Spatial Transcriptomics as a Novel Approach to Redefine Electrical Stimulation Safety

Current standards for safe delivery of electrical stimulation to the central nervous system are based on foundational studies which examined post-mortem tissue for histological signs of damage. This set of observations and the subsequently proposed limits to safe stimulation, termed the “Shannon limits,” allow for a simple calculation (using charge per phase and charge density) to determine the intensity of electrical stimulation that can be delivered safely to brain tissue. In the three decades since the Shannon limits were reported, advances in molecular biology have allowed for more nuanced and detailed approaches to be used to expand current understanding of the physiological effects of stimulation. Here, we demonstrate the use of spatial transcriptomics (ST) in an exploratory investigation to assess the biological response to electrical stimulation in the brain. Electrical stimulation was delivered to the rat visual cortex with either acute or chronic electrode implantation procedures. To explore the influence of device type and stimulation parameters, we used carbon fiber ultramicroelectrode arrays (7 μm diameter) and microwire electrode arrays (50 μm diameter) delivering charge and charge density levels selected above and below reported tissue damage thresholds (range: 2–20 nC, 0.1–1 mC/cm2). Spatial transcriptomics was performed using Visium Spatial Gene Expression Slides (10x Genomics, Pleasanton, CA, United States), which enabled simultaneous immunohistochemistry and ST to directly compare traditional histological metrics to transcriptional profiles within each tissue sample. Our data give a first look at unique spatial patterns of gene expression that are related to cellular processes including inflammation, cell cycle progression, and neuronal plasticity. At the acute timepoint, an increase in inflammatory and plasticity related genes was observed surrounding a stimulating electrode compared to a craniotomy control. At the chronic timepoint, an increase in inflammatory and cell cycle progression related genes was observed both in the stimulating vs. non-stimulating microwire electrode comparison and in the stimulating microwire vs. carbon fiber comparison. Using the spatial aspect of this method as well as the within-sample link to traditional metrics of tissue damage, we demonstrate how these data may be analyzed and used to generate new hypotheses and inform safety standards for stimulation in cortex

Effects of Electric Stimulation on Physiology and Anatomy of Visual Pathway

Retinal degenerative diseases that progressively lead to severe blindness impact the affected individual’s quality-of-life. Visual prosthesis technology aims to provide an individual a potential means of obtaining visual information lost to them by blindness. Since the proof-of-concept success in 1968 of a device implanted in a human, visual prostheses have had sustained academic research and commercial interest. However, commercial failure of two retinal prosthesis device have raised concerns for the visual prosthesis field. To learn from this experience, research in this dissertation is aimed at understanding the impact of electric stimulation on the target neural tissue and investigating technology for a visual cortex prosthesis, which can reach a larger patient population (compared to a retinal prosthesis). My first set of experiments assessed, in an animal model of retinal degeneration, the condition of the brain and its ability to receive artificial vision information. Retinitis Pigmentosa has been proven to impact the human brain. My study investigated the extent to which this was replicated in a rat animal model of a single genetic mutation of Retinitis Pigmentosa. The P23H-1 rat was investigated with electrophysiology and immunohistochemistry to understand the brain’s function and structural condition. Visually evoked potentials changed as a result of blindness progression and electrically evoked response was maintained during retinal degeneration. Histology images show a relatively stable macrostructure of the blind rat brain. Neural activity measure using c-Fos saw a change due to weekly stimulation, but the results may be spurious when put next to an auxiliary analysis using high magnification VGluT2 images. I also created a rodent retinal implant procedure to test newly developed visual prosthesis devices. A retinal device with Parylene-C as its main component was tested and its feasibility in the small eye of a rat animal model was investigated. The device can survive 4-weeks of implantation and is mechanically stable within the eye. In support of the development of a novel cortical visual prosthesis device that fits the need of blind individuals, I used a rat animal model to prove the efficacy and safety of a novel neurostimulation device in preclinical development (StiMote). I worked to characterize the full ability of the neural interface, High-density carbon fiber arrays with electrodeposited Platinum-Iridium. The ability of PtIr-HDCF as a simultaneous recording and stimulation neural interface device was verified with nominal electrochemical measurements before, during, and after a long-duration 7-hour electric stimulation session that simulated a full day of device use. Based on my previous work and prior literature of HDCF as a neural interface that reduces neuroinflammatory response compared to other microelectrode array archetypes, PtIr-HDCF can be used as a device to monitor the brain and can better extract the effect of electric stimulation on the brain alone. I recorded neural electrophysiology to verify the rat brain’s sensitivity to stimuli before and after 7-hour stimulation. Based on the visually evoked potential and intracortical microstimulation data, possible increase in cortical sensitivity to stimuli was hypothesized. To add another layer of information, Spatial Transcriptomics as a novel method was used to define electric stimulation safety. Spatial Transcriptomics showed that PtIr-HDCF, when compared to a conventional microwire array, performs better in reducing proinflammatory cytokines. Findings of this dissertation can be used to better inform future investigations into brain electrophysiology and transcriptomics projects aimed to assist cortical visual prosthesis device development.PHDBiomedical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/174215/1/beomkoo_1.pd

A Wireless Neural Stimulator IC for Cortical Visual Prosthesis

We propose a 0.25 x 0.25 x 0.3 mm (∼0.02 mm 3 ) optically powered mote for visual cortex stimulation to restore vision. Up to 1024 implanted motes can be individually addressed. The complete StiMote system was confirmed fully functional when optically powered and cortex stimulation was confirmed in-vivo with a live rat brain