Abstract
Electrodes for cortical stimulation need to deliver current to neural tissue effectively and safely. We have developed electrodes with a novel annular geometry for use in cortical visual prostheses. Here, we explore a critical question on the ideal annulus height to ensure electrical stimulation will be safe and effective. We implanted single electrodes into the motor cortex of anesthetized rats and measured the current required to evoke a motor response to stimulation, and the charge injection capacity (CIC) of the electrodes. We compared platinum iridium (PtIr) electrodes with different annulus heights, with and without a coating of porous titanium nitride (TiN). Threshold charge densities to evoke a motor response ranged from 12 to 36 μC.cm-2.ph-1. Electrodes with larger geometric surface areas (GSAs) required higher currents to evoke responses, but lower charge densities. The addition of a porous TiN coating did not significantly influence the current required to evoke a motor response. The CIC of both electrode types was significantly reduced in vivo compared with in vitro measurements. The measured CIC was 72 and 18 μC.cm-2.ph-1 for electrodes with and without a TiN coating, respectively. These results support the use of PtIr annular electrodes with annulus heights greater than 100 μm (GSA of 38, 000 μm2). However, if the electrodes are coated with porous TiN the annulus height can be reduced to 40 μm (GSA of 16,000 μm2).
Highlights
Stimulation of nerve cells for the restoration of brain and sensory deficits has been successfully implemented, most prominently with the cochlear implant (Tyler et al, 1988; Svirsky et al, 2000; Cogan, 2008) and in deep brain stimulation (Breit et al, 2004)
Pure Pt is too soft to do so (Brummer et al, 1983) and the electrode shafts need to be made from stronger materials, e.g., platinum iridium (PtIr) alloys, to ensure that electrodes do not bend during insertion (Brummer et al, 1983)
Charge Injection Capacity The measured charge injection capacity (CIC) was determined from the charge that could be injected before the potential exceeded the water window of PtIr when stimulating with single biphasic current pulses
Summary
Stimulation of nerve cells for the restoration of brain and sensory deficits has been successfully implemented, most prominently with the cochlear implant (Tyler et al, 1988; Svirsky et al, 2000; Cogan, 2008) and in deep brain stimulation (Breit et al, 2004) This has been extended to vision restoration for the blind by the development of prostheses that stimulate nerve cells at some point along the visual pathway – the retina (Schwahn et al, 2001; Zrenner et al, 2009; Humayun et al, 2012; Shepherd et al, 2013), the optic nerve (Delbeke et al, 2002; Veraart et al, 2003), and the visual cortex (Dobelle, 1999; Normann et al, 1999; Troyk et al, 2005; Lowery, 2013) – and produce the perception of spots of light, ‘phosphenes’ (Weiland and Humayun, 2008; Shepherd et al, 2013). Pure Pt is too soft to do so (Brummer et al, 1983) and the electrode shafts need to be made from stronger materials, e.g., platinum iridium (PtIr) alloys, to ensure that electrodes do not bend during insertion (Brummer et al, 1983)
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