Designing Hydrogels by ATRP - In-Situ Gelling Polymers: For Biomedical Applications - page 82

with M w / M n < 1.3 [ 149 , 151 ]. When a hydrophilic divinyl crosslinker is used as
comonomer, the prepared water-dispersible nanogels possess several useful fea-
tures, including the preserved halide initiating groups for further chain extension
and latex modification, relatively narrow MWD of the degraded product (linear
primary chains), and more homogenous nanogel structure with better swelling
properties and degradation behavior [ 152 , 153 ]. These crosslinked nanogels pre-
pared by ATRP in inverse heterogeneous systems have been applied to encapsu-
lation and delivery of various hydrophobic drugs [ 152 ] carbohydrates [ 153 ] and
siRNA molecules [ 154 ]. Detailed discussion of the nanogels prepared by ATRP in
inverse miniemulsion will be presented in Sect. 6 .
5 Hydrogels Prepared by ATRP
Hydrogels are three-dimensional crosslinked networks of hydrophilic polymers
possessing unique properties such as tunable chemical and physical structure,
good mechanical properties, high water content, and biocompatibility [ 1 , 2 ]. These
unique properties offer great potential for the use of hydrogels as the material of
choice for various applications in the field of regenerative medicine, tissue engi-
neering, drug delivery, and bio-nanotechnology [ 155 ]. Recently, several efforts
have been made to develop advanced hydrogels as microfluidic biomaterials for
tissue scaffolds [ 156 ] inverse opal for photonic crystals [ 157 ] and biosensors for
sensing and detection applications [ 158 ]. Hydrogels are generally prepared from
hydrophilic polymer matrices that are crosslinked through physical or chemical
crosslinking [ 4 , 159 ]. Physical crosslinking utilizes supramolecular association
typically through hydrogen bonds, crystallized domains, ionic interactions, hydro-
phobic interactions, stereo-complexation, host-guest interactions, and temperature-
induced sol-gel transitions. These physically crosslinked gels feature the absence
of toxic crosslinkers and provide reversible degradation into the corresponding
precursors upon application of external stimuli. In contrast, chemical crosslink-
ing is a more versatile method that allows for the synthesis of a stable crosslinked
gel network through the formation of new covalent bonds. Well-defined organic
reactions such as click-type reactions as well as various polymerization methods
such as free radical polymerization and step-growth polycondensation in the pres-
ence of various crosslinkers have been explored. Recently, ATRP has been utilized
for the construction of well-defined crosslinked nanomaterials in the presence of
crosslinkers. This section describes how ATRP, as a single polymerization method
or in combination with other methods, has been utilized for the development of
advanced hydrogels, including thermoresponsive hydrogels, nanostructured hybrid
hydrogels, and degradable hydrogels.
Thermoresponsive hydrogels can swell or deswell in response to changes in
temperature, thus undergoing a volume change at low critical solution tempera-
ture (LCST) in water. Above the LCST, the hydrogels are hydrophobic and expel
water; below the LCST, they are hydrophilic and absorb water [ 160 , 161 ]. Due