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    Now the Good News? Part 1: The Cochlear Implant

    Introduction

    This paper is based on the lecture given to the RACI Chemical Education Division conference held in Queensland in June 1998. The theme was 'Bridging the gap' and three strands were identified -'Bridging the gap between chemistry and the community', 'Bridging the gap between secondary and tertiary 'and 'Bridging the gap to industry'. I had been invited to talk in the 'Community' strand about my work on the Cochlear implant, an Australian-invented prosthesis for the profoundly deaf, as a way of bringing good scientific news to the community. I started work on the paper with an up-beat message, determined to bring the benefits of science to the apparently ungrateful public. By the time the paper was delivered, the title had acquired a question mark, and the talk was less about implants but more about the sophistication needed when explaining any science, no matter how obviously beneficial.
    As the readers of this resource book can probably cope with a reasonable amount of science. I have split the original paper into two. In part one I shall describe the implant and particularly the work my laboratory carried out on its electrochemistry. In the following paper I shall review the discussion about how science should be communicated to the public, and argue that a scientist should be sensitive to different opinions and should join the public debate with the understanding that he or she is not going to persuade all of the people instantly.

    The Cochlear Implant

    The Cochlear implant is a prosthetic device that stimulates the auditory nerve in response to sound. It is designed for the profoundly or severely deaf following certain illnesses or accidents in which the cilia in the cochlea are damaged or destroyed. Research into cochlear implants can be traced back to 1957, but the first breakthrough in multichannel devices was made by Professor Graeme Clark from the University of Melbourne in the 1970s (1). The first 10-channel implant was first switched on in 1978, and the more sophisticated 22 channel implant was ready for commercialization in 1982(2).
    The device is surgically implanted after which the recipient undergoes extensive training to learn to interpret the current pulses in terms of hearing. The implant, surgery and following training is expensive (around A$20,000), but may be covered by public medical benefits.

    The device has two parts. Externally, sound is received by a microphone and the signal is then processed by electronics worn on a belt. The output is a radiofrequency signal that is sent through the skull behind the ear to a receiver, which responds by applying short (200 ms) current pulses between specific pairs of electrodes located in the spiral cochlea (Fig. 1). In a present design there are 22 such electrodes.

     The technology has been extensively developed in recent years and the company now claims that users of the device score nearly 80% in standard hearing tests. Research is being conducted into a device that will stimulate the brain stem directly, for those whose auditory nerve is not functioning. In 1995 Cochlear became the only company to gain clearance for the use of its device in adults who are Severely Hearing Impaired (i.e. not completely deaf).

    The company has financed research (with the Australian Government) into the electrochemistry of the device. The work provides an interesting example of how understanding the fundamental science can aid the development of the technology.

    Electrical and electrochemical operation of the implant (3)

    The stimulating waveform for the Cochlear implant is shown below (Fig. 2). Square wave current, charge balanced, symmetrical pulses of width up to 200 ms and pulse amplitude up to about 1.75 mA are applied to electrode pairs on the array, and the frequency of the pulse train is generally of the order of 100Hz. Also a short interpulse delay of about 10 ms may be applied.

     The aim is to use a waveform that will utilise only the "safe" reactions to inject charge, or at least to limit the production of harmful species. It is generally accepted that if the potential response is kept within the limits of hydrogen and oxygen evolution the only reactions that occur will be contained at the electrode surface. The reactions that inject charge without diffusion of dangerous by products include, platinum oxidation and oxide reduction [1], hydrogen plating and stripping [3] and the charging of the double layer and specific adsorption of anions such as the chloride ion (6).

    Electrochemistry of platinum in perilymph

    The liquid inside the cochlea is known as perilymph. It is a saline solution containing amino acids and proteins. The implant operates by passing current between platinum electrodes through the perilymph, and so the research started by investigating the electrochemistry at platinum in such a solution. The composition of perilymph is known (Table 1), so the experiments could be carried out in vitro in an artificial solution. It is of interest that the concentration of amino acids in perilymph increases markedly after the round window has been pierced. (This occurs when the device is implanted).

     Two series of experiments were conducted. An investigation of the electrochemistry of platinum in artificial perilymph was carried out using cyclic voltammetry, and the dissolution of platinum during operation was measured. Experimental details may be found in Weitzner and future publications of the author (4).

    Cyclic voltammetry is a technique in which the potential at an electrode is cycled linearly with time between set lower and upper limits. The measured current is displayed against the voltage (Fig. 3) and from this it is possible to gain insights into the electrochemical mechanisms at the electrode. In phosphate buffered saline peaks due to platinum oxidation and reduction (peaks PO and PR respectively in Fig.3) and hydrogen ion reduction and oxidation of adsorbed hydrogen atoms (HR and HO respectively) may be seen. At the extremes of voltage, water is broken down to hydrogen gas (H2) and oxygen gas (O2).

    It is seen from Fig. 3, that the platinum oxide reduction peak (PR) is reduced in the presence of artificial perilymph. This indicates that the extent of platinum oxidation as the voltage is swept more positively, is not as great. However the oxidation current in the region of voltage that platinum oxide is increased (PO). These facts may be rationalised by arguing that the amino acids are irreversibly oxidised in preference to the platinum. Experiments on the amino acids and proteins separately showed that the effect comes from the sulfur-containing amino acids, particularly cysteine. It is known that thiols (the sulfur analog of an alcohol) adsorb strongly at platinum. A similar mechanism probably operates as the electrode becomes a cathode. Adsorption causes a lowering of hydrogen evolution which is reflected in the smaller current as adsorbed hydrogen is re-oxidised (HO), but the reduction current is enhanced as the adsorbed amino acids are reduced (HR). Experiments with and without added human serum albumen, show that there is no extra effect due to protein. To avoid the breakdown of the perilymph, the potential at the electrodes should be maintained between about -500 and +600 mV.

    Dissolution of platinum

    A second aspect of the electrochemistry of interest to Cochlear was the extent to which platinum may dissolve during the operation of the device. Platinum is a toxic heavy metal, and although it is known to be chemically inert, long term use as an electrode does raise questions of dissolution and release of platinum into the body.
    After cleaning in acid, a triangular wave potential sweep was applied to the working electrode, giving a series of cyclic voltammograms between -200 and +1200 mV. The sweep was stopped at the potential at which the experiment was to be carried out. The electrode was then removed from this solution, rinsed in distilled water and transferred to a beaker containing 15 mL or 20 mL of the test solution, where the test potential was applied usually for up to 31 hours with periodic sampling (0.5 mL)of the solution. This solution was not replaced as the experiment progressed. Samples were stored in plastic specimen tubes and frozen until analysis. Each experiment was performed at least twice. In all cases the working electrodes used were "home made" platinum flag electrodes. Geometrical areas were approximately 2cm2, however real areas were measured after cleaning in 0.1 M H2SO4 by determining the area under the hydrogen adsorption peak in the CV. For the dissolution experiments a thin silver wire auxiliary, and a Ag|AgCl|0.lM NaCl reference electrode were used.

    The Pourbaix diagram in Fig. 4 indicates the domains of passivity, immunity and corrosion for platinum metal in aqueous solution. The smallest wedge shaped area enclosed by the dotted lines, at low values of pH, indicates the pH/potential range in which platinum corrosion is thought to occur when no complexing agents are present. However, in solutions containing 0.15 M NaCl, this region would theoretically be expanded to the larger wedge-shaped area, due to the formation of tetrachloroplatinum (II),

    Pt2+ 4Cl- ⇌ PtCl42- E° = 0.47V

    The larger wedge-shaped region delineated by the dashed lines enclose the pH/ potential range in which the concentration of tetrachloroplatinate ion would be 10-9 M.

     The rates of platinum dissolution at pH 7 at a series of applied potentials is shown in Fig. 5. Measurements were made in plain phosphate-buffered solutions, as well as in a solution of artificial perilymph and in solution containing 200 mg/mL HSA. For plain PBS solution, the behaviour of the platinum dissolution fits with expected passivation. Between about 0.6 to 1.1 V the rate of dissolution is low and at 1.2V and 0.4V there is evidence of the dissolution rate beginning to rise. Solutions containing HSA seemed to show similar behaviour, however the presence of free amino acids in the artificial perilymph solution seemed to cause a greater degree of platinum dissolution.

    In solutions of pH 3 and pH 4 (Fig. 5) there seems to be a more obvious correlation between the rise in applied potential and the elevation in measured dissolved platinum.

     Modelling the behaviour of the implant

    The ubiquity of modern computers has allowed development of sophisticated modelling techniques that can be used for a range of scientific and technological problems. Simulating the response of a device can save considerable time and cost in bench experiments and, in the medical field, testing on animals or humans. A computer program was written in C++ (Borland C++ Builder V1.0) to simulate the voltage output of two electrodes given the current profile, and parameters of the different electrochemical reactions (Fig. 6). Double layer charging, platinum oxidation and reduction, and other electrochemical reactions were programmed. Using the program it is possible to determine voltage limits during operation, what electrochemistry is happening and what parameter ranges are safe.

    Conclusions

    Despite the complexity of the system in which the Cochlear implant operates, it is now known that the stimulation achieved by the current pulses is largely double layer charging with the formation and reduction of platinum oxide. Under normal conditions, electrochemistry operates in the 'safe region', where the reactions are reversible with no adverse products. Dissolution of platinum is not a cause for concern.

    Ackowledgments

    Ms K Weitzner (UNSW) and Dr P Carter and Mr B Tabor (Cochlear Pty Ltd) are acknowledged for their work on the electrochemistry of the Cochlear implant.

    References

    1. Tong Y.C., Black R.C., Clark G.M., Forster I.C., Millar, J.B., O'Loughlin B.J. and Patrick, J.F. (1979). The Journal of Laryngology and Otology, 93.679-695
    2. Cochlear (1998) , The growth of Cochlear, Web page, URL http ://www.cochlear.com.au/about
    3. Weitzner K., Hibbert D.8., Carter P. and Tabor B. (1997), Electrochemistry of in vivo platinum electrodes, 10th Australian Electrochemical Conference, Surfers Paradise, February 1997. Abstracts. Eds D.M. Druskovich and G. Hope, Griffith University, ISBN 1 875902 45 7
    4. Weitzner K. (1998). Electrochemical studies of auditory prostheses. PhD thesis, University of New South Wales, Sydney.
    5. Haruta A., Tono T. and Morimitsu T. (1995). Acta Otolaryngol., 115, 504-508
    6. Salt A.N. (1996), Cochlear Fluids Research Laboratory, Washington University http://lab9924.wustl.edu/

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