Self-reinforced composites of bioabsorbable polymer and bioactive glass with different bioactive glass contents. Part II: In vitro degradation

The in vitro degradation behavior of self-reinforced bioactive glass-containing composites was investigated comparatively with plain self-reinforced matrix polymer. The materials used were spherical bioactive glass 13-93 particles, with a particle size distribution of 50-125 microm, as a filler material and bioabsorbable poly-L,DL-lactide 70/30 as a matrix material. The composites containing 0, 20, 30, 40 and 50 wt.% of bioactive glass were manufactured using twin-screw extruder followed by self-reinforcing. The samples studied were characterized determining the changes in mechanical properties, thermal properties, molecular weight, mass loss and water absorption in phosphate-buffered saline at 37 degrees C for up to 104 weeks. The results showed that the bioactive glass addition modified the degradation kinetics and material morphology of the matrix material. It was concluded that the optimal bioactive glass content depends on the applications of the composites. The results of this study could be used as a guideline when estimating the best filler content of other self-reinforced osteoconductive filler containing composites which are manufactured in a similar way.     

The effect of nanofiller on the opacity of experimental composites

The purpose of this study was to evaluate the effect of the nanofiller in experimental composites on opacity (contrast ratio). Thirteen experimental composites were prepared with three different sizes of fillers: barium glass minifiller (1 microm; 69-76 wt %), silica microfiller (0.04 microm; 0-6 wt %), and silica nanofiller (7 nm; 0-7 wt %). After disk-type specimens were irradiated with a halogen light curing unit at 500 mW/cm(2) for 30 s, the specimens were aged for 6 h at room conditions and were stored in deionized water for 1, 7, 14, 21, 28, 56, and 84 days. The contrast ratios of the specimens were measured as a function of aging period using a spectrophotometer. The distribution morphology of the filler particles in the resin matrix was also examined using energy-filtering transmission electron microscopy. The experimental composites that contained more than 3% nanofiller had significantly lower contrast ratios (p < 0.05). The composites that contained 6 wt % nanofiller had contrast ratios 34-65% lower than the composite that did not contain nanofiller. The values of the contrast ratio from the composites that excluded microfiller were lower than the values from the composites that included microfiller. From the comparison with the 3 different sizes of filler, the contrast ratio of the composite that contained 70 wt % minifiller and 6 wt % microfiller was the highest, the contrast ratio of the composite that contained only 76 wt % minifiller was the median value, and the contrast ratio of the composite that contained 70 wt % minifiller and 6 wt % nanofiller was the lowest. When the microfiller content was decreased from 6 wt % to 0 wt %, the contrast ratio decreased 6-9%. Energy-filtering transmission electron microscopy images indicated that the contrast ratio of experimental composites is related to the distribution morphology of the filler particles in the resin matrix.     

Organic overlayer model of a dental composite analyzed by laser desorption postionization mass spectrometry and photoemission

Some dental composites consist of a polymerizable resin matrix bound to glass filler particles by silane coupling agents. The resin in these composites includes bisphenol A diglycidyl methacrylate (Bis-GMA) as well as other organic components. Silane coupling agents such as 3-(trimethoxysilyl) propyl methacrylate (MPS) have been used to improve the mechanical properties of the dental composites by forming a covalent bond between the glass filler particles and the resin. These resin-glass composites undergo material property changes during exposure to the oral environment, but degradation studies of the commercial composites are severely limited by their chemical complexity. A simplified model of the dental composite has been developed, which captures the essential chemical characteristics of the filler particle-silane-resin interface. This model system consists of the resin matrix compound Bis-GMA covalently bound via a methacryloyl overlayer to amorphous silicon oxide (SiO2) surface via a siloxane bond. Scanning electron microscopy shows the porous characteristic and elemental composition of the SiO2 film, which approximately mimics that of the glass filler particles used in dental composites. LDPI MS and XPS verify the chemistry and morphology of the Bis-GMA-methacryloyl overlayer. Preliminary results demonstrate that LDPI MS will be able to follow the chemical processes resulting from aging Bis-GMA-methacryloyl overlayers aged in water, artificial saliva, or other aging solutions.     

Initiators based on poly(ethylene glycol) for starting heterophase polymerizations: generation of block copolymers and new particle morphologies

Radical heterophase polymerizations with poly(ethylene glycol) radicals lead to the formation of block copolymer particles where the block copolymer architecture and the particle morphology depend on the number of radical per poly(ethylene glycol) chain, the radical termination mode, and the polarity of the monomer, respectively. The thermal decomposition of symmetrical poly(ethylene glycol) azo-initiators following the classical recipes of Heitz results in one radical per poly(ethylene glycol) chain whereas the number of radicals can be adjusted between one or two in the redox system poly(ethylene glycol)/cerium ions. The polymerization of styrene results in latex particles with an almost spherical morphology, consisting of block copolymers, only. In case of a methyl methacrylate polymerization the latex morphology depends on the architecture of the block copolymers formed. Heterophase polymerization with poly(ethylene glycol) azoinitiators in oligo(ethylene glycol) (average molecular weight 200 g · mol⁻¹) instead of in water results in particles with random shape and a strongly indented interface which is explained by the surface tension between polymers and solvent being close to zero.     

Copolymerization of ethylene and 1-hexene with in-situ supported Et[Ind]2ZrCl2

Copolymerization of ethylene and 1-hexene was carried out with different catalysts (homogeneous Et[Ind]₂ZrCl₂, supported Et[Ind]₂ZrCl₂ and in-situ supported Et[Ind]₂ZrCl₂). The novel in-situ supported metallocene catalyst showed higher activity than the corresponding supported metallocene catalyst. ¹³C NMR, gel permeation chromatography and crystallization analysis fractionation studies showed that the microstructure of ethylene/1-hexene copolymers depends upon catalyst type. At the same polymerization conditions, the relative reactivity of 1-hexene increases in the following order: supported metallocene ≈ in-situ supported metallocene < homogeneous metallocene. The molecular weights of the produced copolymers with the three different catalysts are similar, but the molecular weight distribution of the copolymer made with the in-situ supported metallocene is broader than that of those made with the other catalysts. The short chain branching distribution (SCBD) of the copolymer produced with the in-situ supported metallocene catalyst is the broadest with a shoulder in the high crystallinity range, while the copolymers produced with the homogeneous and supported metallocene catalysts show unimodal SCBD. This may indicate that there are at least two different active species with the in-situ supported metallocene catalyst in the copolymerization of ethylene and 1-hexene.     

Nondestructive quantification of leakage at the tooth–composite interface and its correlation with material performance parameters

Current methods to determine debonding/leakage at the tooth–composite interface are qualitative or semi-quantitative. Our previous work introduced a 3D imaging technique to determine and visualize leakage and its distribution at the interface of cavity wall and composite restoration in model cavities. In this study, an automated program was developed to quantify leakage in terms of area and volume. 3D leakage distribution obtained via the image analysis program was shown to have excellent agreement with leakage visualized by dye penetration. The relationship between leakage and various material performance parameters including processability, shrinkage, stress, and shrinkage strain-rate was determined using a series of experimental composites containing different filler contents. Results indicate that the magnitude of leakage correlated well with polymerization stress, confirming the validity of the common approach utilizing polymerization stress to predict bonding durability. 3D imaging and image analysis provide insight to help understand the relations between leakage and material properties.     

Controlled Synthesis and Characterization of Poly[ethylene‐block‐(L,L‐lactide)]s by Combining Catalytic Ethylene Oligomerization with “Coordination‐Insertion” Ring‐Opening Polymerization

Model poly[ethylene‐block‐(L ,L ‐lactide)] (PE‐block‐PLA) block copolymers were successfully synthesized by combining metallocene catalyzed ethylene oligomerization with ring‐opening polymerization (ROP) of L ,L ‐lactide (LA). Hydroxy‐terminated polyethylene (PE‐OH) macroinitiator was prepared by means of ethylene oligomerization on rac‐dimethyl‐silylen‐bis(2‐methyl‐benz[e]indenyl)‐zirconium(IV)‐dichloride/methylaluminoxane (rac‐MBI/MAO) in presence of diethyl zinc as a chain transfer agent, and subsequent in situ oxidation with synthetic air. Poly[ethylene‐block‐(L ,L ‐lactide)] block copolymers were obtained via ring‐opening polymerization of LA initiated by PE‐OH in toluene at 100 °C mediated by tin octoate. The formation of block copolymers was confirmed by ¹H NMR spectroscopy, fractionation experiments, thermal behavior, and morphological characterization using AFM and light microscopy techniques.

     

Synthesis of hairy acrylic core‐shell particles as toughening agents for epoxy networks

Hairy poly(butyl acrylate) (PBuA) core‐shell particles with a crosslinked shell were prepared by soap‐free emulsion polymerization. The incorporation of the hairy structure was obtained either by surface polymerization of a methacrylate (MMA) terminated poly(ethylene oxide) (PEO), or by physical adsorption of poly(ethylene oxide)‐poly(propylene oxide) (PPO)‐poly(ethylene oxide) triblock copolymers. The particle shell was crosslinked during the synthesis so as to keep the integrity and morphology of the particle upon curing the epoxy network. Particle sizes and size distributions were determined both by quasi‐elastic light scattering and transmission electron microscopy. Particle morphology was investigated by electron and atomic force microscopies. The presence of the poly(ethylene oxide) layer was evidenced by direct analysis of the latexes by means of ¹H NMR spectroscopy. Mixing of the core shell rubber particles with the reactive epoxy and processing of the toughened‐epoxy networks are described. The influence of the hairy layer (with regards to density and PEO chain length) on particle dispersion within the epoxy network, the resulting rheological behaviour of the core‐shell (CS)/prepolymer epoxy blends, and mechanical properties of the modified epoxy were examined and discussed.     

Polyethylene/phospholipid polymer alloy as an alternative to poly(vinylchloride)-based materials

To develop new biomaterials for making medical devices, polymer alloys composed of a phospholipid polymer, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), and polyethylene (PE) were prepared. The PE/PMPC alloy membrane could be obtained by a combination of solution mixing and solvent evaporation methods using xylene and n-butanol mixture as a solvent. Moreover, thermal treatment was applied to improve the mechanical properties of the PE/PMPC alloy membrane. In the PE/PMPC alloy membrane, the PMPC domains were located not only inside the membrane but also at the surface. Surface analysis of the PE/PMPC alloy membrane with X-ray photoelectron spectroscopy, wettability evaluation, and dynamic contact angle measurements revealed that the phospholipid polar groups in the PMPC covered the surface even after thermal treatment. Blood compatibility tests with attention to platelet adhesion and change in morphology of adhered platelets showed that the PE/PMPC alloy membrane had excellent platelet adhesion resistance. We finally concluded that the PE/PMPC alloy could be used as biomaterials instead of poly(vinyl chloride)-based materials.     

Bioactive composites consisting of PEEK and calcium silicate powders

Bioactive bone-repairing materials with mechanical properties analogous to those of natural bone can be obtained through the combination of bioactive ceramic fillers with organic polymers. Previously, we developed novel bioactive microspheres in a binary CaO-SiO2 system produced through a sol-gel process as filler for the fabrication of composites. In this study, we fabricate bioactive composites in which polyetheretherketone is reinforced with 0-50 vol% 30CaO x 70SiO2 (CS) microspheres. The prepared composites reinforced with CS particles form hydroxyapatite on their surfaces in simulated body fluid. The induction periods of hydroxyapatite formation on the composites decrease with increasing amount of CS particles. The mechanical properties of the composites are evaluated by three-point bending test. The composites reinforced with 20 vol% CS particles show 123.5 MPa and 6.43 GPa in bending strength and Young's modulus, respectively.     

Simultaneous Deconvolution of the Bivariate Molecular Weight and Chemical Composition Distribution of Ethylene/1‐Hexene Copolymers

The bivariate molecular weight and chemical composition distribution (MWD×CCD) of ethylene/1‐hexene copolymers can be measured using TREF×GPC cross fractionation characterization (CFC). In this work, the experimental MWD×CCD of ethylene/1‐hexene copolymers made with a Ziegler–Natta catalyst under different polymerization conditions are measured by CFC and deconvoluted to identify the minimum number of site types present in the catalyst. Blends of ethylene/1‐hexene copolymers produced with a metallocene catalyst with known MWD×CCD are used to validate the proposed technique. This is a powerful methodology to better understand the nature of active sites on multisite catalysts, and can be beneficial for the development of copolymers with precisely controlled microstructures.     

Comparison of nanoscale and microscale bioactive glass on the properties of P(3HB)/Bioglass composites

This study compares the effects of introducing micro (m-BG) and nanoscale (n-BG) bioactive glass particles on the various properties (thermal, mechanical and microstructural) of poly(3hydroxybutyrate) (P(3HB))/bioactive glass composite systems. P(3HB)/bioactive glass composite films with three different concentrations of m-BG and n-BG (10, 20 and 30 wt%, respectively) were prepared by a solvent casting technique. The addition of n-BG particles had a significant stiffening effect on the composites, modulus when compared with m-BG. However, there were no significant differences in the thermal properties of the composites due to the addition of n-BG and m-BG particles. The systematic addition of n-BG particles induced a nanostructured topography on the surface of the composites, which was not visible by SEM in m-BG composites. This surface effect induced by n-BG particles considerably improved the total protein adsorption on the n-BG composites compared to the unfilled polymer and the m-BG composites. A short term in vitro degradation (30 days) study in simulated body fluid (SBF) showed a high level of bioactivity as well as higher water absorption for the P(3HB)/n-BG composites. Furthermore, a cell proliferation study using MG-63 cells demonstrated the good biocompatibility of both types of P(3HB)/bioactive glass composite systems. The results of this investigation confirm that the addition of nanosized bioactive glass particles had a more significant effect on the mechanical and structural properties of a composite system in comparison with microparticles, as well as enhancing protein adsorption, two desirable effects for the application of the composites in tissue engineering.     

Effect of hydrogen on ethylene polymerization using in‐situ supported metallocene catalysts

The effect of hydrogen on ethylene polymerization using two different in‐situ supported metallocene catalysts (Et[Ind]₂ZrCl₂ and Cp₂ZrCl₂) was investigated. In the presence of hydrogen, both catalysts showed enhancement of the polymerization rate. Surprisingly, the molecular weight distribution (MWD) changed from unimodal to bimodal with addition of hydrogen.     

Poly(epsilon-caprolactone-co-D,L-lactide) /silk fibroin composite materials: preparation and characterization

Poly(epsilon-caprolactone-co-D,L-lactide) copolymers with 10, 30, and 50% by weight of silk particles (size range: 5-250 microm) derived from Bombyx mori were blended in acetone solution. After evaporation of the solvent, the morphology, thermal behavior, and mechanical properties of the composites were examined. The composites were transparent and the silk fibroin particles were homogeneously distributed within the composite structure. The particles appeared as bright reflected images under the optical microscope, suggesting that they were in a crystalline state. DSC thermograms of the composites revealed that the glass transition of the matrix was at ca. -18 degrees C. Degradation of the silk fibroin occurred beyond 270 degrees C. The decomposition temperatures and degradation rate decreased with increasing silk fibroin content as revealed by TGA analysis. FTIR spectra of the composites showed absorption bands at 1730 and 1088 cm(-1) for the copolymer and at 3273 and 1617 cm(-1) for the silk fibroin. Although the characteristic lines of poly(epsilon-caprolactone-co-D,L-lactide) were independent of filler concentration. the absorption bands of the beta-sheet form of the silk fibroin increased slightly due to the interaction of silk fibroin with the copolymer.     

Use of coupling agents to enhance the interfacial interactions in starch-EVOH/hydroxylapatite composites.

Different zirconate, titanate and silane coupling agents were selected in an effort to improve the mechanical properties of starch and ethylene-vinyl alcohol copolymer (EVOH) hydroxylapatite (HA) composites, through the enhancement of the filler particles-polymer matrix interactions and the promotion of the interfacial adhesion between these two phases. The mechanical performance was assessed by tensile tests and discussed on the basis of the respective interfacial morphology (evaluated by scanning electron microscopy). The main relevant parameters were found to be the surface properties and reactivity of the filler (non-sintered HA) and the chemical nature (pH and type of metallic centre) of the added coupling agent. Significant improvements in the stiffness were achieved (about 30% increase in the modulus) when using the acidic zirconate coupling agents. The acidic zirconate combined the capability of crosslinking the polymer matrix with the establishment of donor-acceptor interactions and hydrogen bonding between it and the ceramic particles, leading to very good interfacial adhesion. The optimization of these coupling processes associated with the introduction of higher amounts of filler, may be an effective way to produce composites with mechanical properties analogous to those of the human cortical bone.     

Polymerization of alkylene oxide by alumina

Amorphous γ-alumina of small particles size has been prepared by the hydrolysis of aluminum isopropoxide (AIP) powder with a gas mixture of steam and nitrogen and subsequent calcination of aluminum hydroxide at 600°C.; it has proved to be of quite high catalyst activity for the polymerization of ethylene oxide and propylene oxide. Several other alumina species have also been prepared by different procedures and examined for catalyst activity. The properties of these alumina species, such as surface area, acid strength, acidity, and crystal form were surveyed in relation to the catalyst activity. It has been assumed that the high catalytic activity of amorphous γ-alumina prepared by the above specific procedure is due to the amorphous nature and to high acidity.     

Poly(ethylene oxide) surfactant polymers

We report on a series of structurally well-defined surfactant polymers that undergo surface-induced self-assembly on hydrophobic biomaterial surfaces. The surfactant polymers consist of a poly(vinyl amine) backbone with poly(ethylene oxide) and hexanal pendant groups. The poly(vinyl amine) (PVAm) was synthesized by hydrolysis of poly(N-vinyl formamide) following free radical polymerization of N-vinyl formamide. Hexanal and aldehyde-terminated poly(ethylene oxide) (PEO) were simultaneously attached to PVAm via reductive amination. Surfactant polymers with different PEO:hexanal ratios and hydrophilic/hydrophobic balances were prepared, and characterized by FT-IR, 1H-NMR and XPS spectroscopies. Surface active properties at the air/water interface were determined by surface tension measurements. Surface activity at a solid surface/water interface was demonstrated by atomic force microscopy, showing epitaxially molecular alignment for surfactant polymers adsorbed on highly oriented pyrolytic graphite. The surfactant polymers described in this report can be adapted for simple non-covalent surface modification of biomaterials and hydrophobic surfaces to provide highly hydrated interfaces.     

Composites of poly(lactide-co-glycolide) and the surface modified carbonated hydroxyapatite nanoparticles

To improve the mechanical properties of the composites of poly(lactide-co-glycolide) (PLGA, LA/GA = 80/20) and the carbonate hydroxyapatite (CHAP) particles, the rice-form or claviform CHAP particles with 30-40 nm in diameter and 100-200 nm in length were prepared by precipitation method. The uncalcined CHAP particles have a coarse surface with a lot of global protuberances, which could be in favor of the interaction of the matrix polymer to the CHAP particles. The nanocomposites of PLGA and surface grafted CHAP particles (g-CHAP) were prepared by solution mixing method. The structure and properties of the composites were subsequently investigated by the emission scanning electron microscopy, the tensile strength testing, and the cell culture. When the contents of g-CHAP were in the range of 2-15 wt %, the PLGA/g-CHAP nanocomposites exhibited an improved elongation at break and tensile strength. At the 2 wt % content of g-CHAP, the fracture strain was increased to 20% from 4-5% for neat PLGA samples. Especially at g-CHAP content of 15 wt %, the tensile strength of PLGA/g-CHAP composite was about 20% higher than that of neat PLGA materials. The tensile moduli of composites were increased with the increasing of filler contents, so that the g-CHAP particles had both reinforcing and toughening effects on the PLGA composites. The results of biocompatibility test showed that the higher g-CHAP contents in PLGA composite facilitated the adhesion and proliferation properties of osteoblasts on the PLGA/g-CHAP composite film.     

High density polyethylene/graphite nano-composites for total hip joint replacements: processing and in vitro characterization

The main objective of the present study is to investigate how the thermal, rheological, mechanical and cytotoxicity behavior of High Density Polyethylene (HDPE) can be changed by the addition of graphite nano particles (GNPs) at different contents. The HDPE/GNPs composites were prepared using melt blending in a co-rotating intermeshing twin screw extruder. The in vitro tests results showed that the original material (HDPE) and all HDPE/GNPs composites do not exhibit any cytotoxicity to the WISH cell line. The microscopic examination of the nano-composite tensile-fractured surface found a good distribution of GNPs in the HDPE matrix. The Differential Scanning Calorimetry (DSC) results indicated that the crystallization percentage increased by adding GNPs to HDPE up to 4%. The XRD patterns of the HDPE/GNPs composites showed an increase in peak intensity compared to neat HDPE. This increase echoed the crystallinity results obtained from DSC. The rheological tests showed that the complex viscosity of the HDPE increased as the percentage of GNPs increased due to the restriction of the molecular mobility. The tensile test results showed that with increasing the GNPs content, Young’s modulus and the yield strength of the HDPE/GNPs composite increased while the strain at fracture decreased. Finally, the preliminary results of the abrasion test indicated that the abrasion rate decreased by increasing the GNPs ratio up to 4% content. The prepared HDPE/GNPs composites appear to have fairly good comprehensive properties that make them a good candidate as a bearing material for the total joint replacement.     

Hydroxyapatite-polyethylene composites for bone substitution: effects of ceramic particle size and morphology

Synthetic hydroxyapatite particles of two median sizes and different morphologies have been used to manufacture hydroxyapatite reinforced high density polyethylene composites (HAPEX^TM) for medical applications. The effects of hydroxyapatite particle size on properties of the resultant composites were investigated using various techniques. It was found that composites with smaller hydroxyapatite particles had higher torsional modulus, tensile modulus and tensile strength, but lower strain to failure. Examination of fracture surfaces revealed that only a mechanical bond existed between the filler and the matrix. It was shown that dynamic mechanical analysis is useful in studying the viscoelastic behaviour of the composite.