Wearable Modular Focused Ultrasound Systems for Non-Invasive Stimulation of the Human Brain During Deep Space Exploration

Deep space exploration will cast unprecedented and multi-faceted health challenges for crew members. Limited access to resources calls for non-pharmacological alternatives to improve the performance and wellness of astronauts in the midst of stressful environments during a long space flight mission. Non-invasive regional brain stimulation techniques may expand options that can benefit their mental as well as physical health. Focused ultrasound (FUS) techniques enable the delivery of highly-focused (with a focal size measuring a few millimeters) acoustic energy to biological tissue in a non-invasive fashion. A series of recent studies, including our own, have found that transcranial FUS (tFUS) sonication to the brain, given in batches of pulses at a low intensity (below the threshold for heat generation or mechanical damage to the tissue), can reversibly modulate the excitability of regional brain tissue without elevating its temperature. We have also demonstrated that FUS can modulate the degree of neural conduction through myelinated nerves and can elicit tactile sensations by directly stimulating the nerve endings, suggesting its further applicability to the peripheral nervous system. Based on the advantages over existing brain stimulation modalities, especially in terms of the spatial selectivity and depth penetration, tFUS technique is anticipated to provide greater flexibility in non-invasive neuromodulation. In addition to the implementation and optimization of the technique for a past decade, we have developed light-weight wearable FUS transducers and their control systems, augmented with image-guidance for targeting the sonication focus to individual neuroanatomy. On-site computer simulation is warranted to characterize the acoustic propagation through the skull, informing the user of the focal location and its intensity. The proposed ground-based study is intended to advance our knowledge on neuromodulatory potentials of tFUS while addressing crucial technical innovations necessary to achieve the accurate delivery of the acoustic focus to specific brain areas (both cortical and subcortical), with specifications suitable for its routine use during deep space exploration. We will develop a modular tFUS system equipped with optical-based individual image-guidance and on-site numerical simulation of the acoustic propagation. The FUS transducer will be made wearable, adopting a light-weight ergonomic headgear configuration. Prior to its use in modulating brain neural circuitries associated with performance-enhancing cognitive functions, we intend to examine the operating characteristics and safety of the technique in stimulating the primary somatosensory (S1) area and its thalamic projection (i.e., ventral posterolateral nucleus-VPL) among healthy humans. The range of sonication parameters that stimulate these brain regions at the lowest possible acoustic intensity will be examined via closed-loop monitoring of the electrophysiological responses (electroencephalograph sensory evoked potentials - SEP) and through subject self-reporting. Computer-generated, randomized/balanced inclusion of passive and active sham FUS conditions will provide a double-blind experimental design. We will assess the safety of the procedure through neuroradiological and neurological evaluations performed at variable time points, covering acute, delayed, and long-term periods after the sonication.

The study was intended to advance our knowledge regarding neuromodulatory potentials of tFUS while addressing crucial technical innovations for deep space exploration. We developed a tFUS system using single-element transducer configuration (rather than a more error-prone multi-array transducer) for the reliable operation in spaceflight. A reconfigurable ergonomic headgear allowed for sonicating both cortical and deep brain structures. Light-weight, wearable FUS-enabled headgear and its adaptation to optical-based image-guidance were particularly suitable as a translational platform for human applications.

Development of embedded real-time numerical acoustic simulation software also served as a critical innovation to ensure the accuracy of sonication in a spaceflight environment. All hardware components were prepared in modular configurations for maintenance, replacement, and troubleshooting in space. These state-of-the art technical innovations were used to subsequently demonstrate the efficacy and safety of the tFUS-mediated, non-invasive brain stimulation.

We demonstrated the utility of our tFUS system in stimulating the primary sensory (corresponding to the unilateral hand area representation) and its thalamic projections among healthy humans as well as for stimulating primary motor (corresponding to the unilateral hindlimb) and thalamus in awake animals. Because there is very limited data regarding tFUS neuromodulation in humans to date, a range of pulse duration sonication parameters that transiently modulate the function of the targeted neural circuitries were investigated.

We found (1) the existence of specific pulse duration that yielded effective stimulation (which was found to be similar across species), (2) successfully stimulated the targeted brain areas, and (3) demonstrated the safety of the technique through comprehensive histological analysis of the ovine brain and extensive neurological assessment of the human volunteers. From human study using functional connectivity analysis, we also found, for the first time, that (4) the sonication may yield long-lasting neuromodulatory effects beyond the sonication itself, which may generate therapeutic effects via neural plasticity. Therefore, our study not only conferred firm technical foundations for stimulating somatosensory circuits among healthy human volunteers, but also established initial safety profiles prior to its ultimate use in spaceflight.

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