Long before Paul Steen was a Cornell University professor, he was a boy climbing trees in his backyard – and there were always moments where he couldn’t quite grab on to a nearby branch.
Years later, Steen is experimenting with ways that a simple concept — surface tension — can help us hang on and scale a variety of complex surfaces.
Steen and his team developed a centimeter-by-centimeter patch that supports hundreds of grams of weight.
“If you scale that up, you can imagine being able to hold yourself against a wall or a ceiling,” said Steen.
The professor spoke with Tech Briefs about how the technology can be used – and how soon you’ll find “Spider-Man” shoes at your local Wal-Mart. His edited responses are below.
Tech Briefs: What was the inspiration behind your adhesion technology?
Prof. Paul Steen: The palm beetle, native to the southeast United States, has the ability to withstand attack from its primary enemy, the ant, by adhering to the palm leaf, its main habitat.
My colleague Professor Tom Eisner discovered this adhesion strength could be on the order of a hundred times its body weight. So, this is like you or I being able to hold six or seven Toyota Corollas just through gripping and hanging on to them. Eisner was able to bring these reluctant beetles back to the cold climates of Ithaca, New York, and get them to live long enough so he could understand the physical basis for their adhesion
Tech Briefs: How does the palm beetle have this kind of strength?
Steen: These guys are the size of your little fingernail, and they're able to articulate 120,000 little droplets at the end of feet. Each of these little droplets, through parallel action, give a little tug.
But if you multiply those tugs together, you get the strength of adhesion that was being observed on the beetle scale. So, we sat down and said, "Look, we want to imitate the beetle."
Tech Briefs: So, how did you begin trying to get your technology to match the beetle?
Steen: The challenge became finding a way to get the droplets out. What we envisioned was an array of droplets that would present themselves against the surface we were trying to adhere to, and then control that array of droplets.
When the droplets grab the surface, they become bridges to the surface; and then when you pull the liquid back, those liquid bridges break and there’s no more contact or adhesion.
It's a really simple concept, if you wish. You can do this simple experiment in your home. You just take a little drop of water and push it out of a small tube; it can grab a piece of aluminum foil and pull it up.
Tech Briefs: Is the technology itself that simple?
Steen: One of the beauties of this pump: it has no mechanical moving parts. It's all done through electro-osmotic forces and just water. The water does move, of course, but otherwise everything's solid materials and porous materials. Very simple materials can make this device. There's individual addressability to every droplet if you want it. It's low voltage. You can do switching on the order of milliseconds.
Tech Briefs: How is the device powered?
Steen: The actual device we've built can be powered by ten volts. When you grab, it takes a little power; and when you release, it takes a little power. When it's adhered, it requires no energy. Once it's, for example, hanging from the ceiling, it's essentially there for good. So, one of the advantages is that it’s very stingy in its power requirements.
Tech Briefs: How well does the adhesive conform to surfaces?
Steen: At this stage, it doesn't conform to strongly curved surfaces. Right now, it's essentially been demonstrated for flat surfaces. The strength of adhesion is key, and that often gets up to a strength that's potentially on the order of epoxy glue. This switching adhesion is reversible, so you can flip, add, and change the polarity of the pump to get it to release itself. It's a lightweight grab-and-release device controlled with low voltages.
Tech Briefs: How is the catch and release achieved?
Steen: Let’s say you have a very smooth countertop in your kitchen and you have a flat glass pane that you're trying to clean off. If it happens to have some liquid underneath it and gets squished against the countertop, then you can have a very hard time getting it off. That is due to the surface tension that is acting around the perimeter of the piece of glass.
Tech Briefs: What other applications exist, beyond scaling walls like Spider-Man?
Steen: One of the things that I think is most interesting is using this very lightweight, low-power adhesion device as a place for drones to find shelter from the storm, so to speak. You can imagine a drone has a hard time in, say, a 15-mile-an-hour gust of wind and so forth, so it might be very useful to be able to settle and rest up underneath a porch or on a ceiling overhang somewhere.
Tech Briefs: What about manufacturing possibilities?
Steen: A simple idea would be a “pick and place” situation where you'd pick up a part on a manufacturing factory floor, and you’d move it around and place it somewhere else. Nowadays that's done easily by suction. But it turns out that this wet adhesion, or the switchable electronically controlled adhesion device, can carry larger weights for a smaller cross section.
Tech Briefs: Are there medical applications?
Steen: Another application would be precise metering pumps for biomedical applications. We can use these single droplets, or 2 x 2 arrays of droplets, to pump very precise amounts of liquid, say, for insulin injections and replacing some of the mechanical pumps.
Tech Briefs: What kinds of surfaces have you tested?
Steen: Reversible adhesion to brick and plywood was a lot of fun. We took any material we could think of, roof shingles, brick, plywood, linoleum, sandpaper, and it worked for all these.
Tech Briefs: Given the Spider-Man comparisons, what do you think drives researchers to create “super” technologies?
Steen: I think science is most fun when it's coupled with imagination. I always tell those that are working with me, “Let's just dream a little but more about this.” I was not a comic reader as a youngster, but I think this is important. Any kind of imagination that is stimulated by superheroes is important.
Tech Briefs: Do you have any guess as to when we'll see sticky shoes and gloves in Wal-Mart?
Steen: Paying for the engineering, materials, and labor to get this done actually turns out to be significant once you get to a certain level of sophistication. Within three years, if we had someone who was sufficiently interested to help us out with those resources, we could produce adhesion strengths necessary for being able to hold a human with two gloved hands.
We basically have a pause in terms of the fundamental research and are hoping to have an interest from somebody interested in licensing the technology and developing it for their particular application.
What “particular applications” do you envision? Share your comments and questions below.