Optimal Gait for Bioinspired Climbing Robots Using Dry Adhesion： A Quasi-Static Investigation

Legged robots relying on dry adhesives for vertical climbing are required to preload their feet against the wall to increase contact surface area and consequently maximize adhesion force. Preloading a foot causes a redistribution of forces in the entire robot, including contact forces between the other feet and the wall. An inappropriate redistribution of these forces can cause irreparable detachment of the robot from the vertical surface. This paper investigates an optimal preloading and detaching strategy that minimizes energy consumption, while retaining safety, during locomotion on vertical surfaces. The gait of a six-legged robot is planned using a quasi-static model that takes into account both the structure of the robot and the character- istics of the adhesive material. The latter was modelled from experimental data collected for this paper. A constrained optimi- zation routine is used, and its output is a sequence of optimal posture and motor torque set-points.

Theoretical optimization of micropillar arrays for structurally stable bioinspired dry adhesives

Inspired by the excellent adhesion performances of setae structure from organisms,micro/nano-pillar array has become one of the paradigms for adhesive surfaces.The micropillar arrays are composed of the resin pillars for adhesion and the substrate with different elastic modulus for supporting.The stress singularity at the bi-material corner between the pillars and the substrate can induce the failure of the micropillar-substrate corner and further hinder the fabrication and application of micropillar arrays,yet the design for the stability of the micropillar array lacks systematical and quantitative guides.In this work,we develop a semi-analytical method to provide the full expressions for the stress distribution within the bi-material corner combining analytical derivations and numerical calculations.The predictions for the stress within the singularity field can be obtained based on the full expressions of the stress.The good agreement between the predictions and the FEM results demonstrates the high reliability of our method.By adopting the strain energy density factor approach,the stability of the pillar-substrate corner is assessed by predicting the failure at the corner.For the elastic mismatch between the pillar and substrate given in this paper,the stability can be improved by increasing the ratio of the shear modulus of the substrate to that of the micropillar.Our study provides accurate predictions for the stress distribution at the bi-material corner and can guide the optimization of material combinations of the pillars and the substrate for more stable bioinspired dry adhesives.

A Mechanical Model for the Adhesion of Spiders to Nominally Flat Surfaces

In dry attachment systems of spiders and geckos, van der Waals forces mediate attraction between substrate and animal tarsus. In particular, the scopula of Evarcha arcuata spiders allows for reversible attachment and easy detachment to a broad range of surfaces. Hence, reproducing the scopula's roughness compatibility while maintaining anti-bunching features and dirt particle repellence behavior is a central task for a biomimetic transfer to an engineered model. In the present work we model the scopula of E. arcuata from a mechano-elastic point of view analyzing the influence of its hierarchical structure on the attachment behavior. By considering biological data of the gecko and spider, and the simulation results, the adhesive capabilities of the two animals are compared and important confirmations and new directives in order to reproduce the overall structure are found. Moreover, a possible suggestion of how the spider detaches in an easy and fast manner is proposed and supported by the results.

Switchable dry adhesive based on shape memory polymer with hemispherical indenters for transfer printing

Transfer printing based on switchable adhesive is essential for developing unconventional systems,including flexible electronics,stretchable electronics,and micro light-emitting diode(LED)displays.Here we report a design of switchable dry adhesive based on shape memory polymer(SMP)with hemispherical indenters,which offers a continuously tunable and reversible adhesion through the combination of the preloading effect and the thermal actuation of SMP.Experimental and numerical studies reveal the fundamental aspects of design,fabrication,and operation of the switchable dry adhesive.Demonstrations of this adhesive concept in transfer printing of flat objects(e.g.,silicon wafers),three-dimensional(3D)objects(e.g.,stainless steel balls),and rough objects(e.g.,frosted glasses)in two-dimensional(2D)or 3D layouts illustrate its unusual manipulation capabilities in heterogeneous material integration applications.

Recent advances in gecko adhesion and friction mechanisms and development of gecko-inspired dry adhesive surfaces

The remarkable ability of geckos to climb and run rapidly on walls and ceilings has recently received considerable interest from many researchers.Significant progress has been made in understanding the attachment and detachment mechanisms and the fabrication of articulated gecko-inspired adhesives and structured surfaces.This article reviews the direct experiments that have investigated the properties of gecko hierarchical structures,i.e.,the feet,toes,setae,and spatulae,and the corresponding models to ascertain the mechanical principles involved.Included in this review are reports on gecko-inspired surfaces and structures with strong adhesion forces,high ratios of adhesion and friction forces,anisotropic hierarchical structures that give rise to directional adhesion and friction,and“intelligent”attachment and detachment motions.

Gecko-inspired composite micro-pillars with both robust adhesion and enhanced dry self-cleaning property

Self-cleaning surfaces are desirable in many engineering applications where low energy consumption,reusability and sustainability are of the biggest concerns.Inspired by the gecko’s unique ’dry selfcleaning’ hierarchical structures.Here we fabricated artificial Fe304/PDMS composites that show robust self-cleaning capabilities.The enhanced adhesion performance is attributable to the decrease of PDMS polymerization degree and the load transfer between PDMS matrix and Fe304 magnetic particles.The self-cleaning surfaces showed up to 24.3% self-cleaning rate with as few as 4 steps.Simulation result indicated that the changing of cross linking between Fe304 and PDMS is the main reason for the enhanced self-cleaning surfaces.This work reveals an alternative route of making high-performance self-cleaning smart surfaces that are applicable in the textile industry,robotic locomotion/gripping technology,outerspace explorations and tissue engineering.

Multi-Scale Compliant Foot Designs and Fabrication for Use with a Spider-Inspired Climbing Robot

Climbing robots are of potential use for surveillance, inspection and exploration in different environments. In particular, the use of climbing robots for space exploration can allow scientists to explore environments too challenging for traditional wheeled designs. To adhere to surfaces, biomimetic dry adhesives based on gecko feet have been proposed. These biomimetic dry adhesives work by using multi-scale compliant mechanisms to make intimate contact with different surfaces and adhere by using Van der Waals forces. Fabrication of these adhesives has frequently been challenging however, due to the difficulty in combining macro, micro and nanoscale compliance. We present an all polymer foot design for use with a hexapod climbing robot and a fabrication method to improve reliability and yield. A high strength, low-modulus silicone, TC-5005, is used to form the foot base and microscale fibres in one piece by using a two part mold. A macroscale foot design is produced using a 3D printer to produce a base mold, while lithographic definition of microscale fibres in a thick photoresist forms the ＇hairs＇ of the polymer foot. The adhesion of the silicone fibres by themselves or attached to the macro foot is examined to determine best strategies for placement and removal of feet to maximize adhesion. Results demonstrate the successful integration of micro and macro compliant feet for use in climbing on a variety of surfaces.

Enhanced Compliant Adhesive Design and Fabrication with Dual-Level Hierarchical Structure

Synthetic dry adhesives inspired by the nano-and micro-scale hairs found on the feet of geckos and some spiders have beendeveloped for almost a decade. Elastomeric single level micro-scale mushroom shaped fibres are currently able to function evenbetter than natural dry adhesives on smooth surfaces under normal loading. However, the adhesion of these single level syntheticdry adhesives on rough surfaces is still not optimal because of the reduced contact surface area. In nature, contact area ismaximized by hierarchically structuring different scales of fibres capable of conforming surface roughness. In this paper, weadapt the nature’s solution arid propose a novel dual-level hierarchical adhesive design using Polydimethylsiloxane (PDMS),which is tested under peel loading at different orientations. A negative macro-scale mold is manufactured by using a laser cutterto define holes in a Poly(methyl methacrylate) (PMMA) plate. After casting PDMS macro-scale fibres by using the obtainedPMMA mold, a previously prepared micro-fibre adhesive is bonded to the macro-scale fibre substrate. Once the bondingpolymer is cured, the micro-fibre adhesive is cut to form macro scale mushroom caps. Each macro-fibre of the resulting hierarchicaladhesive is able to conform to loads applied in different directions. The dual-level structure enhances the peel strengthon smooth surfaces compared to a single-level dry adhesive, but also weakens the shear strength of the adhesive for a given areain contact. The adhesive appears to be very performance sensitive to the specific size of the fibre tips, and experiments indicatethat designing hierarchical structures is not as simple as placing multiple scales of fibres on top of one another, but can requiresignificant design optimization to enhance the contact mechanics and adhesion strength.

Fabrication and Testing of Self Cleaning Dry Adhesives Utilizing Hydrophobicity Gradient

In this paper we present a method to create a hydrophobicity gradient on the surface of a Polydimethylsiloxane （PDMS） dry adhesive. The method consists of the partial silanization of the surface of the dry adhesive by Chemical Vapour Deposition （CVD） of octadecyltrichlorosilane （OTS）. The partial silanization of the surface of the sample results in a hydrophobic to hy- drophilic gradient across the surface of the dry adhesive. The resulting change in hydrophobicity across the surface of the dry adhesive results in the uphill motion of a droplet of water, which appears to be directly proportional to the area of contact between the droplet and the adhesive. Normal adhesion testing is performed to quantify the effect of the hydrophobic gradient across the surface of the sample. While a variation in adhesion are only minimally affected by the silanization, and the motion strength across the sample is measured, the adhesive properties of the droplet of water doesn＇t cause any loss of adhesion.

Magnetic-field-driven switchable adhesion of NdFeB/PDMS composite with gecko-like surface

Switchable adhesives have attracted widespread attention due to their strong reusability and adaptability to operate stably in complex environments.However,the simple fabrication of adhesive structures and reliable control of adhesion remain challenging.Here,we developed a neodymium iron boron/polydimethylsiloxane(NdFeB/PDMS)magnetic composite with optimal mechanical and magnetic performance.Then we fabricated lamellar structures and setal arrays using a molding and magnetic field-induced process,imitating the multi-level adhesion system of gecko feet.The lamellar can be deformed under the action of a magnetic field to control the adhesion,the setal array is used to enhance adhesion and provide self-cleanability to the adhering surface.Switchable adhesion was realized by applying an external magnetic field,where the maximum adhesion strength was 5.1 kPa,the switchable range was within 40%.Through finite element analysis simulations and experimental verification,it was proved that the adhesion force variation was ascribed to the magnetic field-induced surface deformation.Finally,we installed the adhesive on the end of the robotic arm,realizing the transfer of the target object.This work provides a simple method to fabricate a gecko-like surface and a practical strategy to realize switchable adhesion,which sheds light on broad application potential in production lines,medical products,more.

Advanced Electro-active Dry Adhesive Actuated by an Artificial Muscle Constructed from an Ionic Polymer Metal Composite Reinforced with Nitrogen-doped Carbon Nanocages

An advanced electro-active dry adhesive, which was composed of a mushroom-shaped tibrillar dry adhesive array actuated by an Ionic Polymer Metal Composite （IPMC） artificial muscle reinforced with nitrogen-doped carbon nanocages （NCNCs）, was developed to imitate the actuation of a gecko＇s toe. The properties of the NCNC-reinforced Nation membrane, the electro- mechanical properties of the NCNC-reinforced IPMC, and the related electro-active adhesion ability were investigated. The NCNCs were uniformly dispersed in the 0.1 wt% NCNC/Nafion membrane, and there was a seamless connection with no clear interface between the dry adhesive and the IPMC. Our 0.1 wt% NCNC/Nation-IPMC actuator shows a displacement and force that are 1.6 - 2 times higher than those of the recast Nafion-IPMC. This is due to the increased water uptake （25.39%） and tensile strength （24.5 MPa） of the specific 3D hollow NCNC-reinforced Nation membrane, as well as interactions between the NCNCs and the sulfonated groups of the Nation. The NCNC/Nation-IPMC was used to effectively actuate the mushroom-shaped dry adhesive. The normal adhesion forces were 7.85 raN, 12.1 mN, and 51.7 mN at sinusoidal voltages of 1.5 V, 2.5 V, and 3.5 V, respectively, at 0.1 Hz. Under the bionic leg trail, the normal and shear forces were approximately 713.5 mN （159 mN·cm^-2） and 1256.6 mN （279 mN·cm^-2）, respectively, which satisfy the required adhesion. This new electro-active dry adhesive can be applied for active, distributed actuation and flexible grip in robots.

Gecko-Like Dry Adhesive Surfaces and Their Applications: A Review

Gecko has the ability to climb flexibly on various natural surfaces because of its fine layered adhesion system of foot,which has motivated researchers to carry out a lot of researches on it.Significant progresses have been made in the gecko-like dry adhesive surfaces in the past 2 decades,such as the mechanical measurement of adhesive characteristics,the theoretical modeling of adhesive mechanism and the production of synthetic dry adhesive surfaces.Relevant application researches have been carried out as well.This paper focuses on the investigations made in recent years on the gecko-like dry adhesive surfaces,so as to lay the foundation for further research breakthroughs.First,the adhesion system of gecko’s foot and its excellent adhesive characteristics are reviewed,and the adhesive models describing the gecko adhesion are summarily reviewed according to the diff erent contact modes.Then,some gecko-like dry adhesive surfaces with outstanding adhesive characteristics are presented.Next,some application researches based on the gecko-like dry adhesive surfaces are introduced.Finally,the full text is summarized and the problems to be solved on the gecko-like dry adhesive surfaces are prospected.

Additive manufacturing of flexible 3D surface electrodes for electrostatic adhesion control and smart robotic gripping

Mechanically flexible surface structures with embedded conductive electrodes are attractive in contact-based devices,such as those used in reversible dry/adhesion and tactile sensing.Geometrical shapes of the surface structures strongly determine the contact behavior and therefore the resulting adhesion and sensing functionalities;however,available features are often restricted by fabrication techniques.Here,we additively manufacture elastomeric structure arrays with diverse angles,shapes,and sizes;this is followed by integration of conductive nanowire electrodes.The fabricated flexible three-dimensional(3D)surface electrodes are mechanically compliant and electrically conductive,providing multifunctional ability to sense touch and to switch adhesion via a combined effect of shear-and electro adhesives.We designed soft,anisotropic flexible structures to mimic the gecko’s reversible adhesion,which is governed by van der Waals forces;we integrated nanowires to further manipulate the localized electric field among the adjacent flexible 3D surface electrodes to provide additional means to digitally tune the electrostatic attraction at the contact interface.In addition,the composite surface can sense the contact force via capacitive sensing.Using our flexible 3D surface electrodes,we demonstrate a complete soft gripper that can grasp diverse convex objects,including metal,ceramic,and plastic products,as well as fresh fruits,and that exhibits 72%greater electroadhesive gripping force when voltage is applied.

Effective metal mold method for the production of bionic adhesives based on electrochemical modifications

Bionic adhesives with tip-expanded microstructural arrays have attracted considerable interest owing to their high adhesive performance at low preloads.Their mainstream manufacturing method is molding.Due to most molds are made of silicon or silicon-based soft templates,and have poor wear resistant or vulnerability to high temperature,limiting their use in large-scale manufacturing.Nickel is widely used as an embossing mold in the micro/nano-imprint industrial process owing to its good mechanical properties.However,the processing of metal molds for the fabrication of tip-expanded microstructural arrays is extremely challenging.In this study,using electrodeposition techniques,the shape of the micropores is modified to obtain end-controlled pores.The effect of the non-uniformity of the electric field on the microporous morphology in the electrodeposition process is systematically investigated.Furthermore,the mechanism of non-uniformity evolution of the microporous morphology is revealed.The optimized microporous metal array is used as a mold to investigate the cavity evolution laws of the elastic cushions under pre-load during the manufacturing process.As a result,typical bionic adhesives with tip-expansion are obtained.Moreover,corresponding adhesion mechanics are analyzed.The results show that electrochemical modifications have broad application prospects in the fabrication of tip-expanded microstructures,providing a new method for the large-scale fabrication of bionic adhesives based on metal molds.

Abigaille-Ⅲ： A Versatile, Bioinspired Hexapod for Scaling Smooth Vertical Surfaces

This paper presents a novel, legged robot, Abigaille-Ⅲ, which is a hexapod actuated by 24 miniature gear motors. This robot uses dual-layer dry adhesives to climb smooth, vertical surfaces. Because dry adhesives are passive and stick to various surfaces, they have advantages over mechanisms such as suction, claws and magnets. The mechanical design and posture of Abigaille-Ⅲ were optimized to reduce pitchback forces during vertical climbing. The robot＇s electronics were designed around a Field Programmable Gate Array, producing a versatile computing architecture. The robot was reconfigured for vertical climbing with both 5 and 6 legs, and with 3 or 4 motors per leg, without changes to the electronic hardware. Abigaille-Ⅲ demonstrated dexterity through vertical climbing on uneven surfaces, and by transferring between horizontal and vertical sur- faces. In endurance tests, Abigaille-Ⅲ completed nearly 4 hours of continuous climbing and over 7 hours of loitering, showing that dry adhesive climbing systems can be used for extended missions.

Autonomous self-healing 3D micro-suction adhesives for multi-layered amphibious soft skin electronics

Autonomously self-healing, reversible, and soft adhesive microarchitecturesand structured electric elements could be important features in stable and versatilebioelectronic devices adhere to complex surfaces of the human body(rough, dry, wet, and vulnerable). In this study, we propose an autonomousself-healing multi-layered adhesive patch inspired by the octopus, which possessself-healing and robust adhesion properties in dry/underwater conditions.To implement autonomously self-healing octopus-inspired architectures, adynamic polymer reflow model based on structural and material design suggestscriteria for three-dimensional patterning self-healing elastomers. In addition,self-healing multi-layered microstructures with different moduli endowsefficient self-healing ability, human-friendly reversible bio-adhesion, and stablemechanical deformability. Through programmed molecular behavior ofmicrolevel hybrid multiscale architectures, the bioinspired adhesive patchexhibited robust adhesion against rough skin surface under both dry andunderwater conditions while enabling autonomous adhesion restoring performanceafter damaged (over 95% healing efficiency under both conditions for24 h at 30℃). Finally, we developed a self-healing skin-mountable adhesiveelectronics with repeated attachment and minimal skin irritation by laminatingthin gold electrodes on octopus-like structures. Based on the robust adhesionand intimate contact with skin, we successfully obtained reliable measurements during dynamic motion under dry, wet, and damagedconditions.