Electrical Stability of Biocompatible Electrodes
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ELECTRICAL STABILITY OF BIOCOMPATIBLE ELECTRODES L. HE AND WA. ANDERSON State University of New York at Buffalo, Center for Electronic and Electro-Optic Materials, Department of Electrical and Computer Engineering, 217 Bonner Hall, Amherst, NY 14260 ABSTRACT Biocompatible electrodes were fabricated using ordinary semiconductor device processing techniques. For the purpose of identifying electrically stable electrode materials, which are biocompatible for long life, several kinds of contact metal were used. From a comparison of the metal elemental constants, it is suggested that the metal atom ionization energy, cohesive energy and the metal material conductivity may be the major factors to decide the material biocompatibility. Electrical testing shows Pd, with high atom ionization energy and low electrical conductivity, to have the best long life stability. In a test of 140 hours in saline solution with electrical power, the average maximum resistance deviation for a Pd electrode was 5%. The metal Ru, with high atom cohesive energy, has the best ability to withstand surface erosion for biocompatibility requirements. In a microscopic examination, the surface erosion for the Ru layer was invisible after 140 hours testing. The double-layer structures Pd/Ru and Mo/Ru were also tested. The maximum resistance deviations were found to be 7% and 9%, respectively. INTRODUCTION
Electrical stimulation for biomedical applications is being utilized for cochlear implants, bladder control, stimulation of limb movement and pain control[I-3]. Electrodes are required to pass the electrical signal to the proper nerve. Electrodes must be biocompatible, of small size and able to give specific stimulation. Electrode materials do not now meet these requirements, nor do they provide wireless signal transfer. Our purpose is to find certain materials which could fulfill the above requirements and to fabricate such an electrode. The work presented here is based on previous work[4] in our laboratory. Ordinary semiconductor device fabrication techniques such as photolithography, chemical etching, impurity diffusion, metal evaporation and/or sputtering were used. The samples were tested for stability in saline under a continuous current condition. Optical microscope examinations were performed on the samples before and after testing for evaluation of surface erosion. EXPERIMENTAL
P-type doped silicon wafers with a thick Si02 layer( about 2 micrometers thick) were first cleaned by acetone, methanol and deionized water accompanied by using an ultrasonic cleaner. An ultraviolet lamp photolithography was conducted for mesa confinement. Buffered HF was used to etch the surface Si02 layer away in the electrode area while the photoresist could preserve the area without patterns from being etched to provide electrical isolation. The conductive channel was formed by boron dry dif16 m3 fusion. The surface carrier concentration was higher than lxlO cm after diffusion. Different kinds of metal including: Al, Ni, Pd, Cr, Ru, RuO2, Au and Mo, were used for
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