Poly(d,l-Lactic acid) Composite Foams Containing Phosphate Glass Particles Produced via Solid-State Foaming Using CO2 for Bone Tissue Engineering Applications.

This study reports on the production and characterization of highly porous (up to 91%) composite foams for potential bone tissue engineering (BTE) applications. A calcium phosphate-based glass particulate (PGP) filler of the formulation 50P2O5-40CaO-10TiO2 mol.%, was incorporated into biodegradable poly(d,l-lactic acid) (PDLLA) at 5, 10, 20, and 30 vol.%. The composites were fabricated by melt compounding (extrusion) and compression molding, and converted into porous structures through solid-state foaming (SSF) using high-pressure gaseous carbon dioxide. The morphological and mechanical properties of neat PDLLA and composites in both nonporous and porous states were examined. Scanning electron microscopy micrographs showed that the PGPs were well dispersed throughout the matrices. The highly porous composite systems exhibited improved compressive strength and Young's modulus (up to >2-fold) and well-interconnected macropores (up to ~78% open pores at 30 vol.% PGP) compared to those of the neat PDLLA foam. The pore size of the composite foams decreased with increasing PGPs content from an average of 920 µm for neat PDLLA foam to 190 µm for PDLLA-30PGP. Furthermore, the experimental data was in line with the Gibson and Ashby model, and effective microstructural changes were confirmed to occur upon 30 vol.% PGP incorporation. Interestingly, the SSF technique allowed for a high incorporation of bioactive particles (up to 30 vol.%-equivalent to ~46 wt.%) while maintaining the morphological and mechanical criteria required for BTE scaffolds. Based on the results, the SSF technique can offer more advantages and flexibility for designing composite foams with tunable characteristics compared to other methods used for the fabrication of BTE scaffolds.

Toughening mechanisms in interfacially modified HDPE/thermoplastic starch blends.

The mechanical behavior of polymer blends containing 80 wt% of HDPE and 20 wt% of TPS and compatibilized with HDPE-g-MA grafted copolymer was investigated. Unmodified HDPE/TPS blends exhibit high fracture resistance, however, the interfacial modification of those blends by addition of HDPE-g-MA leads to a dramatic drop in fracture resistance. The compatibilization of HDPE/TPS blends increases the surface area of TPS particles by decreasing their size. It was postulated that the addition of HDPE-g-MA induces a reaction between maleic anhydride and hydroxyl groups of the glycerol leading to a decrease of the glycerol content in the TPS phase. This phenomenon increases the stiffness of the modified TPS particles and stiffer TPS particles leading to an important reduction in toughness and plastic deformation, as measured by the EWF method. It is shown that the main toughening mechanism in HDPE/TPS blends is shear-yielding. This article demonstrates that stiff, low diameter TPS particles reduce shear band formation and consequently decrease the resistance to crack propagation.

Osteoblastic differentiation under controlled bioactive ion release by silica and titania doped sodium-free calcium phosphate-based glass.

Sodium-free phosphate-based glasses (PGs) doped with both SiO2 and TiO2 (50P2O5-40CaO-xSiO2-(10-x)TiO2, where x=10, 7, 5, 3, and 0mol%) were developed and characterised for controlled ion release applications in bone tissue engineering. Substituting SiO2 with TiO2 directly increased PG density and glass transition temperature, indicating a cross-linking effect of Ti on the glass network which was reflected by significantly reduced degradation rates in an aqueous environment. X-ray diffraction confirmed the presence of Ti(P2O7) in crystallised TiO2-containing PGs, and nuclear magnetic resonance showed an increase in Q(1) phosphate species with increasing TiO2 content. Substitution of SiO2 with TiO2 also reduced hydrophilicity and surface energy. In biological assays, MC3T3-E1 pre-osteoblasts effectively adhered to the surface of PG discs and the incorporation of TiO2, and hence higher stability of the PG network, significantly increased cell viability and metabolic activity indicating the biocompatibility of the PGs. Addition of SiO2 increased ionic release from the PG, which stimulated alkaline phosphatase (ALP) activity in MC3T3-E1 cells upon ion exposure. The incorporation of 3mol% TiO2 was required to stabilise the PG network against unfavourable rapid degradation in aqueous environments. However, ALP activity was greatest in PGs doped with 5-7mol% SiO2 due to up-regulation of ionic concentrations. Thus, the properties of PGs can be readily controlled by modifying the extent of Si and Ti doping in order to optimise ion release and osteoblastic differentiation for bone tissue engineering applications.

A new interface element with progressive damage and osseointegration for modeling of interfaces in hip resurfacing.

Finite element models of orthopedic implants such as hip resurfacing femoral components usually rely on contact elements to model the load-bearing interfaces that connect bone, cement and implant. However, contact elements cannot simulate progressive degradation of bone-cement interfaces or osseointegration. A new interface element is developed to alleviate these shortcomings. This element is capable of simulating the nonlinear progression of bone-cement interface debonding or bone-implant interface osseointegration, based on mechanical stimuli in normal and tangential directions. The new element is applied to a hip resurfacing femoral component with a stem made of a novel biomimetic composite material. Three load cases are applied sequentially to simulate the 6-month period required for osseointegration of the stem. The effect of interdigitation depth of the bone-cement interface is found to be negligible, with only minor variations of micromotions. Numerical results show that the biomimetic stem progressively osseointegrates (alpha averages 0.7 on the stem surface, with spot-welds) and that bone-stem micromotions decrease below 10 microm. Osseointegration also changes the load path within the femoral bone: a decrease of 300 microepsilon was observed in the femoral head, and the inferomedial part of the femoral neck showed a slight increase of 165 microepsilon. There was also increased stress in the implant stem (from 7 to 11 MPa after osseointegration), indicating that part of the load is supported through the stem. The use of the new osseointegratable interface element has shown the osseointegration potential of the biomimetic stem. Its ability to model partially osseointegrated interfaces based on the mechanical conditions at the interface means that the new element could be used to study load transfer and osseointegration patterns on other models of uncemented hip resurfacing femoral components.

A non-damaging chemical amination protocol for poly(ethylene terephthalate) - application to the design of functionalized compliant vascular grafts.

Bioengineering approaches have been intensively applied to create small diameter vascular grafts using artificial materials. However, a fully successful, high performing and anti-thrombogenic structure has not been achieved yet. In this study, we present the first step of a process aiming at biofunctionalizing previously designed compliant polyethylene terephthalate (PET) scaffolds (Moreno et al., 2011). The main challenge of such a surface modification is to prevent the bulk polymer from any damage, so that it preserves the mechanical properties that the structures have been designed for. In that endeavor, an aminated long-chain polymer (polyvinylamine, PVAm) was used as an aminolysis reagent to get amine (-NH2) moieties only on the very surface of PET. Different reaction conditions were assayed, leading to a large range of amino group densities associated with slight variations of the planar tensile properties. These results were in stark contrast with those generated with a common small diamine substrate (ethylenediamine, EtDA), as the latter yielded a strong degradation of the mechanical properties for comparable amine densities. Tubular mechanical assays were then carried out on PVAm-functionalized PET scaffolds. The latter showed a compliance match with arteries under the chosen reaction conditions, as initially observed for pristine PET tubular scaffolds.

Effect of phosphate-based glass fibre surface properties on thermally produced poly(lactic acid) matrix composites.

Incorporation of soluble bioactive glass fibres into biodegradable polymers is an interesting approach for bone repair and regeneration. However, the glass composition and its surface properties significantly affect the nature of the fibre-matrix interface and composite properties. Herein, the effect of Si and Fe on the surface properties of calcium containing phosphate based glasses (PGs) in the system (50P(2)O(5)-40CaO-(10-x)SiO(2)-xFe(2)O(3), where x = 0, 5 and 10 mol.%) were investigated. Contact angle measurements revealed a higher surface energy, and surface polarity as well as increased hydrophilicity for Si doped PG which may account for the presence of surface hydroxyl groups. Two PG formulations, 50P(2)O(5)-40CaO-10Fe(2)O(3) (Fe10) and 50P(2)O(5)-40CaO-5Fe(2)O(3)-5SiO(2) (Fe5Si5), were melt drawn into fibres and randomly incorporated into poly(lactic acid) (PLA) produced by melt processing. The ageing in deionised water (DW), mechanical property changes in phosphate buffered saline (PBS) and cytocompatibility properties of these composites were investigated. In contrast to Fe10 and as a consequence of the higher surface energy and polarity of Fe5Si5, its incorporation into PLA led to increased inorganic/organic interaction indicated by a reduction in the carbonyl group of the matrix. PLA chain scission was confirmed by a greater reduction in its molecular weight in PLA-Fe5Si5 composites. In DW, the dissolution rate of PLA-Fe5Si5 was significantly higher than that of PLA-Fe10. Dissolution of the glass fibres resulted in the formation of channels within the matrix. Initial flexural strength was significantly increased through PGF incorporation. After PBS ageing, the reduction in mechanical properties was greater for PLA-Fe5Si5 compared to PLA-Fe10. MC3T3-E1 preosteoblasts seeded onto PG discs, PLA and PLA-PGF composites were evaluated for up to 7 days indicating that the materials were generally cytocompatible. In addition, cell alignment along the PGF orientation was observed showing cell preference towards PGF.

Titania-hydroxyapatite nanocomposite coatings support human mesenchymal stem cells osteogenic differentiation.

In addition to mechanical and chemical stability, the third design goal of the ideal bone-implant coating is the ability to support osteogenic differentiation of mesenchymal stem cells (MSCs). Plasma-sprayed TiO(2)-based bone-implant coatings exhibit excellent long-term mechanical properties, but their applications in bone implants are limited by their bioinertness. We have successfully produced a TiO(2) nanostructured (grain size <50 nm) based coating charged with 10% wt hydroxyapatite (TiO(2)-HA) sprayed by high-velocity oxy-fuel. On Ti64 substrates, the novel TiO(2)-HA coating bond 153× stronger and has a cohesive strength 4× higher than HA coatings. The HA micro- and nano-sized particles covering the TiO(2)-HA coating surface are chemically bound to the TiO(2) coating matrix, producing chemically stable coatings under high mechanical solicitations. In this study, we elucidated the TiO(2)-HA nanocomposite coating surface chemistry, and in vitro osteoinductive potential by culturing human MSCs (hMSCs) in basal and in osteogenic medium (hMSC-ob). We assessed the following hMSCs and hMSC-ob parameters over a 3-week period: (i) proliferation; (ii) cytoskeleton organization and cell-substrate adhesion; (iii) coating-cellular interaction morphology and growth; and (iv) cellular mineralization. The TiO(2) -HA nanocomposite coatings demonstrated 3× higher hydrophilicity than HA coatings, a TiO(2)-nanostructured surface in addition to the chemically bound HA micron- and nano-sized rod to the surface. hMSCs and hMSC-ob demonstrated increased proliferation and osteoblastic differentiation on the nanostructured TiO(2)-HA coatings, suggesting the TiO(2)-HA coatings nanostructure surface properties induce osteogenic differentiation of hMSC and support hMSC-ob osteogenic potential better than our current golden standard HA coating.

Nano/micro electro-spun polyethylene terephthalate fibrous mat preparation and characterization.

Electro-spun polyethylene terephthalate (PET) fibrous mats are potential substrates for biotechnological and biomedical applications. In this regard, substrate characteristics including, fiber diameter, orientation and mechanical properties play an important role in controlling the interaction of substrate with biological entities. However, few studies reporting the preparation of electro-spun PET substrates with such controlled characteristics have been published. In this study, electro-spun PET fibrous mats with fiber diameters in the nanometer and micrometer range were produced by varying polymer solution concentration and flow rate. Fiber orientation within the mats was also varied by varying collector surface velocities in rotation and translation. Their morphological, mechanical, thermal and structural properties were evaluated as a function of fiber diameter and collector speed using scanning electron microscopy (SEM), a micromechanical tester, differential scanning calorimetry (DSC) and X-ray diffraction (XRD), respectively. Varying polymer solution concentration and flow rate allowed the production of matrices with fiber diameters ranging from 400 nm to 2 µm. Tensile properties increased with fiber diameter and collector surface velocity. Thermal properties of electro-spun PET fibers were different from the structure of as received raw PET in the form of pellets, revealing an amorphous structure for the entire electro-spun PET. This was also confirmed by XRD analysis. No considerable differences were observed between electro-spun PET fibers, in terms of crystalline and thermal properties, produced under various conditions. These electro-spun mats with different fiber diameters, orientation and mechanical properties can be used for various applications including tissue engineering scaffolds.

Low thrombogenicity coating of nonwoven PET fiber structures for vascular grafts.

Vascular PET grafts (Dacron) have shown good performance in large vessels (≥ 6 mm) applications. To address the urgent unmet need for small-diameter (2-6 mm) vascular grafts, proprietary high-compliance nonwoven PET fiber structures were modified with various PEG concentrations using PVA as a cross-linking agent, to fabricate non-thrombogenic mechanically compliant vascular grafts. The blood compatibility assays measured through platelet adhesion (SEM and mepacrine dye) and platelet activation (morphological changes, P-selectin secretion, and TXB2 production) demonstrate that functionalization using a 10% PEG solution was sufficient to significantly reduce platelet adhesion/activation close to optimal literature-reported levels observed on carbon-coated ePTFE.

Effect of sterilization on non-woven polyethylene terephthalate fiber structures for vascular grafts.

Non-woven polyethylene terephthalate (PET) fibers produced via melt blowing and compounded into a 6 mm diameter 3D tubular scaffold were developed with artery matching mechanical properties. This work compares the effects of ethylene oxide (EtO) and low temperature plasma (LTP) sterilization on PET surface chemistry and biocompatibility. As seen through X-ray photoelectron spectroscopy (XPS) analysis, LTP sterilization led to an increase in overall oxygen content and the creation of new hydroxyl groups. EtO sterilization induced alkylation of the PET polymer. The in vitro cytotoxicity showed similar fibroblastic viability on LTP- and EtO-treated PET fibers. However, TNF-α release levels, indicative of macrophage activation, were significantly higher when macrophages were incubated on EtO-treated PET fibers. Subcutaneous mice implantation revealed an inflammatory response with foreign body reaction to PET grafts independent of the sterilization procedure.

Preparation and characterization of NaOH treated micro-fibrous polyethylene terephthalate nonwovens for biomedical application.

Recently, micro-fibrous polyethylene terephthalate nonwovens have been investigated and applied in many biotechnological and biomedical applications. NaOH treatment has been used as a simple and cost effective method to alter surface properties, in order to overcome their surface inertness. However, the effects of this treatment on the matrices mechanical and physical properties; particularly, those composed of fibers with small diameter (<20 microm); have been poorly investigated. This study investigates the variations, imposed by the NaOH treatment, in the physical and tensile properties of micro-fibrous polyethylene terephthalate mats. Polyethylene terephthalate webs with two different average fiber diameters of 6+/-2.5 and 10+/-4 microm were produced by melt blowing process. A number of these webs were consolidated to prepare fibrous matrices using a thermal treatment. The matrices were treated using NaOH 1 N at 65 degrees C for various durations (ranging from 20 min to 24 h). In addition to their physical properties such as weight loss, thickness, porosity, shrinkage and surface density; their morphology and tensile properties were also evaluated using scanning electron microscopy and micromechanical tester, respectively. In general, by increasing treatment duration, weight loss, porosity, and shrinkage increased, while thickness and density decreased. As a result of treatment duration, pores appeared on the surface of individual fibers, and tensile stress and Young's modulus decreased while tensile strain increased. Mats with different fiber diameters showed different physical and mechanical properties. These findings suggested that the structure of the matrices and the properties required for its end use, for biomedical applications including scaffolding materials for tissue engineering, should be considered in selecting NaOH treatment condition.

Modulation of polycaprolactone composite properties through incorporation of mixed phosphate glass formulations.

Phosphate-based glasses (PGs) and their composites are of interest as bone repair and tissue engineering scaffolds due to the totally degradable nature of the materials. This study has investigated the effect of Si and Fe on the properties of PG particulate-filled polycaprolactone (PCL) matrix composites. Two glass compositions were investigated (mol.%): 50P(2)O(5), 40CaO and 10SiO(2) or Fe(2)O(3) (Si(10) and Fe(10), respectively). All composites contained 40 vol.% particulate filler, either Si(10), Fe(10), or a blend (40Si(10)/0Fe(10), 30Si(10)/10Fe(10), 20Si(10)/20Fe(10), 10Si(10)/30Fe(10) or 0Si(10)/40Fe(10)). Ion release, weight loss and composite mechanical properties were characterised as a function of time in deionised water (DW) and phosphate-buffered saline (PBS), respectively. The potential for calcium phosphate deposition was assessed in simulated body fluid (SBF). Calcium and phosphate ion release in DW increased in tandem with the rate of composite weight loss, which increased with Si(10) content. A Si(10) content dependent rate of pH reduction was observed in DW. After 56 days the PG in the 40Si(10)/0Fe(10) composite was completely dissolved, whereas 67% of that in the 0Si(10)/40Fe(10) composite remained. The initial flexural strength of 40Si(10)/0Fe(10) composites was significantly lower when compared with the other materials. An increase in Si(10) content led to an increase in Young's modulus and a concomitant decrease in flexural strain. It was found that the PCL molecular weight (M(w)) decreased dramatically with increasing Si(10) content. FTIR analysis showed that Si incorporation into PG led to reaction with the PCL ester bonds, resulting in a reduction in PCL M(w) when processed at elevated temperatures. Changes in mechanical properties with time in PBS were glass blend dependent and a more rapid rate of reduction was observed in higher Si(10) content composites. After 28 days in SBF surface deposited brushite was formed in 20Si(10)/20Fe(10) PG containing composites. Thus, the properties of PCL-PG composites could be tailored by controlling the phosphate glass blend composition.

Bone remodeling in a new biomimetic polymer-composite hip stem.

Adaptive bone remodeling is an important factor that leads to bone resorption in the surrounding femoral bone and implant loosening. Taking into account this factor in the design of hip implants is of clinical importance, because it allows the prediction of the bone-density redistribution and enables the monitoring of bone adaptation after prosthetic implantation. In this article, adaptive bone remodeling around a new biomimetic polymer-composite-based (CF/PA12) hip prosthesis is investigated to evaluate the amount of stress shielding and bone resorption. The design concept of this new prosthesis is based on a hollow substructure made of hydroxyapatite-coated, continuous carbon fiber (CF)-reinforced polyamide 12 (PA12) composite with an internal soft polymer-based core. Strain energy density theory coupled with 3D Finite Element models is used to predict bone density redistributions in the femoral bone before and after total hip replacement (THR) using both polymer-composite and titanium (Ti) stems. The result of numerical simulations of bone remodeling revealed that the CF/PA12 composite stem generates a better bone density pattern compared with the Ti-based stem, indicating the effectiveness of the composite stem to reduce bone resorption caused by stress-shielding phenomenon. This may result in an extended lifetime of THR.

Performance of CF/PA12 composite femoral stems.

This study presents the microstructural and mechanical behavior of the CF/PA12 composite material developed as well as its biomechanical performance when used for the fabrication of femoral stems. The static tests were performed to evaluate the compressive and flexural modulus as well as the ultimate compressive and bending strength. It was found that CF/PA12 composite had bone-matching properties in the same order of magnitude as cortical bone in the femur. Density and void content measurements were also done to assess the consolidation quality. Dynamic fatigue testing was conducted on both CF/PA12 cylinders and femoral stems to evaluate the long term durability and mechanical reliability of the composite. Compression-compression cyclic loading was used at a frequency of 6 Hz with loads varying between 17 kN and 22 kN for the composite cylinders while a frequency of 10 Hz and load of 2300 N was employed for the femoral stems. Results indicate that the fatigue performance of CF/PA12 composite surpasses by far the required fatigue performance for total hip prosthesis (THP) stems. The overall performance of the CF/PA12 femoral stems confirms that this composite is an excellent candidate material for orthopedic applications such as THP stems.

Biocompatibility of novel polymer-apatite nanocomposite fibers.

On the basis of the bioactivity of hydroxyapatite (HA) and the excellent mechanical and biocompatible performance of polyethylene terephthalate (PET), composite microfibers made of nanograde HA with PET was designed and fabricated to mimic the structure of biological bone, which exhibits a composite of nanograde apatite crystals and natural polymer. The PET/HA nanocomposite was molded into fibers so that the bulk structures' mechanical properties can be custom tailored by changing the final 3D orientation of the fibbers. This study focused on the in vitro biocompatibility evaluation of the PET/HA composite fibers as potential bone fixation biomaterial for total hip replacement prosthesis surfaces. The MTT assay was performed with the extracts of the composite fibers in order to evaluate the short-term effects of the degradation products. The cell morphology of L929 mouse fibroblast cell line was analyzed after direct contact with the fiber scaffolds for different time periods, and the cell viability was also analyzed by the Alamar Blue assay. The release of the inflammatory cytokine, tumor necrosis factor-alpha (TNF-alpha), from RAW 264.7 macrophages in the presence of fiber extracts and fibers was used as a measure of the inflammatory response. The ability of the fiber matrices to support L929 attachment, spreading, and growth in vitro, combined with the compatible degradation extracts and low inflammation potential of the fibers and extracts, suggests potential use of these fibers as load-baring bone fixation biomaterial structures.