Coil

In progress.


In the end, our goal is to stimulate the brain. This means we need to be able to generate electric fields of the appropriate intensity, at the appropriate depth, that last the appropriate duration. See Rheobase and chronaxie for these values. Coil design for the central nervous system generally centers around the desire to have a very focused area of large electric field, with minimum heating of the coil, or mechanical stress in the coil.

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What principles govern the design of a focal electric field?

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Watch this space!

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Most studies agree that figure-8 coils, in which a single wire is wound repeatedly in a figure-8 fashion, give the most focused induced electrical fields in a volume conductor for a simple design, as shown in the following figures:


Left, Hsu, K.-H. and Durand, D. M. (2001) IEEE Trans Biomed Eng 48(10):1162-1168; Right, Maccabee, P. J., et al. (1993) J Physiol 460:201-209.

The core coil that we are focusing on for the beginning is a figure-8 coil, accordingly. While not the theoretically most focal coil, it is very easy to make and is much more focal than, say, a single loop of wire (which stimulates all around its periphery, a very large area). Nevertheless, different coil designs can be useful for driving different brain structures — which may be at different depths, or of different sizes.

Preferred coil diameters for each part of the figure-8 coil are 2.5 cm. Less than 2 cm becomes a very hard circuit to drive with a large current to get the requisite field, due to the very small inductance. We should explore this parameter space. Most models assume that the coil inductance is at least 25 microHenries, and then optimize from there.

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One idea for making a more focal stimulator is to put a high magnetic-permeability core, e.g. iron, in the empty spaces in the coil. At least one paper suggests that the field increase with a proper-geometry iron core can be double the value without the core:

Epstein, C. M. and Davey, K. R. (2002) J Clin Neurophys 19(4):376-81.
Accordingly we can machine, or order, steel cores that fit the coils appropriately designed.

Note that this design may be incompatible with a wearable design. Even a small iron-core stimulator could be very heavy.

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One idea has been to print, etch, or otherwise create coils, so that novel geometries can be created. In addition, it is possible that the coils could be put onto flexible circuitry, so that they could be wrapped conformally around a subject's head. For example, many coils can be placed in an array:

Left, a "center-surround coil" that could in principle allow more focal stimulation, or reduce scalp pain. Middle, a more complex coil arrangement that would allow stimulation of two sites, while reducing the fields farther away. Right, an array of coils that can be individually driven, allowing stimulation of many sites. From Boyden, E. S. (1999), "Quantum Computing: Theory and Implementation," M. Eng. thesis, MIT.

Note that, in reference to the above information on coil dimensions — each of these novel coil proposals is rather inefficient, in terms of needing a lot of current to generate the necessary field. At the very least, we would need to make coils of much larger caliber (which may not be practical for being printed), to reduce heating and resistive losses. Also, we must worry about coupling across coils, which would load the system and also possibly induce severe heating.

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OTHER THOUGHT 1 (from Foo Camp): Write software to model the electric fields induced by a particular coil. What is the simplest piece of code to do this?

OTHER THOUGHT 2 (from Foo Camp): Coils can be noisy, because the current causes the wires to contract and mechanically deform. Shall we try to embed them in a damping material? Use active noise cancellation methods? Place a near-vacuum around the coil to reduce vibration transmission to the outside?


Not yet resolved. Can wind by hand around a milled piece with iron core.


None yet.


For wound coils, 10-gauge beryllium copper wire is best. This seems to be a best-practices consensus. Round wires do not necessarily space-fill the best in a coil — you can eke a little bit more out if you use square wire. Where can you get square wire? Here is a list of places.

Printed coils are the way to go in the future, but it's not clear on the way to print a coil of such low resistance.


None yet.


None yet.

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