Electromagnetic and mechanical characterization of a flexible coil for transcranial magnetic stimulation

Transcranial magnetic stimulation is a painless and noninvasive method for treating brain disorders. The coil-geometries that match the head topologies achieve more effective stimulation, however it is difficult for a rigid coil to fit the skulls of all patients. We propose and develop a rubber-like flexible coil that can be shaped into different geometries to reduce the inter-individual variabilities in its clinical use. The main challenge is attributed to the fact that the external bending and induced Lorentzian forces cause coil deformation and fatigue. Herein, we investigated the influence of bending on the electromagnetic characteristics of the flexible coil. The magnetic field distribution was calculated and measured using a search coil. The induced Lorentzian force was calculated and the induced eddy current density was simulated using the scalar potential finite difference (SPFD) method. For a mechanical characterization, we fixed the center of the coil, and external bending forces were applied on the two wings of the coils, while Lorentzian forces were applied in a direction normal to the side wall of the wire groove. Fatigue analyses of these forces were also conducted. The results show that the eddy current density induced in the brain by the flexible coil was significantly higher compared to that of the figure-eight and butterfly coils. Fatigue analyses show that the bending force required to achieve a close coil fit on the human head and the generated Lorentzian force would not lead to fatigue problem.