Wireless Power Transfer Techniques for Implantable Medical Devices:A Review

Wireless power transfer (WPT) systems have become increasingly suitable solutions for the electrical powering of advanced multifunctional micro-electronic devices such as those found in current biomedical implants. The design and implementation of high power transfer efficiency WPT systems are, however, challenging. The size of the WPT system, the separation distance between the outside environment and location of the implanted medical device inside the body, the operating frequency and tissue safety due to power dissipation are key parameters to consider in the design of WPT systems. This article provides a systematic review of the wide range of WPT systems that have been investigated over the last two decades to improve overall system performance. The various strategies implemented to transfer wireless power in implantable medical devices (IMDs) were reviewed, which includes capacitive coupling, inductive coupling, magnetic resonance coupling and, more recently, acoustic and optical powering methods. The strengths and limitations of all these techniques are benchmarked against each other and particular emphasis is placed on comparing the implanted receiver size, the WPT distance, power transfer efficiency and tissue safety presented by the resulting systems. Necessary improvements and trends of each WPT techniques are also indicated per specific IMD

Neurostimulator with Waveforms Inspired by Nature for Wearable Electro-Acupuncture

The work presented here has 3 goals: establish the need for novel neurostimulation waveform solutions through a literature review, develop a neurostimulation pulse generator, and verify the operation of the device for neurostimulation applications. The literature review discusses the importance of stimulation waveforms on the outcomes of neurostimulation, and proposes new directions for neurostimulation research that would help in improving the reproducibility and comparability between studies. The pulse generator circuit is then described that generates signals inspired by the shape of excitatory or inhibitory post-synaptic potentials (EPSP, IPSP). The circuit analytical equations are presented, and the effects of the circuit design components are discussed. The circuit is also analyzed with a capacitive load using a simplified Randles model to represent the electrode-electrolyte interface, and the output is measured in phosphate-buffered saline (PBS) solution as the load with acupuncture needles as electrodes. The circuit is designed to be used in different types of neurostimulators depending on the needs of the application, and to study the effects of varying neurostimulation waveforms. The circuit is used to develop a remote-controlled wearable veterinary electro-acupuncture machine. The device has a small form-factor and 3D printed enclosure, and has a weight of 75 g with leads attached. The device is powered by a 500 mAh lithium polymer battery, and was tested to last 6 hours. The device is tested in an electro-acupuncture animal study on cats performed at the Louisiana State University School of Veterinary Medicine, where it showed expected electro-acupuncture effects. Then, a 2-channel implementation of the device is presented, and tested to show independent output amplitude, frequency, and stimulation duration per channel. Finally, the software and hardware requirements for control of the wearable veterinary electro-acupuncture machine are detailed. The number of output channels is limited to the number of hardware PWM timers available for use. The Arduino software implements PWM control for the output amplitude and frequency. The stimulation duration control is provided using software timers. The communications protocol between the microcontroller board and Android App are described, and communications are performed via Bluetooth

A Versatile Hermetically Sealed Microelectronic Implant for Peripheral Nerve Stimulation Applications

This article presents a versatile neurostimulation platform featuring a fully implantable multi-channel neural stimulator for chronic experimental studies with freely moving large animal models involving peripheral nerves. The implant is hermetically sealed in a ceramic enclosure and encapsulated in medical grade silicone rubber, and then underwent active tests at accelerated aging conditions at 100°C for 15 consecutive days. The stimulator microelectronics are implemented in a 0.6-μm CMOS technology, with a crosstalk reduction scheme to minimize cross-channel interference, and high-speed power and data telemetry for battery-less operation. A wearable transmitter equipped with a Bluetooth Low Energy radio link, and a custom graphical user interface provide real-time, remotely controlled stimulation. Three parallel stimulators provide independent stimulation on three channels, where each stimulator supports six stimulating sites and two return sites through multiplexing, hence the implant can facilitate stimulation at up to 36 different electrode pairs. The design of the electronics, method of hermetic packaging and electrical performance as well as in vitro testing with electrodes in saline are presented

In vivo measurements with a 64-channel extracellular neural recording integrated circuit

This paper presents in vivo measurements obtained from an implantable 64-channel neural recording Application Specific Integrated Circuit (ASIC) developed at IMSE and gives details of the computer interface used for real-time data acquisition. This interface connects the ASIC to a conventional 2.0 USB port by means of a Field Programmable Gate Array (FPGA). Communications are bidirectional and employ custom protocols both for delivering commands to the ASIC and for recording neural information under different channel selection and operation modes. The link is controlled by a user-friendly programming interface written in C++ which includes a built-in routine to efficiently index and store the captured data. Measurements demonstrate the suitability of the ASIC for capturing local field and action potentials with two different microelectrode array platforms.Ministerio de Economía y Competitividad TEC2012-3363

Neurostimulateur hautement intégré et nouvelle stratégie de stimulation pour améliorer la miction chez les paraplégiques

RÉSUMÉ Une lésion de la moelle épinière est un problème dévastateur médicalement et socialement. Pour la population des États-Unis seulement, il y a près de 10 000 nouveaux cas chaque année. A cause des nombreux types de lésions possibles, divers degrés de dysfonctionnement du bas appareil urinaire peuvent en découler. Une lésion est dite complète lors d’une perte totale des fonctions sensorielles et motrices volontaires en dessous du niveau de la lésion. Une lésion incomplète implique que certaines activités sensorielles et/ou motrices soient encore présentes. Si la lésion se produit au dessus du cône médullaire, la vessie développera une hyperréflexie qui se manifeste par des contractions réflexes non-inhibées. Ces contractions peuvent être accompagnées d’une augmentation de l’activité du sphincter externe. Par conséquent, cela mène à un état d’obstruction fonctionnelle de la vessie, qui induit une forte pression intravésicale à chacune des contractions réflexes et qui peut potentiellement endommager le haut appareil urinaire. Dans ce contexte, la neurostimulation est l'une des techniques les plus prometteuses pour la réhabilitation de la vessie chez les patients ayant subi une lésion de la moelle épinière. Le seul neurostimulateur implantable commercialisé, ciblant l'amélioration de la miction et ayant obtenu des résultats satisfaisants, nécessite une rhizotomie (section de certains nerfs) afin de réduire la dyssynergie entre la vessie et le sphincter. Cependant, la rhizotomie est irréversible et peut abolir les réflexes sexuels, de défécation ainsi que les sensations sacrales si encore présents dans le cas de lésions incomplètes. Afin d'éviter la rhizotomie, nous proposons une nouvelle stratégie de stimulation multi-site appliquée aux racines sacrées, et basée sur le blocage de la conduction des nerfs à l'aide d'une stimulation à haute fréquence comme alternative à la rhizotomie. Cette approche permettrait une meilleure miction en augmentant sélectivement la contraction de la vessie et en diminuant la dyssynergie. Huit expériences en phase aigüe ont étés menées sur des chiens pour vérifier la réponse de la vessie et du sphincter urétral externe à la stratégie de stimulation proposée. Le blocage à haute-fréquence (1 kHz) combiné à la stimulation basse-fréquence (30 Hz), a augmenté la différence de pression intra-vésicale/intra-urétrale moyenne jusqu'à 53 cmH2O et a réduit la pression intra-urétrale moyenne jusqu'à hauteur de 86 % relativement au niveau de référence. Dans l’objectif de tester la stratégie de neurostimulation proposée avec des expériences animales en phase chronique, un dispositif de neurostimulation implantable est requis. Un prototype discret implémentant cette stratégie de stimulation a été réalisé en utilisant uniquement des composants discrets disponibles commercialement. Ce prototype est capable de générer des impulsions à une fréquence aussi basse que 18 Hz tout en générant simultanément une forme d’onde alternative à une fréquence aussi haute que 8.6 kHz, et ce sur de multiples canaux. Lorsque tous les étages de stimulation et leurs différentes sorties sont activés avec des fréquences d’impulsions (2 mA, 217 μs) et de sinusoïdes de 30 Hz et 1 kHz respectivement, la consommation de puissance totale est autour de 4.5 mA (rms). Avec 50 mW de puissance inductive disponible par exemple et 4.5 mA de consommation de courant, le régulateur haute-tension peut être réglé à 10 V permettant ainsi une stimulation de 2 mA avec une impédance nerf-électrode de 4.4 kΩ. Le nombre effectif de sorties activées et le maximum réalisable des paramètres de stimulation sont limités par l’énergie disponible fournie par le lien inductif et l’impédance des interfaces nerf-électrode. Cependant, une plus grande intégration du neurostimulateur devient de plus en plus nécessaire à des fins de miniaturisation, de réduction de consommation de puissance, et d’augmentation du nombre de canaux de stimulation. Comme première étape vers une intégration totale, nous présentons la conception d’un neurostimulateur hautement intégré et qui peut être assemblé sur un circuit imprimé de 21 mm de diamètre. Le prototype est basé sur trois circuits intégrés, dédiés et fabriqués en technologie CMOS haute-tension, ainsi qu’un FPGA miniature à faible puissance et disponible commercialement. En utilisant une approche basée sur un abaisseur de tension, où la tension induite est laissée libre jusqu’à 20 V, l’étage d’entrée de récupération de puissance inductive et de données est totalement intégré.----------ABSTRACT Spinal cord injury (SCI) is a devastating condition medically and socially. For the population of USA only, the incidence is around 10 000 new cases per year. SCI leads to different degrees of dysfunction of the lower urinary tract due to a large variety of possible lesions. With a complete lesion, there is a complete loss of sensory and motor control below the level of lesion. An incomplete lesion implies that some sensory and/or motor activity is still present. Most patients with suprasacral SCI suffer from detrusor over-activity (DO) and detrusor sphincter dyssynergia (DSD). DSD leads to high intravesical pressure, high residual urine, urinary tract infection, and deterioration of the upper urinary tract. In this context, neurostimulation is one of the most promising techniques for bladder rehabilitation in SCI patients. The only commercialized implantable neurostimulator aiming for improved micturition and having obtained satisfactory results requires rhizotomy to reduce DSD. However, rhizotomy is irreversible and may abolish sexual and defecation reflexes as well as sacral sensations, if still present in case of incomplete SCI. In order to avoid rhizotomy, we propose a new multisite stimulation strategy applied to sacral roots, and based on nerve conduction blockade using high-frequency stimulation as an alternative to rhizotomy. This approach would allow a better micturition by increasing bladder contraction selectively and decreasing dyssynergia. Eight acute dog experiments were carried out to verify the bladder and the external urethral sphincter responses to the proposed stimulation strategy. High-frequency blockade (1 kHz) combined with low-frequency stimulation (30 Hz) increased the average intravesical-intraurethral pressure difference up to 53 cmH2O and reduced the average intraurethral pressure with respect to baseline by up to 86 %. To test the proposed neurostimulation strategy during chronic animal experiments, an implantable neurostimulateur is required. A discrete prototype implementing the proposed stimulation strategy has been designed using commercially available discrete components. This prototype is capable of generating a low frequency pulse waveform as low as 18 Hz with a simultaneous high frequency alternating waveform as high as 8.6 kHz, and that over different and multiple channels

Doctor of Philosophy

dissertationMagnetic fields are permeable to the biological tissues and can induce electric field in the conductive structures. Some medical devices take advantage of this ability to transfer energy from the source to the receiving site without direct contact. Prosthetic devices such as retinal implants use time-varying magnetic field to achieve wireless power transfer to the implanted magnetic coil. However, devices such as magnetic stimulators use the induction principle to create an electric field at the stimulation site. Efficiency of these devices is primarily dependent on the design of the magnetic coils. Therefore, in this work, we designed and validated efficient magnetic coils for wireless power transfer to implanted devices and magnetic stimulation of the peripheral nerves. Typical wireless power transfer (WPT) systems uses two-coil based design to achieve contactless power transfer to the implanted electronics. These systems achieve low power transfer efficiency (< 30%) and frequency bandwidth. Moreover, efficient wireless system requires high coupling and load variation tolerance during device operation. To design an electromagnetic safe WPT system, the power absorbed by the tissue and radiated field due to the proximal magnetic coils needs to be minimized. In this work, we proposed a multi-coil power transfer system which solves some of the current challenges. The proposed multi-coil WPT system achieves more than twice the power transfer efficiency, controllable voltage gain, wider frequency bandwidth, higher tolerance to coupling and load variations, lower absorbed power in the tissue and lower radiated field from the magnetic coil than a comparable two-coil system. In this work, we have developed analytic models of the multi-coil WPT system and validated the accuracy of the solutions using experiments. Magnetic coils play an important role in controlling the distribution of induced electric field inside the nerve during magnetic stimulation. In the past, homogeneous models were used to estimate the field profile inside conductive tissue due to the time varying current in the magnetic coil. Moreover, the effect of the surrounding media and stimulation mechanisms was understudied, which limits the optimization accuracy of the magnetic coils. In this work, we developed anatomically correct tissue models to study the effect of tissue heterogeneity and the surrounding media on the induced electric field. We also developed an optimization algorithm for designing energy efficient cm-size magnetic coils, that were then used for ex-vivo magnetic stimulation of the frog's sciatic nerve

Design of Integrated Neural/Modular Stimulators

Ph.DDOCTOR OF PHILOSOPH