Biomedical Engineering Reference
In-Depth Information
Fig. 4.30.
Representative methods for the formation of electrode patterns (
a
-
c
)
and microscopic structures (
d
-
f
) used for micro-electrochemical sensors.
a
etching,
b
lift-off,
c
screen-printing,
d
anisotropic etching of silicon,
e
field-assisted bonding,
and
f
application of photosensitive polymer
In constructing some kinds of chemical sensors and systems, three-dimen-
sional structures are required. A couple of examples include a container for
an electrolyte solution and a flow channel. Anisotropic etching of silicon
is promising and is used very often (Fig. 4.30d). The etching utilizes sig-
nificant differences in etching rates among orientations of crystal lattices.
Field-assisted bonding or anodic bonding is also useful in constructing three-
dimensional structures (Fig. 4.30e). With a combination of a #7740 glass
substrate and a silicon substrate, a negative high voltage is applied to the
glass substrate against silicon. Hermetical sealing is instantly achieved even at
around 250
◦
C. Photosensitive polymers such as polyHEMA (poly (2-hydroxy-
ethylmethacrylate)), polyimide, and thick-film photoresist such as SU-8
TM
are useful in forming microscopic three-dimensional structures (Fig. 4.30f).
The photosensitive polymers can be patterned following a process similar to
that used to produce a photoresist. In addition to these methods, various
promising techniques have been used, as summarized in Table 4.3.
4.6.3
Microsensors for Dissolved Gases and Electrolytes
In conducting the measurement of pO
2
and pCO
2
in whole blood, commonly
used devises are the Clark-type oxygen electrode and the Severinghaus-type
carbon dioxide electrode. The Clark-type oxygen electrode consists of a ca-
thode and anode pair housed in a container. A hydrophobic gas-permeable
membrane is placed over the sensitive area. The oxygen electrode measures
the current generated by the electrochemical oxygen reduction on the ca-
thode at an overpotential. The structure of the Severinghaus-type electrode
is similar to that of the Clark-type oxygen electrode. A pH-glass electrode
and a reference electrode are incorporated in a container. A local change in
pH caused by the permeation of carbon dioxide through the gas-permeable
