Arrays of microfabricated metal electrodes can be used to monitor tissue slices, or can even be implanted into the brain. But the electrodes in these arrays are the same size as, or bigger than, single neurons. Lieber says the signals picked up by such electrodes are small because they represent average electrical activity over an entire cell or multiple cells.
Using Lieber’s system, “you can sense inputs to a neuron on dendrites, and sense how it responds at the axon,” says Todd Pappas, director of sensory and molecular neuroengineering at the University of Texas Medical Branch. “This is a great leap forward.”
Florida’s Ditto says the nanowires will be an important tool for studying neural circuits – the networks of communicating neurons – in great detail. And understanding neural circuits could provide insight into learning and memory, as well as advancing computer science.
Lieber hopes the nanowires will find applications in medical devices. They might be used to “build an interface to the brain that’s much more sophisticated” than current ones, which rely on large electrodes, he says. Such devices might help control epilepsy or pain, or, like cochlear implants for hearing, substitute for damaged sensory nerves.
Lieber’s group is also developing the nanowires to make even more sophisticated connections with neurons. The area where two neurons meet, a synapse, is characterized not just by electrical signaling but also by chemical signaling. In fact, the transfer of chemicals known as neurotransmitters at synapses is what allows the transmission of electrical signals from one neuron to another. Lieber has already demonstrated that his nanowires can act as sensitive chemical sensors (see “Drugstore Cancer Tests”). His group is now working to make nanowires that can detect – and someday possibly release – neurotransmitters.
Zhong Lin Wang, director of the Center for Nanostructure Characterization at Georgia Tech, says Lieber’s arrays of 50 nanowires are a remarkable achievement. “From a nanowire point of view, an array of 50 is a landmark,” he says. Indeed, to attain this level, Lieber had to fine-tune his manufacturing process to end up with 90 percent functioning nanowires in each batch.
Wang views Lieber’s nanowire work as a validation of nanotechnology’s growing relevance. “It shows that work in the nanotechnology field can become revolutionary and important” in other fields, such as biology, he says.