Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 138

units react to form linear chains or three-dimensional networks of polymer chains. If only one type of
polymer is used, the material is called homopolymer. Polymerization of two or more monomer units
results in a copolymer. Polymers containing specific additives are called plastics. Polymers exist in two
basic forms: amorphous and microcrystalline. The macromolecules in a polymeric material have
different lengths. Thus, there is no fixed melting temperature for polymers. Several temperatures exist
in the melting process of a polymeric material. The characteristic lower and upper temperatures of
a polymeric material are the glass transition temperature and the decomposition temperature. At the
glass transition temperature, the material still keeps its solid shape but loses its crosslinking strength.
A further increase in temperature damages the bondage between the monomers, and the plastic will
lose its solid shape. Above the glass transition temperature, a polymeric material becomes soft and can
be machined by molding or hot embossing. The glass transition temperature can be adjusted by mixing
a softener with the original polymeric material. Above the decomposition temperature, the polymeric
material starts to degrade and ceases to function.
Based on their molding behavior, polymers can be categorized into three groups: elastomeric
materials, duroplastic materials, and thermoplastic materials. Elastomeric materials or elastomers
have weakly crosslinked polymer chains. These polymer chains can be stretched under external stress,
but regain their original state if the stress is removed. Elastomeric polymer does not melt before
reaching decomposition temperature. Elastomeric materials are suitable for prototyping of micro-
fluidic devices. The elastic property is ideal for sealing of the fluidic interfaces. In contrast to elas-
tomeric materials, duroplastic materials or duroplastics have strong crosslinked polymer chains.
Duroplastics do not soften much before decomposition temperature. They are strong and brittle. The
properties of thermoplastic materials are ranked between the above two extremes. The material
consists of weakly linked polymer chains. Thus, thermoplastics can be softened and structured at
temperatures between the glass transition point and decomposition point. Due to this characteristic,
thermoplastic polymers are commonly used for micromolding.
Most micromixers used for chemical analysis and life sciences require an optically transparent
material. Many polymers are self-fluorescent at low excitation wavelengths. Self-fluorescence may
affect the sensitivity of microfluidic applications with fluorescent detection. The next drawback of
polymers is their poor chemical resistance to solvents. With applications in the chemical industry and
drug discovery, micromixers may need to handle a variety of solvents. In this case, glass and silicon are
the materials of choice. Polymers are usually not a material of great endurance. For long-term
applications, aging, low chemical resistance, and low UV resistance will be the main problems of using
polymers as substrate material.
Surface properties play an important role for devices utilizing electroosmotic pumping. A high
charge density on the surface assures a stable and controllable electroosmotic flow. Furthermore,
a surface with patterned zeta potential is vital for designing electrokinetic micromixer based on chaotic
advection. Due to the lack of ionizable groups, most polymers have a lower surface charge density
compared to glass. Thus, for applications with electroosmotic flows, such as CE separation, the surface
of the polymeric substrate should be treated accordingly.
The major advantage of polymeric micromixers compared to silicon-based or glass-based coun-
terparts is their superior biocompatibility. Polymeric devices are best for DNA analysis, polymerase
chain reactions, cell handling, and clinical diagnostics. Many polymers are compatible to blood and
tissue. Micromachining of these materials may make implantable microfluidic devices for applications
such as drug delivery possible. Table 4.7 lists the properties of some typical polymers.