Effects of annealing temperature on both radial supporting performance and axial flexibility of poly(L-lactic acid) braided stents

In this paper, the effects of annealing temperature on both radial supporting performance and axial flexibility of poly(L-lactic acid) (PLLA) braided stents are studied. Stents are annealed at a series of temperatures ranging from 80 to 160°C for 1 h, then indicators of shaping effect, radial supporting performance, and axial flexibility are compared. Stents not annealed and annealed at 80°C cannot be completely shaped. In contrast, stents annealed at 100 to 160°C are well shaped showing radial shrinkage rate of 1.0 ± 0.2% and almost no axial elongation. The radial compressive force and axial force are gradually increased by 51.2% and 89.2%, respectively with the annealing temperature gradually increasing from 80 to 160°C, indicating that the radial supporting performance is improved but axial flexibility is weakened by a higher annealing temperature. Taken together, PLLA braided stents can be annealed at 100 to 120°C to obtain sufficient radial force and lower axial force simultaneously for clinical applications. Moreover, tensile test, X-ray diffraction, and differential scanning calorimetry are performed for monofilaments annealed at different temperatures to further explore the effect mechanism of annealing temperature on the mechanical properties of stents. This study may provide helpful suggestions for the manufacture of biodegradable braided stents.     

Experimental investigation on the properties of poly (L-lactic acid) vascular stent after accelerated in vitro degradation

Poly (L-lactic acid) (PLLA) vascular stents are promising to be used to treat vascular stenosis. However, the degradation of these implanted stents has a great influence on the mechanical properties. In this work, we studied the factors that can influence the mechanical properties by accelerating the degradation of PLLA stents with crimping and expansion in phosphate buffer solution (PBS) at 70°C. The typical influence factors such as the real-time weight-average molecular weight (M_w), crystallinity, surface morphologies, and mass loss of these samples were analyzed in detail. Results showed that radial strength of the stents increased from 877 to 1204 mmHg after accelerated degradation for 2 days and then decreased to almost 0 mmHg after 10 days. Meanwhile, radial stiffness showed an upward trend which is caused by the decrease in M_w, increase in crystallinity and local damage during the crimping and expansion process. Moreover, negligible mass loss of stents was observed in this period and the bulk degradation behavior predominated in the whole process.     

Biodegradation evaluation of poly (lactic acid) for stent application: Role of mechanical tension and temperature

In this study, an experimental investigation is conducted on mechanical characteristics of poly(lactic acid) (PLA), before, and during degradation for stent application. A bioreactor is designed and fabricated to mimic in-vivo environment of the body for studying degradation behavior of PLA fibers manufactured by melt spinning method. Beside PLA fibers, the degradation of PLA braided stents is investigated as control samples. To measure stress–strain and stress relaxation properties of PLA fibers, tensile, and relaxation tests are conducted. The decreasing trend of Young's modulus, variations in residual stress value after relaxation and pattern of stress relaxation are found during degradation. The influence of effective parameters, that is, temperature and stress, on PLA degradation is also studied. Moreover, the PLA degradation is analyzed by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), Thermogravimetric analysis (TGA) and microscopic images. GPC results indicate the molecular weight decreases from 196,000 to 80,000 due to degradation while DSC analysis confirmed that the degradation promote an increase in PLA degree of crystallinity (from 43.3% to 59.8%). In addition, TGA results show that the PLA thermal stability decreases during degradation. This study provides useful information on PLA properties during degradation to assess the material in context of degradable stents.     

In vitro degradation of poly(L-lactic acid) fibers produced by melt spinning

In vitro degradation of poly(L -lactic acid) fibers was investigated for a period of 16 weeks in Ringer solution at 37°C. Two sets of fibers, with similar initial mechanical properties, molar mass, and crystallinity content, but markedly different in diameter (72 and 120 μm) were studied. Viscometric molar mass decreased during the immersion time at a faster rate for the thinner fibers compared to the thicker ones. As a consequence, the fiber mechanical properties changed; the elastic modulus was only slightly affected by the molar mass decrease whereas ultimate mechanical properties (stress and strain at break) showed a strong decrease. A quantitative correlation between tensile strength and viscometric-average molar mass was attempted. A possible explanation of the faster degradation rate of the thinner fibers was proposed on the basis of the higher surface/volume ratio and water uptake. Dynamic mechanical properties were also measured as a function of immersion time. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 213–223, 1997     

Dynamic mechanical and thermal properties of plasticized poly(lactic acid)

The blends of low molecular weight triacetin (TAC) and oligomeric poly(1,3‐butylene glycol adipate) (PBGA) were used as multiple plasticizers to lubricate poly(lactic acid) (PLA) in this study. The thermal and mechanical properties of plasticized polymers were investigated by means of dynamic mechanical analysis and differential scanning calorimetry. Atomic force microscopy (AFM) was used to analyze the morphologies of the blends. Multiple plasticizers were effective in lowering the glass transition temperature (T_g) and the melting temperature (T_m) of PLA. Moreover, crystallinity of PLA increased with increasing the content of multiple plasticizers. Tensile strength of the blends decreased following the increasing of the plasticizers, but increased in elongation at break. AFM topographic images showed that the multiple plasticizers dispersed between interfibrillar regions. Moreover, the fibrillar crystallite formed the quasicrosslinkings, which is another cause for the increase in elongation at break. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1583–1590, 2006     

生物医用材料聚L-乳酸的生物相容性及降解性能研究

通过DSC、SEM、TGA等分析手段对合成得到的聚L-乳酸(PLLA)进行了细胞种植、动物体内包埋及热降解实验,其生物相容性及体内、体外降解情况。实验证明PLLA材料具有良好的生物相容性和组织相容性。用其薄膜埋植于兔子皮下,兔子的生存状态及生长良好;其薄膜可为细胞在其表面上生长、增殖、分泌基质提供良好的微环境。PLLA材料分子量越大降解速度越慢,且体内降解的速度要比体外降解的速度快。     

Effect of trimethylene carbonate segments on the structure and properties of poly(L-lactide-glycolide-trimethylene carbonate) terpolymers for biodegradable vascular stents

The total biodegradable vascular stent made of poly(L-lactide-glycolide-trimethylene carbonate) terpolymers with excellent performances has the potential to be used in a new generation of medical implant devices. In this work, the properties and structure of terpolymers were investigated, which were found to be controlled by the composition of the copolymer chain. The amorphous trimethylene carbonate segments not only disturbed the sequence regularity of the L-lactide segment, but also reduced the crystallization ability of the material. Furthermore, it is the flexible segments in the terpolymer chains that improve the plastic flow capacity, increase the flexibility of chains as well as reduce the apparent activation energy of thermal degradation of materials. Such a chain structured terpolymer exhibited good extrudability and buildability in 3D printing and formed a vascular stent with a cohesion entanglement characteristic microstructure during the process. The crush resistance with radially applied load and crush resistance with parallel plates of stent were 2561.4 mm Hg and 0.24 N/mm, respectively.     

Effect of surface acetylated‐chitin nanocrystals on structure and mechanical properties of poly(lactic acid)

In this work, the miscibility between chitin nanocrystals (ChNs) and poly(lactic acid) (PLA) was expected to be improved by surface acetylation of ChN. The reaction of acetic anhydride onto the ChN surface was confirmed by FTIR and ¹³C NMR, while XRD and TEM proved the crystalline structure and rod‐like morphology were maintained. The acetylated ChN (AChN) was incorporated into a PLA matrix by solution blending, and resulted in an increase of tensile strength and Young's modulus and they reached to the maximum value as 45 and 37% higher than neat PLA film, respectively, with the loading level of AChN reaching to 4 wt %. The enhancement could be attributed to that acetylation improved dispersion of AChN in the PLA matrix and interfacial adhesion between AChN and PLA. The performances of the nanocomposites based on PLA and chitin nanocrystals derived from renewable resources have good potential for industrial applications. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39809.     

Gelatin multilayers assembled on poly(L-lactic acid) surface for better cytocompatibility

Gelatin multilayers were assembled on PLLA substrate at pH 3, 5, and 7, which was below, around, and above the isoelectric point of the amphoteric polymer, using the layer-by-layer assembly technique. The multilayer deposition on the PLLA substrate was monitored by X-ray photoelectron spectroscopy (XPS) and water contact angle measurement. The XPS, water contact angle, and atomic force microscopy data indicated that the layer thickness, surface hydrophilicity, and surface morphology of the gelatin multilayers assembled strongly depended on the pH at which the layers were deposited. Chondrocyte culture was used to test the cell attachment, cell morphology, and cell viability on the PLLAs modified with gelatin multilayers. All these modified PLLAs exhibited dramatically improved cytocompatibility compared with the virgin PLLA, and even better performance than that of tissue culture polystyrene (TCPS). The best cell compatibility was observed for the gelatin multilayers assembled at pH = 5. These results indicate that the cell compatibility of biodegradable polyesters can be effectively and facilely improved by assembly of gelatin multilayers, and the performance can be controlled by adjusting the solution pH at which the multilayers are deposited. This may help the design and fabrication of better scaffolds for tissue engineering. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Synthesis of bio-based poly(butylene carbonate-co-glycolic acid) with tunable degradation rate and mechanical properties

Bio-based copolymers with excellent mechanical properties and biodegradability are highly desired for packaging and agricultural applications. In this work, a series of high molecular weight bio-based poly(ester-carbonate)s are synthesized using glycolic acid (GA), dimethyl carbonate (DMC), and 1,4-butanediol (BDO). The intrinsic viscosity ([η]) of these copolymers ranges from 0.70 to 1.11 dL/g. The tensile strength, elastic modulus, and elongation at break of PBCGA10 are 19.69 ± 2.40 MPa, 151.20 ± 37.17 MPa, and 1173 ± 142%, respectively, which are superior to the mechanical properties of PBAT. The degradation weight loss of PBCGA10 and PBCGA20 after 30 days can reach 8% and 40%, respectively, indicating excellent degradation performance of the copolymers. Moreover, copolymers undergo degradation through a surface degradation mechanism. The main products of copolymer degradation are BDO, CO₂, and H₂O, which do not cause harm to organisms and the environment.     

可生物降解聚乳酸类胶粘剂的研究进展

对可生物降解的聚乳酸类胶粘剂的种类、应用及最新的研究进展等几个方面进行了综述。特别介 绍了工业用聚乳酸类胶粘剂和医学用聚乳酸类胶粘剂的研究进展。     

Superstructure and mechanical properties of poly(L-lactic acid) microfibers prepared by CO2 laser-thinning

Poly(L-lactic acid) (PLLA) microfibers were obtained by a carbon dioxide (CO₂) laser-thinning method. A laser-thinning apparatus used to continuously prepare microfibers was developed in our laboratory; it consisted of spools supplying and winding the fibers, a continuous-wave CO₂-laser emitter, a system supplying the fibers, and a traverse. The laser-thinning apparatus produced PLLA microfibers in the range of 100-800 m min⁻¹. The diameter of the microfibers decreased as the winding speed increased, and the birefringence increased as the winding speed increased. When microfibers, obtained through the laser irradiation (at a laser power of 8.0 W cm⁻²) of the original fiber supplied at 0.4 m min⁻¹, were wound at 800 m min⁻¹, they had a diameter of 1.37 μm and a birefringence of 24.1×10⁻³. The draw ratio calculated from the supplying and winding speeds was 2000×. The degree of crystal orientation increased with increasing the winding speed. Scanning electron microscopy showed that the microfibers obtained with the laser-thinning apparatus had smooth surfaces not roughened by laser ablation that were uniform in diameter. The PLLA microfiber, which was obtained under an optimum condition, had a Young's modulus of 5.8 GPa and tensile strength of 0.75 GPa.     

Effects of low molecular weight compounds with hydroxyl groups on properties of poly(L-lactic acid)

The effects of low molecular weight compounds with hydroxyl groups on the properties of poly(L -lactic acid) (PLLA) were investigated. The specific interaction between PLLA and 4,4′-thiodiphenol (TDP) was investigated by Fourier transform IR spectroscopy. The spectra of the blends suggested that there were interassociated hydrogen bonds between the PLLA chains and TDP molecules. The thermal and dynamic mechanical properties of PLLA/TDP blends were investigated by differential scanning calorimetry and dynamic mechanical thermal analysis, respectively. The results revealed that the thermal and dynamic mechanical properties of PLLA were greatly modified through blending with TDP and that PLLA/TDP blends possessed eutectic phase behavior. The effects of the thermal history on the formation of hydrogen bonds and the thermal and mechanical properties were evaluated. It was found that the properties of the blends were strongly dependent on the thermal history. In addition to TDP, PLLA blends with other low molecular weight hydroxyl compounds, including aromatic and aliphatic compounds, were also characterized. The effects of the chemical structure of the low molecular weight compounds on the properties of PLLA were discussed. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 640–649, 2001     

聚乳酸多孔微球的制备及其表征

以聚乳酸（PLLA）/四氢呋喃（THF）为淬火溶液,无其他添加剂条件下,通过低温淬火、萃取、洗涤和干燥得到直径为30.92μm±1.55μm的PLLA多孔微球,多孔微球由直径为0.34μm±0.06μm向外辐射的纤维组成。偏光显微镜表明多孔微球为球晶结构。XRD结果表明,多孔微球属于α晶型,晶粒尺寸大小为17.25nm。DSC结果表明,PLLA多孔微球的结晶度为36.05%。与熔融挤出造粒得到PLLA原料（结晶度小于10%）相比,低温淬火得到的多孔微球的结晶度大大提高。N₂吸附-脱附结果分析表明,多孔微球的平均孔径和孔体积分别为42.92nm和0.1135cm³/g,大部分为大孔和介孔结构,比表面积和孔隙率分别为14.18cm²/g和93.15%。采用等温DSC模拟低温淬火过程研究了PLLA在THF溶液中结晶动力学,利用Avrami方程得到Avrami指数n平均值为2.29,说明PLLA在THF溶液中为异相成核和三维生长。     

Interplay between physicochemical and mechanical properties of poly(ethylene terephthalate) meshes for hernia repair

Non‐medical‐grade industrial nets are often implemented in developing countries as affordable alternative to surgical meshes for hernia repair. Even if there is clear evidence about their repairing reliability, their physicochemical and mechanical properties have been not fully investigated. This works compares three industrial nets with different textile patterns, and a surgical mesh of the same polymer. Nets are autoclave‐sterilized and characterized through scanning electron microscope, Raman spectroscopy, thermogravimetric analysis , differential scanning calorimetry, and uniaxial tensile tests. Spectral and thermal analyses reveal that all samples are based on poly(ethylene terephthalate). Differences are found in phase conformations with modifications in amorphous, ordered amorphous, and crystalline domains. Changes in material characteristics do not affect mechanical properties, which are mainly ascribable to the textile pattern. Industrial nets show a stiffening behavior different from the almost linear anisotropic response of surgical mesh. However, non‐medical‐grade nets could be potentially applied for surgeries once their biocompatibility and in vivo stability have been evaluated. Non‐medical‐grade industrial nets are used as low‐cost alternative to surgical meshes for hernia repair in less developed countries. Nets sterilization modifies their phase conformation, due to the interaction with water, but do not affect mechanical properties. Even tough industrial nets show a different stiffening behavior, physicochemical characterization confirms a similarity to standard surgical meshes. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46014.     

Effect of cooling conditions on the mechanical properties of crystalline poly(lactic acid)

Appropriate cooling conditions in melt processing were found to provide crystalline poly(lactic acid) (PLA) with greater mechanical toughness in tensile tests. PLA films cooled near its glass transition temperature T_g showed ductile behavior, whereas those obtained by a quenching process exhibited brittle fracture. The content of gauche‐gauche (gg) conformer, which leads to low critical onset stress for shear yielding, increased in the films cooled near T_g. The crystallinity of the films hardly affected their mechanical toughness and proportion of gg conformer except for that with a high degree of crystallinity (>50%). © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44960.     

Effects of molten poly(3-hydroxybutyrate) on crystalline morphology in stereocomplex of poly(L-lactic acid) with poly(D-lactic acid)

Effects of poly(3-hydroxybutyrate) (PHB) on crystalline morphology of stereocomplexing capacity of poly(L- and D-lactic acid) (PLLA and PDLA) were studied by differential scanning calorimetry (DSC), polarizing-light optical microscopy (POM), atomic-force microscopy (AFM) and wide-angle X-ray diffraction (WAXD). When crystallized at high T_c (130 °C or above), morphology transition of stereocomplexed PLA (sc-PLA) occurs from original well-rounded Maltese-cross spherulites to dendritic form in blends of high PHB contents (50 wt.% or higher), where PHB acts as an amorphous species. Microscopy characterizations show that morphology of sc-PLA in PHB/sc-PLA blends crystallized at T_c = 170 °C no longer retain original complexed Maltese-cross well-rounded spherulites; instead, the spherulites are disintegrated and restructured into two types of dendrites: (1) edge-on feather-like dendrites (early growth) and (2) flat-on wedge-like crystal plates (later growth) by growing along different directions and exhibiting different optical brightness. The concentration and/or distribution of amorphous PHB at the crystal growth front, corresponding to variation of the slopes of spherulitic growth rates, is a factor resulting in alteration and restructuring of the sc-PLA spherulites in the blends. Despite of spherulite disintegration, WAXD result shows that these two PHB-induced dendrites still retain the original unit cells of complexes, and thus these two new dendrites are sc-PLA.     

Synthesis,physico-chemical properties and biomedical applications of poly(amidoamine)s

The aim of this article is to provide a comprehensive survey on synthesis, chemical and physico-chemical properties, and applications in several fields, including the biomedical field, of a family of tertiary amino polymers, the poly(amido-amine)s.     

静电纺丝法纺制聚乳酸纳米纤维无纺毡

采用静电纺丝法制备了生物降解聚乳酸(PLLA)纳米纤维无纺毡。分析了纺丝液浓度、电压、接收距离、挤出速度等因素对纤维形态的影响。结果表明:纺丝液的浓度和挤出速度对纤维直径的影响较为明显,溶液挤出速度增大,所得纤维微孔含量及尺寸也增大;适当的电压和接收距离有利于收集无液滴纤维;随着纤维直径的减小,无纺毡的孔径呈减小趋势。在PLLA质量分数为5．7％、挤出速度0．8 mL／h、接受距离 15．5 cm、电压8 kV的静电纺丝条件下,可制备纤维直径为200-400 nm的PLLA纳米纤维无纺毡。     

Thermal and mechanical properties of biodegradable blends of poly(L‐lactic acid) and lignin

The thermal and mechanical properties of the biodegradable blends of poly(L ‐lactic acid) (PLLA) and lignin prepared by manual mixing have been investigated. Both the results of DSC and FTIR spectral analyses indicated the existence of intermolecular interaction between PLLA and lignin. Furthermore, FTIR elucidated the existence of an intermolecular hydrogen‐bonding interaction. Study of the mechanical properties of the blends indicated that the maximum strength and the elongation of the material decreased and the Young's modulus remained almost constant when the lignin content reached 20%. Thermogravimetry indicated that lignin would accelerate the thermal degradation of PLLA when the content of lignin was more than 20%. Compared with some other biodegradable polymers, the blend of PLLA and lignin is thought to be a promising material, because the material properties, as well as the price, are at an acceptable level when the content of lignin is less than about 20%. Copyright © 2003 Society of Chemical Industry