reaction (PCR) and for bioreactions. However, silicon substrates are relatively
expensive when compared with other materials, such as glass and polymers.
Furthermore, the fabrication process for silicon-based microfluidic devices
involving substrate cleaning, resist coating, photolithography, development, and
wet/dry etching are relatively time consuming and costly. These limitations
hinder its practical applications in commercial immunoassays. Another drawback
is silicon’s optical opacity, which limits its direct applications in real-time optical
detection.
2.1.2 Glass
After the initial focus on silicon, glass has emerged as the substrate of choice for
the time. The reason for this is that wet and dry etch methods were developed
for generating microstructures and microchannels in glass at a high level of
resolution. Glass has the advantage of being transparent and chemically stable,
which is particularly attractive when optical detection methods such as fluores-
cence or surface plasmon resonance (SPR) are used. Several types of glass are
used in microfluidic devices such as soda lime, quartz, and borosilicate. Soda
lime glass is one of the cheapest and most commonly used forms of glass, but it
can contain a large amount of contaminants such as aluminum. Quartz is the
most suitable glass material for optical sensing devices since it is transparent
between the ultraviolet and infrared wavelength spectrum [ 17 ]. Unfortunately,
the etching rate and cost of the quartz substrates are 2–3 time higher than that of
soda-lime glass. Borosilicate glass orpyrex is the most commonly used material
in microfluidic and nanofluidics devices because of its optical characteristics
(transparent from approximately 350 through 700 nm) and its physical proper-
ties (annealing temperature of 640C, resistant to most chemicals). However,
borosilicate glass is also more expensive than soda lime glass. In the end,
selecting the material will affect either the device cost or the device function.
The key to selecting an adequate material for a microfluidic device comes down
to the device’s application. In many cases, manymaterialsneedtoworktogether
to yield the desired functionality. An example of this can be seen with glass,
whose elastic modulus is highly dependent on the glass’s composition; as a
result constructing active componentswith more dynamic valves and pumps
require multiple materials to form hybrid devices such as the one seen in
Fig.6.3[ 18 ]. Glass has many other favorable traits that become paramount to
a microfluidic device’s function. One of the most important and well-known
traits is its compatibility with biological samples. Since glass has the property of
relatively low nonspecific adsorption and is not gas permeable, it is an ideal
material for working with biologics where cell kinetics and gas incubation need
to be controlled.
150 P. Manickam et al.