Hydrogel nanoparticles(8)

PNIPAM in water,exhibit a low critical solution temperature.This very sharp transition is attributed to the disruption of hydrogen bonding of water molecules around the amide group of the side polymer chains.

Hydrogel NP networks containing dextran have been developed by G.Huang et al.[231].In their study,PNIPAM-co-allylamine NP networks and PNIPAM-co-acrylic acid NP networks are formed by covalently crosslinking.Also,Gan and Lyon[232]have synthesized thermoresponsive core-shell PNIPAM NPs via seeding and feeding precipitation polymerization method.The in?uence of chemical differentiation between the core and the shell polymers on the phase transition kinetic and thermodynamic behavior,has been examined in their study.

3.7.Hydrogel nanoparticles of other origins

As noted in the hydrogel section,responsive hydrogel systems have devoted a great contribution to the drug delivery?eld.Sahoo et al. [233]have prepared pH-and temperature-sensitive hydrogel NPs from copolymers including vinylpyrrolidone(VP)and acrylic acid (AA),crosslinked by N,N methylene bis acrylamide(MBA),with particle sizes up to50nm in diameter loaded with a marker compound FITC-dextran.The release of FITC-dextran was slow in acid solution, but it increased considerably as the pH of the medium was increased. The release rate also rose with the increment of temperature.

Moreover,magnetically responsive hydrogel networks based on composites of magnetic nanoparticles and temperature responsive hydrogels were developed[234].These systems show great promise as active components of microscale and nanoscale devices and are expected to have a wide applicability in various biomedical applica-tions.In this context,nanocomposite hydrogel systems based on the temperature-sensitive N-isopropylacrylamide hydrogels crosslinked with ethylene glycol dimethacrylate,tetraethylene glycol dimetha-crylate,and poly(ethylene glycol)400dimethacrylate(PEG400DMA) were synthesized and characterized.The composite systems were synthesized by UV free radical polymerization.Iron oxide magnetic nanoparticles were incorporated into the hydrogel systems by polymerizing mixtures of the nanoparticles and monomer solutions. The swelling response of these composite systems to different crosslinking molecular weights,temperature,and the effect of the presence of the magnetic nanoparticles were examined.

Pullulan-based hydrogel NPs have been prepared as a drug delivery carrier.In a study dealing with self-assembled hydrogel NPs of cholesterol-bearing pullulan which led to the production of20–30nm NPs,Kazunari et al.evaluated the complexation and stabilization of insulin[235].They demonstrated that spontaneous dissociation of insulin from the complex and thermal denaturation/aggregation,were effectively suppressed upon complexation.In another study,Gupta et al.[236]provided a method for enhancing the delivery of nucleic acid molecules to cells by encapsulating them within the hydrogel pullulan NPs.In this work,pullulan NPs bearing plasmids were entrapped inside the aqueous droplets of a w/o microemulsion.Transmission electron microscopy(TEM)images showed spherical particles with diameter of45±0.80nm.

Poly(methacrylic acid-grafted-poly(ethylene glycol))(P(MA-g-PEG))hydrogel NPs were prepared by a thermally-initiated free radical polymerization method[237].These hydrogel NPs show pH-sensitive swelling behavior,which is strongly in?uenced by the crosslinker dosage.

Self-assembled nanogels composed of dextran and PEG macromers prepared by Kim et al.[238]from glycidyl methacrylate dextran (GMD)and dimethyl methacrylate poly(ethylene glycol)(DMP)via radical polymerization has been exploited as a drug delivery system. Moreover,preparation of stable polymeric NPs composed of PEG and poloxamer407(Pluronic®F127)through inverse emulsion photo-polymerization resulted in successful encapsulation of doxorubicin (loading ef?ciency=8.7%)[239].References

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