The present invention relates to composition, processes, techniques, and apparatus for synthesizing monodisperse microgels using a precipitation polymerization method. These non-toxic and anti-immunogenic microgels are applicable to controlled drug delivery and other biomedical applications.
Poly-N-isopropylacrylamide (PNIAPM) is one of the most studied thermo-responsive polymers with a lower critical solution temperature (LCST) at 32° C.[1] Free radical polymerization of NIPAM monomer under various conditions has been used to produce polymer, bulk gel, microgel (or nanoparticle).[2] At room temperature, PNIAPM gel is in a swollen state and at body temperature it changes into a collapsed state. This change is due to an entropy effect, resulting from a balance between hydrogen-bond formation with water and intramolecular hydrophobic forces.[3] The combination of the sharp transition and easy accessible, tunable LCST near the body temperature has made PNIPAM very attractive for both scientific studies and technological applications. Specifically, PNIPAM gels and their derivatives have been intensively studied and were found very promising for pulsatile drug delivery.[4-9] However, the extraordinary thermo-sensitive properties of PNIPAM have not been transferred into a biomedical breakthrough in controlled drug delivery devices for human body. The major hurdle is that NIPAM monomer is carcinogenic or teratogenic.[10] Recently, Lutz, et al have reported that that copolymers of 2-(2-methoxyethoxy)ethyl methacrylate and oligo(ethylene glycol) methacrylate (P(MEO2MA-co-OEGMA)) exhibit a thermoresponsive behavior generally comparable, and in some cases, superior to PNIPAM.[11-13] The present invention relates to microgels of P(MEO2MA-co-OEGMA) which have been synthesized using free radical polymerization. The microgels with a variety of particle radii have been obtained with different surfactant concentrations. The particle size distribution is extremely narrow and even better than PNIPAM microgels. The new P(MEO2MA-co-OEGMA) microgels show thermo-reversible volume phase transition near the LCST and can easily self-assemble into crystalline structures, similar to PNIPAM microgels.[14-18] Considering that PEG is nontoxic and anti-immunogenic and has been approved by the FDA [11-13, 19-20], thermo-responsive P(MEO2MA-co-OEGMA) microgels may lead to many biomedical applications.
The present invention comprises 1) The processes, techniques and apparatus for synthesizing of monodisperse microgels of poly(ethylene glycol) analogues-based polymers by using precipitation polymerization method. The microgels with a variety of particle radii ranging from 82 nm to 412 nm have been obtained with different surfactant concentrations. The LCST corresponding to the molar ratio of oligo(ethylene glycol) methacrylate (OEGMA) to 2-(2-methoxyethoxy)ethyl methacrylate (MEO2MA) at 10 and 20% are 31 and 37° C., respectively. 2) The microgels in water self-assemble into various phases including a crystalline structure with iridescent colors, which are the result of Bragg diffraction from different oriented crystalline planes. 3) The crystal structures of microgels can be made permanent by either covalently bonding neighboring particles or entrapping microgels into another hydrogel matrix. 4) Considering that PEG is nontoxic and anti-immunogenic and has been approved by the FDA, thermo-responsive PEG-based microgels may lead to many biomedical applications including controlled drug delivery.
The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of specific embodiments presented herein.
The present invention relates to composition, processes, techniques, and apparatus for synthesizing monodisperse microgels based on poly(ethylene glycol) (PEG) derivative polymers by using precipitation polymerization. These microgels are hydrophilic and have the adjustable volume phase transition temperature in aqueous environment. Microgels can be added with various functional groups. These microgels in water can self-assemble into various phases, including a crystalline phase. Hydrogel films with iridescent colors were formed using these microgels as crosslinkers to connect poly(ethylene glycol) chains. The colors of these hydrogel films change with changes of environment such temperature, pH, salt concentration, etc.
2-(2-Methoxyethoxy)ethyl methacrylate (MEO2MA 95%), poly(ethylene glycol) methyl ether methacrylate (OEGMA 475 Mn=475 g mol-1), poly(ethylene glycol) methyl ether methacrylate (OEGMA 300 Mn=300 g mol-1), dodecyl sulfate sodium salt 98% (SDS), potassium persulfate (KPS) were purchased from Aldrich. Ethylene glycol dimethacrylate (EGDMA 97%) was purchased from Fluka. Water for sample preparation was distilled and deionized to a resistance of 18.2 MW by a Millipore system and filtered through a 0.22 μm filter to remove particulate matter.
The copolymerization of MEO2MA and OEGMA was carried out in a three neck flask equipped with a magnetic stirrer and a nitrogen feed (Table 1): 0.016 mol of MEO2MA, different moles and molecular weights of OEGMA, 4.6×10−4 mol EGDMA, different concentrations of SDS were dissolved in 245 g deionized water. The solution was purged with nitrogen gas for 40 minutes at 70° C. Potassium persulfate (0.10 g), which was dissolved in 5 mL of water, was then added to initiate the emulsion copolymerization. The reaction lasted for 6 hours under nitrogen atmosphere. The reaction temperature was kept at 70+0.5° C. All copolymerization of MEO2MA and OEGMA microgels were purified via dialysis tube (MWCO 13 000) against frequent changes of stirring water for 1 weeks at room temperature. The final microgels were collected by centrifuge.
Dynamic Light Scattering Characterization. A laser light scattering spectrometer (ALV, Germany) equipped with an ALV-5000 digital time correlator was used with a helium-neon laser (Uniphase 1145P, output power of 22 mW and wavelength of 632.8 nm) as the light source. The hydrodynamic radius distribution of the microgels in water was measured at a scattering angle of 60°.
UV-Visible Spectroscopy Measurements. The turbidity (a) of the gels was measured as a function of the wavelength using a diode array UV-visible spectrometer (Agilent 8453) by calculating the ratio of the transmitted light intensity (It) to the incident intensity (I0) a=−(1/d)ln(It/I0), where d is the thickness (1 mm) of the sampling cuvette.
The free radical copolymerization of MEO2MA and OEGMA was carried out in a three neck flask equipped with a magnetic stirrer and a nitrogen feed. Typically, 0.016 mol of MEO2MA, different moles and molecular weights of OEGMA, 4.6×10−4 mol EGDMA as a crosslinker, different concentrations of SDS as surfactant were dissolved in 245 g dionized and distilled water. The solution was purged with nitrogen gas for 40 minutes at 70° C. Potassium persulfate (0.10 g), which was dissolved in 5 mL of water, was then added to initiate polymerization. The reaction lasted for 6 hours under a nitrogen atmosphere at 70° C. The resulting P(MEO2MA-co-OEGMA) microgels were purified via dialysis tube (MWCO 13,000) against frequent changes of stirring water for one week at room temperature. The final microgels were collected by an ultracentrifuge.
The average hydrodynamic radius (Rh) and the radius distribution function, f(Rh), of these microgels were characterized using a laser light scattering spectrometer (ALV Co., Germany). The dynamic light scattering experiments were performed at the scattering angle è=60°.
a shows typical results of the hydrodynamic radius distributions of P(MEO2MA-co-OEGMA) microgels prepared by using different surfactant (SDS) concentrations. As surfactant concentration increases, the particle size decreases. Hydrodynamic radius distributions of a typical PNIPAM microgel and the P(MEO2MA-co-OEGMA(475)) microgel (batch 5) in water are compared in
Temperature dependence of normalized hydrodynamic radii (Rh) of P(MEO2MA-co-OEGMA(475)) microgels with three different molar ratios of OEGMA to MEO2MA is shown in
The new microgels have been concentrated using ultracentrifugation with the speed of 13,000 rpm for 4 h. The dispersion of the microgels was then diluted to different polymer concentrations. These dispersions were then shaken with a vibrator and then allowing them to reach an equilibrium state at 18° C. As shown in
This procedure of shaking and then keeping dispersions in a certain temperature was repeated for several temperatures. The results of the phase behavior as functions of both temperature and polymer concentration are summarized in
The most interesting phase is the crystalline structure. We have grown the crystal structures with different interparticle distance by first preparing microgel dispersions with different polymer concentrations, then heating these dispersions above their respective melting point and finally letting them to cool down naturally to 18° C. The results are shown in
The use of thermal responsive PEG colloidal dispersions based on their crystalline structures is limited because the structures can be easily destroyed by any external disturbance such as small vibrations. Here we show schematics to chemically bond self-assembled PEG microgels. The covalent bonding contributes to the structural stability, while self-assembly provides crystal structures that diffract light, resulting in colors.
As shown in
Materials. Poly(ethylene glycol)ethyl ether methacrylate (PEGETH2MA, Mn˜246 g poly(ethylene glycol) methyl ether methacrylate (PEGEMAPEGMEA, Mn˜300 g poly(ethylene glycol) acrylate (PEGA, Mn˜375 g mol−1), acryloyl chloride, dodecyl sulfate sodium salt 98% (SDS), and potassium persulfate (KPS) were purchased from Aldrich. Ethylene glycol dimethacrylate (EGDMA 97%) was purchased from Fluka. All chemicals were used as received.
PEGETH2MA-co-PEGMA-co-PEGA Microgel Preparation. The copolymerization of PEGETH2MA, PEGMAPEGMEA and PEGAAPEGA was carried out in a three-necked flask equipped with stirrer and a nitrogen feed. 5.63 g of PEGETH2MA(Mn˜246 g mol−1), 1.72 g PEGMEA (Mn˜300 g mol−1), 1.07 g PEGAAPEGA (Mn˜375 g mol−1), 0.064 g SDS and 4.6×10−4 mol of EGDMA were dissolved in 400 ml of DI water. The solution was purged with nitrogen gas for 40 min at 70° C. Ammonium persulfate (0.20 g), which was dissolved in 5 mL of water, was then added to initiate the emulsion copolymerization. The reaction lasted for 12 hours under the nitrogen atmosphere. The reaction temperature was kept at 70° C. Then the microgels were purified via a dialysis tube (MWCO13 000) against frequent changes of stirring water for 1 week at room temperature. The microgels were collected by ultra centrifugation.
Vinyl thermo-responsive PEG based microgel preparation. The collected PEGETH2MA-PEGMAPEGMEA-co-PEGA microgel 10 g (10 wt. %) were dried by freeze-dry method. Then the microgels were re-dispersed in 100 ml CH2Cl2. 1 g acryloyl chloride and trace amount tri-ethylamine (compared with acryloyl chloride) were slowly added into microgels solution. The molar ratio between acrylate PEG (OH group) to acryloyl chloride was 1 to 32. The reaction was carried out under dark at room temperature with anhydrous environment for 24 hours. The reaction in darkness was just precaution for protecting the vinyl group. The reaction was stopped by adding 300 ml absolute ethyl alcohol. The vinyl-PEG based microgels were collected by ultra centrifugation. Then the vinyl microgels were dispersed in ethyl alcohol and put into a dialysis tube under dark for a week in absolute ethyl alcohol, 50 vol. % ethyl alcohol, 25 vol. % ethyl alcohol and DI water at temperature 4° C. Vinyl group was confirmed by IR spectra.
Vinyl PEG based microgel/PEG acrylate (PEGA, Mn˜375 g mol−1) crystalline hydrogel film preparation. 0.45 g 20 wt. % PEG acrylate with UV initiator 2-hydroxy-1-[4-(2-hydroxyothoxy)phenyl]-2-methyl-1-propanone (CIBA) (0.2 wt %), 0.55 g 12 wt. % vinyl PEG microgel were mixed. The suspension was de-oxygen by freeze-thaw method. The suspension was injected into a cell consisting of two clean quartz disks separated a 125 μm Parafilm film. The crystalline structures were formed by slowly changing temperature from 29° C. to 4° C. in 24 hours. If this change was too rapidly, there would be no crystallization. The crystalline structure was then stabilized by UV irradiation triggered free radical polymerization at 0° C. for 30 min. All chemicals were used as received. The resultant hydrogels was washed out with DI water that was changed twice a day for 10 days to clean monomer and un-reacted small molecules.
Because the building blocks here are environmentally responsive colloidal spheres, their sizes as well as the lattice spacing should be tunable by external stimuli. As a result, the crystalline hydrogel can serve as an optical sensor to visually inspect environmental changes. One of the examples is shown in
It has been already established that colors of microgel dispersions are related to the Bragg diffraction from periodic arrays of microgels, which is shown as a peak in UV-visible spectrum in
5.2 Entrapping Microgels into Another Hydrogel Matrix
Firstly monodisperse microgels were prepared. Random copolymers of 2-(2-methoxyethoxy)ethylmethacrylate (MEO2MA) and oligo (ethylene glycol) methacrylate (OMGMA or O300 Mn=300) exhibited LCST 37° C. with mole ratio=1:1. 1.98 g MEO2MA, 3.06 g O300, 0.035 g sodium dodecyl sulfate (SDS, work as surfactant), 0.10 g ethylene glycol dimethacrylate (EGDMA M=198, 2.45% of monomer work as the cross-linker) and 0.18 g acrylic acid (AA) were mixed together in a reactor. 195 g deionized water are added and bubbled with nitrogen for 40 min at 70° C. Then a solution of 0.16 g potassium persulfate in 5 g deionized water was added to initiate the reaction. The reaction was carried out at 70° C. for 4 hours. The resulting particle dispersions were dialyzed for 7 days to remove small molecules and surfactant. Then the particle dispersions were concentrated with centrifuging at 14000 rpm. Solid percentage in microgels was 11 wt %.
Second, these microgels were entrapped into a polymer network, a hydrogel (such as polyacrylamide or PEG with cross-linker). 4.66 g sample above and 0.25 g acrylamide, 0.19 g photo initiator (0.1% 2-hydroxyl-1-[4-(2-hydroxy ethoxy)phenyl]-2-methyl propanone water solution) and 0.005 g BIS (methylenebisacrylamide) were mixed and bubbled with nitrogen for 40 min. Then the sample was handled under nitrogen protection and exposed under ultra violet light for 30 min.
In summary, the P(MEO2MA-co-OEGMA) microgels have been synthesized by using free radical polymerization. The microgels with a variety of particle radii ranging from 82 nm to 412 nm have been obtained with different surfactant concentrations. As surfactant concentration increases, the particle size decreases. The particle size distribution is extremely narrow and even better than PNIPAM microgels. The pure MEO2MA microgel has the LCST about 22° C. The LCST corresponding to the molar ratio of OEGMA to MEO2MA at 10 and 20% are 31 and 37° C., respectively. The LCST can be also tuned by fixing the molar ratio of OEGMA to MEO2MA at 10% but changing molecular weight of OEGMA. The microgels in water self-assemble into various phases including a crystalline with iridescent colors, which are the result of Bragg diffraction from different oriented crystalline planes. The UV-visible spectra from microgel dispersions show that the sharp Bragg peak from 620 to 480 nm as the polymer concentration increases from 4.8 to 10.2 wt %. This crystalline structure was fully stabilized by either trapping microgels into a hydrogel matrix or covalently linking neighboring microgels. The thin films of these interlinked microgels have been taken out from the test tubes. UV-visible spectroscopy has been used to monitor the change of the Bragg diffraction form these films as a function of temperature. The change of the peak wavelength is due to the shrinkage of the particle size with the temperature, which causes the decrease of inter-particle spacing in crystalline hydrogels. As a result, the crystalline hydrogel may serve as an optical sensor to visually inspect environmental changes.
The following references, to the extent that they provide exemplary procedural or other details supplementary to those set forth herein, are specifically incorporated herein by reference.
This application claims priority to U.S. Provisional Patent Application Ser. No. 61/135,318, entitled “MONODISPERSE THERMO-RESPONSIVE MICROGELS OF POLY(ETHYLENE GLYCOL) ANALOGUE-BASED BIOPOLYMERS, THEIR MANUFACTURE, AND THEIR APPLICATIONS” filed on Jul. 18, 2008, the entire content of which is hereby incorporated by reference.
This invention was made in part during work supported by a grant from the National Science Foundation (DMR-0507208), to Zhibing Hu, entitled “Novel polymer microgel dispersions with an inverse thermoreversible gelation”. The government may have certain rights in the invention.
Number | Date | Country | |
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61135318 | Jul 2008 | US |