Low power circuits and systems for wireless neural stimulation

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 155-161).Electrical stimulation of tissues is an increasingly valuable tool for treating a variety of disorders, with applications including cardiac pacemakers, cochlear implants, visual prostheses, deep brain stimulators, spinal cord stimulators, and muscle stimulators. Brain implants for paralysis treatments are increasingly providing sensory feedback via neural stimulation. Within the field of neuroscience, the perturbation of neuronal circuits wirelessly in untethered, freely-behaving animals is of particular importance. In implantable systems, power consumption is often the limiting factor in determining battery or power coil size, cost, and level of tissue heating, with stimulation circuitry typically dominating the power budget of the entire implant. Thus, there is strong motivation to improve the energy efficiency of implantable electrical stimulators. In this thesis, I present two examples of low-power tissue stimulators. The first type is a wireless, low-power neural stimulation system for use in freely behaving animals. The system consists of an external transmitter and a miniature, implantable wireless receiver-and-stimulator utilizing a custom integrated chip built in a standard 0.5 ptm CMOS process. Low power design permits 12 days of continuous experimentation from a 5 mAh battery, extended by an automatic sleep mode that reduces standby power consumption by 2.5x. To test this device, bipolar stimulating electrodes were implanted into the songbird motor nucleus HVC of zebra finches. Single-neuron recordings revealed that wireless stimulation of HVC led to a strong increase of spiking activity in its downstream target, the robust nucleus of the arcopallium (RA). When this device was used to deliver biphasic pulses of current randomly during singing, singing activity was prematurely terminated in all birds tested. The second stimulator I present is a novel, energy-efficient electrode stimulator with feedback current regulation. This stimulator uses inductive storage and recycling of energy based on a dynamic power supply to drive an electrode in an adiabatic fashion such that energy consumption is minimized. Since there are no explicit current sources or current limiters, wasteful energy dissipation across such elements is naturally avoided. The stimulator also utilizes a shunt current-sensor to monitor and regulate the current through the electrode via feedback, thus enabling flexible and safe stimulation. The dynamic power supply allows efficient transfer of energy both to and from the electrode, and is based on a DC-DC converter topology that is used in a bidirectional fashion. In an exemplary electrode implementation, I show how the stimulator combines the efficiency of voltage control and the safety and accuracy of current control in a single low-power integrated-circuit built in a standard 0.35 pm CMOS process. I also perform a theoretical analysis of the energy efficiency that is in accord with experimental measurements. In its current proof-of-concept implementation, this stimulator achieves a 2x-3x reduction in energy consumption as compared to a conventional current-source-based stimulator operating from a fixed power supply.by Scott Kenneth Arfin.Ph.D

A Travelling Wave Zeeman Decelerator For Atoms and Molecules

The design of a modular moving trap Zeeman decelerator capable of decelerating gas pulses produced from a supersonic source is presented here. Unlike the conventional form of Zeeman decelerator, paramagnetic particles are confined in a 3D potential throughout the deceleration process. The decelerator field is produced by flattened helical coils and currents of up to 1000 A peak. As the coils are periodic in nature, each coil produces a number of deep, quadrupole traps along the molecular beam axis. The resultant periodic field is described as a travelling wave. The application of the appropriate time dependent current allows the traps to move through the four coil modules. In order to compensate for the weaker transverse confinement, a quadrupole guide, operating at 700 A DC, is required to prevent further losses during the deceleration process. The operation of the decelerator relies on the power electronics developed specifically for the quadrupole and the decelerator coils. Due to the electromagnetic interference generated through the switching of the large currents, much of the electronics used to control the power electronics had to be developed specifically. The quadrupole power electronics have been designed to produce fast switching edges. This is necessary to minimise the interaction of the particles within the fringe field regions while maximising the interaction time within the pure quadrupole field. Even at a modest voltage applied to the circuitry, the rise time in current to 700 A has been reduced by a half. The decelerator power electronics must be capable of producing an alternating waveform with an amplitude of at least 500 A for each of the coil phases. Furthermore, the frequency of the waveforms must be tunable within a range of 10 kHz to 0 Hz. Through a combination of pulse width modulation and knowledge of the electrical properties of the coil it is possible to synthesise an alternating current waveform from a 800 V DC supply using a suitable switching circuit. %The challenge of switching such high powers is the transient voltage spikes that can be produced, however, these can be avoided through careful consideration of the layout of the circuit. Decelerators such as this do not cool the sample but instead reduces the mean velocity of a subset of particles which remained trapped. This maintains the phase space density of trapped particles. Modelling the magnetic fields generated by the decelerator coils has been necessary in order to understand the phase space acceptance of the decelerator. The helical nature of the coils required the development of a specific algorithm in order to calculate the field generated by each wire element. The resultant potential can then be interpolated using a tricubic interpolator to extract the field gradients necessary for numerical simulations of the particle trajectories. Including the effects of the pulse width modulated on the trap facilitates the characterisation of the acceptance of the decelerator and the limitations of the current iteration of the design. The numerical simulations can also be compared to experimental results gathered for metastable argon. The 3D guiding, or velocity bunching, of the gas packet over a range of velocities has been demonstrated. The ability to 3D guide and decelerate were severely hampered by the failure of key electronic components, limiting three coils to 100 A peak, moreover, these traps were sub-optimally loaded. Deceleration from 350 to 347 m/s and 342 to 310 m/s has been observed. The design of a trap capable of simultaneously loading samples of decelerated CaH and Li while allowing the cooling of Li would potentially allow for the sympathetic cooling of a molecular species with an atomic refrigerant. This particular atom-molecule system would also facilitate the examination of controlled chemistry and collisions over a range of temperatures through state selection of the reactants. The loading of the trap has been optimised in 1D for CaH with a loading efficiency of 52.2 % while only 7.3 % of Li is loaded when each of the gas packets has a mean velocity of 11 m/s. This implies that the source of the Li must be at least 130 times brighter than that of the CaH

Novel MRI Technologies for Structural and Functional Imaging of Tissues with Ultra-short T₂ Values

Conventional MRI has several limitations such as long scan durations, motion artifacts, very loud acoustic noise, signal loss due to short relaxation times, and RF induced heating of electrically conducting objects. The goals of this work are to evaluate and improve the state-of-the-art methods for MRI of tissue with short T₂, to prove the feasibility of in vivo Concurrent Excitation and Acquisition, and to introduce simultaneous electroglottography measurement during functional lung MRI

Acoustic power distribution techniques for wireless sensor networks

Recent advancements in wireless power transfer technologies can solve several residual problems concerning the maintenance of wireless sensor networks. Among these, air-based acoustic systems are still less exploited, with considerable potential for powering sensor nodes. This thesis aims to understand the significant parameters for acoustic power transfer in air, comprehend the losses, and quantify the limitations in terms of distance, alignment, frequency, and power transfer efficiency. This research outlines the basic concepts and equations overlooking sound wave propagation, system losses, and safety regulations to understand the prospects and limitations of acoustic power transfer. First, a theoretical model was established to define the diffraction and attenuation losses in the system. Different off-the-shelf transducers were experimentally investigated, showing that the FUS-40E transducer is most appropriate for this work. Subsequently, different load-matching techniques are analysed to identify the optimum method to deliver power. The analytical results were experimentally validated, and complex impedance matching increased the bandwidth from 1.5 to 4 and the power transfer efficiency from 0.02% to 0.43%. Subsequently, a detailed 3D profiling of the acoustic system in the far-field region was provided, analysing the receiver sensitivity to disturbances in separation distance, receiver orientation and alignment. The measured effects of misalignment between the transducers are provided as a design graph, correlating the output power as a function of separation distance, offset, loading methods and operating frequency. Finally, a two-stage wireless power network is designed, where energy packets are inductively delivered to a cluster of nodes by a recharge vehicle and later acoustically distributed to devices within the cluster. A novel dynamic recharge scheduling algorithm that combines weighted genetic clustering with nearest neighbour search is developed to jointly minimise vehicle travel distance and power transfer losses. The efficacy and performance of the algorithm are evaluated in simulation using experimentally derived traces that presented 90% throughput for large, dense networks.Open Acces

Measurements, Models, Systems and Design

531 s.

An Investigation of Cylindrical Liner Z-pinches as Drivers for Converging Strong Shock Experiments

A cylindrical liner z-pinch configuration has been used to drive converging radia- tive shock waves into different gases. Experiments were carried out on the MAGPIE (1.4 MA, 250 ns rise-time) pulsed-power device at Imperial College London [1]. On application of the current pulse, a series of cylindrical shocks moving at typical velocities of 20 km s-1 are consecutively launched from the inside liner wall into an initially static gas- ll. The drive current skin depth calculated prior to resis- tive heating was slightly less than the liner wall thickness and no bulk motion of the liner occurred. Axial laser probing images show the shock fronts to be smooth and azimuthally symmetric, with instabilities developing downstream of each shock. Evidence for a radiative precursor ahead of the first shock was seen in laser inter- ferometry imaging and time-gated spatially resolved optical spectroscopy. In addition to investigating the shock waves themselves, the timing of the shocks was used together with their trajectories to gain insight into launching mechanisms. This provided information on the response of the liner to the current pulse, which is useful for the benchmarking of magneto-hydrodynamics (MHD) codes. A new load voltage diagnostic provided evidence for two phase transitions occurring within the liner wall. The voltage probe was also fielded on various other z-pinch loads for measurements of energy deposition and inductance. The response of magnetically thick liners was found to differ significantly from the case where the liner wall was thin with respect to the initial skin depth of the current. In the later case the evolution of the liner is dominated by the ablation of plasma much like during the ablation phase of a wire array z-pinch

Physics of intense light ion beams, production of high energy density in matter, and pulsed power applications. Annual report 1996/97

Physik intensiver Strahlen leichter Ionen, Erzeugung hoher Energiedichten in Materie und Anwendungen der Pulsed Power Technik Jahresbericht 1995 In dem Bericht werden die in 1995 erzielten Ergebnisse zum Arbeitsthema "Physik intensiver Ionenstrahlen und gepulster dichter Plasmen" dargestellt. Zus"tzlich wurden die neu hinzugekommenen Arbeiten zu industriellen Anwendungen der Pulsed Power Technik aufgenommen

Doctor of Philosophy

dissertationThis dissertation comprises two separate studies: 1) efficacy of an anabolic steroid, oxandrolone, on the energy utilization of the heart of a lamb born with single ventricle (SV) physiology using 31P MR spectroscopy (MRS) and 2) signal behavior of ultra-high-b radial diffusion weighted imaging (UHb-rDWI) in healthy and multiple sclerosis (MS) subjects. SV infants have the highest mortality of all infants that have congenital heart defects. Their inability to gain weight appropriately may be due to high cardiac energy requirements from their shunt dependent physiology. We hypothesize that oxandrolone, which is already known to markedly improve the nutritional state of burn patients, will improve the energy utilization in the heart. We tested our hypothesis on SV modelled lambs using 31P MRS, home built 1H/31P double tuned radio frequency (RF) coil, and 1H and 31P T/R switches. We monitored cardiac energy in the lamb by quantitatively evaluating the first-order forward reaction rate (kf) of the creatine-kinase (CK) reaction in the heart. Spinal cord injury due to pathologies, such as MS, may include demyelination and/or axonal damage and lead to varying degrees of neurologic deficit. Noninvasive imaging biomarkers for earlier disease detection and monitoring in the follow-up and treatment stages would be a significant advancement in patient care. Moreover, imaging of the cervical spinal cord (CSC) is technically challenging because of the low signal to noise ratio from the small cross section of the cord, susceptibility artifact due to tissue-bone interface, and motion induced artifact from breathing and swallowing. To resolve these challenges, we used the UHb-rDWI technique and a CSC dedicated phased array RF coil. We studied the behavior of UHb-rDWI signal over the range of b-values from 0 sec/mm2 to 7348 sec/mm2 in the CSC of healthy and MS subjects over multiple time points. In the normal CSC, the signal decays fast at low b and slowly at UHb (b>4000 sec/mm2). In MS patients, the region affected by active lesions revealed a marked decrease in signal intensities in UHb region. UHb-rDWI could, therefore, be used for establishing an imaging biomarker to distinguish inflammation, demyelination, and axonal loss in the CSC