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The brain is a massively-interconnected and constantly-evolving network of specialized circuits; a systematic understanding of the circuits requires a probe-tissue interface that can record and modulate brain activities at diverse spatial and temporal scales. Implanted electrodes provide a unique approach to decipher brain circuitry by allowing for time-resolved electrical detection of individual neuron activity. However, conventional intracortical recordings are often sparse, and importantly, unstable over long term. These pose limitations on their scientific and clinical applications. Through the use of less-rigid polymer materials and the ten-to-a-hundred fold reduction on probe size, the flexibility of a neural probe can be improved by four orders of magnitude, which results in a friendly probe-tissue interfaces, long-term recording performance, and greater potential for scaling up in implantation density. Here we present our progress on the development of a novel neural recording platform including 1. A stable neural interface named nanoelectronic threads (NETs) which demonstrated long-term stable recording over four months, seamless chronic probe-tissue integration and easy compatibility with optical imaging. 2. A facile implantation method to apply the flexible NETs in scalable, reliable neural recording in rodent brain. 3. A dense and high-bandwidth NET platform towards volumetric mapping and large-scale distributed recordings in the neocortex and subcortical structures, and 4. A low-cost and versatile multifunctional neural probe platform to achieve optogenetic stimulation and controlled drug infusion with simultaneous, spatially resolved neural recording. These capabilities will drive new long-term studies of brain circuits across different spatiotemporal dimensions and modalities
Interest in restoring lost function using neuro-prosthetic devices and treating neurological disorders or neurodegenerative diseases through electrical stimulation of neural activity has increased in recent years. For example, implantable cortical neural interfaces allow investigation of sensorimotor learning, and control of both natural and prosthetic limbs through recording of volitional intent and stimulation of neural activity. However, these interfaces decline rapidly in performance over chronic timescales. Foreign body response is believed to limit their recording and stimulation reliability. The resulting glial scar isolates the indwelling microelectrodes from healthy neuronal cells. The consequence is recording from large populations of weak neural signals and the requirement for high current amplitudes to deliver the necessary charge for neural activation. Recently, carbon fiber microelectrodes with small cross-sectional dimensions (below 10 μm) have been shown to reduce insertion damage to neurons and microvasculature, minimize adverse tissue reaction, and provide stable neural recording over chronic timescales. Despite these achievements, the development of carbon fiber MEAs faces the issue of micro-assembly, micromanipulation, and the general lack of control of the geometric surface area (GSA) of the active sites. This dissertation addresses these issues by developing ultrathin cellular or sub-cellular scale microelectrode arrays (MEAs) based on amorphous silicon carbide. Amorphous silicon carbide (a-SiC) deposited by plasma enhanced chemical vapor deposition has similar mechanical properties to carbon fiber but is amenable to thin-film microfabrication methods, thus permitting a wide variety of designs, control of GSA, and batch fabrication of microelectrode arrays. Challenges associated with residual stress control in the a-SiC and those associated with metal patterning needs to be addressed to use the a-SiC in ultrathin MEA designs. Also, implantation strategies for ultrathin MEA shanks and the burden of using small contact sites for electrochemical measurement, electrical stimulation and electrophysiology need to be addressed. In this dissertation, microelectrode arrays based on a-SiC were fabricated, characterized for their electrochemical properties in a saline model of the interstitial fluid, and evaluated functionally in songbird and rat brain. We describe stress engineering in the multilayered structure to regulate the curvature of the a-SiC MEAs. Engineering challenges associated with process controls to produce penetrating probes of reduced cross-sectional shank dimensions are discussed. We have developed implantation strategies to insert ultrathin a-SiC MEAs into rat motor cortex. We show that a minimum a-SiC thickness of 6 μm is required to insert 2 mm long a-SiC MEAs shanks into rat cortex without the need for insertion guides or temporary support structures. Below this thickness, we demonstrate that a-SiC MEAs will require temporary support structures such as polyethylene glycol or in situ designs that increases the critical buckling load of the implanted shanks. With the reduced shank dimensions, the electrode sites on the a-SiC MEA are small with high electrode impedance and low charge injection properties. We investigated low impedance coatings such as titanium nitride, sputtered iridium oxide and electrodeposited iridium oxide films as a means of improving the electrochemical performance for neural stimulation and recording. We show that cathodal charge injection capacities greater than 17 mC/cm2 can be achieved with the coated ultramicroelectrode site with appropriate biasing.
Flexible electrode arrays are a crucial technology for neural interfaces. In this thesis, the continued development of a flexible based electrode array is discussed. Two types of strain relief are introduced into the leads to allow the base to deform like the spinal cord in a repeatable manner. The resulting sandwich and embedded lead configurations were characterized using a tensile tester. In addition, the base material was characterized after exposure to physiological solutions and a sterilization protocol. Results show that the sandwich lead configuration had properties that were better matched to those of the spinal cord tissue. Parametric finite element modelling was also performed to identify the effect of the base modulus after varying several properties associated with the base. The results of the modelling would help in the long term manufacturing of these flexible based electrode arrays.
This paper describes the development of microelectrodes with integrated three-dimensional electrode structures. The integration of three-dimensional structures aims at an improvement of the electrode/tissue interface. Due to the increase in surface area the electrode impedance is reduced, while the density of microelectrodes per area remains the same as with flat electrodes. Two different types of electrodes have been developed: Flexible, implantable microelectrodes with pyramidal, protruding structures and tip-shaped electrode arrays on glass substrates. The protrusion heights of the electrode sites can easily be adjusted depending on the actual application. For the flexible structures we used a polyimide- based process to fabricate microelectrodes with sharp or flat pyramidal tips and with electrode arrangements on front and backside of the devices. The tip-shaped electrode arrays were fabricated from a glass substrate by isotropic wet chemical etching and subsequent metallization and passivation. Data from impedance measurements and acute brain slice recordings indicate a considerable improvement regarding electrode impedance and obtainable signal strength. Is - Three-dimensional, Electrode array, Microelectrodes, Polyimide, Class.
Recent commercialization efforts made in the fields of flexible sensors, low-energy Bluetooth transmitters, and low-power thin-film electronics have contributed to significant fast-paced growth in the smart wearable industry. This drastic paradigm shift in the flexible electronics component design has fuelled an evolution in the flexible personal electronics, biomedical, athletics, and logistics industries as more flexible, thin-film products are offered. Flexible thin-film electrochemical capacitors (EC) or supercapacitors are energy storage solutions that offer both high energy and power densities resulting from the exceptional high electrode specific surface area and appropriately tuned electrode/electrolyte interface. To meet both the electrochemical and mechanical requirements, several aspects of the electrode design was to be considered in the proposed flexible EC devices: a) Areal- and gravimetric specific capacitance; b) Charge-discharge cycling properties; c) Mechanical bending and flexing behaviours; and d) Environmental stability. In this work, novel facile techniques in fabricating flexible EC electrodes with micro- and nano-structured surface modification have been proposed to produce high-performing flexible EC electrodes with additional intrinsic multi-functionalities, such as piezoresistive sensors and breathing sensors, along with energy storage. Herein, to create facile pathways for fabricating flexible, high-performance EC electrodes, this thesis has been divided into the following studies: The first study focuses on the design of a novel environmental-controlled self-assembly method, where polyaniline (PAni) nanorod structures were grown on polyacrylonitrile (PAN) nanofibers for high-surface-area textile electrochemical capacitor electrodes with intrinsic piezoresistive tactile sensing capabilities. The second study relied on a novel laser-assisted photochemical reduction method to produce reduced graphene oxide micro-ribbon textile electrode directly on a liquid surface, which can be transfer printed onto any substrate for both supercapacitors and breath sensor applications. The third study validated a method for creating hierarchically structured multilayer reduced graphene oxide for flexible intercalated supercapacitor electrodes where simultaneous reduction and nitrogen-doping were successfully achieved. The last study explored the possibility of creating a multi-layered flexible polypyrrole micro-foam/carbon nanotube composite electrodes for flexible supercapacitor devices via the creation of three-dimensional polypyrrole microsphere architecture.
Neuroprosthetic devices are widely employed in clinical and research settings. However, most of these devices suffer from diminishing device performance over time, likely due to the reactive tissue response of the central nervous system and scarring to biological tissue from the implantation procedure. In an effort to minimize acute tissue trauma while maintaining high spatial resolution, this thesis presents a novel miniaturized neural implant device capable of recording high-density neural signals from cortical tissue with penetrating silicon microwire sensing elements on a flexible conformable substrate. In addition to reduced tissue scarring, it is possible to improve on current commercially available neural probe designs by reducing the mechanical property mismatch between the electrode interface and neural tissue. Neural electrodes fabricated on flexible substrates have received an increase in attention in recent years; however, these designs offer moderate spatial resolution because signals are typically recorded from the surface of neural tissue in thin film style electrode designs. The MEA presented in this thesis is fabricated by a hybrid integration technology that takes advantage of both rigid penetrating pillars, similar yet smaller than commercially available technology, and is fabricated on thin film polyimide substrates. The thin film polyimide substrate allows for the electrode to make intimate contact with surrounding cortical tissue around sulci and gyri of the brain, and the silicon microwire sensing elements, which penetrate into the cortex, enable this MEA to have the potential for the best spatial resolution. The impedance of these MEAs was characterized and found to be in the range of a few hundred kilohms making them suitable for both local field potentials and single unit recordings.
Implanted neural electrodes require high stiffness to ensure the insertion into the target tissue and to provide connections to multichannel contacts. However, recent studies suggest that the stiffness of the implanted device can significantly influenced the tissue response, and subsequently the chronic performance of the electrode. Current methods to improve flexibility methods include adding biodegradable or biomimetic substrate materials. But adding those materials also raise issues such as remaining material following biodegradation or increasing the electrode neural distance. The goal of thesis is to design a flexible electrode whose dimension and flexibility should be similar to that of a single axon, as well as an effective insertion method that can minimize the neural trauma. To achieve the goal, carbon nanotube yarns were chosen for their high flexibility and biocompatibility. The flexible carbon nanotube yarns were combined with a removable stiff needle such as a tungsten needle for the insertion. The diameter (10μm)and flexural rigidity of the designed neural interface (3.3×10−12N·m2) are similar to that of a single axon. This neural interface was implanted into the rat sciatic nerve and could record neural activity for long periods of time. This novel device provides the basis for a platform technology for long term recording of neural signals in small nerves such as those of the autonomic nervous system which so far has not been achieved.