NEWS: Laser Probes for Brain Experiments
http://www.spectrum.ieee.org/may09/9235
19 May 2009—Understanding how the brain works typically involves
sticking sharp metal electrodes into an animal’s brain and zapping its
neurons with electricity. But researchers at Case Western Reserve University,
in Cleveland, are working on what could be a more benign, efficient, and
effective way to study brain circuits: using light.
The researchers have created a new kind of brain probe by coating the inside
of a tiny hollow glass needle with nanoparticles of lead selenide, a
semiconductor commonly used in infrared detectors. They insert the needle tips
into slices of rat brains and shine infrared light from an
830-nanometer-wavelength titanium-sapphire laser on the probes. The
nanoparticles absorb photons and generate an electric field that stimulates
neurons, whose signals are recorded using another electrode placed next to
them.
Metal electrodes activate only brain cells that are in a tight cluster
around the electrodes, which is not what happens when you naturally stimulate
your gray matter. “When you smell a rose, you’re activating many,
many brain cells, but they’re dispersed all over your olfactory
system,” says Ben Strowbridge, a Case Western neuroscience professor who
took part in the research. “Our technique can get much closer to
activating lots of different areas instead of activating many axons that are
close together,” he says.
Ordinary electrodes can damage tissue, and they need wires to connect to
power sources outside the brain. The light probes, on the other hand, could be
made with thin, flexible optical fibers, tiny polymer microcapsules, or
nanoparticle-coated flexible patches, says Clemens Burda, a Case Western
chemistry professor who collaborated with Strowbridge. Once the probes are
embedded in a certain part of the brain, you could wirelessly trigger neurons
by scanning a laser beam on that area. (The near-infrared light used in the
experiments is good at penetrating brain tissue.)
Beyond their use in neuroscience research, the photoelectrodes could also
play a role in medicine. “If one has a spinal cord lesion or a brain
defect, we could excite the remaining brain cells in a biologically realistic
way to restore function,” says Strowbridge.
Activating neurons with
light instead of electricity isn’t a new idea, but the Case Western
researchers are the first to use real brain tissue instead of neuron cultures,
says Spencer Smith, a neuroengineer at the University College London. Other
groups have grown neurons on top of thin silicon films connected to electrodes.
The setup uses photoconductivity: Shining light on a patch of the film
increases its conductivity, increasing current flow and triggering neurons on
that patch.
Michael Colicos, a physiology and biophysics professor at the University of
Calgary, in Canada, has worked on the photoconductive approach but sees promise
in the Case Western technology. “The advantage of the new technique is
that you don’t need the current pulse from the material,” he says.
“The fact that you can liberate yourself from having to be directly
electrically coupled with the electrode is a big step forward.”
Colicos finds the trick of coating the inside of the glass tip innovative,
since the nanoparticles don’t touch the brain cells directly, reducing
toxicity concerns. Strowbridge says his team did not see any evidence of damage
to neurons over a few hours of testing with the electrodes nestled right next
to the neurons. But they still need to do extensive toxicity studies.
There’s a ways to go before the researchers will be able to implant
and control the circuits in a living brain. For now, they plan to make a lead
selenide–coated glass surface and test brain slices placed on top.
“From that we can create flexible substrates that we can place right over
the surface of the brain,” Strowbridge says. “We’re certainly
not there yet, but that’s where the technology is headed.”
About the Author
Prachi Patel is a contributing editor at IEEE Spectrum.
In the April 2009 issue, she reported on engineers’ efforts to track volcanic lightning at Mt.
Redoubt in Alaska. Hear the story on Spectrum
radio too.
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