Pulse-Clamp Technique for Characterizing Neural-Stimulating Electrodes

Abstract

A reliable test methsod is required to ensure adequate charge-injection capability for high-resolution neural-stimulation applications that demand both a large amount of charge injection and a small electrode size. A pulse-clamp circuit is designed and employed to characterize the electrode charge-storage capability that will allow different electrodes to be quickly and accurately compared. The custom circuit, which consists of commercial components, has a switching charge noise of 15 pC and a switching time of 1 s. Pulse-clamp measurements are performed on flat platinum electrodes in 0.15 M saline solution in air with 1 ms long pulses at a charge density of up to 1 mC/cm 2 . Results indicate a safe charge-injection limit of 0.1 mC/cm 2 , and a hydrolysisdominated regime above 0.5 mC/cm 2 . Comparing different electrode sizes indicates that the charge-storage capability of an electrode is proportional to its surface area. The scalability of the pulse-clamp technique allows it to be used to accurately quantify the roughness of a surface modification. Simulation program with integrated circuits emphasis simulation is used to extract the electrode capacitance and resistance parameters. These parameters, which can vary, depend strongly on the injected charge density.In recent years, neural-stimulating devices have been used to restore lost function to patients suffering from neurodegenerative diseases. 1 By electrically stimulating neurons, devices such as pacemakers and cochlear implants allow patients to overcome congenital defects and gain functionality and independence. 2 These prosthetic devices typically use platinum, iridium oxide, or titanium nitride electrodes to inject the necessary current to elicit a neural response. These materials are chosen because of their ability to inject large amounts of charge with negligible electrode degradation. 3,4 To ensure long device lifetimes, most devices incorporate electrodes with a large surface area and operate at well below their charge-injection limit to avoid unnecessary dissolution and gas evolution which damages biological cells. 5 However, in order to increase the spatial resolution of the prosthetic device, the electrode density needs to be increased ͑i.e., smaller individual electrodes͒. An example of an application that needs high-resolution stimulation is the epi-retinal prosthesis, which is designed to reintroduce a form of vision to patients blinded by macular degeneration or retinitis pigmentosa through electrical stimulation of remaining retinal neurons. 6,7 Because of the physiology of the retina, substantial charge injection ͑1 C͒ is required to elicit a visual response. 8 For a flat platinum electrode, an area of 1 mm 2 is required to inject a charge of 1 C without accelerated dissolution. Because the area of the retina used for central vision is only 10 mm 2 , using 1 mm 2 area electrodes on the retinal surface is clearly not suitable for a high-resolution retinalprosthesis system. To reduce the electrode area while maintaining the required charge-in...

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