Biomedical Engineering Reference
In-Depth Information
potential drift but their stability is improved after soaking for a certain amount of time
ranging from hours to days [44]. Shelf-life also affects the storage stability of some
sensors, such as glass electrodes. When all or most of the above properties are opti-
mized, a reliable pH sensor system can be achieved for in-vivo measurements.
10.2.8 Biocompatibility
Biomaterials are inert substances that are used in contact with living tissue, resulting in
an interface between living and non-living substances [45, 46]. Biocompatibility of this
interface is achieved by using such biomaterials for encapsulation in the construction
of sensor devices.
The interaction in an interface of device/tissue is limited by two factors. There is
the corrosive environment, such as biological fl uid, which contains salts and proteins
among other cellular structures in which the sensor device must survive [47, 48].
Second, there is the encapsulation material which may induce a toxic reaction due to
poor biocompatibility and hemocompatibility [49, 50]. It is crucial to use a biomaterial
that can overcome both limiting factors to maintain the lifetime of the sensor device
and protect the body [51, 52].
10.3 FABRICATION OF MICROELECTRODES FOR
pH DETERMINATION
Various pH microelectrodes and numerous fabrication techniques have been devel-
oped. Some selected examples of microelectrodes based on the formats of pH sensitive
materials are discussed in this section.
10.3.1 Glass-based pH microelectrodes
Glass electrodes are the most commonly used potentiometric sensors for pH measure-
ments [53]. The pH response of glass electrodes was fi rst observed by Max Cremer in
1906. While studying liquid-solid interactions, Cremer discovered that the solid-liquid
interface, which was separated by blowing a thin bubble of glass, created an electric
potential that could be measured. Later, Fritz Haber and Zygmunt Klemensiewicz
applied this potential to hydrogen ion activity in 1909. They used the glass bulb to
measure hydrogen ion activity and named it the glass electrode [54, 55].
A schematic diagram of a combination glass electrode is shown in Fig. 10.2. The
glass electrode consists of a glass tube with a thin glass bulb at the tip. Inside is a known
solution of potassium chloride (KCl), buffered at a pH of 7.0. A silver wire with an Ag/
AgCl electrode tip makes contact with the inner solution. An outer jacket contains a
second Ag/AgCl in an electrolyte with known Cl concentration, such as saturated KCl.
This Ag/AgCl serves as the reference electrode during pH measurements. A combined
thermocouple is used to measure and compensate for temperature effects.
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