Advanced age is not a barrier to chronic intracortical single-unit recording in rat cortex.

Introduction: Available evidence suggests that as we age, our brain and immune system undergo changes that increase our susceptibility to injury, inflammation, and neurodegeneration. Since a significant portion of the potential patients treated with a microelectrode-based implant may be older, it is important to understand the recording performance of such devices in an aged population. Methods: We studied the chronic recording performance and the foreign body response (FBR) to a clinically used microelectrode array implanted in the cortex of 18-month-old Sprague Dawley rats. Results and discussion: To the best of our knowledge, this is the first preclinical study of its type in the older mammalian brain. Here, we show that single-unit recording performance was initially robust then gradually declined over a 12-week period, similar to what has been previously reported using younger adult rats and in clinical trials. In addition, we show that FBR biomarker distribution was similar to what has been previously described for younger adult rats implanted with multi-shank recording arrays in the motor cortex. Using a quantitative immunohistochemcal approach, we observed that the extent of astrogliosis and tissue loss near the recording zone was inversely related to recording performance. A comparison of recording performance with a younger cohort supports the notion that aging, in and of itself, is not a limiting factor for the clinical use of penetrating microelectrode recording arrays for the treatment of certain CNS disorders.

Unprotected sidewalls of implantable silicon-based neural probes and conformal coating as a solution.

Silicon-based implantable neural devices have great translational potential as a means to deliver various treatments for neurological disorders. However, they are currently held back by uncertain longevity following chronic exposure to body fluids. Conventional deposition techniques cover only the horizontal surfaces which contain active electronics, electrode sites, and conducting traces. As a result, a vast majority of today's silicon devices leave their vertical sidewalls exposed without protection. In this work, we investigated two batch-process silicon dioxide deposition methods separately and in combination: atomic layer deposition and inductively-coupled plasma chemical vapor deposition. We then utilized a rapid soak test involving potassium hydroxide to evaluate the coverage quality of each protection strategy. Focused ion beam cross sectioning, scanning electron microscopy, and 3D extrapolation enabled us to characterize and quantify the effectiveness of the deposition methods. Results showed that bare silicon sidewalls suffered the most dissolution whereas ALD silicon dioxide provided the best protection, demonstrating its effectiveness as a promising batch process technique to mitigate silicon sidewall corrosion in chronic applications.

Fabrication and modeling of recessed traces for silicon-based neural microelectrodes.

OBJECTIVE: Chronically-implanted neural microelectrodes are powerful tools for neuroscience research and emerging clinical applications, but their usefulness is limited by their tendency to fail after months in vivo. One failure mode is the degradation of insulation materials that protect the conductive traces from the saline environment. APPROACH: Studies have shown that material degradation is accelerated by mechanical stresses, which tend to concentrate on raised topographies such as conducting traces. Therefore, to avoid raised topographies, we developed a fabrication technique that recesses (buries) the traces in dry-etched, self-aligned trenches. MAIN RESULTS: The fabrication technique produced flatness within approximately 15 nm. Finite element modeling showed that the recessed geometry would be expected to reduce intrinsic stress concentrations in the insulation layers. Finally, in vitro electrochemical tests confirmed that recessed traces had robust recording and stimulation capabilities that were comparable to an established non-recessed device design. SIGNIFICANCE: Our recessed trace fabrication technique requires no extra masks, is easy to integrate with existing processes, and is likely to improve the long-term performance of implantable neural devices.

Gelatin embedding and LED autofluorescence reduction for rodent spinal cord histology.

We present two innovations in histological technique for rodent spinal cord: gelatin embedding and LED photobleaching. Gelatin embedding uses liquid gelatin solution to permeate delicate biological structures then solidify to provide mechanical support throughout dissection, vibratome sectioning, and staining. LED photobleaching uses high-intensity visible light during blocking and primary incubations to reduce autofluorescence in tissue sections before fluorescent secondaries are added. We found gelatin embedding improved mechanical stability without interfering with immunohistochemical staining. Gelatin embedding also preserved some spinal roots and provided an opportunity for dye-less and cut-less tracking of left/right orientation during free-floating staining, which is valuable for tissue samples that have no spare areas that can be marked. LED photobleaching greatly reduced autofluorescence and added essentially no extra time or labor to the process. Descriptions of the techniques and characterization data are provided.

3D Reconstruction of the Intracortical Volume Around a Hybrid Microelectrode Array.

Extensive research using penetrating electrodes implanted in the central and peripheral nervous systems has been performed for many decades with significant advances made in recent years. While penetrating devices provide proximity to individual neurons in vivo, they suffer from declining performance over the course of months and often fail within a year. 2D histology studies using serial tissue sections have been extremely insightful in identifying and quantifying factors such as astroglial scar formation and neuronal death around the implant sites that may be contributing to failures. However, 2D histology has limitations in providing a holistic picture of the problems occurring at the electrode-tissue interface and struggles to analyze tissue below the electrode tips where the electrode tracks are no longer visible. In this study, we present 3D reconstruction of serial sections to overcome the limitations of 2D histological analysis. We used a cohort of software: XuvStitch, AutoAligner, and Imaris coupled with custom MATLAB programming to correct warping effects. Once the 3D image volume was reconstructed, we were able to use Imaris to quantify neuronal densities around the electrode tips of a hybrid microelectrode array incorporating Blackrock, Microprobes, and NeuroNexus electrodes in the same implant. This paper presents proof-of-concept and detailed methodological description of a technique which can be used to quantify neuronal densities in future studies of implanted electrodes.

Recessed Traces for Planarized Passivation of Chronic Neural Microelectrodes.

Implantable microfabricated neural electrodes have numerous neuroscientific research and clinical applications. However, these devices are prone to failure after several months in vivo. One mechanism is failure of passivation layers followed by corrosion of metal traces in the saline environment. It has been suggested that mechanical stress accelerates passivation layer failure and that stress is concentrated whenever passivation layers have a non-planar topography. Therefore, we developed a simple process for recessing metal traces within the substrate so that overlying passivation layers are planar. The process requires no extra masks and no post-passivation planarization steps.

BBB leakage, astrogliosis, and tissue loss correlate with silicon microelectrode array recording performance.

The clinical usefulness of brain machine interfaces that employ penetrating silicon microelectrode arrays is limited by inconsistent performance at chronic time points. While it is widely believed that elements of the foreign body response (FBR) contribute to inconsistent single unit recording performance, the relationships between the FBR and recording performance have not been well established. To address this shortfall, we implanted 4X4 Utah Electrode Arrays into the cortex of 28 young adult rats, acquired electrophysiological recordings weekly for up to 12 weeks, used quantitative immunohistochemical methods to examine the intensity and spatial distribution of neural and FBR biomarkers, and examined whether relationships existed between biomarker distribution and recording performance. We observed that the FBR was characterized by persistent inflammation and consisted of typical biomarkers, including presumptive activated macrophages and activated microglia, astrogliosis, and plasma proteins indicative of blood-brain-barrier disruption, as well as general decreases in neuronal process distribution. However, unlike what has been described for recording electrodes that create only a single penetrating injury, substantial brain tissue loss generally in the shape of a pyramidal lesion cavity was observed at the implantation site. Such lesions were also observed in stab wounded animals indicating that the damage was caused by vascular disruption at the time of implantation. Using statistical approaches, we found that blood-brain barrier leakiness and astrogliosis were both associated with reduced recording performance, and that tissue loss was negatively correlated with recording performance. Taken together, our data suggest that a reduction of vascular damage at the time of implantation either by design changes or use of hemostatic coatings coupled to a reduction of chronic inflammatory sequela will likely improve the recording performance of high density intracortical silicon microelectrode arrays over long indwelling periods and lead to enhanced clinical use of this promising technology.

Acute monitoring of genitourinary function using intrafascicular electrodes: selective pudendal nerve activity corresponding to bladder filling, bladder fullness, and genital stimulation.

OBJECTIVE: To investigate the use of a microelectrode array with a high spatial density of penetrating intrafascicular electrodes for selective recording of pudendal nerve activity evoked by a variety of genitourinary stimuli. MATERIALS AND METHODS: Felines were anesthetized with alpha-chloralose and high-density Utah slanted electrode arrays (48 microelectrodes; 200-µm spacing) were implanted into the pudendal nerve for acute experimentation. Neural activity was recorded during bladder filling, spontaneous reflexive distention-evoked bladder contractions, and tactile somatosensory stimulation. RESULTS: The intrafascicularly implanted pudendal nerve electrodes were able to selectively record neural activity that corresponded to various genitourinary stimuli. Across all seven experimental animals, a total of 10 microelectrodes recorded neural units that were selectively driven by bladder filling or distention-evoked bladder contractions. Twenty-two electrodes were selectively driven by tactile stimulation. CONCLUSION: Microelectrode arrays implanted intrafascicularly into the pudendal nerve can be used to selectively record the neural responses that reflect bladder status and urogenital tactile stimulation. This work sets the stage for developing future implantable closed-loop neuroprosthetic devices for restoration of bladder function.