Transcranial magnetic stimulation (TMS) is a non-invasive way to stimulate the motor cortex and other parts of the brain. A coil energized by a pulse generator is placed near the head of a human or animal, creating a pulsed magnetic field which induces small electric currents in the part of the brain just under the coil. By observing the resultant motor activity of the patient or subject, medical professionals can assess the damage from a brain injury or disorder, such as a stroke or multiple sclerosis.
Scientists at Bar-Ilan University in Israel designed a study to explore exactly how magnetic stimulation acts on nerve cells in the brain.1 Thin slices of the somatosensory cortex of rats’ brains were prepared for use in the study. A common-used procedure for studying the electrical activity of neurons, known as the patch-clamp technique, was modified to facilitate magnetic stimulation of individual neurons. The main component of the modification was a custom made magnetic coil.
Standard lacquer-coated copper wire was used to make the coil, which consisted of 14 turns of wire in each of two layers. The coil was constructed using a wet-winding technique, in which the coil is impregnated with an epoxy compound during the winding process. Researchers selected low viscosity Master Bond EP29LPSP epoxy for the wet-winding process. The epoxy was mixed with 25μm alumina particles to enhance heat transfer, increase electrical insulation, and strengthen the coil. The magnetic coil was positioned below the neuron under test during the experiment, which gave researchers important insights into how TMS affects neurons.
In an earlier study, researchers at the same university fabricated a custom-made mini coil for use in a TMS experiment on an awake monkey.2 In this case, the coil was immersed in a saline solution and placed inside a chamber designed to record brain activity via multiple micro-electrodes attached to various regions of the monkey’s brain. A wet-winding technique was used to build the coil, which included 32 turns of standard copper wire. Again, the coil was impregnated with Master Bond EP29LPSP epoxy mixed with 25μm alumina particles during the winding process. For this application, electrical insulation of the coil and its windings was especially important in order to minimize the risk of electric breakdown. The insulated mini-coil was tested to voltage levels up to 1200V.