Isothermal and Non-isothermal Crystallization Kinetics of Poly(L-Lactide)/Carbonated Hydroxyapatite Nanocomposite Microspheres

Medical-profession-accepted and the US Food and Drug Administration (FDA)-approved biodegradable polymers have been used for tissue engineering applications over the last two decades due to their good biocompatibility and acceptable biodegradation properties. Poly(L-lactide) (PLLA) is a linear aliphatic biodegradable polymer and has been widely studied for use as a scaffolding material for human body tissue regeneration (Wei and Ma 2004; Chen, Mak et al. 2006; Wang 2006). The enzymatic and non-enzymatic hydrolysis rate of PLLA strongly depends on its chemical properties (such as molecular weight and weight distribution) and physical properties (such as crystallinity and morphology). Crystallinity plays an important role in the degradation behavior of biodegradable polymers. It is well known that the crystallinity and morphology of semicrystalline polymers such as PLLA are greatly influenced by their thermal history. Therefore, the crystallization kinetics of PLLA should be carefully studied and correlated to its processing method as it forms a basis for the interpretation of the scaffold properties. The isothermal bulk crystallization kinetics of PLLA has been studied by a number of research groups, covering a temperature range from 70 to 165 °C (Marega, Marigo et al. 1992; Iannace and Nicolais 1997; Miyata and Masuko 1998; Di Lorenzo 2005). But only a few studies were conducted on the non-isothermal crystallization kinetics of neat PLLA. Miyata and Masuko (1998) reported that PLLA could not crystallize and remained amorphous when the cooling rate was higher than 10 °C/min. The knowledge on non-isothermal crystallization kinetics is useful for modelling real industrial processes such as cast film extrusion, which generally takes place at a nonconstant cooling rate (Piorkowska, Galeski et al. 2006). Particulate bioceramic reinforced polymer composites can combine the strength and stiffness of bioactive inorganic fillers with the flexibility and toughness of biodegradable organic matrices. Carbonated hydroxyapatite (CHAp) is a desirable bioactive material for bone substitution as it is bioresorbable and also more bioactive in vivo than stoichiometric hydroxyapatite. PLLA/CHAp nanocomposite has been developed and used for constructing bone tissue engineering scaffolds through selective laser sintering (SLS) (Zhou, Lee et al. 2007; Zhou, Lee et al. 2008). In the SLS process, the laser beam selectively fuses