A Miniaturized Ultra-Focal Magnetic Stimulator and Its Preliminary Application to the Peripheral Nervous System

Colella, Micol; Liberti, Micaela; Apollonio, Francesca; Bonmassar, Giorgio

doi:10.1007/978-3-030-45623-8_9

Cited by 6 publications

(6 citation statements)

References 30 publications

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“…For a good trade-off between overall coil height and the number of turns, the trace thickness was selected equal to 35 µm or 1 Oz., which allowed a single trace height of 1 mm with a 2.847 m in length and overall target resistance of 1.4 Ω. Supported by previous FEM numerical simulations, [75][76][77] we fabricated the mcoil as four copper micro traces (four-layered) etched using photolithography on a polyimide substrate (Figure 1). The overall height of the mcoil was only 5.1 mm, given the trace-to-trace spacing of just 0.38 mm (Figure 3a).…”

Section: The Mcoil Fabrication and The F-nims Driving Systemmentioning

confidence: 99%

A study of flex miniaturized coils for focal nerve magnetic stimulation

et al. 2022

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Background: Peripheral magnetic stimulation (PMS) is emerging as a complement to standard electrical stimulation (ES) of the peripheral nervous system (PNS). PMS may stimulate sensory and motor nerve fibers without the discomfort associated with the ES used for standard nerve conduction studies. The PMS coils are the same ones used in transcranial magnetic stimulation (TMS) and lack focality and selectiveness in the stimulation. Purpose: This study presents a novel coil for PMS, developed using Flexible technologies, and characterized by reduced dimensions for a precise and controlled targeting of peripheral nerves. Methods: We performed hybrid electromagnetic (EM) and electrophysiological simulations to study the EM exposure induced by a novel miniaturized coil (or mcoil) in and around the radial nerve of the neuro-functionalized virtual human body model Yoon-Sun, and to estimate the current threshold to induce magnetic stimulation (MS) of the radial nerve. Eleven healthy subjects were studied with the mcoil, which consisted of two 15 mm diameter coils in a figure-ofeight configuration, each with a hundred turns of a 25 µm copper-clad four-layer foil. Sensory nerve action potentials (SNAPs) were measured in each subject using two electrodes and compared with those obtained from standard ES. The SNAPs conduction velocities were estimated as a performance metric. Results: The induced electric field was estimated numerically to peak at a maximum intensity of 39 V/m underneath the mcoil fed by 70 A currents. In such conditions, the electrophysiological simulations suggested that the mcoil elicits SNAPs originating at 7 mm from the center of the mcoil. Furthermore, the numerically estimated latencies and waveforms agreed with those obtained during the PMS experiments on healthy subjects, confirming the ability of the mcoil to stimulate the radial nerve sensory fibers. Conclusion: Hybrid EM-electrophysiological simulations assisted the development of a miniaturized coil with a small diameter and a high number of turns using flexible electronics. The numerical dosimetric analysis predicted the threshold current amplitudes required for a suprathreshold peripheral nerve sensory stimulation, which was experimentally confirmed. The developed and now validated computational pipeline will be used to improve the performances (e.g., focality and minimal currents) of new generations of mcoil designs.

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Section: The Mcoil Fabrication and The F-nims Driving Systemmentioning

confidence: 99%

A study of flex miniaturized coils for focal nerve magnetic stimulation

et al. 2022

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“…Previous 3-D FEM approaches have either used idealized geometries such as a cylindrical arm [44][45][46] or derived from 2-D anatomical images by extruding geometry [35] and limiting to certain cross sections [32]. The modeling methodology applied here, from the geometry, applying EM simulations into a dynamic Neuron solver, using the MRG model, and subsequently using titration analysis, mimics the one employed in the context of magnetic stimulation (MS) [22]. The simulation setup used in the aforementioned MS study has been further validated using clinical experiments.…”

Section: Discussionmentioning

confidence: 99%

Understanding the effect of pulse width on activation depth in TENS: A computational study

Guillen,

Truong,

Cakmak

et al. 2024

Preprint

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Background: Transcutaneous electrical nerve stimulation (TENS) has been a commonly used modality to relieve aches and pain for over 40 years. Commercially available devices provide multiple therapy modes involving a different combination of frequency and pulse width with intensity. While frequency sets sensation, intensity helps determine tolerability, longer pulse width is reported to induce a feeling of deeper stimulation. In fact, longer pulse width has been empirically shown to deliver current into deeper tissues, but in context of other electrical stimulation modalities. The goal of this study was to unpack the relationship between pulse width and activation depth in TENS. Methods: A highly realistic, anatomically-based, 3D finite element model of the forearm was used to simulate the electric field (E-field) distribution, as the pulse width is varied. A typical titration-guided mechanism was used to obtain the strength-duration (S-D) curves of a sensory McIntyre-Richardson-Grill (MRG) axonal model simulating the pain-transmitting A-delta fibers. The pulse widths tested ranged from 30 μs to 495 μs. Results: As expected, shorter pulse widths required more current to achieve activation, resulting in a larger E-field. The S-D curve of the target median nerve indicates a rheobase of 1.75 mA and a chronaxie of 232 µsec. When the applied currents are the same, shorter pulse widths result in a smaller volume of tissue activated (VTA) compared to the longer pulse widths. A 21 fold difference in VTA was found between the longest and shortest pulse widths considered. We observed a linear relationship between pulse width and activation depth for the conditions tested in the study. Conclusion: Our findings highlight the impact of pulse width on activation depth. While choice of a given therapy mode is usually based on an ad-hoc desirable sensation basis, medical professionals may consider advocating a certain therapy mode based on the depth of the intended target nerve.

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“…A recent work simulated the micro-coil stimulation of the radial nerve fibers in arms with a circular micro-coil and a figure-eight micro-coil. The authors found that the threshold current in the figure-eight micro-coil was significantly less than the circular micro-coil for neural activation ( Colella et al, 2021 ). Our results are generally in agreeance with this report.…”

Section: Discussionmentioning

confidence: 99%

“…Saito constructed a figure-eight micro-coil using commercially available chip inductors, which effectively suppressed synchronized bursting activity in a cultured neural network ( Saito, 2021 ). Colella et al (2021) simulated axonal stimulation of peripheral nerves with the figure-eight micro-coil, and found that the threshold for axonal activation was lower for the figure-eight coil than for a circular micro-coil.…”

Section: Introductionmentioning

confidence: 99%

“…So far, activating function analysis has not been performed to estimate the focality and consistency of stimulation in a figure-eight coil system for the activation of a straight axon. Furthermore, although it was believed that the figure-eight micro-coil could provide a better "coupling between nerve fibers and the induced electric field" (Colella et al, 2021), there is a lack of understanding of the neuronal mechanisms underlying axonal activation by such a coil. This paper will examine the consistency of axonal activation by a figure-eight micro-coil, and investigate the underlying ion channel mechanisms using combined activating function analysis and NEURON modeling.…”

Section: Introductionmentioning

confidence: 99%

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Improving focality and consistency in micromagnetic stimulation

Hall

Hendee

2023

Front. Comput. Neurosci.

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The novel micromagnetic stimulation (μMS) technology aims to provide high resolution on neuronal targets. However, consistency of neural activation could be compromised by a lack of surgical accuracy, biological variation, and human errors in operation. We have recently modeled the activation of an unmyelinated axon by a circular micro-coil. Although the coil could activate the axon, its performance sometimes lacked focality and consistency. The site of axonal activation could shift by several experimental factors, including the reversal of the coil current, displacement of the coil, and changes in the intensity of the stimulation. Current clinical practice with transcranial magnetic stimulation (TMS) has suggested that figure-eight coils could provide better performance in magnetic stimulation than circular coils. Here, we estimate the performance of μMS by a figure-eight micro-coil, by exploring the impact of the same experimental factors on its focality and consistency in axonal activation. We derived the analytical expression of the electric field and activating function generated by the figure-eight micro-coil, and estimated the location of axonal activation. Using NEURON modeling of an unmyelinated axon, we found two different types (A and B) of axon activation by the figure-eight micro-coil, mediated by coil currents of reversed direction. Type A activation is triggered by membrane hyperpolarization followed by depolarization; Type B activation is triggered by direct membrane depolarization. Consequently, the two types of stimulation are governed by distinct ion channel mechanisms. In comparison to the circular micro-coil, the figure-eight micro-coil requires significantly less current for axonal activation. Under figure-eight micro-coil stimulation, the site of axonal activation does not change with the reversal of the coil current, displacement of the coil, or changes in the intensity of the stimulation. Ultimately, the figure-eight micro-coil provides a more efficient and consistent site of activation than the circular micro-coil in μMS.

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A Miniaturized Ultra-Focal Magnetic Stimulator and Its Preliminary Application to the Peripheral Nervous System

Cited by 6 publications

References 30 publications

A study of flex miniaturized coils for focal nerve magnetic stimulation

A study of flex miniaturized coils for focal nerve magnetic stimulation

Understanding the effect of pulse width on activation depth in TENS: A computational study

Improving focality and consistency in micromagnetic stimulation

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