Microelectrodes for in-vivo determination of pH - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 283

10.4.6 Wireless pH measurement systems
Wireless pH measurement systems have been used in clinical studies. Unlike a stand-
alone pH electrode, a power source, control electronics, and a signal transmitter have
to be incorporated into a wireless system. Watanabe et al. [138] have reported a wire-
less pH sensor to record salivary pH continuously. The sensor system transmits pH
data via a telemetry system for about 19 hours with a 3 V lithium battery (190 mAh).
The error of transmitted pH data was less than 0.15 pH in the range of pH 5.0 to 9.0.
In another study by Okij et al. [139], a wireless pH sensing system developed for
in-vivo pH measurement in the digestive system consists of a pH measurement chip,
a wireless communication chip, and a power management chip. The radio frequency
(RF) for signal transmission was 2.45 GHz and the signal attenuation was found to
be 60-90 dB through the human body equivalent. An ISFET was used as the pH sen-
sor and a capacitor type battery as the energy source. The power consumption was
reported to have a low 50 nJ with the help of a differential amplifi cation and a pulse
operation design.
Liao et al. [62] reported a different telemetric system that was designed to meas-
ure pH potentiometrically or ethanol amperometrically. In this system, the pH was
measured from an all-solid-state pH sensitive glass membrane electrode with a slope
of 45.19 mV/pH using a homemade potentiostat. The very thin planar glass membrane
(only 0.7 m) was sputtered on top of Pt seed layer on a ceramic substrate. The meas-

ured pH data was wirelessly transmitted through a PDA-based telemeter to a remote
receiver. A fast A/D converter showed benefi ts in transferring pH signal with less
damping for peak pH response.
Rao et al. [140] described a study using a commercial pH sensitive radio-telemetry
capsule (RTC) to evaluate small bowel and colonic transit time in athletes with gas-
trointestinal symptoms. The RTC (type 7006 Remote Control Systems, London, UK)
consists of a glass electrode with an integral reference cap and battery. RF transmis-
sions from the capsule are detected by a solid-state receiver worn on the belt of the
patient. The recorder samples the pH from the capsule at 6 second intervals for a period
of 24 hours. They used pH changes as an indication of the pH capsule's movement. A
sharp rise in pH from around pH 2 to pH 6 indicates that the capsule has moved into the
duodenum from the stomach. Then the pH progressively rises to a plateau around pH
8, which indicates that the capsule has moved into the terminal ileum. Another com-
mercially available wireless pH sensor (Bravo) from Medtronic Inc. has been used to
measure esophageal pH for a period of 48 hours and it will be discussed later.

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10.5 IN-VIVO APPLICATIONS OF pH MICROELECTRODES

10.5.1 pH in the body
The normal pH range for most body chemistry is small and close to neutral.
Extracellular fl uid, which is about 20% of body water (typically 14 liters), has a nor-
mal pH of 7.4 [141]. This value is close to that of blood and is slightly alkaline. On the