title: Deep brain stimulation through engineered micro-coils
Creators: name: Masalehdan, Tahereh
Publisher: name: University of Glasgow
Year: 2024

description: Neurostimulation techniques are crucial in advancing neuroscience research and addressing neurological disorders. Within this realm, magnetic neurostimulation stands out by offering numerous benefits compared to conventional methods like electrical stimulation. One of its key strengths is its enhanced orientational selectivity, which holds great potential as a noninvasive approach for deep brain neurostimulation (DBS), enabling precise targeting of specific brain circuits. The capability of magnetic neurostimulation to focus the magnetic field and penetrate deeply into the brain contributes to its effectiveness and therapeutic potential. However, attaining a high level of spatial resolution to precisely target specific sub-regions of the brain, particularly in the axial direction, poses a significant challenge. For instance, transcranial magnetic stimulation typically offers a spatial resolution in the range of ~ 0.5 – 1 line pairs per centimetre, which is unsuitable for DBS applications.

This work explored the feasibility of employing spiral micro-coils for magnetic neurostimulation, utilising both computational methods, employing the finite element method in COMSOL Multiphysics and Sim4Life software platforms, and experimental micro-coils fabrication studies. Simulations were run at a constant 100 mA current with frequencies from 1 to 3 kHz, followed by a broader sweep examining frequencies of 5 Hz to 100 kHz and currents of 1 A to 5 A in two different current input models. Furthermore, heat generation across the micro-coils for various applied frequencies and currents were modelled. In the fabrication of micro-coils, the process commenced with electroplating gold onto a polyimide substrate. Additionally, the potential of using copper as a more feasible and potentially cheaper substitute for gold in micro-coil construction was explored. The investigation of the laser parameters optimising for micro-coil fabrication involved testing three different laser power levels (2.5 W, 2.6 W, and 2.7 W) in conjunction with specific pulse settings and wavelengths.

The results revealed that the applied frequencies had a negligible effect on the coils' selfinductance, which is consistent with the predictions of the quasi-static approximation. Because the coil wire diameter was 10 times smaller than the minimum skin depth, there were no significant changes observed in magnetic flux density across different frequencies. Magnetic field strength calculations around the coils demonstrated a direct correlation between increasing current and B-field strength at all measured locations. Simulations predicted average B-field values ranging from 0.1 to 0.8 mT when using currents of 100 mA and regardless of current direction, ranged from 8.41 to 42.05 mT for currents between 1 and 5 A across all frequencies tested. However, the B-field strength above the coil surface exhibited a dependence on the direction of the current. When using the same current direction, the B-field ranged from 0.31 mT to 1.56 mT, conversely, applying the current in the opposite direction resulted in a stronger B-field ranging from 2.04 mT to 10.18 mT for currents of 1 A and 5 A. The simulations conducted in this study suggest that this magnetic neurostimulation approach has the potential to reach depths of up to 2 cm within brain tissue. Furthermore, it has been shown that regardless of the specific measurement location on the coil surface, temperature increased proportionately with increasing current, ranging from 30.42 °C to 165.38 °C for currents ranging from 1 A to 5 A across all tested frequencies.

Analysis of the electroplated gold layer revealed inconsistencies in thickness across the polyimide substrate. This variation in thickness hindered precise laser cutting during the micro-coil fabrication process. The analysis revealed that the lowest power setting (2.5 W) resulted in incomplete copper removal, verified through surface roughness measurements and visual inspection using optical and scanning electron microscopy (SEM) analyses. Higher laser power settings (2.6 W and 2.7 W) successfully achieved complete copper removal, creating clean separation between the coils. However, the highest power setting also caused minor burning of the wires. Results of electrical characteristics measurements revealed that impedance increased with frequency (in the range of 0 to 550 kHz) for all power levels. Inductance measurements showed minimal variation, remaining relatively constant at approximately 5 µH. The phase angles exhibited an increase with frequency for all coils and reached to 81.12° at 550 kHz, confirming their inductive behaviour. Power level influenced the overall resistance values, but the variations were minor. For instance, at 550 kHz, the difference in resistance between coils fabricated at 2.7 W and 2.5 W was only 1.18 Ω. The results indicate that the magnetic field (B-field) exhibited variability, ranging from 429.26 nT at the coil's centre to 1678.01 nT at 25 mm from the centre.

The aim the current study was to offer improved precision and selectivity and assess the depth and spread magnetic field's attenuation. The simulation results indicated that the magnetic neurostimulation technique being proposed could generate magnetic fields, enabling penetration into the brain model up to a depth of around 2 cm for neurostimulation. In summary, the optimal laser cutting power was identified as 2.6 W and the results demonstrate that the electrical properties of these micro-coils can be influenced by the laser power used during fabrication. Notably, the impedance values were significantly higher than the resistance across all frequencies, indicating that the micro-coils primarily behave as inductors.