Multichannel implantable stimulator for control of paralyzed muscle

Electrical connectors for neural implants: design, state of the art and future challenges of an underestimated component

Technological advances in electrically active implantable devices have increased the complexity of hardware design. In particular, the increasing number of stimulation and recording channels requires innovative approaches for connectors that interface electrodes with the implant circuitry.

OBJECTIVE

This work aims to provide a common theoretical ground for implantable connector development with a focus on neural applications.

APPROACH

Aspects and experiences from several disciplines are compiled from an engineering perspective to discuss the state of the art of connector solutions. Whenever available, we also present general design guidelines.

MAIN RESULTS

Degradation mechanisms, material stability and design rules in terms of biocompatibility and biostability are introduced. Considering contact physics, we address the design and characterization of the contact zone and review contaminants, wear and contact degradation. For high-channel counts and body-like environments, insulation can be even more crucial than the electrical connection itself. Therefore, we also introduce the requirements for electrical insulation to prevent signal loss and distortion and discuss its impact on the practical implementation.

SIGNIFICANCE

A final review is dedicated to the state of the art connector concepts, their mechanical setup, electrical performance and the interface to other implant components. We conclude with an outlook for possible approaches for the future generations of implants.

Neurophysiology and neural engineering: a review

Neurophysiology is the branch of physiology concerned with understanding the function of neural systems. Neural engineering (also known as neuroengineering) is a discipline within biomedical engineering that uses engineering techniques to understand, repair, replace, enhance, or otherwise exploit the properties and functions of neural systems. In most cases neural engineering involves the development of an interface between electronic devices and living neural tissue. This review describes the origins of neural engineering, the explosive development of methods and devices commencing in the late 1950s, and the present-day devices that have resulted. The barriers to interfacing electronic devices with living neural tissues are many and varied, and consequently there have been numerous stops and starts along the way. Representative examples are discussed. None of this could have happened without a basic understanding of the relevant neurophysiology. I also consider examples of how neural engineering is repaying the debt to basic neurophysiology with new knowledge and insight.

Early History and Challenges of Implantable Electronics

Implantable systems for biomedical research and clinical care are now a flourishing field of activities in academia as well as industrial institutions. The broad field includes experimental explorations in electronics, mechanical, chemical, and biological components and systems, and the combination of all these. Today virtually all implants involve both electronic circuits and micro-electro-mechanical-systems (MEMS). This article offers a very brief glance back at the early history of implant electronics in the period from the 1950s to the 1970s, by employing selected examples from the author's research. This short review also discusses the challenges of implantable electronics at present, and suggests some potentially important trends in the future research and development of implantable microsystems. It is aimed as an introduction of implantable/attached electronic systems to research engineers that are interested in implantable systems as a section of Biomedical Instrumentations.

A single-channel implantable microstimulator for functional neuromuscular stimulation

This paper describes a single-channel implantable microstimulator for functional neuromuscular stimulation. This device measures 2 x 2 x 10 mm3 and can be inserted into paralyzed muscle groups by expulsion from a hypodermic needle. Power and data to the device are supplied from outside by RF telemetry using an amplitude-modulated 2-MHz RF carrier generated using a high-efficiency class-E transmitter. The transmitted signal carries a 5-b address which selects one of the 32 possible microstimulators. The selected device then delivers up to 2 microC of charge store in a tantalum chip capacitor for up to 200 microseconds (10 mA) into loads of < 800 omega through a high-current thin-film iridium-oxide (IrOx) electrode (approximately 0.3 mm2 in area). A bi-CMOS receiver circuitry is used to: generate two regulated voltage supplies (4.5 and 9 V), recover a 2-MHz clock from the carrier, demodulate the address code, and activate the output current delivery circuitry upon the reception of an external command. The overall power dissipation of the receiver circuitry is 45-55 mW. The implant is hermetically packaged using a custom-made glass capsule.

Peripheral nerve stimulation for restoration of motor function

This review paper discusses the use of electrical stimulation to restore function after upper motor neurone type of paralysis. It describes the basic physiology of electrical stimulation, the electrophysiology and biomaterials associated with using metal electrodes to deliver charge to living tissue, and also the adverse effects of stimulation. The central concepts of electrode applications, stimulus parameters, muscle fatigue, and stimulation control are covered. Next, a survey of clinical applications is made with focus on upper and lower limb applications. A concluding section mentions the current status of commercial products available for stimulation.

Grasp synthesis for upper-extremity FNS. Part 1. Automated method for synthesising the stimulus map

Functional neuromuscular stimulation (FNS) has been used in open-loop upper-extremity neuroprostheses to provide grasp and release for quadriplegic individuals. A principal issue in implementation of the open-loop neuroprosthesis is the synthesis of the grasp stimulus map, which is the relationship between the input control signal and the output stimulus applied to each electrode. The goal of this project was to develop a method of stimulus map synthesis which compensates for the influence of the electrode-position-dependent recruitment properties on the grasp output, and which can be performed using automated procedures. A method called the external moment grasp synthesis procedure (GSP) has been developed which meets these criteria. The GSP uses an external moment of model of the hand which describes the interaction between the active moments produced by electrical stimulation, the passive joint moments and the total joint moment and angle. The GSP was used to develop the stimulus map for three paralysed subjects, and in all cases the GSP-generated grasp patterns had lower errors than grasp patterns developed using the previously implemented rule-based method of stimulus map synthesis. The GSP was successfully implemented by computer and was able to compensate for the effects of the electrode recruitment properties on the grasp pinch force.

Effect of the metallic seal of a hermetic enclosure on the induction of power to an implant

Most neuroprostheses which use integrated circuits protect these chip components in a hermetic package. No satisfactory method of sealing such an enclosure has been found which does not use metals. Therefore, in general, the seal is an electrically conducting ring. If induction is used to supply power to this device, this ring will be a 'short-circuited turn' which will affect the performance of the inductive link. In the paper, theory is presented for a model in which the metallic seal is a tertiary inductance, coupled to the primary and secondary. The tertiary has finite Q. Equations for this model are given from which formulae for the gain and efficiency are derived for the particular condition of tuning that the carrier frequency equals the resonant frequency of both the primary and secondary circuits. From the formulae, gain curves are plotted which show how the seal affects the link. However, it is clear that general solutions to the problem are needed if the theory is to be of practical use.

Electrode characterization for functional application to upper extremity FNS

A quantitative method has been developed to characterize the isometric force vectors of electrically stimulated paralyzed muscles of the thumb. The vectorial force output as a function of the stimulus level was measured for individual electrode/muscle combinations in a number of intramuscular and epimysial electrodes implanted in paralyzed thenar muscles of cervical level spinal cord injury subejcts. Vectors are used to determine the output characteristics of each electrode/muscle combination. The characteristics studied include: the strength of the contraction, the stimulus level at which fibers from other muscles are stimulated, the recruitment gain of force, dependency of the output on the skeletal position, and the direction of force produced. These characteristics can then be used to select stimulus parameters to produce coordinated hand motion and force generation by functional neuromuscular stimulation (FNS). The range of muscle force and direction for each electrode/muscle combination showed considerable variation between subjects and between electrodes in the same subject. This variation is primarily due to differences in electrode placement within the muscle. Comparison between intramuscular and epimysial electrodes demonstrated similar characteristics in the force vector output. Preliminary results show the potential for using the force vector output to predict the cocontracted output of two muscles.

A multichannel FES system for the restoration of motor functions in high spinal cord injury patients: a respiration-controlled system for multijoint upper extremity

A multichannel functional electrical stimulation (FES) system for the restoration of quadriplegic upper extremity function is described. The system is composed of a personal computer NEC PC-8801mkII, peripheral electronic circuits, CRT display and respiratory sensors for volitional control by the patient, and percutaneous electrodes. A C4 quadriplegic patient could drink canned tea by herself by using this FES system. Distinct features of the system are as follows: 1) Versatile volitional control was realized by controlling the memory allocation of the stored stimulation data by voluntary respiratory signals. 2) Sophisticated fine control of the fingers, wrists, and elbow was realized by creating the multichannel stimulation data from recorded myoelectric activities of normal subjects during movements of the upper limb.

Synthesis of hand grasp using functional neuromuscular stimulation

A functional neuromuscular stimulation system has been developed to provide grasp-release function in quadriplegic individuals. A single command input from the subject controls the stimulus levels to a number of electrodes, thus simultaneously activating several muscles. A method for synthesizing the command input to stimulus output relationship has been developed. The first step involves electrode profiling, which is a method for characterizing the output of an individual electrode/muscle combination. The electrodes are then grouped according to function and a set of rule based procedures is used to synthesize the basic grasp parameters. Results are presented demonstrating the output from lateral and palmar grasps developed by this method. The method has successfully resulted in grasping patterns that can be utilized functionally. Limitations of the method and future improvements are discussed.

Functional electrical stimulation for the control of the upper extremities

A multi-channel functional electrical stimulation (FES) system for the restoration of hand function of the quadriplegic is described. The system is composed of a personal computer NEC PC-880lmkII, peripheral electronic circuits and two kinds of sensors, i.e. an analog displacement sensor for volitional control (channel 1) and a logical sensor (high pitch sound or head switch, channel 2). Combination of the two channel signals allow three major function: 1) designation of the desired prehension pattern among cylindrical grasp, key grip and parallel extension grip; 2) selection of the operation status--'start', 'proportional control', 'hold', 'stop'--and, 3) volitional control which can be controlled by the shoulder movement. In the clinical application, Caldwell-Reswick type multistrand stainless steel percutaneous electrodes were used. In this FES system, standard multi-channel stimulation patterns were obtained from electromyographical analysis of joint movement of the upper extremities in normal subjects which gave us precise information about a role of each muscle during various kinds of motion. Such stimulation patterns have enabled us to restore motor function of the paralyzed upper extremities for activities of daily living (ADL).

Recruitment properties of monopolar and bipolar epimysial electrodes

Muscle force was studied as a function of stimulus parameters, epimysial electrode position relative to nerve supply, and muscle length to provide insight into the properties of motor prostheses that employ epimysial electrodes. The results of the acute experiments indicated that the dependence of recruitment (force versus stimulus amplitude) on muscle length was minimal for a monopolar electrode positioned close to nerve entrance or 5 mm proximal to the motor point. The selectivity of stimulation (minimal activation of adjacent muscles) was best, and the recruitment rate the highest, for an electrode placed close to the nerve entrance. A bipolar pair of electrodes placed on the superficial surface of the muscle opposite to the nerve entrance gave better selectivity than the monopolar electrode at the low end of the recruitment range, and poorer selectivity at the high end. This electrode configuration required greater stimulus currents and exhibited a lower recruitment gain than was obtained for a monopolar electrode in the same position. Examination of tissue surrounding the electrode 30 days after implantation showed that the fibrous tissue encapsulating the electrode had been incorporated into the fascial layer. Slightly larger dependence on muscle length and lower selectivity of stimulation were measured after encapsulation than were measured in the acute experiments.