Microgels with two interpenetrating polymer networks of poly-N-isopropylacrylamide and polyacrylic acid (PNIPAM-IPN-PAAc) were synthesized using a seed method. The IPN microgels in water have an average hydrodynamic radius of about 85 nm at 21 °C, measured by dynamic light scattering method. The atomic force microscope image showed that the particles were much smaller after they were dried but remain their spherical shape. The storage and loss moduli G' and G' ' of dispersions of IPN microgels were measured in the linear stress regime as functions of temperature and frequency at various polymer concentrations using a stress-controlled rheometer. For dispersions with high polymer concentration (3.5 and 6.0 wt%) and at high temperatures (34 and 38 °C), the samples behave as viscoelastic solids and the storage modulus was larger than the loss modulus over the entire frequency range. The loss tangent was measured at various frequencies as a function of temperature. The gelation temperature was determined to be 33 °C at the point where a frequency-independent value of the loss tangent was first observed.Using an animal implantation model, the biocompatibility and drug release properties of the IPN microgl dispersion were evaluated. Fluorescein as a model drug was mixed into an aqueous microgel dispersion at ambient temperature. This drug loaded liquid was then injected subcutaneously in Balb/C mice from Taconic Farms. The test results have shown that the IPN microgels were biocompatible in this acute implantation model and the presence of gelled microgel dispersion substantially slowed the release of fluorescein.
Photonic materials and devices that manipulate photons in visible and near-infrared spectra are the focus of research in diverse fields ranging from nanomaterials technology to optical computing. 1 Creating photonic hydrogels is part of this major thrust because they can not only control or detect photons by their ordered structures but also change propagation of photons in response to external stimuli. 2 The photonic hydrogel has been made by polymerizing a poly-N-isopropylacryalmide (PNIPAM) gel around a mesoscopically crystalline colloidal array of polystyrene (PS) colloids. 2 Inverse crystalline hydrogels have been obtained by using silica or PS colloidal arrays as templates. 3-5 A PNIPAM microgels array has been either entrapped by another gel matrix 6 or interconnected by cross-linkers. 7 Here we show that the microgels based on poly(ethylene glycol) (PEG) derivative polymers have been successfully used as building blocks for preparation of photonic hydrogels. These hydrophilic particles not only have thermal responsive behavior like PNIPAM particles but also can self-assemble into crystalline structures like PS, silica, or PNIPAM particles. Hydrogels are well-known for their hydrophilic and environmentally responsive properties. 8-11 Constructing photonic hydrogels with nontoxic, anti-immunogenic, thermally responsive PEG derivative polymers 12,13 could open a door for new applications.Our approach is illustrated in Scheme 1. PEG derivative microgels are prepared and then attached with vinyl groups. After these particles self-assemble into an ordered array, they are used to connect neighboring polymer chains to stabilize the crystalline structure. Particles as cross-linkers can significantly enhance the mechanical properties of the gel. 14,15 PEG derivative microgels were prepared by copolymerization of poly(ethylene glycol)ethyl ether methacrylate (PEGETH 2 MA), poly(ethylene glycol) methyl ether methacrylate (PEGMEMA), and poly(ethylene glycol) acrylate (PEGA) using the free radical polymerization method. 13 The first two components have different values of the low critical solution temperature (LCST) due to their different molecular weights. 12 The combination of these two components made the LCST of the sample near the physiological temperature. The last component provided a functional group that will be used for the attachment of vinyl group after the microgel is prepared. Ethylene glycol dimethacrylate as cross-linker, dodecyl sulfate sodium as surfactant, and ammonium persulfate as initiator were mixed with monomer solution. The polymerization was carried out at 70°C under a nitrogen atmosphere for 12 h. To attach vinyl groups, PEG derivative microgels were dried and redispersed in CH 2 Cl 2 . 1 g of acryloyl chloride and trace amount of triethylamine were slowly added into microgels solution. The reaction was carried out under dark at room temperature in an anhydrous environment for 24 h. a Green spheres represent PEG derivative particles, brown spheres vinyl group, and curved lined polymer chai...
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