Thermal stability and melt rheology of poly(<Emphasis Type="Italic">p</Emphasis>-dioxanone)

Melt viscosities of poly(p-dioxanone) (PPDO) samples having different molecular weights were studied using a controlled-strain rotational rheometer under a nitrogen atmosphere. First, PPDO’s thermal stability was evaluated by recording changes in its viscosity with time. The result, that samples’ viscosities decreased with time when heated, demonstrated that PPDO is thermally unstable: its degradation activation energy, obtained by using a modified MacCallum equation, was a relatively low 71.8 kJ/mol K. Next, viscoelastic information was acquired through dynamic frequency measurements, which showed a shear thinning behavior among high molecular weight PPDOs, but a Newtonian flow behavior in a low molecular weight polymer (M _w = 18 kDa). Dynamic viscosity values were transferred to steady shear viscosities according to the Cox–Merz rule, and zero shear viscosities were derived according to the Cross model with a shear thinning index of 0.80. Then flow activation energy (48 kJ/mol K) was extrapolated for PPDO melts using an Arrhenius type equation. This activation energy is independent of polymer molecular weight. A linear relationship between zero shear viscosity and molecular weight was obtained using a double-logarithmic plot with a slope of 4.0, which is near the usually observed value of 3.4 for entangled linear polymers. Finally, the rheological behaviors of PPDO polymer blends having bimodal molecular weight distributions were investigated, with the results indicating that the relationship between zero shear viscosity and low molecular weight composition fraction can be described with a Christov model.     

In vivo evaluation of a novel dexamethasone-heparin-double-coated stent for inhibition of artery restenosis and thrombosis

To evaluate the efficacy and safety of dexamethasone-heparin-double-coated stent (DHDCS) on inhibition of artery lumen reduction and neointimal hyperplasia in porcine model we carried out this study. Bare mental stents (BMS, n = 12), protein-coated stents (PCS, n = 12), heparin microballoon-coated stents (HMCS, n = 12), and DHDCS (n = 12), prepared by the spray drying method, were implanted into the selected internal iliac artery, external iliac artery, sacrococcygeal artery, and femoral artery of each of the selected pigs (n = 12), which were randomly divided into four groups on average. Thirty days and ninety days after the implantation, aorta angiography was performed on all the 12 mini-pigs to evaluate the artery lumen reduction. Subsequently, in order to analyze their histological appearance, the pigs were killed, and their arteries with the stents inside were taken out, embedded in plastic for hard histological section and hematoxylin-eosin (H.E.) staining, and examined by light microscopy and scanning electron microscopy (SEM). The artery lumen reduction and average neointimal hyperplasia in the group of DHDCS were significantly lesser than those in the other three groups of BMS, PCS, and HMCS. This study shows that DHDCS is capable of inhibiting the proliferation of intima and lumen area reduction of the target artery within stents, and effectively and safely reducing the incidence of regional thrombosis and restenosis for a short term.     

Characterization and in vivo evaluation of a bio-corrodible nitrided iron stent

A bio-corrodible nitrided iron stent was developed using a vacuum plasma nitriding technique. In the nitrided iron stents, the tensile strength, radial strength, stiffness and in vitro electrochemical corrosion rate were significantly increased compared with those of the control pure iron stent. To evaluate its performance in vivo, the deployment of the nitrided iron stents in juvenile pig iliac arteries was performed. At 3 or 6 months postoperatively, the stented vessels remained patent well; however, slight luminal loss resulting from intimal hyperplasia and relative stenosis of the stented vessel segment with piglets growth were observed by 12 months; no thrombosis or local tissue necrosis was found. At 1 month postoperatively, a nearly intact layer of endothelial cells formed on the stented vessel wall. Additionally, a decreased inflammation scoring, considerably corroded struts and corrosion products accumulation were seen. These findings indicate the potential of this nitrided iron stent as an attractive biodegradable stent.     

Bioinspired,Anticoagulative, 19F MRI-Visualizable Bilayer Hydrogel Tubes as High Patency Small-Diameter Vascular Grafts

The clinical patency of small-diameter vascular grafts (SDVGs) (ID < 6 mm) is limited, with the formation of mural thrombi being a major threat of this limitation. Herein, a bilayered hydrogel tube based on the essential structure of native blood vessels is developed by optimizing the relation between vascular functions and the molecular structure of hydrogels. The inner layer of the SDVGs comprises a zwitterionic fluorinated hydrogel, avoiding the formation of thromboinflammation-induced mural thrombi. Furthermore, the position and morphology of the SDVGs can be visualized via ¹⁹F/¹H magnetic resonance imaging. The outer poly(N-acryloyl glycinamide) hydrogel layer of SDVGs provides matched mechanical properties with native blood vessels through the multiple and controllable intermolecular hydrogen-bond interactions, which can withstand the accelerated fatigue test under pulsatile radial pressure for 380 million cycles (equal to a service life of 10 years in vivo). Consequently, the SDVGs exhibit higher patency (100%) and more stable morphology following porcine carotid artery transplantation for 9 months and rabbit carotid artery transplantation for 3 months. Therefore, such a bioinspired, antithrombotic, and visualizable SDVG presents a promising design approach for long-term patency products and great potential of helping patients with cardiovascular diseases.     

ZrO<Subscript>2</Subscript> foams for porous radiant burners

In this work, Y₂O₃-stabilized ZrO₂ (YSZ) foams with low relative density were developed through the replication method, for application as porous radiant burners. The ceramic foams were produced by impregnation of open-cell polyurethane foams with aqueous suspensions and different fractions of raw materials: ZrO₂–8% Y₂O₃ (8YSZ) powder, and additives. The materials were milled for 10–40 min. The impregnated foams were dried and submitted to a heat treatment for polyurethane elimination at 1000 °C for 1 h, with subsequent sintering of the remaining ceramic structure at 1600 °C for 2 h, which resulted in YSZ foams with low relative density (0.07). The structural analysis revealed a cellular structure with an average mechanical strength of 95.6 kPa. The radiation efficiency (>19%) was obtained by tests with different air/fuel ratio. The ceramic matrixes exhibited high performance and structural integrity at high operation temperatures (1400 °C).     

Body distribution of poly(<Emphasis Type="SmallCaps">d</Emphasis>,<Emphasis Type="SmallCaps">l</Emphasis>-lactide-<Emphasis Type="Italic">co</Emphasis>-glycolide) copolymer degradation products in rats

Poly (d,l-lactide-co-glycolide) (PLGA) copolymers are among the few synthetic polymers approved for human use, but the biocompatibility of PLGA-derived oligomers and particles remains questionable. Here, high molecular weight PLGA (Mw = 32,000) was radiolabeled with ¹²⁵I in chloroform solution, and the body distribution of PLGA copolymer degradation products was examined following subcutaneous implantation of round ¹²⁵I-PLGA films on the back of Sprague Dawley rats. Autoradiographic images of the PLGA implant taken at 2, 4, 6, 8, 10, and 12 weeks revealed that the central portion of the film degraded much more rapidly than the marginal portions. Examination of the body compartment distribution at these time points revealed that over one-half of the radioactivity was recovered from skin. The remaining radioactivity was concentrated in the blood, liver, and kidneys. Radioactivity steadily appeared in the blood and remained elevated up to 12 weeks after implantation, while the liver to kidney distribution began to decrease after 6 weeks. Cumulatively, these results indicate that the clearance of degraded particles and fragments from the implantation site is extremely delayed. Moreover, the degraded particles and fragments were selectively concentrated in the liver and kidneys, following release of degraded products into the bloodstream from the implantation site.     

不同释放程度下下肢动脉支架的疲劳强度及寿命预测

目的 探究释放程度和支架结构对支架疲劳寿命的影响。方法 利用有限元法分析3款镍钛合金下肢动脉支架（Complete SE、E–luminexx–B和Pulsar–35）在脉动载荷作用下的力学性能,基于疲劳应变理论评估支架的疲劳强度,使用Fe–safe和Abaqus软件预测支架在不同释放程度（80%、85%、90%）下的疲劳寿命及安全性。应用Origin软件对支架应力与疲劳寿命进行拟合。结果 支架在释放尺度为80%～90%下的疲劳强度均满足10年疲劳寿命的要求。3种支架释放程度为90%时,疲劳强度大,寿命长;释放程度为80%时,疲劳强度、寿命及安全系数小。相同释放程度下,偏置型波峰结构支架变形后的交变应变及弹性应力值最大,支架寿命及安全系数最小;对称型波峰结构支架CE的寿命及安全系数小于EB。通过LangevinMod函数拟合PR支架疲劳寿命,拟合优度R2>0.99。结论 随着释放程度的增大,3款支架疲劳强度、寿命及安全系数呈增大趋势。在相同释放程度下,支撑单元对称型波峰结构支架疲劳强度、寿命及安全系数比偏置型波峰结构支架大,减小对称型波峰结构支架的连接体宽度有利于提高支架寿命。     

Properties of Ti–6Al–4V non-stochastic lattice structures fabricated via electron beam melting

This paper addresses foams which are known as non-stochastic foams, lattice structures, or repeating open cell structure foams. The paper reports on preliminary research involving the design and fabrication of non-stochastic Ti–6Al–4V alloy structures using the electron beam melting (EBM) process. Non-stochastic structures of different cell sizes and densities were investigated. The structures were tested in compression and bending, and the results were compared to results from finite element analysis simulations. It was shown that the build angle and the build orientation affect the properties of the lattice structures. The average compressive strength of the lattice structures with a 10% relative density was 10 MPa, the flexural modulus was 200 MPa and the strength to density ration was 17. All the specimens were fabricated on the EBM A2 machine using a melt speed of 180 mm/s and a beam current of 2 mA. Future applications and FEA modeling were discussed in the paper.     

Practicability and Limitations of Finite Element Simulation of the Dilation Behaviour of Coronary Stents

After first implantation of a metallic stent into cardiac vessels in 1986 stent implantation has become a standard technique for treatment of coronary heart disease. During implantation of balloon‐expandable stents, the structure of the stent undergoes high plastic deformation. Despite the fact, that stents are used for more than 15 years nearly no information about the mechanical and micro structural process during dilation are known. The present paper presents a detailed study and comparison of the experimental and the simulated expansion behaviour of metallic stents. Used material models are discussed and crystallographic details are presented. Dilation curves describe the behaviour of balloon‐expandable coronary stents. The dilation behaviour depends on both the materials properties and the design of the stent. A numerical simulation of the dilation process by means of FE is suitable. A comparison of the experimental measurement and the numerical simulation demonstrates, that a Cauchy stress‐strain material model should be used for numerical simulations. A local failure criterion is introduced, which considers void initiation as a criterion for mechanical failure.     

Crystal structure and methane oxidation on perovskite-type (La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Nd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)CoO<Subscript>3</Subscript> synthesized using citric acid

Perovskite-type (La_1−x Nd _x )CoO₃ was synthesized using citric acid at 700 °C. The Rietveld method indicated that the crystal structure changed from a rhombohedral to an orthorhombic system at x = 0.4. The Co–O distance of the rhombohedral structure connected continuously with the average Co–O(2) distance of the orthorhombic structure, and the Co–O–Co angle of the rhombohedral structure and the Co–O(2)–Co angle of the orthorhombic structure were continuous. The average particle size of the samples was approximately 55 nm. CH₄ oxidation started above 300 °C, and the temperature corresponding to the 50% conversion (T _1/2) of CH₄ increased linearly with increases in x. It is considered that the amount of adsorbed oxygen decreased in response to the steric hindrance, and that T _1/2 increased as a result.     

In vitro hydrolytic degradation of poly(para-dioxanone)/poly(d,l-lactide) blends

A series of biodegradable polymers were prepared by solution coprecipitation of poly(para-dioxanone) (PPDO) and poly(d,l-lactide) (PDLLA) in various blend ratios. Samples were compression molded into bars using a platen vulcanizing press. The in vitro hydrolytic degradation of PPDO/PDLLA blends was studied by examining the changes in weight, water absorption, tensile strength, breaking elongation, thermal properties, and morphology of the blends in phosphate buffered saline (PBS; pH 7.44) at 37 °C for 8 weeks. During the hydrolytic degradation, the weight loss and water absorption increased significantly for all samples, whereas the hydrolysis rate varied with the blend composition. The weight loss of PPDO/PDLLA 80/20, which showed the smallest degradation rate, was lower than that of pure PPDO for almost all of the hydrolytic degradation period. The results showed that the blend composition played an important role in determining the degradation behaviors of blends.     

Natural Weathering and Sea Water Effects on the Durability of Glass Fiber Reinforced Vinylester: Fractographic Analysis

This paper presents a study of the effects of harsh outdoor weather and warm sea water on the tensile behavior of Glass-Fiber Reinforced Vinylester (GFRV) pipe materials destined for sea water handling and transportation. The effect of Dhahran’s outdoor weather for exposure periods ranging from 3 to 36 months revealed an improvement in tensile strength when compared with the as received GFRV sample. A significant increasing trend of tensile strength from 3 to 12 months was noted. This is attributed mainly to the post curing effects resulting in higher cross linking density. After 12 months of exposure the tensile strength showed a decreasing trend, but remaining still higher than the average tensile strength of as received (baseline) GFRV sample. Similar results of enhanced tensile strength were noted after immersion of GFRV pipes in warm Gulf sea water for 12 months. Fractographic analysis was performed on the tensile tested GFRV samples using optical microscope followed by scanning electron microscope (SEM). The characterization of the controlling failure mechanisms involved from fracture initiation to fracture propagation through the gage section of the specimen were predicted and were justified by correlating the optical and SEM pictures.     

Microstructure,fluidity, and mechanical properties of semi-solid processed ductile iron

This research is directed towards studying the effect of semi-solid processing (using cooling plate technique) on the microstructure, fluidity, and mechanical properties of ductile iron (DI). Sand mold castings with constant width of 25 mm and length of 150 mm with the thicknesses of 6, 12, 18, and 25 mm were used in this study. Microstructure, fluidity, and tensile strength properties were investigated as a function of fraction of solid. The results indicated that by increasing fraction of solid microstructure becomes finer and more globular. However, increasing primary fraction of solid increases the cementite content in the matrix. Above a certain fraction of solid (f _s = 0.28, f _s = 0.1, and f _s = 0.05 for 25, 18, and 12 mm wall thickness, respectively), the fluidity of semi-solid processed DI decreases steeply. For low fraction of solid (f _s ≤ 0.15), increasing the fraction of solid results in an increment in the tensile strength, comparing with the ordinary DI due to the fine and globular structure formation. Any further increment of fraction of solid (f _s > 0.15) leads to the cementite increment and gas porosity formation, consequently the tensile strength decreased. The fraction of solid of DI and casting wall thickness should be considered in order to obtain the best combination of microstructure, fluidity, and mechanical properties of semi-solid processed DI.     

Synthesis and characterization of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–ZrO<Subscript>2</Subscript> nanocomposite powder by sucrose process

Nanocrystalline alumina–zirconia powders were prepared by a modified chemical route using sucrose, polyvinyl alcohol (PVA) and metal nitrates followed by a post calcination process. The process involved dehydration of Al³⁺–Zr⁴⁺ ions-sucrose–PVA solution to a highly viscous liquid which on decomposition process produced a black precursor material. The obtained precursor were then calcined at various temperatures: 1,050, 1,100, 1,150, 1,200 and 1,250 °C for different soaking times (1, 2, 4 h) in air. The formation of a nanocomposite composed of α-alumina (~20 nm) and tetragonal (t) zirconia (~19 nm) crystallites were confirmed for the sample calcined at 1,200 °C for 2 h, based on our XRD and TEM results. However, for the samples calcined below 1,150 °C the composite formed were composed of metastable alumina (γ, δ, θ) as well as t-zirconia phases. Interestingly, the zirconia phase retained its tetragonal structure for all the samples calcined above 1,050 °C. This is possibly related to the “size effect” and reduction of surface enthalpy of the zirconia crystallites surrounded by Al³⁺ cations.     

Influence of design parameters on the mechanical behavior and porosity of braided fibrous stents

In spite of all innovations in stent design, commonly used metallic stents present several problems such as corrosion, infection and restenosis, leading to health complications or even death of patients. In this context, the present paper reports a systematic investigation on designing and development of 100% fiber based stents, which can eliminate or minimize the problems with existing metallic stents. For this purpose, braided stents were produced by varying different materials, structural and process parameters such as mono-filament type and diameter, braiding angle and mandrel diameter. The influence of these design parameters on mechanical behavior as well as stent's porosity was thoroughly investigated, and suitable parameters were selected for developing a stent with mechanical characteristics and porosity matching with the commercial stents. According to the experimental results, the best performance was achieved with a polyester stent designed with 0.27 mm monofilament diameter, braiding angle of 35° and mandrel diameter of 6 mm, providing similar properties to commercial Nitinol stents.     

Effect of particle size on microstructure and strength of porous spinel ceramics prepared by pore-forming <Emphasis Type="Italic">in situ</Emphasis> technique

The porous spinel ceramics were prepared from magnesite and bauxite by the pore-forming in situ technique. The characterization of porous spinel ceramics was determined by X-ray diffractometer (XRD), scanning electron microscopy(SEM), mercury porosimetry measurement etc and the effects of particle size on microstructure and strength were investigated. It was found that particle size affects strongly on the microstructure and strength. With decreasing particle size, the pore size distribution occurs from multi-peak mode to bi-peak mode, and lastly to mono-peak mode; the porosity decreases but strength increases. The most apposite mode is the specimens from the grinded powder with a particle size of 6·53 μm, which has a high apparent porosity (40%), a high compressive strength (75·6 MPa), a small average pore size (2·53 μm) and a homogeneous pore size distribution.     

Natural Weathering and Sea Water Effects on the Durability of Glass Fiber Reinforced Vinylester: Fractographic Analysis

This paper presents a study of the effects of harsh outdoor weather and warm sea water on the tensile behavior of Glass-Fiber Reinforced Vinylester (GFRV) pipe materials destined for sea water handling and transportation. The effect of Dhahran’s outdoor weather for exposure periods ranging from 3 to 36 months revealed an improvement in tensile strength when compared with the as received GFRV sample. A significant increasing trend of tensile strength from 3 to 12 months was noted. This is attributed mainly to the post curing effects resulting in higher cross linking density. After 12 months of exposure the tensile strength showed a decreasing trend, but remaining still higher than the average tensile strength of as received (baseline) GFRV sample. Similar results of enhanced tensile strength were noted after immersion of GFRV pipes in warm Gulf sea water for 12 months. Fractographic analysis was performed on the tensile tested GFRV samples using optical microscope followed by scanning electron microscope (SEM). The characterization of the controlling failure mechanisms involved from fracture initiation to fracture propagation through the gage section of the specimen were predicted and were justified by correlating the optical and SEM pictures.     

Crystal chemistry and domain structure of rare-earth doped BiFeO<Subscript>3</Subscript> ceramics

Bi_(1−x)RE _x FeO₃ (BREF100x, RE = La, Nd, Sm, Gd) has been investigated with a view to establish a broad overview of their crystal chemistry and domain structure. For x ≤ 0.1, the perovskite phase in all compositions could be indexed according to the rhombohedral, R3c cell of BiFeO₃. For Nd and Sm doped compositions with 0.1 < x ≤ 0.2 and x = 0.15, respectively, a new antipolar phase was stabilised similar in structure to PbZrO₃. The orthoferrite, Pnma structure was present for x > 0.1, x > 0.15, and x > 0.2 in Gd, Sm, and Nd doped BiFeO₃, respectively. For x > 0.2, La doped compositions became pseudocubic at room temperatures but high angle XRD peaks were broad and asymmetric. These compositions have been indexed as the orthoferrite structure. It was concluded therefore that the orthoferrite phase appeared at lower values of x as the RE ferrite, end member tolerance factor decreased. However, the compositional window over which the PbZrO₃-like phase was stable increased with increasing end member tolerance factor but was not found as single phase in La doped compositions at room temperature. On heating, the PbZrO₃-like phase in BNF20 transformed to the orthoferrite, Pnma structure. T _C for all compositions decreased with decreasing A-site, average ionic polarizabilty and tolerance factor. For compositions with R3c symmetry, superstructure and orientational, and translational (antiphase) domains were observed in a manner typical of an antiphase-tilted, ferroelectric perovskite. For the new PbZrO₃-like phase orientational domains were observed.     

Effect of the chemical structure of aromatic polyimides on their thermal aging,relaxation behavior and mechanical properties

This article examines the effects of structural changes and thermal aging treatments on the relaxation processes and mechanical properties of three polyimides differing for their molecular structure i.e. PMDA-ODA, 6FDA-ODA, and 6FDA-6FpDA. These polyimides were obtained by thermal imidization of their polyamic acid precursors, which were synthesized from the respective dianhydrides [pyromellitic anhydride (PMDA), hexahydrofluoroisopropylidene diphthalic anhydride (6FDA)], and diamines [4,4′-diaminodiphenyl ether (ODA), 4,4′-(hexafluoroisopropylidene) dianiline (6FpDA)]. After the curing process, the polyimides were thermally aged at a fixed temperature for various times Dynamic mechanical measurements performed in a multi-frequency mode, were used to determine the glass-rubber and sub-glass transitions, as well as the activation energy of the β transition. It was found that the T _g decreased in the order PMDA-ODA > 6FDA-6FpDA > 6FDA-ODA as a result of an increased chain rigidity and molecular packing induced by charge transfer interactions during the thermal imidization process. The β sub-glass transition showed two relaxation processes identified as β′ and β′′. The β′ process was attributed to the local motion of the diamine constituents while the β′′ process was caused by the local motion of the dianhydride moiety. The cooperativity of these molecular motions was also assessed via the Starkweather method. The thermal aging enhanced the state of aggregation of polyimide chains and thus the T _g and the sub-glass transition properties. This effect was particularly marked for the PMDA-ODA polyimide. Also the mechanical properties were significantly affected by chemical structure and aging treatments. For non-aged samples the more influenced parameter was the elongation at break, which decreased in the order PMDA-ODA > 6FDA-ODA > 6FDA-6FpDA. The aging enhanced the elastic modulus and the tensile strength and reduced the elongation at break.     

A shape memory stent of poly(ε-caprolactone-co-dl-lactide) copolymer for potential treatment of esophageal stenosis

Biodegradable polymer stent with shape memory effect is expected to be developed in the treatment of esophageal stenosis, most likely due to traditional stents having such shortages as considerable rigidity and nondegradation. A tubular stent with the inner and outer diameters of 28 and 30 mm was manufactured from biodegradable poly(ε-caprolactone-co-dl-lactide) (PCLA) copolymer consisting of ε-caprolactone and dl-lactide at a weight ratio of 10/90. A series of tests were accomplished to investigate its properties including shape memory effects (SMEs), compression property and influence of in vitro degradation of polymer matrix on its shape recovery and dilation force. Significantly, an implantation of the stent into a dog model was performed to evaluate its function for the treatment of esophageal stenosis. The deformed stent needs about 36 s to recover its initial shape in vitro in 37°C warm water. The primary animal experiment in vivo has revealed that the implanted deformed stent could be triggered by body temperature and expectedly returned to a nearly-round shape to support esophageal wall. Therefore, the biodegradable intelligent polymer stent may be great potential to displace the conventional metallic stents for the esophageal stenosis therapy.