The "iRings" wheel (as in "iron rings," the symbol of Canadian engineering--this is not iPod-compatible) uses a chain-mail-type fabric encasing granular particulate material, which gives it some pretty fantastic properties. Engineers seem to be embracing the philosophy of the beanbag more lately, as seen in the coffee-balloon-handed robot--it allows strength and flexibility of shape at the same time. In the case of the iRings wheels, that translates to wheels that mold to the shape of the terrain, rather than bumping along above it, as traditional wheels would.