F. Terry Hambrecht

Bio: F. Terry Hambrecht is an academic researcher from National Institutes of Health. The author has contributed to research in topics: Tantalum pentoxide & Neural Prosthesis. The author has an hindex of 4, co-authored 6 publications receiving 296 citations.

Sensors for Use with Functional Neuromuscular Stimulation

Abstract: Functional neuromuscular stimulation (FNS) designates artificially applied electrical activation of muscles to restore function lost as a result of neurological lesions. FNS prostheses are currently being designed to restore urinary bladder control, standing, walking, and hand function. All of these prostheses need sensors for interaction with the human users and the environment. This paper discusses each of these prostheses with special regard to the use of sensors and the design specifications that the sensors must meet.

Theory and design of capacitor electrodes for chronic stimulation.

Abstract: Conventional metal electrodes generate electrochemical byproducts during stimulation of nerve or muscle. These byproducts may cause tissue damage, especially with the long-term stimulation necessary with neural prosthetic devices. To prevent the possibility of such damage, completely insulated electrodes have been devised which deliver current pulses by capacitive charging of the electrode surface, not involving electrochemical reactions. Anodised discs of porous tantalum, 1·0 mm in diameter and 0·25 mm thick, can deliver 0·5 ms, 5 mA pulses. Such electrodes are available as components of commercial capacitors and are easily adapted for biological use. The design may be optimised by mathematical analysis of an equivalent electrical circuit.In vitro tests demonstrate a clear advantage of these electrodes over capacitively coupled platinum-iridium electrodes in preventing oxidation-reduction reactions. The electrodes are stable on chronic implantation and should provide a safer interface between neural prosthetic devices and human tissue.

Capacitor electrode stimulates nerve or muscle without oxidation-reduction reactions.

Abstract: Porous tantalum disks, available as "slugs" from the capacitor industry, have large available surface area and a thin insulating coating of tantalum pentoxide. When implanted, they fill with extracellular fluid and operate as capacitor-stimulating electrodes having high capacitance per unit volume. Capable of stimulating excitable tissute without generating electrochemical by-products, these electrodes should provide a safer interface between neural prosthetic devices and human tissue.

Functional electrical stimulation: an overview.

Abstract: The development of future neural prostheses involves much more than connecting commercially available stimulators to disabled individuals. Safe and effective operation of prostheses requires fundamental studies of the electrode-tissue interface. The electrochemistry of the interface must be controlled to prevent toxic byproducts. Histopathological studies of stimulated tissue are necessary to establish safe limits of stimulation and to determine mechanisms of neural damage when it does occur. Electrophysiological studies elucidate which neural pathways are excited and help in the design of more selective electrode arrays. Biomaterials are required that protect the implant from the hostile environment of the body. Presently available materials are being improved and totally new materials are being developed. One of the goals of neural prostheses developers is a nonhermetic packaging material that can be applied to miniature implants without appreciably increasing their size. The techniques used to make integrated circuits on silicone substrates are ideally suited to making ultraminiature electrodes with self-contained electronic signal processing. Both integrated circuit stimulating and recording electrodes are being designed and fabricated.

Neural stimulation and recording electrodes.

Abstract: Electrical stimulation of nerve tissue and recording of neural electrical activity are the basis of emerging prostheses and treatments for spinal cord injury, stroke, sensory deficits, and neurological disorders. An understanding of the electrochemical mechanisms underlying the behavior of neural stimulation and recording electrodes is important for the development of chronically implanted devices, particularly those employing large numbers of microelectrodes. For stimulation, materials that support charge injection by capacitive and faradaic mechanisms are available. These include titanium nitride, platinum, and iridium oxide, each with certain advantages and limitations. The use of charge-balanced waveforms and maximum electrochemical potential excursions as criteria for reversible charge injection with these electrode materials are described and critiqued. Techniques for characterizing electrochemical properties relevant to stimulation and recording are described with examples of differences in the in vitro and in vivo response of electrodes.

Structure and method of manufacture of an implantable microstimulator

Abstract: An implantable microstimulator has a structure which is manufactured to be substantially encapsulated within a hermetically-sealed housing inert to body fluids, and of a size and shape capable of implantation in a living body, by expulsion through a hypodermic needle. The internal structure of the microstimulator comprises a coil adapted to function as the secondary winding of a transformer and receive power and control information. Circuit means, including control electronics, a capacitor and electrodes are provided. The electrodes, which may be made one of iridium and the other of tantalum and placed on opposite ends of the microstimulator, or alternatively, an iridium electrode at each end of the microstimulator, are at least partially exposed and provide electrical, stimulating pulses to the body.

Detection of static and dynamic activities using uniaxial accelerometers

Abstract: Rehabilitation treatment may be improved by objective analysis of activities of daily living. For this reason, the feasibility of distinguishing several static and dynamic activities (standing, sitting, lying, walking, ascending stairs, descending stairs, cycling) using a small set of two or three uniaxial accelerometers mounted on the body was investigated. The accelerometer signals can be measured with a portable data acquisition system, which potentially makes it possible to perform online detection of static and dynamic activities in the home environment. However, the procedures described in this paper have yet to be evaluated in the home environment. Experiments were conducted on ten healthy subjects, with accelerometers mounted on several positions and orientations on the body, performing static and dynamic activities according to a fixed protocol. Specifically, accelerometers on the sternum and thigh were evaluated. These accelerometers were oriented in the sagittal plane, perpendicular to the long axis of the segment (tangential), or along this axis (radial). First, discrimination between the static or dynamic character of activities was investigated. This appeared to be feasible using an rms-detector applied on the signal of one sensor tangentially mounted on the thigh. Second, the distinction between static activities was investigated. Standing, sitting, lying supine, on a side and prone could be distinguished by observing the static signals of two accelerometers, one mounted tangentially on the thigh, and the second mounted radially on the sternum. Third, the distinction between the cyclical dynamic activities walking, stair ascent, stair descent and cycling was investigated. The discriminating potentials of several features of the accelerometer signals were assessed: the mean value, the standard deviation, the cycle time and the morphology. Signal morphology was expressed by the maximal cross-correlation coefficients with template signals for the different dynamic activities. The mean signal values and signal morphology of accelerometers mounted tangentially on the thigh and the sternum appeared to contribute to the discrimination of dynamic activities with varying detection performances. The standard deviation of the signal and the cycle time were primarily related to the speed of the dynamic activities, and did not contribute to the discrimination of the activities. Therefore, discrimination of dynamic activities on the basis of the combined evaluation of the mean signal value and signal morphology is proposed.

Implanted Neural Interfaces: Biochallenges and Engineered Solutions

Abstract: Neural interfaces are connections that enable two-way exchange of information with the nervous system. These connections can occur at multiple levels, including with peripheral nerves, with the spinal cord, or with the brain; in many instances, fundamental biophysical and biological challenges are shared across these levels. We review these challenges, including selectivity, stability, resolution versus invasiveness, implant-induced injury, and the host-interface response. Subsequently, we review the engineered solutions to these challenges, including electrode designs and geometry, stimulation waveforms, materials, and surface modifications. Finally, we consider emerging opportunities to improve neural interfaces, including cellular-level silicon to neuron connections, optical stimulation, and approaches to control inflammation. Overcoming the biophysical and biological challenges will enable effective high-density neural interfaces for stimulation and recording.