A computational simulation and synaptic response to planar coil-based implanatable magnetic stimulation

Abstract

Electrical brain stimulation is electrotherapy used in clinical neurobiology or research to stimulation a neural cell or network in the brain. However, it has several limitations as implantable devices. In electrical stimulation, electrodes have shown a decline in electrical performance over time due to inflammatory responses. Moreover, the deep brain stimulation can damage the brain tissue by electrode heating under magnetic resonance imaging examination. Micromagnetic stimulation using implantable coils was recently suggested as a new method to overcome the limitations of implantable electrical stimulators. Even though the tissue encapsulation is formed around the implanted coil, implantable magnetic stimulation can maintain the stimulation performance because the induced current by the magnetic field is used to elicit neuronal activities without the necessity for direct contact between the target tissue and the metal pad of electrode emitting the electron. At first, we conducted computational simulation study on electromagnetic induction and heat transfer of implantable magnetic stimulation in the brain using finite element analysis (FEA), according to varying geometric parameters of the planar coil and conditions of simulation source applied to the coil. In simulations, there were two criteria for the allowable temperature rise in the brain and the threshold of the electric field intensity to elicit neural activations. The temperature increases and the induced electric field was mainly determined by the energy contained in the waveform. Low coil resistance, short pulse width and short rising time of voltage were both capable of reducing temperature and providing sufficient nerve stimulation. In addition, the insulation layer at the micrometer has suppressed the rapid temperature rise in the surrounding tissue. We found the optimal range of variables through the simulation study. Subsequently, we fabricated the coil based on the simulation results using a MEMS fabrication process. The fabricated coils have 10 mm outer diameter, 1 mm inner diameter, 30 μm line width and spacing, and 75 turns of the coil. Compared to existing TMS, the coil size was sharply reduced and simultaneously increased the coil resistance. Hence, the coil has the potential of cell damages by excessive heat generation from the coil. It is therefore necessary to study the stimulation performance and safety issue. We devised contact-mode magnetic stimulation (CMS), where the magnetic field was transferred to the hippocampal slices through a customized planar coil attached underneath the slice in the contact mode. We conducted the synaptic potential recording in hippocampal slice and temperature measurement during CMS to Electrical brain stimulation is electrotherapy used in clinical neurobiology or research to stimulation a neural cell or network in the brain. However, it has several limitations as implantable devices. In electrical stimulation, electrodes have shown a decline in electrical performance over time due to inflammatory responses. Moreover, the deep brain stimulation can damage the brain tissue by electrode heating under magnetic resonance imaging examination. Micromagnetic stimulation using implantable coils was recently suggested as a new method to overcome the limitations of implantable electrical stimulators. Even though the tissue encapsulation is formed around the implanted coil, implantable magnetic stimulation can maintain the stimulation performance because the induced current by the magnetic field is used to elicit neuronal activities without the necessity for direct contact between the target tissue and the metal pad of electrode emitting the electron. At first, we conducted computational simulation study on electromagnetic induction and heat transfer of implantable magnetic stimulation in the brain using finite element analysis (FEA), according to varying geometric parameters of the planar coil and conditions of simulation source applied to the coil. In simulations, there were two criteria for the allowable temperature rise in the brain and the threshold of the electric field intensity to elicit neural activations. The temperature increases and the induced electric field was mainly determined by the energy contained in the waveform. Low coil resistance, short pulse width and short rising time of voltage were both capable of reducing temperature and providing sufficient nerve stimulation. In addition, the insulation layer at the micrometer has suppressed the rapid temperature rise in the surrounding tissue. We found the optimal range of variables through the simulation study. Subsequently, we fabricated the coil based on the simulation results using a MEMS fabrication process. The fabricated coils have 10 mm outer diameter, 1 mm inner diameter, 30 μm line width and spacing, and 75 turns of the coil. Compared to existing TMS, the coil size was sharply reduced and simultaneously increased the coil resistance. Hence, the coil has the potential of cell damages by excessive heat generation from the coil. It is therefore necessary to study the stimulation performance and safety issue. We devised contact-mode magnetic stimulation (CMS), where the magnetic field was transferred to the hippocampal slices through a customized planar coil attached underneath the slice in the contact mode.