Posted: October 22, 2009 |
Synthetic cells act as battery |
(Nanowerk News) Trying to understand the complex workings of a biological cell by teasing out the function of every molecule within it is a daunting task. But by making synthetic cells that include just a few chemical processes, researchers can study cellular machinery one manageable piece at a time. A new paper ("Synthetic Protocells to Mimic and Test Cell Function") from researchers at Yale University and the National Institute of Standards and Technology (NIST) describes a highly simplified model cell that not only sheds light on the way certain real cells generate electric voltages, but also acts as a tiny battery that could offer a practical alternative to conventional solid-state energy-generating devices.
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Image of two artificial cells that can act as a tiny battery. Each cell has a droplet of a water-based solution containing a salt -- potassium and chloride ions -- enclosed within a lipid wall. If the solutions in the two cells start with different salt concentrations, then poking thin metal electrodes into the droplets creates a small electric battery. (Image: NIST)
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Each synthetic cell built by NIST engineer David LaVan and his colleagues has a droplet of a water-based solution containing a salt—potassium and chloride ions—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. 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.
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If the researchers only did that much, nothing interesting would happen, but they also inserted into the bilayer a modified form of a protein, alpha-hemolysin, made by the bacterium Staphylococcus aureus. 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."
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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.
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Building synthetic versions of complex real cells—such as those that enable an electric eel to zap its prey—is far too difficult a task for now, says LaVan. So the researchers instead created this far simpler system whose performance they could understand in terms a handful of basic properties, including the size of the droplets, the concentration of the aqueous solutions, and the number of ion channels in the barrier between the two cells.
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A tiny battery with two droplets, each containing just 200 nanoliters of solution, could deliver electricity for almost 10 minutes. A bigger system, with a total volume of almost 11 microliters, lasted more than four hours. In terms of the energy it can deliver for a given volume, the biological battery is only about one-twentieth as effective as a conventional lead-acid battery. But in its ability to convert chemical into electrical energy, the synthetic cell has an efficiency of about 10 per cent, which compares well with solid-state devices that generate electricity from heat, light, or mechanical stress—so that synthetic cells may one day take their place in the nanotechnology toolbox.
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