Design of a Synthetic Gecko Adhesive

My primary research involves the design, fabrication, and testing of synthetic gecko adhesives. These are nano- and micro-scale structures composed of millions of elastic fibers that mimic the setal arrays on the digits of natural geckos. A comparison of setal arrays and our synthetic products are shown in the figure below.

Figure 1 (left) scanning electron micrograph of setal array from the Anolis lizard; (right) array of vertically aligned polimide fibers; both structures are composed of 20 micron long, 0.6 micron diameter polymeric fibers

Design

Designs for micro fiber adhesives are inspired by the setal arrays in gecko lizards. Individual setal stalks independently bend to conform to a surface. In this way, the array achieves intimate contact and forms millions of bonds at the setal tips through weak surface forces. By selecting the appropriate geometry, this same effect can be achieved with synthetic microstructures for a wide range of materials. Of particular interest are stiff, hydrophobic materials since they are resistant to wear and fouling.

An important design principle is the global compliance of a micro fiber array and the ability of fibers to uniformly share load during attachment and detachment. One mechanism that allows uniform load sharing is side contact, in which individual fibers make contact with a surface along there side and must be peeled off during detachment [1,2]. This mechanism has been postulated to explain the adhesion of high aspect ratio structures such as multi-walled carbon nanotubes [3], silicon nanowires [4], and polyimide nano fibers (unpublished).

Figure 2 (left) array of 60 micron long, 200 nanometer diameter polyimide fibers; (right) postulated mode of contact for nano fibers

Nano fibers are susceptible to adhering to there neighbors and this may lead to the formation of large clumps. Based on a mathematical model similar to that used to study side contact, we can predict the occurrence and extent of clumping in an array of nano fibers [5].

Figure 3 (left) clumps of 20 micron long, 0.6 micron diameter polyimide fibers; (right) model of adhesion between two adjacent fibers

Fabrication

Arrays of polymer micro fibers are fabricated by casting uncured polymer in a commercially available mold. We typically use a nuclear-track etched porous polycarbonate membrane for our mold.

Testing

Although reliable adhesion has not yet been observed, we have measured high friction with arrays of polypropylene micro fibers. Friction measurements are obtained with a traditional pulley apparatus as well as with a two degree-of-freedom optical force sensor. Remarkably, high friction is maintained even under larger pressures of >100 kPa. Since no adhesion is observed, these structures follow an Amonton friction law. A mathematical model of the structure suggests behavior that is consistent with the experimental result [6].

Figure 4 (left) array of 20 micron long, 0.6 micron diameter polypropylene fibers; (right) US quarter suspended on an 82 degree incline with the array of polypropylene micro fibers.

[1] C. Majidi, R. E. Groff, R. S. Fearing, "Attachement of fiber array adhesive through side contact," J. Applied Physics 98 103521 (2005).

[2] C. Majidi, "Remarks on formulating an adhesion problem using Euler's elastica," Mechanics Research Communications in press (2006).

[3] Y. Zhao, et al. "Interfacial energy and strength of multiwalled-carbon-nanotube-based dry adhesive," J. Vacuum Sci. & Tech. B 24 331-335 (2006).

[4] R. Dubrow, US Patent No. US2004/0250950 A1 (2004).

[5] C. Majidi, R. E. Groff, R. S. Fearing, "Clumping and Packing of Hair Arrays Manufactured by Nanocasting," Proc. IMECE, ASME, Anaheim (2004).

[6] C. Majidi, et al. "High Friction from a Stiff Polymer using Micro-Fiber Arrays," Physical Review Letters 97 076103 (2006).

Last Updated August 14, 2006