Tonini, D. | Search bioRxiv

bioRxiv2022

Micromagnetic Stimulation (μMS) Dose-Response of the Rat Sciatic Nerve

ObjectiveThe objective of this study was to investigate the effects of micromagnetic stimuli strength and frequency from the Magnetic Pen (MagPen) on the rat right sciatic nerve. The nerves response would be measured by recording muscle activity and movement of the right hind limb. ApproachThe MagPen was custom-built such that it can be held over the sciatic nerve in a stable manner. Rat leg muscle twitches were captured on video and movements were extracted using image processing algorithms. EMG recordings were also used to measure muscle activity. Main resultsThe MagPen prototype when driven by alternating current, generates time-varying magnetic field which as per Faradays Law of Electromagnetic Induction, induces an electric field for neuromodulation. The orientation dependent spatial contour maps for the induced electric field from the MagPen prototype has been numerically simulated. Furthermore, in this in vivo work on MS, a dose-response relationship has been reported by experimentally studying how the varying amplitude (Range: 25 mVp-p through 6 Vp-p) and frequency (Range: 100 Hz through 5 kHz) of the MagPen stimuli alters the hind limb movement. The primary highlight of this dose-response relationship is that at a higher frequency of the MS stimuli, significantly smaller amplitudes can trigger hind limb muscle twitch. This frequency-dependent activation can be justified following directly from the Faradays Law as the magnitude of the induced electric field is directly proportional to frequency. SignificanceThis work reports that MS can successfully activate the sciatic nerve in a dose-dependent manner. The MagPen probe, unlike electrodes, does not have a direct electrochemical interface with tissues rendering it much safer than an electrode. Magnetic fields create more precise activation than electrodes because they induce smaller volumes of activation. Finally, unique features of MS such as orientation dependence, directionality and spatial selectivity have been demonstrated.

bioengineering↗

bioRxiv2021

Strength-frequency curve for micromagnetic neurostimulation through EPSPs on rat hippocampal neurons and numerical modeling of magnetic microcoil (μcoil)

ObjectiveThe objective of this study was to measure the effect of micromagnetic stimulation (MS) on hippocampal neurons, by using single microcoil (coil) prototype, Magnetic Pen (MagPen). MagPen will be used to stimulate the CA3 region magnetically and excitatory post synaptic potential (EPSP) response measurements will be made from the CA1 region. The threshold for micromagnetic neurostimulation as a function of stimulation frequency of the current driving the coil will be demonstrated. Finally, the optimal stimulation frequency of the current driving the coil to minimize power will be estimated. ApproachA biocompatible, watertight, non-corrosive prototype, MagPen was built, and customized such that it is easy to adjust the orientation of the coil and its distance over the hippocampal tissue in an in vitro recording setting. Finite element modeling (FEM) of the coil design was performed to estimate the spatial profiles of the magnetic flux density (in T) and the induced electric fields (in V/m). The induced electric field profiles generated at different values of current applied to the coil can elicit a neuron response, which was validated by numerical modeling. The modeling settings for the coil were replicated in experiments on rat hippocampal neurons. Main resultsThe preferred orientation of MagPen over the Schaffer Collateral fibers was demonstrated such that they elicit a neuron response. The recorded EPSPs from CA1 region due to MS at CA3 region were validated by applying tetrodotoxin (TTX). Application of TTX to the hippocampal slice blocked the EPSPs from MS while after prolonged TTX washout, a partial recovery of the EPSP from MS was observed. Finally, it was interpreted through numerical analysis that increasing frequency of the current driving the coil, led to a decrease in the current amplitude threshold for micromagnetic neurostimulation. SignificanceThis work reports that micromagnetic neurostimulation can be used to evoke population EPSP responses in the CA1 region of the hippocampus. It demonstrates the strengthfrequency curve for MS and its unique features related to orientation dependence of the coils, spatial selectivity and stimulation threshold related to distance dependence. Finally, the challenges related to MS experiments were studied including ways to overcome them.

bioengineering↗