Randles Model of Vitreous Humor

Tjerignimin Silue, Saugandhika Minnikanti and Nathalia Peixoto

Electrical and Computer Engineering, George Mason University, Fairfax, VA, U.S.A.

Keywords: Vitreous Humor, Stainless Steel Electrodes, Electrochemistry, Characterization, Cyclic Voltammetry,

Electrochemical Impedance Spectroscopy, Randles Model.

Abstract: The vitreous is a gel-like structure found in the eyes. It is located above the retina to prevent the passage of

fluids. As aging occurs, the vitreous can liquefy and can cause retinal detachment. The literature has little

characterization of the vitreous, as it is often a less interesting structure than the retinal tissue. We

investigate the impedance properties of the stimulation electrodes such as the constant phase element (Q)

and the resistance of the solution (R

sol

). We show results on vitreous characterization through

electrochemical methods as a first step toward understanding the role of electrical stimulation in retinal

prosthetics applications as it pertains to vitreous liquefaction. Our objective is to characterize the vitreous

for a wide frequency range and to determine how charge is distributed through its conductive structure. Our

electrochemical experiments were performed using insulated stainless steel electrodes (1) in phosphate

buffered saline (PBS) and (2) in thimerosal as controls, (3) in vitreous without thimerosal, as well as (4) in

vitreous preserved with thimerosal. We also performed cyclic voltammetry to measure the cathodic charge

storage capacity for the electrodes for all experimental groups. Our results showed that the resistivity of the

vitreous increases as thimerosal is added and that the cathodic charge storage capacity of the vitreous does

not show any significant difference in the means as thimerosal is added.

1 INTRODUCTION

Retinal prostheses for the blind are in clinical trials

world-wide. Each retinal prosthesis attempts to

position the stimulus electrode array in close

proximity to the retina in order to effectively

activate the remaining retina neural circuitry (Majdi

et al., 2014). Stimulus pulses can be remarkably

attenuated by resistive and capacitive barriers in the

eye wall as well as the vitreous (Majdi et al., 2014),

which in some cases could prevent effective

activation of the retinal tissue. Poorly placed

electrode arrays can lose charge through this gel like

conductive structure called the vitreous humor.

The vitreous humor is a virtually acellular, jelly

adhesive-like, highly hydrated extracellular

substance located above the retina to prevent rapid

passage of fluid into holes which may exist in the

retina (Joseph, 1990). A network of thin unbranched

collagen fibrils that are mixed in composition,

comprising collagen types II, V/XI and IX,

maintains the gel-like structure in a rabbit model

(Bishop, 2000). The vitreous humor contains two

chemically specific proteins: mucoid and vitrein.

The mucoid fills the gap of the micelles of the

vitrein, which prevents it from diffusing away. The

chemical distribution of the vitrein in the vitreous

humor suggests that the vitreous humor is not

strictly a uniform gel and that the vitreous

membrane is composed of vitrein (Krause, 1934).

During ageing, the gel can liquefy. In about 30%

(Bishop, 2000) of the population, the residual gel

structure eventually collapses away from the

posterior retina in a process called vitreous

detachment. This process plays an important role in

a number of common blinding conditions including

rhegmatogenous retinal detachment, proliferative

diabetic retinopathy and macular hole formation

(Bishop, 2000). Thus, understanding molecular

events underlying vitreous liquefaction and vitreous

detachment may lead to new retinal prostheses to

improve retina degenerative diseases caused by

vitreous detachment.

Retinal prostheses are implants with custom

circuits that electrically stimulate retinal cells

through electrodes. Recent results point to

confounding variables that might have been

overlooked as significant in earlier designs such as

the influence of the vitreous humor (Shah et al.,

2007). For example, sticking of the vitreous to the

156

Silue T., Minnikanti S. and Peixoto N.

Randles Model of Vitreous Humor.

DOI: 10.5220/0006166001560162

In Proceedings of the 10th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2017), pages 156-162

ISBN: 978-989-758-216-5

retina may change the impedance seen by the

electrodes when they are positioned at the surface of

the retina. Recent studies examined the electrical

properties of the retinal-electrode interface and

showed that the tissue resistance of the retina is

greater than that of the vitreous humor in the eye but

little is known about the electrical behavior of the

vitreous humor (Shah et al., 2007). In order to

perform experiments of reasonable length in any

biological tissue, traditionally the tissue is kept in

media. For vitreous humor, previous research has

demonstrated that the use of a preservative does not

impact viscosity or other morphological

characteristics of the sample (Kawano et al, 1982).

Thimerosal is a methiolate sodium used in vaccines.

In retinal research, thimerosal in a concentration of

0.005% is added in a small quantity (1 µl) to the

vitreous for long-term observations. The addition of

thimerosal at this concentration does not affect the

viscosity of the vitreous (Kawano et al., 1982) which

will be shown through vitreous characterization

using electrochemical methods as a first step toward

understanding its role during electrical stimulation in

retinal prosthetics applications.

The objective is to investigate methods to

reliably determine the characteristics of the vitreous

from a stimulation electrode to the sample tissue

extracted from rabbit eyes. As previously stated,

characterizing the vitreous will aid in the

determination of the conductivity of the structure,

the location of the electrode within the eye, during

the implant period. Both of these variables are

critical for the long term success of implants.

Currently available technology will be used and we

hypothesized that, by measuring and analyzing

impedance and charge delivery, we can establish the

electrochemical characterization of the vitreous

humor.

We will investigate how the impedance

properties of the stimulation electrodes. In order to

determine which factors influence the impedance,

experimental data will be fitted to a Randles model

and evaluated from the physical characteristics

perspective. Solution resistance, charge transfer

resistance, and constant phase elements, when taken

together, determine the interface between the

electrode and the electrolyte. The model can, in the

future, be scaled to determine impedance models of

the human eye can be used to demonstrate the

effects of stimulation waveforms on the tissue.

In this study, we measured the electrical

properties of the vitreous of the rabbit eye with

implantable electrodes. We studied the

electrochemical impedance spectroscopy of stainless

steel electrodes of 250 µm diameter, insulated with

Teflon (10 µm). We tested the impedance of the

electrodes in four different scenarios: (1) in

phosphate buffered saline (PBS), (2) in thimerosal

(an organomercury compound), (3) in vitreous

without thimerosal; as well as (4) in vitreous with

thimerosal. We also performed cyclic voltammetry

and measured the cathodic charge storage capacity

for the electrodes for all experimental groups.

Several electrical equivalent models were tested and

fitted to the data to describe the physical system and

provide insight into the actual model for the

vitreous. We hypothesized that vitreous with and

without thimerosal would therefore not change its

electrical characteristics, given the low

concentration of the preservative.

2 METHODS

Here we describe the utilized substrates and the

electrochemical methods that were applied in order

to characterize the vitreous samples.

2.1 Preparation of Vitreous and

Solutions

A 1 ml of vitreous humor, stored in a 2 ml vial, was

bought from a biological supply company

(BioChemed Services, Winchester, VA).

Thimerosal was acquired from Sigma-Aldrich (St.

Louis, MO). A 0.005% of thimerosal solution was

prepared with 5 mg of thimerosal (C9H9HgNaO2S)

dissolved in 100 ml of distilled water. 1 µl of the

prepared 0.005% thimerosal solution was added to

the extracted vitreous for preservation according to

Kawano et al (Kawano et al., 1982). The final

concentration of thimerosal in vitreous humor is

5x10

-5

mg/ml.

2.2 Electrochemical Measurements

We manufactured clinically relevant stainless steel

electrodes of 250 µm core-diameter, insulated with

Teflon (10 µm) and characterized their

electrochemical behavior in phosphate buffered

saline, thimerosal and in vitreous. The electrode is

micromanipulated and positioned in the vitreous

sample, and electrochemical measurements were

simultaneously taken. We recorded electrochemical

impedance spectroscopy (EIS) and cyclic

voltammetry (CV).

A high efficiency, research-grade potentionstat

designed for fast and low-current measurements

Randles Model of Vitreous Humor

157

(Reference 600+, Gamry, Warminster, PA) was used

to perform measurements for the electrochemical

characterization of the vitreous humor.

Electrochemical Impedance Spectroscopy (EIS) and

Cyclic Voltammetry (CV) was performed. To

achieve EIS, a frequency response analyzer (FRA) is

used to impose an AC signal to a cell. The AC

voltage and current response of the cell is analysed

by the FRA to determine the resistive, capacitive and

inductive behaviour - the impedance - of the cell at

that particular frequency (Loveday et al., 2001). On

the other hand, CV is achieved by cycling potential

to measure current. Characterization of the vitreous

humor as well as the stainless steel electrode was

performed in Phosphate Buffer Saline (PBS) and in

the 0.005% thimerosal solution. The stainless steel

electrodes were scanned from 1 Hz to 100 kHz for

the EIS measurements and the CV curves were ran

for 5 cycles at a scan rate of 100 mV/s. In addition,

impedance spectra and CV of the stainless steel

electrodes were performed in thimerosal diluted

(0.005 %) in PBS and water. This was undertaken to

investigate the properties of thimerosal and its effect

on the electrodes.

For the purpose of our experiment, a two

electrode setup was employed to measure the

potential across the cell where the counter and

reference terminals of the potentiostat were shorted

and connected to an Ag/AgCl wire. The working and

working sense were connected to a stainless steel

wire electrode (250 µm in diameter, n = 5) for EIS

and CV measurements of the vitreous humor (figure

1).

Figure 1: Schematic of experimental setup.

2.3 Data Analysis

The electrode is modelled by a Randles circuit,

shown in figure 2, containing a constant phase

element, indicated by (Q), and modelled as a non-

linear capacitor which maintains the phase

difference between current and voltage constant

Figure 2a: Equivalent circuit model for PBS, vitreous,

vitreous w/ thimerosal (distilled water) and thimerosal w/

PBS.

Figure 2b: Equivalent circuit model for thimerosal w/

distilled water.

throughout the frequency spectrum. The constant

phase element is parallel to a resistor (R

), and these

two elements are commonly called pseudo-

capacitance or polarization impedance in electrodes.

The solution resistance of the vitreous, which is

mainly water, is denoted by R

sol

. The Randles circuit

was generated for measuring the impedance between

the electrode and ground, we applied a small (10

mVrms) sinusoidal wave of varying frequency, and

measure the current through the circuit, that is,

between the electrode and the Ag/AgCl wire.

Reference and counter electrodes are shorted, given

the low currents involved. The modulus of the

measured impedance is then the amplitude

difference between the applied voltage and

measured current, and the phase angle of the

impedance is the phase difference of both sinusoidal

waves. By applying this method from 1 Hz to 100

kHz one obtains Nyquist and Bode plots (modulus

and phase) of the combined impedance of the whole

system: electrode, insulation, solution resistance.

ZSimpWin (EChem Software, Ann Arbor, MI)

was used to develop the Randles circuit model

(figure 2) from the EIS data. ZSimpWin employs the

down-hill simplex method for optimizing the fits.

Data were exported as text files from the Echem

Analyst software (Gamry Instruments, Warminster,

PA). The models presented with visual fit to Bode,

Nyquist, real, and imaginary impedance values

versus frequency plots, chi-squared value (χ

) <

10×10

-4

, relative standard errors < 15% and non-

trending residual plots. As a two- electrode set up

was employed, the measured EIS data will reflect

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the properties of the working electrode (WE), of the

media, and of the reference electrode (RE). Initially,

impedance spectra obtained from PBS was fit to the

model. The electrodes were modelled as a parallel

connection of a constant phase element (Q) and the

charge transfer resistance (R

). The reference and

working electrodes were connected via a series

resistance (R

sol

) representing the media. The

constant phase element arises due to surface non-

uniformity and roughness of the interface. The

impedance of the constant phase element is given by

Z = Yo (iω)α, where (Yo)-1 is a constant with

dimension Fcm-2s(α-1), ω is angular frequency

(2πf), i =√-1, and 0 < α < 1, where α = 1 for an ideal

capacitor. The results were obtained by averaging

results from five electrodes (n=5) in each

experimental group.

The measurement of how much charge can be

delivered to tissue, by any electrode, is the integral

of current over time, as dQ/dt = I, where Q is the

charge in Coulombs, t is the time in seconds, and I is

the current in Amperes. When the CV excitation

voltage is applied at a slow rate, the hysteresis

observed (difference between the positive and

negative cycles of the voltage) denotes how much

charge can held by the electrode. The area under the

CV curve, is the charge storage capacity of the

electrode, and a common parameter reported in the

literature is the cCSC, cathodic charge storage

capacity. This parameter is relevant in implants used

for tissue stimulation because it sets the maximum

possible charge transfer by a given electrode

material and its geometrical area. This will allow for

the evaluation of how well that specific electrode is

performing during the implant period. CV tests to

our experimental data will be shown and charge

transfer mechanisms in vitro (with the electrode

immersed in vitreous) will be evaluated.

3 RESULTS

The interplay between the electrode surface

chemistry and the charge transfer into the vitreous

solution was examined in detail by using

voltammetric techniques. The system was stimulated

with a quasi-DC voltage that ranged from -0.8V to

0.7V, this could vary depending on the electrode

size and type. As the voltage is varied slowly over

time (at 100 mV/s), the current flowing through the

two electrodes was measured and plotted. Stability

was determined and reduction and oxidation peaks

on the current waveforms were observed. The cyclic

voltammetry (CV) spectrum gave insights when the

Figure 3: (Blue) thimerosal + PBS (0.005%), (Red)

thimerosal + Distilled Water (0.005%), (Grey) PBS,

(Orange) vitreous w/o thimerosal (0.005%), (Dark Blue)

vitreous w/ Thimerosal (0.005%).

Table 1: Mean and standard deviation of the vitreous with

and without thimerosal of all 5 electrodes. All values are

in Coulombs (C ).

Vit+Thim Vit-Thim

Mean -1.63×10

-6

-1.46×10

-6

st. dev. ±5.63×10

-7

±2.35×10

-7

electrode was immersed in phosphate buffered

saline, thimerosal versus when it is dipped into the

vitreous.

The integration of the CV curve using

EchemAnalyst provided the cathodic charge storage

capacity (cCSC), expressed Coulombs (C), of the

vitreous with thimerosal and the vitreous without

thimerosal solutions (table 1). The mean cCSC was

-1.63×10

-6

C for vitreous with thimerosal and -

1.46×10

-6

C for vitreous without thimerosal while

the standard deviation was ±5.63×10

-7

C for vitreous

with thimerosal and ±2.35×10

-7

C for vitreous

without thimerosal.

The mean and standard deviation of the charge

storage capacity determined to characterize the

electrode in vitreous with and without thimerosal

provides information about the performance of the

electrode in the electrolyte. Figure 3 shows the CV

curves of the stainless steel electrode in the vitreous

with and without thimerosal. A one-way ANOVA

test was performed to determine the statistical

significance of the means for each electrode. The p-

value lower than 0.05 (p = 0.000, 0.012, 0.40, 0.008,

0.022 respectively) indicated that the mean

difference between the vitreous with and without

thimerosal is statistically different for all electrodes.

A Tukey test was then performed to determine if

there is a significant difference between the mean of

the two groups. Results showed that the means are

not significantly different. The insignificant

Randles Model of Vitreous Humor

159

difference between the means is also observed in

figure 3. The “vitreous w/o thimerosal” and

“vitreous w/ thimerosal” voltammetric curves are

similar, revealing that the charge transfer from the

electrode to the vitreous is not significantly altered

by the addition of thimerosal. If there had been any

proteins adsorbed onto the surface of the electrode,

then the “vitreous w/ thimerosal” spectrum would

show significant differences to the “vitreous w/o

thimerosal” spectrum.

Figure 4A: Overlay of EIS impedance curves of a random

electrode (Blue) thimerosal + PBS (0.005%), (Red)

thimerosal + Distilled Water (0.005%), (Grey) PBS,

(Orange) vitreous w/o thimerosal (0.005%), (Dark Blue)

vitreous w/ thimerosal (0.005%).

Figure 4B: Overlay of EIS phase curves of a random

electrode (Blue) thimerosal + PBS (0.005%), (Red)

thimerosal + Distilled Water (0.005%), (Grey) PBS,

(Orange) vitreous w/o thimerosal (0.005%), (Dark Blue)

vitreous w/ thimerosal (0.005%).

Table 2: Summary of the fitted parameter values with

corresponding standard deviations to impedance spectra of

stainless steel electrodes measured in PBS, vitreous and

vitreous with 0.005% tthimerosal solution.

PBS Vitreous Vitreous +

thimerosal

(0.005 % in

dist. Water)

Q-Y

-SS

(uF)

6.49± 4.59 1.18 ± 1.28 1.59 ± 1.55

Q-n-SS 0.49± 0.13 0.54 ± 0.07 0.50 ± 0.07

(k Ω)

1.82 ±

1.15

24.49 ±

10.60

18.34 ±

8.56

sol

(Ω)

473.55 ±

56.64

659.83 ±

310.60

1333.98 ±

1258.39

Q-Y

-RE

(uF)

0.12± 0.02 0.10 ±0.03 0.10 ± 0.03

Q-n-RE 0.85± 0.01 0.83 ± 0.02 0.83 ±0.02

Table 3: Summary of the fitted parameter values with

corresponding standard deviations to impedance spectra of

stainless steel electrodes measured in 0.005% thimerosal

diluted in distilled water versus in PBS solutions.

Thimerosal

(0.005% in

distilled water)

Thimerosal

(0.005 % in

PBS)

Q-Y

-SS 60.40 ± 1.01 (nF) 6.84 ±3.28 (uF)

Q-n-SS 0.80 ± 0.10 0.43 ± 0.05

(kΩ) 65.36 ± 62.566 12.92 ± 18.31

sol

(uF) 0.145 ± 0.04 N/A

sol

11.93 ± 5.54 GΩ 940.9±276.78

kΩ

Q-Y

-RE 0.38 ± 0.22 (nF) 0.11 ± 0.03 (uF)

Q-n-RE 0.60 ± 0.07 0.85 ±0.01

Electrochemical impedance spectra of the

electrodes were acquired for PBS, vitreous with and

without thimerosal diluted with distilled water.

Equivalent circuit model (figure 2) was fitted to the

data (figure 4). The averaged parameters (n=5) are

shown in table 2. The “solution” resistance (R

sol

) is

greater for vitreous in comparison to PBS. The

estimated model parameters depict this trend

(473.55 ± 56.64 Ω (PBS), 659.83 ± 310.60 Ω

(vitreous)). The solution resistance further increases

when thimerosal (0.005 %) is added to the vitreous

(1.34 ± 1.25 kΩ). The RE electrode parameters are

the same across all media. The constant phase

element (Q) describes the deviation of the interfacial

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impedance from the ideal behavior. The Q-n

exponent values for the stainless steel electrode (SS)

for PBS, vitreous and vitreous with thimerosal

(0.005% in water) represent a deviation from a

capacitive nature towards resistive (<0.6). The

charge transfer occurring at the electrode-electrolyte

interface is highest for vitreous and lowest in PBS

while R

is lower when thimerosal is added to the

vitreous. All the values are reported in the form of

value ± standard deviation.

Impedance spectra were compared for

thimerosal in distilled water versus PBS with the

same dilution to understand its properties and effect

on the electrodes performance. The averaged

parameters (n=5) are shown in table 3. The

“solution” resistance (R

sol

) is not only greater when

thimerosal (11.93 ± 5.54 GΩ) is diluted in water in

comparison to PBS (940.90 ± 276.78 kΩ) but also

presents a capacitive nature (0.145 ± 0.04 uF).

Interestingly, the reference electrode parameters are

different when the dilution media is changed (table

2). While the double layer capacitance is two orders

of magnitude lower in the distilled water dilution

(60.4 nF), versus PBS (6 µF), the Q-n exponent

values for the stainless steel electrode (SS) is higher

0.80 ± 0.10, indicating the behaviour of a capacitor.

As expected, the charge transfer resistance

occurring at the electrode-electrolyte interface is

higher for water dilution when comparing to PBS

because the media is less conductive as expected.

All the values are reported in the form of value ±

standard deviation.

4 DISCUSSION

The effects of thimerosal diluted with PBS versus

distilled water is determined with the “solution”

resistance (R

sol

). From our data, it is noticed that R

sol

is only greater when thimerosal (11.93 ± 5.54 GΩ) is

diluted in water in comparison to PBS (940.90 ±

276.78 kΩ) indicating its capacitive nature. This

may be attributed to the low amount of free ions in

distilled water in comparison to PBS. Moreover, the

double layer capacitance lower in the distilled water

dilution (60.40 ± 1.01 nF) with the higher Q-n

exponent values (0.80 ± 0.10Ω) persists to conclude

that thimerosal diluted with water has a capacitive

behavior.

Though the RE electrode parameters is the same

across all media, the “solution” resistance (R

sol

)

increases further when thimerosal (0.005 %) is

added to the vitreous (1.34 ± 1.25 kΩ). This is true

due to thimerosal containing ethylmercury, a

compound that acts as both a substrate and an

inducer (Clark et al., 1977). This property affects to

the resistivity of the vitreous. This also explains why

the Q-n exponent values for vitreous with thimerosal

(0.005% in distilled water) deviated from capacitive

nature towards resistive (<0.6) due to the added

thimerosal.

The difference in the charge storage capacity of

the vitreous with and without thimerosal shows that

the ability of the electrode to deliver charge to the

solution increases when dipped in the vitreous with

thimerosal (table 1). The “solution” resistance

reveals high conductivity for the vitreous solution

w/o thimerosal versus the vitreous solution w/

thimerosal. The addition of the thimerosal solution

w/ distilled water to the vitreous increased the

mixture’s resistivity while decreasing its electrical

conductivity because of the resistance (11.93 ± 5.54

GΩ) of thimerosal diluted with distilled water.

5 CONCLUSION

The determined electrical properties of the vitreous

with and without an organomercury compound with

implantable electrodes allowed for a more detailed

electrical representation of the vitreous. In retinal

prostheses, the issue of the vitreous sticking to the

retina presents itself. Since the resistivity of the

vitreous increases when the thimerosal is added, it is

expected to notice an increase in the impedance seen

by the electrodes when they are positioned at the

surface of the retina. Though the charge storage

capacity is not significantly different as thimerosal is

added, its addition leads to the increase of the

solution resistance. This in turn decreases the

electrical conductivity of the vitreous solution with

thimerosal.

Our randles model and findings can be used to

provide a starting point to assist regulators in

understanding safety and effectiveness issues with

the vitreous. It can also assist the retinal implant

industry and device evaluators by establishing

common metrics of device effectiveness and aid in a

better understanding of the design issues that cause

loss of effectiveness of retinal stimulus electrodes of

retinal prostheses in blind subjects.

For future endeavors, biochemical analyses

could be attained to detect the different ions

concentration as well as the chemical processes of

the vitreous. This includes determining the

correlation between the temperature, viscosity,

liquefaction, oxidation and reduction potentials. This

Randles Model of Vitreous Humor

161

will enhance our knowledge about this gel-like

substance called the vitreous humor.

ACKNOWLEDGEMENTS

We would like to acknowledge the NSF grant FDA

SIR: 1445684.

REFERENCES

Bishop, P. N., 2000. Structural macromolecules and

supramolecular organisation of the vitreous gel.

Progress in Retinal and Eye Research, 19, 323-344

ISSN 1350-9462

Krause, C., 1934. Structural macromolecules and

supramolecular organisation of the vitreous gel. Arch

Ophthalmology. 1934;11(6):960-963.

doi:10.1001/archopht.1934.00830130044006.

Joseph, N. H., 1990. Patent US4902292 - Vitreous Body

Prosthesis Device.

Shah, S., Hines, A., Zhou, D., Greenberg, J. R., Humayun,

S. M., Weiland, D. J., 2007. Electrical Properties of

Retinal–electrode Interface. Journal of Neural

Engineering.

Kawano, S. I., Honda, Y., and Negi, A., 1982. Effects of

the Biological Stimuli of the Viscosity of the Vitreous,

National Center for Biotechnology Information. U.S.

National Library of Medicine.

Clark, D. L., Weiss, A. A., Silver, S., 1977. Mercury and

Organomercurial Resistances Determined by

Plasmids in Pseudomonas. Journal of Bacteriology

132.1 (1977): 186–196. Print.

Loveday D. et al., 2001. Potentiostatic EIS Tutorial –

Getting Started. https://www.gamry.com/application-

notes/EIS/potentiostatic-eis-tutorial/”. Web.

Peixoto, N., Jackson, K., Samiyi, R., Minnikanti, S., 2009.

Charge Storage: Stability measures in implantable

electrodes. Engineering in Medicine and Biology

Society, 2009. EMBC 2009. Annual International

Conference of the IEEE, Minneapolis, MN, 2009, pp.

658-661.

doi: 10.1109/IEMBS.2009.5333449

Majdi, J. A., Minnikanti, S., Peixoto, N., Agrawal, A.,

Cohen, E. D., 2014. Access resistance of stimulation

electrodes as a function of electrode proximity to the

retina. Journal of neural engineering 12, no. 1:

016006.

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162