Microelectrodes for in-vivo determination of pH - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 280

10.4.3 Microelectrode arrays for pH mapping
Implantable microelectronic devices for neural prosthesis require stimulation elec-
trodes to have minimal electrochemical damage to tissue or nerve from chronic stimu-
lation. Since most electrochemical reactions at the stimulation electrode surface alter
the hydrogen ion concentration, one can expect a stimulus-induced pH shift [17].
When translated into a biological environment, these pH shifts could potentially have
detrimental effects on the surrounding neural tissue and implant function. Measuring
depth and spatial profi les of pH changes is important for the development of neural
prostheses and safe stimulation protocols.
Even though the single needle type pH electrode can detect the pH change around
the stimulation electrode, it has limitations. One such limitation is that a needle type
pH electrode cannot be used to detect the pH change in the electrode-electrolyte inter-
face when the stimulation electrode array is tight against the tissue surface. Another
limitation of such needle type pH electrodes is that they cannot detect the pH change
when they are very close to the stimulation electrode surface without disturbing cur-
rent and fi eld distributions. And fi nally, using a single electrode to map a pH change
requires probe scanning. This is impractical in most cases for those in-vivo measure-
ments [17, 18]. To overcome these limitations, microelectrode arrays can be used to
measure spatial pH profi le.
Planar microelectrode arrays were constructed in our group to monitor pH changes
during electrical stimulation. The microelectrode arrays consist of both stimulating
electrodes and pH sensing electrodes. Stimulating electrodes are Pt, or other noble
metals, and their alloys while the pH sensing electrodes are iridium oxide based. The
array can be in both thick fi lm and thin fi lm forms. The iridium oxide is reactively
sputtered on the metal seed layer. The electrode sizes range from 50 to 350 m diam-

eter disks. Figure 10.7 shows an example of such a micro pH electrode array in thin
fi lm construction. The array was made of fl exible polymer with imbedded thin fi lm
metal traces. There were 16 electrodes 200

µ

4 arrangement
on the array. The needle type pH electrode can be seen on the left corner of the array
as a control. The pH response of the iridium oxide electrodes was calibrated in three
buffer solutions with a nearly ideal Nernstian response 56.9

µ m in diameter with a 4

1.1 mV/pH in the tested

pH range of 4 to 10.
The pH changes due to electric stimulation were recorded successfully in saline by the
planar micro pH electrode array [19]. A two-dimensional distribution of pH change can
be established by using such combined microelectrode arrays. Figure 10.8 shows a typi-
cal 2D pH change profi le after stimulation for only one minute. The electrode site in dark
color is the Pt stimulating electrode. All other 15 electrodes are IrOx pH sensing elec-
trodes. The charge density of stimulation pulses applied on the Pt electrode was 0.14 mC/
cm 2 . The pH changes on electrodes presented a clear 2D distribution. The four elec-
trodes closest to the Pt stimulation electrode had the most pH changes (2.9

0.3 pH),

while the four corner ones had slightly fewer 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).

These studies have revealed that the pH changes around the electrode-electrolyte interface