For as long as humans have needed to stick things together, nature has provided inspiration. The modern suction cup mimics the suckers on the ends of octopus arms. The origin of velcro’s loops and hooks lie in their inventor’s frustration with burrs—those pesky seeds that nestle themselves into sweaters and coats. Now a team of scientists at UMass Amherst has developed a super adhesive they say can hang heavy objects on smooth surfaces like glass. This time a small lizard is leading the way.
Duncan Irschick’s lab is filled with reptiles that creep and crawl, dart, flit and hang. Irschick is a biologist at UMass who’s been studying small lizards, including the gecko, which has an outsized reputation for sticking to its surroundings. Irschick and his team of polymer scientists have been closely observing the animals to determine just why their clinging abilities are so powerful.
“Geckos have tendons that go from bone into skin…that’s very unusual because actually tendon usually goes from bone to muscle.”
Irschick is using that tendon model to create a synthetic adhesive he calls “geckskin.” Irschick reaches into an aquarium tank and takes out a creature he calls “Big Mama”. She’s a blue and orange spotted Tokay gecko—which is the largest gecko species. Big Mama weighs in at a hefty three and half ounces.
Big Mama’s toe-pads feel soft and velvety. Each of her feet is about the size of a fingertip. And her legs can hold fast against an unusual amount of force given her size.
Imagine giving her a one-fingered high-five. Pull straight down and her toe remains strongly attached to your finger. Pull back and the attachment is slightly weaker. Irschick says what’s notable is that you can can easily disengage from Big Mama by rolling or peeling your fingers apart. And she’s got a residue free grip.
Biologist Al Crosby–who’s also on the geckskin team–says that’s a defining feature of what’s called dry adhesion. He says unlike viscous adhesives such as scotch tape, geckos use elastic structures to adhere.
“If you use materials that are elastic, then when you adhere them to a wall you can remove them and then you can restick them. There’s really nothing changing about the material between attach and detach.”
In the past, Crosby says, scientists have tried creating adhesive strips made from a synthetic form of the millions of tiny hairs covering gecko feet. But they haven’t yet proved capable of supporting heavier loads. Rather than focusing on microscopic surface features, Crosby says, the UMass team took a more integrative approach. He says they believe the geckos’ adhesive strength has to do with how the animals’ tendons attach directly to their soft toe-pads while supported by skin that’s particularly rigid. He says gecko skin has a relatively high level of keratin – the same substance that makes your fingernails rigid.
“So we started thinking. And it lead into this idea of draping…and if you think about a table cloth…it has the ability to kind of drape and conform over a table very easily. But if you pull on a table cloth it’s very stiff. And these were the two attributes we wanted to build and that’s what we saw in the gecko anatomy.”
Actually building the geckskin began with a trip to a fabric store. Graduate student Michael Bartlett says he bought a few yards of nylon and polyester, and cut them into pieces to create gecko-like-toe pads . He cured them with bathroom caulk. Attach a piece of stiff fabric to the pads, and you’ve got a pseudo “tendon.”
Bartlett made thousands of experiments with twenty or thirty different types of fabric. The latest prototype, is a 16-square inch piece of laminated cloth with a flap on it that can hold up to seven-hundred pounds.
“What you’re looking at is a plain weave of carbon fiber and kevlar…this is just your typical over under weave. If you touch the surface you can see that it’s not sticky…but if we take our adhesive material and put it on a piece of glass…and you try to pull on it, its very strong, and if you pull really hard you can’t get it off.”
Not only is the adhesive secure–even when pulling from different angles. But as Bartlett and Crosby illustrate, when compared to more traditional adhesives, removing the geckskin is effortless and silent. In fact, we had to amplify its peeling sound more than ten times just to make it audible.
The scientists say there are many potential uses for the geckskin outside the lab. The material could be used to hold computers to the wall, for instance, or aid would-be human flies out for a climb. But the team doesn’t talk much about real-world applications — they say one of the project’s major funders, the Department of Defense’s research arm, DARPA, has asked them to keep such details quiet.
The scientists have demonstrated one use by hanging a 42-inch flatscreen television on a glass panel at UMass. A version of the product could be commercially available in a year.