Bioinspiration: Cockleburs, Geckos and Porcupines

"Bioinspiration" is the relatively new engineering skill that takes a leaf from nature's book. It involves building intricate structures with surprising new properties derived from nature's own nanotechnology. Examples include using spider silk to manufacture tendons and ligaments, using the leaves of the lotus plant as a model for self-cleaning surfaces and mimicking the spiny fruit of the cocklebur to fashion the familiar hook-and-loop fastener.

I have previously written about spider silk and much of the developmental work is ongoing.¹ This article describes the serendipitous experience of George de Mestral following his brush with cockleburs. It also covers the amazing adhesion power of a gecko's foot and how these properties can be applied to apparel and wearable prosthetics. And last, I discuss how the barbs of a porcupine's quills can serve as a basis for development of devices such as tissue adhesives, trocars and needles.

Cockleburs

De Mestral, an inventor trained as an electrical engineer, lived in Lausanne, Switzerland. His pastime of hunting in the lower slopes of the Jura Mountains led to his discovery. He was a curious person and fascinated by fasteners, apparently as a result of difficulty in connecting the large hooks and eyes on his wife's dresses.

In 1948, when de Mestral returned home from one of his walks, his dog and his pants were covered in burs. Cockleburs are plant seed sacs that cling to animal fur as a means of traveling to fertile new planting grounds.² Instead of just picking them off, he marveled at their tenacity and examined the burs microscopically. He noticed all the small sharp hooks that enabled the burs to cling to the tiny loops in the fabric of his pants.

This observation led de Mestral to design a unique, two-sided fastener: one side with stiff hooks like the burs and the other with soft loops like the fabric of his trousers. Culmination of his design came with the introduction of nylon. It had optimal softness and strength, and reproducible material properties. No previous substance was suitable for fashioning an effective hook-and-loop fastener.³

He filed the original patent in 1951 with the help of a weaver at a textile plant in Lyon, France, and a Swiss loom maker in Basel, Switzerland. The product, called Velcro (a combination of velour and crochet) came on the market in 1955. The American patent expired in 1978 and de Mestral died in 1990. Hook and loop fasteners are now a major product worldwide and the Velcro Corporation (which he started) is still the major producer and a multimillion-dollar industry.⁴

Gecko Setae

Geckos are a group of lizards that includes about 930 species in 88 subgroups (genera).⁵ Among the more common types are the tokay gecko, crested gecko, gargoyle gecko, dwarf gecko, leopard gecko, Mediterranean gecko, Western banded gecko and New Caledonian gecko. Most are petite, slender-bodied lizards with broad, flat heads. Many are tree-dwelling creatures.

Geckos have always astonished observers-including Aristotle in the 4^th century BC-with their ability to run vertically and upside down at will. They can scale a perfectly smooth wall, even glass, and walk across a ceiling whether the surface is rough or smooth, or wet or dry.⁶ This ability is truly amazing to see, but is even more impressive when considering the microstructures on the gecko's feet.

An unaided visual inspection offers few clues to their adhesive ability. However, an electron microscope shows many bristles or "setae," almost 500,000 on each foot. Each seta, which can be 30 to 130 µm long, is only one-tenth the diameter of a human hair.⁷ The ends of the bristles fork into 100 to 1,000 mini-bristles with enlarged and flattened spoon-like endings (spatulas). Spatulas are from 0.2 to 0.5 µm in diameter. It is these spatulas that make contact with the surface. A single gecko has about one billion of these points of contact.⁸

Measurements have revealed that "a seta is 10 times more effective at adhesion than predicted from maximal estimates on whole animals."⁹ Adhesive force values support the hypothesis that individual setae operate by van der Waals forces. These forces are the result of intermolecular attractions and defined as the weak attractive force between atoms or nonpolar molecules caused by a temporary change in dipole moment arising from a brief shift of orbital electrons to one side of one atom or molecule, creating a similar shift in adjacent atoms or molecules.¹⁰ Van der Waals forces are generated by the ability of billions of spatulas to maintain close contact by molding themselves to the contours of any surface.

The setae are adhesive even when not attached to a gecko. Tape can be made from the setae, which can be harvested harmlessly from a live animal and grow back.¹¹ There are already countless potential uses for gecko adhesives and no doubt many that are unforeseen. Microsurgery and clean room processing to attract submicroscopic particles from the product's surface are potential uses.¹²

Porcupine Quills

A team at Brigham and Women's Hospital in Boston discovered how the quills of North American porcupines easily puncture tissue and why, once stuck in flesh, they are difficult to remove. The team did this work using natural porcupine quills and molded synthetic polyurethane quills.¹³

The North American porcupine has approximately 30,000 quills on the dorsal (back) surface that are released in self-defense when a predator makes contact. It has been well documented that removing porcupine quills lodged in tissue is difficult.

The reason is that the quills have two distinct regions. The conical black tip contains a layer of microscopic, backward-facing barbs on its surface, while the cylindrical white base contains smooth, scale-like structures. The natural quill's geometry enables easy penetration and high tissue adhesion.

"Reduced penetration force is achieved by topography that appears to create stress concentrations along regions of the quill where the cross-sectional diameter grows rapidly. This facilitates cutting of tissue suggesting that the tissue absorbs less energy and is damaged less by the quill," according to an article written in Proceedings of the National Academy of Sciences.¹⁴ Such findings should serve as the basis for the development of bioinspired devices such as tissue adhesives or needles, trocars and vascular tunnelers where minimizing penetration force is important to prevent collateral damage.¹⁵

Final Thoughts

Cockleburs, gecko setae and porcupine quills are examples of plant and animal attributes used in the field of bioinspiration, a growing body of techniques for making materials with novel and startling properties. The mechanisms described above are based on physical structures ranging from one-billionth to one millionth of a meter. This size range is the nanoregion, and the natural structures at this level are called "nanostructures."

There will surely be similar discoveries in the future. For example, the flashing light of the firefly is caused by a chemical reaction that produces almost no heat and has been mimicked to produce biomedical diagnostic tests. Nanotechnology has brought nature and engineering far closer together and other surprises are in store for us.

References

1. Sherman M. "Spiders." Regulatory Focus. 2011: 5, 38-41.
2. About.com. "The Invention of VELCRO - George de Mestral." inventors.about.com/library/weekly/aa091297.htm. Accessed 25 December 2012.
3. Ibid.
4. Ibid.
5. About.com. "Geckos." animals.about.com/od/Lizards/p/geckos.htm. Accessed 29 December 2012.
6. Forbes P. "The Gecko's Foot." New York, NY: WW Norton & Co.; 2005.
7. Autumn K et al. "Adhesive force of a single gecko foot-hair." Nature. http://www.nature.com/nature/journal/v405/n6787/full/405681a0.html. Accessed 22 January 2013.
8. Op cit 6.
9. Answers. "van der Waals force." www.answers.com/topic/van-der-waals-force. Accessed 17 December 2012.
10. Ibid.
11. Ibid.
12. Op cit 6.
13. HealthDay. "Porcupine Quill Could Foster New Medical Devices." consumer.healthday.com/article.asp?AID=671379. Accessed 13 December 2012.
14. Cho WK et al. "Microstructured barbs on the North American porcupine quill enable easy tissue penetration and difficult removal." Proceedings of the National Academy of Sciences. http://www.pnas.org/content/early/2012/12/04/1216441109. Accessed 22 January 2013.
15. Ibid.