Browsing by Author "Cogan, Stuart F."
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Item Amorphous Silicon Carbide Ultramicroelectrode Arrays for Neural Stimulation and Recording(2018-10-22) Deku, Felix; Cohen, Yarden; Joshi-Imre, Alexandra; Kanneganti, Aswini; Gardner, Timothy J.; Cogan, Stuart F.; 0000-0002-4915-1200 (Deku, F)); 0000-0002-8149-6954 (Cohen, Y); Deku, Felix; Cohen, Yarden; Joshi-Imre, Alexandra; Kanneganti, Aswini; Cogan, Stuart F.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 m² and electrode sites with a geometric surface area varying from 20 to 200 m². Electrode coatings of TiN and SIROF reduced 1 kHz electrode impedance to less than 100 kΩ from ~2.8 MΩ for 100 m² 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.Item Amorphous Silicon Carbide-based Intraneural Ultramicroelectrode Array for Selective Interfacing to the Rat Cervical Vagus Nerve(2020-07-16) Ghazavi, Atefeh; Cogan, Stuart F.With the ever-increasing applications of neuroprosthetics, improving their effectiveness and reducing their side effects are essential. This requires developing more selective and less invasive interfaces. The aim of this dissertation was to improve the selectivity and reduce the invasiveness of current peripheral nerve interfaces by reducing the electrode dimensions and array shafts. To that end, a 16-channel intraneural electrode array incorporating small electrodes and small cross-sectional area shafts was developed. Sputtered iridium oxide (SIROF) electrodes as small as 20µm2 are feasible to be used for neural recording and evoking functional responses in neural stimulation based on their electrochemical properties in a model of interstitial fluid. The unusual behavior attributed to ultramicroelectrodes (UMEs) was observed for SIROF electrodes smaller than 200µm2 in ferrocene and PBS electrolytes. This behavior is due to hemispherical diffusion of electroactive species to the electrode. In order to evaluate the effect of large current densities in UMEs on electrode degradation, the SIROF UMEs were subjected to continuous current pulses. After exposing to 1.7 billion pulses the electrochemical properties of SIROF UMEs (50µm2 ) remained stable and no effect of degradation was observed. While having smaller cross-sectional area shafts decrease the invasiveness and increases the flexibility of neural probes, it complicates the insertion. To facilitate the implantation and improve the nerve stability during acute surgeries, a polymeric structure named Neurocase using thin film technology was developed. Device’s functional selectivity was demonstrated by performing acute recordings from rat cervical vagus nerve (cVN) which is a multifunctional nerve. Different electrodes on the array recorded distinct activity upon evoking neural activity in the cVN by manipulating blood pressure, inducing oxygen restriction, and electrical stimulation of the subdiaphragmatic vagus nerve. As complete control over the nerve functionality is only possible through both stimulation and blocking, an equation for determining the electrode polarization during kilohertz frequency alternating current (KHFAC) nerve conduction blocking was derived. The effect of DCfiltering, electrode material, and stimulation parameters on the electrode polarization was investigated. It was observed that the choice of electrode material (platinum, titanium nitride or SIROF) did not affect the electrode polarization at frequencies higher than 10 kHz and the DC filter had a significant impact on reducing the electrode polarization.Item Electrodeposited Iridium Oxide on Carbon Fiber Ultramicroelectrodes for Neural Recording and Stimulation(Electrochemical Society Inc.) Deku, Felix; Joshi-Imre, Alexandra; Mertiri, A.; Gardner, T. J.; Cogan, Stuart F.; 0000-0002-4915-1200 (Deku, F); 43420545 (Cogan, SF); Deku, Felix; Joshi-Imre, Alexandra; Cogan, Stuart F.Host encapsulation decreases the ability of chronically implanted microelectrodes to record or stimulate neural activity. The degree of foreign body response is thought to depend strongly on the cross-sectional dimensions of the electrode shaft penetrating neural tissue. Microelectrodes with cellular or sub-cellular scale shaft cross-sectional dimensions, such as carbon fiber ultramicroelectrodes have been previously demonstrated to elicit minimal tissue response, but their small geometric surface area results in high electrode impedances for neural recording, and reduced charge injection capacity during current pulsing for neural stimulation. We investigated electrodeposited iridium oxide films (EIROF) on carbon fiber ultramicroelectrodes as a means of enhancing the charge injection capacity and reducing electrode impedance. EIROF coatings reduced the electrode impedance measured at 1 kHz by a factor of 10 and improved charge storage and charge injection capacities. The maximum charge injection capacity was also strongly dependent on the interpulse bias and pulse width, and reflected a potential-dependent EIROF impedance. The charge injection capacity of the EIROF-coated carbon fiber ultramicroelectrodes measured in an inorganic buffered saline model of interstitial fluid exceeded 17 mC/cm2 with appropriate biasing, allowing charge-injection at levels well above reported charge/phase thresholds for intraneural microstimulation.Item Evaluation of Local Field Potentials and Inflammatory Response to Chronic Microelectrode Arrays in Rat Motor Cortex(2018-08-30) Shih, Ellen; Cogan, Stuart F.Neural interface devices are being developed for applications encompassing communication interfaces between prosthetics and patients and investigative tools for understanding complex neural circuitry. This work investigates encapsulation materials and strategies for chronic recording of neural electrical signals for intracortical electrodes. These devices could be used for brain-computer interfacing in applications related to the recording of volitional intent in conditions such as brainstem stroke, spinal cord injury, and locked-in syndrome. Intracortical microelectrode arrays (MEAs), such as the Utah-style electrode array (UEA) which is currently in clinical trials for neural recording in brain-computer interfacing, suffer from a lack of chronic reliability. A number of abiotic and biotic factors have been identified as contributors to the decline in performance. The primary biotic mechanism for loss of device performance is associated with the inflammatory response that follows implantation and chronic residence in the brain parenchyma. This foreign body response is characterized by glial scarring, loss of viable neurons, and persistent astrogliosis. A significant abiotic failure mechanism involves loss of integrity of polymer encapsulation coatings that may delaminate or become ineffective as barrier coatings resulting in parasitic electrical leakage pathways and corrosion. How adverse tissue reaction and material failure in MEAs interact and affect device performance is yet to be fully understood. This thesis investigates two elements of the chronic performance of neural interfaces: 1) the use of local field potentials (LFPs) as an alternative to single-units as a quantification of recording performance of cortical interfaces, and 2) the adverse foreign body response to amorphous silicon carbide (a-SiC), as an alternative encapsulation material for intracortical devices. The performance of neural electrodes is typically quantified by the capability of the device to measure neuron single-unit activity. However, single-unit activity is challenging to use as a volitional control signal due to an observed variability of recorded action potentials at electrodes during chronic studies. It is known that LFPs represent the sum of the low frequency (<300 Hz) electrical activity surrounding an electrode, and have drawn interest as a signal for brain-computer interface control. However, the long-term stability of LFPs is less well-established. We describe a method of evaluating the trends in LFPs over time and show they reflect the decline in performance as shown by single-unit activity. We identify a time window in which the decline is most prominent, which also correlates with changes in the longitudinal electrochemical properties of the recording electrodes measured in vivo in the same animal preparations. Towards the second goal, we aim to minimize the immune response to intracortical devices. It is known that some encapsulation materials for intracortical devices on the market are not optimal. For example, the current Utah-style MEAs employ Parylene-C, a poly(xylylene) polymer, as an encapsulation material.. This material has been documented to delaminate and therefore result in leakage and shunting of current, reduced signal-to-noise ratio of neural data, and corrosion. Additionally, biocompatibility of the encapsulation influences the extent of the foreign body response. Amorphous SiC is a material with several desirable electrical and material properties as an encapsulation for implanted MEAs, including high electrical resistivity, a low bioreactivity, and extremely low dissolution rate. We compare the foreign body response to a-SiC and Parylene-C encapsulated arrays implanted in rat cortical tissue through progressive histochemical analysis. Our results show that Parylene-C shows a reduced inflammatory response compared to a-SiC or bare Si over a period of 120 days as measured by the spatial distribution of reactive astrocytes, microglial and neurons around implanted electrodes. This thesis discusses alternative methods of evaluating cortical electrode performance and offers insight into a different encapsulation material.Item Silicon Nanocrystals and Defect States in Silicon Rich Silicon Nitride for Optoelectronic Applications(2016-12) Mohammed, Shakil; Cogan, Stuart F.Research interest in silicon nanocrystals (Si-NC) has increased significantly as a result of the desire to improve the light emission efficiency of bulk silicon. Si-NCs embedded in silicon nitride have desirable characteristics for optoelectronic applications since they can increase the tunneling probability and have a lower tunneling barrier than silicon oxide. Higher tunneling probability is an important feature as it can be used to develop more efficient electroluminescent and photovoltaic devices. In this dissertation, the Si-rich Si3N4 (SRN) was prepared using low pressure chemical vapor deposition (LPCVD) and RF sputtering followed by high temperature treatment in order to precipitate Si-NCs within the silicon nitride matrix. Several different characterization techniques were used on the Si-NC samples in order to understand the physical, structural, optical and electrical behavior of the nanocrystals. Characterization techniques used in this analysis included photoluminescence (PL), time resolved PL, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, transmission electron microscopy, ellipsometry and capacitance-voltage (C-V) measurements. Silicon nitride was found to contain a high defect density which suppressed the PL effect from the Si-NC. The PL observed from each different SRN sample correlated to defect states, namely dangling bonds and oxygen related bonding. Although substantial evidence suggested that Si-NC had formed within the SRN sample, a PL effect due to the quantum confinement effect (QCE) from the nanocrystals could not be detected. However, Si rich SiOx samples exhibited excellent PL which correlated with the QCE for an indirect bandgap semiconductor. Further experiments were conducted using forming gas in order to passivate the defects in the SRN. Though significant changes in PL was not achieved due to passivation, the electrical behavior from the SRN indicated that the intrinsically charged defects may have been passivated.Item Stability of Softening Neural Interfaces With a-SIC Thin Film Interlayer(2021-12-01T06:00:00.000Z) Duran Martinez, Adriana C; Voit, Walter E.; Cogan, Stuart F.; Hsu, Julia; Di Prima, Matthew; Joshi-Imre, Alexandra; Prasad, ShaliniNeural interfaces are implantable devices that enable communication between a computer and nervous tissue to read, write and block neural activity within targeted nerves. To improve the chronic use of neural interfaces, the materials used to develop them have been evolving with time, leading to softer and thinner layers of the involved materials to minimize the foreign body response from the body caused by the implanted device. Recently, researchers have studied many biocompatible polymers that promise to extend the lifetime of neural interfaces. An emerging materials class of interest, softening polymers (SPs), has performance advantages (while stiff and rigid) similar to Parylene-C and Polyimide during fabrication, handling, and insertion, but after softening (e.g. once implanted into the body), this class of polymers demonstrates enhanced conformability. This dissertation work (1) describes the flexibility and performance as an insulator of thiol-ene based softening polymers, (2) details a fabrication process of SP-based devices integrating amorphous silicon carbide (a-SiC) as an encapsulation layer and (3) elucidates structure-property-processing relationships of a-SiC SP neural interfaces via long-term electrical stability after accelerated aging and cyclic bending for future use in chronic animal studies.Item Thinking Small: Progress on Microscale Neurostimulation Technology(Wiley, 2018-10-22) Pancrazio, Joseph J.; Deku, Felix; Ghazavi, Atefeh; Stiller, Allison M.; Rihani, Rashed; Frewin, Christopher L.; Varner, Victor D.; Gardner, Timothy J.; Cogan, Stuart F.; 0000 0000 2895 2047 (Cogan, SF); 43420545 (Cogan, SF); Pancrazio, Joseph J.; Deku, Felix; Ghazavi, Atefeh; Stiller, Allison M.; Rihani, Rashed; Frewin, Christopher L.; Varner, Victor D.; Gardner, Timothy J.; Cogan, Stuart F.Objectives: Neural stimulation is well-accepted as an effective therapy for a wide range of neurological disorders. While the scale of clinical devices is relatively large, translational, and pilot clinical applications are underway for microelectrode-based systems. Microelectrodes have the advantage of stimulating a relatively small tissue volume which may improve selectivity of therapeutic stimuli. Current microelectrode technology is associated with chronic tissue response which limits utility of these devices for neural recording and stimulation. One approach for addressing the tissue response problem may be to reduce physical dimensions of the device. "Thinking small" is a trend for the electronics industry, and for implantable neural interfaces, the result may be a device that can evade the foreign body response. Materials and Methods: This review paper surveys our current understanding pertaining to the relationship between implant size and tissue response and the state-of-the-art in ultrasmall microelectrodes. A comprehensive literature search was performed using PubMed, Web of Science (Clarivate Analytics), and Google Scholar. Results: The literature review shows recent efforts to create microelectrodes that are extremely thin appear to reduce or even eliminate the chronic tissue response. With high charge capacity coatings, ultramicroelectrodes fabricated from emerging polymers, and amorphous silicon carbide appear promising for neurostimulation applications. Conclusion: We envision the emergence of robust and manufacturable ultramicroelectrodes that leverage advanced materials where the small cross-sectional geometry enables compliance within tissue. Nevertheless, future testing under in vivo conditions is particularly important for assessing the stability of thin film devices under chronic stimulation.