Metin Sitti

Evidence for van der Waals adhesion in gecko setae

Geckos have evolved one of the most versatile and effective adhesives known. The mechanism of dry adhesion in the millions of setae on the toes of geckos has been the focus of scientific study for over a century. We provide the first direct experimental evidence for dry adhesion of gecko setae by van der Waals forces, and reject the use of mechanisms relying on high surface polarity, including capillary adhesion. The toes of live Tokay geckos were highly hydrophobic, and adhered equally well to strongly hydrophobic and strongly hydrophilic, polarizable surfaces. Adhesion of a single isolated gecko seta was equally effective on the hydrophobic and hydrophilic surfaces of a microelectro-mechanical systems force sensor. A van der Waals mechanism implies that the remarkable adhesive properties of gecko setae are merely a result of the size and shape of the tips, and are not strongly affected by surface chemistry. Theory predicts greater adhesive forces simply from subdividing setae to increase surface density, and suggests a possible design principle underlying the repeated, convergent evolution of dry adhesive microstructures in gecko, anoles, skinks, and insects. Estimates using a standard adhesion model and our measured forces come remarkably close to predicting the tip size of Tokay gecko seta. We verified the dependence on size and not surface type by using physical models of setal tips nanofabricated from two different materials. Both artificial setal tips stuck as predicted and provide a path to manufacturing the first dry, adhesive microstructures.

Synthetic gecko foot-hair micro/nano-structures as dry adhesives

This paper proposes techniques to fabricate synthetic gecko foot-hairs as dry adhesives for future wall-climbing and surgical robots, and models for understanding the synthetic hair design issues. Two nanomolding fabrication techniques are proposed: the first method uses nanoprobe indented flat wax surface and the second one uses a nano-pore membrane as a template. These templates are molded with silicone rubber, polyimide, etc. type of polymers under vacuum. Next, design parameters such as length, diameter, stiffness, density, and orientation of hairs are determined for non-matting and rough surface adaptability. Preliminary micro/nano-hair prototypes showed adhesion close to the predicted values for natural specimens (around 100 nN each).

Gecko‐Inspired Directional and Controllable Adhesion

The structure that allows gecko lizards and insects to climb vertical and inverted surfaces with ease has been studied extensively since the mechanism of attachment was shown to be due dominantly to intermolecular surfaces forces. Gecko toes have been shown to adhere with high interfacial shear strength to smooth surfaces (88–200 kPa) using microscale angled fiber structures on their feet. These structures exploit the weak van der Waals interaction forces at the tips of the branching keratinous fiber arrays through their conformation into intimate contact with climbing surfaces, creating a large overall adhesion through millions of sub-micrometer scale contact points. These many contacts also resist peeling by disrupting crack propagations at the interface. Since these discoveries, many attempts have been made to replicate the structures seen in animals using synthetic materials to create adhesives with similar adhesive characteristics. Applications for such adhesives include wall-climbing robots, tissue adhesives for medical applications, and grippers for manipulation. Autumn et al. molded the first synthetic mimics by creating templates using an sharp probe followed by nanomolding. This effort was followed by attempts using electron-beam lithography, carbon nanotubes, nanodrawing, and micro/nanomolding to form high aspect ratio fibrillar structures. Higher adhesion was seen in structures with wider flat mushroom tips, demonstrating that tip size is an important parameter for generating large forces. The wider flat tips increase the contact area and may eliminate the stress singularities along the edge of the interface, as described by Bogy. Adhesion strengths as high as 180 kPa have been demonstrated with vertically aligned mushroom tipped microfibers, although damage occurs to the structures during detachment. Fibrillar structures have also been fabricated to increase (or decrease) friction. In addition, fiber surfaces have been created that provide shear adhesion using vertical arrays of single and multi-walled carbon nanotubes. Unfortunately, these fibrillar structures require very high preloads in order to provide interfacial shear strength. Stiff polypropylene sub-micrometer diameter fibers have been shown to exhibit shear adhesion without requiring high preloading.

Biologically inspired polymer microfibers with spatulate tips as repeatable fibrillar adhesives

Being inspired by gecko foot hairs, microfibers with flat spatulate tips are proposed as repeatable adhesives. They are fabricated by molding a master template fabricated using deep reactive ion etching and the notching effect. Fabricated polyurethane fiber arrays with 4.5μm fiber and 9μm tip diameter demonstrated macroscale adhesion pressures up to 18N∕cm2 and overall work of adhesion up to 11J∕m2 on a 6mm diameter glass hemisphere for a preload pressure of 12N∕cm2. These results show around four times higher adhesion and five times higher overall work of adhesion as compared to the flat polyurethane surface.

Controlled pushing of nanoparticles: modeling and experiments

A nano-robotic manipulation system using an atomic force microscope probe as the pushing manipulator and force and topology sensor is proposed. The task is the two-dimensional positioning of nanometer-size particles on a substrate in ambient conditions. Thus, the modeling of interaction forces and dynamics during the pushing operation is analyzed, and compared with the experimental results for an improved understanding of the nano scale physical phenomenon which is different from macro scale physics. Simulations and experiments are held for determining the conditions and strategies for reliable manipulation, and determining the affecting parameters. The results show that the latex particles with 242- and 484-nm radii can be positioned on Si substrates successfully with around 30-nm accuracy, and the behavior of the particle motion during pushing can be predicted from the experimental data.

Enhanced adhesion by gecko-inspired hierarchical fibrillar adhesives

The complex structures that allow geckos to repeatably adhere to surfaces consist of multilevel branching fibers with specialized tips. We present a novel technique for fabricating similar multilevel structures from polymer materials and demonstrate the fabrication of arrays of two- and three-level structures, wherein each level terminates in flat mushroom-type tips. Adhesion experiments are conducted on two-level fiber arrays on a 12-mm-diameter glass hemisphere, which exhibit both increased adhesion and interface toughness over one-level fiber samples and unstructured control samples. These adhesion enhancements are the result of increased surface conformation as well as increased extension during detachment.

Wing transmission for a micromechanical flying insect

Flapping wings provide unmatched manoeuvrability for flying microrobots. Recent advances in modelling insect aerodynamics show that adequate wing rotation at the end of the stroke is essential for generating adequate flight forces. We developed a thorax structure using four bar frames combined with an extensible fan-fold wing to provide adequate wing stroke and rotation. Flow measurements on a scale model of the beating wing show promising aerodynamics. Calculations using a simple resonant mechanical circuit model show that piezoelectric actuators can generate sufficient power, force and stroke to drive the wings at 150 Hz.

Modeling and Experimental Characterization of an Untethered Magnetic Micro-Robot

Here we present the control, performance and modeling of an untethered electromagnetically actuated magnetic micro-robot. The microrobot, which is composed of neodymium—iron—boron with dimensions 250 μm 1 130 μm 1 10 μm , is actuated by a system of six macro-scale electromagnets. Periodically varying magnetic fields are used to impose magnetic torques, which induce stick—slip motion in the micro-robot. These magnetic forces and torques are incorporated into a comprehensive dynamic model, which captures the behavior of the micro-robot. By pivoting the micro-robot about an edge, non-planar obstacles with characteristic sizes comparable to the robot length can be surmounted. Actuation is demonstrated on several substrates with different surface properties, in a fluid environment, and in a vacuum. Observed micro-robot translation speeds can exceed 10 mm s-1 .

Microstructured elastomeric surfaces with reversible adhesion and examples of their use in deterministic assembly by transfer printing

Reversible control of adhesion is an important feature of many desired, existing, and potential systems, including climbing robots, medical tapes, and stamps for transfer printing. We present experimental and theoretical studies of pressure modulated adhesion between flat, stiff objects and elastomeric surfaces with sharp features of surface relief in optimized geometries. Here, the strength of nonspecific adhesion can be switched by more than three orders of magnitude, from strong to weak, in a reversible fashion. Implementing these concepts in advanced stamps for transfer printing enables versatile modes for deterministic assembly of solid materials in micro/nanostructured forms. Demonstrations in printed two- and three-dimensional collections of silicon platelets and membranes illustrate some capabilities. An unusual type of transistor that incorporates a printed gate electrode, an air gap dielectric, and an aligned array of single walled carbon nanotubes provides a device example.

Design Methodology for Biomimetic Propulsion of Miniature Swimming Robots

Miniature and energy-efficient propulsion systems hold the key to maturing the technology of swimming microrobots. In this paper, two new methods of propulsion inspired by the motility mechanism of prokaryotic and eukaryotic microorganisms are proposed. Hydrodynamic models for each of the two methods are developed, and the optimized design parameters for each of the two propulsion modes are demonstrated. To validate the theoretical result for the prokaryotic flagellar motion, a scaled-up prototype of the robot is fabricated and tested in silicone oil, using the Buckingham PI theorem for scaling. The proposed propulsion methods are appropriate for the swimming robots that are intended to swim in low-velocity fluids.