StanfordDryAdhesives? : Work to develop dry adhesive patches at Stanford based on methods from Ron Fearing's lab.

Geckos!

Evidence for van der Waals adhesion in gecko setae Research by Autumn et al. has provided the first direct experimental evidence for dry adhesion of gecko setae by van der Waals forces, rejecting theories of mechanisms relying on high surface polarity including capillary action. Experiments proved that geckos adhere to both hydrophobic and hydrophilic polarizable surfaces, thus supporting the van der Waals mechanism theory. This theory implies that adhesive properties are merely the result of the geometry of spatualar tips of the hundreds of thousands of hairs on the geckos toes, and not of surface chemistry.

Adhesive force of a single gecko foot-hair This study reports the first direct measurement of single setal force by using a two-dimenstional micro-electromechanical systems force sensor and a wire as a force gauge. Measured adhesive force values further support the hypothesis of a van der Waals mechanism and reveal that a seta is ten times more effective at adhesion than predicted from maximal estimates on whole animals! The study also shows how the gecko's particular behavior of toe uncurling and peeling indicates two aspects of setal function which increase their effectiveness.

seta v. spatula? To clarify the questions about the structure of hairs on the gecko foot, microscopy studies have shown that each hair has almost 500,000 keratin hairs called setae. Each seta is 30-130 micrometers in length. These branch into hundreds of projections, which terminate in 0.2-0.5 micrometer spatula shaped structures. Further description of the structure of the toe can be found in:

• Rusell, A. P. A contribution to the functional morphology of the foot of the tokay, Gekko gecko (Reptillia, Gekkonidae). J. Zool. Lond. 176, 437-476 (1975).
• Ruibal, R. & Ernst, V. The structure of the digital setate of lizards. J. Morphol. 117, 271-294 (1965)

barnacles?? look into Robert Pearsall's work at Duke University on barnacle adhesion. Doesn't seem to check out on Google...

Technology

Various people have been working on technololgies for Dry Adhesion

Foremost among them, Ron Fearings Lab at U.C. Berkeley is pursuing several techniques as part of the RiSE project. Using the K. Autumn, et al. papers above as outlines for design criterion, they explore two main methods of fabrication: nanorobotic imprinting and parallel fabrication.

• Nanorobotic Imprinting uses a single Atomic Force Microscope (AFM) probe to imprint a flat bed of wax, which is used as a template and filled with a polymer, which can be removed by peeling. Experiments were performed using two different hydrophobic polymers--silicone rubber compared with a polyester resin. Using the Young's modulus and adhesive force of arrays formed using these materials, it was found that the polyester resin array closely approximated measured values from actual gecko spatulae. This fabrication method is proved to be independent of material properties, repeatable, and consistent with a standard adhesive model.
• Parallel Fabrication uses a pre-existing membrane as a mold for a liquid polymer wich is poured and then cured. In the case of the alumina membrane, after curing the membrane was etched away with HCl. The largest problem with this method was the attraction of hydrophobic compliant hairs. As a result, a polycarbonate membrane was used in a subsequent trial because of its lower pore density and aspect ratio. While hairs did not stick together, their dimensions (diameter and length) and adhesive force was less representative of an actual gecko setae. Thus while this method has great potential for mass production, further optimization is needed either geometrically or in the etching process, such as the use of external excitation or a surfactant for the given etchant.

Ron Fearing's work:

• Ron Fearing's page on Synthetic Gecko Spatula Array Fabrication
• Sitti and Fearing's paper on Nanomolding Based Fabrication of Synthetic Gecko Foot-Hairs
• Tiffany's power point slides on the paper for the 6/4/03 Biomimetics meeting

Geim's work (at the University of Manchester):

• Brief Yahoo News article
• University of Manchester news article
• Brief article in Nature Materials (copied to Docushare)
• Picture of Spiderman action figure hanging from ceiling using artificial adhesive (from Nature Materials article)
• Geim. "Microfabricated adhesive mimicking gecko foot-hair" (PDF of complete Nature Materials article) (Docushare)

-- MarkCutkosky - 04 Jun 2003