One pitfall of utilizing silicon and glass substrates is the costly conditions under
which the micromachining of channels and structures need to take place. Most of
these microstructures require the use of cleanroom facilities and equipment to
maintain a small structural resolution; which includes the use of wet and dry
etching, photolithography, electron beam lithography, and a variety of other tech-
niques that can only be done properly in a particle-free environment. The high cost
involved in processing glass and other materials like it, will likely limit their usage
as disposable devices. In addition to this, glass may not be a very suitable material
for more complex multi-layered devices. This challenge stems from the fact that
bonding glass-based device layers to create sealed channels is a difficult to
do. Often high temperatures and/or large electric fields may be needed to achieve
the desired outcome and therefore new fabrication methods for biomaterials have to
be adapted to meet these device-bonding requirements.
2.1.3 Ceramics
Ceramics are another inorganic material that offer many novel structural and
functional capabilities in microfluidic device fabrication. Ceramics such as,
Low-Temperature Cofired Ceramic (LTCC) are a commonly used aluminum
oxide based material that comes in laminated sheets that can be patterned, assem-
bled, and then fired at elevated temperatures to construct microfluidic platforms
(Fig.6.4). LTCC has inherent properties that make it another viable material for
microfluidic structure construction. It has high-temperature stability, chemical
inertness, biocompatibility, low thermal conductivity, excellent dielectric proper-
ties, mechanical strength, packaging capabilities, and three-dimensional structuring
characteristics that are ideal for developing multi-layered microfluidic devices
coupled with electronic components [ 19 , 20 ]. A representative LTCC device
consists of a multilayered stack of sintered ceramic tapes, each of which contains
passive electronic elements such as resistors, capacitors and inductors buried within
it. The various layers are interconnected via channels filled with conducting mate-
rials such as gold (Au) or silver (Ag); topical gold or silver paste can also be utilized
to bridge device components. LTCC-based fabrication aids rapid prototyping with a
significantly low turn-around time in a semi-cleanroom environment and with
minimal use of expensive tools when compared to conventional cleanroom based
microfabrication techniques (Fig.6.5).
2.2 Elastomers and Plastics
Polymer-based microfluidic chips were introduced several years after silicon/glass
chips. The vast variety of polymers offers great flexibility in choosing a suitable
material with specific properties for any microfluidic application. Compared to
inorganic materials, polymers are easy to access and inexpensive, which is why
6 Materials and Surfaces in Microfluidic Biosensors 151