Change in Pull-Out Force during Resorption of Magnesium Compression Screws for Osteosynthesis of Mandibular Condylar Fractures

Background: Magnesium has been used as degradable fixation material for osteosynthesis, but it seems that mechanical strength is still a current issue in these fixations. The aim of this study was to evaluate the axial pull-out force of compression headless screws made of magnesium alloy during their resorption. Methods: The tests included screws made for osteosynthesis of the mandible head: 2.2 mm diameter magnesium alloy MgYREZr (42 screws) and 2.5 mm diameter polylactic-co-glycolic acid (PLGA) (42 pieces, control). The screws were resorbed in Sørensen’s buffer for 2, 4, 8, 12, and 16 weeks, and force was measured as the screw was pulled out from the polyurethane block. Results: The force needed to pull the screw out was significantly higher for MgYREZr screws than for PLGA ones (p < 0.01). Within eight weeks, the pull-out force for MgYREZr significantly decreased to one third of its initial value (p < 0.01). The dynamics of this decrease were greater than those of the pull-out force for PLGA screws (p < 0.05). After these eight weeks, the values for metal and polymer screws equalized. It seems that the described reduction of force requires taking into account when using magnesium screws. This will provide more stable resorbable metallic osteosynthesis.     

Quaternized chitosan-coated nanofibrous implants loaded with gossypol prepared by electrospinning and their efficacy against Graffi myeloid tumor

Nanofibrous poly(L-lactide-co-D,L-lactide) (coPLA) or coPLA/poly(ethylene glycol) implants loaded with plant polyphenolic compound gossypol (GOS) with anti-tumor activity were fabricated by electrospinning. Implants containing quaternized chitosan (QCh) were prepared by coating of the obtained fibrous materials with a thin film of cross-linked QCh. The morphology of the implants and chemical composition of the implant surface were studied by means of scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). In vitro cytotoxicity assay showed that GOS-loaded nanofibrous implants, both non-coated and QCh-coated displayed about two-fold higher inhibitory activity against Graffi tumor cells than that of free GOS at the 72nd h of incubation. As evidenced by the performed fluorescence microscopy analyses and SEM observations, the anti-tumor activity of the fibrous implants was mainly due to induction of apoptosis. The experiments in which the implants containing both QCh and GOS were placed locally into the tumor site after the tumor extirpation showed an increase in the survival rate and a lower rate of recurrence in the operative field and of metastases in regional lymph nodes. In this case, 40% of hamsters were alive on the 45th day of implantation and they did not show any clinical sign of recurrence in the operative field and metastases in the regional lymph nodes.     

Tuning of Sol–Gel Transition in the Mixed Polymer Micelle Solutions of Copolymer Mixtures Consisting of Enantiomeric Diblock and Triblock Copolymers of Polylactide and Poly(ethylene glycol)

Poly(ethylene glycol) (PEG) is partially furanylated with different feed ratios of furfuryl isocyanate and used as the macro initiator of ring‐opening polymerization of l ‐ and d ‐lactides to synthesize copolymer mixtures of furan‐terminated AB diblock and ABA triblock copolymers (poly(oxyethylene)–poly(l ‐lactide)/poly(l ‐lactide)–poly(oxyethylene)–poly(l ‐lactide) and poly(oxyethylene–poly(d ‐lactide)/poly(d ‐lactide)–poly(oxyethylene)–poly‐(d ‐lactide)) having different diblock/triblock ratios. The mixed micelle solutions of these enantiomeric copolymer mixtures undergo sol‐to‐gel or gel‐to‐sol transition depending on the diblock/triblock ratio of the copolymer mixtures. The rheological properties of the mixed micelle solutions could also be controlled by changing the diblock/triblock ratios or the initial furanylation ratio of PEG.

     

Effects of chitosan and bioactive glass modifications of knitted and rolled polylactide‐based 96/4 L/D scaffolds on chondrogenic differentiation of adipose stem cells

The performance of biodegradable knitted and rolled 3‐dimensional (3D) polylactide‐based 96/4 scaffolds modified with bioactive glass (BaG) 13‐93, chitosan and both was compared with regard to the viability, proliferation and chondrogenic differentiation of rabbit adipose stem cells (ASCs). Scaffold porosities were determined by micro‐computed tomography (μCT). Water absorption and degradation of scaffolds were studied during 28‐day hydrolysis in Tris‐buffer. Viability, number and differentiation of ASCs in PLA96/4 scaffolds were examined in vitro. The dimensions of the scaffolds were maintained during hydrolysis and mass loss was detected only in the BaG13‐93 containing scaffolds. ASCs adhered and proliferated on each scaffold type. Cell aggregation and expression of chondral matrix components improved in all scaffold types in chondrogenic medium. Signs of hypertrophy were detected in the modified scaffolds but not in the plain PLA96/4 scaffold. Chondrogenic differentiation was most enhanced in the presence of chitosan. These findings indicate that the plain P scaffold provided a good 3D‐matrix for ASC proliferation whereas the addition of chitosan to the PLA96/4 scaffold induced chondrogenic differentiation independent of the medium. Accordingly, a PLA96/4 scaffold modified by chitosan could provide a functional and bioactive basis for tissue‐engineered chondral implants. Copyright © 2012 John Wiley & Sons, Ltd.     

聚乙二醇-柠檬酸共聚物(PEGCA)对聚乳酸的增韧及其机理

通过缩聚反应制备聚乙二醇-柠檬酸共聚物(PEGCA),通过密炼制备了PEGCA与聚乳酸(PLA)的共混物,研究了PEGCA对共混物的形貌结构、热性能和力学性能的影响。结果表明,PEGCA与PLA部分相容,PEGCA呈球状颗粒分散在PLA基体中。PEGCA对PLA有良好的增韧效果,当PEGCA质量分数为15%时,材料的断裂伸长率提高到327.9%,冲击强度提高到63.0 J/m。同时利用空穴化理论解释PEGCA增韧PLA的机理,用临界韧带厚度理论和J积分定量描述增韧的效果。     

Porous polylactide/β‐tricalcium phosphate composite scaffolds for tissue engineering applications

Porous polylactide/β‐tricalcium phosphate (PLA/β‐TCP) composite scaffolds were fabricated by freeze‐drying. The aim of this study was to characterize these graded porous composite scaffolds in two different PLA concentrations (2 and 3 wt%). Also, three different β‐TCP ratios (5, 10 and 20 wt%) were used to study the effect of β‐TCP on the properties of the polymer. The characterization was carried out by determining the pH, weight change, component ratios, thermal stability, inherent viscosity and microstructure of the scaffolds in 26 weeks of hydrolysis. This study indicated that no considerable change was noticed in the structure of the scaffolds when the β‐TCP filler was added. Also, the amount of β‐TCP did not affect the pore size or the pore distribution in the scaffolds. We observed that the fabrication method improved the thermal stability of the samples. Our results suggest that, from the structural point of view, these scaffolds could have potential for the treatment of osteochondral defects in tissue engineering applications. The porous bottom surface of the scaffold and the increased osteogenic differentiation potential achieved with β‐TCP particles may encourage the growth of bone cells. In addition, the dense surface skin of the scaffold may inhibit the ingrowth of osteoblasts and bone tissue, while simultaneously encouraging the ingrowth of chondrocytes. Copyright © 2010 John Wiley & Sons, Ltd.     

均聚物PS共混对PI-PS-PLA薄膜中微纳米结构的影响

将均聚物聚苯乙烯（PS）与三嵌段共聚物聚异戊二烯聚苯乙烯聚乳酸（PIPSPLA）以一定摩尔比共混,采用原子力显微镜（AFM）研究了均聚物浓度对其薄膜样品中微纳米结构的影响。结果表明：当n(PS)∶n(PIPSPLA)为1∶2和1∶1时,薄膜呈现出规则排列的柱状微结构,且柱状微区的尺寸较混合前的样品有所改变;当n(PS)∶n(PIPSPLA)为2∶1时,PS与嵌段共聚物PIPSPLA的分子链发生了相分离,其中均聚物PS相形成了较大尺寸的岛状微区。     

Influence of polymer molecular weight in osteoinductive composites for bone tissue regeneration

In bone tissue regeneration, certain polymer and calcium-phosphate-based composites have been reported to enhance some biological surface phenomena, facilitating osteoinduction. Although the crucial role of inorganic fillers in heterotopic bone formation by such materials has been shown, no reports have been published on the potential effects the polymer phase may have. The present work starts from the assumption that the polymer molecular weight regulates the fluid uptake, which determines the hydrolysis rate and the occurrence of biological surface processes. Here, two composites were prepared by extruding two different molecular weight l/d,l-lactide copolymers with calcium phosphate apatite. The lower molecular weight copolymer allowed larger fluid uptake in the composite thereof, which was correlated with a higher capacity to adsorb proteins in vitro. Further, the large fluid absorption led to a quicker composite degradation that generated rougher surfaces and enhanced ion release. Following intramuscular implantation in sheep, only the composite with the lower molecular weight polymer could induce heterotopic bone formation. Besides influencing the biological potential of composites, the molecular weight also regulated their viscoelastic behaviour under cyclic stresses. The results lead to the conclusion that designing biomaterials with appropriate physico-chemical characteristics is crucial for bone tissue regeneration in mechanical load-bearing sites.     

In Situ Aggregates of Enantiomeric Poly(styrene)‐block‐Poly(lactide) Diblock Copolymers via Stereocomplexation in a Non‐selective Solvent

A comprehensive investigation of in situ aggregation of structurally well‐defined enantiomeric poly(styrene)‐block‐poly(lactide) (PS‐b‐PLLA and PS‐b‐PDLA) in a non‐selective solvent, tetrahydrofuran (THF), is presented. The isolated aggregates are found to form poly(L ‐lactide) (PLLA)/poly(D ‐lactide) (PDLA) racemic crystals by differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), and Fourier transform infrared (FTIR) spectroscopy. The kinetic study reveals that the growth rate of the aggregates depends on the molecular weight of the enantiomeric PLA blocks, as well as the preparation conditions. The proposed mechanism demonstrates a new PS (shell)–PLA (core) structural hierarchy solely driven by stereocomplexation between enantiomeric PLLA and PDLA blocks.

     

In vitro biocompatibility evaluation of bioresorbable copolymers prepared from l-lactide, 1, 3-trimethylene carbonate,and glycolide for cardiovascular applications

PLLA-TMC-GA terpolymer was prepared by ring-opening polymerization of l-lactide, 1, 3-trimethylene carbonate (TMC), and glycolide (GA). The biocompatibility of terpolymer was evaluated in comparison with PLLA and PLLA-TMC with the aim of assessing their potential in the development of bioresorbable cardiovascular stents. Various aspects of in vitro biocompatibility were considered, including MTT assay, hemolytic test, dynamic clotting time, platelet adhesion, platelet activation, protein adsorption, plasma recalcification time and release of cytokines. The results revealed that the terpolymer presents good cytocompatibility and hemocompatibility. Moreover, no significant increase in the release of cytokines was detected. It is thus concluded that these polymers, in particular PLLA-TMC-GA terpolymer present good biocompatibility for cardiovascular applications.