Introduction 3
on the catheter surfaces, is said to create a favorable environment for bacteria
to grow. While the catheters should provide required strength to maintain their
mechanical structures integrity, the prevention of surface fouling is also critical for
device design.
1.2 MICROMACHINING OF BIOMATERIALS
MEMS is a technology that uses micromachining techniques to form microscale
devices, such as accelerometers,
9
optical lens,
10
RF (radio frequency) devices
11,12
and microfluidic devices. Micromachining uses thin-film deposition and etching
techniques developed by the IC (integrated circuit) industry to fabricate small
mechanical parts monolithically on a substrate. Limitations, such as processing
temperature and chemicals used in established IC fabrication processes, restrict
materials to silicon, silicon dioxide, silicon nitride, aluminum and refractory
metals. Conventional micromachining methods start with a bulk material, for
example, a silicon wafer, following by thin-film deposition, photolithography and
etching. The process can be repeated, with some materials forming sacrificial
layers and structures, to form more complex 2-D or 3-D structures. The level of
complexity of a micromachining process is often determined by the number of
photolithography masks used in the processes.
Historically, micromachining process can be categorized into two groups: (1)
bulk micromachining and (2) surface micromachining. Bulk micromachining is often
involved in etching of a silicon wafer itself and use the single crystalline planes
in silicon to form mechanical structures. Examples include early ink-jet printing
nozzles, inertial sensors, as well as microfluidic valves and pumps. On the other
hand, surface micromachining uses silicon substrates only as a mechanical sup-
port, the add-on thin-films, such as polycrystalline silicon (poly-si), silicon nitride
and metals are the actual mechanical components. Examples include scanning
optical mirrors, inertial sensors and radio frequency switches. Typically, bulk
micromachining consumes more silicon real estate than surface micromachining,
but may require less processing steps. Bulk-micromachined structures are gen-
erally simpler and provide generic advantages using single crystalline structures
such as smoother planes, higher Young’s Modulus and reliability. Surface micro-
machining, on the other hand, offers potentials for complex structures to satisfy
nonconventional functionality and requirements, possibility to be integrated with
complementary metal oxide semiconductor (CMOS) circuitry and ability to adapt
to exotic materials.
Micromachining of biomaterials initially used bulk-micromachined silicon
molds.
13
Silicon wafers were etched into micro-scale structures as molds as
they offer precision in the molds by which the conventional metal drilling or
quick prototyping methods may not offer. Casting, hot-embossing and micro-
molding have been used
14,15
to mold biomaterial (mainly polymers) to desired
shapes. Hot-embossing and micro-molding are rather mature technologies and
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