Narrow Results

The large size of the crystallites in poly(L-lactide) and the low growth rate enable detailed time- and temperature-dependent X-ray scattering studies of the ordering processes to be carried out. A layer located intermediate between crystals and melt-like regions is observed which finally takes on crystalline order. Recrystallization processes during heating change the complete stack structure rather than the crystallites individually and produce voids in the stacks. Establishment of a new stable structure after a temperature jump in the melting range can be followed in time. DSC experiments indicate times of melting of the order of minutes.

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.

In this study the response surface methodology (RSM) coupled with the central composite design (CCD) were used to optimize the mechanical strength properties of poly(L-lactide)/multi-walled carbon nanotube (MWCNT) scaffolds. The scaffolds were prepared by the freeze-extraction method. MWCNTs were incorporated into PLLA composite as a reinforcement agent in order to improve the strength properties of the scaffolds. The effect of process parameters such as ratio of PLLA/(PLLA + MWCNT) (93–100%), solvent amount (100–200 ml), freezing time (5–7 h) and immersing time (2–4 days) were studied using the design of experiment (DOE). Based on CCD, quadratic model was obtained and developed to correlate the process parameters to the strength of the scaffolds. An analysis of variance (ANOVA) was applied to determine the significant factors affecting the experimental design response (strength) of the scaffolds. The predicted values after optimization process were in good agreement with the experimental values. The model was able to accurately predict the response of strength with less than 5% error.

High-strength hydroxyapatite (HA)/poly (L-lactide) (PLLA) composites for use as biodegradable fracture-fixation devices have been produced using a new compression molding method. The purpose of this study was to investigate the bone-bonding ability of these HA/PLLA composites. The composites contained 30% by weight of HA particles with a mean diameter of 3 μm. The initial bending strength and flexural elastic modulus of this composite were 280 MP a and 7.8 GP a, respectively. Rectangular plates made of this composite and of unfilled PLLA were implanted into the proximal tibiae of mature rabbits. Detaching tests and histological examinations were performed at 8 and 25 weeks after implantation. The tensile failure loads of the composite and unfilled PLLA were 1.39 kgf and 0.05 kgf at 8 weeks, and 2.60 kgf and 0.25 kgf at 25 weeks, respectively. There were significant differences between the composite and unfilled PLLA during the experiment (p< 0.05 at each point: two-way ANOVA). Histological examination showed that HA/PLLA bonded directly to bone throughout the experimental period, while unfilled PLLA exhibited intervening fibrous tissue between bone and plate. No inflammatory cells were found in the vicinity of either composite or unfilled PLLA plates, even at 25 weeks. These results indicate that the bone-bonding ability of the HA/PLLA composite could enhance fixation stability in the treatment of fractured bones.

The kinetics of ring-opening polymerization of L-lactide initiated by Sn(Oct)2/ triphenylphosphine catalyst system in a co-rotating twin-screw extruder was investigated at a temperature of 150°C - 190 °C and a monomer to initiator molar ratio of 2000:1 - 6000:1. The results showed that the polymerization of L-lactide is first-order in monomer and catalyst concentration. The apparent activation energy is 58.4 kJ/mol, indicating that less energy consumption was needed by using a twin-screw extruder as the polymerization reactor. Therefore, it is suggested that a twin-screw extruder is a promising apparatus for ring-opening polymerization of L-lactide.