Abstract
We report the synthesis and properties of temperature- and pH-responsive p([NIPAm-co-PEGMA] (core)/[NIPAm-co-AAc] (shell)) nanogels with narrow size distributions, tunable sizes and increased drug loading efficiencies. The core-shell nanogels were synthesized using an optimized two-stage seeded polymerization methodology. The core-shell nanogels show a narrow size distribution and controllable physico-chemical properties. The hydrodynamic sizes, charge distributions, temperature-induced volume phase transition behaviors, pH-responsive behaviors and drug loading capabilities of the core-shell nanogels were investigated using transmission electron microscopy, zeta potential measurements, dynamic light scattering and UV-Vis spectroscopy. The size of the core-shell nanogels was controlled by polymerizing NIPAm with crosslinker poly(ethylene glycol) dimethacrylate (PEGDMA) of different molecular weights (Mn-200, 400, 550 and 750 g/mol) during the core synthesis. It was found that the swelling/deswelling kinetics of the nanogels was sharp and reversible; with its volume phase transition temperature in the range of 40–42 °C. Furthermore, the nanogels loaded with l-3,4-dihydroxyphenylalanine (L-DOPA), using a modified breathing-in mechanism, showed high loading and encapsulation efficiencies, providing potential possibilities of such nanogels for biomedical applications.
Highlights
Stimuli-responsive polymers belong to a class of smart materials that have the ability to respond to a variety of external stimuli; like pH [1], temperature [2], mechanical force [3], the presence of various molecules [4] and electric/magnetic fields [5,6,7]
Setbacks related to increasing the NIPAm’s volume phase transition temperature (VPTT) and hydrophilic drug loading can be resolved by polymerizing NIPAm with ionic comonomer (acrylic acid (AAc)) and hydrophilic crosslinker (poly(ethylene glycol) dimethacrylate (PEGDMA))
PEGMA and NIPAM are responsible for maintaining the hydrophilicity and hydrophobicity balance of the core nanogels, respectively
Summary
Stimuli-responsive polymers belong to a class of smart materials that have the ability to respond to a variety of external stimuli; like pH [1], temperature [2], mechanical force [3], the presence of various molecules [4] and electric/magnetic fields [5,6,7]. NIPAm-based gels can be modified to have higher or lower transition temperature by incorporating various comonomers and crosslinkers [22,23]. NIPAm with ionic monomers and comonomers results in temperature-sensitive nanogels with an increased volume phase transition temperature (VPTT) [21,24]. Setbacks related to increasing the NIPAm’s VPTT and hydrophilic drug loading can be resolved by polymerizing NIPAm with ionic comonomer (acrylic acid (AAc)) and hydrophilic crosslinker (poly(ethylene glycol) dimethacrylate (PEGDMA)). One way to overcome the above-mentioned shortcomings is by designing NIPAm nanogels with a core-shell architecture; the core comprising NIPAM and PEG and the shell comprising NIPAm and AAc. The resulting nanogels will have increased VPTT and higher hydrophilic drug loading capacity. NIPAm nanogels with such a core-shell morphology would illustrate dual responsive properties with respect to both temperature and pH. The nanogels showed high loading and encapsulation efficiencies with l-3,4-dihydroxyphenylalanine (L-DOPA), a drug for Parkinson’s disease [45]
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