On 26 Apr 2003 07:10:09 -0700, y.k.y at lycos.com (yan king yin) wrote:
>I have another question which is related to making brain-computer
>interfaces. Suppose I make some nano-scale tubes of diameters on
>the order of 10-100nm (assuming we know how to do that). Inside
>these tubes would be an ionic solution. The tubes contain no
>ionic channels; basically they are closed. Will these tubes
>conduct electric signals efficiently? Given that ions in solution
>are different from electrons in metals, what would be the
>differences between these tubes and electrodes? In terms of
>conductance, and frequency-dependent impedence?
>>(Another assumption is that the surface of the tubes are
>electrical insulators -- but this is probably not true
>because carbon nanotubes, for one, are known to be conductive.)
I am not sure what you are really trying to accomplish here.
To build a brain-computer interface, it is sufficient to put
electrodes into the proper locations. The electrode are technically
interfaces between two conducting media -- the metallic (or
semiconductor) materials that connect to the computer and that
function with mobile electrons (or "holes"), and the saline solutions
of the brain that conduct with ion movements.
There are two types of electrode. In one, there is a redox reaction
that occurs at the surface of the electrode translating electron
movement into ionic movement. The tradional system for experimental
work is to use a silver wire electrode covered with insoluble silver
chloride. Then at the interface, Ag(s) gives up an electron to the
metallic side and binds a Cl- from the solution to form AgCl(s). On
the other electrode, the reverse reaction accepts an electron from the
metal to reduce AgCl to Ag(s) and release Cl- into the solution.
In the other kind of electrode, there is no redox reaction at the
electrode surface. Instead, there is an electrical bilayer,
essentially a capacitance, so that electrons arriving at the surface
in the metal accumulate a negative charge on the electrode surface,
driving away Cl- ions in the solution and attracting Na+ (assuming the
extracellular NaCl solution -- other ions are involved inside the
cells). Similarly, electrons moving in the other direction allow a
positive charge to accumulate on the electrode and drive the ions in
solution the other direction.
The first, reversible, electrode is suitable for recording and
stimulating all frequencies down to DC or steady currents and
potentials. In order to record a cell resting potential, you need a
system that works this way. Similarly, in order to voltage clamp a
membrane, you need this kind of electrode to be able to pass steady
currents for some substantial time period.
The second kind of electrode is suitable only for AC stimulation and
recording. It acts like a capacitor in series with the circuit. The
lower frequency limit depends on the details of electrode size and
construction. However, this is perfectly adequate for measuring
potential changes associated with action potentials and, in many
cases, even synaptic potentials. It is also quite suitable for
stimulating neurons with brief "biphasic" pulses -- that is, pulses
with no DC component which is the usual method of stimulation.
The type of electrode you need depends also on the stability of the
electrode over time. The electrodes with the reversible redox
reactions tend to be unstable over long periods of time and also to be
potentially toxic or damaging to nerve tissue. "Inert" materials
suitable for long-term implantation are inert specifically because
they dont react! Therefore virtually all long term electrodes work
the second way. I really am not sure just how semiconductor
electrodes work or their long-term stability and toxicity (I mean
growing tissue culture neurons directly onto semiconductor chips or
implanting naked silicon devices directly into the brain). Still the
mechanism of transduction with these interfaces must be one of the two
methods described above (or, more realistically, some combination of
the two mechanisms).
Carbon nanotubes would be no different. No matter what the conduction
method used inside the nanotube, the interaction between the
"electrode" and the brain will either be through a redox reactions or
a capacitative process at the interface.
To see whether any scheme will work, you replace the biological system
with an equivalent electrical circuit (a voltage source with a
characteristic source impedance). Your electrode also has an
equivalent electrical circuit (ordinarily a series resistance with
stray capacitance to ground). Finally, the electrode-solution
interface has an equivalent circuit (a series resistance and voltage
source for the reversible electrode, a series capacitance for the
non-reversible). Add the electrical circuit for your stimulating or
recording device and you can easily figure out just how well the
system will work.