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* 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.
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.
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."
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.
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.
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.
*J. Xu, F.J. Sigworth, and D.A. LaVan. Synthetic Protocells to Mimic and Test
Cell Function. Advanced Materials, published online Oct. 1, 2009 (DOI: 10.1002/adma.200901945).