Articles

Walking Up Walls-- Insects Inspire a Better ‘Sticky Tape’

By David Catchpoole

Seemingly effortlessly, a fly can not only walk vertically up walls, but also upside down on ceilings—even ones made of glass.

For years, scientists have strived to work out the ‘secret’ allowing insects to do this—with an eye to potential commercial applications. Imagine, for example, an adhesive tape as effective as an insect’s foot—able to attach super-strongly yet also be readily detached, again and again, without losing any of its ‘stickiness’.

An early suggestion was that insects used ‘microsuckers’. But this doesn’t explain all insect ‘stickiness’. With the advent of electron microscopes and other sophisticated micro-and nano-level technology used to examine insect foot pads in fine detail, researchers now have a much better understanding of insect adhesive mechanisms.

Scientists now know that insect feet have an incredibly intricate micro-structure with a range of different basic physical forces contributing to the overall adhesion. Micro-fine setae/hairs on the insect foot pad allow van der Waals and Coulomb forces to do their part, just like the feet of geckos 1,2 and some spiders. 3 This is not only very efficient, but also self-cleaning.

Some insects also secrete a tiny amount of fluid from their foot pads, allowing additional strongly cohesive capillary forces (surface tension and molecular adhesion). And some insects have sophisticated mechanical-hydraulic machinery that enables them to stick this way. 4

Armed with this information, materials engineers have developed a micropatterned polymer tape modelled on the foot pad surface of certain insects. 5,6 Subsequent testing revealed the vastly superior performance of the ‘bioinspired’ 7 tape, relative to a commercially-available pressure-sensitive adhesive tape.

Compared to a flat PVS tape, the microstructure patterned tape demonstrated considerably higher adhesion in a peeling test, with a higher pull-off force per unit apparent contact area.

What’s more, the microstructured tape ‘is less sensitive to contamination by dust particles than the flat tape or a regular scotch tape’. And even if the ‘insect tape’ is contaminated, it can be washed with a soap solution in water, completely recovering its initial adhesive properties.

Window-cleaning robots!

Robots that could climb vertical surfaces as insects and lizards do could be used in exploration, inspection of tall buildings and bridges and for high-rise window cleaning. Wall-walking robots are indeed being developed with such applications in mind. This ‘bioinspired’ tape was ‘successfully applied’ 8 to the multi-spoke wheel-leg appendages of a 120 g wall-climbing robot Mini-Whegs TM. 9

Despite being only at the pre-prototype stage, no-one would say that the wall-walking robot, or the adhesive ‘insect tape’, were not designed.

The moral of the story? Let me put it into rhyme:

Walking up walls is quite an art,
So, that robot’s designer must be pretty smart.
But looking at an insect’s skill—
The insect’s Designer must be smarter still.

References

• 1. Sarfati, J., Great gecko glue? Creation 23(1): 54–55, 2000; <creation.com/gecko>..
• 2. Sarfati, J., Gecko foot design could it lead to a real spiderman? Creation 26(1): 22–23, 2003; <creation.com/geckoman>..
• 3. Sarfati, J., Spectacular spider stickiness, Creation 27(4):54–55, 2005; <creation.com/spiderstick>..
• 4. Sarfati, J., Startling stickiness, Creation 24(2):37, 2002; <creation.com/stickiness>.
• 5. Gorb, S., Varenberg, M., Peressadko, A., and Tuma, J., Biomimetic mushroom-shaped fibrillar adhesive microstructure, Journal of the Royal Society Interface 4(13):271–275, 2007..
• 6. The insect pad surface copied consisted of a pattern of hairs (fibres, pillars) with broad flattened tips and a narrowed flexible region just below the flattened tip. The pillars exhibit a hexagonal distribution pattern—therefore a high packing density..
• 7. Gorb, S., Sinha, M., Peressadko, A., Daltorio, K., and Quinn, R., Insects did it first: a micropatterned adhesive tape for robotic applications, Bioinspiration & Biomimetics 2(4):S117–S125, 2007..
• 8. According to the authors of ref. 7. Their paper has a photograph (Figure 9, on p. S122) of the Mini-Whegs TM, with ‘insect tape’ affixed, scaling a wall. (At the time of writing (April 2008), the photograph could be viewed online at http://www.iop.org/EJ/article/1748-3190/2/4/S01/bb7_4_s01.pdf?request=id=44d2e4.).
• 9. There are many other potential practical applications for the new ‘insect tape’ too, e.g. fastening objects onto glass surfaces, and for protecting and gripping optically sensitive materials with smooth surfaces such as lenses, CDs and DVDs.

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