Page 27 - Optofluidics Fundamentals, Devices, and Applications

8 Cha pte r T w o

make possible the fabrication of devices with a useful range of func-
tions, ranging from molecular analysis to frequency-tunable lasing.

2-2 Historical Background
Microfluidic systems have the properties required for applications in a
wide range of areas: molecular analysis, biodefense, molecular biology,
microelectronics, clinical diagnostics, and drug development [1]. There
are many benefits resulting from the miniaturization of devices for use
in these areas, including decreased cost in manufacture, use, and
disposal; decreased time of analysis; reduced consumption of reagents
and analytes; reduced production of potentially harmful by-products;
increased separation efficiency; decreased weight and volume; and
increased portability [1]. The growth of molecular biology has stimu-
lated the development of systems for analysis of biomolecules, DNA,
and proteins. The first microfluidic device was a miniaturized gas
chromatography (GC) system developed by Terry et al. [2] at Stanford
University in the 1970s. The laboratories of Manz [3–5], Harrison
[6–10], Ramsey [11–15], and Mathies [16–18] were among the first to
develop microfluidic systems to analyze aqueous solutions. The tech-
nology used to fabricate these early systems—photolithography and
etching in silicon and glass—was derived from microelectronics, as
these technologies were available and highly developed. These materi-
als and techniques are expensive and time-consuming, however, they
require access to specialized facilities. They are therefore only margin-
ally useful in research requiring rapid evaluation of prototypes. Their
major advantage—chemical inertness—is so far required only in the
still-undeveloped area of organic synthesis.

2-3 Materials for Fabricating Microfluidic Devices
Most research in microfluidic systems is now carried out in PDMS
and other polymers. Fabrication in polymers is easier, more flexible,
and much less expensive than in silicon or glass. It also avoids other
problems of hard materials (e.g., formation of sharp shards on break-
age) and enables certain components (e.g., pneumatic valves) that
cannot be fabricated in rigid materials. In the following sections, we
will focus on the use of PDMS for the development of microfluidic
systems. PDMS has several attractive properties that make it suitable
as a material for rapid prototyping of microfluidic devices capable of
supporting a wide range of applications. Table 2-1 summarizes some
of these properties and consequences.

2-3-1 Mechanical Properties of PDMS
PDMS is elastomeric. It has tunable Young’s modulus, typically
around 750 kPa [19]. It deforms easily, conforms to surfaces, and