hydrogels form networks through ionic or physical interactions under physiologi-
cally relevant conditions. Synthetic hydrogels (e.g., polyethylene glycol,
polyacrylic acid, and polyvinyl alcohol) are hydrophilic polymers and synthesized
covalently by radical chain polymerization or step-growth polymerization. Studies
byMirzabekovand his collaborators showed that immobilizing nucleic acid probe
molecules into hydrogel networks rather than grafting them directly onto solid
surfaces (such as in a traditional microarray) led to significant detection advantages;
with respect to specificity and sensitivity [ 36 , 37 ].Jinseok et al. describe a
microfluidic biosensor that uses an array of hydrogel-entrapped enzymes to quan-
titatively determine the concentration of an analyte and simultaneously detect
multiple analytes [ 38 ]. The hydrogel, poly(N-isopropylacrylamide) (PNIPAAm)
is a thermosensitive polymer that exhibits a reversible phase transition from a
swollen hydrated state to a dehydrate state. The reversible phase transition can
also be described theoretically as a gas-liquid (hydrated to dehydrated state) phase
transition [ 39 ]. Microvalve actuators are the simplest hydrogel-based components
in microfluidic systems.Beebe et al.presented a hydrogel-based microvalve con-
cept for autonomous flow control inside microfluidic channels corresponding to
different pH values [ 40 ]. The hydrogel components were fabricated inside the
microchannels using a liquid phasein situphotopolymerization process. Owing
to low density at the macromolecule scale (and low strength), hydrogels support
only lower resolutions (micrometer scale) in microfabrication than other polymers
(nanometer scale). While there is no complete study of hydrogel as microfabricated
material, numerous methods have been developed to integrate hydrogel into
microfabricated devices. The applications of hydrogel devices are mostly cell
related.
2.4 Paper
Microfluidic paper-based analytical devices (μPADs) is a growing research field
first described by Whitesides and his collaborators in 2007 [ 41 ]. At the initial stage,
microfluidic channels were designed on chromatography paper to allow the simul-
taneous colorimetric detection of glucose and proteins in the same sample. Paper
has become an alternative starting material to inorganic or polymer materials for
fabricating low-cost micro-analytical devices for following three reasons: (1) it is a
ubiquitous and inexpensive cellulosic material; (2) it is compatible with many
chemical/biochemical/medical applications; and (3) it transports liquids using
capillary forces without the assistance of external forces.
Filter paper (Whatman Grade 1) and chromatography paper are the most widely
used substrates forμPADs. They are composed of pure cellulose, while many other
papers contain structure-reinforcing additives that are potentially detrimental in
analytical assays. For example, surface coatings can prevent the analyte from
diffusing into the paper matrix. Cellulose is the primary component which has
abundant hydroxyl groups (─OH) and a few carboxylic acid groups (─COOH) on
6 Materials and Surfaces in Microfluidic Biosensors 157