Processing of porous polymer composites for bone tissue engineering

Bone is a natural cellular composite, with a gradient structure going from a loose interconnected core to an outer dense wall, thus minimising bone weight while maintaining a high mechanical performance. Bone can be repaired using either auto- or allo- grafts, which, however, have a limited availability, or present pathogen transmission risks, respectively. Synthetic tissue grafts are therefore investigated in order to provide porous scaffolds, seeded with the appropriate type of cells, as templates for tissue regeneration. This study focused on composite scaffolds based on polymer matrices. The objectives were to understand how different foaming phenomena are affected by the addition of fillers into the polymer, to investigate their effect on the structural and mechanical properties of foams, and then to propose and optimise a solvent free process in order to prepare bioresorbable composite scaffolds suitable for bone tissue engineering applications. The comparison of several processes resulting in porous matrices led to choose supercritical CO2 foaming as a flexible technique to manufacture cellular structures, with the possibility of adding fibres or fillers. First, the integration of glass fibres into polyurethane (PUR) and polyethylene terephtalate (PET) foams, processed by reactive and gas foaming respectively, was investigated in order to study the potential improvement of mechanical properties of porous polymers. With the PUR system, modulus was shown to increase with fibre content, and an optimum fibre length existed. With PET, dense and heterogeneous structures were obtained. In both cases a homogeneous mixing of the two components was required before foaming. Gas foaming of bioresorbable neat polymers, poly(L-lactic acid) (PLA) and poly(D,L-lactic-co-glycolic acid) (PLGA), was carried out in a high-pressure vessel. A wide variety of PLA cellular structures was obtained, with open and closed pores, of diameters between 0.2 and 1.0 mm, and a range of porosity (70-92 %) and modulus (10-180 MPa). On the contrary, using PLGA led to closed, larger pores, and a low modulus. Only PLA was therefore considered as a matrix for composite foaming. Hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP), were then added to PLA in order to improve mechanical resistance, to regulate pH during scaffold resorption, and, owing to their intrinsic osteoconductivity, to favour mineralisation. Composite preforms were prepared by three different techniques: mixing powders in the dry state, dispersion in polymer solution, and melt-extrusion. Observation of the ceramic distribution, measurements of glass transition and melting temperatures, and tensile tests led to select and optimise melt-extrusion as a clean technique for preparing homogeneous ceramic-polymer preforms. Structural analysis of composite foams produced in the highpressure chamber enabled us to define processing windows as a function of pressure release and cooling rates. HA or β-TCP filled PLA foams were obtained, with porosity between 74 and 83 %, pore size between 0.1 and 1 mm, and modulus between 30 and 250 MPa. Microcomputed tomography confirmed the anisotropy in cellular morphology. Compression tests were also performed and revealed the anisotropic and viscoelastic behaviour of the porous polymer and composite structures. Finally, in collaboration with the Laboratory of Orthopaedic Research (LRO-EPFL) and the Hôpital Orthopédique de Suisse Romande (HO-CHUV), the biocompatibility of the developed structures with human primary foetal bone cells was demonstrated, as they proliferated on the template surfaces and differentiated. Foaming parameters could thus be defined to create composite scaffolds suitable for bone tissue engineering applications. Structural and mechanical properties similar to those of cancellous bone were demonstrated and promising in vitro tests were conducted. Therefore the developed scaffolds can now be evaluated in vivo.

Processing of fibre and porosity gradients in cellular thermoplastic composites

Manuel Bühler

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.

EPFL2008

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

Polylactic acid-phosphate glass composite foams as scaffolds for tissue engineering

Pierre-Etienne Bourban, Jan-Anders Månson, Dominique Pioletti

Phosphate glass (PG) of the composition 0.46(CaO)-0.04(Na(2)O)- 0.5(P(2)O(5)) was used as filler in poly-L-lactic acid (PLA) foams developed as degradable scaffolds for bone tissue engineering. The effect of PG on PLA was assessed both in bulk and porous composite foams. Composites with various PG content (0, 5, 10, and 20 wt %) were melt-extruded, and either compression-molded or foamed through supercritical CO(2). Dynamic mechanical analysis on the bulk composites showed that incorporating 20 wt % PG resulted in a significant increase in storage modulus. Aging studies in deionized water in terms of weight loss, pH change, and ion release inferred that the degradation was due to PG dissolution, and dependent on the amount of glass in the composites. Foaming was only possible for composites containing 5 and 10 wt % PG, as an increase in PG increased the foam densities; however, the level of porosity was maintained above 75%. PLA-T(g) in the foams was higher than those obtained for the bulk. Compressive moduli showed no significant reinforcement with glass incorporation in either expansion direction, indicating no anisotropy. Biocompatibility showed that proliferation of human fetal bone cells was more rapid for PLA compared to PLA-PG foams. However, the proliferation rate of PLA-PG foams were similar to those obtained for foams of PLA with either hydroxyapatite or beta-tricalcium phosphate.