Recently, University of Wisconsin engineer, Tom Krupenkin, has designed a shoe capable of generating electricity. In a process called reverse electrowetting, two plastic bladders are filled with an oil water mixture.

The bladder under the heel of the shoe is connected to the one under the toe by a thin tube, so that when you step down on the heel, the rear bladder is compressed and the liquid travels through the tube to the front bladder. When you step on the toe, the liquid travels back.

The liquid moves over electrodes, converting electrical energy to mechanical energy, and can create enough electricity to charge a smartphone. In fact, a small battery would store the energy, and can be accessed by a micro-USB port on the heel of the shoe.

Generally when we walk, we waste nearly 20 watts per second, turning most of the calories into heat that quickly dissipates. A small prototype showed that each step taken could move 1,000 droplets of liquid, generating up to 10 watts of power.

A new real-size prototype is currently in the works, as designers push to come up with a marketable product that people would actually want to wear. If successful, it could extend the life of phone batteries by a factor of ten.

Its not the first energy-saving shoe attempted, though. In 1998, MIT researchers designed Nikes capable of broadcasting a stride-powered radio signal. Also, inventor Trevor Baylis designed a boot-heel insert to charge his cell phone during a long trek through a South African desert.

This newest shoe design, however, is said to be much more efficient. Previous attempts used a technology called piezoelectrics, tiny crystals that create a current as theyre compressed and expanded. In reality they didnt produce nearly enough power, generating only a few thousandths of a watt. Reverse elctrowetting is the next evolution, and many think that it will prove successful in the near future.