Synthetic cells that act as a battery could one day be used to power nanotech devices. Scientists from Yale University and the National Institute of Standards and Technology (NIST) created a very simple cell model in order to study the way certain real cells generate electric voltages. In the process they produced a minute working battery that converts chemical energy into electrical energy at an efficiency of about 10 per cent - a figure that's high enough to make cell batteries a practical alternative as a nano power source.

Lead by NIST engineer David LaVan, the research team built synthetic cells consisting of a water-based salt solution enclosed within a wall made of a lipid (a molecule with one end that is attracted to water molecules while the other end repels them).

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When two of these "cells" come into contact, the water-repelling lipid ends that form their outsides touch, creating a stable double bilayer that separates the two cells' interiors, just as actual cell membranes do.

Next the researchers a modified form of a protein inserted into the bilayer. These embedded proteins create pores that act as channels for ions, mimicking the pores in a biological cell.

"This preferentially allows either positive or negative ions to pass through the bilayer and creates a voltage across it," LaVan says. "We can harness this voltage to generate electric current."

If the solutions in the two cells start with different salt concentrations, then poking thin metal electrodes into the droplets creates a small battery: electrons will flow through a circuit connected to the electrodes, counterbalancing the ion flow through the channels. As this happens, the ion concentrations in the droplets eventually equalize as the system discharges its electric potential.

The synthetic cells were developed to study the processes of far more complex living cells like those found in an electric eel, but researchers believe the system does have real world potential as a battery. A system of two droplets, each containing just 200 nanoliters of solution, could deliver electricity for almost 10 minutes while an 11 microliter system lasted more than four hours.

Per given volume, the researchers say the biological battery is only about one-twentieth as effective as a conventional lead-acid battery, but its 10 per cent efficiency compares well with solid-state devices that generate electricity from heat, light, or mechanical stress. Given that an advantage of solid state devices is their ability to be produced on a tiny scale, this could puts cell batteries in the frame for nanotechnology applications.