A microchip that uses chemicals instead of pulses of electricity to
stimulate neurons has been created. It could open the way to implants
that interact with our nervous system in a far more subtle way than is
electrical pulses convey impulses along neurons, the cells communicate
with each other and with other cells such as muscles by releasing
chemical messengers. These neurotransmitters are released from one side
of a cell junction, or synapse, and picked up by receptors on the other
side, triggering another electrical pulse.
Since synapses are
typically around 50 nanometres across, and each chemical puff contains
just a few thousand molecules, building an artificial synapse is a huge
challenge. But Mark Peterman and Harvey Fishman at Stanford University
in California are getting close. They told a biophysics conference in
Texas earlier in March that they have created four "artificial synapses"
on a silicon chip one centimetre square.
To cells on the surface
of the device, the artificial synapse is simply a hole in the silicon.
But each hole opens into a pipeline etched into a plastic layer on the
back of the chip, connected at both ends to a reservoir of
neurotransmitter. When an electric field is applied, the
neurotransmitter is pumped through the pipeline, and a little of it
squeezes out of the hole, stimulating nearby cells (see graphic).
At 5000 nanometres wide,
these artificial synapses are closer in size to a whole cell than a real
synapse, but even so, the pair have fine-tuned the device so that it can
stimulate just one cell in the layer above the chip.
The ultimate ambition is to develop neural prosthetics - implanted
devices that can interact with our nervous system. Devices that use
electrical stimulation are already commonplace, such as the cochlear
implants that partially restore hearing.
But electric pulses
stimulate nerve cells indiscriminately. Different neurotransmitters, in
contrast, have different effects on a given cell. What is more, a single
neurotransmitter can affect one cell in one way and a different cell in
another. The neurotransmitter used by retinal cells, for example, turns
some cells on and others off.
It means that devices
that use neurotransmitters could interact with cells in more subtle and
precise ways. Biomedical engineer Gerald Loeb from the University of
Southern California speculates that powerful devices could be produced
by combining chemical and electrical stimulation in one implant.
But there are still
formidable obstacles. How densely can you pack the synapses in when each
needs its own pipeline? How do you stop the pipelines from clogging with
immune cells when the device is implanted? And how often would you need
to refill the neurotransmitter reservoir?
To answer this last
question, Peterman estimates that a thousand artificial synapses firing
a thousand times a second would need as little as half a millilitre of
fluid to function for 250 years. The other problems may prove tougher.
"But it's early days," Loeb points out.
In the meantime, the most
immediate application of the technique could be in tissue research.
Drugs could be delivered to individual cells in a tissue sample to see
how this affects the entire system.