Brain probes from nanotubes

Purdue University researchers have shown that nanotubes might be used to create brain probes and implants to study and treat neurological damage and disorders.

Probes made of silicon are currently used to study brain function and disease but may one day be used to apply electrical signals that restore damaged areas of the brain. A major drawback to these probes, however, is that they cause the body to produce scar tissue that eventually accumulates and prevents the devices from making good electrical contact with brain cells called neurons, said Thomas Webster, an assistant professor of biomedical engineering.

New findings showed that the nanotubes not only caused less scar tissue but also stimulated neurons to grow 60 percent more fingerlike extensions, called neurites, which are needed to regenerate brain activity in damaged regions, Webster said.

The nanotubes were specially designed so that their surfaces contained tiny bumps measured in nanometres. Conventional silicon probes do not contain the nanometer-scale surface features, causing the body to regard them as foreign invaders and surround them with scar tissue. Because the nanometer-scale features mimic those found on the surfaces of natural brain proteins and tissues, the nanotubes induce the formation of less scar tissue.

The scar tissue is produced by cells called astrocytes, which attach to the probes. The Purdue researchers discovered that about half as many astrocytes attach to the nanofibres compared to nanotubes that don’t have the small features.

‘These astrocytes can’t make scar tissue unless they can adhere to the probe,’ Webster said. ‘Fewer astrocytes adhering to the nanotubes means less scar tissue will be produced.’

The Purdue researchers pressed numerous nanofibres together to form discs and placed them in petri plates. Then the petri plates were filled with a liquid suspension of astrocytes. After one hour the nanotube disks were washed and a microscope was used to count how many of the dyed astrocytes washed out of the suspension, which enabled the researchers to calculate how many astrocytes stuck to the nanotubes.

About 400 astrocytes per square centimetre adhered to the nanotubes containing the small surface features, compared to about 800 for nanotubes not containing the small surface features. The researchers repeated the experiment while leaving the nanotubes in the cell suspension for two weeks, yielding similar results.

When the nanotubes were placed in a suspension with neurons, the brain cells sprouted about five neurites, compared with the usual three neurites formed in suspensions with nanotubes that didn’t have the small surface features.

Researchers plan to make brain probes and implants out of a mixture of plastics and nanotubes. The findings demonstrated that progressively fewer astrocytes attached to this mixture as the concentration of nanotubes was increased and the concentration of plastics was decreased.

‘That means if you increase the percentage of carbon nanofibres you can decrease the amount of scar tissue that might form around these electrodes,’ Webster said.

The nanometer-scale bumps mimic features found on the surface of a brain protein called laminin.

‘Neurons recognise parts of that protein and latch onto it,’ Webster said.

The crucifix-shaped protein then helps neurons sprout neurites, while suppressing the formation of scar tissue.

The tube-shaped molecules of carbon have unusual properties that make them especially promising for these and other applications. Researchers theorise that electrons might flow more efficiently over extremely thin nanotubes than they do over conventional circuits, possibly enabling scientists to create better brain probes as well as non-silicon-based transistors and more powerful, compact computers.