Biochips – fundamentals and applications - Electrochemical Sensors, Biosensors and Their Biomedical Applications - page 365

motivated by fast response times, well-controlled reaction conditions, small power
and chemical consumption, and highly parallel screening and testing ability. Since
each technique has its own advantages and disadvantages, the most suitable method of
device fabrication often depends on the specifi c application of the device [170].
Silicon micromachining is based on a lithography process and is discussed in section
11.4.2.2. The use of glass instead of silicon in an LOC system is prompted by its unique
properties such as chemical resistance, optical transparency, high dielectric coeffi cient,
and biocompatibility. Borofl oat glass is the most popular material in the fabrication of
a LOC system. Fabrication of a glass microfl uidic network involves the use of the tra-
ditional silicon-processing technique as discussed in section 11.4.2.2, such as photo-
lithography and wet chemical etching. The wet etching of glass is mostly done with
hydrofl uoric acid (HF) or buffered hydrofl uoric acid (BHF). The electronic ELISA chip
in Fig. 11.28 (see Plate 11 for color version) made in the author's lab is based on glass
substrate. A glass cover plate is often used to seal the etched microfl uidic channel net-
work. The three most frequently used glass-glass bonding methods are thermal fusion,
anodic and adhesion bonding, and the most popular one among them is the fusion
method due to its direct bonding without an intermediate layer. Many types of polymers
show better chemical resistance and biocompatibility than silicon. These polymers can
be mass produced at signifi cantly lower cost than silicon microstructure. The methods to
fabricate polymer microfl uidic systems include thermal embossing, injection molding,
casting, laser machining, milling, and x-ray/UV polymer lithography. The theory for pol-
ymer lithography is that some polymer materials are affected by energetic radiation such
as UV and x-ray radiation, which may break chemical bonds or lead to chemical changes
of the polymer and be removed by following solution treatment. There are quite a few
well-developed polymer bonding techniques including gluing, laminating, thermal bond-
ing, and ultrasonic and laser welding. One of the best methods for joining two plastic sur-
faces is gluing, due to its simplicity and low cost. Fabrication efforts include using optical
lithography to make a silicon wafer mold master with complicated 3D structure, and
then replicating thousands of polymer microfl uidic devices by molding. This is an inex-
pensive method and the manufacturing process allows mass productions. Figure 11.34
shows a plastic fl ow-through ELISA microfl uidic system made in the author's lab [171].
Taking advantage of microfabrication technology, the silicon wafer master (Fig. 11.34a)
with 3D-structured multichannels was fabricated and served as a mold for plastic replica
molding. The molded device is shown in Fig. 11.34b. The resulting images from different
channels showed high sensitivity and specifi city without cross-over interference from dif-
ferent detection sites. Furthermore, combining lithography approaches with electroplating
will result in a robust metal structure suitable as a mold insert for a polymer replica-
tion process. The resist pattern is also used directly as a molding template to cast PDMS
into the form for a replicating pattern. Recent research has started to focus on techniques
like electron and ion beam lithography for patterning nanostructured fl uidic systems. In
all these individual or combining technologies, design and fabrication of 3D-structured
LOC is essential. 3D structured systems can solve very complicated biochip tasks, such
as separation, reaction, and sensing. As discussed above, it is crucial for addressable
arrays and patterned electrodes to have inexpensive electronic biochips. However, most