Fabrication technologies - Micromixers Fundamentals Design and Fabrication - page 132

4.2 POLYMERIC MICROTECHNOLOGIES
Micromixers based on silicon and other inorganic materials have the drawbacks of higher cost and
biocompatibility. For mass production of the relatively large devices for applications in analytical
chemistry and biomedical diagnostics, polymers offer a real alternative to silicon-based substrates.
Polymers are macromolecular materials, which are formed through polymerization reactions. In
a polymerization reaction, the monomer units connect each other either in linear chains or in three-
dimensional network chains and form a macromolecule. For devices in biomedical applications,
materials such as silicone rubber, polycarbonate, and polyimide are inexpensive and disposable [44] .
According to the behaviors of interconnects between monomers and subsequently molding behaviors,
polymers are categorized as thermoplastics, elastomers, and thermosets.
Polymers as functional materials fulfill a number of requirements of devices for chemical and
biomedical applications:
Polymers are suitable for bulk and surface micromachining.
Many polymers are optically transparent.
Most polymers are good electrical insulators. They can also be modified to be electrically
conductive.
The surface chemistry of polymers can be easily modified for a certain application.
Shape memory polymers (SMPs) are interesting materials with possible applications in active
micromixers. Similar to shape memory alloys, SMPs are activated thermally. SMPs have a morphology
consisting of a shape-fixing matrix phase and a shape-memorizing dispersed phase. The shape-
memorizing phase consists of crosslinks that remember a primary shape of the SMP. The polymer
can be brought from a primary shape into a secondary shape at a temperature above the transition. The
secondary shape is locked by cooling the SMP under the transition temperature. Heating the SMP
above the transition temperature again brings it back to the primary shape [45] .
The advances in polymeric electronics allow the integration of electronic components into an all-
polymeric system. For recent works on polymeric electronics, the reader is referred to the recent
review by Facchetti et al. [46] . In the field of bioengineering, polymeric technologies can provide
scaffold structures for growing and harvesting tissues.
4.2.1 Thick-film polymeric materials
4.2.1.1 Polymethylmethacrylate (PMMA) resist
Polymethylmethacrylate (PMMA) is well known by a variety of trade names such as Acrylic, Lucite,
Oroglas, Perspex, and Plexiglas. PMMA can be used as a substrate material or as a thick-film resist for
the LIGA technique [47] .
A thick PMMA film can be deposited on a substrate by different methods: multiple spin coating,
prefabricated sheets, casting, and plasma polymerization. Since multilayer spin coating is achieved with
several coating steps, the multiple layers cause high interfacial stresses and lead to cracks. The problem
with the cracks can be avoided by using a preformed PMMA sheet, which is bonded to the substrate
[48] . Monomer MMA (methylmethacrylate) can be used as the adhesive material for the bonding
process [49] . PMMA can also be polymerized in situ with casting resin [50] or with plasma [51] .