Spider Legs Lead To Better Post-it Notes

By Roland Piquepaille

European scientists have found why spiders stick to a ceiling by using a scanning electron microscope (SEM) to take pictures of the foot of a jumping spider. According to their findings, a spider can carry 170 times its own weight without falling. As one scientist said, "That's like Spiderman clinging to the flat surface of a window on a building by his fingertips and toes only, whilst rescuing 170 adults who are hanging on to his back!" One possible application will be to make Post-it notes that don't fall off, even if they're wet. This also paves the way for other new kinds of adhesive materials.

This is the first time anyone has measured exactly how spiders stick to surfaces, and how strong the adhesion force is. The team used a scanning electron microscope (SEM) to make images of the foot of a jumping spider, Evarcha arcuata. There is a tuft of hairs on the bottom of the spider's leg, and each individual hair is covered in more hairs. These smaller hairs are called setules, and they are what makes the spider stick.

Here is a couple of pictures taken by a Scanning Electron Microscope (SEM) (Credit: The Institute of Physics).

	On this view of the underside of the scopula, taken at a magnification of 270x, you can see the single hairs (setae) that make up the scopula. The oval represents the estimated scopula area.
	This one, shot at a magnification of 8750x, shows the very dense setules on the underside of one seta.

What kind of application can we expect from this discovery?

What makes the van der Waals force an interesting form of adhesion is that, unlike many glues, the surrounding environment does not affect it. The only thing that affects it is the distance between the two objects.

"One possible application of our research would be to develop Post-it® notes based on the van der Waals force, which would stick even if they got wet or greasy," said Professor Antonia Kesel, head of the research group in Bremen. "You could also imagine astronauts using spacesuits that help them stick to the walls of a spacecraft -- just like a spider on the ceiling."

But if such a note is so sticky, how will you remove it from your monitor?

The total van der Waals force on the spider's feet is very strong, but it is the sum of many very small forces on each molecule. The researchers believe the spider lifts its leg so that the setules are lifted successively, not all at once, and it does not need to be very strong to do this. All you would have to do to lift a future kind of Post-it® note is peel it off slowly.

The research paper has been published by Smart Materials and Structures on April 19, 2004. The full paper is available for free for registered users for a duration of one month. Here is a link to the abstract of this paper named "Getting a grip on spider attachment."

Source: Institute of Physics, April 25, 2004, via EurekAlert!; and various websites