Biomedical Engineering Reference
In-Depth Information
molecules which serve to sensitize the base sensor to a preselected species (e.g., am-
monium ion). Also, such selective layers may function as adhesion promoters by
which the preselected ligand receptor is immobilized to the wholly microfabricated
ligand/ligand receptor-based biosensor embodiment. DNA electrochemical biosen-
sors are developed using graphite or carbon electrodes. Carbon-based electrodes,
however, are generally not adaptable to MEMS technology when small (less than a
micrometer) dimensions are needed.
9.5.3.3 Microfl uidic Devices
Microfluidics refers to the fluid flow in microchannels (i.e., one of the dimensions
of flow is measured in micrometers). They are developed using the elastomeric tem-
plates of the required channel patterns by sealing both the ends and creating inlet
and outlet points in the top layer (Figure 9.17). Microstructured substrates provide
control over the flow of fluid through a geometric shape and the surface chemistry.
For example, the passive manipulation of fluids' centrifugal forces can be used to
control flow. A number of devices are under development for use in various ap-
plications including bioseparations, microdialysis, high-throughput drug screening,
DNA analysis, mass spectroscopy ensuring the safety of air, food, and water, and
combating terrorism and biowarfare.
There are technical barriers that must be overcome for microfluidic devices
to reach their full potential for biosensing applications. A common problem often
encountered in microfluidics is the clogging of channels due to particle contamina-
tion, which requires the prefiltering of liquid samples. Since microfluidic devices
engage fluid in motion, capillary effects, micropumping, and surface tension have
to be considered along with the design and fabrication of the micro-sized flow con-
duits. Surface area is an important factor at the microscale. For example, a 35-mm
diameter dish half-filled with 2.5-mL water has a surface area-to-volume ratio of
4.2 cm
2
/cm
3
, whereas a microchannel that is 50
m wide, and 30 mm
long filled with 75-nL water has a surface area-to-volume ratio of 800 cm
2
/cm
3
.
A very large surface area-to-volume ratio makes capillary electrophoresis more
efficient in microchannels by removing excess heat more rapidly. Adhesive forc-
es (van der Waals force, electrostatic forces, surface tension) are more dominant
than gravity in the microscale. Microfluidic flows have a normally low Reynolds
number (N
Re
) due to the very small length scales. This makes turbulent flow virtu-
ally impossible to achieve and the mixing of fluids is generally slow and diffusion
controlled. Mixing can be enhanced in the laminar flow regime by subjecting the
fluid to a chaotic flow pattern. In a chaotic flow, complex patterns are created that
allow the fluid to stretch and fold. Mixing is greatly enhanced by the tendency of
fluid particles to become homogeneously dispersed and by a decrease in the length
scale for diffusion between unlike components.
There are two common methods by which fluid actuation through microchan-
nels is achieved. In a
pressure-driven flow
, the fluid is pumped through the device
via positive displacement pumps, such as syringe pumps. With the assumption of
a no-slip boundary condition (the fluid velocity at the walls must be zero), the
laminar flow produces a parabolic velocity profile within the channel (Chapter
4). The parabolic velocity profile has significant implications for the distribution
μ
m tall, 50
μ
