A STUDY OF STRUCTURE AND PROPERTY CHANGES OF BIODEGRADABLE POLYGLYCOLIDE AND POLY(GLYCOLIDE-co-LACTIDE)FIBERS DURING PROCESSING AND IN VITRO DEGRADATION

Structure and properties of bioabsorbable polyglycolide (PGA) and poly(glycolide-co-lactide) (PGA-co-PLA)fibers were investigated during several industrial processing stages and in vitro degradation by means of wide-angle X-raydiffraction (WAXD), dynamic mechanical analysis (DMA) and mechanical property tests. In the orientation stage, the PGAfibers were found to have higher degrees of crystallinity than corresponding PGA-co-PLA samples produced under similarconditions. In the hot-stretching and post-annealing stages, after fibers were braided, PGA samples were found to gain morecrystallinity and higher T_g than PGA-co-PLA samples. The higher crystallinity in PGA fibers resulted in a slower rate ofdegradation. DMA results showed that a great deal of internal stress that was built during orientation and hot-stretchingstages was released in the post-annealing stage for a1l PGA and PGA-co-PLA samples. During earlier stages of in vitrodegradation, both PGA and PGA-co-PLA samples exhibited the typical cleavage-induced crystallization mechanism. Theheat shrinkage in the glass transition area was found to disappear after 6-8 days of degradation for all PGA and PGA-co-PLAsamples, indicating the amorphous portions of the polymers lost orientation after a short period in the buffer solution, mostlikely due to relaxation of the cleaved chains.

Bioactivity of bioresorbable composite based on bioactive glass and poly-L-lactide

Bioactive and bioresorbable composite was fabricated by a solvent evaporation technique using poly-L-lactide(PLLA) and bioactive glass (average particle size: 6.8 μm). Bioactive glass granules are homogeneously distributed in the composite with microcrack structure. The formation of hydroxyapatite(HA) on the composite in simulated body fluid(SBF) was analyzed by scanning electron microscopy(SEM), energy dispersive spectroscopy(EDS), X-ray diffraction(XRD), and Raman spectra. Rod-like HA crystals deposit on the surface of PLLA/bioactive glass composite after soaking for 3 d. Both rod-like crystals and HA layer form on the surface for 14 d in SBF. The high bioactivity of PLLA/bioactive glass composite indicates the potential of materials for integration with bone.

Poly(L-lactide)-Poly(ethylene glycol) Multiblock Copolymers: Synthesis and Properties

Poly (L-lactide)-poly(ethylene glycol) multiblock copolymers with predetermined block lengths were synthesized by polycondensation of PLA diols and PEG diacids. These copolymers presented special properties, such as better miscibility between the two components, low crystallinity and better hydrophilicity, which can be modulated by adjusting the block lengths of the two components.

NON-ISOTHERMAL CRYSTALLIZATION OF POLY(L-LACTIDE)(PLLA) UNDER QUIESCENT AND STEADY SHEAR CONDITIONS

The non-isothermal crystallization of poly（L-lactide） （PLLA） under quiescent and steady shear flow conditions was in situ investigated by using polarizing optical microscopy （POM） with a hot shear stage and wide-angle X-ray diffraction （WAXD）. The shear rate and the cooling rate both play a significant role in the final crystalline morphology and crystallinity. Under quiescent conditions, the morphology assumes different sized spherulites, and its crystallinity dramatically reduces with increasing the cooling rate. On the other hand, the shear flow increases the onset crystallization temperature, and enhances the final crystallinity. When the shear rate is above 5 s^-1, cylindrite-like crystals are observed, furthermore, their content depends on the cooling rate.

Preparation of high viscosity average molecular mass poly-L-lactide

Poly-L-lactide(PLLA) was synthesized by ring-opening polymerization from high purity L-lactide with tin octoate as initiator, and characterized by means of infrared, and 1H-nuclear magnetic resonance. The influences of initiator concentration, polymerization temperature and polymerization time on the viscosity average molecular mass of PLLA were investigated. The effects of different purification methods on the concentration of initiator and viscosity average molecular mass were also studied. PLLA with a viscosity average molecular mass of about 50.5×104 was obtained when polymerization was conducted for 24 h at 140 ℃ with the molar ratio of monomer to purification initator being 12 000. After purification, the concentration of tin octoate decreases; however, the effect of different purification methods on the viscosity average molecular mass of PLLA is different, and the obtained PLLA is a typical amorphous polymeric material. The crystallinity of PLLA decreases with the increase of viscosity average molecular mass.

Different crystallinity of poly(d,l-lactide-co-p-dioxanone) copolymers acquired by control of chain microstructure

Poly（d,l-lactide-co-p-dioxanone） （P（LA-co-PDO）） copolymers with different chain microstructures were synthesized by onestep or two-step bulk ring-opening polymerizations of d,l-lactide （LA） and p-dioxanone （PDO） monomers using stannous octoate [Sn（Oct）2]/n-dodecanol as the initiating system. The average sequence lengths of the lactidyl （LLA） and dioxanyl （LpDo） units were calculated from the ^1H NMR spectra. It was found that both LLA and Lpoo values from the two-step syntheses were significantly longer than those from the corresponding one-step syntheses, indicating more blocky structure achieved for the twostep copolymers. Corresponding to this difference in microstructure, the two-step copolymers were semi-crystalline, while the one-step copolymers were completely amorphous. In conclusion, the crystallinity of P（LA-co-PDO） copolymers could be adjusted conveniently to meet specific applications by changing the microstructure of the copolymers via different polymerization routes.

RING-OPENING COPOLYMERIZATION OF DL-LACTIDE AND POLY(ETHYLENE GLYCOL) INITIATED BY LANTHANUM ACETATE AND APPLICATION OF THE COPOLYMER AS MATRIX OF MICROSPHERES CONTAINING VIBRIO CHOLERA ANTIGENS

Poly-dl-lactide-poly(ethylene glycol) (PELA) triblock copolymers were synthesized with lanthanum acetate as the initiator. PELA microspheres with entrapped Vibrio Cholera antigen and outer membrane protein (OMP) were prepared by a double emulsion W/O/W based on solvent extraction methods. The obtained microspheres showed smooth and spherical surface and their size varied between 0.5 and 5.0 mu m, which are suitable for oral targeting delivery system. The distribution tests in rabbits and mice through scanning electronic micrography and fluorescence microscope indicated that microspheres have successfully reached the immunization-related tissues, such as the liver, spleen and intestinal peyer's patches, following oral administration. The PELA microspheres were also evaluated as an efficient antigen delivery system by enhancing a higher protective ratio against live Vibrios Cholera.

Silk Fibroin/Poly-L-lactide Lactone Bi-layered Membranes for Guided Bone Regeneration

After removal of the caries or diseased teeth,the alveolar ridge will undergo absorption and atrophy.When the amount of alveolar bone is insufficient,it will cause an inability to perform effective dental implant restoration.In order to control the absorption and promote the repair and regeneration of alveolar ridge,a method of implanting guided bone regeneration(GBR)membranes at the extraction site is often used.In this study,silk fibroin(SF)and poly-L-lactide lactone(PLCL)were used to prepare bilayered guided bone regeneration membranes,and its morphology,hydrophilicity,surface roughness and mechanical properties were studied.At the same time,the drug release behaviors and cell compatibility of the bilayered membranes were studied.The results showed that SF/PLCL bi-layered membranes had good mechanical properties and surface hydrophilicity,and the drug-loaded bi-layered membranes had good cell compatibility.The bilayered membranes fabricated in this study are of potential for applying in the oral health field to promote bone regeneration.

Biodegradation of Absorbable Hydroxyapatite/Poly-DL-lactide Composites in Different Environment

To develop a new generation of absorbable fracture fixation devices with enhanced biocompatibility, the biodegradation mechanism and its influence on the cellular response at the tissue/implant interface of hydroxyapatite/ poly DL lactide (HA/PDLLA) composites were investigated in vitro and in vivo.HA/PDLLA rods were immersed in phosphate buffered saline,or implanted in muscle and bony tissue for 52 weeks.Scanning electron microscopic and histological studies were done.The degradation rate was the slowest in vitro,slower in muscle tissue and fast in bone.In vitro, the composites degraded heterogeneously and a hollow structure was formed.In bone,the limited clearing capacity leads to the accumulation of oligomeric debris,which contribute totally to the autocatalytic effect.So,the fastest degradation and intense tissue response were seen.In muscle tissue,oligomeric debris migrated into vicinal fibers over a long distance from the original implant cavity and the tissue reactions were,however, quite moderate.For the same size organic/inorganic composite,the environment where it was placed is the major factor in determining its biodegradation process and cellular reaction.In living tissue,factors such as cells,enzymes and mechanical stress have an obvious influence on the biodegradation and biological process at the tissue/implant interface.The biocompatibility of the HA/PDLLA composites is enhanced with the incorporating of the resorbable HA microparticles.

MISCIBILITY AND CRYSTALLIZATION BEHAVIOR OF BIODEGRADABLE BLEND OF POLY(β—HYDROXYBUTYRATE)AND POLY(D,L—LACTIDE)—CO—POLY(ETHYLENE GLYCOL)

Studies on the miscibility of PHB/PELA blends showed that PHB and PELA were miscible in amorphous state.The crystallization behavior of PHB in the blend was strongly de- pendent on the addition of PELA component.

MISCIBILITY AND MORPHOLOGICAL STRUCTURE OF BIODEGRADABLE BLEND OF POLY(β—HYDROXYBUTYRATE)AND POLY(D,L—LACTIDE)

The poly(β-hydroxybutyrate)/poly(d,1-lactide)(PHB/PLA)blend was found to be immiscible,the melting point and the phase crystallinity of PHB were independent of the blend composition.Comparing with plain PHB,the blend exhibited the crystallization change and a certain improvement of the mechanical property.

Poly (L-lactide-co-e-caprolactone)/Functionalized CNTs Nanocomposite,Thermal and Mechanical Properties

Multiwalled carbon nanotubes(MWCNTs) grafted with poly(L-lactide-e-caprolactone)(PCLA) were synthesized by ring opening polymerization reaction and used as a reinforcement for neat PCLA.Scanning electron microscopy(SEM) revealed that the applied tensile load on the composite was transferred to the MWNT-OH-g-PCLA,loading to a strain failure of the MWCNTs rather than an adhesive failure between the MWCNTs and the matrix.

Preparation and characterization of biodegradable nanoparticles from methoxy poly(ethylene glycol)-poly(D,L-lactide)block copolymers as novel drug carriers

Methoxy poly(ethylene glycol)-poly(D,L-lactide) block copolymers (PEG-PLA) were prepared through ring-opening polymerization.The oil in water suspension method was used to prepare block copolymer micelles. The critical micelle concentration (CMC) by fluorescence spectroscopy was 0.0056 mg·ml -1 . The physical state of the inner core region of micelles was characterized with 1HNMR. The size of indomethacin (IMC) loaded micelles measured by dynamic light scattering (DLS) showed narrow monodisperse size distribution and the average diameters were less than 50 nm. In addition, the nanoparticles with relatively high drug loading content (DLC) were obtained.

NOVEL COMPOSITES OF POLY(L-LACTIDE)AND SURFACE MODIFIED BIOACTIVE SiO_2-CaO-P_2O_5 GEL NANOPARTICLES:MECHANICAL AND BIOLOGICAL PROPERTIES

Bioactive SiO2-CaO-P2O5 gel (BAG) nanoparticles with 40 nm in diameter were synthesized by the sol-gel route and further modified via the ring-opening polymerization of lactide on the surface of particles. Surface modified BAG (mBAG) was introduced in poly(L-lactide) (PLLA) matrix as bioactive filler. The dispersibility of mBAG in PLLA matrix was much higher than that of rough BAG particles. Tensile strength of the mBAG/PLLA composite could be increased to 61.2 MPa at 2 wt% filler content from 53.4 MPa for pure PLLA. The variation of moduli of the BAG/PLLA and mBAG/PLLA composites always showed an enhancement tendency with the increasing content of filler loading. The SEM photographs of the fracture surfaces showed that mBAG could be homogeneously dispersed in the PLLA matrix, and the corrugated deformation could absorb the rupture energy effectively during the breaking of materials. In vitro bioactivity tests showed that both BAG and mBAG particles could endow the composites with ability of the calcium sediment in SBF, but the surface modification of BAG particles could weaken this capability to some extent. Biocompatibility tests showed that both BAG and mBAG particles could facilitate the attachment and proliferation of the marrow cells on the surface of the composite. All these results showed that the mBAG/PLLA composite would be a promising material for bone tissues regeneration.

THE RING-OPENING POLYMERIZATION OF D,L-LACTIDE WITH ONE COMPONENT RARE EARTH CATALYST

(D, L)-Lactide (LA) was first polymerized with one component of rare earth catalysts [Nd(naph)(3), Nd(oct)(3), Nd(O-iPr)(3), Nd(AcAc)(3), Y(AcAc)(3), Sm(AcAc)(3), Er(AcAc)(3))] respectively in solution and in melt state. The effects of [Cat]/[La] molar ratio, solvents, polymerization time, temperature, various rare earth Elements and ligands were investigated in detail. The results showed that both the conversion of polymerization and the molecular weight (MW) of poly (D, L-Lactide) (PLA) in melt polymerization are higher than that in solution polymerization, but the polymerization rate in melt was lower than in solution. The molecular weight distribution (MWD) of PLA is broader with increasing temperature. X-ray study indicated that PLA obtained by Nd(AcAc)(3) in melt polymerization is an amorphous polymer.

Synthesis and Characterization of Poly(L-lactide-co-glycolide)

Poly （ l- lactide- co-glycolide ） （ PLGA ） with different compositions was prepared using stannous octaoate as catalyst by bulk ring-opening copolymerization of l-lactide and glycolide. The structure and properties of the PLGA copolymers were cbaracterized by means of attenuated total reflectance-Fourier transform infrared （ATR-FTIR）, ^1H NMR, differential scanning calorimeter （ DSC and X-ray diffraction （XRD） methods, The experimented results indicate that the synthetic conditions and the composition of copolymers have art obvious influence on the structure of PLGA copolymers, The degradation rate of eopolymers increased with the increasing of the glycolide component in the copolymers.

INVESTIGATION OF POLYDL-LACTIDE-b-POLY(ETHYLENE GLYCOL)-b-POLYDL-LACTIDE MICROSPHERES CONTAINING PLASMID DNA

PolyDL-lactide (PDLLA) and the block copolymer, polyDL-lactide-b-poly(ethylene glycol)-b-polyDL-lactide (PELA) were used as the microsphere matrix to encapsulate plasmid DNA. The PDLLA, PELA, pBR322-loaded PDLLA and pBR322-loaded PELA microspheres were prepared by solvent extraction method based on the formation of multiple w1/o/w2 emulsion. The microspheres were characterized by surface morphology, mean particle size, particle size distribution and loading efficiency. The integrity of DNA molecules after being extracted from microspheres was determined by agarose gel electrophoresis. The result suggested that plasmid DNA molecules could retain their integrity after being encapsulated by PELA. The PELA microspheres could prevent plasmid DNA from being digested by DNase. The in vitro degradation and release profiles of plasmid DNA-loaded microspheres were measured in pH = 7.4 buffer solution at 37 °C. The in vitro degradation profiles of the microspheres were evaluated by the deterioration in microspheres surface morphology, the molecular weight reduction of polymer, the mass loss of microspheres, the changes of pH values of degradation medium, and the changes of particle size. The in vitro release profiles of the microspheres were assessed by measurement of the amount of DNA presented in the release medium at determined intervals. The release profiles were correlation with the degradation profiles. The release of plasmid DNA from PELA microspheres showed a similar biphasic trend, that is, an initial burst release was followed by a slow, but sustained release.

Preparation of poly-L-lactide/bioactive glass composite and evaluation of cytotoxicity in vitro

Bioactive and bioresorbable composite materials were fabricated from poly-L-lactide and bioactive glass (average particle size 6.8 μm) by a solvent evaporation technique.Cellular cultivation in vitro and MTT assay were conducted for evaluating the influence on morphology,growth and proliferation of cultured fibroblasts.The results of cytotoxicity testing show that cells cultured in extracts of PLLA/BG and on the surface of composites demonstrate normal growth and proliferation.The bioactive glass in PLLA composite facilitates both adhesion and proliferation of rat fibroblasts on PLLA/bioactive glass composite film.

Surface-modified Biphasic Calcium Phosphate/Poly (L-lactide) Biocomposite

Biphasic calcium phosphate （BCP） powders were prepared by hydrolyzation proc-ess and surface-modified by directly grafted L-lactide （LLA） onto the surface of BCP through a chemical linkage. The grafting ratio of organic groups was 9 wt%. After surface modification, the surface of BCP powders was covered by the lamella-shaped crystal. Poly （L-lactide） was mixed with BCP to form the BCP/PLLA biocomposite. Modified BCP （mBCP） particles could be uniformly dis-persed in PLLA matrix. The compressive strength of the mBCP/PLLA composite is 115 MPa, 28% higher than that of unmodified-BCP/PLLA composite. The improved mechanical strength is attributed to the enhanced adhesion between the inorganic BCP filler and the organic PLLA matrix.

Toughening of Poly (L-lactide) with Branched Multiblock Poly (ε-caprolactone)/poly (D-lactide) Copolymers

A total biodegradable elastomer,branched multiblock poly( ε-caprolactone)/poly( D-lactide)( BMCD) was prepared using 3-isocyanatopropyltriethoxysilane( IPTS) as a coupling agent.To improve the toughness of poly( L-lactide)( PLLA),PLLA/BMCD blends were prepared via a simple solvent evaporation method at various BMCD loadings. Tensile test showed that the elongation at break of PLLA blends increased to 50. 97% and104. 55% at the loadings of 5% and 7%( mass fraction) BMCD respectively, with no sacrifice of their biodegradability. This approach allowed for simultaneous control of mechanical and biodegradable properties of PLLA with a few additives in actual production. Furthermore, UV-VIS test showed that the light transmittance of the films at the loadings of 5%( mass fraction)BMCD was almost the same as pure PLLA at 400 nm.