2.2.2 Thermoplastics
Thermoplastics are a class of synthetic polymers that exhibit softening behavior
above a characteristic glass transition temperature (Tg) resulting from long-range
motion of the polymer backbone, while also allowing them to return to their
original chemical state upon cooling. Thermoplastic polymers differ from elasto-
mers by their ability to be softened, fully melted, and reshaped upon heating, while
remaining chemically stable over a wide range of operational temperatures and
pressures [ 29 ]. Polymethylmethacrylate (PMMA), polycarbonate (PC), and cyclic
olefin polymers (COP) or copolymers (COC) are most common examples of
thermoplastics used in the microfluidic design. A short list of other engineered
thermoplastics which have been used for microfluidic chips include polystyrene
(PS), polyetheretherketone (PEEK), polyesters, polyethylene terephthalate (PET),
polyethylene (PE), polyvinylidene chloride (PVDC), polyvinyl chloride (PVC),
polypropylene (PP), polysulfone (PSU), and fluoropolymers such as polytetrafluor-
oethylene (PTFE) and fluorinated ethylene propylene (FEP). The microchannel
fabrication process with thermoplastics involves a different variety of replication
methods including hot or cold embossing, injection molding, or thermoforming.
Early developments in thermoplastic microfluidic systems were largely focused on
using PC and PMMA due to their wide availability in a variety of grades, good
solvent and chemical compatibility, and well-characterized molding parameters. A
chart summarizing several important material properties for common microfluidic
thermoplastics is provided in Table6.1.
PMMA, a commonly used thermoplastic, is widely known under the commercial
names of Plexiglas and Lucite. It has an elastic modulus of 3.3 GPa and good
optical clarity from the visible into the UV [ 30 ]. Other advantages of this material
include biological compatibility, gas impermeability, and ease of micromachining
at relatively low temperatures ( 100 C). Yang et al. demonstrated that
α-fetoprotein can be quantified in blood serum using immuno-affinity extraction
Fig. 6.6 Some commercial
elastomer based
microfluidic devices
154 P. Manickam et al.