Ординатура / Офтальмология / Английские материалы / Artificial Sight Basic Research, Biomedical Engineering, and Clinical Advances_Humayun, Weiland, Chader_2007

Effects from intensity and frequency of stimulation pulses were also examined. The pH changes with stimulation time were monitored at different pulse currents and frequencies. To avoid gas evolution reactions, very low charge densities (30 to 120 C/cm2) were used for monophasic stimulations. It was noticed that pH changes increased with the increased pulse current and pulse frequencies. Figure 12.4 shows both cathodic-induced pH increase and anodic-induced pH decrease under different pulse current amplitudes. It is clear that the higher the pulse current, the more the pH changes are detected at any given time. This indicates that an electrode with a lower threshold in clinical applications, which requires less pulse current, will minimize possible damage due to the pH shift. The frequency effects on the pH changes during pulse stimulation were measured at 60 C/cm2 and the results showed similar effects (data not shown). Higher frequency pulse (60–120 Hz) induced more pH shift for the electrodes pulsed with lower frequency (10–30 Hz) at the same current amplitudes. For neural stimulation, pulsing should use low frequencies to improve the pH balance at the electrode–electrolyte interface.

The pH changes in the test solution were confirmed using a pH indicator, phenol red. Time for color change of phenol red indicator was recorded. For cathodic-only pulses, phenol red color change was noted around the stimulating electrode to change from yellow to pink in about 10 minutes, which indicates an increased pH, shown in Figure 12.5a. For anodic-only stimulation, phenol red color change could not be easily detected due to small contrast of yellow to light yellow color change; however, color change could be seen around the Pt counter

Figure 12.4. Induced pH shifts with various stimulation current intensities. Cathodic pulse current is shown to cause an increased pH while anodic pulse current caused a decreased pH, correlating with amplitude of current.

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A

B

Figure 12.5. (a) pH indicator color change around cathodically stimulating electrodes. Phenol red pH indicator changed color from yellow to pink, due to increase in pH, around working electrode. (b) pH indicator color change around counter electrode of anodically stimulating electrodes. pH decrease due to anodically stimulating electrodes could not be verified by phenol red pH indicator; however, phenol red around return electrode of test cell showed yellow to pink color change, indicating the increase in pH.

electrode, shown in Figure 12.5b. A different pH indicator which gives a sharp color change in lower pH range < 6 should be employed in future work to detect pH changes around anodic stimulating electrodes.

Typical neural prosthesis stimulation use charge-balanced, biphasic pulses with the intention that by-products produced in the first phase of the current pulse are reversed by the second phase [8, 14]. Figure 12.6 shows the minimal pH shift over 15 minutes of biphasic pulsing at 0 35 mC/cm2 with a cathodic-first pulse followed by anodic pulse stimulation in unbuffered saline solution. No phenol red color change was noted and the decrease in pH was less than 1 unit. Anodic-first, followed by cathodic counter pulse biphasic stimulation, showed similar results. pH shift was minimal over 15 minutes stimulation. No phenol red color change was observed and the change in pH was less than 1 unit. These results demonstrate the importance to use biphasic pulses for neural stimulations to minimize pH changes.

While the single needle-type pH electrode can detect the pH change around the stimulation electrode, it cannot be used to detect the pH change in the electrode–electrolyte interface when the stimulation electrode array is tightly against the tissue or retinal surface. Another limitation of such needle-type pH electrodes is that it cannot detect the pH change very close to the stimulation electrode surface without disturbing current and field distributions. During our preliminary studies, we found that pH changes were higher when the sensor tip is closer to the stimulation electrode (Data not shown). Additional factors influencing measurements by the needle electrode included the size of the pH senor, which contributed a blocking effect, and the movement (vibration) of solution around the stimulation electrode, due to the stirring effect. For a sensitive

Figure 12.6. A typical pH shift from biphasic, charge-balanced, cathodic-first stimulation. Minimal pH shift was detected from stimulation using biphasic pulse at 0 35 mC/cm2 in unbuffered saline solution.

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pH measurement, the pH-sensing electrode closely arranged around the stimulating electrode is important. Planar microelectrode arrays were constructed in our group to monitor pH changes during pulse stimulation. The microelectrode arrays consist of both stimulating electrodes and pH-sensing electrodes. Stimulating electrodes on these arrays could be made from Pt or other noble metals and their alloys, while the pH-sensing electrodes are IrOx based. The array can be in both thick-film and thin-film forms. The IrOx is reactively sputtered on the metal seed layer. The electrode sizes range from 50 to 350 m disks in diameter.

The IrOx pH-sensing electrodes were characterized by cyclic voltammetry. Cyclic Voltammetry (CV) was measured using an EG&G M273 potentiostat and M270 Electrochemical analysis software. The Cyclic Voltammogram for a typical IrOx electrode is shown in Figure 12.7. The potential scan range was from −0 6 to +0 9V against an Ag/AgCl reference electrode at 50 mV/s. The electrolyte was 100 mM phosphate buffered saline (PBS). The thin-film IrOx electrode exhibited characteristic redox peaks in the voltammogram. Two pairs of anodic and cathodic peaks within −0 4 to +0 8 V vs. Ag/AgCl are believed to be from redox reactions of IrOx.

Although the redox reaction mechanisms of IrOx are still not clear, most researchers believe that during electrochemical reactions, oxidation states changes in the hydrated IrOx layer is accompanied by the injection/ejection of

E/mV vs AgCl

Figure 12.7. A typical cyclic voltammogram of Electroplated IrOx. The voltammogram is recorded with a voltage scan rate of 50 mV/sec in 0.1 M phosphate-buffered saline, pH 7.4.

H+ [10, 15]. The electrode potential changes with the hydrogen ion concentration and is expressed by Nernstian equation:

where E is the standard electrode potential with the value of 926 mV vs. standard hydrogen electrode (SHE), or 729 mV vs. Ag/AgCl reference electrode, R is the gas constant, T is the absolute temperature (K) and F is the Faraday constant. The potential/pH slope is expected to be 58 mV/pH at 22 C.

An example of such micro-pH electrode array in thin-film construction is shown in Figure 12.8. The array is made of flexible polymer with imbedded thin-film metal traces. There are 16 electrodes with a 4 x 4 arrangement on the array. The pH response of the IrOx electrodes were calibrated in three standard solutions. The buffer solutions are air-saturated without stirring.

Using a series of calibration solutions, the response curve or calibration curve of a pH electrode as per the Nernstian Eq. (1) can be experimentally determined by plotting the electrode voltage versus the pH of the calibration solution. The linear range of the calibration curve is applied to determine the pH in any unknown solution. The slope of the calibration curve within the linear range is used to determine the response (Nernstian) slope or electrode sensitivity in mV/pH. Figure 12.9 shows the open circuit electrode voltages change with pH and an average Nernstian slope calculated for 16 electrodes in a micro-pH electrode array. The electrode based on this oxide film exhibits very promising pH-sensing performance, with a nearly ideal Nernstian response 57+/− 1.1 mV/pH in the tested pH range of 4–10. The intercept at pH 0 gives an E value of 756 +/− 15 mV against an Ag/AgCl reference electrode, or 953 mV vs. SHE. The experimentally determined E of 953 mV is slightly higher than the theoretical value (926 mV). The E value is related in the stoichiometry of

Figure 12.8. An example of a micro-pH electrode array in thin-film construction. There are 16 sputtered IrOx electrodes with a 4 × 4 arrangement on the array.

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Figure 12.9. A calibration curve of an IrOx-based pH electrode array measured in standard pH solutions vs. an Ag/AgCl reference electrode at room temperature. The slope of the calibration curve within the linear range is used to determine the response (Nernstian) slope or electrode sensitivity in mV/pH.

the oxide film and the difference of its oxidation state. The higher E value measured in the standard pH buffers suggests the higher oxidation states in the stoichiometry of the oxide films [10].

The potential response of the planar micro-pH electrodes is fast, with a 90% response time obtained in less than 1 second for all pH changes. The open-circuit potential of the electrode is stable during calibrations. It is evident from Figure 12.9 that the potential/pH slopes and the electrode potentials show excellent agreement among electrodes from the same array.

In order to measure pH profiles in 2D distributions, all, except one, of the electrodes in the array were coated with pH-sensing materials. The stimulusinduced pH shifts were measured with this type of multi-pH-sensing electrode array. The stimulating electrode was pulsed at monophasic pulses of either cathodic-only or anodic-only pulses. The pH changes on 15 electrodes were recorded throughout a duration of stimulation of about 10 minutes. Similar to the needle pH electrode experiments, for cathodic-only pulses, pH increase was noted around the stimulating electrode while for anodic-only stimulation, pH decrease is detected.

Figure 12.10 shows a 2D pH changes with the electrode positions after stimulation for only one minute [16]). The electrode site in dark color is the Pt-stimulating electrode. All other 15 electrodes are pH-sensing electrodes. The charge density of stimulation pulses applied on the Pt electrode was 0 15 mC/cm2. It is interesting to note that the pH changes on electrodes presented a clear 2D distribution. The four electrodes closest to the Pt-stimulation electrode had most pH changes 2 9 + / − 0 3 pH , while the four corner ones had slightly less pH changes 2 0 + / − 0 4 pH . The electrodes far away from the stimulating site showed relatively small pH changes 0 2 + / − 0 15 pH .