title: Access resistance of stimulation electrodes as a function of electrode proximity to the retina.
abstract: OBJECTIVE
  Epiretinal prostheses seek to effectively stimulate the retina by positioning electrode arrays close to its surface so current pulses generate narrow retinal electric fields. Our objective was to evaluate the use of the electrical impedance of insulated platinum electrodes as a measure of the proximity of insulated platinum electrodes to the inner surface of the retina.
  
  
  METHODS
  We examined the impedance of platinum disk electrodes, 0.25 mm in diameter, insulated with two widths (0.8 and 1.6 mm outer diameter) of transparent fluoropolymer in a rabbit retinal eyecup preparation. Optical coherence tomography measured the electrode's proximity to the retinal surface which was correlated with changes in the voltage waveform at the electrode. Electrode impedance changes during retinal deformation were also studied.
  
  
  RESULTS
  When the 1.6 mm diameter insulated electrodes advanced towards the retinal surface from 1000 μm, their voltage step at current pulse onset increased, reflecting an access resistance increase of 3880 ± 630 Ω, with the 50% midpoint averaging 30 μm, while thin 0.8 mm insulated electrode advancement showed an access resistance increase 50% midpoint averaging 16 μm. Using impedance spectroscopy, electrode-retina proximity differences were seen in the 1.6 mm insulated electrode impedance modulus between 1 and 100 kHz and the waveform phase angle at 0.3-10 kHz, while thin 0.8 mm insulated electrode advancement produced smaller impedance modulus changes with retinal proximity between 3 and 100 kHz. These impedance changes with retinal proximity may reflect different sized zones of eye wall being coupled in series with the insulated platinum electrode.
  
  
  CONCLUSIONS
  The proximity of stimulus electrodes to neural tissue in fluid-filled spaces can be estimated from access resistance changes in the stimulus pulse waveform. Because many prosthetic devices allow back telemetry communication of the stimulus electrode waveform, it is possible these series resistance increases observed with retinal proximity could be used as a metric of stimulus electrode placement.
authors.affiliation: Division of Biomedical Physics, Office of Science and Engineering Labs, Center for Devices and Radiological Health, Food and Drug Administration, Rm 1204 Bldg 62. White Oak Federal Research Labs, Silver Spring, MD 20993-0002, USA.
authors.forename: Joseph A
authors.initials: JA
authors.lastname: Majdi
authors.forename: Saugandhika
authors.initials: S
authors.lastname: Minnikanti
authors.forename: Nathalia
authors.initials: N
authors.lastname: Peixoto
authors.forename: Anant
authors.initials: A
authors.lastname: Agrawal
authors.forename: Ethan D
authors.initials: ED
authors.lastname: Cohen
mesh_terms: D000818:Animals; D020099:Coated Materials, Biocompatible / Q000737:chemistry*; D017097:Electric Impedance; D004599:Electric Stimulation Therapy / Q000295:instrumentation*; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005260:Female; D066298:In Vitro Techniques; D008297:Male; D010984:Platinum / Q000737:chemistry*; D019919:Prosthesis Implantation / Q000379:methods*; D011817:Rabbits; D012160:Retina / Q000502:physiology*; D057488:Visual Prosthesis
chemical_list: D020099:Coated Materials, Biocompatible; D010984:Platinum