Preparation and performance evaluation of castor oil-based polyurethane prepolymer/polylactide blends

Polylactide (PLA) is an important biodegradable polymer, used for numerous applications ranging from industrial packaging to tissue engineering. However, its inherent brittleness and limited thermal stability have restricted its penetration to niche markets. In this communication, the authors demonstrate that blending of PLA with castor oil-based polyurethane prepolymer (COPUP), with the addition of COPUP, dispersed in the PLA matrix can overcome the inherent brittleness of the matrix polymer. NCO-terminated COPUP was successfully synthesized and subsequently mixed with variable concentration of PLA matrix using melt blending technique. The interfacial compatibilization between COPUP and PLA phase happened by the reaction of ?NCO groups with terminal hydroxyl groups of PLA was confirmed by FT-IR peak deconvolution technique. As indicated by the results of DMA and DSC, the glass transition temperature (T _g) of both PLA and COPUP shifted closer together, indicating that the blend compatibility increased. The tensile properties and notched Izod impact strength of the PLA and toughened PLA are also investigated. With the addition of 30 % COPUP concentration, the elongation at break of the blend reached 377.46 %, and a notched Izod impact 269.62 J m^?1. With improved toughness, the PLA/COPUP blends could be used as replacements for some traditional petroleum-based polymers.     

The mechanical response of glutaraldehyde-fixed bovine pericardium to uniaxial load

Bovine pericardium, treated with glutaraldehyde, is used in the construction of heart valve substitutes. This study examines the mechanical properties of this tissue by using a continuum physics approximation of the material. A consideration of the relative magnitudes of the characteristic deformation time of a heart valve leaflet and the measured relaxation time of the tissue suggests that it can be effectively represented by a non-linear elastic solid. A compressible isotropic strain energy function is used to characterize the homogeneous deformation of the tissue when it is subjected to uniaxial load. The initial elastic material which is characterized by only two elastic constants, undergoes a transition to a second elastic material which is governed by a strain energy function of different magnitude by the same functional form as that associated with the initial elastic solid. This model is used to investigate the pericardial sac-to-sac and within-sac directional variation of the response to load in the unstrained state. Analysis of variance shows that glutaraldehyde treated pericardium possesses no preferred directional strength properties in the unstrained state. Any observed differences in the mechanical properties of different test specimens can be attributed to random biological variation alone.     

Synthesis of cross-linked polyurethane elastomers with fluorescein linkages

The reaction of polyurethane prepolymers with fluorescein (free acid) and glycerin or castor oil-cross-linker has been performed in N,N-dimethylformamide and in such way a cross-linked elastomer material has been obtained. Polyurethane prepolymers were prepared by reaction of the poly(1,4-butane)diols (Terathane 1400) with aliphatic 1,6 hexamethylene diisocyanate. The above polymerization reactions lead to the formation of novel aromatic–aliphatic polyurethane elastomers having fluorescein moieties in the main chain. The resulting novel cross-linked polyurethanes have excellent mechanical properties, especially in the castor oil polyurethane elastomer formulations and show light-violet fluorescence property and could be used as photoactive polymers for labeling materials. These polyurethanes were characterized through IR, TGA, DSC, and mechanical analyses. Some aspects of the fluorescent emission in fluorescein-polyurethane elastomers were studied.     

Employment of polyurethane foam for the adsorption of Methylene Blue in aqueous medium

This work presents a detailed study about the adsorption of Methylene Blue (MB) onto polyether type polyurethane foam (PUF). The adsorption process is based on the formation of a hydrophobic ionic-pair between cationic dye MB and dodecylsulfate anion (SDS), which present high affinity by PUF. Set-up employed in the study was built up by adjusting a 200mg cylinder of PUF to the arm of an overhead stirrer. The system was characterized in relation to equilibrium, kinetic and thermodynamic aspects and it was modeled by employing Langmuir and Freundlich isotherms. Obtained results showed that the ratio between SDS and MB concentrations plays an important role on the adsorption process. According to Langmuir isotherm, a maximum adsorption capacity of 7.20 x 10(-5) mol MBg(-1) was achieved when optimized operational conditions were employed. The adsorption rate seems to be regulated by an intraparticle diffusion mechanism. Adsorption process was spontaneous (negative DeltaG) at ambient temperature and presented an endothermic characteristic (positive DeltaH). Sequential extraction experiments were carried out by changing PUF plugs in 30 min time intervals and around 96% of the MB present in solution could be removed through consecutive extractions with six 200 mg PUF cylinders.     

微量水对聚氨酯预聚体结构与溶解性的影响研究

为了更充分地了解微量水对聚氨酯预聚反应及预聚体结构的影响,用一步法首先合成了聚氨酯初始预聚体,然后研究了微量水及扩链温度等对聚氨酯预聚体的特性粘数、在THF中的溶解性和分子量分布等的影响,采用FT-IR对水扩链预聚体的聚集态结构进行了初步的研究。研究结果表明,水和—NCO基团反应生成聚脲结构,表现在聚氨酯硬段间的氢键作用增强,而硬段与软段间的氢键作用力减弱。微量水的存在可以对聚氨酯预聚体起到扩链作用,但随水量的增加预聚体会生成交联结构,产生不溶性凝胶。聚合温度的提高会加速聚脲结构的形成。     

Interpenetrating polymer networks of polyurethane cross-linked epoxy and polyurethanes

Interpenetrating polymer networks (IPNs) were prepared from polyurethane (PU) cross-linked epoxy and polyurethanes based on the mixture of polydiol and polytriol by the one-shot method. The mechanical properties, dynamic mechanical properties, morphology and damping behaviour were investigated. The results show that the damping ability and mechanical strength are enhanced through the introduction of PU cross-linked epoxy into the PU matrix to form the IPN structure. As the epoxy content increases, the tensile strength of the two types of the IPNs decrease in low composition, then increase in high composition. The damping properties of the PU (polyether type) cross-linked epoxy/PU IPNs are much better than those of the PU (polyester type) cross-linked epoxy/PU IPNs, but the mechanical properties reveal an opposite tendency. The sample with 20 wt% epoxy content in the PU cross-linked epoxy/PU IPNs shows particle–matrix morphology and exhibits good damping properties.     

Calcification and identification of metalloproteinases in bovine pericardium after subcutaneous implantation in rats

The purpose of this study was to evaluate the influence of two anticalcification pre-treatments (chloroform/methanol and ethanol) and serum conditioning of glutaraldehyde-crosslinked bovine pericardium on the calcification degree and the presence of gelatinase activities in a subcutaneous implantation model in rats. Regarding calcification of the implants, glutaraldehyde control treatments showed a significatively higher calcification degree than pericardium treated with anticalcification reagents. Serum conditioning of glutaraldehyde treated tissues did not influence the calcification degree; moreover, no differences were found in these samples with the time of implantation (30 and 90 days). On the other hand, anticalcification treatments resulted in a very significant decrease in the calcium content in the implanted membranes. Gelatinase activities were detected by gelatin zymography in almost all the implanted samples. However, control tissues with and without serum conditioning showed less gelatinase activities than those samples pre-treated with anticalcification treatments. Metalloproteinase (MMP-2) activity was detected in all the samples analyzed but a higher expression of MMP-9 was detected in those implants treated with chloroform/methanol and ethanol. Additional gelatinase activities showing lower molecular weight than MMP-2 were also detected in both anticalcification treated samples. The presence of these gelatinase activities is probably due to host cellular infiltrates and could contribute to the biomaterial degradation.     

Inhibition of calcification of bovine pericardium after treatment with biopolymers,E-beam irradiation and in vitro endothelization

This work has investigated the in vitro calcification of bovine pericardium (BP) treated with chitosan (C), silk fibroin (SF) and electron beam irradiation after its endothelization in vitro. For this purpose, freeze-dried BP membranes treated with mixtures of C and SF (1:3, 1:1 and 3:1) and then irradiated by electron beam irradiation were seeded with human umbilical vein endothelial cells (HUVEC) in vitro. After 3 weeks of cultivation these membranes were submitted to in vitro calcification tests using simulated body fluid as the calcifying agent. Control membranes were also studied (without endothelial cells exposure). The results have shown that the membrane compatibility with HUVECs in vitro prevent such biomaterial from calcifying, showing a potential application in biomaterial area, such as cardiac valves and repair patches.     

Influence of aqueous medium on mechanical properties of conventional and new environmentally friendly regenerated cellulose fibers

Comparative investigations between the new lyocell fibers and the regular viscose and modal types were made in order to explain the reasons for the differences in the mechanical properties of the fibers. The purpose was a systematic analysis of structure characteristics and of influence of aqueous medium on the mechanical properties. The properties determined in the wet state reflect the effect of the aqueous medium on the changes in the supermolecular structure during wet treatments [1, 2]. The new lyocell fibers consist of longer molecules and have a higher degree of crystallinity. Smaller but longer crystallites are oriented in the fiber axis direction and the voids structure is similar to that of viscose fibers [3]. Good mechanical properties are conditioned by the structure of the lyocell fibers, above all by high values of the orientation factor and crystallinity index. Sorption properties place lyocell fibers between the viscose and modal fibers. The water influence on the mechanical properties of lyocell fibers is considerably smaller compared to the viscose and modal fibers. Received: 18 September 2000 / Reviewed and accepted: 20 September 2000     

Processing and mechanical characterization of lightweight polyurethane composites

A simple procedure was established to fabricate polyurethane-cenosphere particulate composite materials. Composites having four different volume fractions of cenospheres (hollow ceramic microspheres) ranging from 10 to 40% in increments of 10% were prepared and their mechanical properties were evaluated. A predictive model to estimate the fracture toughness of the composite was developed. The dynamic constitutive behavior of the composite in compression was investigated using the split Hopkinson pressure bar (SHPB) technique in conjunction with high-speed photography. The results of the material characterization indicated that addition of cenospheres decreased the density of the composite. The quasi-static stiffness, both in tension and compression, and the quasi-static fracture toughness of the composite increased with addition of cenospheres. The high strain rate constitutive behavior of 100% polyurethane showed monotonic stiffening whereas the composite at higher cenosphere volume fractions (40%) exhibited a stiffening-softening-stiffening behavior. Scanning Electron Microscopy (SEM) studies were also carried out to determine the failure mechanisms of the composite.     

Ultrasound evaluation of mechanical injury of bovine knee articular cartilage under arthroscopic control

A local cartilage injury can trigger development of posttraumatic osteoarthritis (OA). Surgical methods have been developed for repairing cartilage injuries. Objective and sensitive methods are needed for planning an optimal surgery as well as for monitoring the surgical outcome. In this laboratory study, the feasibility of an arthroscopic ultrasound technique for diagnosing cartilage injuries was investigated. In bovine knees (n = 7) articular cartilage in the central patella and femoral sulcus was mechanically degraded with a steel brush modified for use under arthroscopic control. Subsequently, mechanically degraded and intact adjacent tissue was imaged with a high frequency (40 MHz) intravascular ultrasound device operated under arthroscopic guidance. After opening the knee joint, mechanical indentation measurements were also conducted with an arthroscopic device at each predefined anatomical site. Finally, cylindrical osteochondral samples were extracted from the measurement sites and prepared for histological analysis. Quantitative parameters, i.e., reflection coefficient (R), integrated reflection coefficient (IRC), apparent integrated backscattering (AIB), and ultrasound roughness index (URI) were calculated from the ultrasound signals. The reproducibilities (sCV %) of the measurements of ultrasound parameters were variable (3.7% to 26.1%). Reflection and roughness parameters were significantly different between mechanically degraded and adjacent intact tissue (p < 0.05). Surface fibrillation of mechanically degraded tissue could be visualized in ultrasound images. Furthermore, R and IRC correlated significantly with the indentation stiffness. The present results are encouraging; however, further technical development of the arthroscopic ultrasound technique is needed for evaluation of the integrity of human articular cartilage in vivo.     

Screening biomaterials with a new in vitro method for potential calcification: Porcine aortic valves and bovine pericardium

Calcification is still a major cause of failure of implantable biomaterials. A fast and reliable in vitro model could contribute to the study of its mechanisms and to testing different anticalcification techniques. In this work, we attempted to investigate the potential calcification of biomaterials using an in vitro model. We purposed to test the ability of this model to screening possible anticalcification efficacy of different biomaterials. Porcine heart valve (PAV) and bovine pericardial (BP) tissues, fixed with glutaraldehyde were immersed into biological mimicking solution, where the pH and the initial concentrations of calcium and phosphoric ions were kept stable by the addition of precipitated ions during calcification. Kinetics of calcification was continuously monitored. The evaluation of biomaterials was carried out by comparing the kinetic rates of formation of calcific deposits. After 24 h, the calcific deposits on PAVs were found to be developed at significant higher rates (ranged from 0.81 x 10(-4)-2.18 x 10(-4)mol/min m2) than on BP (0.19 x 10(-4)-0.52 x 10(-4)mol/min m2) (one-way ANOVA, p < 0.05) depending on the experimental conditions (supersaturation of the solution). Parallel tests for similar biomaterials implanted subcutaneously in animal (rat) model showed after 49 days that significant higher amounts of total minerals deposited on PAV (236.73+/-139.12, 9 animals mg minerals/g dry net tissue) (mean+/-standard deviation) compared with that formed on BP (104.36+/-79.21, #9 mg minerals/g dry net tissue) (ANOVA, p < 0.05). There is evidence that in vitro calcification was correlated well with that of animal model and clinical data.     

阴离子型水性聚氨酯的制备及进展

综述了阴离子型水性聚氨酯亲水基团的研究状况。亲水基团的引入方式有多种,可以通过小分子扩链剂引入,也可通过含有离子基团的聚酯或聚醚引入,或在水性聚氨酯的分子结构中同时引入磺酸盐和羧酸亲水基团,形成混合内乳化剂,可提高水性聚氨酯的耐水性和其他物理性能,降低溶剂的使用量。     

Apatite mineralization abilities and mechanical properties of covalently cross-linked pectin hydrogels

Natural bone has features such as high fracture toughness and bone-bonding bioactivity, and is organic–inorganic hybrid composed of collagen and apatite crystals. Therefore, apatite-polymer hybrids designed to mimic the structure of bone represent candidates for high-performance bone substitutes. In this study, we prepared pectin hydrogels through covalent cross-linking using divinylsulfone (DVS) and investigated their apatite-forming abilities of the gels in simulated body fluid (SBF) and mechanical properties by tensile test. The obtained results were interpreted in terms of surface charge of the gels and chemical reaction with SBF. The apple- and citrus-derived gels formed the apatite on their surfaces in SBF within 3 days. These gels showed tensile strength around 30 MPa.     

Thermal and mechanical behaviour of flexible polyurethane foams modified with graphite and phosphorous fillers

Flexible polyurethane foams (FPF) are polymer materials that have high flammability. Fyrol PNX (FPNX) and expandable graphite (EG), have been used to modify the properties of these materials. The aim of this study was to assess the possibility of improving the thermal stability and flame retardancy of FPF by the addition of FPNX and EG fillers. The prepared foams were characterised by their apparent density, hardness, flexibility, irreversible strain and linear flammability, as well as thermogravimetric analysis (TGA), dynamic mechanical analysis, Fourier transform infrared spectroscopy (FT-IR) and pyrolysis combustion flow calorimetry (PCFC) measurements. The apparent density, hardness, flexibility and irreversible strain results showed that the addition of graphite and phosphorous fillers to the FPF makes slight changes to the mechanical properties, which remain within the acceptable norms. It was also observed that reducing the amount of Fyrol PNX and replacing it with the same amount of EG allowed similar values of linear flammability to be obtained with a simultaneous increase in thermal stability, as shown in the TGA study and the PCFC test. Moreover, it was found that the modification of flexible polyurethane foam by the addition of a mixture of FPNX and EG fillers allows the best properties of this type of materials to be obtained. This result indicates that this type of modification could be an effective way to improve the thermal stability of FPF.