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Device might allow “spider-man” walk

Feb. 2, 2010
Courtesy Cornell University
and World Science staff

Could hu­mans one day walk on walls, like Spider-Man? A palm-sized de­vice in­vented at Cor­nell Uni­vers­ity in New York just might make it pos­si­ble, its de­vel­op­ers claim.

The mech­an­ism could lead to such ap­plica­t­ions as shoes or gloves that stick and un­stick to walls, or Post-it-like notes that can bear loads, ac­cord­ing to Cor­nell chem­i­cal en­gi­neer Paul Steen, who in­vented the de­vice with Mi­chael Vo­gel, a form­er post­doc­tor­al as­so­ci­ate.

The de­vice is the re­sult of in­spira­t­ion drawn from a bee­tle na­tive to Flor­i­da, which can ad­here to a leaf with a force 100 times its own weight, yet al­so in­stantly un­stick it­self. Re­search be­hind the de­vice is pub­lished on­line Feb. 1 in the jour­nal Pro­ceed­ings of the Na­tional Acad­e­my of Sci­ences.

The in­ven­tion ex­ploits the stick­i­ness of sim­ple wa­ter. Wa­ter mo­le­cules are at­tracted to each oth­er and al­so to those of many oth­er ma­te­ri­als; this is why, for ex­am­ple, two wet glass slides stick to­geth­er.

“In our eve­ry­day ex­pe­ri­ence, these forc­es are rel­a­tively weak,” Steen said. “But if you make a lot of them and can con­trol them, like the bee­tle does, you can get strong ad­he­sion forc­es.”

The de­vice con­sists of a flat plate pat­terned with tiny or micro­scop­ic holes. A bot­tom plate holds a liq­uid res­er­voir, and in the mid­dle is anoth­er po­rous lay­er. An elec­tric field ap­plied by a com­mon nine-volt bat­tery pumps wa­ter through the de­vice and causes droplets to squeeze through the top lay­er. The ex­posed droplets make the de­vice grip anoth­er sur­face.

One of the re­search­ers’ pro­to­types was made with about 1,000 holes, each about three-tenths of a mil­li­meter wide. They found it could hold about 30 grams – the weight of more than 70 pa­per clips. They found that as they scaled down the holes and packed more of them on­to the de­vice, the ad­he­sion got stronger. They es­ti­mate that a de­vice the size of a larg­ish post­age stamp, with mil­lions of holes each a thous­andth of a mil­li­me­ter wide, could hold more than 15 pounds (about 7 kilos).

To turn the ad­he­sion off, the elec­tric field is simply re­versed, and the wa­ter is pulled back through the pores, break­ing the ti­ny “bridges” cre­at­ed be­tween the de­vice and the oth­er sur­face by the in­di­vid­ual droplets.

One of the big­gest chal­lenges in mak­ing these de­vices work, Steen said, was keep­ing the droplets from co­a­lesc­ing, as wa­ter droplets tend to do when they get close to­geth­er. To solve this, they de­signed their pump to re­sist wa­ter flow while it’s turned off.

Steen en­vi­sions fu­ture pro­to­types on a grander scale, once the pump mech­an­ism is per­fected, and the ad­he­sive bond can be made even stronger. He al­so ima­gi­nes cov­er­ing the droplets with thin mem­branes – thin enough to be con­trolled by the pump but thick enough to elim­i­nate wet­ting. The en­cap­su­lat­ed liq­uid could ex­ert sim­ul­ta­ne­ous forc­es, like ti­ny punches. “You can think about mak­ing a cred­it card-sized de­vice that you can put in a rock fis­sure or a door, and break it open with very lit­tle volt­age,” Steen said. “It’s a fun thing to think about.”


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Could humans one day walk on walls, like Spider-Man? A palm-sized device invented at Cornell University in New York just might make it possible, its developers claim. The rapid adhesion mechanism could lead to such applications as shoes or gloves that stick and unstick to walls, or Post-it-like notes that can bear loads, according to Cornell chemical engineer Paul Steen, who invented the device with Michael Vogel, a former postdoctoral associate. The device is the result of inspiration drawn from a beetle native to Florida, which can adhere to a leaf with a force 100 times its own weight, yet also instantly unstick itself. Research behind the device is published online Feb. 1 in the journal Proceedings of the National Academy of Sciences. The invention exploits the stickiness of simple water. Water molecules are attracted to each other and also to those of many other materials; this is why, for example, two wet glass slides stick together. “In our everyday experience, these forces are relatively weak,” Steen said. “But if you make a lot of them and can control them, like the beetle does, you can get strong adhesion forces.” The device consists of a flat plate patterned with holes, each a few thousandths or hundredths of a millimeter wide. A bottom plate holds a liquid reservoir, and in the middle is another porous layer. An electric field applied by a common nine-volt battery pumps water through the device and causes droplets to squeeze through the top layer. The exposed droplets make the device grip another surface. For example, one of the researchers’ prototypes was made with about 1,000 300-micron-sized holes, and it can hold about 30 grams – more than 70 paper clips. They found that as they scaled down the holes and packed more of them onto the device, the adhesion got stronger. They estimate, then, that a one-square-inch device with millions of 1-micron-sized holes could hold more than 15 pounds. To turn the adhesion off, the electric field is simply reversed, and the water is pulled back through the pores, breaking the tiny “bridges” created between the device and the other surface by the individual droplets. The research builds on previously published work that demonstrated the efficacy of what’s called electro-osmotic pumping between surface tension-held interfaces, first by using just two larger water droplets. One of the biggest challenges in making these devices work, Steen said, was keeping the droplets from coalescing, as water droplets tend to do when they get close together. To solve this, they designed their pump to resist water flow while it’s turned off. Steen envisions future prototypes on a grander scale, once the pump mechanism is perfected, and the adhesive bond can be made even stronger. He also imagines covering the droplets with thin membranes – thin enough to be controlled by the pump but thick enough to eliminate wetting. The encapsulated liquid could exert simultaneous forces, like tiny punches. “You can think about making a credit card-sized device that you can put in a rock fissure or a door, and break it open with very little voltage,” Steen said. “It’s a fun thing to think about.”