Plasticization of polylactide foams for tissue engineering

Supercritical CO2 foaming of amorphous and semicrystalline polylactides plasticized by blending with 20 wt% polyethylene glycol has been investigated as a means of producing low-stiffness bioresorbable scaffolds for tissue engineering. For a given set of foaming parameters, the presence of polyethylene glycol generally resulted in a significant reduction in foam density and increased cell diameters, effects that could be accounted for by a large decrease in melt viscosity with respect to that of the pure polylactides. The compression moduli of plasticized foams under simulated physiological conditions (immersion in deionized water at 37 degrees C)were also substantially lower than those of the neat foams. This was attributed primarily to a decrease in the glass transition temperature from about 60 degrees C for the pure polylactides to about 30 degrees C on addition of the polyethylene glycol, implying the blends to be in the rubbery state at the test temperature of 37 degrees C. Under the conditions investigated, the processing window for homogeneous foams was found to be wider for blends based on semicrystalline polylactides than for blends based on amorphous polylactides. However, by tailoring the processing parameters, open cell architectures with porosities of more than 75% and cell diameters in the range 200 to 700 mu m could be achieved in both types of blend. Such morphologies have already been shown to be suitable for bone repair and, when combined with a low-stiffness matrix, may offer exciting new possibilities for applications in soft tissue engineering, such as cartilage repair.

Hybrid Polymer Foams for Osteochondral Tissue Engineering

Matthieu Cuénoud

Articular cartilage is a soft tissue that covers the ends of articulating bones, and is essential for maintaining joint motion, i.e. translation and rotation between the bones. Articular cartilage nevertheless has a limited capacity for self-repair, and cartilage lesions following traumatic injury may consequently initiate an irreversible degenerative process. This ultimately leads to progressive loss of joint function, pain and medical costs. Many different clinical treatments have been investigated, but these have not so far led to satisfactory long-term solutions. Tissue engineering research has therefore focused on the preparation of synthetic bioresorbable scaffolds that provide a template for cells and guide the process of tissue regeneration. In the present study, it is proposed to develop an osteochondral scaffold using novel solvent-free processing methods based on supercritical carbon dioxide (CO2) foaming. The approach is first to tailor the mechanical properties of medical grade polylactides (PLA), using poly(ethylene glycol) (PEG) as a plasticizer, to produce scaffolds with reduced elastic moduli intended for cartilage repair. These polymer scaffolds are then combined with an existing biocomposite scaffold (PLLA/bioceramic), developed for bone repair, to give a bilayered scaffold suitable for osteochondral repair. The investigation thus covers: (I) the preparation of plasticized polylactide and the dependence of its properties on composition; (II) the design of processing windows for the production of porous plasticized polylactide scaffolds by supercritical CO2 foaming; (III) the preparation of hybrid structures with gradients in composition and properties, and tailored interfaces; (IV)the properties and in vitro response of sterilized scaffolds. Plasticization of semicrystalline poly-L-lactide (PLLA) with PEG reduces Tg to below 37 °C, depending on the composition, implying the amorphous regions to be in the rubbery state at body temperature. While immersion in an aqueous medium tends to promote phase separation in blends containing low molar mass PEG, relatively good stability is observed with PEG with a molar mass of 35,000 g/mol. It has thus been possible to prepare interconnected, open-pore foams with porosities of more than 75 %, cell diameters in the range 200 to 700 pm and compression moduli of a few MPa. It was also observed that addition of PEG resulted in foams with increased pore diameters and lower foam densities than the neat polylactide, but also increased pore coalescence at high saturation pressures, effects that were accounted for by the large decrease in melt viscosity on addition of PEG. Furthermore, the compression moduli of the foams were shown to be strongly reduced on addition of PEG, which was assumed to be a direct consequence of the plasticization of the polylactide by PEG and the accompanying decrease in glass transition temperature, implying the amorphous content to be in the rubbery state at in vivo temperature, as required. Thus, foams with moduli ranging from 0.3 to 15 MPa could be prepared from the different PLA/PEG blends and copolymers studied. Bilayered scaffolds were produced by assembling the plasticized foams with a biocomposite foam using an adapted fusion bonding method. This allowed control of interfacial properties such as the bonding strength, and the interface density and permeability. These hybrid scaffolds exhibited well-defined layers with suitable properties for the repair of bone and cartilage, respectively. Finally, X-ray and y irradiation was shown to strongly degrade the polymer matrix. However, it was found that treatment with ethylene oxide gas could be used to sterilize the scaffolds without significantly modifying their properties, or compromising their biocompatibility, as borne out by in vitro tests. The approach proposed in this work was hence concluded to be highly promising for cartilage repair, and the resulting hybrid osteochondral scaffolds are ready for the next stage of the developmental work, which will consist of extended in vivo studies.

EPFL2012

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.

EPFL2011

Plasticization of poly-L-lactide for tissue engineering

Pierre-Etienne Bourban, Matthieu Cuénoud, Jan-Anders Månson, John Christopher Plummer

Plasticization of medical grade poly-L-lactide (PLLA) by addition of polyethylene glycol (PEG) with various molar masses has been evaluated as means of producing low stiffness matrices for bioresorbable scaffolds for soft-tissue engineering applications. As reported previously, the Tg of injection molded specimens of the PLLA/PEG blends decreased strongly with PEG content, so that at PEG contents of 15 and 25 wt % it became significantly lower than normal human body temperature, implying an essentially rubber-like mechanical response in vivo. The degree of crystallinity of the moldings also increased strongly with PEG content, reaching a maximum of about 60 wt % at 25 wt % PEG. Moreover, after the immersion in phosphate-buffered saline for 5 days in 37°C to simulate conditions in vivo, the moldings with the highest PEG contents showed increased water uptake and, for relatively low molar mass PEG, significant mass loss, associated with phase separation and leaching of the PEG. Blends with relatively low PEG contents also showed large increases in their degree of crystallinity. The implications of these changes for the in vivo performance of the blends and their potential for development as matrices for bioresorbable scaffolds are discussed in the light of results from a series of PLLA/PEG copolymers.