Materials for Implantable Electrodes
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Corrosion can also lead to chronic electrical stimulation of adjacent neurons and tissues with associated undesirable side effects. These include increased metabolic activity, vasoconstriction, formation of calcium deposits, epileptic seizures, and breakdown of the blood-brain barrier which normally protects the brain from circulating microorganismsand toxic substances. Other Problems to Overcome Damage to the tissue and damage to the electrode are not the only potential problems. In addition, the electrodes must not induce the body to respond with one of its traditional defense mechanisms—encapsulation. Normally the human body shields itself from foreign bodies by encapsulating them in connective tissue. However, this can render an implantable electrode nonfunctional by blocking access to the appropriate nerve. Different types of neural prostheses, with their different functional and physiological requirements, demand different mechanical and electrical solutions to these problems. For simpler types of implantable electrodes, the so-called single-channel devices, the pacemaker industry has developed reliable materials and manufacturing procedures. These devices—cardiac pacemakers, bone growth stimulators, some muscle stimulators and some pain reduction devices—generally use large-surface-area, low-impedance electrodes. The more complex prostheses—multichannel devices that restore auditory or visual perception, or functional neuromuscular stimulation (FNS) devices that must incorporate sensory as well as motor nerve function—require tiny, precisely located and high-impedance stimulation contracts. Such devices place more stringent demands on the materials used. Insulation of the electrode becomes increasingly important; a poorly insulated electrode will not only fail to deliver sufficient current but in addition, places the surrounding tissue at greater risk. Insertion of the electrode and prevention of breakage also become more troublesome as the electrodes become smaller. And the highercurrents necessary as a result of smaller surface areas mean increased risk of electrochemical corrosion. Biocompatibility Scientists at EIC Laboratories in Norwood, Massachusettes, led by S. Barry Brummer, have conducted a series of experiments on the electrolysis of various metals in a saline e n v i r o n m e n t . They concluded that the noble metals platinum and iridium can safely induce ionic currents in body fluids at specifically designated charge densities. Platinum or platinum-
iridium alloys have since been generally accepted as optimal electrode materials. Newer electrodes using iridium oxide surfaces are still in the research phase. Given the parameters defined by Brummer et al., scientists still had to determine whether current levels which don't induce electrolysis are sufficiently low to prevent the problems of chronic electrical stimulation. "We've been able to define safe levels using accelerated tests in animals," says F. T e r r y Hambrecht, head of the Neural Prosthesis Program, National Institute of Neurologica
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