Processing of fibre and porosity gradients in cellular thermoplastic composites

The aim of this study has been to develop processing techniques for novel bioresorbable thermoplastic cellular composites, reinforced with continuous inorganic fibres, for use as biodegradable scaffolds for bone tissue engineering. As the implanted composite scaffold starts to degrade, it should be progressively replaced by natural bone tissue, which should consequently support increasing proportions of the applied mechanical loads. In order to simulate natural high performance porous structures such as bone or light wood, composites with simultaneous gradients in fibre content and porosity were studied. In order to carry out investigations over wide ranges of fibre content and porosity and to ensure biocompatibility and bioresorbability, it was decided to focus on foamable poly(L-lactic acid) (PLA) and continuous glass fibres. As well as improved mechanical reinforcement due to the high aspect ratio, the use of continuous glass fibres offers the possibility of achieving higher loadings than with short fibres or particles, increasing the potential range of mechanical properties. For processing the composites, a fibre winding technique developed in house was used to prepare unidirectional preforms with gradient structures tailored at the fibre bundle level. PLA fibre bundles, containing 36 monofilaments were prepared on a spinning line, and mingled with glass fibre bundles comprising 250 or 1000 monofilaments. During the winding process, the PLA fibres were intimately mixed with the reinforcing fibres, to ensure a good impregnation during subsequent processing steps. Moreover it was possible to maintain the relative positions of the fibres during pre-consolidation of graded composites. The resulting preforms had a porosity of 25% and could be further consolidated to decrease the porosity, or foamed in an autoclave to obtain cellular composites (foams) with porosities of up to 90%. It was observed that, independently of the applied foaming conditions, the presence of continuous fibres reduced the porosity by an amount equivalent to twice the fibre volume fraction. This processing method provided composites with a simultaneous gradient in fibre content and porosity. In order to predict the Young's modulus and the collapse strength of the cellular composites a model was proposed, which combines the buckling of composite columns within a porous structure with the property predictions for neat polymer foams. In neat foams with a porosity of 60%, the Young's modulus was calculated to be 0.25 GPa. By adding a fibre volume fraction of 14% the modulus reached 1.2 GPa. The respective collapse strengths were 5 MPa and 16 MPa. Experimental results were consistent with the predictions and validated the model. The model determines the fibre content required for composites with a given porosity and stiffness. Gradient cellular composites were obtained with a range of porosity between 50% to 90% and a range of stiffness between 100 and 1500 MPa. Furthermore the processing parameters can be adjusted to maintain a desired porosity when fibres are added. Thus, composites that fulfill the mechanical requirements for bone scaffolds can be achieved in a rational manner. The long term behaviour of the foams was tested under physiological conditions. Viscoelasticity tests provided evidence for the important role of the unidirectionally oriented fibres. Whereas the cells in neat foams collapsed after at most two days when loaded at 0.69 MPa, the fibre reinforced foams resisted longer and their creep strain was well described by a power law. The fatigue properties were compared to reported values of natural trabecular bone. None of the neat foams with a porosity of 74% resisted to the first load cycle of 7.3 MPa. With a fibre volume fraction of 11% and a comparable porosity of 72%, the composite scaffolds resisted for at least 11'000 loading cycles. To compare, trabecular human bone failed after 1909 cycles, when loaded with 8.5 MPa. It was concluded that the reinforced composites have the potential to replace natural bone. Preliminary in vitro results with human foetal osteoblasts and in vivo tests on femurs of rats were promising because the cells proliferated well on the composite substrates. Further investigations for applications in bone tissue engineering can be planned. Furthermore, the studied process and the obtained results are envisaged for other material systems to prepare cellular composites for novel lightweight applications in transportation and building industry.

Processing of Sustainable Cellular Biocomposites

Carole Boissard

Petroleum-based polymers and composites are commonly used in a wide variety of application fields. However ecological concerns, increasing oil prices and dwindling natural resources explain why nowadays industrial and public interest is more than ever focused on new bio-based – “greener”- products. In order to find viable substitutes for petroleum-based materials, major efforts are devoted to the development of sustainable solutions combining fully compostable matrices and fibres from renewable resources. Polylactide (or polylactic acid) (PLA) is currently one of the most versatile biodegradable polymers derived from renewable resources such as non-food crops of corn or sugar cane. Its transformation into foamed cellular structures would decrease its density and at the same time may improve its specific mechanical properties. This could provide cost-effective and sustainable products for packaging and construction. In the context of developing “green” foams, the focus in the present work has been on a solvent-free process based on physical foaming with supercritical carbon dioxide (ScCO2) as a foaming agent. This foaming process includes thermally driven but kinetically controlled phenomenon. Solubility and diffusion of the gas in the molten polymer, cell nucleation and growth, as well as stabilization of the final foam structures were studied for neat and composite materials. At low processing temperatures and low depressurization rates, homogeneous structures were obtained thanks to a balance between cell growth mechanisms and cell stabilization, thus limiting uncontrolled cell coalescence. The density of the resulting polylactide foams ranged from 0.12 to 0.3 g/cm3 with corresponding compression moduli of 6 to 73 MPa respectively. The processing windows were strongly dependent on the rheological melt properties of the PLAs. PLA is generally considered to be a high strength, high modulus thermoplastic but its brittleness has limited its industrial use. PLA was therefore combined with wood fibres (WF) and microfibrillated cellulose (MFC) in order to improve its mechanical performance without compromising its environmental impact. Two compounding methods, a wet mixing papermaking-like process and a solvent casting process were investigated for the production of WF/PLA and MFC/PLA biocomposites. Degradation of PLA occured during wet-mixing, due to the water used as a dispersant, and to several heating steps applied during compounding. Improvement of the drying and processing conditions and/or the replacement of water by isopropanol limited this degradation. The presence of cellulose fibres considerably modified foaming mechanisms; gas solubility was decreased and the coefficient of diffusion increased, leading to lower quantities of CO2 available for foaming. In addition, stiffness of the fibre network impeded full expansion of the polymer matrix. Thus smaller cells were created. Densities from 0.15 (neat PLA) to 0.37 and 0.55 g/cm3 and moduli ranging from 35 (neat PLA) to 125.5 and 373 MPa were obtained by the addition of 5wt% WF and MFC respectively. Addition of a chain extender enabled viscosity of the matrix to be increased causing increased orientation of the fibre network during flow and hence fibre alignment in the cell walls. Density was thus decreased by 95 % to 0.017 g/cm3 and compression modulus decreased to 0.32 MPa for 5wt% WF composite. Cell wall shrinkage was observed in the neat PLA foams obtained with the chain extender. This was reduced by adding wood fibres while maintaining similar density and modulus. Cellulose fibres act as an extra scaffold in the cellular structures. Industries are currently considering the use of these materials for applications in packaging e.g. as cushioning materials, or in architecture as display panels, or as sandwich core materials for the replacement of expanded polystyrene, polyvinyl chloride or polyurethane foams.

EPFL2012

Cellular thermoplastic composites with microstructural gradients of fibres and porosity

Pierre-Etienne Bourban, Manuel Bühler, Jan-Anders Månson

New thermoplastic composites with their composition tailored at the yarn level were proposed and successfully prepared. A specific processing method was developed for composites exhibiting different fibre and porosity concentration gradients. Mechanical properties of such composites were determined by three-point bending tests. In order to carry out investigations on wide ranges of fibre and porosity fractions, a material system based on poly(lactic acid) (PLA), and continuous glass fibres was processed into a composite foam structure with up to 85% porosity and a modulus reaching 1.5 GPa. Micrographs confirmed that the fibre placement could also be controlled at a microscale as well as at the usual millimetre range. Furthermore it was shown that porosity distribution is indeed influenced by the presence of continuous reinforcing fibres. Gradient cellular composites are envisaged for bone replacement and for lightweight sandwich structures.

2008

Bioresorbable Nanocomposite Foams for Bone Tissue Engineering

Claire Diane Delabarde

The present work forms part of an ongoing effort at the Laboratory of Polymer and Composite Technology (LTC) to develop novel solvent-free processing methods for next generation resorbable porous scaffolds for bone replacement, based on supercritical carbon dioxide (CO2) foaming. Although adequate mechanical performance and in vitro biocompatibility have been demonstrated for such scaffolds and results from in vivo testing have been promising, osteoconduction and osteogeneration remain inadequate. The approach proposed here is to use highly dispersed synthetic poly-L-lactide (PLLA)/bioceramic nanocomposites, in which the modifier particles are comparable in size and composition to those in natural bone, combined with targeted surface treatments, as a means of enhancing the osteoconductivity of the scaffolds. The investigation has covered: (i) the preparation, basic structure-property relationships and resorption behavior of the nanocomposites; (ii) the optimization of processing parameters for the production of porous nanocomposite scaffolds by supercritical CO2 foaming; (iii) the in vitro and in vivo response of the scaffolds with emphasis on the influence of selected surface treatments. The bulk of the work involved hydroxyapatite nanoparticle fillers (nHA), although some attention was also given to nanoparticles prepared from Bioglass® (nBG), which has been reported to show superior osteoconduction and osteinduction to hydroxyapatite. Mechanical mixing with a miniature twin screw extruder was found to give excellent dispersions of certain grades of nHA in the PLLA matrix without the need for an organic solvent or surfactants. Initial studies of the crystallization behavior of the resulting PLLA/nHA nanocomposites showed nHA addition to result in a decrease in spherulite growth rates but an increase in nucleation density, leading to a significant increase in the global crystallization rates. Addition of nHA particles also resulted in increases in the rate of mass loss during in vitro ageing of bulk films, identified with accelerated resorption at the matrix/particle interfaces. However, the tensile strengths and strains to fail were significantly higher in the aged films in the presence of nHA, which indicated the nHA to act as an effective toughener of the bulk material and hence to retard the loss in tensile strength during resorption. It was also shown that the initial presence of a coarse spherulitic morphology may be deleterious to mechanical performance during ageing, owing to large scale internal cracking associated with the spherulite radii. Finally, the accelerated ageing treatments used for the resorption tests were found to result in significantly increased hydrophilicity and, in the case of the PLLA/nHA nanocomposites, significant pitting at the scale of the nanoparticles and increased nanoparticle concentrations at the film surfaces, suggesting that such treatments may also be beneficial for the interactions between the nanocomposites and bone cells during implantation. In foams prepared by supercritical CO2 processing using a new dedicated autoclave designed specifically for this purpose, the addition of nHA resulted in reduced foam cell sizes and improved homogeneity in the cell size distribution, but did not significantly affect the degree of crystallinity, which remained of the order of 50 wt% in all the foams. Indeed, the compressive modulus and strength were found to be primarily influenced by the porosity of the foams regardless of the nHA content. Even so, the mechanical properties of the foams were comparable with those of trabecular bone, and by adjusting the saturation pressure and depressurization rate it was possible to generate porosities of about 76 %, an interconnected morphology and cell diameters in the range 200-400 µm from PLLA containing 10 wt% nHA, satisfying established geometrical requirements for bone replacement scaffolds. On the basis of the resorption studies and comparison with other potential surface treatments, selected scaffolds were also aged in aqueous NaOH in an attempt to improve cell-scaffold affinity. After 8 weeks of implantation in vivo in rat femoral condyles, growth of new bone was confirmed to be significantly enhanced in the NaOH treated PLLA/10 wt% nHA scaffolds, as compared with untreated controls and previous generation scaffolds containing βTCP microparticles. The results of these pre-clinical studies are considered to be highly promising, although they remain to be completed and extended to large animal models.