Renal Nerve Activity and Arterial Depressor Responses Induced by Neuromodulation of the Deep Peroneal Nerve in Spontaneously Hypertensive Rats.

Hypertension is a main cause of death in the United States with more than 103 million adults affected. While pharmacological treatments are effective, blood pressure (BP) remains uncontrolled in 50-60% of resistant hypertensive subjects. Using a custom-wired miniature electrode, we previously reported that deep peroneal nerve stimulation (DPNS) elicited acute cardiovascular depressor responses in anesthetized spontaneously hypertensive rats (SHRs). Here, we further study this effect by implementing a wireless system and exploring different stimulation parameters to achieve a maximum depressor response. Our results indicate that DPNS consistently induces a reduction in BP and suggests that renal sympathetic nerve activity (RSNA) is altered by this bioelectronic treatment. To test the acute effect of DPNS in awake animals, we developed a novel miniaturized wireless microchannel electrode (w-µCE), with a Z-shaped microchannel through which the target nerves slide and lock into the recording/stimulation chamber. Animals implanted with w-µCE and BP telemetry systems for 3 weeks showed an average BP of 150 ± 14 mmHg, which was reduced significantly by an active DPNS session to 135 ± 8 mmHg (p < 0.04), but not in sham-treated animals. The depressor response in animals with an active w-µCE was progressively returned to baseline levels 14 min later (164 ± 26 mmHg). This depressor response was confirmed in restrained fully awake animals that received DPNS for 10 days, where tail-cuff BP measurements showed that systolic BP in SHR lowered 10% at 1 h and 16% 2 h after the DPNS when compared to the post-implantation baseline. Together, these results support the use of DPN neuromodulation as a possible strategy to lower BP in drug-resistant hypertension.

Miniature electroparticle-cuff for wireless peripheral neuromodulation.

OBJECTIVE: Recent developments in peripheral nerve electrodes allow the efficient and selective neuromodulation of somatic and autonomic nerves, which has proven beneficial in specific bioelectronic medical applications. However, current most clinical devices are wired and powered by implantable batteries which suffer from several limitations. We recently developed a sub-millimeter inductively powered neural stimulator (electroparticle; EP), and in this study, we report the integration of the EP onto commercial cuff electrodes (EP-C) allowing the wireless activation of peripheral nerves. APPROACH: The current output of this device was defined at different magnetic field strenghts, and with respect to external antenna distance and activation angles. In acute in vivo testing, stimulation of the rat sciatic nerve (ScN) with the EP-C was able to evoke motor responses quantified by 3D tracking of the hind limb movement. Motor recruitment curves were obtained in response to variations in magnetic field strength (0-92.91 A m-1), stimulation frequencies (2-7 Hz), and pulse widths (50-200 µs). MAIN RESULTS: The results show constant output voltage throughout 50 400 stimulating cycles on a benchtop setting, and successful ScN motor activation with a 4 cm distance between external antenna and receiver. We achieved optimal motor recruitment indicated by maximizing range of hindlimb movement (6.01 ± 2.92 mm) with a magnetic field of 40.02 ± 2.85 A m-1 and 150 µs pulse width. Stimulating pulse width or frequency did not significantly influence motor recruitment. SIGNIFICANCE: We confirmed that continuous stimulation for 14 min using monophasic pulses did not deleteriously affect the evoked motor responses when compared to wired charge-balanced biphasic electrical stimulation. We observed, however, a 36%-44% decrease in the evoked limb movement in both groups over time due to muscle fatigue. This study shows that the EP-C device can be used effectively for peripheral nerve neuromodulation.

Chronic recording and electrochemical performance of amorphous silicon carbide-coated Utah electrode arrays implanted in rat motor cortex.

OBJECTIVE: Clinical applications of implantable microelectrode arrays are currently limited by device failure due to, in part, mechanical and electrochemical failure modes. To overcome this challenge, there is significant research interest in the exploration of novel array architectures and encapsulation materials. Amorphous silicon carbide (a-SiC) is biocompatible and corrosion resistant, and has recently been employed as a coating on biomedical devices including planar microelectrode arrays. However, to date, the three-dimensional Utah electrode array (UEA) is the only array architecture which has been approved by the food and drug administration (FDA) for long-term human trials. APPROACH: Here, we demonstrate, for the first time, that UEAs can be fabricated with a-SiC encapsulation and sputtered iridium oxide film (SIROF) electrode coatings, and that such arrays are capable of single-unit recordings over a 30 week implantation period in rat motor cortex. Over the same period, we carried out electrochemical measurements, including voltage transients, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS), to evaluate potential failure modes. Furthermore, we evaluated chronic foreign body response via fluorescence immunohistochemistry following device explantation. MAIN RESULTS: During the indwelling period, we observed a reduction in active electrode yield percentage from 94.6 ± 5.4 (week 1) to 16.4 ± 11.5% (week 30). While the average active electrode yield showed a steady reduction, it is noteworthy that 3 out of 8 UEAs recorded greater than 60% active electrode yield at all times through 24 weeks and 1 out of 8 UEAs recorded greater than 60% active electrode yield at all times through the whole implantation period. SIGNIFICANCE: In total, these findings further suggest that a-SiC may serve as a mechanically and electrochemically stable device encapsulation alternative to polymeric coatings such as Parylene-C.

Thin Film Multi-Electrode Softening Cuffs for Selective Neuromodulation.

Silicone nerve cuff electrodes are commonly implanted on relatively large and accessible somatic nerves as peripheral neural interfaces. While these cuff electrodes are soft (1-50 MPa), their self-closing mechanism requires of thick walls (200-600 µm), which in turn contribute to fibrotic tissue growth around and inside the device, compromising the neural interface. We report the use of thiol-ene/acrylate shape memory polymer (SMP) for the fabrication of thin film multi-electrode softening cuffs (MSC). We fabricated multi-size MSC with eight titanium nitride (TiN) electrodes ranging from 1.35 to 13.95 × 10-4 cm2 (1-3 kΩ) and eight smaller gold (Au) electrodes (3.3 × 10-5 cm2; 750 kΩ), that soften at physiological conditions to a modulus of 550 MPa. While the SMP material is not as soft as silicone, the flexural forces of the SMP cuff are about 70-700 times lower in the MSC devices due to the 30 µm thick film compared to the 600 µm thick walls of the silicone cuffs. We demonstrated the efficacy of the MSC to record neural signals from rat sciatic and pelvic nerves (1000 µm and 200 µm diameter, respectively), and the selective fascicular stimulation by current steering. When implanted side-by-side and histologically compared 30 days thereafter, the MSC devices showed significantly less inflammation, indicated by a 70-80% reduction in ED1 positive macrophages, and 54-56% less fibrotic vimentin immunoreactivity. Together, the data supports the use of MSC as compliant and adaptable technology for the interfacing of somatic and autonomic peripheral nerves.

Glial-derived growth factor and pleiotrophin synergistically promote axonal regeneration in critical nerve injuries.

The repair of nerve gap injuries longer than 3 cm is limited by the need to sacrifice donor tissue and the morbidity associated with the autograft gold standard, while decellularized grafts and biodegradable conduits are effective only in short nerve defects. The advantage of isogenic nerve implants seems to be the release of various growth factors by the denervated Schwann cells. We evaluated the effect of vascular endothelial growth factor, neurotrophins, and pleiotrophin (PTN) supplementation of multi-luminal conduits, in the repair of 3 and 4 cm nerve gaps in the rabbit peroneal nerve. In vitro screening revealed a synergistic regenerative effect of PTN with glial-derived neurotrophic factor (GDNF) in promoting sensory axon density, and motor axonal growth from spinal cord explants. In vivo, pleiotrophins were able to support nerve regrowth across a 3 cm gap. In the 4 cm lesions, PTN-GDNF had a modest effect in the number of axons distal to the implant, while increasing the mean axon diameter (1 ±â€¯0.4; p ≤ 0.001) over PTN or GDNF alone (0.80 ±â€¯0.2, 0.84 ±â€¯0.5; respectively). Some regenerated axons reinnervated muscle targets as indicated by neuromuscular junction staining. However, many were wrapped in Remak bundles, suggesting a delay in axonal sorting, explaining the limited electrophysiological function of the reinnervated muscle, and the modest recovery in toe spreading in the PTN-GDNF repaired animals. These results support the use of synergistic neurotrophic/pleiotrophic growth factors in long gap repair and underscore the need for re-myelination strategies distal to the injury site. STATEMENT OF SIGNIFICANCE: Nerve injuries due to trauma or tumor resection often result in long gaps that are challenging to repair. The best clinical option demands the use of autologous grafts that are associated with serious side effects. Bioengineered nerves are considered a good alternative, particularly if supplemented with growth factors, but current options do not match the regenerative capacity of autografts. This study revealed the synergistic effect of neurotrophins and pleiotrophins designed to achieve a broad cellular regenerative effect, and that GDNF-PTN are able to mediated axonal growth and partial functional recovery in a 4 cm nerve gap injury, albeit delays in remyelination. This report underscores the need for defining an optimal growth factor support for biosynthetic nerve implants.

Chronic recording and electrochemical performance of Utah microelectrode arrays implanted in rat motor cortex.

Multisite implantable electrode arrays serve as a tool to understand cortical network connectivity and plasticity. Furthermore, they enable electrical stimulation to drive plasticity, study motor/sensory mapping, or provide network input for controlling brain-computer interfaces. Neurobehavioral rodent models are prevalent in studies of motor cortex injury and recovery as well as restoration of auditory/visual cues due to their relatively low cost and ease of training. Therefore, it is important to understand the chronic performance of relevant electrode arrays in rodent models. In this report, we evaluate the chronic recording and electrochemical performance of 16-channel Utah electrode arrays, the current state-of-the-art in pre-/clinical cortical recording and stimulation, in rat motor cortex over a period of 6 mo. The single-unit active electrode yield decreased from 52.8 ± 10.0 ( week 1) to 13.4 ± 5.1% ( week 24). Similarly, the total number of single units recorded on all electrodes across all arrays decreased from 106 to 15 over the same time period. Parallel measurements of electrochemical impedance spectra and cathodic charge storage capacity exhibited significant changes in electrochemical characteristics consistent with development of electrolyte leakage pathways over time. Additionally, measurements of maximum cathodal potential excursion indicated that only a relatively small fraction of electrodes (10-35% at 1 and 24 wk postimplantation) were capable of delivering relevant currents (20 µA at 4 nC/ph) without exceeding negative or positive electrochemical potential limits. In total, our findings suggest mainly abiotic failure modes, including mechanical wire breakage as well as degradation of conducting and insulating substrates. NEW & NOTEWORTHY Multisite implantable electrode arrays serve as a tool to record cortical network activity and enable electrical stimulation to drive plasticity or provide network feedback. The use of rodent models in these fields is prevalent. We evaluated chronic recording and electrochemical performance of 16-channel Utah electrode arrays in rat motor cortex over a period of 6 mo. We primarily observed abiotic failure modes suggestive of mechanical wire breakage and/or degradation of insulation.

Amorphous silicon carbide ultramicroelectrode arrays for neural stimulation and recording.

OBJECTIVE: Foreign body response to indwelling cortical microelectrodes limits the reliability of neural stimulation and recording, particularly for extended chronic applications in behaving animals. The extent to which this response compromises the chronic stability of neural devices depends on many factors including the materials used in the electrode construction, the size, and geometry of the indwelling structure. Here, we report on the development of microelectrode arrays (MEAs) based on amorphous silicon carbide (a-SiC). APPROACH: This technology utilizes a-SiC for its chronic stability and employs semiconductor manufacturing processes to create MEAs with small shank dimensions. The a-SiC films were deposited by plasma enhanced chemical vapor deposition and patterned by thin-film photolithographic techniques. To improve stimulation and recording capabilities with small contact areas, we investigated low impedance coatings on the electrode sites. The assembled devices were characterized in phosphate buffered saline for their electrochemical properties. MAIN RESULTS: MEAs utilizing a-SiC as both the primary structural element and encapsulation were fabricated successfully. These a-SiC MEAs had 16 penetrating shanks. Each shank has a cross-sectional area less than 60 µm2 and electrode sites with a geometric surface area varying from 20 to 200 µm2. Electrode coatings of TiN and SIROF reduced 1 kHz electrode impedance to less than 100 kΩ from ~2.8 MΩ for 100 µm2 Au electrode sites and increased the charge injection capacities to values greater than 3 mC cm-2. Finally, we demonstrated functionality by recording neural activity from basal ganglia nucleus of Zebra Finches and motor cortex of rat. SIGNIFICANCE: The a-SiC MEAs provide a significant advancement in the development of microelectrodes that over the years has relied on silicon platforms for device manufacture. These flexible a-SiC MEAs have the potential for decreased tissue damage and reduced foreign body response. The technique is promising and has potential for clinical translation and large scale manufacturing.

A Sub-millimeter, Inductively Powered Neural Stimulator.

Wireless neural stimulators are being developed to address problems associated with traditional lead-based implants. However, designing wireless stimulators on the sub-millimeter scale (<1 mm3) is challenging. As device size shrinks, it becomes difficult to deliver sufficient wireless power to operate the device. Here, we present a sub-millimeter, inductively powered neural stimulator consisting only of a coil to receive power, a capacitor to tune the resonant frequency of the receiver, and a diode to rectify the radio-frequency signal to produce neural excitation. By replacing any complex receiver circuitry with a simple rectifier, we have reduced the required voltage levels that are needed to operate the device from 0.5 to 1 V (e.g., for CMOS) to ~0.25-0.5 V. This reduced voltage allows the use of smaller receive antennas for power, resulting in a device volume of 0.3-0.5 mm3. The device was encapsulated in epoxy, and successfully passed accelerated lifetime tests in 80°C saline for 2 weeks. We demonstrate a basic proof-of-concept using stimulation with tens of microamps of current delivered to the sciatic nerve in rat to produce a motor response.

Asymmetric Sensory-Motor Regeneration of Transected Peripheral Nerves Using Molecular Guidance Cues.

Neural interfaces are designed to decode motor intent and evoke sensory precepts in amputees. In peripheral nerves, recording movement intent is challenging because motor axons are only a small fraction compared to sensory fibers and are heterogeneously mixed particularly at proximal levels. We previously reported that pain and myelinated axons regenerating through a Y-shaped nerve guide with sealed ends, can be modulated by luminar release of nerve growth factor (NGF) and neurotrophin-3 (NT-3), respectively. Here, we evaluate the differential potency of NGF, glial cell line-derived neurotrophic factor (GDNF), brain-derived neurotrophic factor (BDNF), pleiotrophin (PTN), and NT-3 in asymmetrically guiding the regeneration of sensory and motor neurons. We report that, in the absence of distal target organs, molecular guidance cues can mediate the growth of electrically conductive fascicles with normal microanatomy. Compared to Y-tube compartments with bovine serum albumin (BSA), GDNF and NGF increased the motor and sensory axon content, respectively. In addition, the sensory to motor ratio was significantly increased by PTN (12.7:1) when compared to a BDNF + GDNF choice. The differential content of motor and sensory axons modulated by selective guidance cues may provide a strategy to better define axon types in peripheral nerve interfaces.

Chronic and low charge injection wireless intraneural stimulation in vivo.

Functional stability and in-vivo reliability are significant factors determining the longevity of a neural interface. In this ongoing study, we test the performance of a wireless floating microelectrode array (WFMA) over a period of 143 days. The topography of the microelectrodes has allowed for selective stimulation of different fascicles of the rat sciatic nerve. We confirmed that motor evoked thresholds remain stable over time and that the nerve stimulation charges were within tissue safety limits. Importantly, motor evoked responses were elicited at threshold currents in fully awake animals without causing pain or discomfort. These data validate the use of the WFMA system for intraneural interfacing of peripheral nerves for neuroprosthetic and bioelectronics medical applications.

Chronic in-vivo testing of a 16-channel implantable wireless neural stimulator.

Here, we report on chronic in-vivo testing of a 16-channel wireless floating microelectrode array (WFMA) in a rat sciatic nerve model. Muscle threshold currents, charge injection levels, and charge density were monitored for electrodes of two WFMA devices implanted into animal subjects over a five month period. This type of wireless stimulation device could eliminate problems associated with percutaneous connectors for a variety of neural prostheses and other medical devices.