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Introduction
As a Science and Technology librarian, it can be a challenge to answer research questions without oversimplifying the science, yet provide an answer that a non-scientist can grasp. It usually means reviewing a range of materials on a subject in order to make sense of what may be highly technical subjects. It also means articulating an answer that makes sense within the context of the everyday world that most people experience. This article attempts to illustrate this challenge, first by presenting a narrative answer to a science reference question, followed by a short summary of the research process involved.
How Does A Spider Walk Across a Ceiling Without Falling?
Last year it was discovered how spiders are able, not only to walk across a ceiling, but to carry up to 170 times their own body weight. The study that accomplished this was performed by a German and Swiss scientific team at the University of Zurich, using atomic force microscopy (AFM) to examine spider legs, and thus "clarify the fundamental basics of a biological attachment system and to supply potential input for the development of novel technical devices" (Kesel et al. 2004).
In other words, someone wants to make a Spiderman suit.
Spiders, like other animals from the phylum Arthropoda (insects, crabs, etc.), have an external skeleton called the arthropod cuticle. This exoskeleton is the armor that keeps their insides in and the outside world out. It consists of a layered material with some unusual characteristics, and happens to have a lot of potential for duplication and use in artificial materials, suggesting some interesting technological possibilities.
Coated with a waxy layer, the spider's exoskeleton is water repellant and has extremely low adhesion, which means that spiders don't just stick to whatever they happen to be on. Though this might seem a limitation for a spider wanting to crawl across the ceiling, in fact a much more complex and sophisticated system has evolved to enable it to do that. A spider has claws that it can use to cling to a roughly textured surface, but on a smooth surface a wonderfully adapted system of microscopic proportions on the spider's feet enables it to hold on. The precise nature of this "anti-gravity" attachment system was the subject of the work done at the University of Zurich, the first study of its kind.
The spiders studied at Zurich are members of a family of jumping spiders that hunt down prey without building a web, so holding onto surfaces while carrying significant weight (i.e., food) is obviously crucial to their survival. On each of the spider's feet there are hair-like tufts, called scopulae, and using a scanning electron microscope it was discovered that a single scopula is itself composed of many, many, much smaller, single hairs. It is these minuscule hairs, or setules, that actually represent the direct contact points with a surface. The number of setules per foot is estimated to be 78,000 each, and since spiders have eight feet, they have upwards of 600,000 individual points of contact with any given surface. The ability of the spider to cling to overhead smooth surfaces is due to lots and lots of extremely miniaturized contact elements, or as the scientists explain: "Branched hairs and progressive structural miniaturization, broadened contact elements, as well as the absence of adhesive secretions, are characteristic features of ... the spider attachment system."
Underlying this system of course, are principles of physics. Though each of these points of contact is extremely small, together they combine into a powerful force. In this case they are known as van der Waals forces. Essentially, the spider's leg hairs and the surface it is sitting on are bonding with each other, at the molecular level. The ceiling is holding on to the spider and the spider is holding on to the ceiling. Extremely week individually, when all of these forces act in concert on these hundreds of thousands of hairs, the total adhesive force is extremely powerful, up to 170 times the weight of the spider, if all eight legs are in contact. Jumping spiders however, as their name would indicate, rarely have all eight legs on a surface at once, yet even one leg can exert approximately enough force to support 21 times the weight of the spider. It's not hard to start thinking about how much a person could carry if they wore a suit that exerted roughly the same degree of force. A lot of pizzas!
Due to the power of the van der Waals forces, "adhesion becomes independent not only of material characteristics but also of conditions of the surroundings," surroundings such as a vacuum or a weightless environment like outer space, or even underwater. Technologies such as the already-proposed waterproof "Spider-Post-Its®" illustrate some of the potential applications of this discovery. Imagine a suit that enables astronauts to stick to the wall of a space vehicle, inside or out, peeling off as necessary to move around and do their work. It isn't much of a stretch beyond that to imagine fabric for personal Spiderman suits, complete with special adhesive Spidey-powers!
Then there is the Gecko Factor. Though not arthropods themselves, it turns out that geckos, those cute little lizards that can run up walls, across ceilings, and dance in television commercials, have some of the same abilities as the jumping spider, and for the same reason: Lots of little hairs on their little green feet. Again, each individual hair produces only a minuscule amount of the amazing van der Waals force, but cumulatively they are powerful enough to create a formidable stickiness. And geckos have only have half as many legs as the spider. Duplicating this phenomenon into some sort of artificial material has already resulted in prototype 'gecko tape,' in which researchers at the University of Manchester used "microfabrication of dense arrays of flexible plastic pillars, optimized to ensure their collective maximum adhesion" (Geim et al. 2003). But that is another question and another answer.
Notes
Unlike the branching, progressively organized system of scopula and setules on the legs of the jumping spider and the gecko, the research for this piece followed a much more random pattern. The original inspiration came from a Science Notebook piece in the Washington Post, from April 19, 2004. This blurb provided a short summary of the discovery, along with the name of the primary researcher and the source of publication of their results, Smart Materials & Structures. The Institute of Physics Electronic Journals page linked to the journal's home page where a quick search provided a complete citation (Kesel 2004). Unable to locate a full-text copy online, either open-source or in one of our subscription databases, I did it the old-fashioned way; by photocopying from the print journal.
Meanwhile, I Googled "van der Waals" and got more than enough information, both definitions and biographies (Johannes Diderik van der Waals N.D.). I also searched the LC catalog, and some of these titles are listed below. And finally, reading the various related pieces I stumbled across led to the gecko connection, specifically the article on "Microfabricated adhesive mimicking gecko foot-hair."
Web Resources:
Kesel, A.B., Martin, A., and Seidl, T. 2004. Getting a grip on spider attachment. Smart Materials & Structures. [Abstract online.] Available: {http://iopscience.iop.org/0964-1726/13/3/009} [Accessed July 25, 2005].
The Hungry Jumping Spider Climbed Up the Waterspout. 2004. [Online]. Available: http://whyfiles.org/shorties/152sticky_spider/ [Accessed July 25, 2005].
Scanning electron microscope image of the foot of the jumping spider. 2004. [Online]. Available: {http://web.archive.org/web/20060208170842/http://physics.iop.org/IOP/Press/spiders1a.jpg In: Spiders make best ever Post-It notes. Available: {http://web.archive.org/web/20060202095306/http://physics.iop.org/IOP/Press/PR2904.html} [Accessed July 25, 2005].
Intermolecular Bonding - van der Waals Forces. 2000. [Online]. Available: http://www.chemguide.co.uk/atoms/bonding/vdw.html [Accessed July 25, 2005].
Selected Internet Resources - Physics. 2004. [Online]. Available: http://www.loc.gov/rr/scitech/selected-internet/physics.html [Accessed July 26, 2005].
Science Tracer Bullets Online: Nanotechnology [Online]. Available: http://www.loc.gov/rr/scitech/tracer-bullets/nanotechnologytb.html [Accessed July 26, 2005].
Books:
Parsegian, V. Adrian. 2005. Van der Waals Forces. New York: Cambridge University Press.
Langbein, Dieter W. 1974. Theory of Van der Waals Attraction. Berlin, New York: Springer-Verlag.
Additional Articles:
Knight, Will. 2003. Gecko tape will stick you to the ceiling. NewScientist.com news service. [Online]. Available: {https://www.newscientist.com/article/dn3785-gecko-tape-will-stick-you-to-ceiling/} [Accessed July 26, 2005].
Graham-Rowe, Duncan. 2003. Synthetic gecko hairs promise walking up walls. NewScientist.com news service. [Online]. Available: {https://www.newscientist.com/article/dn3726-synthetic-gecko-hairs-promise-walking-up-walls/} [Accessed July 26, 2005].
Kaufman, Marc. 2004. Science; notebook. Washington Post. (Apr. 19, 2004, Final ed. p. A-8).
References
Kesel, A.B., et al. 2004. Getting a grip on spider attachment: an AFM approach to microstructure adhesion in arthropods. Smart Materials & Structures 13(3): 512-518.Geim, A.K., et al. 2003. Microfabricated adhesive mimicking gecko foot-hair. Nature Materials 2: 461-463.
Johannes Diderik van der Waals -- Biography N.D. IN: Nobel Lectures, Physics 1901-1921 (Amsterdam: Elsevier, 1967). [Online]. Available: http://nobelprize.org/physics/laureates/1910/waals-bio.html [Accessed: August 5, 2005]
