Electrochemical considerations for safe electrical stimulation of the nervous system with platinum electrodes

Theoretical design and implementation of a transcutaneous, multichannel stimulator for neural prosthesis applications

The development of a transcutaneous, implantable, multichannel neural stimulator is described. This was originally dedicated to the stimulation of the auditory nerve in profoundly deaf persons, but is sufficiently flexible in design and operation to be applicable to other areas of neural prosthetics. Control of both the amplitude and time of stimulation for up to fifteen independent channels is possible with a maximum stimulation rate of 1 kHz. Particular attention is given to the design of the transcutaneous link stage which allows both power and data to be transferred to the implanted device using a compact coupling inductor configuration. All circuit timing is derived from a single clock in the external transmitter unit, resulting in stable operation with predictable stimulus output characteristics. The implantable device, realised using thick film hybrid techniques, employs CMOS logic extensively to reduce power consumption. One such device has been implanted in a profoundly deaf volunteer for a period exceeding two years and has continued to operate reliably in conjunction with the complete prosthesis system.

Electrical stimulation with pt electrodes. IV. Factors influencing Pt dissolution in inorganic saline

Trace analysis has shown that Pt electrodes can suffer appreciable dissolution when used to apply biphasic current pulses of the type used in neural stimulation. The dissolution occurs even under conditions where other irreversible faradaic reactions, e.g., H2O electrolysis, are avoided. In the present study, factors influencing the dissolution of Pt electrodes during biphasic pulsing in neutral inorganic saline have been examined. The findings are consistent with the behaviour of Pt electrodes reported in other inorganic media. In a given test, the quantity of Pt dissolved was found to be a linear function of the aggregate anodic or cathodic charge injected. Therefore, dissolution 'rates' can be conveniently expressed in terms of nanograms of Pt per coulomb injected, e.g., 100 ng C-1. Most of the Pt went into solution as Pt (II) species, so the above rate would correspond approximately to 100 p.p.m. of the anodic charge per pulse. For anodic-first (AF) pulses, charge density was the major factor controlling dissolution, whereas for cathodic-first (CF) pulses, pulse duration had the greater influence. Depending on the polarity (AF or CF), charge density, and duration of the biphasic pulse, the dissolution rate for smooth bead electrodes ranged from 30 to 300 ng C-1. Lower rates were achieved with platinized electrodes but in the absence of organic solutes, it is unlikely that Pt dissolution can be completely suppressed.

Electrical stimulation with Pt electrodes. V. The effect of protein on Pt dissolution

The effect of protein on the charge-induced dissolution of Pt stimulation electrodes was studied in an in vitro system. Biphasic pulses (+/- 400 mA, +/- 400 microC cm-2 geom.) were applied to smooth Pt bead electrodes (surface area = 0.03-0.08 cm2) in a buffered saline solution containing 0.075-0.2 mg/ml human serum albumin; pulse solutions were analysed periodically for dissolved Pt by flameless atomic absorption spectrometry. The incorporation of protein in the pulse solution drastically altered the dissolution behaviour of Pt electrodes. Whereas dissolution in organic saline solutions proceeded essentially in a linear fashion, the rate in protein solution decreased with time and asymptotically approached zero. The passage of electric charge was required for the observed inhibition to develop but was not required to maintain it. In contrast, the constant presence of at least 0.15 mg/ml protein in the pulse solution was required for inhibition to develop and continue. Higher concentrations of protein did not enhance the observed inhibition. These findings bear favourably on the prospect for in vivo use of Pt stimulation electrodes: the dissolution process now appears to place no greater restriction on the acceptable pulse parameters than do other irreversible reactions such as H2O electrolysis.

Some practical considerations in development of multichannel scala tympani prostheses

There are several basic issues regarding the safety and feasibility of implanting multichannel scala tympani cochlear prostheses in humans. Physiological and technical studies have been designed to resolve some of these questions. Results of physiological investigations demonstrate that (1) nerve viability is not affected by the presence (without activation) of a multichannel array; (2) use of a highly specified bipolar electrode configuration permits discrete and predictable stimulation of sectors of the auditory nerve array, and (3) some deleterious effects (i.e., nerve damage and calcification products) may result when stimulus parameters (intensity, duration and waveform symmetry) are not well controlled over long periods of stimulation. Results of technical investigations regarding the specifications (materials and design) of multi-electrode arrays and engineering studies regarding hardware and software for safe and efficient stimulators for humans have provided devices with which 2 patients could be implanted and tested psychophysically. Some preliminary results of testing with these provide data regarding threshold, loudness discrimination and pitch perception.

A circuit for safe diagnostic electrical stimulation of the human brain

An electronic circuit for controlled electrical stimulation of the human brain has been designed to optimize safety in charge transfer from electrodes to brain and to eliminate the likelihood of unwanted currents from the neurostimulator resulting from component failure. The circuit schematics feature the following designs: (1) a highly accurate and versatile rate generator 0.5 p.p.s. to 99 in increments of 0.5 p.p.s., (2) symmetrically-biphasic lead and lag pulses of 100 musec duration, (3) photo-isolated driver amplifiers with accurate waveform, reproduction, (4) true biphasic passive-current regulators driven by an isolated battery supply voltage for switch-selectable currents of 0.25 to 5 ma or variable current regulation, (5) accurate current and voltage waveform monitors that isolate the stimulating electrodes from the monitors, and (6) capacity-coupled outputs to guarantee no net D.C. component at the end of each biphasic pulse. Relay-switching circuits are also shown that allow sequential stimulation and recording from one or several electrodes. This neurostimulator has been in use for over two years without evidence of electrolytic damage at identifiable electrode tips. The utility of simultaneous stimulation of several different electrodes at safe charge levels is described.

Biomedical engineering specifications for epidural spinal cord stimulation to augment motor performance

Stimulating electrodes were placed in the posterior portion of the epidural space in the upper thoracic spinal region in 28 patients with upper motor neuron disorders. Parameters of stimulation commonly used for chronic stimulation were a 200 microsecond pulse width, 22 Hz repetition rate, and 5 ma amplitude. Using a 16 gauge Touhy needle, platinum electrodes were passed between the posterior vertebral processes into and up the spinal epidural space to upper thoracic locations. Passive implanted receivers, powered and controlled by external RF transmitter/pulse-generator devices, were employed to provide the stimulus current to the electrodes. A description of available systems, problems and diagnosis of problems, and future directions is presented. This is based on studies in patients with stimulation systems implanted for more than six months.

Indications and ethical considerations of deep brain stimulation

Electrical impulses through chronically implanted electrodes in the human brain are used today in the relief of chronic crippling pain, motor movement disorders and behavior disturbances. A great number of reports from animal studies and several descriptions of the results produced by human stimulation seem to suggest that pain can often be adequately controlled. However, the perspectives of such methods raise many ethical problems and ask for caution. Nervous tissue damage after long-term stimulation, biochemical modifications, electrode migration and long-term follow-up of its clinical value are still under study and are the factors which might limit for the time being the indications. A restrictive role for this procedure is advocated for: 1. cases of chronic facial pain, 2. cases of chronic pain in other locations where other neurosurgical operations have failed and where the patient is not too young and is not suffering from pain of benign origin.

[Physiological basis for a cochlear prosthesis (author's transl)]

For the attempt to develop a cochlear prosthesis, which allows some understanding of speech, it seems--at least for the first attempts--to be appropriate to mimic natural conditions as far as possible. The auditory nerve contains about 30,000 afferent fibres. Qualitatively, their behavior is similar but quantitative measures show considerably differences (2.3). Nothing certain can be said at present however about the spiral fibres originating from the outer hair cells. The quantitative differences between single afferents concern tuning, frequency selectivity, thresholds, intensity functions and--of particular interest for electrical stimulation--differences in timing of the activity pattern, brought about by differences in travelling time along the cochlear duct (2.3). The time differences seen in the activity pattern of different fibres are in the order of several ms (2.3.6;2.3.7). Actionpotentials elicited by natural acoustic stimuli show probabilistic behavior, that is they are not strictly determined. It is obvious that with artificial electrical stimulation not every surviving single fibre can be selectively stimulated. An electrode will always stimulate a group of fibres simultaneously. With any conceivable electrical stimulation all fibres in the suprathreshold region of the electrode will be synchronously activated (3.2); a fundamental difference to the natural situation. To estimate the number of channels, necessary to stimulate the auditory nerve with sufficient accuracy to allow speech perception we consider some psychoacoustic data. These have shown that the auditory system possesses the ability to differentiate a great number of different pitches, but on the other hand it is capable of integrating different frequency areas to a so called critical bandwidth. Sound energy falling into one critical bandwidth is integrated to a uniform auditory sensation. If one is to integrate various fibres of the auditory nerve to one channel of stimulation it seems to be adequate to use the critical bandwidth as a measure (3.1). According to this criterion 15 channels would have to be introduced into the speech region of the cochlea. This would allow 1.2 mm of cochlear length for each channel. Perfect electrical separation of the channels is required. Considering the severe distortions in neuronal activity pattern, introduced by electrical stimulation in comparison to the natural conditions it is not clear even whether the number given would be sufficient. On the other hand, current spreading would appear to prohibit any higher electrode density. As far as coding of sound parameters within one channel is concerned it is proposed that full use should be made of frequency analysis according to the place principle. In respect to coding of periodicity and loudness it is proposed to approach natural conditions as far as possible (3.3). Here delay times between the individual channels and a probabilistic character of the stimuli should be introduced to avoid dominance of periodicity pitch...