic, and immunohistochemical methods. Setae terminate with spatulated ends responsible for adhesion that allow these lizards to climb vertical substrates and conquer arboreal niches. Setae derive from a complex interfaced molding between two specialized epidermal layers of the shedding complex that determines the cyclical skin molting, Oberhautchen and clear layers. Setae result from the action of setae cytoskeleton and the surrounding cytoplasm of clear cells. An intense protein synthesis, indicated by histidine and proline autoradiography, takes place during setae formation. Corneous and cytoskeletal proteins such as beta-proteins (CBPs), intermediate filament keratins (IFKs), actin, RhoV, tubulin, plakophilin-1, are produced during setae formation. Microfilaments of actin and microtubules of tubulin grow inside the elongating setae. Microtubules associated with filaments of unknown IKFs are produced in the cytoplasm of clear cells, forming a helical cytoskeleton that surrounds the growing setae. Oberhautchen and clear cells are tightly joined by numerous desmosomes and plakophilin-1 is mainly localized along the perimeter of these cells. These observations suggest that actin and tubulin are present in a dynamic form in the forming setae and in the surrounding cytoplasm of clear cells. Aside the localized micro-deformations of the cell membranes leading to setae formation the cytoskeleton determines the accumulation of CBPs inside the growing setae and the spatula. How the genome determines the specific pattern of cytoskeletal organization with the resulting species-specific setae branching remains unknown.

Reversible Underwater Adhesion: The Unique C-shaped Suckers of Net-winged Midge Larvae (Blepharicera sp.)

Aquatic insects living in fast-flowing streams have developed various types of attachment systems to resist being carried away by strong currents. Combinations of various attachment devices offer aquatic insects advantages in underwater adhesion on substrates with different surface properties. In this study, the net-winged midge (Blepharicera sp.) larvae were investigated to understand micro-/nano-structural attachment mechanisms. The hierarchical structure of insect adhesive surfaces was characterized using Optical Microscopy (OM), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Centrifugal measurements were also conducted to measure the critical rotational velocity at which the larvae of Blepharicera sp. can adhere to substrates with varying roughness. Commercial suckers require smooth substrate surface to maintain a pressure that is lower than the surrounding pressure for adhesion under the sucker cup while the suckers of net-winged midge larvae possess hierarchical micro-/nano-structures, which attach closely to rough surfaces underwater. Furthermore, the functions of microstructures observed on the sucker, including wrinkled surface, inward setae, outer fibers, and nick were explored and may contribute to underwater adhesion. The aligned C-shaped suckers can attach and detach effectively by closing or opening the gap. The unique microstructure and adhesion capability of such suckers could shed light on the design and synthesis of novel bio-inspired devices for reversible underwater adhesion.

Liquid-Superrepellent Bioinspired Fibrillar Adhesives

Bioinspired elastomeric fibrillar surfaces have significant potential as reversible dry adhesives, but their adhesion performance is sensitive to the presence of liquids at the contact interface. Like their models in nature, many artificial mimics can effectively repel water, but fail when low-surface-tension liquids are introduced at the contact interface. A bioinspired fibrillar adhesive surface that is liquid-superrepellent even toward ultralow-surface-tension liquids while retaining its adhesive properties is proposed herein. This surface combines the effective adhesion principle of mushroom-shaped fibrillar arrays with liquid repellency based on double re-entrant fibril tip geometry. The adhesion performance of the proposed microfibril structures is retained even when low-surface-tension liquids are added to the contact interface. The extreme liquid repellency enables real-world applications of fibrillar adhesives for surfaces covered with water, oil, and other liquids. Moreover, fully elastomeric liquid-superrepellent surfaces are mechanically not brittle, highly robust against physical contact, and highly deformable and stretchable, which can increase the real-world uses of such antiwetting surfaces.

Recent Advances in Tissue Adhesives for Clinical Medicine

Tissue adhesives have attracted more attention to the applications of non-invasive wound closure. The purpose of this review article is to summarize the recent progress of developing tissue adhesives, which may inspire researchers to develop more outstanding tissue adhesives. It begins with a brief introduction to the emerging potential use of tissue adhesives in the clinic. Next, several critical mechanisms for adhesion are discussed, including van der Waals forces, capillary forces, hydrogen bonding, static electric forces, and chemical bonds. This article further details the measurement methods of adhesion and highlights the different types of adhesive, including natural or biological, synthetic and semisynthetic, and biomimetic adhesives. Finally, this review article concludes with remarks on the challenges and future directions for design, fabrication, and application of tissue adhesives in the clinic. This review article has promising potential to provide novel creative design principles for the generation of future tissue adhesives.

Non-sticky and Free-forward Performances of Grubs against Soil

The intriguing non-sticky and free-forward performances of grubs against soil deeply attract our interests. In this study, the life cycle and body morphology of a kind of grubs, larvae of Japanese rhinoceros beetles, are introduced. The uniformly oriented hierarchical micro structures pattern on the back epidermis is firstly reported. The rotating and forwarding motion configuration of grubs in soil is unraveled. The friction and adhesion properties of grubs are evaluated and compared with typical materials. The biological electroosmosis induced adhesion reduction effect and the hierarchical structures pattern induced anisotropic friction feature are highlighted.

Immunolocalization of corneous proteins including a serine-tyrosine-rich beta-protein in the adhesive pads in the tokay gecko

Adhesive pads of geckos contain many thousands of nanoscale spatulae for the adhesion and movement along vertical or inverted surfaces. Setae are composed of interlaced corneous bundles made of small cysteine-glycine-rich corneous beta proteins (CBPs, formerly indicated as beta-keratins), embedded in a matrix material composed of cytoskeletal proteins and lipids. Negatively charged intermediate filament keratins (IFKs) and positively charged CBPs likely interact within setae, aside disulphide bonds, giving rise to a flexible and resistant corneous material. Using differernt antibodies against CBPs and IFKs an updated model of the composition of setae and spatulae is presented. Immunofluorescence and ultrastructural immunogold labeling reveal that one type of neutral serine-tyrosine-rich CBP is weakly localized in the setae while it is absent from the spatula. This uncharged protein is mainly present in the thin Oberhautchen layer sustaining the setae, although with a much lower intensity with respect to the cysteine-rich CBPs. These proteins in the spatula likely originate a positively charged or neutral contact surface with the substrate but the influence of lipids and cytoskeletal proteins present in setae on the mechanism of adhesion is not known. In the spatula, protein-lipid complexes may impart the pliability for the attachment and adapt to irregular surfaces. The presence of cysteine-glycine medium rich CBPs and softer IFKs in alpha-layers sustaining the setae forms a flexible base for compliance of the setae to substrate and improved adhesion.

Boston Ivy Disk-Inspired Pressure-Mediated Adhesive Film Patches

Boston ivy (Parthenocissus tricuspidata) climbs brick walls using its tendril disks, which excrete a sticky substance to perform binding and attachment. While the cellular structures and adhesive substances involved have been identified for decades, their practical applicability as an adhesive has not yet been demonstrated. A Boston ivy disk-inspired adhesive film patch system is reported in which structural and compositional features of the Boston ivy disk are mimicked with a form of thin adhesive film patches. In analogy to the sticky disk of a mature ivy in which porous microchannels are occupied by catechol-containing microgranules on the bound site, 3,4-dihydroxylphenylalanine bolaamphiphile nanoparticle (DOPA-C7 NP)-coated alginate microgels are two-dimensionally positioned into the cylindrical holes that are periodically micropatterned on the flexible stencil film. Finally, it is demonstrated that the pressurization of the patch breaks the microgels filled in the holes, releasing the polysaccharides and leading to crosslinking with DOPA-C7 NPs via ligandation with combined Ca²⁺ and Fe³⁺ ions, thus enabling development of a pressure-mediated adhesion technology.

Soft dendritic microparticles with unusual adhesion and structuring properties

The interplay between morphology, excluded volume and adhesivity of particles critically determines the physical properties of numerous soft materials and coatings^1-6. Branched particles² or nanofibres³, nanofibrillated cellulose⁴ or fumed silica⁵ can enhance the structure-building abilities of colloids, whose adhesion may also be increased by capillarity or binding agents⁶. Nonetheless, alternative mechanisms of strong adhesion found in nature involve fibrillar mats with numerous subcontacts (contact splitting)^7-11 as seen in the feet of gecko lizards and spider webs^12-17. Here, we describe the fabrication of hierarchically structured polymeric microparticles having branched nanofibre coronas with a dendritic morphology. Polymer precipitation in highly turbulent flow results in microparticles with fractal branching and nanofibrillar contact splitting that exhibit gelation at very low volume fractions, strong interparticle adhesion and binding into coatings and non-woven sheets. These soft dendritic particles also have potential advantages for food, personal care or pharmaceutical product formulations.

Stick or Slip: Adhesive Performance of Geckos and Gecko-Inspired Synthetics in Wet Environments

The gecko adhesive system has inspired hundreds of synthetic mimics principally focused on replicating the strong, reversible, and versatile properties of the natural system. For geckos native to the tropics, versatility includes the need to remain attached to substrates that become wet from high humidity and frequent rain. Paradoxically, van der Waals forces, the principal mechanism responsible for gecko adhesion, reduce to zero when two contacting surfaces separate even slightly by entrapped water layers. A series of laboratory studies show that instead of slipping, geckos maintain and even improve their adhesive performance in many wet conditions (i.e., on wet hydrophobic substrates, on humid substrates held at low temperatures). The mechanism for this is not fully clarified, and likely ranges in scale from the chemical and material properties of the gecko's contact structures called setae (e.g., setae soften and change surface confirmation when exposed to water), to their locomotor biomechanics and decision-making behavior when encountering water on a substrate in their natural environment (e.g., some geckos tend to run faster and stop more frequently on misted substrates than dry). Current work has also focused on applying results from the natural system to gecko-inspired synthetic adhesives, improving their performance in wet conditions. Gecko-inspired synthetic adhesives have also provided a unique opportunity to test hypotheses about the natural system in semi-natural conditions replicated in the laboratory. Despite many detailed studies focused on the role of water and humidity on gecko and gecko-inspired synthetic adhesion, there remains several outstanding questions: (1) what, if any, role does capillary or capillary-like adhesion play on overall adhesive performance of geckos and gecko-inspired synthetics, (2) how do chemical and material changes at the surface and in the bulk of gecko setae and synthetic fibrils change when exposed to water, and what does this mean for adhesive performance, and (3) how much water do geckos encounter in their native environment, and what is their corresponding behavioral response? This review will detail what we know about gecko adhesion in wet environments, and outline the necessary next steps in biological and synthetic system investigations.

Competing forces on a liquid bridge between parallel and orthogonal dissimilar fibers

This paper presents a detailed investigation on the mechanical forces acting on a liquid bridge between dissimilar fibers in parallel and orthogonal configurations. These forces were measured experimentally, using a sensitive scale, and were also predicted computationally, via numerical simulation. Special attention was paid to the fiber-fiber spacing at which the liquid bridge detached from the fibers, and to how a transition from an equilibrium liquid bridge to a spontaneously (time-dependent) detaching bridge took place. It was found that, while varying the spacing between the fibers affects a liquid bridge differently for fibers with different relative angles with respect to one another, the spacing at which the bridge detaches from the fibers is independent of the fibers' relative angle. This paper also formulates the contribution of the geometrical and wetting properties of the fibers competing for the droplet that results from a liquid bridge detachment, and presents a mathematical expression to predict the fate of that droplet.

A transparent, glue-free, skin-attachable graphene pressure sensor with micropillars for skin-elasticity measurement

Strong peeling resistance and water-drainable properties on rough and wet skin surfaces are highly desirable for realizing wearable and skin-attachable electronic sensors. Here, we propose a transparent, sensitive, glue-free pressure sensor for skin electronics. To achieve a thin, light-weight, transparent, and stretchable sensor patch, we laminated a single-layer graphene film as a sensing element on a thin polymeric supporter of polydimethylsiloxane. By assembling the graphene layer with densely populated micropillars, the pressure sensor achieved 10 times the sensitivity of a similar sensor without micropillars in the low-pressure range (<6 kPa). We then employed hexagonal patterns inspired by the toe pads of a tree frog, giving the assembled patch sensor with strong peeling resistance under both dry and wet conditions on surfaces such as silicon (15.5 J cm^-2 for dry and 11.6 J cm^-2 for wet conditions) and pig skin (2.0 J cm^-2 for dry and 1.4 J cm^-2 for wet conditions) without contamination after detachment. Our layered sensor patch also demonstrated successful measurement of water-dependent skin elasticity with transparent, conformal, and residual-free attachment, suggesting a variety of cosmetic and medical applications.

Bioinspired Adhesive Architectures: From Skin Patch to Integrated Bioelectronics

The attachment phenomena of various hierarchical architectures found in nature have extensively drawn attention for developing highly biocompatible adhesive on skin or wet inner organs without any chemical glue. Structural adhesive systems have become important to address the issues of human-machine interactions by smart outer/inner organ-attachable devices for diagnosis and therapy. Here, advances in designs of biologically inspired adhesive architectures are reviewed in terms of distinct structural properties, attachment mechanisms to biosurfaces by physical interactions, and noteworthy fabrication methods. Recent demonstrations of bioinspired adhesive architectures as adhesive layers for medical applications from skin patches to multifunctional bioelectronics are presented. To conclude, current challenges and prospects on potential applications are also briefly discussed.

Capillarity-Enhanced Organ-Attachable Adhesive with Highly Drainable Wrinkled Octopus-Inspired Architectures

Mimicking the attachment of octopus suction cups has become appealing for the development of skin/organ adhesive patches capable of strong, reversible adhesion in dry and wet conditions. However, achieving high conformity against the three-dimensionally (3D) rough and curved surfaces of the human body remains an enduring challenge for further medical applications of wound protection, diagnosis, or therapeutics. Here, an adhesive patch inspired by the soft wrinkles of miniaturized 3D octopus suction cups is presented for high drainability and robust attachment against dry and wet human organs. Investigating the structural aspects of the wrinkles, a simple model is developed to maximize capillary interactions of the wrinkles against wet substrates. A layer of soft siloxane derivative is then transferred onto the wrinkles to enhance fixation against dry and sweaty skin as well as various wet organ surfaces. Our bioinspired patch offers opportunities for enhancing the versatility of adhesives for developing skin- and/or organ-attachable devices.

Rapidly responsive smart adhesive-coated micropillars utilizing catechol-boronate complexation chemistry

Smart adhesive hydrogels containing 10 mol% each of dopamine methacrylamide (DMA) and 3-acrylamido phenylboronic acid (APBA) were polymerized in situ onto polydimethylsiloxane (PMDS) micropillars with different aspect ratios (AR = 0.4, 1 and 2). Using Johnson-Kendall-Roberts (JKR) contact mechanics tests, the adhesive-coated pillars demonstrated strong wet adhesion at pH 3 (Wadh = 420 mJ m-2) and can be repeatedly deactivated and reactivated by changing the pH value (pH 9 and 3, respectively). When compared to the bulk adhesive hydrogel of the same composition, the adhesive-coated pillars exhibited a significantly faster rate of transition (1 min) between strong and weak adhesion. This was attributed to an increased surface area to volume ratio of the adhesive hydrogel-coated pillars, which permitted rapid diffusion of ions into the adhesive matrix to form or break the catechol-boronate complex.

The Integrative Biology of Gecko Adhesion: Historical Review, Current Understanding, and Grand Challenges

Geckos are remarkable in their ability to reversibly adhere to smooth vertical, and even inverted surfaces. However, unraveling the precise mechanisms by which geckos do this has been a long process, involving various approaches over the last two centuries. Our understanding of the principles by which gecko adhesion operates has advanced rapidly over the past 20 years and, with this knowledge, material scientists have attempted to mimic the system to create artificial adhesives. From a biological perspective, recent studies have examined the diversity in morphology, performance, and real-world use of the adhesive apparatus. However, the lack of multidisciplinarity is likely a key roadblock to gaining new insights. Our goals in this paper are to 1) present a historical review of gecko adhesion research, 2) discuss the mechanisms and morphology of the adhesive apparatus, 3) discuss the origin and performance of the system in real-world contexts, 4) discuss advancement in bio-inspired design, and 5) present grand challenges in gecko adhesion research. To continue to improve our understanding, and to more effectively employ the principles of gecko adhesion for human applications, greater intensity and scope of interdisciplinary research are necessary.

Improving the adhesion, flexibility, and hemostatic efficacy of a sprayable polymer blend surgical sealant by incorporating silica particles

Commercially available surgical sealants for internal use either lack sufficient adhesion or produce cytotoxicity. This work describes a surgical sealant based on a polymer blend of poly(lactic-co-glycolic acid) (PLGA) and poly(ethylene glycol) (PEG) that increases wet tissue adherence by incorporation of nano-to-microscale silica particles, without significantly affecting cell viability, biodegradation rate, or local inflammation. In functional studies, PLGA/PEG/silica composite sealants produce intestinal burst pressures that are comparable to cyanoacrylate glue (160 mmHg), ∼2 times greater than the non-composite sealant (59 mmHg), and ∼3 times greater than fibrin glue (49 mmHg). The addition of silica to PLGA/PEG is compatible with a sprayable in situ deposition method called solution blow spinning and decreases coagulation time in vitro and in vivo. These improvements are biocompatible and cause minimal additional inflammation, demonstrating the potential of a simple composite design to increase adhesion to wet tissue through physical, noncovalent mechanisms and enable use in procedures requiring simultaneous occlusion and hemostasis. STATEMENT OF SIGNIFICANCE: Incorporating silica particles increases the tissue adhesion of a polymer blend surgical sealant. The particles enable interfacial physical bonding with tissue and enhance the flexibility of the bulk of the sealant, without significantly affecting cytotoxicity, inflammation, or biodegradation. These studies also demonstrate how silica particles decrease blood coagulation time. This surgical sealant improves upon conventional devices because it can be easily deposited with accuracy directly onto the surgical site as a solid polymer fiber mat. The deposition method, solution blow spinning, allows for high loading in the composite fibers, which are sprayed from a polymer blend solution containing suspended silica particles. These findings could easily be translated to other implantable or wearable devices due to the versatility of silica particles.

Swelling effects on localized adhesion of an elastic ribbon

We investigate the adhesion mechanism between an elastic strip of vinylpolysiloxane bent in a racquet-like shape, and a thick elastomeric substrate with the aim to understand how local swelling modifies adhesion. Using a modified loop-tack adhesion test, we place a droplet of silicone oil in between the two materials, vary the dwell time and measure the force required to separate the two interfaces. The experiments are then compared with an analytical model that describes how the critical peel force is modified as the interfacial surface energy changes over time. Our study reveals that in certain circumstances swelling can enhance adhesion. More specifically, strong adhesion is obtained when most of the droplet is absorbed by the solid. By contrast, when the droplet remains at the interface a small adhesive force is measured.

Gecko Adhesion in Space and Time: A Phylogenetic Perspective on the Scansorial Success Story

An evolutionary perspective on gecko adhesion was previously hampered by a lack of an explicit phylogeny for the group and of robust comparative methods to study trait evolution, an underappreciation for the taxonomic and structural diversity of geckos, and a dearth of fossil evidence bearing directly on the origin of the scansorial apparatus. With a multigene dataset as the basis for a comprehensive gekkotan phylogeny, model-based methods have recently been employed to estimate the number of unique derivations of the adhesive system and its role in lineage diversification. Evidence points to a single basal origin of the spinulate oberhautchen layer of the epidermis, which is a necessary precursor for the subsequent elaboration of a functional adhesive mechanism in geckos. However, multiple gains and losses are implicated for the elaborated setae that are necessary for adhesion via van der Waals forces. The well-supported phylogeny of gekkotans has demonstrated that convergence and parallelism in digital design are even more prevalent than previously believed. It also permits the reexamination of previously collected morphological data in an explicitly evolutionary context. Both time-calibrated trees and recently discovered amber fossils that preserve gecko toepads suggest that a fully-functional adhesive apparatus was not only present, but also represented by diverse architectures, by the mid-Cretaceous. Further characterization and phylogenetically-informed analyses of the other components of the adhesive system (muscles, tendons, blood sinuses, etc.) will permit a more comprehensive reconstruction of the evolutionary pathway(s) by which geckos have achieved their structural and taxonomic diversity. A phylogenetic perspective can meaningfully inform functional and performance studies of gecko adhesion and locomotion and can contribute to advances in bioinspired materials.

Micropipette force sensors for in vivo force measurements on single cells and multicellular microorganisms

Measuring forces from the piconewton to millinewton range is of great importance for the study of living systems from a biophysical perspective. The use of flexible micropipettes as highly sensitive force probes has become established in the biophysical community, advancing our understanding of cellular processes and microbial behavior. The micropipette force sensor (MFS) technique relies on measurement of the forces acting on a force-calibrated, hollow glass micropipette by optically detecting its deflections. The MFS technique covers a wide micro- and mesoscopic regime of detectable forces (tens of piconewtons to millinewtons) and sample sizes (micrometers to millimeters), does not require gluing of the sample to the cantilever, and allows simultaneous optical imaging of the sample throughout the experiment. Here, we provide a detailed protocol describing how to manufacture and calibrate the micropipettes, as well as how to successfully design, perform, and troubleshoot MFS experiments. We exemplify our approach using the model nematode Caenorhabditis elegans, but by following this protocol, a wide variety of living samples, ranging from single cells to multicellular aggregates and millimeter-sized organisms, can be studied in vivo, with a force resolution as low as 10 pN. A skilled (under)graduate student can master the technique in ~1-2 months. The whole protocol takes ~1-2 d to finish.

Ultrastable Underwater Anti-Oil Fouling Coatings from Spray Assemblies of Polyelectrolyte Grafted Silica Nanochains

Surfaces that have superhydrophilic characteristics are known to exhibit extreme oil repellency under water, which is attractive for applications including anti-fogging, water-oil separations, and self-cleaning. However, superhydrophilic surfaces can also be easily fouled and lose their extreme oil repellency, which limits their usage in practical applications. In this work, we create an anti-oil fouling coating by spray coating poly(acrylic acid) (PAA)-grafted SiO₂ nanochains (approximately 45 nm wide and 300 nm long) onto solid surfaces, forming a nanoporous film exhibiting superhydrophilicity (water contact angle in air ≈ 0°) and underwater superoleophobicity (dichloroethane contact angle ≥ 165°). The polymer-grafted nanochain assemblies exhibit extremely low contact angle hysteresis (<1°) and small adhesion hysteresis (-0.05 mN m^-1), and thus, oil can readily roll off from the surface when the coating is immersed in water. Compared to other superhydrophilic surfaces, we show that both the unique structure of spray-assembled nanochains and the hygroscopic nature of PAA are essential to enable ultrastable anti-oil fouling. Even after the PAA-grafted nanochain coating is purposely fouled by oil, oil can be readily and completely expelled and lifted-off from the coating within 10 s when placed under water. Further, we show that our coating retains underwater superoleophobicity even after being subjected to shearing under water for more than 168 h. Our approach offers a simple yet versatile method to create an ultrastable superhydrophilic and anti-oil fouling coating via a scalable manufacturing method.

Re-designing materials for biomedical applications: from biomimicry to nature-inspired chemical engineering

Gathering inspiration from nature for the design of new materials, products and processes is a topic gaining rapid interest among scientists and engineers. In this review, we introduce the concept of nature-inspired chemical engineering (NICE). We critically examine how this approach offers advantages over straightforward biomimicry and distinguishes itself from bio-integrated design, as a systematic methodology to present innovative solutions to challenging problems. The scope of application of the nature-inspired approach is demonstrated via examples from the field of biomedicine, where much of the inspiration is still more narrowly focused on imitation or bio-integration. We conclude with an outlook on prospective future applications, offered by the more systematic and mechanistically based NICE approach, complemented by rapid progress in manufacturing, computation and robotics. This article is part of the theme issue 'Bioinspired materials and surfaces for green science and technology'.

Review: mapping proteins localized in adhesive setae of the tokay gecko and their possible influence on the mechanism of adhesion

The digital adhesive pads that allow gecko lizards to climb vertical surfaces result from the modification of the oberhautchen layer of the epidermis in normal scales. This produces sticky filaments of 10-100 μm in length, called setae that are composed of various proteins. The prevalent types, termed corneous beta proteins (CBPs), have a low molecular weight (12-20 kDa) and contain a conserved central region of 34 amino acids with a beta-conformation. This determines their polymerization into long beta-filaments that aggregate into corneous beta-bundles that form the framework of setae. Previous studies showed that the prevalent CBPs in the setae of Gekko gecko are cysteine-rich and are distributed from the base to the tip of adhesive setae, called spatulae. The molecular analysis of these proteins, although the three-dimensional structure remains undetermined, indicates that most of them are charged positively and some contain aromatic amino acids. These characteristics may impede adhesion by causing the setae to stick together but may also potentiate the van der Waals interactions responsible for most of the adhesion process on hydrophobic or hydrophilic substrates. The review stresses that not only the nanostructural shape and the high number of setae present in adhesive pads but also the protein composition of setae influence the strength of adhesion to almost any type of substrate. Therefore, formulation of dry materials mimicking gecko adhesiveness should also consider the chemical nature of the polymers utilized to fabricate the future dry adhesives in order to obtain the highest performance.

Material gradients in fibrillar insect attachment systems: the role of joint-like elements

Insects have developed elaborate fibrillar (or hairy) attachment systems that allow them to attach reliably on a variety of different and unpredictable surfaces. These hairy adhesive pads consist of fine and long surface outgrowths (setae), terminated by thin plate-like tips of different shapes. Besides structural adaptations, recent work revealed material gradients along the length of the setae with spatula-shaped and pointed tip structures. It was shown that these setae have a rigid base and soft setal tips, which is assumed to enhance the adaptability to rough surfaces and prevent clustering of the setae. Here, we show a not yet described type of material gradient found in discoidal (or mushroom-shaped) setae of male leaf beetles. In contrast to the previously shown gradient, the setal tips and the basal and central seta sections are composed of relatively stiff chitinous materials, whereas the transition zones between the central seta sections and the setal tips contain large proportions of the rather soft and elastic protein resilin, forming a joint-like element. Having performed adhesion measurements with artificial macroscopic mushroom-shaped adhesive structures with different implemented joint-like elements, we show that the main functional role of these joint-like elements is to facilitate the adaptability to uneven and non-parallel substrates, rather than to provide detachment tolerance towards pull-off forces applied under various tilt angles.

Biomimetic wall-shaped adhesive microstructure for shear-induced attachment: the effects of pulling angle and preliminary displacement

To date, a handful of different gecko-like adhesives inspired by spatula-shaped attachment hairs have been suggested based on wedge and flap geometry of contact elements. However, while these surface designs have been shown to have directionality in adhesion, high friction, long lifetime and the ability to work in vacuum, an experimental verification of the very basic concept of the pulling angle effect has not yet been reported. To close this gap, here we use wall-shaped adhesive microstructures of three different flap heights to systematically study the effect of pulling angle on the normal and tangential components of the pull-off force tested at different preliminary tangential displacements. In accord with the prediction of the Kendall model for the normal component of peeling force, there is an optimal normal force that is required to detach the wall-shaped adhesive microstructure. The optimum is obtained at about half the distance needed to initiate sliding and at pulling angles that range within 60-90°, which suggests that the wall-shaped microstructure can tolerate relatively large inaccuracies in the loading direction. The increase of the attachment force with increasing flap height is found to correlate with the flap thickness, which decreased with increasing flap height.

Bioinspired Further Enhanced Dry Adhesive by the Combined Effect of the Microstructure and Surface Free-Energy Increase

Silicone elastomers are known for having low surface free energies generally leading to poor adhesive performances. This surface characteristic can be enhanced by plasma treatments. The microstructured silicone elastomer surfaces can demonstrate superior adhesive performance that is more than 10 times higher in terms of pull-off forces, compared to their unstructured counterpart. Here, we have demonstrated that the combination of these two methods further enhances adhesive performance, especially when the surfaces are biomimetic micro/nanopatterned with, e.g., beetle-inspired mushroom-shaped adhesive microstructure (MSAMS). The plasma treatment time and pressure parameters were varied for the unstructured and MSAMS poly(vinylsiloxane) surfaces to find optimum parameters for maximum adhesion performance. Air plasma treatment induced average adhesive enhancement forces up to 30% on the unstructured surface, but the MSAMS surface demonstrated an enhancement of adhesive forces up to 91% higher than that of an untreated, microstructured control, despite the plasma-treated surface area of the structured surface being only 50% of that of the unstructured surface. High-speed video-recordings of individual microstructures in contact with a glass surface shows that the origin of the adhesion enhancement is due to the special detachment mechanism of individual microstructures that allows sustaining a wider contact area at detachment. We believe that this integration of the plasma treatment with MSAMS suggests a versatile way of functionalization that can further advance the adhesive ability of low-surface-energy polymer surfaces.

The contact mechanics challenge: tribology meets soft matter

In the fall of 2015, Martin Müser suggested a Contact Mechanics Challenge for the Tribology community. The challenge was an ambitious effort to compare a wide variety of theoretical and computational contact-mechanics approaches, and involved researchers voluntarily tackling the same hypothetical contact problem. The result is an impressive collection of innovative approaches - including a surprise experimental effort - that highlight the continuing importance of surface contact mechanics and the challenges of solving these large-scale problems. Here, we describe how the Contact Mechanics Challenge also reveals exciting opportunities for the Soft Matter community to engage intensely with classical and emerging problems in tribology, surface science, and contact mechanics.

Remote Control over Underwater Dynamic Attachment/Detachment and Locomotion

Despite extensive efforts to mimic the fascinating adhesion capability of geckos, the development of reversible adhesives underwater has long been lagging. The appearance of mussels-inspired dopamine chemistry has provided the feasibility to fabricate underwater adhesives; however, for such a system, imitating the reversible and fast dynamic attachment/detachment mechanism of gecko feet still remains unsolved. Here, by synthesizing a thermoresponsive copolymer of poly(dopamine methacrylamide-co-methoxyethyl-acrylate-co-N-isopropyl acrylamide) and then decorating it onto mushroom-shaped poly(dimethylsiloxane) pillar arrays, a novel underwater thermoresponsive gecko-like adhesive (TRGA) can be fabricated, yielding high adhesion during the attachment state above the lower critical solution temperature (LCST) of the copolymer, yet low adhesion during the detachment state below the LCST of the copolymer. By integrating the Fe₃ O₄ nanoparticles into the TRGA, TRGAs responsive to near-infrared laser radiation can be engineered, which can be successfully used for rapid and reversible remote control over adhesion so as to capture and release heavy objects underwater because of the contrast force change of both the normal adhesion force and the lateral friction force. It is also demonstrated that the material can be assembled on the tracks of an underwater mobile device to realize controllable movement. This opens up the door for developing intelligent underwater gecko-like locomotion with dynamic attachment/detachment ability.

Differences in Nanostructure and Hydrophobicity of Cicada (Cryptotympana atrata) Forewing Surface with the Distribution of Precipitation

Although the cicada wing has a variety of functions and the nanostructure and surface properties of many species have been extensively investigated, there are no reports investigating diversity of nanostructures and wetting properties within a single species collected at locations with different rainfall conditions. In this study, the hydrophobicity and nanostructure dimensions of the forewing surface of Cryptotympana atrata were measured, based on specimens collected from 12 distributions with varying precipitation averages in China and Japan. The relationships among hydrophobicity, nanostructures, and precipitation were analyzed, and the adaption of hydrophobic nanostructures under different wet environments is discussed. The precipitation of locations in the years the samples of C. atrata were collected only has an effect on the diameter and spacing of wing surface nanostructure, and the multiple years of precipitation may have an influence on the basic diameter and spacing, as well as the height of protrusions. The rougher the wing surface, the stronger the hydrophobicity which was observed from samples taken where the rainfall conditions of the collection years are high. To our knowledge, this is one special example providing evidence of hydrophobic nanostructures found on a biological surface of a single species which shows adaption for specific wet environments.

Bioinspired reversible hydrogel adhesives for wet and underwater surfaces

A hydrogel-based wet adhesive with bioinspired microstructures can exhibit strong and reversible adhesion to wet and underwater surfaces.

Chemical mediated elasto-capillarity of elastic sheets

Capillary forces can cause outstanding deformation of slender structures, and this behavior holds great potential in a plethora of areas. In this study, we propose a facile strategy to mediate the deformation of elastic structures via chemicals, which is named chemo-elasto-capillarity. The experiment shows that the added surfactant can significantly change the interfacial energy of the system, and then modulate the configuration of the adhered structures. The wetted length and deflection of each sheet can be predicted using the large and infinitesimal deformation theory of beams, and these theoretical values are in excellent agreement with the experimental results. Moreover, the proposed method can be successfully extended to unfold two adhered sheets, and one racket like sheet. The present scheme is accessible to accurately regulate elasto-capillarity, and provide some inspirations for engineering some chemical-sensitive devices and humidity-stimulated actuators.

Effect of counterface roughness on the friction of bionic wall-shaped microstructures for gecko-like attachments

Hairy adhesive systems involved in gecko locomotion have drawn the interest of many researchers regarding the development of bionic solutions for fast and reversible adhesive technologies. To date, despite extensive efforts to design gecko-inspired adhesive surfaces, adhesion and friction capacities are often evaluated using smooth and rigid counterfaces, in general glass, whereas most natural and artificial surfaces inevitably have a certain level of roughness. For that reason, in this study experiments tested the effects of the substrate roughness on the friction of bionic wale-shaped microstructures for gecko-like attachments. To this end, 12 substrates with different isotropic roughness were prepared using the same Epoxy material. Friction force was measured under various normal loads. It was concluded that classical roughness parameters, considered separately, are not appropriate to explain roughness-related variations in friction force. This has led us to develop a new integrative roughness parameter that combines characteristics of the surface. The parameter is capable of classifying the obtained experimental results in a readable way. An analytical model based on the experimental results has been developed to predict the variation of the friction force as a function of counterface roughness and applied normal load.

Quantifying Liquid Transport and Patterning Using Atomic Force Microscopy

Atomic force microscopy (AFM) provides unique insight into the nanoscale properties of materials. It has been challenging, however, to use AFM to study soft materials such as liquids or gels because of their tendency to flow in response to stress. We propose an AFM-based technique for quantitatively analyzing the transport of soft materials from an AFM probe to a surface. Specifically, we present a method for loading an AFM probe with a single 0.3 to 30 pL droplet of liquid and subsequently measuring the mass of this liquid by observing the change in the vibrational resonance frequency of the cantilever. Using this approach, the mass of this liquid was detected with picogram-scale precision by a commercial AFM system. Additionally, sub-femtoliter droplets of liquid were transferred from the probe to a surface with agreement found between the real-time change in mass of the liquid-loaded probe and the volume of the feature written on the surface. To demonstrate the utility of this approach in studying nanoscale capillary and transport phenomena, we experimentally determine that the quantity of liquid transported from the tip to a surface in a given patterning operation scales as the mass of liquid on the probe to the 1.35 power. In addition to providing new avenues for studying the dynamics of soft materials on the nanoscale, this method can improve nanopatterning of soft materials by providing in situ feedback.

Scaling Effects on Materials Tribology: From Macro to Micro Scale

The tribological study of materials inherently involves the interaction of surface asperities at the micro to nanoscopic length scales. This is the case for large scale engineering applications with sliding contacts, where the real area of contact is made up of small contacting asperities that make up only a fraction of the apparent area of contact. This is why researchers have sought to create idealized experiments of single asperity contacts in the field of nanotribology. At the same time, small scale engineering structures known as micro- and nano-electromechanical systems (MEMS and NEMS) have been developed, where the apparent area of contact approaches the length scale of the asperities, meaning the real area of contact for these devices may be only a few asperities. This is essentially the field of microtribology, where the contact size and/or forces involved have pushed the nature of the interaction between two surfaces towards the regime where the scale of the interaction approaches that of the natural length scale of the features on the surface. This paper provides a review of microtribology with the purpose to understand how tribological processes are different at the smaller length scales compared to macrotribology. Studies of the interfacial phenomena at the macroscopic length scales (e.g., using in situ tribometry) will be discussed and correlated with new findings and methodologies at the micro-length scale.

Influence of Humidity on Grip and Release Adhesion Mechanisms for Gecko-Inspired Microfibrillar Surfaces

Geckos have developed foot pads that allow them to maintain their unique climbing ability despite vast differences of surfaces and environments, from dry desert to humid rainforest. Likewise, successful gecko-inspired mimics should exhibit adhesive and frictional performance across a similarly diverse range of climates. In this work, we focus on the effect of relative humidity (RH) on the "frictional-adhesion" behavior of gecko-inspired adhesive pads. A surface forces apparatus was used to quantitatively measure adhesion and friction forces of a microfibrillar cross-linked polydimethylsiloxane surface against a smooth hemispherical glass disk at varying relative humidity, from 0 to 100% (including fully submerged under water). Geometrically anisotropic tilted half-cylinder microfibers yield a "grip state" (high adhesion and friction forces after shearing along the tilt of the fibers, F_ad⁺ and F_∥⁺) and a "release state" (low adhesion and friction after shearing against the tilt of the fibers, F_ad^- and F_∥^-). By appropriate control of the loading path, this allows for transition between strong attachment and easy detachment. Changing the preload and shear direction gives rise to differences in the effective contact area at each fiber and the microscale and nanoscale structure of the contact while changing the relative humidity results in differences in the relative contributions of van der Waals and capillary forces. In combination, both effects lead to interesting trends in the adhesion and friction forces. At up to 75% RH, the grip state adhesion force remains constant and the ratio of grip to release adhesion force does not drop below 4.0. In addition, the friction forces F_∥⁺ and F_∥^- and the release state adhesion force F_ad^- exhibit a maximum at intermediate relative humidity between 40% and 75%.

A Review of the State of Dry Adhesives: Biomimetic Structures and the Alternative Designs They Inspire

Robust and inexpensive dry adhesives would have a multitude of potential applications, but replicating the impressive adhesive organs of many small animals has proved challenging. A substantial body of work has been produced in recent years which has illuminated the many mechanical processes influencing a dry adhesive interface. The especially potent footpads of the tokay gecko have inspired researchers to develop and examine an impressive and diverse collection of artificial fibrillar dry adhesives, though study of tree frogs and insects demonstrate that successful adhesive designs come in many forms. This review discusses the current theoretical understanding of dry adhesive mechanics, including the observations from biological systems and the lessons learned by recent attempts to mimic them. Attention is drawn in particular to the growing contingent of work exploring ideas which are complimentary to or an alternative for fibrillar designs. The fundamentals of compliance control form a basis for dry adhesives made of composite and “smart,” stimuli-responsive materials including shape memory polymers. An overview of fabrication and test techniques, with a sampling of performance results, is provided.

Influence of substrate modulus on gecko adhesion

The gecko adhesion system fascinates biologists and materials scientists alike for its strong, reversible, glue-free, dry adhesion. Understanding the adhesion system’s performance on various surfaces can give clues as to gecko behaviour, as well as towards designing synthetic adhesive mimics. Geckos encounter a variety of surfaces in their natural habitats; tropical geckos, such as Gekko gecko, encounter hard, rough tree trunks as well as soft, flexible leaves. While gecko adhesion on hard surfaces has been extensively studied, little work has been done on soft surfaces. Here, we investigate for the first time the influence of macroscale and nanoscale substrate modulus on whole animal adhesion on two different substrates (cellulose acetate and polydimethylsiloxane) in air and find that across 5 orders of magnitude in macroscale modulus, there is no change in adhesion. On the nanoscale, however, gecko adhesion is shown to depend on substrate modulus. This suggests that low surface-layer modulus may inhibit the gecko adhesion system, independent of other influencing factors such as macroscale composite modulus and surface energy. Understanding the limits of gecko adhesion is vital for clarifying adhesive mechanisms and in the design of synthetic adhesives for soft substrates (including for biomedical applications and wearable electronics).