Nanotechnology listening device for neuronal talks
(Nanowerk Spotlight) Carbon nanotubes, like the nervous cells of our brain, are excellent electrical signal conductors and can form intimate mechanical contacts with cellular membranes, thereby establishing a functional link to neuronal structures. There is a growing body of research on using nanomaterials in neural engineering ("Nanotechnology to repair the brain"). Most studies simply grow carbon nanotubes (CNTs) over microelectrodes to interface with neurons extracellularly (outside the neural membrane). Such an extracellular interface is non-invasive, but it only allows the action potential of neurons to be recorded. In contrast, an intracellular interface allows all of the sophisticated neural activity to be probed, but it is an invasive approach that usually destroys the neuron.
Now, new research by scientists in Taiwan is the first to explore the feasibility of using CNTs to probe neural activity intracellularly (inside the neural membrane), opening the way for intracellular neural probes that minimize damage to the neuron.
Scientists experimenting with CNT-based neural probes are still challenged by fully understanding the mechanisms underlying the electrical coupling at the CNT-electrolyte interface. Most studies indicated that CNT electrodes were like metal electrodes, relying on capacitive coupling to record and to stimulate neural activity.
The new study by the Taiwanese team brings two important findings for future nanotechnology work with CNT-based neural probes. Firstly, the electrical conduction at the CNT-electrolyte interface involves not only capacitive coupling but also resistive conduction to a comparable extent. Secondly, both the resistive and capacitive conductivity improves (increases) towards favoring neural recording after the CNTs conduct direct currents for a long time.
Single-walled carbon nanotubes grown on an AFM tip. CNT length: 2µm; CNT diameter: <10nm. (Image: Prof. Tri-Rung Yew, NTHU)
"An intracellular interface not only improves the signal-to-noise ratio by several tens of times but also makes it possible to record action potentials and postsynaptic potentials with better selectivity," Hsin Chen explains to Nanowerk. "By placing CNT bundles in a glass pipette and using the pipette tip to penetrate the neuronal membrane, we demonstrated that the CNTs are capable of recording and stimulating neurons intracellularly, with a performance comparable to the conventional silver/silver chloride electrodes used in physiological experiments."
Chen, an assistant professor in the Department of Electrical Engineering at National Tsing Hua University (NTHU), also points out that, interestingly, the recording capability of the CNTs was found to improve – instead of degrading – after delivering direct-current stimuli for a long period of time. "The long-term endurance of CNTs makes CNT probes particularly suitable for long-term usage, superior to the silver/silver chloride (Ag/AgCl) electrodes which normally wear out after the silver chloride is reduced into silver."
Chen's team, which included scientists from NTHU's Institute of Molecular Medicine, Institute of Electronics Engineering, Department of Materials Science and Engineering, and Institute of Nano Engineering and Micro System, as well as the Microsystem Technology Center, ITRI, further investigated the mechanisms supporting the intriguing properties of CNTs with impedance measurement, cyclic voltammetry, and high-resolution imaging.
"We found that the CNT-electrolyte interface has non-negligible resistive conductivity, which allowed the CNTs to record the equilibrium potentials of neurons intracellularly, or to deliver direct-current stimuli" says Chen. "We were able to determine that the resistive conduction relied mainly on the abundant functional groups on the CNTs' surface. More interestingly, we found that the impedance of the CNT-electrolyte interface improved with the delivery of current stimuli, apparently because it induces hydrolysis reactions and polishing effects at the CNT surface."
For their study, Chen's team made two types of CNT probes – in both cases bundles of multi-walled CNTs connected to a silver wire; one coated with insulating epoxy, one inserted into a sharp glass pipette – to measure neural activity not only extracellularly but also intracellularly. But unlike other CNT-coated microelectrodes, these probes had only carbon nanotubes at the probe tips involved in interfacing neurons, facilitating the characterization of the CNT-electrolyte interface.
The researchers compared the performance of their CNT probes with that of conventional Ag/AgCl electrodes with the well-characterized escape neural circuit of the crayfish, Procambarus clarkia. These tests showed that the CNT probes have comparable performance to the currently used Ag/AgCl electrodes, as well as a superior long-term endurance, making them a new tool for general neural physiological experiments as well as therapeutic devices such as brain-machine interfaces.
Having demonstrated CNTs' capability to interface with neurons intracellularly, the main goal of the Taiwanese team now is to develop fully functional nanoscale neural probes based on the CNTs.
"Our main challenge is that the interface impedance will increase significantly if the probe consists of only one or a few CNTs" says Chen. "Therefore, techniques for growing CNTs longer than 10µm are important to us. In addition, as the nanoscale probe penetrates the neuronal membrane, one segment of the CNTs will always remain outside the membrane. This segment needs to be insulated from extracellular fluids to ensure that the CNTs record only the potentials of the intracellular fluid. Therefore, we also need to develop a technique for coating an insulating nanoscale layer over CNT probes."