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PubMed2005

Investigations of step-growth thiol-ene polymerizations for novel dental restoratives.

OBJECTIVES: The goal of this work was to investigate the feasibility of formulating novel dental restorative materials that utilize a step-growth thiol-ene photopolymerization. Particularly, we are aiming to significantly reduce the polymerization shrinkage and shrinkage stress while retaining adequate physical properties as compared to current dimethacrylatre-based systems. METHODS: The thiol-ene system is composed of a 4:3 molar mixture of triallyl-1,3,5-triazine-2,4,6-trione (TATATO) and pentaerythritol tetramercaptopropionate (PETMP). The simultaneous measurement of shrinkage stress and functional group conversion was performed. Solvent extraction of unreacted monomers and dynamic mechanical analysis on the polymer networks that were formed were also studied. Flexural strength was measured for both filled and unfilled PETMP/TATATO and Bis-GMA/TEGDMA systems. RESULTS: Photopolymerization of PETMP/TATATO occurs at a much higher rate, with the maximum polymerization rate six times faster, than Bis-GMA/TEGDMA cured under the identical conditions. The results from the simultaneous measurement of shrinkage stress and conversion showed that the onset of shrinkage stress coincides with the delayed gel point conversion, which is predicted to be 41% for the 3:4 stoichiometric PETMP/TATATO resin composition. The maximum shrinkage stress developed for PETMP/TATATO was about 0.4 MPa, which was only approximately 14% of the maximum shrinkage stress of the Bis-GMA/TEGDMA system. Adequate flexural strength and flexural modulus values were obtained for both filled and unfilled PETMP/TATATO systems. SIGNIFICANCE: The dramatically reduced shrinkage stress, increased polymerization rate, significance increased functional group conversion, and decreased leachable species are all benefits for the use-of thiol-ene systems as potential dental restorative materials.

3-Mercaptopropionic Acid↗
PubMed2005

A physico-chemical explanation of the post-polymerization shrinkage in dental resins.

UNLABELLED: The main problem of a methacrylated dental resin's photopolymerization is the shrinkage phenomenon. This occurs, as expected, during light irradiation but also, unexpectedly, during about 24h after photopolymerization (i.e. during the so-called 'post-polymerization' stage). During this period, the conversion degree does not change significantly (no more initiation, very limited, if any, propagation reaction) but free radicals concentration decreases. OBJECTIVES: To better understand what happens during the 24h after the photopolymerization, a thermal study of these resins is investigated at first and an explanation is then discussed. METHODS: In this paper, the glass transition temperatures (T(g)) are measured at 0 and 24h by DMA. The post-shrinkage phenomenon is observed by TMA. Conversion degree (DC) is followed by Raman and free radical decay by ESR spectroscopy. RESULTS: T(g) increases significantly during post-polymerization (55-80 degrees C). The same samples were studied by TMA at room temperature and shrinkage is observed. The fact that the degree of conversion (DC) does not increase significantly and that the 'post-shrinkage' occurs at T<T(g) leads to the hypothesis that a physical phenomenon should occur which can, in turn, be responsible for a secondary chemical phenomenon (post-polymerization). SIGNIFICANCE: The proposed explanation is that, as photopolymerization of dental resins is very fast, a large excess of free volume is trapped in non-equilibrated samples. As they have no time to return to an equilibrium state, free volume should decrease below T(g) and samples do physically shrink during the first 24h. As a consequence, free radicals can come into 'contact' and undergo limited propagation but significant termination justifying the decrease in overall radical concentration.

Composite Resins↗
PubMed2005

Polymerization characteristics of light- and auto-curing resins for individual splints.

OBJECTIVES: The aim of this in vitro study was to assess different resins for occlusal splints regarding a number of polymerization characteristics. METHODS: One autopolymerizing resin (Palapress) and four light-curing resins (Acrylight, Primosplint, Triad TranSheet Colorless and Pink) were tested for flexural strength, modulus of elasticity, water sorption, volumetric shrinkage, and conversion of double bonds. These resins were polymerized for 10 and 15 min in a light oven at frequencies between 320 and 400 nm or 400 and 500 nm, respectively. Data were treated statistically by ANOVA and by Scheffé test. RESULTS: Concerning flexural strength and modulus of elasticity, Triad TranSheet Colorless showed significantly greater values than Primosplint and Palapress (p<0.05). The light curing resins Acrylight and Primosplint featured significantly higher water sorption when cured for 10 min compared to all other resins (p<0.05). Acrylight displayed significantly higher volumetric shrinkage and a higher quantity of remaining double bonds compared to all other light-curing resins (p<0.05). Primosplint, Triad TranSheet Colorless and Pink showed significantly lower shrinkage values than autopolymerizing Palapress (p<0.05). For all light curing resins, polishing of the samples resulted in a significantly lower quantity of remaining double bonds (p<0.05) compared to unpolished samples. SIGNIFICANCE: With respect to the polymerization properties evaluated, the light-curing resins reached similar material properties to the autopolymerizing resin or even surpassed them. Light-curing resins seem to be suitable for the fabrication of occlusal splints and can been seen as an alternative to autopolymerizing polymethylmethacrylates.

Acrylic Resins↗
PubMed2005

The effect of fiber orientation on the polymerization shrinkage strain of fiber-reinforced composites.

OBJECTIVE: The aim of this study was to characterize the linear polymerization shrinkage strain of glass fiber-reinforced composite (FRC) according to the fiber orientation. METHODS: Test specimens (n=5) (10.0 x 10.0 x 1.5mm) were prepared from different brands of photopolymerizable resin-preimpregnated FRC; unidirectional continuous FRC, experimental random-oriented FRC, and bidirectional continuous FRC. As control materials, particulate filler composite resin and unfilled dimethacrylate monomer resin were used. Two uniaxial strain gages (gage length 2mm) were used to measure shrinkage strains in two directions: longitudinally and transversally to the fiber direction. The uncured composite or resin was placed on top of the strain gages, covered with a separating sheet and a glass plate, and irradiated for 40s with a light-curing unit. The shrinkage strain was monitored for 300 s. ANOVA and Tukey's posthoc test were used at a significance level of 0.05. RESULTS: ANOVA revealed that orientation of fiber and brand of material had a significant effect (P<0.05) on shrinkage strain. The unidirectional FRC revealed no shrinkage longitudinally to the fiber direction, whereas the shrinkage occurred transversally to the fiber direction. Particulate filler composite resin and unfilled resin revealed equal shrinkage strain in both of the measured directions. SIGNIFICANCE: Anisotropic nature of FRC exists with regard to polymerization shrinkage strain. The variation of polymerization shrinkage strains of FRC compared to those of particulate filler composites and unfilled resin might be important for future clinical applications.

Anisotropy↗
PubMed2006

Comparison of two methods for measuring the polymerization characteristics of flowable resin composites.

OBJECTIVES: The purpose of this study was to compare two methods for assessing the polymerization characteristics of flowable resin composites. METHODS: Two different flowable resin composites and a hybrid resin composite control were investigated. In order to measure the volumetric shrinkage (DeltaV), each material was placed into a mould and extruded into a water-filled dilatometer. The specimens were then light irradiated for 30s using a curing unit with the power density adjusted to either 100 or 600mW/cm(2). For the speckle contrast measurement, each resin composite was condensed into a glass tube and irradiated. The laser-speckle field was recorded in a digital frame. The calculated values were obtained for each pair of adjacent patterns and the changes in speckle contrast as a function of time were obtained. RESULTS: The average DeltaV values of the resins after 180s ranged from approximately 3.3 to 4.4% for the flowable composites and from approximately 1.8 to 2.3% for the hybrid composite control. The overall magnitude of the speckle contrasts decreased soon after the initial light exposure commenced and gradually increased thereafter. The speckle contrast measurements revealed changes in the pastes due to the polymerization of the flowable resins that were greater than those obtained with the water-filled dilatometer. SIGNIFICANCE: These data suggest that the polymerization characteristics of flowable resins can be measured successfully using two different methods. Moreover, our findings are of clinical relevance, as the data obtained under laboratory conditions might give an indication of the suitability of flowable resin composites for specific clinical applications.

Analysis of Variance↗
PubMed2006

The influence of curing times and light curing methods on the polymerization shrinkage stress of a shrinkage-optimized composite with hybrid-type prepolymer fillers.

OBJECTIVES: The aim of the study was to determine the influence of different light curing units (LCU) and regimes on the polymerization shrinkage stress (PSS) and the mechanical properties of a nano-hybrid composite. MATERIAL AND METHODS: The polymerization shrinkage force (PSF) was measured continuously with compliance compensation for 300s after photo-initiating the composite, Tetric EvoCeram (Ivoclar Vivadent, Schaan, FL, Shade A3) in a Stress-Strain Analyser. Astralis 10, Bluephase and MiniL.E.D LCU with exposure times 10, 20 and 40s were used (C-factor=0.33, n=8 per group). Immediately after the PSF measurements, mechanical properties of the samples were measured at the top and the bottom using a Fischerscope H100C (Helmut Fischer GmbH, Sindelfingen, Germany). Statistical analyses were done using one-way ANOVA (p<0.05) and Tukey post hoc test. RESULTS: Significant differences in the PSS for 10, 20 and 40s polymerization using Astralis 10 were found. The MiniL.E.D recorded low stress values. Modulus of elasticity is high after curing the composite with Astralis 10 at 10, 20 and 40s and for Bluephase 40s. Low moduli of elasticity were recorded for the MiniL.E.D and for the Bluephase 20 and 10s. The hardness values (HV) followed the same pattern as the modulus of elasticity. The Ramping mode of the MiniL.E.D had prolonged gel point. CONCLUSIONS: High intensity LCU produce not just high HV but also high shrinkage, making it important to balance both the effects by choosing the appropriate curing time. Soft-start regimes have no paramount benefit in a LED regarding stresses in the clinical situation.

Analysis of Variance↗
PubMed2005

Skin permeation of different steroid hormones from polymeric coated liposomal formulations.

In this study, the effect of various polymers (polycarbophil, chitosan-EDTA, polymeric emulsifier and carrageenan) on the permeation, the chemical and microbial stability of 17-beta-estradiol, progesterone, cyproterone acetate (cpa) and finasteride incorporated in DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) liposomes has been investigated. The liposomes contained 1% (w/w) of the steroid hormones. Standard diffusion experiments were performed. Drug stability was monitored by analysing the steroid hormone content in the different formulations over a time period of 8 weeks and visually inspecting for microbial contamination. In addition, viscosity measurements were performed. The permeation rate could be improved by addition of polymeric agents depending on their type and drug. In all tested formulations, finasteride exhibited the highest diffusion. Both the chemical and the microbial stability of the hormones were significantly improved by the polymers in comparison to the pure liposomes after an observation period of 8 weeks. After that time microbial stability was still evident for all semisolid formulations. In contrast to this in the pure liposomes already after 2 weeks the steroid drugs showed complete insufficient chemical stability and microbial contamination. Additional rheological measurements indicated an influence of the polymers and drugs on the viscosity in all formulations. The elasticity predominated in nearly all polymeric formulations.

Acrylic Resins↗
PubMed2004

Polymerization of nucleotide-free, GDP- and GTP-bound cell division protein FtsZ: GDP makes the difference.

Stable, more than 98% nucleotide-free apo-FtsZ was prepared from purified Methanococcus jannaschhi FtsZ. This facilitates the study of the functional mechanisms of this FtsZ, an assembling GTPase, which shares a common fold with eukaryotic tubulin. Apo-FtsZ underwent cooperative magnesium-induced polymerization with a similar critical concentration and morphology related to that of reconstituted GTP-bound FtsZ, suggesting that the binding of GTP contributes insignificantly to the stability of the FtsZ polymers. On the other hand, reconstituted GDP-FtsZ polymerized with a larger critical concentration than GTP-FtsZ, indicating that GDP binding destabilizes FtsZ polymers. Upon GTP hydrolysis by FtsZ polymers, in the absence of a continued GTP supply and under macromolecular crowding conditions enhancing FtsZ polymerization, the straight GTP polymers disappeared and were replaced by characteristic helically curved GDP-bound polymers. These results suggest that the roles of GTP binding and hydrolysis by this archaeal FtsZ are simply to facilitate disassembly. In a physiological situation in GTP excess, GDP-bound FtsZ subunits could again bind GTP, or trigger disassembly, or be recognized by FtsZ filament depolymerizing proteins, allowing the Z-ring dynamics during prokaryotic cell division.

Bacterial Proteins↗
PubMed2006

Synthesis of cholesterol imprinted polymeric particles.

The aim of this study is to prepare cholesterol-imprinted polymeric particles. N-Methacryloyl-(L)-tyrosinemethylester (MAT) was chosen as the complexing monomer. In the first step, functional monomer MAT was synthesized by the reaction of L-tyrosine methylester and methacryloyl chloride and characterized by FTIR and NMR. Then, cholesterol was complexed with MAT in different mol ratios and the cholesterol-imprinted poly(2-hydroxyethyl methacrylate-N-methacryloyl-(L)-tyrosine methylester) [MIP] particles were synthesized by bulk polymerization. After that, the template molecules (i.e., cholesterol) were removed using chloroform. MIP particles were characterized by elemental analysis, FTIR, SEM, swelling tests and surface area measurements. Cholesterol adsorption experiments were performed in a batch experimental set-up. Adsorption medium was methanol or intestinal mimicking solution. Stigmasterol and estradiol were used as competing molecules in selectivity tests. Obtained results were as follows: swelling ratio of MIP and non-imprinted (NIP) particles were 60.8% and 44.1% in water. With the increase in the amount of MAT in the polymerization medium, incorporation of MAT was increased (16.6-78.0 micromol/g). SEM photographs showed the surface roughness and porosity. Specific surface area of NIP and MIP particles were found as 19.2 and 31.5 m(2)/g, respectively. Template molecules (i.e., cholesterol) were removed from the polymer structure in the ratio of 76-84% of the initial concentration. Cholesterol adsorption increased with the increase in cholesterol concentration up to 1.5 mg/mL. MIP particles prepared using higher amounts of cholesterol exhibit significantly higher capacity to the NIP particles (i.e., control polymer). MIP particles were 3.09 and 3.60 times selective with respect to the stigmasterol and estradiol, respectively. Reusability of MIP particles was also investigated. MIP particles showed negligible loss in the cholesterol adsorption capacity after five adsorption-desorption cycles with the same adsorbent.

Adsorption↗
PubMed2006

Encapsulation of amphotericin B in poly(ethylene glycol)-block-poly(epsilon-caprolactone-co-trimethylenecarbonate) polymeric micelles.

The aim of this work was to evaluate the potential of self-assembling poly(ethyleneglycol)(750)-block-poly(epsilon-caprolactone-co-trimethylenecarbonate)(4500) 50/50 copolymers (PEG-p(CL-co-TMC)) to solubilize amphotericin B in polymeric micelles and to disaggregate the drug to the less toxic monomeric form. Amphotericin B was encapsulated in the micelles upon dilution of a mixture of the liquid polymer and the drug in water. Its solubility was increased by two orders of magnitude depending on polymer concentration. The aggregation state of amphotericin B was decreased by PEG-p(CL-co-TMC). The preparation method and the loading of the polymeric micelles influenced it. The antifungal activity of the drug was reduced by encapsulation in the polymeric micelles whereas the onset of amphotericin B-induced hemolysis was delayed. PEG-p(CL-co-TMC) micelles could be an easy method for amphotericin B encapsulation.

Amphotericin B↗
PubMed2006

Encapsulation of hydrophobic drugs in polymeric micelles through co-solvent evaporation: the effect of solvent composition on micellar properties and drug loading.

This study was designed to develop an optimized co-solvent evaporation procedure for the efficient encapsulation of hydrophobic drugs in polymeric micelles of methoxy poly(ethylene oxide)-block-poly(epsilon-caprolactone) (MePEO-b-PCL). MePEO-b-PCL block copolymers having varied MePEO and PCL molecular weights were synthesized, assembled to polymeric micelles, and used for the encapsulation of cyclosporine A (CyA) by a co-solvent evaporation method. The co-solvent composition was varied by changing the type of organic co-solvent (using acetone, acetonitrile and tetrahydrofuran), the ratio of organic to aqueous phase, and their order of addition. Carrier size, morphology and encapsulated CyA levels were defined by dynamic light scattering (DLS), transmission electron microscopy (TEM) and HPLC, respectively, and the effect of co-solvent composition on micellar properties and loaded CyA levels was evaluated. Application of acetone and acetonitrile as the selective co-solvent for the core-forming block led to a decrease in the average diameter of self-assembled structures. When acetone was added to water, a decrease in the ratio of organic to aqueous phase led to an increase in the loading efficiency of CyA in MePEO-b-PCL micelles. A similar trend in CyA loading was observed for MePEO-b-PCL micelles of varied MePEO and PCL block lengths. The ratio of organic to aqueous phase did not affect CyA loading when water was added to acetone. Irrespective of the order of addition, the decrease in the organic to aqueous phase ratio caused a reduction in the average diameter of the empty and CyA loaded micelles. We conclude that the co-solvent evaporation method may be optimized to improve the efficiency of drug encapsulation in polymeric micelles. For CyA encapsulation in MePEO-b-PCL micelles, addition of acetone to water at lower organic to aqueous phase ratio is shown to be the optimum procedure leading to higher drug encapsulation and smaller average diameter for the self-assembled structures.

Cyclosporine↗
PubMed2006

Preparation and characterization of surfactant-free polystyrene/layered double hydroxide exfoliated nanocomposite via soap-free emulsion polymerization.

The soap-free emulsion polymerization has been applied for preparing the surfactant-free polystyrene/layered double hydroxide exfoliated nanocomposite. The XRD and TEM determinations have been used to monitor the changes of interlayer spacing and morphology during polymerization. The results show that the obtained nanocomposite has the homogeneous structure of polymeric and inorganic components. Due to the absence of organic surfactant, the PS/LDH nanocomposite shows a remarked improvement on the onset decomposition temperature compared with virgin PS.

Emulsions↗
PubMed2004

Comparative study of morphine diffusion from sustained release polymeric suspensions.

In recent years, great efforts have been devoted to the design of drug delivery systems. Many polymeric excipients have been studied in order to make drug release fit the desired profiles. The aim of this work was to design a morphine oral suspension, as sustained release pharmaceutical formulations. To this end, two different ethylcellulose suspensions were prepared: one with the drug incorporated during synthesis (suspension A) so that the drug was inside the polymeric microparticles. In the second group of suspensions the drug was incorporated after synthesis (suspension B), thus resulting in the drug being adsorbed on the surface. The analytical technique used, spectrophotometry, showed that suspensions A were able to spontaneously encapsulate approximately 92% of the drug, whereas suspensions B adsorbed only 15% dose on the particle surface. Moreover, the diffusion results obtained with Franz-cells showed that suspensions A offered the possibility of easy control of the release rate of the active substance. This system transfers morphine hydrocloride during 24 h in accordance with a Weibull kinetic model. This dosage form presents the clinical advantage of less frequent dosing, with increased quality of life for patients. This report documents the suitability of our ethylcellulose polymeric suspension for encapsulated morphine with a controlled release rate.

Analgesics, Opioid↗
PubMed2004

Hydrotropic agents for study of in vitro paclitaxel release from polymeric micelles.

A new experimental method for in vitro release studies of poorly soluble drugs from polymeric micelle systems was developed using a hydrotropic agent, sodium salicylate. It is difficult to maintain a good sink condition for poorly water-soluble drugs, such as paclitaxel (PTX), because of their low aqueous solubility. In this study, a good sink condition for PTX was achieved by using aqueous sodium salicylate solution which solubilized more than 10 times the total amount of PTX incorporated in polymeric micelles. Sodium salicylate at 1 M concentration increased the aqueous PTX solubility by 100 times without destroying the micellar structure of poly(ethylene glycol)-block-poly(phenylalanine) (PEG-b-PPhe) copolymer. PTX was continuously released from PEG-b-PPhe micelles in the hydrotropic release medium. The hydrotropic solution presents a simple method for studying in vitro release behavior of poorly soluble drugs from polymeric micelles in aqueous media.

Antineoplastic Agents, Phytogenic↗
PubMed2004

Polymeric micellar pH-sensitive drug delivery system for doxorubicin.

A novel polymeric micellar pH-sensitive system for delivery of doxorubicin (DOX) is described. Polymeric micelles were prepared by self-assembly of amphiphilic diblock copolymers in aqueous solutions. The copolymers consist of a biocompatible hydrophilic poly(ethylene oxide) (PEO) block and a hydrophobic block containing covalently bound anthracycline antibiotic DOX. The starting block copolymers poly(ethylene oxide)-block-poly(allyl glycidyl ether) (PEO-PAGE) with a very narrow molecular weight distribution (Mw/Mn ca. 1.05) were prepared by anionic ring opening polymerization using sodium salt of poly(ethylene oxide) monomethyl ether as macroinitiator and allyl glycidyl ether as functional monomer. The copolymers were covalently modified via reactive double bonds by the addition of methyl sulfanylacetate. The resulting ester subsequently reacted with hydrazine hydrate yielding polymer hydrazide. The hydrazide was coupled with DOX yielding pH-sensitive hydrazone bonds between the drug and carrier. The resulting conjugate containing ca. 3 wt.% DOX forms micelles with Rh(a)=104 nm in phosphate-buffered saline. After incubation in buffers at 37 degrees C DOX was released faster at pH 5.0 (close to pH in endosomes; 43% DOX released within 24 h) than at pH 7.4 (pH of blood plasma; 16% DOX released within 24 h). Cleavage of hydrazone bonds between DOX and carrier continues even after plateau in the DOX release from micelles incubated in aqueous solutions is reached.

Doxorubicin↗
PubMed2006

Intestinal uptake and biodistribution of novel polymeric micelles after oral administration.

To determine the fate of polymeric micelles after oral administration, we investigated the possible transport of polymeric micelles across Caco-2 monolayers and their biodistribution in rats after per os administration of [14C]-labelled mmePEG750P(CL-co-TMC) micelles containing risperidone (BCS Class II drug). mmePEG750P(CL-co-TMC) was able to cross Caco-2 monolayer via a saturable transport mechanism. The oral bioavailability of the polymer was 40%. Polymeric micelles based on mmePEG750P(CL-co-TMC) showed very low clearance by the reticuloendothelial system (RES) and a renal excretion. A sustained release of risperidone was observed.

Administration, Oral↗
PubMed2006

A polymeric micelle MRI contrast agent with changeable relaxivity.

Polymeric micelles were formed from cationic polymers (polyallylamine or protamine) and anionic block copolymers (poly(ethylene glycol)-b-poly(aspartic acid) derivative) that bound Gd ions providing high contrasts in Magnetic Resonance Imaging (MRI) by shortening the T(1) longitudinal relaxation time of protons of water. The Gd-binding block copolymer alone showed high relaxivity (T(1)-shortening ability) values from 10 to 11 mol(-1) s(-1), while the polymeric micelles exhibited low relaxivity values from 2.1 to 3.6 mol(-1) s(-1). These findings point to the feasibility of a novel MRI contrast agent that selectively provides high contrasts at solid tumor sites owing to a dissociation of the micelle structures, while selective delivery to the tumor sites is achieved in the polymeric micelle form.

Contrast Media↗
PubMed2006

Molecular design of biodegradable polymeric micelles for temperature-responsive drug release.

We designed thermo-responsive and biodegradable polymeric micelles for an ideal drug delivery system whose target sites are where external stimuli selectively release drugs from the polymeric micelles. The thermo-responsive micelles formed from block copolymers that were composed both of a hydrophobic block and a thermo-responsive block. Poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide) showing a lower critical solution temperature (LCST) around 40 degrees C was synthesized for the thermo-responsive block, while biodegradable poly(D,L-lactide), poly(epsilon-caprolactone), or poly(D,L-lactide-co-epsilon-caprolactone) was used for the hydrophobic block. By changing both the block lengths of the poly(D,L-lactide)-containing block copolymers, physical parameters such as micelle diameter and critical micelle concentration were varied. On the other hand, the choice of the hydrophobic block was revealed to be critical in relation to both on the thermo-responsive release of the incorporated anti-cancer drug, doxorubicin, and the temperature-dependent change of the hydrophobicity of the micelles' inner core. One polymeric micelle composition successfully exhibited rapid and thermo-responsive drug release while possessing a biodegradable character.

Antibiotics, Antineoplastic↗