Engineers at Rice University, Houston, USA, have developed a battery 60,000 times smaller than a conventional AAA. At only 0.5 microns high, and an almost invisible 150 nanometres thick, the battery is formed by nanowires, one half being the cathode and one half being the anode. The battery is halfway between conventional chemical-reaction batteries and a "super-capacitor" that can hold charge and release it as demanded. To put the battery's size into perspective, the wavelength of deep blue light is 400 nanometres, so this battery is less than one half the wavelength of the deepest blue light humans can see. Larger batteries can be made by using a copper-coated nanowire template of tubes, and filling half of each tube with a nickel-tin alloy to create the anode. A layer of polyethylene-oxide gel is then used to insulate the alloy from the cathode formed from a polyaniline material. A layer of aluminium is deposited on top of this structure to complete the circuit. Theoretically, these batteries may be stacked to form any potential size of battery in the future.