False colored scanning electron microscope image of neurons cultured on top of the electrode array. Actual recording experiments are performed with much higher neuron densities containing three to six cell layers covering the entire electrode array (Image courtesy of Harvard SEAS)

How our brain cells, or neurons, use electrical signals to communicate and coordinate for higher brain function is one of the biggest questions in all of science.

For decades, researchers have used electrodes to listen in on and record these signals. The patch clamp electrode, an electrode in a thin glass tube, revolutionized neurobiology in the 1970’s with its ability to penetrate a neuron and to record quiet but telltale synaptic signals from inside the cell. But this tool lacks the ability to record a neuronal network; it can measure only about 10 cells in parallel.

Now, researchers from Harvard University have developed an electronic chip that can perform high-sensitivity intracellular recording from thousands of connected neurons simultaneously. This breakthrough allowed them to map synaptic connectivity at an unprecedented level, identifying hundreds of synaptic connections.

“Our combination of the sensitivity and parallelism can benefit fundamental and applied neurobiology alike, including functional connectome construction and high-throughput electrophysiological screening,” said Hongkun Park, Mark Hyman Jr. Professor of Chemistry and Professor of Physics, and co-senior author of the paper.

“The mapping of the biological synaptic network enabled by this long sought-after parallelization of intracellular recording also can provide a new strategy for machine intelligence to build next-generation artificial neural network and neuromorphic processors,” said Donhee Ham, Gordon McKay Professor of Applied Physics and Electrical Engineering at the John A. Paulson School of Engineering and Applied Sciences (SEAS), and co-senior author of the paper.

The research is described in Nature Biomedical Engineering.

The electronic chip uses the same fabrication technology as computer microprocessors.(Image courtesy of Harvard SEAS)

The researchers developed the electronic chip using the same fabrication technology as computer microprocessors. The chip features a dense array of vertically-standing nanometer-scale electrodes on its surface, which are operated by the underlying high-precision integrated circuit. Coated with platinum powder, each nanoelectrode has a rough surface texture, which improves its ability to pass signals.

Neurons are cultured directly on the chip. The integrated circuit sends a current to each coupled neuron through the nanoelectrode to open tiny holes in its membrane, creating an intracellular access. Simultaneously, the same integrated circuit also amplifies the voltage signals from the neuron picked up by the nanoelectrode through the holes.

“In this way we combined the high sensitivity of intracellular recording and the parallelism of the modern electronic chip,” said Jeffrey Abbott, a postdoctoral fellow in the Department of Chemistry and Chemical Biology and SEAS, and the first author of the paper.