Raymond A. Peck

Micromodular implants to provide electrical stimulation of paralyzed muscles and limbs

Describes the design, fabrication, and output capabilities of a microminiature electrical stimulator that can be injected in or near nerves and muscles. Each single channel microstimulator consists of a cylindrical glass capsule with hermetically sealed electrodes in either end (2-mm diameter/spl times/13-mm overall length). Power and digital control data can be transmitted to multiple implants (256 unique addresses) via a 2-MHz RF field created by an external AM oscillator and inductive coil. In vitro testing demonstrated accurate control of output pulsewidth (3-258 /spl mu/s in 1-/spl mu/s steps) and current (0-30 mA in two linear ranges of 16 steps each, up to 8.5 V available compliance voltage). Microstimulators were used successfully for chronic stimulation in hindlimb muscles of cats. Design and fabrication issues affecting yield and reliability of the packaging and electronics are discussed.

BION™ system for distributed neural prosthetic interfaces

We have developed the first in a planned series of neural prosthetic interfaces that allow multichannel systems to be assembled from single-channel micromodules called BIONs (BIOnic Neurons). Multiple BION implants can be injected directly into the sites requiring stimulating or sensing channels, where they receive power and digital commands by inductive coupling to an externally generated radio-frequency magnetic field. This article describes some of the novel technology required to achieve the required microminiaturization, hermeticity, power efficiency and clinical performance. The BION1 implants are now being used to electrically exercise paralyzed and weak muscles to prevent or reverse disuse atrophy. This modular, wireless approach to interfacing with the peripheral nervous system should facilitate the development of progressively more complex systems required to address a growing range of clinical applications, leading ultimately to synthesizing complete voluntary functions such as reach and grasp.

Cuff electrodes for chronic stimulation and recording of peripheral nerve activity

A comparative study of 5 different designs of nerve cuff electrodes was undertaken to determine their relative merits for stimulating and recording whole-nerve activity over extended periods of chronic implantation on large and small peripheral nerves in 8 cats. Four of the designs represent novel fabrication strategies, including 2 based on flexible, thin-film substrates and 2 based on dip-coating silicone elastomer on a cylindrical mandrel. Various advantages and shortcomings of these materials and designs are discussed in the context of the biophysical factors that influence these electrophysiological interfaces, particularly the problem of recording microvolt-level neurograms in the presence of millivolt-level electromyograms from adjacent muscles in freely behaving subjects. The most effective design was one in which a thin sheath of silicone rubber was wrapped around and intra-operatively sealed to a longitudinally slit, tripolar cuff made by dip-coating silicone over stranded stainless steel leads that were prepositioned on a mandrel using polyvinyl alcohol as a temporary adhesive. When properly installed, these electrodes had stable impedances, recruitment thresholds and relatively interference-free recording properties for the duration of this study (up to 9 weeks).

Toward the ultimate metal microelectrode

The performance of metal microelectrodes for stimulating and recording neuronal action potentials depends on precise control of their geometrical, electrical and mechanical properties. We describe a combination of materials whose properties approach fundamental physical limitations on achievable performance and reproducible fabrication techniques that provide probes with very small dimensions. Pure iridium wire is electrolytically sharpened, vapor-coated with Parylene-C insulation and the tip exposed using an automatically steerable UV laser. Electrochemical activation of the iridium increases the capacitance of the metal-electrolyte interface so that the overall impedance in the relevant frequency band (100-10,000 Hz) is dominated by the access resistance of the surrounding tissues.

BION/sup TM/. Bionic neurons for functional and therapeutic electrical stimulation

The goal of this project is to develop complete clinical systems for the application of functional electrical stimulation to alleviate disabilities and morbidity associated with musculoskeletal paralysis and atrophy. Over the past 8 years, the authors have designed, built and successfully tested in animals the first device in a new class of implantable electronic interfaces with nerve and muscle. BIONs are hermetically encapsulated, leadless electrical devices that are small enough to be injected percutaneously into muscles (2 mm diameter by 15 mm long). They receive their power and digital addressing and command signals from an external transmitter coil that can be worn by or placed under the patient. The authors have designed and built a family of accessory items required for clinical trials, including sterile testing and insertion tools and a bedside control unit so that clinicians can program various patterns of therapeutic stimulation to build strength and bulk in hypotrophic muscles, e.g. following stroke, spinal cord injury, major trauma and orthopedic reconstructions. Other potential applications include urinary incontinence, prevention of deep vein thrombosis and alleviation of chronic pain. Functional recovery of limb movement requires feedback signals about ongoing movement, for which the authors are developing second-generation BIONs that provide outgoing telemetry of such data.

RF-powered BIONs/spl trade/ for stimulation and sensing

Virtually all bodily functions are controlled by electrical signals in nerves and muscles. Electrical stimulation can restore missing signals but this has been difficult to achieve practically because of limitations in the bioelectric interfaces. Wireless, injectable microdevices are versatile, robust and relatively inexpensive to implant in a variety of sites and applications. Several variants are now in clinical use or under development to perform stimulation and/or sensing functions and to operate autonomously or with continuous coordination and feedback control.

Design and Testing of a Percutaneously Implantable Fetal Pacemaker

We are developing a cardiac pacemaker with a small, cylindrical shape that permits percutaneous implantation into a fetus to treat complete heart block and consequent hydrops fetalis, which can otherwise be fatal. The device uses off-the-shelf components including a rechargeable lithium cell and a highly efficient relaxation oscillator encapsulated in epoxy and glass. A corkscrew electrode made from activated iridium can be screwed into the myocardium, followed by release of the pacemaker and a short, flexible lead entirely within the chest of the fetus to avoid dislodgement from fetal movement. Acute tests in adult rabbits demonstrated the range of electrical parameters required for successful pacing and the feasibility of successfully implanting the device percutaneously under ultrasonic imaging guidance. The lithium cell can be recharged inductively as needed, as indicated by a small decline in the pulsing rate.

Microminiature molding techniques for cochlear electrode arrays

We provide a general method for producing a variety of small, complex electrode arrays based on injection molds produced using computer-aided drafting and machining (CAD-CAM) procedures and a novel method for connecting to the very fine electrical leads associated with the individual contacts of such arrays. Cat-sized cochlear electrode arrays with up to eight contacts were built according to these methods and their electrical contacts were characterized in vitro by impedance spectroscopy and in vivo by monitoring impedance for over 1 year of intermittent stimulation in chronically instrumented animals.

Development of BION/spl trade/ technology for functional electrical stimulation: hermetic packaging

BIONs/spl trade/ are chronically implanted, individually addressable, single channel electrical stimulators that are now in clinical trials. They receive power and command signals from an externally worn RF transmission coil. The electronic circuitry is packaged in a glass capsule that can be injected through a hypodermic needle to provide distributed multichannel FES systems. The small size (2 mm OD /spl times/ 16 mm long), external electrodes and long design life (>10 years) pose a challenge for hermetic sealing and testing. We here describe several novel packaging techniques that have been integrated into a single workstation to enable efficient and reliable production of BION implants.

Design and fabrication of hermetic microelectronic implants

Micromodular electrical stimulators called BIONs/sup TM/ (BIOnic Neurons) are injected into paralyzed muscles to treat disuse atrophy. The small, narrow shape required novel solutions to address requirements for wireless power and data transmission, electromechanical assembly and biocompatible hermetic packaging. Analysis of these problems provides some insights into the limits of the constituent microtechnologies.