Implantable Low-Noise Fiberless Optoelectrodes for Optogenetic Control of Distinct Neural Populations

The mammalian brain is often compared to an electrical circuit, and its dynamics and function are governed by communication across different types neurons. To treat neurological disorders like Alzheimer’s and Parkinson’s, which are characterized by inhibition or amplification of neural activity in a particular region or lack of communication between different regions of the brain, there is a need to understand troubleshoot neural networks at cellular or local circuit level. In this work, we introduce a novel implantable optoelectrode that can manipulate more than one neuron type at a single site, independently and simultaneously. By delivering multi-color light using a scalable optical waveguide mixer, we demonstrate manipulation of multiple neuron types at precise spatial locations in vivo for the first time. We report design, micro-fabrication and optoelectronic packaging of a fiber-less, multicolor optoelectrode. The compact optoelectrode design consists of a 7 μm x 30 μm dielectric optical waveguide mixer and eight electrical recording sites monolithically integrated on each shank of a 22 μm-thick four-shank silicon neural probe. The waveguide mixers are coupled to eight side-emitting injection laser diodes (ILDs) via gradient-index (GRIN) lenses assembled on the probe backend. GRIN-based optoelectrode enables efficient optical coupling with large alignment tolerance to provide wide optical power range (10 to 3000 mW/mm2 irradiance) at stimulation ports. It also keeps thermal dissipation and electromagnetic interference generated by light sources sufficiently far from the sensitive neural signals, allowing thermal and electrical noise management on a multilayer printed circuit board. We demonstrated device verification and validation in CA1 pyramidal layer of mice hippocampus in both anesthetized and awake animals. The packaged devices were used to manipulate variety of multi-opsin preparations in vivo expressing different combinations of Channelrhodopsin-2, Archaerhodopsin and ChrimsonR in pyramidal and parvalbumin interneuron cells. We show effective stimulation, inhibition and recording of neural spikes at precise spatial locations with less than 100 μV stimulation-locked transients on the recording channels, demonstrating novel use of this technology in the functional dissection of neural circuits.PHDBiomedical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/137171/1/kkomal_1.pd

Inhomogeneous Aging in Lithium‐Ion Batteries Caused by Temperature Effects

Lithium-ion batteries (LIBs) are widely used as electrochemical energy storage devices due to their advantages in energy and power density as well as their reliability. One research focus is the cyclic lifetime of LIB, where the influence of thermal conditions during cycling is not yet completely understood. In previous publications, investigations on the cyclic aging behavior of LIB under various thermal boundary conditions are presented, including defined temperature gradients and temperature transients. Thereupon, herein, the results of cell openings and an extensive postmortem study with analyses of scanning electron micrographs, electrode thickness, X-ray diffraction, and inductively coupled plasma optical emission spectroscopy are presented, evaluated, and correlated with the thermal conditions. The results reveal significant variations on the electrode and atomic scale for the different thermal boundary conditions during cycling. The electrodes exposed to a temperature gradient exhibit an inhomogeneous distribution of aging behavior that directly correlates with the local temperature. Cycling with superimposed transient temperatures reveals fundamentally different aging effects. With this knowledge, critical temperature conditions can be avoided in applications to prolong cyclic lifetime

Pull-in dynamics of overdamped microbeams

We study the dynamics of MEMS microbeams undergoing electrostatic pull-in. At DC voltages close to the pull-in voltage, experiments and numerical simulations have reported `bottleneck' behaviour in which the transient dynamics slow down considerably. This slowing down is highly sensitive to external forces, and so has widespread potential for applications that use pull-in time as a sensing mechanism, including high-resolution accelerometers and pressure sensors. Previously, the bottleneck phenomenon has only been understood using lumped mass-spring models that do not account for effects such as variable residual stress and different boundary conditions. We extend these studies to incorporate the beam geometry, developing an asymptotic method to analyse the pull-in dynamics. We attribute bottleneck behaviour to critical slowing down near the pull-in transition, and we obtain a simple expression for the pull-in time in terms of the beam parameters and external damping coefficient. This expression is found to agree well with previous experiments and numerical simulations that incorporate more realistic models of squeeze film damping, and so provides a useful design rule for sensing applications. We also consider the accuracy of a single-mode approximation of the microbeam equations --- an approach that is commonly used to make analytical progress, without systematic investigation of its accuracy. By comparing to our bottleneck analysis, we identify the factors that control the error of this approach, and we demonstrate that this error can indeed be very small.Comment: 18 page

Liquid Water Transport in the Reactant Channels of Proton Exchange Membrane Fuel Cells

Water management has been identified as a critical issue in the development of PEM fuel cells for automotive applications. Water is present inside the PEM fuel cell in three phases, i.e. liquid phase, vapor phase and mist phase. Liquid water in the reactant channels causes flooding of the cell and blocks the transport of reactants to the reaction sites at the catalyst layer. Understanding the behavior of liquid water in the reactant channels would allow us to devise improved strategies for removing liquid water from the reactant channels. In situ fuel cell tests have been performed to identify and diagnose operating conditions which result in the flooding of the fuel cell. A relationship has been identified between the liquid water present in the reactant channels and the cell performance. A novel diagnostic technique has been established which utilizes the pressure drop multiplier in the reactant channels to predict the flooding of the cell or the drying-out of the membrane. An ex-situ study has been undertaken to quantify the liquid water present in the reactant channels. A new parameter, the Area Coverage Ratio (ACR), has been defined to identify the interfacial area of the reactant channel which is blocked for reactant transport by the presence of liquid water. A parametric study has been conducted to study the effect of changing temperature and the inlet relative humidity on the ACR. The ACR decreases with increase in current density as the gas flow rates increase, removing water more efficiently. With increase in temperature, the ACR decreases rapidly, such that by 60°C, there is no significant ACR to be reported. Inlet relative humidity of the gases does change the saturation of the gases in the channel, but did not show any significant effect on the ACR. Automotive powertrains, which is the target for this work, are continuously faced with transient changes. Water management under transient operating conditions is significantly more challenging and has not been investigated in detail. This study begins to investigate the effects of changing operating conditions on liquid water transport through the reactant channels. It has been identified that rapidly increasing temperature leads to the dry-out of the membrane and rapidly cooling the cell below 55°C results in the start of cell flooding. In changing the operating load of the PEMFC, overshoot in the pressure drop in the reactant channel has been identified for the first time as part of this investigation. A parametric study has been conducted to identify the factors which influence this overshoot behavior

Integration of micromachined thermal thermal shear stress sensors with microchannels - design, fabrication and testing

Rochester Institute of Technology, 2005 Includes bibliographical references (leaves 91-96) The emerging picture of microvascular flow strongly suggests that local gradients in shear rate along the arteriole walls play an integral role in the ability of a microvascular network to regulate and modify blood flow. The methods to estimate shear stress from approximations of the velocity profiles determined by in vivo particle tracking experiments in the hamster and computational simulation are limited by assumptions made about the flow and experimental techniques. Right now, our ability to relate wall shear stress in microvessels to corresponding biological function is limited by our ability to accurately determine shear stress. A three dimensional computational model was created to simulate the system\u27s thermal response to the constant temperature control circuit. The model geometry included all fabricated layers in thermal shear stress sensor and the microchannel structure (17 microns x 17 microns). This computational technique was used to optimize the dimensions of the system in order to reduce the amount of heat lost to the substrate and maximize the signal response. Hot film thermal shear stress sensors were successfully integrated with microchannel using surface micromachining technique. The entire device was fabricated and tested at Semiconductor Microsystems Fabrication Laboratory (SMFL) at RIT. This thesis discusses the design and optimization of a thermal shear stress sensor using computational techniques to simulate the sensor\u27s performance in microchannel models of arteriole bifurcations. An attempt has been made to verify thermal-transfer principle of hot film shear stress sensors in microchannel

Piezoelectric Transformer and Hall-Effect Based Sensing and Disturbance Monitoring Methodology for High-Voltage Power Supply Lines

Advancements in relaying algorithms have led to an accurate and robust protection system widely used in power distribution. However, in low power sections of relaying systems, standard voltage and current measurement techniques are still used. These techniques have disadvantages like higher cost, size, electromagnetic interference, resistive losses and measurement errors and hence provide a number of opportunities for improvement and integration. We present a novel microsystem methodology to sense low-power voltage and current signals and detect disturbances in high-voltage power distribution lines. The system employs dual sensor architecture that consists of a piezoelectric transformer in combination with Hall-effect sensor, used to detect the disturbances whose harmonics are in the kHz frequency range. Our numerical analysis is based on three-dimensional finite element models of the piezoelectric transformer (PT) and the principle of Hall-effect based “Integrated Magnetic Concentrator (IMC)” sensor. This model is verified by using experimental data recorded in the resonant frequency and low frequency regions of operation of PT for voltage sensing. Actual measurements with the commercial IMC sensor too validate the modelling results. These results describe a characteristic low frequency behaviour of rectangular piezoelectric transformer, which enables it to withstand voltages as high as 150V. In the frequency range of 10Hz to 250Hz, the PT steps down 10-150V input with a linearity of ±1%. The recorded group delay data shows that propagation delay through PT reduces to few microseconds above 1kHz input signal frequency. Similarly, the non-intrusive current sensor detects current with a response time of 8μs and converts the current into corresponding output voltage. These properties, in addition to frequency spectrum of voltage and current input signals, have been used to develop a signal processing and fault detection system for two real-time cases of faults to produce a 6-bit decision logic capable of detecting various types of line disturbances in less than 3ms of delay

Investigation on the Benefits of Safety Margin Improvement in CANDU Nuclear Power Plant Using an FPGA-based Shutdown System

The relationship between response time and safety margin of CANadian Deuterium Uranium (CANDU) nuclear power plant (NPP) is investigated in this thesis. Implementation of safety shutdown system using Field Programmable Gate Array (FPGA) is explored. The fast data processing capability of FPGAs shortens the response time of CANDU shutdown systems (SDS) such that the impact of accident transient can be reduced. The safety margin, which is closely related to the reactor behavior in the event of an accident, is improved as a result of such a faster shutdown process. Theoretical analysis based on neutron dynamic theory is carried out to establish the fact that a faster shutdown process can mitigate accidental consequences. To provide more realistic test cases from a thermalhydraulic perspective, an industry grade simulation tool known as CATHENA is used to generate comparable accident-shutdown transients for different SDS response times. Results from both verification methods explicitly prove the feasibility of improving the safety margin via faster shutdown process. To demonstrate this concept, a prototype of the proposed faster SDS is constructed. The trip logic of CANDU shutdown system No.1 (SDS1) is converted into a digital hardware design and implemented within chosen FPGA platform. The functionality of the FPGA-based SDS1 is implemented, and the response times are tested and compared to those of the existing CANDU SDS1. The achieved 10.5 ms response time of the FPGA-based SDS1 is again applied to the CATHENA simulation process to quantitatively present the 26.98% improvement in the safety margin. To investigate potential improvement in safety margin by using FPGA technology, hardware-in-the-loop (HIL) simulation is performed by connecting the FPGA-based SDS1 to an NPP training simulator. The 6.26% improvement in safety margin has been verified, based on which a 10% potential power upgrade is discussed as another benefit of applying FPGA technology to CANDU NPPs

Native and Radiation-Induced Defects in III-V Solar Cells and Photodiodes

Photodiodes made of III-V materials are ubiquitous with applications for telecommunications, photonics, consumer electronics, and spectroscopy. The III-V solar cell, specifically, is a large-area photodiode that is used by the satellite industry for power conversion due to its unrivaled efficiency and wide range of available materials. As a device driven by its minority carrier diffusion length (MCDL), the performance of a photodiode is sensitive to crystallographic defects that create states in the forbidden energy gap. Defects commonly arise during growth of the crystal and during device fabrication, and they accumulate slowly over time when deployed into the damaging environment of space. Defect-assisted carrier recombination leads to lower MCDL, higher dark current, reduced sensitivity and signal-to-noise ratio, and, in the case of solar cells, reduced power conversion efficiency. Consequently, the development of photodiode technology requires techniques for detection, characterization, and mitigation of defects and the inter-bandgap states they create. In this work, III-V material defects are addressed across a variety of materials and devices. The first half of the work makes use of deep-level transient spectroscopy (DLTS) to deduce the energy level, cross-section, and density of traps the InAlAs, InAlAsSb, and InGaAs lattice-matched to InP. An in situ DLTS system that can monitor defects immediately after irradiation was developed and applied to InGaAs photodiodes irradiated by protons. Evidence of trap annealing was found to occur as low as 150 K. The second half begins with development of GaSb solar cells grown by molecular beam epitaxy on GaAs substrate intended for use in lower-cost monolithic multi-junction cells. Defect analysis by microscopy, dark lock-in thermography, and dark current measurement, among others, was performed. The best GaSb-on-GaAs cell achieved state-of-the-art 4.5% efficiency under concentrated solar spectrum. Finally, light management in III-V photodiodes was explored as a possible route for defect mitigation. Textures, diffraction gratings, metallic mirrors, and Bragg reflectors were simulated by finite difference time domain for single- and multi-junction GaAs-based cells with the aim of reducing the amount of absorber material required and to simultaneously reduce MCDL requirements by generating carriers closer to the junction. The results were inputted into a device simulator to predict efficiency. A backside reflective pyramidal-textured grating was simulated to allow a GaAs cell to be thinned by a factor of \u3e30 compared to a conventional cell

Linear-Quadratic Control of a MEMS Micromirror Using Kalman Filtering

The deflection limitations of electrostatic flexure-beam actuators are well known. Specifically, as the beam is actuated and the gap traversed, the restoring force necessary for equilibrium increases proportionally with the displacement to first order, while the electrostatic actuating force increases with the inverse square of the gap. Equilibrium, and thus stable open-loop voltage control, ceases at one-third the total gap distance, leading to actuator snap-in. A Kalman Filter is designed with an appropriately complex state dynamics model to accurately estimate actuator deflection given voltage input and capacitance measurements, which are then used by a Linear Quadratic controller to generate a closed-loop voltage control signal. The constraints of the latter are designed to maximize stable control over the entire gap. The design and simulation of the Kalman Filter and controller are presented and discussed, with static and dynamic responses analyzed, as applied to basic, 100 micrometer by 100 micrometer square, flexure-beam-actuated micromirrors fabricated by PolyMUMPs. Successful application of these techniques enables demonstration of smooth, stable deflections of 50% and 75% of the gap