Biomedical Engineering Reference
In-Depth Information
19
Fracture Mechanisms of Biodegradable PLA
and PLA/PCL Blends
Mitsugu Todo
1
and Tetsuo Takayama
2
1
Research Institute for Applied Mechanics, Kyushu University
2
Graduate School of Science and Engineering, Yamagata University
Japan
1. Introduction
Poly (lactic acid) (PLA), made from natural resources such as starch of plants, is one of
typical biodegradable thermoplastic polymers and has extensively been used in medical
fields such as orthopedics, neurosurgery and oral surgery as bone fixation devices mainly
due to biocompatibility and bioabsorbability (Higashi et al., 1986; Ikada et al., 1996;
Middleton & Tipton, 2000; Mohanty, 2000). Its importance has led to many studies on its
mechanical properties and fracture behavior which found that the mode I fracture behavior
of PLA is relatively brittle in nature (Todo et al., 2002; Park et al., 2004, 2005, 2006).
Therefore, blending with a ductile biodegradable and bioabsorbable polymer such as poly
(ε-caprolacton) (PCL) has been adopted to improve the fracture energy of brittle PLA (Broz
et al., 2003; Dell'Erba et al., 2001; Chen et al., 2003; Todo et al., 2007; Tsuji & Ikada, 1996,
1998; Tsuji & Ishizuka, 2001; Tsuji et al., 2003); however, it was also found that phase
separation originated by immiscibility of PLA and PCL tends to degrade the mechanical
properties of PLA/PCL blends (Todo et al., 2007). It has recently been found that such phase
separation can dramatically be improved by using an isocyanate group, lysine tri-isocyanate
(LTI) (Takayama et al., 2006; Takayama & Todo, 2006; Harada et al., 2007, 2008), and the
fracture properties of PLA/PCL/LTI are much higher than those of PLA/PCL.
In this chapter, firstly the fracture behavior and micromechanism of pure PLA are
summarized (Park et al., 2004, 2005, 2006; Todo et al., 2002). Effects of crystallization
behavior and loading-rate on the mode I fracture behavior are discussed. Effect of
unidirectional drawing on the fracture energy is also presented as one of the effective ways
to improve the brittleness of PLA (Todo, 2007). Secondly, the fracture behavior of PLA/PCL
blends is discussed on the basis of the relationship between the microstructure and the
fracture property (Todo et al., 2007b). In the third section, improvement of microstructural
morphology of PLA/PCL by using LTI is discussed (Takayama et al., 2006; Takayama &
Todo, 2006; Todo & Takayama, 2007; Todo et al., 2007a). It has been found that addition of
LTI effectively improves the phase morphology of PLA/PCL, resulting in dramatic
improvement of fracture energy. Effects of annealing on the mechanical properties of
PLA/PCL/LTI blend are discussed in the last section (Takayama et al., 2011). It has been
found that a thermal annealing process can effectively improve the mechanical properties of
the polymer blend, as a result of strengthened structures due to crystallization of PLA.
