A flexible, thin-film microchannel electrode array device for selective subdiaphragmatic vagus nerve recording.

The vagus nerve (VN) plays an important role in regulating physiological conditions in the gastrointestinal (GI) tract by communicating via the parasympathetic pathway to the enteric nervous system (ENS). However, the lack of knowledge in the neurophysiology of the VN and GI tract limits the development of advanced treatments for autonomic dysfunctions related to the VN. To better understand the complicated underlying mechanisms of the VN-GI tract neurophysiology, it is necessary to use an advanced device enabled by microfabrication technologies. Among several candidates including intraneural probe array and extraneural cuff electrodes, microchannel electrode array devices can be used to interface with smaller numbers of nerve fibers by securing them in the separate channel structures. Previous microchannel electrode array devices to interface teased nerve structures are relatively bulky with thickness around 200 µm. The thick design can potentially harm the delicate tissue structures, including the nerve itself. In this paper, we present a flexible thin film based microchannel electrode array device (thickness: 11.5 µm) that can interface with one of the subdiaphragmatic nerve branches of the VN in a rat. We demonstrated recording evoked compound action potentials (ECAP) from a transected nerve ending that has multiple nerve fibers. Moreover, our analysis confirmed that the signals are from C-fibers that are critical in regulating autonomic neurophysiology in the GI tract.

Educating the healthcare workforce of the future: lessons learned from the development and implementation of a 'Wearables in Healthcare' course.

Digital health technologies will play an ever-increasing role in the future of healthcare. It is crucial that the people who will help make that transformation possible have the evidence-based and hands-on training necessary to address the many challenges ahead. To better prepare the future health workforce with the knowledge necessary to support the re-engineering of healthcare in an equitable, person-centric manner, we developed an experiential learning course-Wearables in Healthcare-for advanced undergraduate and graduate university students. Here we describe the components of that course and the lessons learned to help guide others interested in developing similar courses.

Fractal Microelectrodes for More Energy-Efficient Cervical Vagus Nerve Stimulation.

Vagus nerve stimulation (VNS) has the potential to treat various peripheral dysfunctions, but the traditional cuff electrodes for VNS are susceptible to off-target effects. Microelectrodes may enable highly selective VNS that can mitigate off-target effects, but they suffer from the increased impedance. Recent studies on microelectrodes with non-Euclidean geometries have reported higher energy efficiency in neural stimulation applications. These previous studies use electrodes with mm/cm-scale dimensions, mostly targeted for myelinated fibers. This study evaluates fractal microelectrodes for VNS in a rodent model (N = 3). A thin-film device with fractal and circle microelectrodes is fabricated to compare their neural stimulation performance on the same radial coordinate of the nerve. The results show that fractal microelectrodes can activate C-fibers with up to 52% less energy (p = 0.012) compared to circle microelectrodes. To the best of the knowledge, this work is the first to demonstrate a geometric advantage of fractal microelectrodes for VNS in vivo.

Human organ donor-derived vagus nerve biopsies allow for well-preserved ultrastructure and high-resolution mapping of myelinated and unmyelinated fibers.

The vagus nerve provides motor, sensory, and autonomic innervation of multiple organs, and electrical vagus nerve stimulation (VNS) provides an adjunctive treatment option for e.g. medication-refractory epilepsy and treatment-resistant depression. The mechanisms of action for VNS are not known, and high-resolution anatomical mapping of the human vagus nerve is needed to better understand its functional organization. Electron microscopy (EM) is required for the detection of both myelinated and unmyelinated axons, but access to well-preserved human vagus nerves for ultrastructural studies is sparse. Intact human vagus nerve samples were procured intra-operatively from deceased organ donors, and tissues were immediately immersion fixed and processed for EM. Ultrastructural studies of cervical and sub-diaphragmatic vagus nerve segments showed excellent preservation of the lamellated wall of myelin sheaths, and the axolemma of myelinated and unmyelinated fibers were intact. Microtubules, neurofilaments, and mitochondria were readily identified in the axoplasm, and the ultrastructural integrity of Schwann cell nuclei, Remak bundles, and basal lamina was also well preserved. Digital segmentation of myelinated and unmyelinated axons allowed for determination of fiber size and myelination. We propose a novel source of human vagus nerve tissues for detailed ultrastructural studies and mapping to support efforts to refine neuromodulation strategies, including VNS.

Graded recruitment of pupil-linked neuromodulation by parametric stimulation of the vagus nerve.

Vagus nerve stimulation (VNS) is thought to affect neural activity by recruiting brain-wide release of neuromodulators. VNS is used in treatment-resistant epilepsy, and is increasingly being explored for other disorders, such as depression, and as a cognitive enhancer. However, the promise of VNS is only partially fulfilled due to a lack of mechanistic understanding of the transfer function between stimulation parameters and neuromodulatory response, together with a lack of biosensors for assaying stimulation efficacy in real time. We here develop an approach to VNS in head-fixed mice on a treadmill and show that pupil dilation is a reliable and convenient biosensor for VNS-evoked cortical neuromodulation. In an 'optimal' zone of stimulation parameters, current leakage and off-target effects are minimized and the extent of pupil dilation tracks VNS-evoked basal-forebrain cholinergic axon activity in neocortex. Thus, pupil dilation is a sensitive readout of the moment-by-moment, titratable effects of VNS on brain state.

Electro-Quasistatic Animal Body Communication for Untethered Rodent Biopotential Recording.

Continuous multi-channel monitoring of biopotential signals is vital in understanding the body as a whole, facilitating accurate models and predictions in neural research. The current state of the art in wireless technologies for untethered biopotential recordings rely on radiative electromagnetic (EM) fields. In such transmissions, only a small fraction of this energy is received since the EM fields are widely radiated resulting in lossy inefficient systems. Using the body as a communication medium (similar to a 'wire') allows for the containment of the energy within the body, yielding order(s) of magnitude lower energy than radiative EM communication. In this work, we introduce Animal Body Communication (ABC), which utilizes the concept of using the body as a medium into the domain of untethered animal biopotential recording. This work, for the first time, develops the theory and models for animal body communication circuitry and channel loss. Using this theoretical model, a sub-inch[Formula: see text] [1″ × 1″ × 0.4″], custom-designed sensor node is built using off the shelf components which is capable of sensing and transmitting biopotential signals, through the body of the rat at significantly lower powers compared to traditional wireless transmissions. In-vivo experimental analysis proves that ABC successfully transmits acquired electrocardiogram (EKG) signals through the body with correlation [Formula: see text] when compared to traditional wireless communication modalities, with a 50[Formula: see text] reduction in power consumption.

A DSP Architecture for Distortion-Free Evoked Compound Action Potential Recovery in Neural Response Telemetry System.

This paper presents a digital signal processing (DSP) architecture for real-time and distortion-free recovery of electrically-evoked compound action potentials (ECAPs) from stimulus artifacts and periodic noises in bidirectional neural response telemetry (NRT) system. In this DSP architecture, a low computation-cost bidirectional-filtered coherent averaging (BFCA) method is proposed for programmable linear-phase filtering of ECAPs, which can be easily combined with the alternating-polarity (AP) stimulation method to reject stimulus artifacts overlapped with ECAP responses. Design techniques including the configurable folded infinite-impulse response (IIR) filter and division-free averaging are also presented for efficient hardware implementation. Implemented in 180-nm CMOS process, the proposed DSP architecture consumes 10.03-mm2 area and 2.35-mW post-layout simulated power. The efficacy of the DSP architecture in recovering ECAPs from recorded neural data contaminated by overlapped stimulus artifacts and periodic noises is validated in in-vivo electrical nerve stimulations. Experiment results show that compared with the previous coherent averaging technique, the proposed DSP architecture improves the signal-to-noise ratio (SNR) of ECAP responses by 11 dB and achieves a 3.1% waveform distortion that is 17.1× lower.

An emerging method to noninvasively measure and identify vagal response markers to enable bioelectronic control of gastroparesis symptoms with gastric electrical stimulation.

BACKGROUND: Gastric electrical stimulation (GES) can be a life-changing, device-based treatment option for drug-resistant nausea and vomiting associated with diabetic or idiopathic gastroparesis (GP). Despite over two decades of clinical use, the mechanism of action remains unclear. We hypothesize a vagal mechanism. NEW METHOD: Here, we describe a noninvasive method to investigate vagal nerve involvement in GES therapy in 66 human subjects through the compound nerve action potential (CNAP). RESULTS: Of the 66 subjects, 28 had diabetic GP, 35 had idiopathic GP, and 3 had postsurgical GP. Stimulus charge per pulse did not predict treatment efficacy, but did predict a significant increase in total symptom score in type 1 diabetics as GES stimulus charge per pulse increased (p < 0.01), representing a notable side effect and providing a method to identify it. In contrast, the number of significant left and right vagal fiber responses that were recorded directly related to patient symptom improvement. Increased vagal responses correlated with significant decreases in total symptom score (p < 0.05). COMPARISON WITH EXISTING METHOD(S): We have developed transcutaneous recording of cervical vagal activity that is synchronized with GES in conscious human subjects, along with methods of discriminating the activity of different nerve fiber groups with respect to conduction speed and treatment response. CONCLUSIONS: Cutaneous vagal CNAP analysis is a useful technique to unmask relationships among GES parameters, vagal recruitment, efficacy and side-effect management. Our results suggest that CNAP-guided GES optimization will provide the most benefit to patients with idiopathic and type 1 diabetic gastroparesis.

Chronic cuffing of cervical vagus nerve inhibits efferent fiber integrity in rat model.

OBJECTIVE: Numerous studies of vagal nerve stimulation (VNS) have been published showing it to be a potential treatment for chronic inflammation and other related diseases and disorders. Studies in recent years have shown that electrical stimulation of the vagal efferent fibers can artificially modulate cytokine levels and reduce systematic inflammation. Most VNS research in the treatment of inflammation have been acute studies on rodent subjects. Our study tested VNS on freely moving animals by stimulating and recording from the cervical vagus with nerve cuff electrodes over an extended period of time. APPROACH: We used methods of electrical stimulation, retrograde tracing (using Fluorogold) and post necropsy histological analysis of nerve tissue, flow cytometry to measure plasma cytokine levels, and MRI scanning of gastric emptying. This novel combination of methods allowed examination of physiological aspects of VNS previously unexplored. MAIN RESULTS: Through our study of 53 rat subjects, we found that chronically cuffing the left cervical vagus nerve suppressed efferent Fluorogold transport in 43 of 44 animals (36 showed complete suppression). Measured cytokine levels and gastric emptying rates concurrently showed nominal differences between chronically cuffed rats and those tested with similar acute methods. Meanwhile, results of electrophysiological and histological tests of the cuffed nerves revealed them to be otherwise healthy, consistent with previous literature. SIGNIFICANCE: We hypothesize that due to these unforeseen and unexplored physiological consequences of the chronically cuffed vagus nerve in a rat, that inflammatory modulation and other vagal effects by VNS may become unreliable in chronic studies. Given our findings, we submit that it would benefit the VNS community to re-examine methods used in previous literature to verify the efficacy of the rat model for chronic VNS studies.

B fibers are the best predictors of cardiac activity during Vagus nerve stimulation: Qing, vagal B fiber activation and cardiac effects.

BACKGROUND: Vagus nerve stimulation (VNS) is a promising therapy for many neurologic and psychiatric conditions. However, determining stimulus parameters for individual patients is a major challenge. The traditional method of titrating stimulus intensity based on patient perception produces highly variable responses. This study explores using the vagal response to measure stimulation dose and predict physiological effect. Clinicians are investigating the use of VNS for heart failure management, and this work aims to correlate cardiac measures with vagal fiber activity. RESULTS: By recording vagal activity during VNS in rats and using regression analysis, we found that cardiac activity across all animals was best correlated to the activation of a specific vagal fiber group. With conduction velocities ranging from 5 to 10 m/s, these fibers are classified as B fibers (using the Erlanger-Gasser system) and correspond to vagal parasympathetic efferents. CONCLUSIONS: B fiber activation can serve as a standardized, objective measure of stimulus dose across all subjects. Tracking fiber activation provides a more systematic way to study the effects of VNS and in the future, may lead to a more consistent method of therapy delivery.

Gastric Electrical Stimulation for Gastroparesis and Chronic Unexplained Nausea and Vomiting.

OPINION STATEMENT: Gastroparesis is a heterogeneous clinical syndrome. Some patients have debilitating vomiting, weight loss, and dehydration, while others have effortless regurgitation of undigested foods or postprandial distress suggestive of functional dyspepsia. Gastric electrical stimulation (GES) has been proposed as an effective treatment option for patients with gastroparesis refractory to medical therapy. Evidence suggests that the clinically available device, a low-energy high-frequency GES, activates the vagal afferent pathways to influence the central control mechanisms for nausea and vomiting. Myoelectrical effects of the stomach are also involved. The results of randomized controlled trials (RCTs) for adults with diabetic and idiopathic gastroparesis are conflicting. There are no RCTs in adults with chronic unexplained nausea and vomiting (CUNV) with normal gastric emptying or in children with gastroparesis. However, there is increasing evidence from large unblinded studies showing the long-term efficacy in selected adults with gastroparesis. Selection criteria should be based on three categories: (a) underlying etiology, (b) clinical presentation and predominant symptoms, and (c) potential risk for complication. Significant abdominal pain, daily opiate use, and idiopathic gastroparesis are identified as negative predictors of success. Temporary GES has been utilized to identify patients who may benefit from surgical GES, but this strategy has yet to be proven in controlled studies. Objectives for this review are to highlight the mechanisms of action for GES, to look at the evidence for clinical efficacy, and to select patients who are likely to benefit.

Burst-Modulated Waveforms Optimize Electrical Stimuli for Charge Efficiency and Fiber Selectivity.

We demonstrate an alternative method of designing electrical stimuli-termed burst modulation--for producing different patterns of nerve fiber recruitment. By delivering electrical charge in bursts of "pulsons"--miniature pulses-instead of as long continuous pulses, our method can optimize the waveform for stimulation efficiency and fiber selectivity. In our in vivo validation experiments, while maintaining C fibers of the rat vagus nerve at ∼ 50% activation with different waveforms, the burst-modulated waveform produced 11% less A fiber activation than the standard rectangular pulse waveform (rectangular: 50.8±1.5% of maximal A response, mean ± standard error of the mean; burst-modulated: 39.8 ±1.3%), which equates to a 20% reduction in A fiber response magnitude. In addition, the burst-modulated waveform required 45% less stimulus charge per phase to maintain 50% C fiber activation (rectangular: 20.7 ±0.86 µC; burst-modulated: 11.3 ±0.41 µC ). Burst-modulated waveforms produced consistent patterns of fiber recruitment within and across animals, which indicate that our methods of stimulus design and response analysis provide a reliable way to study neurostimulation and deliver therapy.

Magnetically Inserted Neural Electrodes: Tissue Response and Functional Lifetime.

Neural recording and stimulation have great clinical potential. Long-term neural recording remains a challenge, however, as implantable electrodes eventually fail due to the adverse effects of the host tissue response to the indwelling implant. Astrocytes and microglia attempt to engulf the electrode, increasing the electrical impedance between the electrode and neurons, and possibly pushing neurons away from the recording site. Faster insertion speed, finer tip geometry, smaller size, and lower material stiffness all seem to decrease damage caused by insertion and reduce the intensity of the tissue response. However, electrodes that are too small result in buckling, making insertion impossible. In this paper, we assess the viability of high-speed (27.8 m/s) deployment of 25 µm, ferromagnetic microelectrodes into rat brain. To characterize functionality of magnetically inserted electrodes, 4 Long-Evans rats were implanted for 31 days with impedance measurements and neural recordings taken daily. Performance was compared to 150 µm diameter PlasticsOne electrodes since 25 µm electrodes buckled during "slow speed" insertion. Platinum-iron magnetically inserted electrodes resolved single unit activity throughout the duration of the study in one rat, and saw no significant change (p=0.970) in impedance (4.54% increase) from day 0 (Z0 ≈ 144 kΩ,Z31 ≈ 150 kΩ). These findings provide a proof-of-concept for magnetic insertion as a viable insertion method that enables nonbuckling implantation of small (25 µm) microelectrodes, with potential for neural recording applications.

A flexible platform for biofeedback-driven control and personalization of electrical nerve stimulation therapy.

Electrical vagus nerve stimulation is a treatment alternative for many epileptic and depressed patients whose symptoms are not well managed with pharmaceutical therapy. However, the fixed stimulus, open loop dosing mechanism limits its efficacy and precludes major advances in the quality of therapy. A real-time, responsive form of vagus nerve stimulation is needed to control nerve activation according to therapeutic need. This personalized approach to therapy will improve efficacy and reduce the number and severity of side effects. We present autonomous neural control, a responsive, biofeedback-driven approach that uses the degree of measured nerve activation to control stimulus delivery. We demonstrate autonomous neural control in rats, showing that it rapidly learns how to most efficiently activate any desired proportion of vagal A, B, and/or C fibers over time. This system will maximize efficacy by minimizing patient response variability and by minimizing therapeutic failures resulting from longitudinal decreases in nerve activation with increasing durations of treatment. The value of autonomous neural control equally applies to other applications of electrical nerve stimulation.

Vagus Nerve Modulation Using Focused Pulsed Ultrasound: Potential Applications and Preliminary Observations in a Rat.

The use of focused ultrasonic waves to modulate neural structures has gained recent interest due to its potential in treating neurological disorders non-invasively. While several papers have focused on the use of ultrasound neuromodulation on peripheral nerves, none of these studies have been performed on the vagus nerve. We present preliminary observations on the effects of focused pulsed ultrasound (FPUS) on the conduction of the left cervical vagus nerve of a Long Evans rat. Ultrasound energy was applied at a frequency of 1.1 MHz, and at spatial-peak, temporal average intensities that ranged from 13.6 to 93.4 W/cm2. Vagus nerve inhibition was observed in most cases. Results of this preliminary study suggested that there is a proportional relationship between acoustic intensity and the level of nerve inhibition.

Evolving refractory major depressive disorder diagnostic and treatment paradigms: toward closed-loop therapeutics.

Current antidepressant therapies do not effectively control or cure depressive symptoms. Pharmaceutical therapies altogether fail to address an estimated 4 million Americans who suffer from a recurrent and severe treatment-resistant form of depression known as refractory major depressive disorder. Subjective diagnostic schemes, differing manifestations of the disorder, and antidepressant treatments with limited theoretical bases each contribute to the general lack of therapeutic efficacy and differing levels of treatment resistance in the refractory population. Stimulation-based therapies, such as vagus nerve stimulation, transcranial magnetic stimulation, and deep brain stimulation, are promising treatment alternatives for this treatment-resistant subset of patients, but are plagued with inconsistent reports of efficacy and variable side effects. Many of these problems stem from the unknown mechanisms of depressive disorder pathogenesis, which prevents the development of treatments that target the specific underlying causes of the disorder. Other problems likely arise due to the non-specific stimulation of various limbic and paralimbic structures in an open-loop configuration. This review critically assesses current literature on depressive disorder diagnostic methodologies, treatment schemes, and pathogenesis in order to emphasize the need for more stringent depressive disorder classifications, quantifiable biological markers that are suitable for objective diagnoses, and alternative closed-loop treatment options tailored to well-defined forms of the disorder. A closed-loop neurostimulation device design framework is proposed, utilizing symptom-linked biomarker abnormalities as control points for initiating and terminating a corrective electrical stimulus which is autonomously optimized for correcting the magnitude and direction of observed biomarker abnormality.

The design and hardware implementation of a low-power real-time seizure detection algorithm.

Epilepsy affects more than 1% of the world's population. Responsive neurostimulation is emerging as an alternative therapy for the 30% of the epileptic patient population that does not benefit from pharmacological treatment. Efficient seizure detection algorithms will enable closed-loop epilepsy prostheses by stimulating the epileptogenic focus within an early onset window. Critically, this is expected to reduce neuronal desensitization over time and lead to longer-term device efficacy. This work presents a novel event-based seizure detection algorithm along with a low-power digital circuit implementation. Hippocampal depth-electrode recordings from six kainate-treated rats are used to validate the algorithm and hardware performance in this preliminary study. The design process illustrates crucial trade-offs in translating mathematical models into hardware implementations and validates statistical optimizations made with empirical data analyses on results obtained using a real-time functioning hardware prototype. Using quantitatively predicted thresholds from the depth-electrode recordings, the auto-updating algorithm performs with an average sensitivity and selectivity of 95.3 +/- 0.02% and 88.9 +/- 0.01% (mean +/- SE(alpha = 0.05)), respectively, on untrained data with a detection delay of 8.5 s [5.97, 11.04] from electrographic onset. The hardware implementation is shown feasible using CMOS circuits consuming under 350 nW of power from a 250 mV supply voltage from simulations on the MIT 180 nm SOI process.

Magnetic insertion system for flexible electrode implantation.

Chronic recording electrodes are a vital tool for brain research and neural prostheses. Despite decades of advances in recording technology, probe structures and implantation methods have changed little over time. Then as now, compressive insertion methods require probes to be constructed from hard, stiff materials, such as silicon, and contain a large diameter shank to penetrate the brain, particularly for deeper structures. The chronic presence of these probes results in an electrically isolating glial scar, degrading signal quality over time. This work demonstrates a new magnetic tension-based insertion mechanism that allows for the use of soft, flexible, and thinner probe materials, overcoming the materials limitations of modern electrodes. Probes are constructed from a sharp magnetic tip attached to a flexible tether. A pulsed magnetic field is generated in a coil surrounding a glass pipette containing the electrode. The applied field pulls the electrode tip forward, accelerating the probe into the neural tissue with a penetration depth that is calibrated against the charge voltage. Mathematical modeling and agar gel insertion testing demonstrate that the electrode can be implanted to a predictable depth given system specific parameters. Trial rodent implantations resulted in discernible single-unit activity on one of the probes. The current prototype demonstrates the feasibility of a tension based, magnetically driven implantation system and opens the door to a wide variety of new minimally invasive probe materials and configurations.

Toward a comparison of microelectrodes for acute and chronic recordings.

Several variations of microelectrode arrays are used to record and stimulate intracortical neuronal activity. Bypassing the immune response to maintain a stable recording interface remains a challenge. Companies and researchers are continuously altering the material compositions and geometries of the arrays in order to discover a combination that allows for a chronic and stable electrode-tissue interface. From this interface, they wish to obtain consistent quality recordings and a stable, low impedance pathway for charge injection over extended periods of time. Despite numerous efforts, no microelectrode array design has managed to evade the host immune response and remain fully functional. This study is an initial effort comparing several microelectrode arrays with fundamentally different configurations for use in an implantable epilepsy prosthesis. Specifically, NeuroNexus (Michigan) probes, Cyberkinetics (Utah) Silicon and Iridium Oxide arrays, ceramic-based thin-film microelectrode arrays (Drexel), and Tucker-Davis Technologies (TDT) microwire arrays are evaluated over a 31-day period in an animal model. Microelectrodes are compared in implanted rats through impedance, charge capacity, signal-to-noise ratio, recording stability, and elicited immune response. Results suggest significant variability within and between microelectrode types with no clear superior array. Some applications for the microelectrode arrays are suggested based on data collected throughout the longitudinal study. Additionally, specific limitations of assaying biological phenomena and comparing fundamentally different microelectrode arrays in a highly variable system are discussed with suggestions on how to improve the reliability of observed results and steps needed to develop a more standardized microelectrode design.