Sergey G. Starodubtsev

Microgels for the Encapsulation and Stimulus-Responsive Release of Molecules with Distinct Polarities

A microfluidic strategy for the encapsulation and stimulus-responsive release of molecules with distinct polarities from the interior of microgels is reported. The approach relies on (i) the generation of a primary O/W emulsion by the ultrasonication method, (ii) MF emulsification of the primary emulsion, and (iii) photopolymerization of the monomer present in the aqueous phase of the droplets, thereby transforming them into microgels. Non-polar molecules are dissolved in oil droplets embedded in the microgels. Polar molecules are physically associated with the hydrogel network. Upon heating, the microgels contract and release polar and non-polar cargo molecules. The approach paves the way for stimuli-responsive vehicles for multiple drug delivery.

Formation of magnetite nanoparticles in poly(acrylamide) gels

Magnetic gels with magnetite nanoparticles incorporated in a matrix of poly(acrylamide) gel were studied. Magnetite was synthesized through coprecipitation of Fe(II) and Fe(III) in the gel phase, in the solution of linear polymer and in aqueous solution without polymer in alkaline media. The effects of network structure and of the concentration of iron salts in the swollen networks on the composition, structure and properties of magnetic gels have been studied by electron diffraction, XRD, transmission electron microscopy and vibration sample magnetometry. The average size of magnetite nanoparticles, D, is of the order of 10 nm. It decreases with the increase of polymer concentration in the gel phase. In the dried gels the particles form spherical aggregates (diameter about 150 nm), whereas in the solution of linear polymer, in the aqueous solution of iron salts and in the gel with high content of polymer the aggregates have irregular shape.

Effect of polymer nature on the structure and properties of gel composites with incorporated bentonite particles

Composite gels based on polyacrylamide and poly(N-isopropyl acrylamide) with incorporated sodium bentonite particles are synthesized. It is shown that the presence of hydrophobic isopropyl groups in a polymer molecule promotes the subsequent formation of highly ordered aggregates of clay and cetylpyridinium chloride in a gel composite. An increase in temperature results in the collapse of composite gels based on poly(N-isopropyl acrylamide); however, no marked changes in the structure of lamellar aggregates of clay and surfactant are observed. It is revealed that the gel can stabilize lamellar structures formed in organoclay suspension prior to the incorporation into swollen polymer network.

Specific features of the polyelectrolyte behavior of weakly charged cryogels of polyacrylamide and poly(N-isopropylacrylamide)

Specific features of the polyelectrolyte behavior of weakly charged common gels and cryogels of copolymers of polyacrylamide and poly(N-isopropylacrylamide) with sodium acrylamido-2-methyl-1-propyl sulfonate are investigated. The cryogels are synthesized in frozen solutions at −15°C. It is shown that the polyelectrolyte swelling is significantly weaker in the case of cryogels than that in the case of gels synthesized in solutions. For thermosensitive gels with isopropylacrylamide groups, collapse occurs during heating. Charging of a common gel leads to a noticeable (18°C) increase in the transition temperature. For a cryogel, this growth is 3°C. During the interaction with cetylpyridinium chloride, the gel contraction is much more pronounced for common weakly charged gels. At the same time, walls of pores of a collapsed cryogel contain a smaller amount of the solvent. Isotherms of the adsorption of a cationic surfactant by anionic common gels and cryogels differ insignificantly. Model gels synthesized in concentrated acrylamide solutions exhibit very weak polyelectrolyte swelling, similar to that of cryogels. The behavior of cryogels is explained by a very high local concentration of crosslinks due to a strong entanglement of polymer chains.

Magnetic alginate beads for the targeted delivery of functional hydrophobic compounds

New composite alginate beads filled with Fe3O4 magnetic particles and Vaseline droplets containing the dissolved hydrophobic dye Sudan 3 are prepared. The structures of beads and Fe3O4 particles are studied with the aid of an optical microscope. The destruction of beads in an inhomogeneous magnetic field is investigated. The effects of the composition of beads and the magnitude of magnetic field on the release of hydrophobic components in the surrounding solution are analyzed. It is shown that the release of Sudan 3 due to the destruction of beads attains 75%.

Supramolecular structures in polyelectrolyte gels

Three physical situations involving the formation of microstructures in polyelectrolyte gels have been considered. (i) The microstructure emerging in weakly charged polyelectrolyte gels upon the addition of a poor solvent. The following possible reasons for the microstructure formation have been analysed: the polyelectrolyte/hydrophobic competition, the formation of ion pairs and their clustering into multiplets, and the partial vitrification. (ii) The ionomer multiplet-type microstructures which appear in polyelectrolyte gels immersed in a solvent of medium polarity upon the increase of the degree of ionization of the gel. (iii) The microstructures which are formed in polyelectrolyte gels interacting with oppositely charged surfactants due to the aggregation of surfactants inside the network and quasicrystalline organization of the aggregates.

Investigation of physical-chemical properties of agarose hydrogels with embedded emulsions.

Composite agarose hydrogels with embedded tetradecane emulsions stabilized by cetylpyridinium chloride were studied. The absorption efficiency of 4-nitrophenyl ethers of carbonic acids by the composite agarose gels increases with the length of the hydrocarbon tail of the ester. The diffusion rate of amphiphilic substances in the composite gels was demonstrated to be much less that than in the standard agarose gels. The reaction kinetics between the esters and dodecylmercaptan dissolved in tetradecane droplets of composite hydrogel was studied. In the region of physiological pH, the reactivity of SH groups embedded in the composite agarose gel in the reaction with the esters is significantly higher than that in a homogeneous solution. Hydrogels with embedded emulsion droplets are of considerable practical importance as drug delivery systems, microreactors, and absorbers. Composite gels filled with emulsions incorporating lipophilic mercaptanes are effective absorbers of heavy metal ions.

Structural design of PNIPA-based intelligent hydrogels via porous silica templates

Abstract Intelligent temperature-sensitive hydrogels have been synthesized using a hierarchically organized, porous silica gel as template. The organic gels were obtained by three-dimensional copolymerization of N-isopropylacrylamide and N,N’- bis(acryloyl)cystamine in the free volume of a monolith of a porous silica gel. After characterization, the silica matrix was dissolved in alkali. Unexpectedly, the remaining organic polymer showed similar properties to a gel obtained in homogeneous solution. The shape of the inorganic mould was retained in the purely organic hydrogel even after dissolution of the inorganic part and it showed the ability to collapse under enhanced temperatures. The collapse rate of the gel prepared via the template synthesis was much higher compared to gels prepared conventionally.