Study of biodegradable and self-expandable PLLA helical biliary stent in vivo and in vitro

Biodegradable stents have advantages for the treatment of benign and malignant biliary stricture, especially eliminating the need for stent removal. In our present work, helical poly-l-lactic acids (PLLA) stent was fabricated and evaluated in vivo and in vitro. For in vivo study, bile duct injury canine models were made by transection of common bile ducts. Duct to duct anastomosis was done with helical PLLA biodegradable stents. Scanning electron microscopy (SEM) and histopathology were performed after three months. For In vitro study, sludge attachment assessment was performed. Polyethylene (PE) and PLLA membranes were immersed in human bile for two months. The samples were taken out and characterized by SEM. Self-expanding property of the helical stent was tested in 37°C water. The results demonstrate that the biodegradable stent had not only good biocompatibility, but also self-clearing effect to clear the attached sludge away. The self-expanding property facilitated stent implantation and also suggested possibility to be implanted endoscopically.     

OPTIMUM SELECTION AND OPERATION OF SLUDGE DEWATERING PROCESSES

The paper presents a life cycle costing model and a mathematical programming model to identify the least cost sludge dewatering process and the optimum polymer dose to be used for sludge dewatering, respectively. The life cycle costing model is based on an infinite horizon and it allows for incremental expansion in the capacities of the sludge dewatering process as demand rises. A new nonlinear formulation for the minimization of the combined costs of polymer dose and transportation is presented. These models were used for the selection of the least cost sludge dewatering process and the determination of the optimum polymer dose for two wastewater treatment plants in Kuwait. These formulations are generic and hence can be used for different types of sludge dewatering processes or polymers. Relationships have also been developed to assess the impact of distances to disposal or reuse site(s) from the wastewater treatment plant location on the optimum polymer dose.     

Fracture micromechanisms of bioabsorbable PLLA/PCL polymer blends

Poly(l-lactide) (PLLA) has actively been used as a biomaterial for resorbable bone fixation devices for use in orthopedic and oral surgeries. Recently, in order to improve the fracture properties of brittle PLLA, polymer blends of PLLA and a ductile bioabsorbable polymer, poly(ε-caprolactone) (PCL), have been developed. The aim of the present study is to elucidate details of the fracture behavior and toughening mechanisms of PLLA/PCL blends. PLLA/PCL blends with different PCL contents were developed, and the critical energy release rate at crack initiation, G_in, was then measured to assess the effect of PCL content. It was shown that G_in is dramatically improved by blending PCL with PLLA, and the maximum 51% of increase of G_in is acheived with 5 wt% of PCL. Polarizing optical microscopy (POM) and scanning electron microscopy (SEM) of crack growth behavior were also performed to characterize the fracture mechanism. PLLA/PCL showed multiple craze formation in the crack-tip region, and elongated fibrils and voids construct the crazes. SEM of fracture surface also indicated that stretched fibril structures are formed on the surface as a result of elongation of PCL spherulites under high tensile stress condition in the crack-tip region. Thus, these damage formations are considered to be the primary energy dissipation mechanisms that resulted in the improvement of fracture energy.     

Porous and dense poly(L-lactic acid) and poly(D,L-lactic acid-co-glycolic acid) scaffolds: <Emphasis Type="Italic">In vitro</Emphasis> degradation in culture medium and osteoblasts culture

The use of bioresorbable polymers as a support for culturing cells has received special attention as an alternative for the treatment of lesions and the loss of tissue. The aim of this work was to evaluate the degradation in cell culture medium of dense and porous scaffolds of poly(L-lactic acid) (PLLA) and poly(D,L-lactic acid-co-glycolic acid) (50:50) (PLGA₅₀) prepared by casting. The adhesion and morphology of osteoblast cells on the surface of these polymers was evaluated. Thermal analyses were done by differential scanning calorimetry and thermogravimetric analysis and cell morphology was assessed by scanning electron microscopy. Autocatalysis was observed in PLGA₅₀ samples because of the concentration of acid constituents in this material. Samples of PLLA showed no autocatalysis and hence no changes in their morphology, indicating that this polymer can be used as a structural support. Osteoblasts showed low adhesion to PLLA compared to PLGA₅₀. The cell morphology on the surface of these materials was highly dispersed, which indicated a good interaction of the cells with the polymer substrate.     

Tailoring impact toughness of poly(L-lactide)/poly(ε-caprolactone) (PLLA/PCL) blends by controlling crystallization of PLLA matrix

Melt blending poly(L-lactide) (PLLA) with various biodegradable polymers has been thought to be the most economic and effective route to toughen PLLA without compromising its biodegradability. Unfortunately, only very limited improvement in notched impact toughness can be achieved, although most of these blends show significant enhancement in tensile toughness. In this work, biodegradable poly(ε-caprolactone) (PCL) was used as an impact modifier to toughen PLLA and a nucleating agent was utilized to tailor the crystallization of PLLA matrix. Depending on the nucleating agent concentrations in the matrix and mold temperatures in injection molding, PLLA/PCL blends with a wide range of matrix crystallinity (10-50%) were prepared by practical injection molding. The results show that there is a linear relationship between PLLA matrix crystallinity and impact toughness. With the increase in PLLA crystalline content, toughening becomes much easier to achieve. PLLA crystals are believed to provide a path for the propagation of shear yielding needed for effective impact energy absorption, and then, excellent toughening effect can be obtained when these crystals percolate through the whole matrix. This investigation provides not only a new route to prepare sustainable PLLA products with good impact toughness but also a fresh insight into the importance of matrix crystallization in the toughening of semicrystalline polymers with a flexible polymer.     

In vitro predegradation at elevated temperatures of poly(lactide)

In this study in vitro predegradation at elevated temperatures, used to obtain an increased degradation rate, was investigated. The in vitro degradation was followed by mass loss, molecular weight loss and changes in thermal properties. Two biodegradable polymers, the homopolymer PLLA and a copolymer PLA96 (96% L4%D lactide), were hydrolytically degraded at 90°C in a phosphate buffered solution. Both polymers, PLLA and PLA96, showed an initial linear degradation rate, but with longer implantation periods the degradation rate decreased and total degradation was best described as an asymptotic. Mass loss of the copolymer PLA96 was twice that of PLLA. The chemical analysis of the in vitro predegraded polymers coincided for both the decrease in molecular weight and the thermal properties with physiologically degraded poly(lactide). The results of this study show that although the degradation temperature is well above the glass transition temperature and not comparable to physiological temperatures, there seems to be good correlation between the in vitro degraded material and physiologically degraded material. In vitro predegradation enables investigation of the entire degradation process of a polymer in a short-term study. Moreover, in vitro predegradation allows direct comparison of the degradation rate of various polymers.     

Tough blends of polylactide and castor oil

Poly(l-lactide) (PLLA) is a renewable resource polymer derived from plant sugars with several commercial applications. Broader implementation of the material is limited due to its inherent brittleness. We show that the addition of 5 wt % castor oil to PLLA significantly enhances the overall tensile toughness with minimal reductions in the modulus and no plasticization of the PLLA matrix. In addition, we used poly(ricinoleic acid)-PLLA diblock copolymers, synthesized entirely from renewable resources, as compatibilizers for the PLLA/castor oil blends. Ricinoleic acid, the majority fatty acid comprising castor oil, was polymerized through a lipase-catalyzed condensation reaction. The resulting polymers contained a hydroxyl end-group that was subsequently used to initiate the ring-opening polymerization of l-lactide. The binary PLLA/castor oil blend exhibited a tensile toughness seven times greater than neat PLLA. The addition of block copolymer allowed for control over the morphology of the blends, and even further improvement in the tensile toughness was realized-an order of magnitude larger than that of neat PLLA.     

Expression of basal lamina components by Schwann cells cultured on poly(lactic acid) (PLLA) and poly(caprolactone) (PCL) membranes

The present in vitro study investigated the expression of basal lamina components by Schwann cells (SCs) cultivated on PCL and PLLA membranes prepared by solvent evaporation. Cultures of SCs were obtained from sciatic nerves from neonatal Sprague Dawley rats and seeded on 24 well culture plates containing the polymer membranes. The purity of the cultures was evaluated with a Schwann cell marker antibody (anti-S-100). After one week, the cultures were fixed and processed for immunocytochemistry by using antibodies against type IV collagen, laminin I and II. Positive labeling against the studied molecules was observed, indicating that such biomaterials positively stimulate Schwann cell adhesion and proliferation. Overall, the present results provide evidence that membrane-derived biodegradable polymers, particularly those derived from PLLA, are able to provide adequate substrate and stimulate SCs to produce ECM molecules, what may have in turn positive effects in vivo, influencing the peripheral nerve regeneration process.     

Poly-l -lactic acid modified by etching and grafting with gold nanoparticles

This work is focused on characterization of plasma treated and consequently etched and grafted biocompatible polymer poly(l-lactide acid) (PLLA). The interaction of biodegradable polymers with cold plasma is of a great importance in a tissue engineering and surface science. Cold plasma exposure, grafting with gold nanoparticles and etching processes were successfully applied to biopolymer substrate. A method for biopolymer nanostructuring as combination of cold plasma treatment and Au nanoparticle grafting for biocompatibility improvement is introduced. Surface roughness, morphology and surface chemistry was determined. The plasma modification leads to significant increase in surface roughness of PLLA and appearance of sharp spikes and ridges on the PLLA surface. Modification by grafting and etching leads to significant changes in PLLA surface morphology and chemistry. The surface ablation of PLLA has been proved to be significant. In etching of plasma-modified PLLA, methanol proves to be stronger etching agent than water. The grafting of PLLA with gold nanoparticles improved mouse embryonic fibroblasts (NIH 3T3) adhesion and proliferation significantly.     

Structure and properties of electrospun PLLA single nanofibres

An electrospinning method was used to spin semi-crystalline poly(L-lactide) (PLLA) nanofibres. Processing parameter effects on the internal molecular structure of electrospun PLLA fibres were investigated by x-ray diffraction (XRD) and differential scanning calorimetry (DSC). Take-up velocity was found as a dominant parameter to induce a highly ordered molecular structure in the electrospun PLLA fibres compared to solution conductivity and polymer concentration, although these two parameters played an important role in controlling the fibre diameter. A collecting method of a single nanofibre by an electrospinning process was developed for the tensile tests to investigate structure-property relationships of the polymer nanofibres. The tensile test results indicated that higher take-up velocity caused higher tensile modulus and strength due to the ordered structure developed through the process.     

Simultaneous time-of-flight secondary ion MS quantitative analysis of drug surface concentration and polymer degradation kinetics in biodegradable poly(L-lactic acid) blends

This paper reports a new quantitative method of analyzing both the earliest stage of degradation of a polymer and the surface concentration of an additive using time-of-flight secondary ion mass spectrometry (TOF-SIMS). The static SIMS spectra of triphenylamine (Ph3N)/poly(L-lactic acid) (PLLA) (20:80 wt %) blend matrixes hydrolyzed in buffered conditions within a short-term (<48 h) period are simultaneously analyzed in the low-mass range for the surface accumulation profile of Ph3N and in the high-mass range to determine the hydrolytic degradation kinetics of PLLA, respectively. The results provide new insight in evaluating the surface concentration of Ph3N (pKb approximately 0) from the blends to see how it relates to the reactions (hydrolytic PLLA degradation) occurring in the surface region in the initial induction period over which negligible loss of polymer weight is observed. The relative PLLA surface degradation at pH 10.0 is approximately 2 times faster than that at pH 7.4. The relative extent of increase in Ph3N surface concentration assayed in pH 10.0 buffer system is 9 times greater than that at pH 7.4. The initial rapid increase in surface concentration of Ph3N is related to but not singularly dependent on the rate of PLLA degradation at the surface of blend matrixes.     

Migration of Acrylic Monomers from Methacrylate Polymers – Establishing Parameters for Migration Modelling

Acrylic acid‐based and methacrylic acid‐based monomers are widely used for the manufacture of polymers, for polymer dispersions or for other specialty resins. Some of these applications cause interactions between the polymer and contact medium such as food contact materials, eyeglasses, contact lenses or toys. More specifically, migration of monomers from the polymer into the contact medium may occur, which needs to be evaluated for safety purposes. The objective of this study was to investigate the basic diffusion properties of acrylic polymers with respect to representative monomers in order to establish a scientific basis for migration modelling simulating the mass transport of monomers from the polymers when they are in contact with foods, human skin or body fluids such as sweat and saliva. For this purpose, 11 representative acrylic polymers containing five different acrylic monomers (MA, EA, BA, MMA and nBMA) were studied in extensive kinetic migrations experiments in contact with five different contact media (simulants) at three different temperatures (20°C, 40°C and 60°C). The simulants were selected according to the applications: toys were simulated by saliva simulant and articles coming in contact with human skin by sweat simulant. For food contact applications, water (aqueous foods), Miglyol 840 (Sasol, Witten, Germany) (fatty food) and Tenax® (Sigma‐Aldrich Corporation, Munich, Germany) (an adsorbent simulating dry foods) were selected. The diffusion coefficients (D) of the monomers in the polymer as well as partition coefficients between polymer and contact media were derived. It was found that those acrylic polymer materials used for rigid plastics applications exhibit extremely low diffusion behaviour, whereas acrylic polymer resins used for coating applications showed somewhat higher diffusion behaviour but this still at very low rates in comparison with other typical polymers used for the manufacture of food packaging materials. As a result, conservative polymer‐specific constants in support of migration modelling were established, and the specifications for the model general applicability were determined and specified. The parameter related to the polymers' intrinsic mobility is applicable to model migration of any other organic chemical substances, which may be present in acrylic polymers as potential migrants when they have comparable or higher molecular weights than the studied monomers. Copyright © 2012 John Wiley & Sons, Ltd.     

The effects of types of degradable polymers on porcine chondrocyte adhesion, proliferation and gene expression

Understanding how a specific biomaterial may influence chondrocyte adhesion, proliferation and gene expression is important in cartilage tissue engineering. In this study several biodegradable polymers that are commonly used in tissue engineering were evaluated with respect to their influence on chondrocyte attachment, proliferation and gene expression. Primary cultures of porcine chondrocytes were performed in films made of poly-L-lactic acid (PLLA), poly-D,L-lactic acid (PDLLA), poly-(lactide-co-glycolide) (PLGA), or polycaprolactone (PCL). Chondrocytes adhered to PDLLA or PLGA after 1-day incubation better than to PLLA or PCL. After 7 or 14 day culture, the cell numbers on PDLLA or PLGA was still higher than PLLA or PCL. The results suggested that cell attachment and growth might depend on degradation rate of biodegradable polymers. Along with the fact that PDLLA or PLGA supported expression of chondrocyte specific genes more than PLLA or PCL, the former two materials seemed to be more suitable for cartilage tissue engineering than the latter ones. Besides, we found that chondrocyte phenotype prior to seeding was important in the expression of ECM proteins.     

Bi-layered constructs based on poly(l-lactic acid) and starch for tissue engineering of osteochondral defects

Articular cartilage has a limited capacity to repair itself, and conventional therapeutic approaches have shown to have limited success as they are deficient and inconsistent in long-term repair. Tissue engineering has shown to be an alternative route to regenerate articular defects. In this work, new bi-layered scaffolds are developed in order to enhance the integration between the engineered cartilage tissue and the corresponding subchondral bone. The concept includes the use of a common polymer in both sides, poly(l-lactic acid), PLLA, to increase the bonding between them, and the use of compression moulding followed by particle leaching to process porous scaffolds with controllable porosities. A compact layer could be observed between the two layers that could be useful for independent cell culturing of the developed osteochondral constructs. A blend of starch and PLLA was used in the cartilage side, which was found to possess adequate hydration capability. For the bone region, where more stiffness and strength was required, PLLA reinforced with hydroxyapatite was used. Preliminary bioactivity tests demonstrated that the bone-layer could induce the formation of a calcium–phosphate layer in vitro, whereas the cartilage layer does not exhibit the ability for calcification.     

Electromechanical stability of semi-crystalline polymer

Semi-crystalline polymers are widely used in manufacturing drivers and capacitors. The two opposite surfaces of semi-crystalline polymer are coated with flexible electrodes and are applied with voltage. Because of static electricity field, semi-crystalline polymer film thins down along the thickness direction while extends along the horizontal direction. Reduction of thickness will lead to a higher electric field, and the positive feedback system has been sustained. When the electric field reaches the critical breakdown of electric field of semi-crystalline polymers, electromechanical coupling system of semi-crystalline polymers becomes unstable. Based on the method on electromechanical instability of semi-crystalline polymers under non-linear field, we use exponential model with two material constants to analyze the electromechanical stability of semi-crystalline polymers. Then, we obtain the relationship among the true critical stress, the true critical electric field of different semi-crystalline polymers and stretching rate. The numerical results show that, when the material ratio C (K₂ = CK₁,K₁ and K₂ are the parameters determined by the experimental stress-strain relationship) of the two material constants increases, the true critical electric field and electromechanical stability of semi-crystalline polymers will decrease. In addition, when C = 0, our result coincides with previous results. It is proved that pre-stretching load can increase the critical breakdown voltage of the polymer. Our conclusions may provide the guidance in designing material and manufacturing semi-crystalline polymer.     

Preparation,crystallization behaviors,and mechanical properties of biodegradable composites based on poly(L-lactic acid) and recycled carbon fiber

The biodegradable composites based on poly(L-lactic acid) (PLLA) and recycled carbon fiber (RCF) were prepared through melting extrusion. The surface-treatment of RCF with silane coupling agent enhanced the interfacial adhesion between RCF and PLLA, and thus the PLLA/RCF composites achieved a significant improvement in mechanical properties. The morphologies of fracture surfaces indicated that the RCF obtained a homogeneous dispersion in PLLA matrix due to a good interfacial interaction. The investigations on the crystallization behaviors and kinetics demonstrated that the RCF acted as a nucleation agent for the crystallization of PLLA, and the crystallization rate and the nucleation density of PLLA matrix were improved remarkably due to the heterogeneous nucleating effect of RCF in the matrix. These features may be advantageous for the enhancement of mechanical properties, heat resistance, and processability of PLLA-based materials. The PLLA-based composites made from RCF can be used as low cost biodegradable materials for many applications.     

Design of novel polymeric adsorbents for metal ion removal from water using computer-aided molecular design

Heavy metals in drinking water act as contaminants that can cause serious health problems. These metal ions in drinking water are generally removed using cation exchange resins that are used as adsorbents. Generally, chelating resins with limited adsorption capacity are commercially available. Manufacturing novel resin polymers with enhanced adsorption capacity of metal ion requires ample experimental efforts that are expensive as well as time consuming. To overcome these difficulties, application of computer-aided molecular design (CAMD) will be an efficient way to develop novel chelating resin polymers. In this paper, CAMD based on group contribution method (GCM) has been used to design novel resins with enhanced adsorption capability of removing heavy metal ions from water. A polymer consists of multiple monomer units that repeat in a polymer chain. Each repeat unit of the polymer can be subdivided into different structural and functional groups. The adsorption mechanism of heavy metals on resin depends on the difference between activities in adsorbents and the bulk fluid phase. The contribution of the functional groups in the adsorption process is found by estimating the activity coefficient of heavy metal in the solid phase and bulk phase using a modified version of the UNIFAC GCM. The interaction parameters of the functional groups are first determined and then they are used in a combinatorial optimization method for CAMD of novel resin polymers. In this work, designs of novel resin polymers for the removal of Cu ions from drinking water are used as a case study. The proposed new polymer resin has an order of magnitude higher adsorption capacity compared to conventional resin used for the same purpose.     

Effect of starch-based biomaterials on the <Emphasis Type="Italic">in vitro</Emphasis> proliferation and viability of osteoblast-like cells

The cytotoxicity of starch-based polymers was investigated using different methodologies. Poly-L-lactic acid (PLLA) was used as a control for comparison purposes. Extracts of four different starch-based blends (corn starch and ethylene vinyl alcohol (SEVA-C), corn starch and cellulose acetate (SCA), corn starch and polycaprolactone (SPCL) and starch and poly-lactic acid (SPLA70) were prepared in culture medium and their toxicity was analysed. Osteoblast-like cells (SaOs-2) were incubated with the extracts and cell viability was assessed using the MTT test and a lactate dehydrogenase (LDH) assay. In addition DNA and total protein were quantified in order to evaluate cell proliferation. Cells were also cultured in direct contact with the polymers for 3 and 7 days and observed in light and scanning electron microscopy (SEM). LDH and DNA quantification revealed to be the most sensitive tests to assess respectively cell viability and cell proliferation after incubation with starch-based materials and PLLA. SCA was the starch blend with higher cytotoxicity index although similar to PLLA polymer. Cell adhesion tests confirmed the worst performance of the blend of starch with cellulose acetate but also showed that SPCL does not perform as well as it could be expected. All the other materials were shown to present a comparable behaviour in terms of cell adhesion showing slight differences in morphology that seem to disappear for longer culture times.The results of this study suggest that not only the extract of the materials but also their three-dimensional form has to be biologically tested in order to analyse material-associated parameters that are not possible to consider within the degradation extract. In this study, the majority of the starch-based biomaterials presented very promising results in terms of cytotoxicity, comparable to the currently used biodegradable PLLA which might lead the biocompatibility evaluation of those novel biomaterials to other studies.     

Coating of collagen on a poly(l-lactic acid) sponge surface for tissue engineering

Porous scaffolds play important roles in tissue engineering. Biodegradable synthetic polymers, such as poly(l-lactic acid) (PLLA), frequently are used in the preparation of porous scaffolds. Pretreating the surface of a PLLA porous scaffold is required to increase its wettability for smooth cell seeding due to the hydrophobic property of the scaffold's surface. In this study, a simple coating method was used to modify the surface of the PLLA sponges. The coating method included three steps: filling the PLLA sponge pores with collagen aqueous solution, centrifuging to remove excess collagen, and, finally, freeze-drying. Compared with the uncoated PLLA sponge, the collagen-coated PLLA sponge demonstrated both improved wettability and high water absorption. Cells were smoothly seeded in the collagen-coated PLLA sponges by dropping a cell suspension solution onto the sponges. Cells adhered to the collagen-coated sponge and were distributed homogeneously throughout the collagen-coated PLLA sponge.     

Use of triethylcitrate plasticizer in the production of poly-L-lactic acid implants with different degradation times

Bioabsorbable materials have been widely used in the repair of damaged tissue as well as in the controlled release of drugs and as a supports for cultured cells. The degradation time of poly-L-(lactic acid) (PLLA) may be controlled by altering the polymer porosity through the addition of the plasticizer triethylcitrate. This in turn influences the extent cellular infiltration. In this study, we examined the degradation of PLLA membranes containing different concentrations of plasticizer. PLLA discs were implanted subcutaneouly in rats and withdrawn 2, 14 and 60 days after implantation. The samples were processed for light microscopy and scanning electron microscopy (SEM). Polymer degradation was proportional to the concentration of plasticizer, indicating that triethylcitrate could affect the degradation time of the implants, without damaging the polymer biocompatibility.