220
G. Keerthiga et al.
ii. Hydrophobic interaction
Water miscible monomers with hydrophobic tail groups on the side chains or
hydrophobic monomers are subjected to thermal induction or ultrasonic treatment
promoting sol-gel transition in hydrogel formation by hydrophobic interaction.
Specific conditions in favor the gel transition are maintained for gel formation, while
removal of the same leads to reversible physical state [62].
Thermal induction based on lower critical solution temperature(LCST) [76–
39]/upper critical solution temperature (UCST) [12, 221] is used for hydrogel fabrication when thermally treated at a critical temperature, the sol-gel transition occurs.
Graft copolymers or amphiphilic blocks can self-assemble to form an organized structure in an aqueous environment with a hydrophobic core such as micelles. Polymers
such as PNIPAM and its derivatives blend with hydrophilic PEO with hydrophobic
PPO/Poly glycolide/Polylactide/PCL are generally fabricated for their thermoresponsive property through LCST. These polymers below LCST are in solution form,
while at higher temperature form an insoluble gel structure through hydrophobic
interactions. Methylcellulose containing Calcium Phosphate (CaP) nanoparticles
using one-pot reaction by LCST gelation, has been reported as an experimental
hydrogel drug reservoir at physiological pH [144].
On the contrary, in UCST induced hydrogel formation, the cooling temperature of the polymer solution plays an important role. Here, the hydrogel state is
achieved at the temperature below UCST. However, at UCST, the hydrogel disintegrates, making the hydrophobic water-soluble micelle cores. Thermo reversible linear
triblock comprising with inner hydrophilic poly-polyethylene glycol methyl ether
methacrylate (PPEGMMA) part and outer polyacrylamide-co-acrylonitrile (PAAmco-AN) units as a cooling induced sol-gel transition. This physical transition can be
modulated by the polymer concentration [45].
Ultrasonic induction [79, 209] induced crosslinking involves phase variation.
Natural or synthetic polymers such as collagen and silk fibroins that possess complex
secondary structures can be physically crosslinked to form interpenetrating networks
(IPNs) with improved physical properties and tunable gelation properties when
treated with physical shear, organic solvents, and heat treatment.
iii. Crosslinking by Crystallization
Crystallites of polymer unit work as sites of physical cross-linking for hydrogel
formation. An aqueous solution of PVA is subjected to repeat freeze-thawing
to yield hydrogel. Parameters such as molecular weight, polymer concentration,
time, and the number of freeze-thawing cycles and freezing temperatures influence
the hydrogel formation [66]. Similarly, exploiting the property of crystallization,
hydrogen bonding in polymers PVA, and hydrophobic polyacrylamide (HAPAM),
physical double network (PDN) was developed by one-potpolymerization followed
by repeated freeze-thaw cycles (Fig. 4).
More compact hydrogel structures are obtained by stereo-complex interaction
between the two enantiomeric polymers. Based on the racemic crystallite Poly LLactic acid (PLLA) and poly d-Lactic acid (PDLA), a unique gel-sol-gel transition occurs upon heating and stereo-complexation. Novel enantiomeric mixtures of
G. Keerthiga et al.
ii. Hydrophobic interaction
Water miscible monomers with hydrophobic tail groups on the side chains or
hydrophobic monomers are subjected to thermal induction or ultrasonic treatment
promoting sol-gel transition in hydrogel formation by hydrophobic interaction.
Specific conditions in favor the gel transition are maintained for gel formation, while
removal of the same leads to reversible physical state [62].
Thermal induction based on lower critical solution temperature(LCST) [76–
39]/upper critical solution temperature (UCST) [12, 221] is used for hydrogel fabrication when thermally treated at a critical temperature, the sol-gel transition occurs.
Graft copolymers or amphiphilic blocks can self-assemble to form an organized structure in an aqueous environment with a hydrophobic core such as micelles. Polymers
such as PNIPAM and its derivatives blend with hydrophilic PEO with hydrophobic
PPO/Poly glycolide/Polylactide/PCL are generally fabricated for their thermoresponsive property through LCST. These polymers below LCST are in solution form,
while at higher temperature form an insoluble gel structure through hydrophobic
interactions. Methylcellulose containing Calcium Phosphate (CaP) nanoparticles
using one-pot reaction by LCST gelation, has been reported as an experimental
hydrogel drug reservoir at physiological pH [144].
On the contrary, in UCST induced hydrogel formation, the cooling temperature of the polymer solution plays an important role. Here, the hydrogel state is
achieved at the temperature below UCST. However, at UCST, the hydrogel disintegrates, making the hydrophobic water-soluble micelle cores. Thermo reversible linear
triblock comprising with inner hydrophilic poly-polyethylene glycol methyl ether
methacrylate (PPEGMMA) part and outer polyacrylamide-co-acrylonitrile (PAAmco-AN) units as a cooling induced sol-gel transition. This physical transition can be
modulated by the polymer concentration [45].
Ultrasonic induction [79, 209] induced crosslinking involves phase variation.
Natural or synthetic polymers such as collagen and silk fibroins that possess complex
secondary structures can be physically crosslinked to form interpenetrating networks
(IPNs) with improved physical properties and tunable gelation properties when
treated with physical shear, organic solvents, and heat treatment.
iii. Crosslinking by Crystallization
Crystallites of polymer unit work as sites of physical cross-linking for hydrogel
formation. An aqueous solution of PVA is subjected to repeat freeze-thawing
to yield hydrogel. Parameters such as molecular weight, polymer concentration,
time, and the number of freeze-thawing cycles and freezing temperatures influence
the hydrogel formation [66]. Similarly, exploiting the property of crystallization,
hydrogen bonding in polymers PVA, and hydrophobic polyacrylamide (HAPAM),
physical double network (PDN) was developed by one-potpolymerization followed
by repeated freeze-thaw cycles (Fig. 4).
More compact hydrogel structures are obtained by stereo-complex interaction
between the two enantiomeric polymers. Based on the racemic crystallite Poly LLactic acid (PLLA) and poly d-Lactic acid (PDLA), a unique gel-sol-gel transition occurs upon heating and stereo-complexation. Novel enantiomeric mixtures of