In pop culture these days the gecko is a punchline, a little animated lizard advertising insurance on TV. But to mechanical engineers it’s a marvel — “the best climber in nature,” says Elliot Hawkes, a graduate student at Stanford University’s Biomimetics and Dexterous Manipulation Lab. “They can basically run up walls at something ridiculous, like a meter per second.”
What gives the gecko its gripping power? That question has long been something of an obsession among engineers and material scientists. In 2000 a biologist first explained the mechanism that allows geckoes to scale sheer surfaces, collections of nanoscale fibers on the bottoms of their feet. Remarkably, geckoes can switch these on and off fast enough to run upward. “It’s pretty magical,” said Hawkes.
Hawkes is the lead author on a new paper that describes one of the best efforts yet to create a climbing tool that gives human beings (or our robot pals) something close to the gecko’s gripping power.
Over the last 14 years more than 400 papers have been published about creating synthetic versions of the gecko’s adhesive feet. All of these efforts have run up against a central, uh, sticking point: as your body weight increases, you end up with more mass relative to your body’s surface area. This means that while geckoes — which weigh just a couple hundred grams — can get by with relatively small adhesive feet, we’d need huge adhesive surfaces to hold our much larger mass. (Incidentally, Hawkes notes that this relationship between mass and surface area “probably [explains] why there are no large climbers in nature.”)
The mass-surface area relationship is the challenge the biomimetics lab has begun to solve. Their innovation is to use what they call a “degressive” spring.” Unlike a typical “progressive” spring, which gets stiffer the farther apart it’s pulled, a degressive spring actually gets softer, “like a piece of chewing gum,” when you stretch it, Hawkes says.
These degressive springs facilitate load sharing, spreading weight evenly over an entire adhesive surface. And when weight is spread this way, a smaller adhesive surface area becomes capable of holding more weight.
Hawkes engineered these degressive springs into adhesive surfaces about the size of a Ping-Pong paddle. The paddles are equipped with another technology, developed in the same lab five years earlier, which causes the paddles to grip as soon as pressure is applied and release as soon as pressure is relaxed — the equivalent of the gecko’s on/off switch. The paddles, which use a synthetic version of the gecko’s gripping fibers, are worn on the hands, and are attached to an apparatus that provides movable platforms for a climber’s feet: As a climber moves his hands up the wall, he moves the platforms up with him, allowing him to step up using his legs rather than having to pull himself up using his arms.
Each paddle is capable of supporting about 200 pounds. They could theoretically allow a person to scale the outside of a skyscraper, but so far Hawkes has only used them to climb 12 feet high. He describes that experience as fatiguing, and says that while this technology may prompt Spiderman-style dreams, its actual application may be with machines, not humans. One use is to affix the paddles to robots in factories, which could use them to pick up glass objects like windshields and television screens.
Another, more far-out application of the gecko’s powers takes place in space. Hawkes and his collaborators have conducted some interesting experiments with NASA. The space agency would like to figure out how to move dead satellites out of useful orbits, and grabbing things in space is difficult. Hawkes says that the paddles perform well in space-like conditions, and could be used to “create grippers that essentially would be on a garbage truck satellite in space.”