Magnetic Stimulation of Neuronal Fibers: From Macroscopic Dorsal Column Neuromodulation for Sciatica to Microscopic L1 Circuit Activation for Connectome Studies

Abstract

Magnetic neuromodulation offers a compelling alternative to pharmacologic therapy and conventional electrode-based stimulation by enabling contactless activation of neural tissue across multiple spatial scales. In this talk, I will present a multiscale engineering framework for magnetic stimulation spanning macroscopic activation of spinal pathways to microscopic manipulation of local neural circuits in the rat.

At the systems level, we developed a high-frequency trans-spinal magnetic stimulation (HF-TSMS) platform designed for noninvasive treatment of chronic neuropathic back pain. The stimulator operates in quasi-resonant mode to generate multi-kA current pulse trains at 10 kHz, driving a custom ribbon-based figure-of-eight coil to induce spatially confined electric fields within the spinal cord. Electromagnetic simulations were used to optimize coil geometry and field distribution for rodent anatomy. In spared nerve injury models, HF-TSMS produced behavioral improvements and modulation of BOLD fMRI responses compared with sham controls, providing preliminary evidence for therapeutic efficacy and supporting future translational development.

At the microscopic scale, we engineered a silicon-based micro-magnetic stimulation (µMS) device (200 × 400 × 7 µm³) fabricated using advanced microfabrication techniques. The µMS coil was deployed in rat cortex and driven with high-current, short-duration pulses to generate highly localized magnetic fields. Neuronal activation was quantified using optical glutamate sensing via fiber photometry in combination with local field potential recordings. µMS elicited stimulus-locked increases in glutamate release in vivo that were absent post-euthanasia, confirming biological specificity. Quantitative analyses demonstrated significant increases in peak amplitude and integrated glutamate response during stimulation epochs.

Together, these results demonstrate that magnetic stimulation can be engineered to operate across anatomical scales—from dorsal column modulation to focal microcircuit activation—using physics-based design, microfabrication, and multimodal validation. This multiscale approach establishes a foundation for next-generation magnetic neuromodulation technologies that are noninvasive at the systems level yet capable of highly localized circuit control.

Bio

Dr. Giorgio Bonmassar is an Associate Professor of Radiology at Harvard Medical School and Director of the Analog Brain Imaging (ABI) Laboratory at the Athinoula A. Martinos Center for Biomedical Imaging at Massachusetts General Hospital. For more than two decades, his research has focused on bioelectromagnetic modeling and the design of MRI-compatible electrophysiology and neuromodulation technologies. His work integrates engineering, physics, and neuroscience to develop safe, high-performance systems that operate seamlessly within the MRI environment, advancing both fundamental brain research and clinical translation. Dr. Bonmassar is internationally recognized for pioneering multimodal neurostimulation platforms, including the first low-frequency focused ultrasound (LiFU) system for spinal cord neuromodulation and a novel high-frequency trans-spinal magnetic stimulation (HF-TSMS) system funded by the NIH. His laboratory develops non-invasive magnetic and ultrasound approaches aimed at transforming the treatment of chronic pain and neurological disorders. In parallel, he has led the design of MRI-conditional deep brain stimulation (DBS) and electrocorticography (ECoG) systems based on metamaterial and resistively tapered technologies, substantially reducing RF-induced heating and imaging artifacts. Earlier contributions include the first demonstration of simultaneous visual evoked potentials and fMRI, MRI-invisible microelectrodes, and polymer-based high-density EEG systems (InkCap and InkNet) that enable safe EEG-fMRI at high field strengths. As Principal Investigator on multiple NIH awards (including U01, R01, SBIR, and BRAIN Initiative mechanisms), Dr. Bonmassar leads interdisciplinary efforts spanning computational modeling, device fabrication, safety validation, and preclinical translation. His work is unified by a central mission: to engineer next-generation neurotechnology platforms that are safe, imaging-compatible, and capable of precise, multimodal control of neural circuits.

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