Yasong Li

Abigaille ii: Toward the development of a spider-inspired climbing robot

This paper presents a novel robotic platform, Abigaille II, designed to climb vertical surfaces using dry adhesion. Abigaille II is a lightweight hexapod prototype actuated by 18 miniaturized motors. The robots feet consist of adhesive patches, which have microhairs with mushroom-shaped caps fixed on the top of millimeter-scale flexible posts. A pentapedal gait is used to climb flat vertical surfaces as this gait maximizes the number of legs in contact to the surface. Abigaille can however also walk by using other gaits, including the tripod gait.

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.

Abigaille-I: Towards the development of a spider-inspired climbing robot for space use

This paper presents the design and testing of Abigaille-I, a spider-inspired robot. The system is miniaturized and has six legs and 18 actively controlled joints. Macro-, micro- and nano-structural design of the robot legs and feet are presented and preliminary experimental results are discussed. The long-term objective of this research is to develop an autonomous and miniaturized robotic system capable to negotiating terrain of any roughness and material and be eventually capable of operating in a space environment.

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 characteristics of the adhesive material. The latter was modelled from experimental data collected for this paper. A constrained optimization routine is used, and its output is a sequence of optimal posture and motor torque set-points.

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

Dry adhesives inspired from climbing animals, such as geckos and spiders, rely on van der Waals forces to attach to the opposing surface. Biological fibrillar dry adhesives have a hierarchical structure closely resembling a tree: the surface of the skin on the animal’s feet is covered in arrays of slender micro-fibrils, each of which supports arrays of fibrils in submicron dimensions. These nano-meter size fibrils can conform closely to the opposing surfaces to induce van der Waals interaction. Bioinspired dry adhesives have been developed in research laboratories for more than a decade. To mimic the biological fibrillar adhesives, fibrillar structures have been prepared using a variety of materials and geometrical arrangements. In this review article, the mechanism and selected fabrication methods of fibrillar adhesives are summarized for future reference in adhesive development. Robotic applications of these bioinspired adhesives are also introduced in this article. Various successful applications of bioinspired fibrillar adhesives can shed light on developing smart adhesives for use in automation.

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 been developed for almost a decade. Elastomeric single level micro-scale mushroom shaped fibres are currently able to function even better than natural dry adhesives on smooth surfaces under normal loading. However, the adhesion of these single level synthetic dry adhesives on rough surfaces is still not optimal because of the reduced contact surface area. In nature, contact area is maximized by hierarchically structuring different scales of fibres capable of conforming surface roughness. In this paper, we adapt the nature’s solution and 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 cutter to define holes in a Poly(methyl methacrylate) (PMMA) plate. After casting PDMS macro-scale fibres by using the obtained PMMA mold, a previously prepared micro-fibre adhesive is bonded to the macro-scale fibre substrate. Once the bonding polymer is cured, the micro-fibre adhesive is cut to form macro scale mushroom caps. Each macro-fibre of the resulting hierarchical adhesive is able to conform to loads applied in different directions. The dual-level structure enhances the peel strength on smooth surfaces compared to a single-level dry adhesive, but also weakens the shear strength of the adhesive for a given area in contact. The adhesive appears to be very performance sensitive to the specific size of the fibre tips, and experiments indicate that designing hierarchical structures is not as simple as placing multiple scales of fibres on top of one another, but can require significant design optimization to enhance the contact mechanics and adhesion strength.

Material versatility using replica molding for large-scale fabrication of high aspect-ratio, high density arrays of nano-pillars

Arrays of high aspect-ratio (AR) nano-pillars have attracted a lot of interest for various applications, such as for use in solar cells, surface acoustic sensors, tissue engineering, bio-inspired adhesives and anti-reflective surfaces. Each application may require a different structural material, which can vary in the required chemical composition and mechanical properties. In this paper, a low cost fabrication procedure is proposed for large scale, high AR and high density arrays of nano-pillars. The proposed method enables the replication of a master with high fidelity, using the subsequent replica molds multiple times, and preparing arrays of nano-pillars in a variety of different materials. As an example applied to bio-inspired dry adhesion, polymeric arrays of nano-pillars are prepared in this work. Thermoset and thermoplastic nano-pillar arrays are examined using an atomic force microscope to assess their adhesion strength and its uniformity. Results indicate the proposed method is robust and can be used to reliably prepare nano-structures with a high AR.

Determining adhesion of nonuniform arrays of fibrils

Dry adhesives containing nonuniform arrays of fibrils were tested for the uniformity of their adhesion strength. These arrays comprised fibrils with nanometer-scale dimensions and lengths tuned from 150 to 1500 nm. The surfaces of the fibrils were rendered hydrophobic through a vapor phase deposition of silane molecules to further tune the adhesion strength of the fibrillar structure. Adhesion force measurements over micrometer-length scales were obtained using a tipless cantilever controlled by a scanning probe microscope. Maps of the adhesion forces depicted diverse variations in adhesion strength with the nonuniform lateral changes in topography. Through an extensive data analysis, differences observed between samples were correlated to changes in processing conditions and surface chemistry modifications. The methods demonstrated in this paper are useful for identifying variations in the adhesion strength of dry adhesives made of nonuniform arrays of fibrils. These advancements are crucial for understanding the correlation between structure and function within nonuniform fibrillar adhesives.

Improved Adhesion and Compliancy of Hierarchical Fibrillar Adhesives

The gecko relies on van der Waals forces to cling onto surfaces with a variety of topography and composition. The hierarchical fibrillar structures on their climbing feet, ranging from mesoscale to nanoscale, are hypothesized to be key elements for the animal to conquer both smooth and rough surfaces. An epoxy-based artificial hierarchical fibrillar adhesive was prepared to study the influence of the hierarchical structures on the properties of a dry adhesive. The presented experiments highlight the advantages of a hierarchical structure despite a reduction of overall density and aspect ratio of nanofibrils. In contrast to an adhesive containing only nanometer-size fibrils, the hierarchical fibrillar adhesives exhibited a higher adhesion force and better compliancy when tested on an identical substrate.

Harnessing tunable scanning probe techniques to measure shear enhanced adhesion of gecko-inspired fibrillar arrays.

The hierarchical arrays of mesoscale to nanoscale fibrillar structures on a geckos foot enable the animal to climb surfaces of varying roughness. Adhesion force between the fibrillar structures and various surfaces is maximized after the gecko drags its foot in one direction, which has also been demonstrated to improve the adhesion forces of artificial fibrillar arrays. Essential conditions that influence the magnitude of these interactions include the lateral distance traveled and velocity between the contacting surfaces, as well as the velocity at which the two surfaces are subsequently separated. These parameters have, however, not been systematically investigated to assess the adhesion properties of artificial adhesives. We introduce a systematic study that investigates these conditions using a scanning probe microscope to measure the adhesion forces of artificial adhesives through a process that mimics the mechanism by which a gecko climbs. The measured adhesion response was different for arrays of shorter and longer fibrils. These results from 9000 independent measurements also provide further insight into the dynamics of the interactions between fibrillar arrays and contacting surfaces. These studies establish scanning probe microscopy techniques as a versatile approach for measuring a variety of adhesion properties of artificial fibrillar adhesives.