Biomedical Engineering Reference
In-Depth Information
Despite many attractive features, ISFET pH sensors still suffer from problems, such
as inherent drift and hysteretic effects, and face some considerable challenges. Among
them, gate material stability and Si sensor chip encapsulation pose great obstacles for
their clinical applications, especially in long-term
in-vivo
pH measurement or as an
implantable sensor. Since a thin gate material is required to ensure high capacitance,
the pH sensitive layer of ISFET sensors is extremely thin. A typical ISFET has about
only 50 nm of Si
3
N
4
. Dissolution of such gate material in the very corrosive biological
environment is unavoidable. This limits the sensor lifetime and causes long-term drift
[48]. Some progress has been made in improving corrosion resistance of gate materials
by using alloys, such as Al
2
O
3
ß
Ta
2
O
5
for acidic resistance, Al
2
O
3
ß
ZrO
2
for alkali
resistance, or Al
2
O
3
ß
Ta
2
O
5
ß
ZrO
2
for both [81]. Although signifi cant progress has
been made during past decades, encapsulation of long-term implantable ISEFT devices
with integrated measurement circuits still remains a challenge. It is evident that the pro-
tection of the ISFET chip with a standard passivation layer provided by CMOS tech-
nology is not enough. Various additional insulation materials such as epoxy, silicone,
and polyimide have been reported for the ISFET encapsulation [52]. In fact, encap-
sulation has been considered a major cost factor in the development of commercial
medical devices including chemical sensors [86].
10.3.4 Metal/metal oxide-based pH microelectrodes
A considerable amount of study has been focused on the development, fabrication,
and characterization of metal/metal oxide pH electrodes. Typically, the pH electrodes
employing metal/metal oxide as sensing materials are all-solid-state, and have several
advantages in comparison to conventional glass electrodes. Unlike the glass pH elec-
trodes, they require a high input impedance pH meter; the metal oxide-based pH sensor
has low electrode impedance. In contrast to the sluggish response of glass pH electrodes,
the solid-state metal oxide pH sensor presents a faster pH response. The method to
prepare a metal/metal oxide-based pH electrode is compatible with thin fi lm and
MEMS manufacturing technologies and provides capability of mass production and
miniaturization.
A variety of metal/metal oxide materials show ideal or near-ideal Nernstian responses;
these materials have been explored for use as pH sensing layers [87, 88]. Some exam-
ples are IrOx [41, 89], RuO
2
[90, 91], nanoporous PtO
2
[43], RuO
2
ß
TiO
2
[92],
TiO
2
ß
PVC [93], PaO
2
[94], Sb/Sb
2
O
3
[95], WO
3
[96], PbO
2
[97], Co
3
O
4
[44], and
SnO
2
[98, 99]. Among these many pH sensitive oxides, iridium oxide (IrOx) shows high
conductivity, good chemical stability, and most importantly superior biocompatibility,
making it the most popular material for the fabrication of all-solid-state pH microelec-
trodes. In addition to IrOx, antimony has received renewed interest recently. Antimony
has been used as a pH sensing material in commercially available pH catheters
for esophageal pH monitoring, as will be discussed later.
Fabrication methods and conditions that determine the structure and composition of
iridium oxide affect pH response characteristics of resultant pH sensors. A comparison
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