Dual-side and three-dimensional microelectrode arrays fabricated from ultra-thin silicon substrates

A method for fabricating planar implantable microelectrode arrays was demonstrated using a process that relied on ultra-thin silicon substrates, which ranged in thickness from 25 to 50 µm. The challenge of handling these fragile materials was met via a temporary substrate support mechanism. In order to compensate for putative electrical shielding of extracellular neuronal fields, separately addressable electrode arrays were defined on each side of the silicon device. Deep reactive ion etching was employed to create sharp implantable shafts with lengths of up to 5 mm. The devices were flip-chip bonded onto printed circuit boards (PCBs) by means of an anisotropic conductive adhesive film. This scalable assembly technique enabled three-dimensional (3D) integration through formation of stacks of multiple silicon and PCB layers. Simulations and measurements of microelectrode noise appear to suggest that low impedance surfaces, which could be formed by electrodeposition of gold or other materials, are required to ensure an optimal signal-to-noise ratio as well a low level of interchannel crosstalk

Development of a Three Dimensional Neural Sensing Device by a Stacking Method

This study reports a new stacking method for assembling a 3-D microprobe array. To date, 3-D array structures have usually been assembled with vertical spacers, snap fasteners and a supporting platform. Such methods have achieved 3-D structures but suffer from complex assembly steps, vertical interconnection for 3-D signal transmission, low structure strength and large implantable opening. By applying the proposed stacking method, the previous techniques could be replaced by 2-D wire bonding. In this way, supporting platforms with slots and vertical spacers were no longer needed. Furthermore, ASIC chips can be substituted for the spacers in the stacked arrays to achieve system integration, design flexibility and volume usage efficiency. To avoid overflow of the adhesive fluid during assembly, an anti-overflow design which made use of capillary action force was applied in the stacking method as well. Moreover, presented stacking procedure consumes only 35 minutes in average for a 4 × 4 3-D microprobe array without requiring other specially made assembly tools. To summarize, the advantages of the proposed stacking method for 3-D array assembly include simplified assembly process, high structure strength, smaller opening area and integration ability with active circuits. This stacking assembly technique allows an alternative method to create 3-D structures from planar components

3-Dimensional Intracortical Neural Interface For The Study Of Epilepsy

Epilepsy is a chronic disease characterized by recurrent, unprovoked seizures, where seizures are described as storms of uncontrollable neuro-electrical activity within the brain. Seizures are therefore identified by observation of electrical spiking observed through electrical contacts (electrodes) placed on the scalp or the cortex above the epileptic regions. Current epilepsy research is identifying several specific molecular markers that appear at specific layers of the epilepsy-affected cortex. However, technology is limited in allowing for live observation of electrical spiking across these layers. The underlying hypothesis of this project is that electrical interictal activity is generated in a layer- and lateral-specific pattern. This work reports a novel neural probe technology for the manufacturing of 3D arrays of electrodes with integrated microchannels. This new technology is based on a silicon island structure and a simple folding procedure. This method simplifies the assembly or packaging process of 3D neural probes, leading to higher yield and lower cost. Various types of 3D arrays of electrodes, including acute and chronic devices, have been successfully developed. Microchannels have been successfully integrated into the 3D neural probes via isotropic XeF2 gas phase etching and a parylene resealing process. This work describes in detail the development of neural devices targeted towards the study of layer-specific interictal discharges in an animal model of epilepsy. Devices were designed utilizing parameters derived from the rat model of epilepsy. The progression of device design is described from 1st prototype to final chronic device. The fabrication process and potential pitfall are described in detail. Devices have been characterized by SEM (scanning electron microscope) imaging, optical imaging, various types of impedance analysis, and AFM (atomic force microscopy) characterization of the electrode surface. Flow characteristics of the microchannels were also analyzed. Various animal tests have been carried out to demonstrate the recording functionality of the probes, preliminary biocompatibility studies, and the reliability of the final chronic device package. These devices are expected to be of great use to the study of epilepsy as well as various other neurological diseases

Implantable Neural Probes for Electrical Recording and Optical Stimulation of Cellular Level Neural Circuitry in Behaving Animals.

In order to advance the understanding of brain function, it is critical to monitor how neural circuits work together and perform computational processing. For the past few decades, a wide variety of neural probes have been developed to study the electrophysiology of the brain. This work is focused on two important objectives to improve the brain-computer interface: 1) to enhance the reliability of recording electrodes by optimizing the shank structure; 2) to incorporate optical stimulation capability in addition to electrical recording for applications involving optogenetics. For the first objective, a flexible 64-channel parylene probe was designed with unique geometries for reduced tissue reactions. In order to provide the mechanical stiffness necessary to penetrate the brain, the miniaturized, flexible probes were coated with a lithographically patterned silk fibroin, which served as a biodegradable insertion shuttle. Because the penetration strength is independent from the properties of the probe itself, the material and geometry of the probe structure can be optimally designed without constraints. These probes were successfully implanted into the layer-V of motor cortex in 6 rats and recorded neural activities in vivo for 6 weeks. For the second objective, either optical waveguides or μLEDs were monolithically integrated on the probe shanks for optogenetic applications. Compared to existing methods, this work can offer high spatial-temporal resolution to record and stimulate from even subcellular neural structures. In the experiments using wild type animals, despite optimized recording of spontaneous neural activities, the cells never responded to illumination. In contrast, for the ChR2 expressed animals, light activation of neural activities was extremely robust and local, which phase-locked to the light waveform whenever the cell was close to the light source. In particular, the probes integrated with μLEDs were capable of driving different neural circuit behaviors using two adjacent μLEDs separated only by a 60-μm-pitch. With 3 μLEDs integrated at the tip of each of the 4 probe shanks, this novel optogenetic probe can provide more than 480 million (12!) different spiking sequences at the sub-cellular resolution, which is ideal to manipulate high density neural network with versatility and precision.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111604/1/wufan_1.pd

Sea of Electrodes Array (SEA): Customizable 3D High-Density High-Count Neural Probe Array Technology

Accurate mapping of neural circuits and interfacing with neurons for control of brain-machine interfaces require simultaneous large-scale and high spatiotemporal resolution recordings and stimulation of neurons in multiple layers and areas of the brain. Conventional penetrating micro-electrode arrays (MEAs) are limited to a few thousand electrodes at best, with limited volumetric 3D spatial resolution. This is mainly due to the types of fabrication technologies and available designs and materials for making such probes. Based on the strengths and shortcomings of the available MEAs, we present a new fabrication technology for a new class of 3D neural electrode array that provides the characteristics of a near-ideal neural interface. This research addresses some of the limitations of previous works in terms of electrode scale, density and spatial coverage (depth and span). In order to realize a scalable 3D out-of-plane array of extremely dense, slender, and sharp needles with recording sites at each of their tips, a number of techniques are developed. These includes: 1- A custom-developed silicon DRIE process to make deep (500 µm) high aspect-ratio (20-30) thru-wafer holes with controlled sidewall slope, 2- A method of extending the thru-wafer holes depth by aligning and then fusion bonding multiple silicon substrates already having holes etched in them, 3- A process for conformal refilling of ultra-deep (~2 mm) ultra-high aspect-ratio (80-100) holes with dielectric and conductive films using LPCVD process, 4- Methods of forming recording sites using self-aligned mask-less metallization processes, and 5- A method based on wet silicon etching to dissolve away the support substrate containing the refilled holes to release the electrodes. Using these technologies, we have fabricated millimeter-long (1.2mm), narrow (10-20µm diameter), sharp (submicron tip size), high-density (400 electrodes/mm2) high-count (5000+) silicon electrode arrays. Electrodes robustness, insertion and recording functionality have been demonstrated by acute in vivo recordings in rats under anesthesia using 2×2 and 3×3 arrays, where local field potentials (LFP) have been recorded. Innovative features of this technology could be utilized to produce arrays with arbitrary 3D design to target specific brain structures to achieve 3D spatial coverage over the convoluted topography of the brain. These include: 1- Length of side-by-side electrodes can be varied from tens of microns to several millimeters independently. 2- Electrodes spacing can be modified by the designer to obtain a desirable density and distribution of the array needles. 3- Electrode cross-sectional size can be controlled to obtain extremely fine, sharp and slender needles, crucial for minimizing tissue damage and improving chronic stability of implanted probes. 4- Any desired distribution of electrodes with customizable length, diameter and pitch across the array can be obtained to realize near-ideal application-specific neural probes. Potential capabilities of this work are investigated. These include integration of optical waveguides and chemical sensors and drug delivery channels to create sophisticated multi-modal multi-channel probes for electrophysiological studies of brain at the cellular levels. Limitations of developed DRIE, bonding, and LPCVD processes and tissue volume displacement by high-density arrays are discussed and a number of solutions are proposed. These results suggest maximum electrode length of ~2.5 mm and 20 µm thick electrodes. A maximum density of ~225 electrodes/mm2 for 10 µm thick electrodes is suggested for chronic applications.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/149834/1/aminsa_1.pd

Fabrication of Silicon Microneedles for Dermal Interstitial Fluid Extraction in Human Subjects

The goal of this project is to design and develop a fabrication process for silicon microneedle arrays to extract dermal interstitial fluid (ISF) from human skin. ISF is a cell- free, living tissue medium that is known to contain many of the same, clinical biomarkers of general health, stress response and immune status as in blood. However, a significant barrier to adoption of ISF as a diagnostic matrix is the lack of a rapid, minimally invasive method of access and collection for analysis. Microfabricated chips containing arrays of microneedles that can rapidly and painlessly access and collect dermal ISF for bioassay could greatly facilitate point-of-care diagnosis and health monitoring, especially in times of crisis or in austere environments, where drawing venous blood poses an unnecessary infection or biohazard risk. Two different fabrication methods were explored. The first method borrows from a previously reported dicing saw process, where a series of parallel and perpendicular cuts of partial depth are made into a thicker silicon wafer, creating arrays of square columns, which are subsequently sharpened into needles. The second method uses a new, entirely-DRIE process to create the arrays of columns. The columns are sharpened into needles using an isotropic wet etch method (HNA etch) which preferentially enhances etching at the tips and diminishes etching at the base, creating remarkably sharp, conical shaped needles capable of piercing skin. The needles contain holes that pass through the wafer to the opposite side, where they connect to a series of microfluidic channels that lead to a reservoir. The back of the wafer is bonded to glass, providing a hydrophilic cap to the channels, as well as a way to see into the device to detect whether the channels are filling with liquid. The fabrication procedures for both methods are presented, along with 2D- and 3D-rendered schematics for the final devices. Needle geometric shape is crucial to their ability to extract ISF. To determine the appropriate pre-sharpened etched shape, needle columns with a variety of different shapes were designed, produced, sharpened, and examined under a scanning electron microscope. The most promising shapes were selected for further processing and testing. Resulting chips were first bench tested to ensure capillary filling capability, and then tested for ISF collection from human skin. Microneedle arrays which were successfully demonstrated to extract ISF are presented, and the unsuccessful shapes are also shown in the interest of completion. Potential means for improving performance and future research directions are discussed

Microfabricated Sampling Probes for Monitoring Brain Chemistry at High Spatial and Temporal Resolution

Monitoring neurochemical dynamics has played a crucial role in elucidating brain function and related disorders. An essential approach for monitoring neurochemicals is to couple sampling probes to analytical measurements; however, this approach is inherently limited by poor spatial and temporal resolution. In this work, we have developed miniaturized sampling probes and analytical technology to overcome these limitations. Conventional sampling probes were handmade and have several disadvantages, including large sizes (over 220 µm in diameter) and limited design flexibility. To address these disadvantages, we have used microfabrication to manufacture sampling probes. By bulk micromachining of Si, microchannels and small sampling regions can be fabricated within a probe, with an overall dimension of ~100 µm. For development of a dialysis probe, nanoporous anodic aluminum oxide was adapted for monolithically embedding a membrane. Coupling the probe to liquid chromatography-mass spectrometry, multiple neurochemicals were measured at basal conditions, including dopamine and acetylcholine. Comparing to conventional dialysis probes, the microfabricated dialysis probe provided at least 6-fold improvement in spatial resolution and potentially had lower tissue disruption. Furthermore, we have continued the development of a microfabricated push-pull probe. We enhanced functionality of the probe by integrating an additional channel into the probe for chemical delivery. Further, we demonstrated that the probe can feasibly be coupled to droplet microfluidic devices for improved temporal resolution. Nanospray ionization mass spectrometry was used for multiplexed measurements of neurochemicals in nanoliter droplet samples. Utility of the integrated system was demonstrated by monitoring in vivo dynamics during potassium stimulation of 4 neurochemicals, including glutamate and GABA. The probe provided unprecedented spatial resolution and temporal resolution as high as ~5 s. Additionally, we highlighted versatility of the method by coupling the probe to another high-throughput assay, i.e., droplet-based microchip capillary electrophoresis for rapid separation (less than 3 s) and measurement of multiple amino acid neurochemicals. This collection of work illustrates that development of the microfabricated sampling probes and their compatible microfluidic systems are highly beneficial for studying brain chemistry. The integrated miniaturized analytical technology can potentially be useful for solving other problems of biological significance.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/144094/1/nonngern_1.pd