A study of flex miniaturized coils for focal nerve magnetic stimulation

Abstract

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.

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

“…The simulation setup used in the aforementioned MS study has been further validated using clinical experiments. Specifically, numerically estimated latencies and waveforms were in agreement with the empirical measurements on subjects undergoing MS on the arm [23]. The only difference to our simulation is the application of electrical stimulation and thereby, related governing equation.…”

“…Furthermore, these predictions have helped in elucidating stimulation parameter choices, understanding mechanism of action, explaining stimulation outcome, and thereby advancing stimulation administration in general [11, 54–57]. We expect this study on the arm to guide researchers in performing future explorations on other body parts, determine ideal pulse width range for target nerve of interest, attempt validation using TENS similar to the one performed in MS [23], and investigate new TENS delivery approaches.…”

“…The model incorporates high resolution data (0.1 x 0.1 x 0.2 mm) making it possible to resolve and thereby segment nerves, arteries, veins, and other small structures. Importantly for this study, it includes all major nerve trajectories from the cranium and spinal cord to internal organs and muscles and has been used in other peripheral nerve stimulation studies [22, 23]. The relevant tissue properties for this study are presented in Table 1 and based on the IT’IS material parameter database [24].…”

Investigation of the mechanisms for wireless nerve stimulation without active electrodes

“…Depending on the shape and design of the magnetic coil, it is possible to modify the stimulation depth and focality of the produced fields. Magnetic stimulation has the advantage of stimulating superficial nerves to a significantly lesser degree than electrical stimulation, avoiding or reducing the uncomfortable tickling sensations in patients (Colella et al, 2023 ). Another advantage of magnetic stimulation devices is that non‐invasive stimulation methods are inherently safer than their invasive counterparts, reducing the risk of infection and further tissue damage or inflammation.…”

A scoping review of current and emerging techniques for evaluation of peripheral nerve health, degeneration, and regeneration: part 1, neurophysiology

“…The technique is appealing given its promise to be able to non-invasively stimulate nerves anywhere in the body due to the lack of attenuation of the field, as well as avoiding the need to inject current and associated stimulation artifact. Nevertheless, compared to electrical stimulation, the technique is limited by its ability to administer a controlled focal stimulus and remains inconsistent, imprecise, and timeconsuming [175][176][177][178]. Micro-coil magnetic probes have been developed for intracortical neural stimulation but have the potential to be successfully repurposed for peripheral nerve stimulation and recording, especially intraoperatively, as discussed further in the section 9 below [179][180][181].…”