Carbon nanotubes for neural engineering

Materials World magazine
,
2 Aug 2007

Scientists at the NASA Ames Research Center, based in Moffett Field, USA, have developed vertical free-standing carbon nanofibres for use in neural-electrical interfaces. These could improve implantable biomedical devices for managing Parkinson’s disease, epilepsy and depression.

Traditional deep brain stimulators involve a battery-powered neurostimulator, encased in a titanium housing, that sends electrical pulses to the brain. Such a device can require multiple surgeries and battery replacements, which increases the risk to the brain.

Carbon nanofibre stimulators are small enough to enable a multiplexed system, combining low current electrical stimulation with neurotransmitter recording, to be implanted in the brain in one step.

The research team has spent the past seven years working on vertically aligned carbon nanofibres (VACNFs).

‘The original idea was to develop a multiplex close-loop electrode so the doctor could tune stimulation through feedback of electrical and electrochemical recordings,’ explains Jun Li, former Senior Scientist at the Ames centre. ‘But we found that the [open] 3D structure forms a better interface with neural tissues, since it is similar to the natural extracellular environment of a tissue.’

Carbon nanostructures usually have a bundled configuration in which fibres are easily entangled and have less freedom to move. This stiff stucture is less mechanically compatible with tissue. The brush-like design of the new substrate should improve electrode-tissue contact.

The VACNF substrate was grown by plasma enhanced chemical vapour deposition using a nickel catalyst on a 200nm thick chromium film on a silicon wafer. The electric field helped to align the nanofibres, generating the free-standing structure.

The scientists worked with nanofibres between 50-150nm in diameter and 5-10 microns in length. The fibres needed to be rigid enough so that they would not collapse into microbundles, yet soft enough to reduce mechanical stress to the neural cells. They were coated with polypyrrole, which improves the neural electrical interface by lowering impedance and increasing biocompatibility and surface stiffness.

Researchers have already successfully cultured PC12 neuron cells on the VACNF substrate. The team is currently looking for partners in biomedicine to help test 3D VACNF electrodes and implants.

‘The most difficult part is the [need for] diverse expertise, from the physical end (materials, fabrication, electrochemistry and electronics), to the biomedical (bioengineering, neurophysiology, animal experiments, neural surgery and clinical research),’ says Li. ‘We have to comply with ethical and safety requirements that are totally new to us.’

Li envisions several applications of this technology. ‘It can be used as the neural electrical interface in all neuroprosthetic devices, such as retinal stimulation, cortical recording and deep brain stimulation. It may also be used for electric stimulation of muscles.’