Accelerated ageing and degradation in poly-L-lactide/hydroxyapatite nanocomposites Claire Delabarde, Christopher J.G. Plummer, Pierre-Etienne Bourban, Jan-Anders E. Månson * Laboratoire de Technologie des Composites et Polymères (LTC), École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland article info Article history: Received 6 October 2010 Received in revised form 7 December 2010 Accepted 27 December 2010 Available online 13 January 2011 Keywords: Ageing Degradation PLLA Hydroxyapatite Nanocomposite Mechanical properties abstract Dry, compression molded films of medical grade poly-L-lactide (PLLA) showed a marked reduction in tensile strength and strain after accelerated ageing in aqueous NaOH at 50  C, accompanied by mass loss, surface erosion, increased hydrophilicity and, in the case of the initially amorphous films, cold crystal- lization owing to the plasticizing effect of the ageing medium. Addition of well dispersed nanosized hydroxyapatite (nHA) particles resulted in increases in the rate of mass loss during ageing, identified with accelerated degradation at the matrix/particle interfaces. However, the associated decreases in tensile strength and strain to fail with ageing time were far less marked in the presence of the nHA than in the unmodified films. This implied that nHA acts as an effective toughener of the bulk material, consistent with TEM observations of the deformed films, which indicated failure of the particleematrix interfaces to promote plastic deformation of the PLLA. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Poly-L-lactide (PLLA) is a semicrystalline thermoplastic that is widely used in biomedical applications, owing to its excellent biocompatibility and inherent biodegradability [1e 12]. It is currently under consideration for use in porous biodegradable scaffolds for bone repair, whose rate of resorption in vivo should ideally match the growth rate of the bone tissue, so that the overall structural integrity of the system is maintained [13,14]. The prin- cipal degradation mechanism of PLLA in aqueous media both in vitro and in vivo is hydrolytic de-esterification, which is reported to result in a progressive reduction in the bulk molar mass with degradation time under fixed conditions [15]. As degradation continues, soluble oligomers are leached from the surface, while those trapped in the interior increase the local acidity owing to the presence of carboxylic acid groups at the chains ends, which cata- lyze hydrolysis of the ester groups [15,16]. The oligomers are subsequently eliminated by natural pathways such as the tricar- boxylic acid cycle and certain grades of PLLA have been approved by the US Food and Drug Administration for applications in vivo [14]. The rate of degradation of PLLA is reported to be dependent on factors such as its initial molar mass, purity, temperature, external dimensions and crystalline morphology [17e23]. Moreover, other physical changes may result from prolonged exposure to an aqueous medium, such as physical ageing, changes in the crystalline morphology and plasticization owing to water absorption, which may also be of crucial importance for the performance of PLLA in vivo [24e27]. There are conflicting reports on the effect of crystal- linity on the resorption rate. For example, Tsuji et al. [19] found the degradation rate to increase with the initial degree of crystallinity in a buffer solution, and argued that a higher degree of crystallinity may result in a higher defect density in the amorphous regions, facilitating the penetration of water. On the other hand, the degra- dation rate has been reported elsewhere to decrease with the degree of crystallinity in both buffer solution and during accelerated ageing in alkaline solution [20,21,28]. A further consideration is the presence of filler particles, which is of particular concern in the context of PLLA-based biodegradable scaffolds, because addition of bioactive ceramic (“bioceramic”) fillers, such as hydroxyapatite (HA), may potentially improve their osteoconductivity (the provision of a suitable substrate for bone growth on a surface, or into pores, channels or pipes [29]) [30,31]. A recent trend is to exploit the improved properties of polymer/ bioceramic nanocomposites in which the modifier particles are of a similar size to those in natural bone, which consists essentially of around 60 wt% ceramic nanocrystals, comparable to HA in compo- sition and structure, embedded in a polymer (collagen) matrix [32e35]. HA (nHA) nanoparticles (mean particle diameters of the * Corresponding author. Tel.: þ41 021 693 4285; fax: þ41 021 693 5880. E-mail address: jan-anders.manson@epfl.ch (J.-A.E. Månson). Contents lists available at ScienceDirect Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab 0141-3910/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2010.12.018 Polymer Degradation and Stability 96 (2011) 595e607