Nanostructured Electrodes for Improved Neural Recording
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Nanostructured Electrodes for Improved Neural Recording Karen C. Cheung1, Yang-Kyu Choi2, Tim Kubow1, and Luke P. Lee1 (1) Berkeley Sensor & Actuator Center and Department of Bioengineering, University of California, Berkeley (2) Department of Electrical Engineering and Computer Science, University of California, Berkeley ABSTRACT We present a new method of increasing the effective electrode surface for improved neural recording. To optimize the electrode, the impedance can be decreased by introducing surface roughness or nanostructures on the electrode. High aspect ratio pillar-like polysilicon nanostructures are created in a reactive ion etch. Nanostructure robustness in cell culture is examined. INTRODUCTION The interface between neurons and electrodes is one of the key issues in implantable microdevices and bioelectronics. The ideal electrode requires maximum selectivity and minimum impedance. During in vivo experiments, electrode insertion inevitably results in tissue damage, and signals from living cells must travel through the attenuating layer of damaged cells. Selectivity refers to the ability to select a single neuron from a multitude of interconnected cells or even damaged cells. High impedance in an electrode attenuates and filters the measured signal. In addition, a sufficient signal-to-noise ratio is required for data analysis; low electrode impedance gives high signal gain [1]. However, an increase in electrode selectivity with smaller geometric electrode size results in increased impedance and noise. To optimize the electrode, the impedance per unit of geometric surface area must be decreased by altering its nanostructure. With the introduction of surface roughness or nanostructures, a square 20 µm x 20 µm electrode has only 400 square microns of geometric (planar) area but a much larger effective area. Effective electrode surface area can be increased through micropatterning, surface roughening, and chemical modification [2]. In micropatterning, structures such as grooves or holes are fabricated on the electrodes. If this pattern is etched into the substrate, the metal that forms the electrode must then be deposited conformally. Although an aspect ratio as high as 100:1 can be achieved using deep reactive ion etching, the metal deposition method (e.g. evaporation or sputtering) ultimately limits the aspect ratio and thus the final effective surface area of the micropatterned structures. These structures are typically on the micro scale. Another method of fabricating such structures is by first photolithographically defining the structures on top of the electrode and then electroplating metal using photoresist as a mold. Electrode materials historically used in neuroscience are gold, platinum, and iridium. The most common surface modifications are wet chemical etching for gold, electroplating platinum black or ion milling the platinum surface, or activating iridium to form iridium oxide. However, they suffer from some drawbacks such as harsh chemical treatment in the case of gold etching, and poor ad
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