Kinetics of Oxidative Degradation: Applicability of Time and Temperature Superposition

New methodology is suggested to quantify the effect of temperature on oxidative degradation and compare test results obtained at different temperatures or evaluate changes in chemical mechanisms. Assumption of the pseudo-first-order kinetics to calculate activation energies in oxidative polymerization of uninhibited 100 percent oleic triglyceride under thin film oxidation provided only semiquantitative results from available oxypolymerization data at six temperatures from 100° to 175°C. As an alternative approach, Time and Temperature Superposition (TTS) was applied to analyze the data. Several TTS forms consistent with Arrhenius equation showed much greater accuracy in determining activation energies and induction periods. Different stages of oxidative degradation were identified after the data from different temperatures were combined into one kinetic map. Kinetic maps of alkali-refined high oleic sunflower oil, fully formulated synthetic oleate-based hydraulic fluid and additive-free super refined mineral oil showed their increasing resistance to oxypolymerization. Limitations of TTS were discussed.     

Influence of microstructure on the in-vitro degradation behaviour of magnesium alloys

The study shows that the microstructural difference between the fine-grained die-cast and coarse-grained sand-cast magnesium-based alloys has no significant effect on the in-vitro degradation behaviour. However, the post-degradation analysis of the alloys suggest that the high volume fraction of secondary phase particles in the die-cast alloy may not be suitable for biodegradable implant applications, primarily due to the high stability of the secondary phase particles in physiological conditions.     

Gate type influence on thermal characteristics of injection molded biodegradable interference screws for ACL reconstruction

In biomedical applications high precision manufacturing is an essential requirement. With precision injection molding it is possible to manufacture implants from thermoplastic materials for both short and long term use. This paper focuses on the injection molding analysis of a biodegradable implant for tendon fixation. Finite element simulation was carried out to compare thermal characteristics of the process, with two different types of gating. All analyses, including the thermal simulations and dimensional stability analysis show, that different gatings do not affect part deformation or shrinkage beyond the accepted limit, although they show some differences.     

Evaluation of Analytical Endpoints to Predict Carcinogenic Potency of Mineral Oils

Abstract

We have compared three sample preparation and analysis methods currently used for estimating the biological activity of mineral oils. The Institute of Petroleum Method 346/80, the Mobil Oil PAC Method and the Multiple PAC Marker approach were evaluated using a number of mineral oils previously assayed by a modification of the Ames Salmonella mutagenesis assay and the two-year mouse skin-painting bioassay. Both the Mobil PAC and IP-346 methods correlated well with the dermal carcinogenic potency of the oils. No substantive difference was observed between the two methods in their ability to provide a measure of the biological activity of oil, sufficient to predict tumorigenic potency. On the other hand, no significant relationship was observed between the levels of individual or summed 10 unsubstituted PAC (Multiple Marker approach) and the biological activity of the oils. The lack of correlation is attributed to the fact that the unsubstituted PAC are not sufficiently representative of the predominantly alkylated PAC that compose the aromatic fraction of mineral oils.     

Magnesium Alloy for Repair of Lateral Tibial Plateau Defect in Minipig Model

Bone graft substitutes are widely-studied as alternatives to bone grafts in the clinic.The currently available products are mostly ceramics and polymers.Considerable progress has been made in the study of the biodegradable magnesium alloys,which possess the necessary attributions of a suitable substitute,including an excellent mechanical property.In the present study,a minipig model of a lateral tibial plateau defect was used to evaluate the effectiveness of a magnesium alloy in the repair of a critical-sized defect.The micro-arc oxidation(MAO)-coated ZK60 alloy tablets and medical-grade calcium sulfate pellets were used as the test and control materials,respectively.Bone morphology was monitored by computed tomography after the implantation for 2 and 4 months.It was found that the bone morphology in minipigs following magnesium treatment was similar to that of the normal bone,whereas an abnormal and concave morphology was displayed following the calcium sulfate treatment.The average bone healing rate for the magnesium-treated defects was higher than that of the calcium sulfate-treated defects at the first 4 months following the implantation.Overall,magnesium treatment appeared to improve the defect repair as compared with the calcium sulfate treatment.Thus,the MAO-coated ZK60 alloy appears to be a useful biocompatible bone graft substitute,and further research on its biological activity in vivo is needed.     

可生物降解材料及其应用研究进展

阐述了可生物降解材料的定义、种类及降解机理,综述了可生物降解材料在国内外各个领域的研究现状和最新应用进展并,对其发展前景进行了展望。     

冷热原油顺序输送管内混油特性数值模拟

针对冷热原油顺序输送工艺,在已有混油理论基础上,建立了冷热原油顺序输送混油计算的数学模型,采用有限容积法进行了数值求解,对水平和竖直管内的混油特性进行了数值模拟和分析研究。结果表明,在水平管段,重力方向与管道轴向垂直,导致混油浓度在径向分布不对称;在竖直管段,重力方向与管道径向垂直,重力作用不会使得混油浓度在径向分布不对称,但在竖直管段,密度大的油品在上方时,混油量相对较大。     

In vitro and in vivo release of naltrexone from biodegradable depot systems

The aim of this study was to prepare poly(d, l-lactide) (PLA) microspheres containing naltrexone (NTX) by a solvent evaporation method, and to evaluate both in vitro and in vivo release characteristics and histopathological findings of tissue surrounding an implant formulation in rats.

This method enabled the preparation of microspheres of regular shape and relatively narrow particle size distribution. The in vitro release profiles of NTX from PLA microspheres showed the release of NTX did not follow zero-order kinetics. An initial burst release was observed, subsequently followed by a nearly constant rate of 0.4% per day after ten days. The cumulative amount of NTX released at the end of 60 days was 80%. Compressed microspheres showed near zero-order sustained release of NTX for 360 days. The plasma NTX levels in rats showed that for compressed microspheres NTX concentrations were constant and exceeded 2 ng/mL for 28 days. Throughout the 28 days of study, the implantations cause a minor inflammatory response, which can be regarded as a normal defence mechanism. The sustained release performance of NTX from the biodegradable depot systems may provide a reliable, convenient, and safe mechanism for the administration of NTX for the long-term treatment of opioid dependence.     

Melt viscoelasticity of biodegradable poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymers

The rheological properties of a series of microbially synthesized poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)s (PHB-HHxs), with varying comonomer (HHx) content, were systematically investigated. Shear viscosities show dependence on the rate of deformation, temperature, molecular weight, and copolymer compositions. The zero-shear viscosity η₀ follows the classical power-law relationship with the weight average molecular weight M_w. The characteristic relaxation time λ, which indicates the onset of shear thinning, ranges from 0.02 to 0.2 s for different PHB-HHxs and is roughly linearly related to η₀. The temperature dependence of rheological properties follows an Arrhenius form. Activation energies for flow E_a are obtained from the slope of the natural logarithm of the shift factor α_T plotted against the inverse of temperature curve, and the values for PHB-HHxs are found to be in the range of 27-36 kJ/mol E_a decreases with HHx content in the copolymer, a trend that can be related to the difference in chemical structure between HHx and HB, according to the method of Vankrevelen and Hoftyzer. A Generalized Maxwell model models the viscoelastic behavior of the PHB-HHx melt well. The value of the plateau modulus obtained suggests a highly entangled configuration. The molecular weight between entanglements M_e decreases from 11,600 to 9400 as HHx content increases from 3.8 to 10.0 mol%. Our results suggest that the presence of propyl groups in HHx increases the steric hindrance of the PHB-HHx chains, thus resulting in increased segmental friction and entanglement density. As a result, viscoelastic parameters for PHB-HHx copolymers, such as η₀ and , are readily tunable by varying the HHx content, making them attractive as “green” substitutes for non-degradable thermoplastics.     

Coating of collagen on a poly(l-lactic acid) sponge surface for tissue engineering

Porous scaffolds play important roles in tissue engineering. Biodegradable synthetic polymers, such as poly(l-lactic acid) (PLLA), frequently are used in the preparation of porous scaffolds. Pretreating the surface of a PLLA porous scaffold is required to increase its wettability for smooth cell seeding due to the hydrophobic property of the scaffold's surface. In this study, a simple coating method was used to modify the surface of the PLLA sponges. The coating method included three steps: filling the PLLA sponge pores with collagen aqueous solution, centrifuging to remove excess collagen, and, finally, freeze-drying. Compared with the uncoated PLLA sponge, the collagen-coated PLLA sponge demonstrated both improved wettability and high water absorption. Cells were smoothly seeded in the collagen-coated PLLA sponges by dropping a cell suspension solution onto the sponges. Cells adhered to the collagen-coated sponge and were distributed homogeneously throughout the collagen-coated PLLA sponge.