Bioresorbable Nanocomposite Foams for Bone Tissue Engineering

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.