The following captured my attention in the announcement of the 11th World Congress of the International Neuromodulation Society, “Technology Transforming Chronic Illness Management.” From June 8 – 13, 2013:
“Micro-Magnetic Stimulation (Monday, June 10) – John T. Gale, Ph.D., has demonstrated for the first time that deep brain stimulation with micro-magnets can activate brain cells in a living organism. Dr. Gale’s research team has shown that placing a micro-magnet on the auditory pathway of hamsters triggers nerve signal transmission. Stimulation from uniquely designed magnetic fields could avoid unintentional activation of nearby brain areas and the associated side effects. Micro-magnets might one day provide stimulation for heart pacing, cochlear implants, Parkinson’s disease, or neural prosthetics.”
I have worked on TMS before, even home-brewed a TMS device (the design of which is detailed in my book “Design and Development of Medical Electronic Instrumentation: A Practical Perspective of the Design, Construction, and Test of Medical Devices”), but it takes a very large amount of energy to induce sufficient current in the tissue to stimulate excitable tissue, so it peaked my attention that to do so at the implantable level would be under consideration.
A bit of research revealed that in experiments published in June 2012 in Nature Communications, neurophysiologist John T. Gale of the Cleveland Clinic and his colleague Giorgio Bonmassar, a physicist at Harvard Medical School and an expert on brain imaging, tested whether micromagnets (which are half a millimeter in diameter) could induce neurons from rabbit retinas to fire. Indeed, they demonstrated that sub-millimeter coils can activate neuronal tissue.
Scientific American summarized the idea very nicely:
“In contrast to the electric currents induced by DBS, which excite neurons in all directions, magnetic fields follow organized pathways from pole to pole, like the magnetic field that surrounds the earth. The researchers found that they could direct the stimulus precisely to individual neurons, and even to particular areas of a neuron, by orienting the magnetic coil appropriately. “That may help us avoid the side effects we see in DBS,” Gale says, referring to, for instance, the intense negative emotions that are sometimes accidentally triggered when DBS is used to relieve motor problems in Parkinson’s.
The micromagnets also solve other problems associated with metal electrodes. The magnetic field easily penetrates the magnets’ plastic coating, which prevents corrosion and the ensuing inflammation of brain tissue. “I’ve been doing DBS research for 14 years now, and this is a totally different way of thinking about activating the brain for me, which is very exciting,” Gale says.
Although the study focused on stimulating neurons, micromagnets could be used to activate other excitable tissues, such as in the heart, inner ear or muscles in our extremities, as part of a pacemaker or prosthetic device. In humans, the micromagnets would be turned on and off by an external control pack, either wirelessly or by connecting to a wire implanted under the skin. A medical company has acquired the rights to manufacture the micromagnets, and if animal research continues to show them to be safe and effective, these devices could be tested in humans within five years, according to Gale.”
The concept is nice, but let’s do a back-of-the-envelope to figure out the compatibility of the technique with current implantable-device technology. According to the Nature Communications paper, the authors were limited to using a pulses with a maximum coil current of 10A for 6 μs before their coils were damaged. With these pulses, they were able to induce an electric field with strength ~6 V/m at a distance of 300 μm from the edge of the coil. This value is on the very edge of the 10 V/m thresholds for neuronal activation measured by Chan and Nicholson.
According to supplementary information, the coils were 100 nH Panasonic ELJ-RFR10JFB, and were driven at 4V per turn x 21 turns = 84 V to achieve a 10 A pulse. So, each magnetic stimulation pulse dissipates 5 mJ of energy. In comparison, a standard 100 μs rectangular electrical DBS pulse has an energy cost of 133 nJ. That is, a typical electrical stimulation pulse used in DBS uses 37,594 times LESS energy than a barely threshold magnetic pulse.
A typical rechargeable lithium-ion cell used in an implantable stimulator has a capacity of 200 to 500 mAh, so assuming a nominal voltage of 2.7 V, the deliverable energy of one of these cells is in the range of 1,944 to 4,860 J. Being generous by assuming an efficiency of 60%, the 500 mAh battery would be able to deliver around 60,000 pulses. At a rate of just 50 Hz (DBS IPGs deliver pulses at a programmable frequency between 30 and 185 pulses per second), so even a 500 mAh cell would last for barely 20 minutes! Even if the performance of the device is improved by an order of magnitude, a fully charged cell with a deliverable capacity of 500 mAh would last only 3.33 hours.
In conclusion, I don’t think that micromagnetic stimulation is a serious candidate to upstage electrical stimulation from implantable devices.