Bioinspired orientation-dependent friction

Bioinspired film-terminated ridges for enhancing friction force on lubricated soft surfaces

Enhancing friction force in lubricated, compliant contacts is of particular interest due to its wide application in various engineering and biological systems. In this study, we have developed bioinspired surfaces featuring film-terminated ridges, which exhibit a significant increase in lubricated friction force compared to flat samples. We propose that the enhanced sliding friction can be attributed to the energy dissipation at the lubricated interface caused by elastic hysteresis resulting from cyclic terminal film deformation. Furthermore, increasing inter-ridge spacing or reducing terminal film thickness are favorable design criteria for achieving high friction performance. These findings contribute to our understanding of controlling lubricated friction and provide valuable insights into surface design strategies for novel functional devices.

Anisotropic Friction in a Ligand-Protein Complex

The effect of an externally applied directional force on molecular friction is so far poorly understood. Here, we study the force-driven dissociation of the ligand-protein complex biotin-streptavidin and identify anisotropic friction as a not yet described type of molecular friction. Using AFM-based stereographic single molecule force spectroscopy and targeted molecular dynamics simulations, we find that the rupture force and friction for biotin-streptavidin vary with the pulling angle. This observation holds true for friction extracted from Kramers' rate expression and by dissipation-corrected targeted molecular dynamics simulations based on Jarzynski's identity. We rule out ligand solvation and protein-internal friction as sources of the angle-dependent friction. Instead, we observe a heterogeneity in free energy barriers along an experimentally uncontrolled orientation parameter, which increases the rupture force variance and therefore the overall friction. We anticipate that anisotropic friction needs to be accounted for in a complete understanding of friction in biomolecular dynamics and anisotropic mechanical environments.

Robust and Elevated Adhesion and Anisotropic Friction in a Bioinspired Bridged Micropillar Array

Bioinspired structured adhesives have promising applications in the fields of robotics, electronics, medical engineering, and so forth. The strong adhesion and friction as well as the durability of bioinspired hierarchical fibrillar adhesives are essential for their applications, which require fine submicrometer structures to stay stable during repeated use. Here, we develop a bioinspired bridged micropillars array (BP), which realizes a 2.18-fold adhesion and a 2.02-fold friction as compared to that of poly(dimethylsiloxane) (PDMS) original micropillar arrays. The aligned bridges offer BP strong anisotropic friction. The adhesion and friction of BP can be finely regulated by changing the modulus of the bridges. Moreover, BP shows strong adaptability to surface curvature (ranging from 0 to 800 m^-1), excellent durability over 500 repeating cycles of attachment/detachment, and self-cleaning ability. This study presents a novel approach for designing robust structured adhesives with strong and anisotropic friction, which may find applications in areas such as climbing robots and cargo transportation.

Switchable Adhesion on Curved Surfaces Mimicking the Coordination of Radial-Oriented Spatular Tips and Motion of Gecko Toes

Bio-inspired structured adhesives have promising applications in many fields, like biomedicine, robotics, and aerospace. However, achieving robust and switchable adhesion in structured adhesives on non-planar surfaces remains highly challenging. Inspired by the gripping and rolling motions of gecko toes, a strong and switchable adhesive, which comprises a pillar array with radial-oriented spatular tips and is named as PROST, is developed. PROST possesses a robust adhesion on flat surfaces and doubles its adhesion on curved surfaces. Moreover, in situ and fast adhesion switching of PROST on flat/curved surfaces in dry and wet conditions has been realized by solvent stimulation, mimicking the bending locomotion of gecko toes. The work here provides a new strategy for designing controllable adhesion on curved substrates.

Tree frog-inspired nanopillar arrays for enhancement of adhesion and friction

Bioinspired structure adhesives have received increasing interest for many applications, such as climbing robots and medical devices. Inspired by the closely packed keratin nanopillars on the toe pads of tree frogs, tightly arranged polycaprolactone nanorod arrays are prepared by mold process and chemical modification. Nanorod arrays show enhanced adhesion and friction on both smooth and rough surfaces compared to the arrays with hexagonal micropillars. The bonding of nanorods results in a larger stiffness of the nanorod surface, contributing mainly to friction rather than adhesion. The results suggest the function of closely packed keratin nanopillars on the toe pad of tree frogs and offer a guiding principle for the designing of new structured adhesives with strong attaching abilities.

Patterned enteroscopy balloon design factors influence tissue anchoring

Balloon-assisted enteroscopy procedures allow visualization and intervention in the small intestine. These balloons anchor an endoscope and/or overtube to the small intestine, allowing endoscopists to plicate the small intestine over the overtube. This procedure can extend examination deeper into the small intestine than the length of the endoscope would allow with direct examination. However, procedures are often prolonged or incomplete due to balloon slippage. Enteroscopy balloons are pressure-limited to ensure patient safety and thus, improving anchoring without increasing pressure is essential. Patterning balloon exteriors with discrete features may enhance anchoring at the tissue-balloon interface. Here, the pattern design space is explored to determine factors that influence tissue anchoring. The anchoring ability of smooth versus balloons with patterned features is investigated by experimentally measuring a peak force required to induce slippage of an inflated balloon inside ex-vivo porcine small intestine. Stiffer materials, low aspect-ratio features, and pattern area/location on the balloons significantly increase peak force compared to smooth silicone balloons. Smooth latex balloons, used for standard enteroscopy, have the lowest peak force. This work demonstrates both a method to pattern curved surfaces and that a balloon with patterned features improves anchoring against a deformable, lubricated tissue interface.

Tree Frog-Inspired Micropillar Arrays with Nanopits on the Surface for Enhanced Adhesion under Wet Conditions

Inspired by the nanoconcave top of epidermal cells on tree frogs' toe pads, an array of composite micropillars with nanopits on the surface (CP_p) has been designed. Polystyrene (PS) nanoparticles are mixed with polydimethylsiloxane (PDMS) and serve as the template for nanopits on the PS/PDMS composite micropillars. CP_p shows much larger wet adhesion compared to the arrays of micropillars without nanopits. Under a certain loading force, most of the liquid between CP_p and the counterpart surface is squeezed out, so the liquid that remained in nanopits forms multiple nanoscale liquid bridges within the contact area of a single micropillar. Moreover, a large loading force could squeeze part of the liquid out of nanopits, resulting in the suction effect during the pull-off. The multiple liquid bridges, the suction effect, and the solid direct contact thus contribute to strong wet adhesion, which could be ∼36.5 times that of tree frogs' toe pads. The results suggest the function of nanoconcaves on the toe pad of tree frogs and offer a new design strategy for structured adhesives to gain strong wet adhesion.

Reversible Adhesion via Light-Regulated Conformations of Rubber Chains

Bio-inspired reversible adhesives have attracted great attention because of their promising applications in the electronic, biomedical, and robotic fields. Here, to achieve in situ reversible adhesion, a new concept is demonstrated by modulating the conformations of polydimethylsiloxane (PDMS) chains. The new adhesive, termed B_GP_P, is composed of the graphene/PDMS composite (GP) as the backing layer and PDMS as the micropillar array. The photothermal effect of graphene under UV irradiation heats up the micropillars, resulting in an increase in the chain conformations of PDMS and thus the contact points with the counterpart surface. The more contact points together with the alignment of PDMS chains during the shearing result in an adhesion much higher than that without UV irradiation. The adhesion switching thus does not rely on the changing of the contact area, and so the macroscopic deformation of structures is avoided. The results suggest a new design principle for light-controllable structured adhesive, which could be conceptualized into other rubbery materials.

Tree frog adhesion biomimetics: opportunities for the development of new, smart adhesives that adhere under wet conditions

Enlarged adhesive toe pads on the tip of each digit allow tree frogs to climb smooth vertical and overhanging surfaces, and are effective in generating reversible adhesion under both dry and wet conditions. In this review, we discuss the complexities of the structure of tree frog toe pads in relation to their function and review their biomimetic potential. Of particular importance are the (largely) hexagonal epithelial cells surrounded by deep channels that cover the surface of each toe pad and the array of nanopillars on their surface. Fluid secreted by the pads covers the surface of each pad, so the pads adhere by wet adhesion, involving both capillarity and viscosity-dependent forces. The fabrication and testing of toe pad mimics are challenging, but valuable both for testing hypotheses concerning tree frog toe pad function and for developing toe pad mimics. Initial mimics involved the fabrication of hexagonal pillars mimicking the toe pad epithelial structure. More recent ones additionally replicate the nanostructures on their surface. Finally we describe some of the biomimetic applications that have been developed from toe pad mimics, which include both bioinspired adhesives and friction-generating devices. This article is part of the theme issue 'Bioinspired materials and surfaces for green science and technology (part 2)'.

Going Out on a Limb: How Investigation of the Anoline Adhesive System Can Enhance Our Understanding of Fibrillar Adhesion

The remarkable ability of geckos to adhere to a wide-variety of surfaces has served as an inspiration for hundreds of studies spanning the disciplines of biomechanics, functional morphology, ecology, evolution, materials science, chemistry, and physics. The multifunctional properties (e.g., self-cleaning, controlled releasability, reversibility) and adhesive performance of the gekkotan adhesive system have motivated researchers to design and fabricate gecko-inspired synthetic adhesives of various materials and properties. However, many challenges remain in our attempts to replicate the properties and performance of this complex, hierarchical fibrillar adhesive system, stemming from fundamental, but unanswered, questions about how fibrillar adhesion operates. Such questions involve the role of fibril morphology in adhesive performance and how the gekkotan adhesive apparatus is utilized in nature. Similar fibrillar adhesive systems have, however, evolved independently in two other lineages of lizards (anoles and skinks) and potentially provide alternate avenues for addressing these fundamental questions. Anoles are the most promising group because they have been the subject of intensive ecological and evolutionary study for several decades, are highly speciose, and indeed are advocated as squamate model organisms. Surprisingly, however, comparatively little is known about the morphology, performance, and properties of their convergently-evolved adhesive arrays. Although many researchers consider the performance of the adhesive system of Anolis lizards to be less accomplished than its gekkotan counterpart, we argue here that Anolis lizards are prime candidates for exploring the fundamentals of fibrillar adhesion. Studying the less complex morphology of the anoline adhesive system has the potential to enhance our understanding of fibril morphology and its relationship to the multifunctional performance of fibrillar adhesive systems. Furthermore, the abundance of existing data on the ecology and evolution of anoles provides an excellent framework for testing hypotheses about the influence of habitat microstructure on the performance, behavior, and evolution of lizards with subdigital adhesive pads.

Force-transmitting structures in the digital pads of the tree frog Hyla cinerea: a functional interpretation

The morphology of the digital pads of tree frogs is adapted towards attachment, allowing these animals to attach to various substrates and to explore their arboreal habitat. Previous descriptions and functional interpretations of the pad morphology mostly focussed on the surface of the ventral epidermis, and little is known about the internal pad morphology and its functional relevance in attachment. In this study, we combine histology and synchrotron micro‐computer‐tomography to obtain a comprehensive 3‐D morphological characterisation of the digital pads (in particular of the internal structures involved in the transmission of attachment forces from the ventral pad surface towards the phalanges) of the tree frog Hyla cinerea. A collagenous septum runs from the distal tip of the distal phalanx to the ventral cutis and compartmentalises the subcutaneous pad volume into a distal lymph space and a proximal space, which contains mucus glands opening via long ducts to the ventral pad surface. A collagen layer connects the ventral basement membrane via interphalangeal ligaments with the middle phalanx. The collagen fibres forming this layer curve around the transverse pad‐axis and form laterally separated ridges below the gland space. The topological optimisation of a shear‐loaded pad model using finite element analysis (FEA) shows that the curved collagen fibres are oriented along the trajectories of the maximum principal stresses, and the optimisation also results in ridge‐formation, suggesting that the collagen layer is adapted towards a high stiffness during shear loading. We also show that the collagen layer is strong, with an estimated tensile strength of 2.0–6.5 N. Together with longitudinally skewed tonofibrils in the superficial epidermis, these features support our hypothesis that the digital pads of tree frogs are primarily adapted towards the generation and transmission of friction rather than adhesion forces. Moreover, we generate (based on a simplified FEA model and predictions from analytical models) the hypothesis that dorsodistal pulling on the collagen septum facilitates proximal peeling of the pad and that the septum is an adaptation towards detachment rather than attachment. Lastly, by using immunohistochemistry, we (re‐)discovered bundles of smooth muscle fibres in the digital pads of tree frogs. We hypothesise that these fibres allow the control of (i) contact stresses at the pad–substrate interface and peeling, (ii) mucus secretion, (iii) shock‐absorbing properties of the pad, and (iv) the macroscopic contact geometry of the ventral pad surface. Further work is needed to conclude on the role of the muscular structures in tree frog attachment. Overall, our study contributes to the functional understanding of tree frog attachment, hence offering novel perspectives on the ecology, phylogeny and evolution of anurans, as well as the design of tree‐frog‐inspired adhesives for technological applications.

Hybrid Surface Patterns Mimicking the Design of the Adhesive Toe Pad of Tree Frog

Biological materials achieve directional reinforcement with oriented assemblies of anisotropic building blocks. One such example is the nanocomposite structure of keratinized epithelium on the toe pad of tree frogs, in which hexagonal arrays of (soft) epithelial cells are crossed by densely packed and oriented (hard) keratin nanofibrils. Here, a method is established to fabricate arrays of tree-frog-inspired composite micropatterns composed of polydimethylsiloxane (PDMS) micropillars embedded with polystyrene (PS) nanopillars. Adhesive and frictional studies of these synthetic materials reveal a benefit of the hierarchical and anisotropic design for both adhesion and friction, in particular, at high matrix-fiber interfacial strengths. The presence of PS nanopillars alters the stress distribution at the contact interface of micropillars and therefore enhances the adhesion and friction of the composite micropattern. The results suggest a design principle for bioinspired structural adhesives, especially for wet environments.

Effective Elastic Modulus of Structured Adhesives: From Biology to Biomimetics

Micro- and nano-hierarchical structures (lamellae, setae, branches, and spatulae) on the toe pads of many animals play key roles for generating strong but reversible adhesion for locomotion. The hierarchical structure possesses significantly reduced, effective elastic modulus (Eeff), as compared to the inherent elastic modulus (Einh) of the corresponding biological material (and therefore contributes to a better compliance with the counterpart surface). Learning from nature, three types of hierarchical structures (namely self-similar pillar structure, lamella⁻pillar hybrid structure, and porous structure) have been developed and investigated.

Monitoring the Contact Stress Distribution of Gecko-Inspired Adhesives Using Mechano-Sensitive Surface Coatings

The contact geometry of microstructured adhesive surfaces is of high relevance for adhesion enhancement. Theoretical considerations indicate that the stress distribution in the contact zone is crucial for the detachment mechanism, but direct experimental evidence is missing so far. In this work, we propose a method that allows, for the first time, the detection of local stresses at the contact area of biomimetic adhesive microstructures during contact formation, compression and detachment. We use a mechano-sensitive polymeric layer, which turns mechanical stresses into changes of fluorescence intensity. The biomimetic surface is brought into contact with this layer in a well-defined fashion using a microcontact printer, while the contact area is monitored with fluorescence microscopy in situ. Thus, changes in stress distribution across the contact area during compression and pull-off can be visualized with a lateral resolution of 1 μm. We apply this method to study the enhanced adhesive performance of T-shaped micropillars, compared to flat punch microstructures. We find significant differences in the stress distribution of the both differing contact geometries during pull-off. In particular, we find direct evidence for the suppression of crack nucleation at the edge of T-shaped pillars, which confirms theoretical models for the superior adhesive properties of these structures.

Stick-Slip Friction of PDMS Surfaces for Bioinspired Adhesives

Friction plays an important role in the adhesion of many climbing organisms, such as the gecko. During the shearing between two surfaces, periodic stick-slip behavior is often observed and may be critical to the adhesion of gecko setae and gecko-inspired adhesives. Here, we investigate the influence of short oligomers and pendent chains on the stick-slip friction of polydimethylsiloxane (PDMS), a commonly used material for bioinspired adhesives. Three different stick-slip patterns were observed on these surfaces (flat or microstructured) depending on the presence or absence of oligomers and their ability to diffuse out of the material. After washing samples to remove any untethered oligomeric chains, or after oxygen plasma treatment to convert the surface to a thin layer of silica, we decouple the contributions of stiffness, oligomers, and pendant chains to the stick-slip behavior. The stick phase is mainly controlled by the stiffness while the amount of untethered oligomers and pendant chains available at the contact interface defines the slip phase. A large amount of oligomers and pendant chains resulted in a large slip time, dominating the period of stick-slip motion.