they have become the most commonly used microfluidic materials. Based on their
physical properties, polymers can be classified into three groups:elastomers,
thermoplastics, and thermosets.
2.2.1 Elastomers
Elastomers consist of cross-linked polymer chains that can stretch or compress
when external force is applied and return to their original shape when the external
force is removed. Elastomers, particularly PDMS-based microfluidic systems have
been used extensively in the control and manipulation of different liquids because
of their remarkable biocompatibility and ease of fabrication. One of the most
common PDMS fabrication processes is soft lithography [ 22 , 23 ], which allows
the patterning of sub-micron sized channels. The process is fast, simple, and does
not require expensive facilities. PDMS offers a number of unique and attractive
features compared to former inorganic materials. These features are as follows:
(1) PDMS has a shear modulus of 0.25 MPa and a Young’s modulus of roughly
0.5 MPa (characteristic of a moderately stiff elastomer). This elastomeric charac-
teristic allows it to conform to a surface and achieve atomic-level contact, a feature
that is useful in forming and in sealing microfluidic systems; (2) PDMS is readily
available from commercial sources at decent prices (~$80/kg); (3) It is optically
transparent which facilitates the observation of fluid transfer and content in the
micro-channels visually or through a microscope; (4) The surface of the PDMS is
hydrophobic (with a water contact angle of ~110) and can be modified to be
hydrophilic (with a water contact angle around 10) by brief exposure to oxygen
plasma; (5) This material can sustain a large temperature range, from 100 to 300C,
without obvious changes in the property. Figure6.6shows the commercially
available elastomer microfluidic devices.
Due to its advantageous features, PDMS has been extensively employed in the
designing of microfluidic devices including. Examples include the detection of
tumor markers [ 24 ], anticancer activity evaluation [ 25 ], the diagnostic of influenza
virus [ 26 ], and HIV-1 infection [ 27 ]. These flexible materials have also been
investigated for use in pressure sensors and wearable healthcare monitoring
devices. For example, using a carbon nanotube (CNT)-PDMS composite, Lee
et al. developed flexible and biocompatible dry electrodes that exhibited good
long-term performance in wearable electrocardiographic (ECG) monitoring when
connected to traditional ECG devices [ 28 ]. Although PDMS has many merits, its
hydrophobicity (due to the repeating OSi(CH 3 ) 2 ) challenges its applications in
biochemical sensing because of the nonspecific adsorption of proteins and other
molecules it exhibits. Furthermore, the PDMS polymer network sometimes absorbs
small molecules, leaches uncured monomers, and swells in solvents. Therefore,
applications for PDMS devices are restricted to aqueous solutions. This disadvan-
tage, however, can be easily overcome by bulk/surface modification and well-
developed functionalization techniques.
6 Materials and Surfaces in Microfluidic Biosensors 153