Relationships between microstructure and mechanical properties of PLA/HA system

An in-depth understanding on the degradation behavior of a kind of resorbable composites, polylactic acid (PLA)/hydroxyapatite (HA) system, particularly in human body environment and under mechanical stress is of particular importance for its design and manufacturing. In the present study, mechanical behaviors and degradations of this composite in simulated physiological environment were investigated. A newly designed three-point bending fixture in simulated body fluid (SBF) tank was used to carry out mechanical tests. The PLA/HA specimens were tested under either static load or constant displacement rate in SBF at 37℃. Fracture behavior of PLA/HA system under simulated physiological environment was dramatically different from that measured in air at room temperature. It is very ductile and crack-insensitive in SBF while it is brittle with low fracture toughness (Kc) (0.34 ± 0.06 MPam1/2) in air. The results indicate the importance of performing mechanical tests in simulated physiological environment for understanding mechanical behavior of PLA/HA in vivo. Degradations of both bending modulus and strength with immersion time in vitro were studied in the testing device, and compared with the results of previous researches in mechanical properties of PLA/HA composite in air at room temperature. Severer degradations of modulus and strength were revealed in this study because of the test environment. It implies that mechanical tests in air could result in overestimation of effective service time of PLA/HA implants in vivo. Synergistic effects of mechanical load and immersion time in SBF were also examined by TGA and SEM. Both factors are proved to take important roles in promoting biodegradation rate of the composite. Besides, high stress level was proved to be an element in accelerating the recrystallization of calcium phosphate crystals