Biomedical Engineering Reference
In-Depth Information
Furthermore, for in vivo application, we have integrated the reference
electrode (counter electrode) with this ultra-microglutamate sensor [83]. An
integrated ultra-microglutamate sensor has been constructed with a 7
mdia-
meter platinized carbon-fiber disk electrode and a platinum thin-film counter
electrode fabricated on the glass capillary tube. The counter electrode shows
good stability and can be used as a substitute for a silver-silver chloride elec-
trode. The sensor shows a stable response to glutamate and a response time
within 12 s. The calibration range for glutamate measurement is 50-800
μ
M.
μ
4.6
The Application of Micromachining
Techniques to Chemical Sensors, Biosensors,
and Microanalysis Systems
4.6.1
Introduction
Considering the application to clinical analyses, the miniaturization of sen-
sors, actuators, and systems is of great importance. In realizing them, se-
miconductor and micromachining technologies are becoming indispensable.
Concurrent advantages accompanying the miniaturization are 1) reduced size,
2) small sample volume, 3) identical, highly uniform, and geometrically well-
defined structures, and 4) ease of integration. Among chemical sensors and
biosensors based on various detection principles, electrochemical sensors will
benefit most from the rapidly advancing technology. In this section, recent
advances in the application of microfabrication techniques to electrochemical
sensors, biosensors, and micro systems will be discussed.
4.6.2
Basic Technologies
Basically, the technologies used to miniaturize chemical sensors and biosen-
sors originate from semiconductor processes. These include pattern formation
by photolithography, thin-film metallization, and chemical etching. In elec-
trochemical sensors and systems, detecting electrodes are formed as patterns
by photolithography. As a representative process, photoresist patterns are
formed on a metal layer and exposed areas are selectively removed by wet
or dry chemical etching (Fig. 4.30a). The metal layers can be deposited by
vacuum-evaporation or sputtering. Lift-off is also used in forming patterns
of noble metals such as platinum and iridium (Fig. 4.30b). After photoresist
patterns are formed on a substrate, a metal layer is deposited. By removing
the photoresist layer in an appropriate solvent such as acetone, a faithful
reproduction of the patterns on a photomask is obtained. Screen-printing
is a technique frequently used to form electrode patterns without expensive
facilities. The patterns are literally printed by casting and squeezing an ap-
propriate paste onto a substrate through a mask (Fig. 4.30c). In forming
mercury microelectrode arrays or an Ag/AgCl electrode, electrodeposition
has also been used supplementally.
