N. de N. Donaldson

Feedback control of unsupported standing in paraplegia. I. Optimal control approach

This is the first of a pair of papers which describe an investigation into the feasibility of providing artificial balance to paraplegics using electrical stimulation of the paralyzed muscles. By bracing the body above the shanks, only stimulation of the plantarflexors is necessary. This arrangement prevents any influence from the intact neuromuscular system above the spinal cord lesion. In this paper, we extend the design of the controllers to a nested-loop LQG (linear quadratic Gaussian) stimulation controller which has ankle moment feedback (inner loops) and inverted pendulum angle feedback (outer loop). Each control loop is tuned by two parameters, the control weighting and an observer rise-time, which together determine the behavior. The nested structure was chosen because it is robust, despite changes in the muscle properties (fatigue) and interference from spasticity.

When are actively balanced biphasic (‘Lilly’) stimulating pulses necessary in a neurological prosthesis? I Historical background; Pt resting potential;Q studies

Actively balanced (‘Lilly’) stimulating current waveforms are generally considered to give very ‘safe’ stimulation. Although this is perfectly true, the specification of the necessary waveform generators in neurological prostheses demands additional complexity, and probably additional expense and development time as well. The paper and its companion enquire whether the use of simple, passively charge-balanced stimulating pulses is equally safe, provided the stimulation parameters and circuitry are designed with appropriate care. It is concluded that, in respect of safe deliverable charge density per pulse at the electrode, release of noxious products and stimulating effectiveness, simple pulses need give no worse performance; in some circumstances they may give better.

Implantable FES Stimulation Systems: What is Needed?

Since their initial development, the performance gains in functional electrical stimulation (FES) systems have been modest. Conceptually, the replacement of normal neural function by artificial electronic systems is attractive, considering the continued technologic advancements in electronics, communication, and control. It is likely that efficacious FES systems will require complete implantation and activation of large numbers of motor units. One approach is to develop a neural interface that has a one‐to‐one relationship between stimulating electrodes and lower motor neurons. While technology may offer solutions to the design of miniaturized implantable stimulators, the high–density neural interface remains more elusive.

Implantable telemeter for long-term electroneurographic recordings in animals and humans

A system is described that amplifies an electroneurographic signal (ENG) from a tripolar electrode nerve cuff and transmits it from the implanted amplifier to an external drive box. The output was raw ENG, bandpass filtered from 800 to 8000 Hz. The implant was powered by radio-frequency induction and operated for coil-to-coil separations up to 30 mm. The testing and performance of the system is described. The input-referred noise was never more than 1μV RMS, and, at some positions of the radio-frequency field, was 0.7μV, close to the expected value for the amplifier used. The common-mode rejection ratio (CMRR) depended on the impedance imbalance from the cuff and the length of input cable. Devices with a short cable and low source impedance had CMRR of 84 dB, but, with 31 cm of cable and a real cuff, the CMRR fell to 66 dB. Recovery from a stimulus artifact took 5ms. The responses of the cuff to external potential gradients and to common-mode signals are described theoretically or by simulation. The devices are available for use in neuroprosthetic or neurophysiological research.

Feedback control of unsupported standing in paraplegia. II. Experimental results

For pt. I see ibid., vol. 5, no. 4, p. 331-40 (1997). This is the second of a pair of papers which describe an investigation into the feasibility of providing artificial balance to paraplegics using electrical stimulation of the paralyzed muscles. By bracing the body above the shanks, only stimulation of the plantar flexors is necessary. This arrangement prevents any influence from the intact neuromuscular system above the spinal cord lesion. Here, the authors present experimental results from intact and paraplegic subjects.

When are actively balanced biphasic ('Lilly') stimulating pulses necessary in a neurological prosthesis? II. pH changes; noxious products; electrode corrosion; discussion.

Actively balanced (‘Lilly’) stimulating current waveforms are generally considered to give very ‘safe’ stimulation. Although this is perfectly true, the specification of the necessary waveform generators in neurological prostheses demands additional complexity, and probably additional expense and development time as well. The paper and its companion enquire whether the use of simple, passively charged-balanced stimulating pulses is equally safe, provided the stimulation parameters and circuitry are designed with appropriate care. It is concluded that, in respect of safe deliverable charge density per pulse at the electrode, release of noxious products and stimulating effectiveness, simple pulses need give no worse performance; in some circumstances they may give better.

Performance of platinum stimulating electrodes mapped on the limitvoltage plane

The paper introduces the electrode limit-voltage plane (ELVP)—a useful tool for describing in electrical terms what a polarisable electrode is doing under a particular set of conditions. The ELVP is then used to consider the charge-injection capability of a platinum stimulating electrode.

An implant for chronic selective stimulation of nerves

An implantable stimulator system has been developed for nerve stimulation. The system is capable of stimulating selectively, either by fibre position, fibre size or by sending action potentials in one direction only, based on the use of nerve cuffs. The stimulator produces either quasi-trapezoidal current pulses, to allow anodal blocking, or conventional rectangular-shaped current pulses, of amplitude 20 microA to 5 mA (in 20 microA steps) with duration of 16 micros to 1 ms (in 8 micros steps). For safety, both active and passive charge balancing is used. The amplitude of the active charge-balancing phase can be varied between 1/7 and 1/47 of the pulse amplitude. During manufacture, each implant is customised so as to drive either 6 quasi-tripolar (dipolar), 4 tripolar or 2 pentapolar cuffs. Possible applications of the device are: improved defaecation and bladder voiding after spinal cord injury, by stimulation of the sacral motor roots; neuromodulation to reduce hyperreflexia without concomitant muscle contractions; in stroke patients, to enable balanced inversion-eversion while dorsiflexing the ankle by stimulating the peroneal nerve. It may also be used in chronic animal experiments.This paper describes the implant system, its hardware and communication protocol, and shows results from in vitro tests of the device and the first acute anodal-blocking experiments in pigs.

Voltage regulators for implants powered by coupled coils.

The relative performances of three types of voltage regulator which could be used in implants powered by coupled coils have been studied theoretically and with apparatus on the bench. Ranking the types according to power loss over the region of coupling coefficient in which the output is regulated, switch mode is the best and shunt the worst; although greater efficiency requires more complex circuits. A switch-mode circuit is given whose own current consumption is small enough for an actual benefit from this type to be shown, compared with an equivalent series regulator. The importance of correct transmitter tuning to good efficiency is illustrated.

Recruitment by motor nerve root stimulators: significance for implant design

Three paraplegics have been implanted with stimulators of the lumbar anterior roots. Twelve roots were trapped in slots, each with three electrodes, a central cathode and two anodes, but the anodes in all the slots were connected together to reduce the number of wires. Cross-talk between roots was observed at lower levels than expected. Cross-talk was assessed from the ratio of the roots threshold to the threshold of the contralateral response (expected ratio: 72). Two hypothetical reasons for this low ratio were: that the cathode current was not equally shared by the anodes; or that the contralateral responses were reflex. Experiments showed that neither explanation was valid. The ratio of the contralateral to ipsilateral threshold for individual slots (K(1)) was sometimes low because the ipsilateral threshold was high. By taking the ratio of the lowest contralateral response to lowest ipsilateral response, for all roots in each subject (K(2)), the ratio should approach the theoretical value. However, for the two subjects with small slots, it was 7.9 and 15.3, much less than 72, suggesting that the original theory was incorrect. Approximate calculations of the activation function suggest that the reason may be that roots which run close to a slot, but not through it, may pass through a virtual anode region outside the ends of the slots, and that anodal break stimulation in those regions causes the cross-talk. Our estimate is that this cross-talk would be expected to occur at intensities above 5.3 times the cathodal threshold. If the roots are stimulated in pairs, below the levels of cross-talk, experimental results show that the moments obtained in response are additive to within 5%.