Biomedical Engineering Reference
In-Depth Information
3D fl ows and time-dependent 2D fl ows, and mixes the fl uid by continuously stretching
different volumes of the fl uid and folding them into one another. In a qualitative sense,
the path taken by a given fl uid element in the fl ow depends in a sensitive way on its
encounters with a series of weak secondary fl ows or eddies, present even at low
Re
in
the corners of channels, which transport the element across the fl ow [167].
An LOC system mainly has four activities: sensing, actuation, heating, and process-
ing. In analytical application, the sensors such as array biochips translate mechanical,
thermodynamic or chemical information from their environments into electrical signals.
That information is then processed, either by the LOC itself, by a nearby IC, or by a
computer. The LOC device may then act on its environment by closing a valve, defl ect-
ing a light beam, pushing a fl uid through a conduit, and moving a wire. These devices
may be heated or cooled quite rapidly - a type of endogenous stimulation from which
any of the other activities may ensue. Thus, except pumps and valves, other functional
components, such as injection, dosing, metering, sensing, and temperature measure-
ment, actuators, and control/sensing circuit components, are all or partially needed in
the LOC. These are discussed in more detail in the literature [168].
11.5.3 Fabrication of BioMEMS
Rapid progress in microfabrication and assembly techniques has led to the develop-
ment of extremely small-scale devices commonly referred to as MEMS (micro-electro-
mechanical systems) or
TAS (micro-total analysis systems). Advances in MEMS
are being applied to biomedical applications and has become a new fi eld of research
known as BioMEMS. In fact, BioMEMS is the technology to fabricate a microfl uidic
system containing functional components for a biological LOC system. Most of these
devices are manufactured in silicon but recent developments have demonstrated that
materials such as glass, quartz, ceramics, and polymers can also be used for MEMS. In
early 1990, silicon and glass substrate-based microfl uidic devices were fi rst made by
using conventional, planar fabrication techniques - photolithography and etching - to
pattern microfl uidic structures [169]. Relative high cost and limited material choice
for the approach resulted in developing alternative techniques such as soft lithography
and polymer molding for fabrication of BioMEMS microfl uidic devices. These non-
photolithographic microfabrication methods are based on printing and molding organic
materials, which are much more straightforward, for making both prototype and special-
purpose devices. They are also much simpler for building three-dimensional networks
of channels and components as compared to photolithography. Precision machining
techniques like milling, grinding, and turning can produce very accurate parts from a
variety of engineering materials allowing to pattern three-dimensional structures with
minimum feature size in the micrometer range when using special microcutting tools.
The machining methods used include high speed cutting (HSC), micromilling as well
as wire and electro discharge machining (EDM). The fabrication is relatively simple
and suitable for rapid prototyping as it requires no optical mask, but its mass produc-
tion with identical structures are cost prohibitive as each structure is machined indi-
vidually (serial process). Generally, the design and microfabrication of BioMEMS is
µ
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