Laura Becerra-Fajardo

Towards addressable wireless microstimulators based on electronic rectification of epidermically applied currents

Electrical stimulation has been explored to restore the capabilities of the nervous system in paralysis patients. This area of research and of clinical practice, known as Functional Electrical Stimulation, would greatly benefit from further miniaturization of implantable stimulators. To that end, we recently proposed and demonstrated an innovative electrical stimulation method in which implanted microstimulators operate as rectifiers of bursts of innocuous high frequency current supplied by skin electrodes, thus generating low frequency currents capable of stimulating excitable tissues. A diode could suffice in some applications but, in order to broaden the methods clinical applicability, we envision rectifiers with advanced capabilities such as current control and addressability. We plan flexible thread-like implants (diameters <; 300 urn) containing ASICs. As an intermediate stage, we are developing macroscopic implants (diameters ~ 2 mm) made of off-the-shelf components. Here we present a circuit which responds to commands modulated within the high frequency bursts and which is able to deliver charge-balanced currents. We show that a number of these circuits can perform independent stimulation of segments of an anesthetized earthworm following commands from a computer.

Demonstration of 2 mm Thick Microcontrolled Injectable Stimulators Based on Rectification of High Frequency Current Bursts

Existing implantable stimulators use powering approaches that result in stiff and bulky systems or result in systems incapable of producing the current magnitudes required for neuromuscular stimulation. This hampers their use in neuroprostheses for paralysis. We previously demonstrated an electrical stimulation method based on electronic rectification of high-frequency (HF) current bursts. The implants act as rectifiers of HF current that flows through the tissues by galvanic coupling, transforming this current into low frequency current capable of performing neuromuscular stimulation. Here, we developed 2-mm-thick, semi-rigid, injectable and addressable stimulators made of off-the-shelf components and based on this method. The devices were tested in vitro to illustrate how they are powered by galvanic coupling. In addition they were tried in an animal model to demonstrate their ability to perform controlled electrical stimulation. The implants were deployed by injection into two antagonist muscles of an anesthetized rabbit and were addressed resulting in independent isometric contractions. Low frequency currents of 2 mA were delivered by the implants. The HF currents are safe in terms of unwanted electrostimulation and tissue heating according to standards. This indicates that the proposed electrical stimulation method will allow unprecedented levels of miniaturization for neuroprostheses.

Bidirectional communications in wireless microstimulators based on electronic rectification of epidermically applied currents

Functional Electrical Stimulation (FES) has been used in order to restore muscle functions in patients suffering from neurological disorders. This therapeutic approach benefits from technological improvements that yield miniaturization. We previously have proposed and demonstrated an innovative electrical stimulation method in which wireless implants act as rectifiers of innocuous high frequency (HF) currents. These currents are conductively supplied to the tissues where the implants are located through external electrodes. Locally, the implants generate low frequency currents capable of stimulating excitable tissues. The method has the potential to enable unprecedented levels of miniaturization. The implant needs only two peripheral electrodes both for picking-up the HF current and for performing electrical stimulation. In addition, a tiny hybrid microcircuit, or a single integrated circuit, may integrate all the necessary electronic components. No bulky parts such as coils or batteries are required. We have demonstrated a number of circuit architectures for the implants with advanced capabilities such as digital addressability. In here, we demonstrate that the proposed method also allows bidirectional communications between the implants and the external system that powers and governs them, enabling proprioception-like sensing capabilities that may be crucial for closed-loop FES systems. We demonstrate a scheme based on amplitude modulation and Manchester encoding.

In Vivo Demonstration of Addressable Microstimulators Powered by Rectification of Epidermically Applied Currents for Miniaturized Neuroprostheses

Electrical stimulation is used in order to restore nerve mediated functions in patients with neurological disorders, but its applicability is constrained by the invasiveness of the systems required to perform it. As an alternative to implantable systems consisting of central stimulation units wired to the stimulation electrodes, networks of wireless microstimulators have been devised for fine movement restoration. Miniaturization of these microstimulators is currently hampered by the available methods for powering them. Previously, we have proposed and demonstrated a heterodox electrical stimulation method based on electronic rectification of high frequency current bursts. These bursts can be delivered through textile electrodes on the skin. This approach has the potential to result in an unprecedented level of miniaturization as no bulky parts such as coils or batteries are included in the implant. We envision microstimulators designs based on application-specific integrated circuits (ASICs) that will be flexible, thread-like (diameters < 0.5 mm) and not only with controlled stimulation capabilities but also with sensing capabilities for artificial proprioception. We in vivo demonstrate that neuroprostheses composed of addressable microstimulators based on this electrical stimulation method are feasible and can perform controlled charge-balanced electrical stimulation of muscles. We developed miniature external circuit prototypes connected to two bipolar probes that were percutaneously implanted in agonist and antagonist muscles of the hindlimb of an anesthetized rabbit. The electronic implant architecture was able to decode commands that were amplitude modulated on the high frequency (1 MHz) auxiliary current bursts. The devices were capable of independently stimulating the target tissues, accomplishing controlled dorsiflexion and plantarflexion joint movements. In addition, we numerically show that the high frequency current bursts comply with safety standards both in terms of tissue heating and unwanted electro-stimulation. We demonstrate that addressable microstimulators powered by rectification of epidermically applied currents are feasible.

In vivo demonstration of injectable microstimulators based on charge-balanced rectification of epidermically applied currents.

OBJECTIVE It is possible to develop implantable microstimulators whose actuation principle is based on rectification of high-frequency (HF) current bursts supplied through skin electrodes. This has been demonstrated previously by means of devices consisting of a single diode. However, previous single diode devices caused dc currents which made them impractical for clinical applications. Here flexible thread-like stimulation implants which perform charge balance are demonstrated in vivo. APPROACH The implants weigh 40.5 mg and they consist of a 3 cm long tubular silicone body with a diameter of 1 mm, two electrodes at opposite ends, and, within the central section of the body, an electronic circuit made up of a diode, two capacitors, and a resistor. In the present study, each implant was percutaneously introduced through a 14 G catheter into either the gastrocnemius muscle or the cranial tibial muscle of a rabbit hindlimb. Then stimulation was performed by delivering HF bursts (amplitude <60 V, frequency 1 MHz, burst repetition frequency from 10 Hz to 200 Hz, duration = 200 μs) through a pair of textile electrodes strapped around the hindlimb and either isometric plantarflexion or dorsiflexion forces were recorded. Stimulation was also assayed 1, 2 and 4 weeks after implantation. MAIN RESULTS The implants produced bursts of rectified current whose mean value was of a few mA and were capable of causing local neuromuscular stimulation. The implants were well-tolerated during the 4 weeks. SIGNIFICANCE Existing power supply methods, and, in particular inductive links, comprise stiff and bulky parts. This hinders the development of minimally invasive implantable devices for neuroprostheses based on electrical stimulation. The proposed methodology is intended to relieving such bottleneck. In terms of mass, thinness, and flexibility, the demonstrated implants appear to be unprecedented among the intramuscular stimulation implants ever assayed in vertebrates.

Flexible Thread-like Electrical Stimulation Implants Based on Rectification of Epidermically Applied Currents Which Perform Charge Balance

Miniaturization of implantable medical electronic devices is currently compromised by the available means for powering them. Most common energy supply techniques for implants – batteries and inductive links – comprise bulky parts which, in most cases, are larger than the circuitry they feed. For overcoming such miniaturization bottleneck in the case of implants for electrical stimulation, we recently proposed and demonstrated a method in which the implants operate as rectifiers of bursts of high frequency current supplied by skin electrodes. In this way, low frequency currents capable of performing stimulation of excitable tissues are generated locally through the implants whereas the auxiliary high frequency currents only cause innocuous heating. The electronics of the prototype we demonstrated previously consisted of a single diode. As a consequence, it caused dc currents through it which made it impractical for clinical applications. Here we present an implantable prototype which performs charge balance for preventing electrochemical damage. It consists of a tubular silicone body with a diameter of 1 mm, two peripheral electrodes and a central electronic circuit made up of a diode, two capacitors and a resistor. We also report that this circuitry works even when water immersed, which may avoid the need for hermetic packaging.

Charge Counter for Performing Active Charge-Balance in Miniaturized Electrical Stimulators

Functional Electrical Stimulation (FES) has been explored in order to restore the capabilities of the nervous system in patients that suffer from paralysis. This area of research and of clinical practice greatly benefits from any technological improvement yielding miniaturization. In this regard, we recently proposed and demonstrated an innovative electrical stimulation method based on implanted microstimulators that operate as rectifiers of bursts of innocuous high frequency current supplied by skin electrodes, generating low frequency currents that are capable of stimulating excitable tissues. We envision flexible ultrathin implants (diameters < 300 μm) containing ASICs that have advanced capabilities, such as addressability and current control. As miniaturization is the main aim of this method, the use of bulky DC-blocking capacitors (e.g. 10 μF) to accomplish zero net charge injection and avoid electrochemical tissue and electrode damage is highly inconvenient. As an alternative, here we present an active charge-balance method based on the use of a digital charge quantifier, whose operation is inspired in the functioning of the tipping bucket rain gauge. The system monitors the charge injection, matching the charge injected in the cathodal phase, with the charge injected in the anodal phase, generating a biphasic current waveform that adapts itself to possible current source mismatches. We have implemented a prototype built with discrete components which uses a capacitor of only 100 pF for the charge counter.

Powering Implants by Galvanic Coupling: A Validated Analytical Model Predicts Powers Above 1 mW in Injectable Implants

While galvanic coupling for intrabody communications has been proposed lately by different research groups, its use for powering active implantable medical devices remains almost non-existent. Here it is presented a simple analytical model able to estimate the attainable power by galvanic coupling based on the delivery of high frequency (>1 MHz) electric fields applied as short bursts. The results obtained with the analytical model, which is in vitro validated in the present study, indicate that time-averaged powers above 1 mW can be readily obtained in very thin (diameter < 1 mm) and short (length < 20 mm) elongated implants when fields which comply with safety standards (SAR < 10 W/kg) are present in the tissues where the implants are located. Remarkably, the model indicates that, for a given SAR, the attainable power is independent of the tissue conductivity and of the duration and repetition frequency of the bursts. This study reveals that galvanic coupling is a safe option to power very thin active implants, avoiding bulky components such as coils and batteries.

First Steps Towards an Implantable Electromyography (EMG) Sensor Powered and Controlled by Galvanic Coupling

In the past it has been proposed to use implanted electromyography (EMG) sensors for myoelectric control. In contrast to surface systems, these implanted sensors provide signals with low cross-talk. To achieve this, miniature implantable devices that acquire and transmit real-time EMG signals are necessary. We have recently in vivo demonstrated electronic implants for electrical stimulation which can be safely powered and independently addressed by means of galvanic coupling. Since these implants lack bulky components as coils and batteries, we anticipate it will be possible to accomplish very thin implants to be massively deployed in tissues. We have also shown that these devices can have bidirectional communication. The aim of this work is to demonstrate a circuit architecture for embedding EMG sensing capabilities in our galvanically powered implants. The circuit was simulated using intramuscular EMG signals obtained from an analytical infinite volume conductor model that used a similar implant configuration. The simulations showed that the proposed analog front-end is compatible with the galvanic powering scheme and does not affect the implant’s ability to perform electrical stimulation. The system has a bandwidth of 958 Hz, an amplification gain of 45 dB, and an output-referred noise of 160 µVrms. The proposed embedded EMG sensing capabilities will boost the use of these galvanically powered implants for diagnosis, and closed-loop control.

Injectable Stimulators Based on Rectification of High Frequency Current Bursts: Power Efficiency of 2 mm Thick Prototypes

To overcome the miniaturization bottleneck imposed by existing power generation/transfer technologies for implantable stimulators, we have proposed a heterodox electrical stimulation method based on local rectification of high frequency (≥1 MHz) current bursts delivered through superficial electrodes. We have reported 2 mm thick addressable injectable stimulators, made of off-the-shelf components, that operate according to this principle. Since a significant amount of high frequency power is wasted by Joule heating, the method exhibits poor energy efficiency. In here we have performed a numerical case study in which the presence of the above implant prototypes is simulated in an anatomically realistic leg model. The results from this study indicate that, despite low power transfer efficiency (~0.05 %), the power consumed by the external high frequency current generator is low enough (<4 W) to grant the use of small portable batteries.