Elliot Hawkes clings to the sheer wall of glass, the ground looming vertiginously below. He reaches higher, touches the hand-held pad to the pane. There is no feeling of suction, no sudden grip – frighteningly, it feels like the pad isn’t going to stick. But Hawkes knows it will stick when he hangs his weight on it. That’s the hardest part, learning to trust it. He holds the pad to the glass, pulls down and – to his relief – a few postage stamps’ worth of silicone rubber hold him up. He reaches with his other gecko hand, eyes skyward …
Visitors to the reptile house are always amazed by the way geckos can cling to the sheer glass walls of their enclosure. Now, thanks to research by engineers at Stanford University in California, humans can do it too.
“The coolest thing about geckos is their feet, how they can adhere to almost anything,” says Hawkes, a graduate student working on the project.
In the last decade, scientists have been studying the millions of tiny hairs that coat the underside of each toe on a gecko’s foot. When the hairs are pressed to a surface, they stick to it due to the van der Waals force – a very weak electrical attraction that exists between all atoms and molecules. The dense coating of extremely tiny hairs maximises the contact area between the gecko’s toes and the wall.
Scientists at the University of Massachusetts had previously found a way to replicate the gecko hairs by forming a surface bristling with wedge-shaped micro-scale ridges made from silicone rubber. When the ridges are pressed on to a surface, they splay out and the van der Waals force kicks in. A new product based on the technology, Geckskin, is now in development.
Scaling up the gecko toe surface to enable human climbing was a huge challenge. In nature, any animal trying to use this adhesion effect has to be small, says Hawkes. “Nothing bigger than about 300 grams is able to climb with adhesion.”
The Stanford team studied tokay geckos and discovered they did not evenly distribute their weight across the whole toe area. Because the nano-scale hairs are somewhat unkempt, the longer ones always take more than their share of the load.
An uneven mat of toe hair is an advantage for geckos, because it gives a good grip on a wall even if its surface is not perfectly smooth. But to hold a human, the team realised they’d have to design their Geckskin pads so that the weight is distributed evenly across the whole device. The adhesive surface is broken up into 24 postage stamp sized tiles, each bearing 416 microscale ridges. Each tile is attached to a special spring, to ensure each tile presses against the glass even if the surface is imperfect.
In late 2014, Hawkes became the first human to scale a sheer glass wall using the gecko-inspired climbing system. The device could support him – but the pads only work on smooth, clean surfaces. For now, scaling a brick or concrete wall is out of the question.
US Defense Advanced Research Projects Agency (DARPA) has shown interest in the material, as they can see an advantage in endowing their soldiers with the spiderman-like ability to climb walls.
The team is also working with NASA to develop arms that can grab on to the solar panels of dead satellites and tug them to safe orbits, a kind of satellite graveyard.
They have already used the same adhesives to build tiny robots with shoes so sticky they can lift or drag objects up to 100 times their own weight. “My Mum wants one to move her potted plants around into the sun,” says Hawkes.
The Stanford team has made great progress, but the gecko is still way ahead of us. While Hawkes could take a few steps per minute, a gecko can take 15 in one second.
“Just watching these little guys run up a wall, especially after I tried, you realise how cool, how smooth, how impressive the whole system is,” says Hawkes.
Gecko glove specifications
Adhesive area: 140 cm2 per hand, spread across 24 postage stamp sized tiles
Load: Each tile (2.5 x 2.5 cm) can support 5 kg
Microstructure: 10,000 silicone ridges per glove, each 100 µm tall (the average diameter of a human hair)
Sticks to: Almost any flat surface, including glass, polished metal, varnished wood and teflon
Material: Silicone rubber
Foot platforms
The adhesiveness of the hand-held pads only kicks in when you slide them. To maximise this sliding action, and to ensure Hawkes never had to hold his whole weight with his arms, the team connected rigid bars running down from the hand-held gecko-pads to foot platforms. While Hawkes positioned the gecko pads with his hands, all his weight was borne by his feet. Once Hawkes lifted his foot, the stickiness switched ‘off’ and he could easily lift the pad from the glass to place it somewhere else.