A tiny spear made of carbon nanotubes can probe the internal electrical activity of a single neuron, giving researchers a more refined look at how brain cells respond to signals from their neighboring cells. Probing the brain at this resolution could be vital to efforts to understand and map its function in new detail (see “Why Obama’s Brain-Mapping Project Matters”).
The neuron “harpoons” are just 5 to 10 micrometers wide and can pierce a living cell to measure electrical changes associated with neuronal signaling. In dissected slices of still-active mouse brain tissue, researchers at Duke University were able to record from inside a single neuron at a time.
“To our knowledge, our paper shows the first intracellular recording with carbon nanotubes from vertebrate neurons,” says Bruce Donald, a biochemist and computer scientist at Duke University and author on the study, which was published in PLoS ONE on Wednesday.
Carbon nanotubes have many desirable properties for brain recordings, says Donald: they are strong, they are compatible with body tissues, and they conduct electricity well. But previous devices built from carbon nanotubes have been too short or wide to be well suited for recording inside cells. The probes built by the Duke researchers, however, were around one millimeter long and lent themselves to monitoring electrical activity more precisely than typical glass or metal electrode setups.
The team was able to detect small changes in electrical activity in the cell—changes corresponding to the input signals the neuron was receiving from other neurons. An average cortical neuron can receive signals from around 10,000 other neurons, says Richard Mooney, a neuroscientist at Duke University and an author on the study. “Individually, those generate very small signals,” he says. Together, the collection of signals is computed by the receiving neuron as it decides whether or not to fire.
Intracellular recordings could be useful for mapping the functional connections between neurons, a goal of the recently launched BRAIN initiative (see “The Brain Activity Map”). “By being able to look inside the cell and measure small voltage changes, you get access to the network that talks to that cell,” say Mooney.
The researchers used a “clever technique” to build their device, says Takashi Kozai, a neural engineer who was not involved in the study. Starting at the tip of a tungsten wire, they built up a long needle-like probe made of tangled carbon nanotubes. Then they coated the probe with an insulating material and used a focused beam of ions to bombard the tip, removing the insulation from that area and shaving it to a fine point.
“With this technique, you can make [probes] as long as you want,” says Kozai, who is also developing microscopic electrodes for recording neuron activity (see “A Carbon Microthread That Makes Contact with the Mind”). The work “sets the stage for making even narrower devices, maybe on the order of 100 nanometers instead of microns,” he says.
In addition to dissected brain slices, the team tested their thin electrode in anesthetized mice, although they were not able to obtain recordings from inside the brain cells of these animals. However, if future versions of the nanotube tip are even sharper, they may be able to better pierce cells in soft and spongy brains, says Kozai. If that’s possible, and if the device is stable in living brains over time, it could help researchers explore how the living brain learns and remembers.
“If they can stably record from the same cell longitudinally,” Kozai says, “it could be applied to map how neurons change during memory formation and learning.”