Magnetic Stimulation Using Microcoils: A Skeptical Reivew

Magnetic Stimulation Using Microcoils: A Skeptical Review.

Over the last decade I’ve written a lot in this blog about magnetic stimulation of nerves using microcoils, which are tiny current-carrying coils generally from one millimeter down to tens of microns in size (see here, here, here, and here). My latest (and probably last) contribution to this topic was published recently in the BOHR International Journal of Neurology and Neuroscience, titled “Magnetic Stimulation Using Microcoils: A Skeptical Review.” The abstract states

Recently several research groups have used sub-millimeter sized microcoils to perform magnetic stimulation of nerves. This review assesses the magnitude of the electric field induced by these microcoils. In some cases magnetic stimulation is a plausible mechanism for neural excitation, but in other cases the induced electric field is far too small to excite a neuron. These results indicate that microcoil magnetic stimulation may not occur via magnetic stimulation, but by some other mechanism. One alternative mechanism is capacitive coupling.

Basically, this paper reviews the entire field of microcoil stimulation, and finds that in some cases the idea is plausible, but in other cases it is not. One thing I like about the article is that I develop a toy model for calculating the electric field produced by the coil. While the model is an approximation, it should determine the electric field correct to at least an order of magnitude. The strengths of the model are that it provides great insight, it is so simple computationally that anyone can reproduce the calculation, and it supplies a common way to analyze a host of different publications. One goal of Intermediate Physics for Medicine and Biology is to train students in forming and analyzing such toy models. I think that the field of microcoil stimulation illustrates what happens when researchers skip the simple model and go straight to complicated numerical calculations using code that is treated as a black box. These computations often produce numbers but little intuition or understanding. The user cannot tell if something goes wrong. Back when I was teaching, I would urge my students to “think before you calculate!” This review shows why.

The conclusion of the review states

Magnetic stimulation by microcoils is an active and growing field in neural stimulation. It may have promise for the development of neural prostheses. This review, however, suggests there are many unanswered questions in this area of research… Frankly, the field appears prone to errors. One must analyze the articles carefully to separate the wheat from the chaff.

Transcranial magnetic stimulation works by magnetic induction. Stimulation using a millimeter-sized, multi-turn coil placed close to the target neuron and carrying several amps of current may work by magnetic induction, but the electric field it generates is sometimes slightly below the threshold value you expect is required to excite neurons. Nevertheless, magnetic induction in these cases seems plausible. On the other hand, when milliamps of current are passed through a single-turn wire, magnetic induction does not seem to be a plausible mechanism for excitation; the electric field appears to be too weak. One alternative mechanism is capacitive coupling. An important goal of future microcoil magnetic stimulation research is to resolve what exactly is the underlying mechanism.

The article is open access, so anyone can read it online without a subscription. Enjoy!

Note added just before this post was published

After my review article was published earlier this week, I discovered a 2025 PhD dissertation that came to a similar conclusion as my review did. Giovanni Calixte working with his graduate advisor Robert Butera at the Georgia Tech published “An Investigation into the Efficacy and Utility of Micromagnetic Stimulation as a Means of Exciting Neural Tissue.” It’s good work, and you can download it here. I reproduce the summary below. I’ve put the critical sentence near the end of the summary in italics.

Micro-magnetic stimulation (μMS) through micro-scale coils is a rapidly advancing form of neuromodulation that possesses qualities that give it the potential to be used as an alternative to electrical stimulation in cases where it cannot be used. However, reliability of the technique is inconsistent as reported in the peer reviewed literature, suggesting that there is not a strong understanding of the basis of the technique. In this thesis, we provide empirical evidence of the efficacy of μMS in dissociated cortical cultures and a review of μMS’ consistency across early pioneering works. We then simulate the electric fields generated by μMS using COMSOL and couple the fields to neuron models in Python to study their response across a range of parameters in neural stimulation. This is followed by an aim to measure the electric fields of these micro-scale coils to determine whether they surpass stimulus thresholds reported in literature. Our key findings suggest that μMS should not be feasible. Finally, we investigate confounding factors during μMS stimulation that may have effects being mis-attributed to μMS.

I’m delighted that it’s not just my former graduate student Mohammed Alzahrani and me who are skeptical of microcoil magnetic stimulation. We are no longer alone!