Thermosets or thermosetting polymers, are covalently cross-linked polymers
and thusdo notmelt. From a manufacturing point of view, thermosets are shaped
during the polymerization and cross-linking process. Because of the covalent bond
formation, thermosets exhibit higher residual stress, shrinkage and crack-formation
compared to thermoplastics. From a lab-on-chip perspective, the main advantages
of the thermosets are their geometrical stability and solvent resistance. Common
thermosets used in microfluidics are poly(dimethylsiloxane) PDMS (an elastomer),
the hard resist SU-8 (MicroChem, USA), and the optical glue NOA81 (Norland
Products, Inc, USA), that has recently been used for solvent resistant
microfluidics [ 34 ].
2.3 Hydrogel
Hydrogels are a class of crosslinked hydrophilic polymer networks and are able to
change their volumes reversibly and reproducibly by more than one order of
magnitude; what’s even better is that this volume expansion can be accomplished
with very small changes of certain environmental parameters [ 35 ]. The volume
change of smart hydrogels can be induced in response to a variety of inputs such as
pH, glucose temperature, electric field, light, as well as by the carbohydrates and
antigens present. Hydrogels can be natural or synthesized in a laboratory. Natural
hydrogels are proteins that are extracted from mammalian or non-mammalian cells
(e.g., collagen, gelatin, and fibrin, and polysaccharides). Most of the natural
Fig. 6.7 Photograph of the assembled, nucleic acid cassette in its storage state [ 32 ] Reproduced
with permission from Springer
156 P. Manickam et al.