The biodegradability of the scaffolds was examined by immersing them in Sorensen’s phosphate-buffered solution and monitoring the percentage mass loss, the viscosity average molecular weight, and bending strength. In one case rapid degradation was simulated by immersing for 3 weeks at 50 °C, while in the other regular degradation was simulated for 56 days at 37 °C. Upon immersion the untreated fiber composites underwent MAO and therefore rapid corrosion was avoided.
For both rapid and normal degradation a low mass loss was observed that was always below 1%. It did increase with an increase of fiber concentration which suggests that the MAO fibers were prone to some degradation, which was however negligible. The viscosity average molecular weight (Mv) decreased faster for both PLA and PLA-fiber composites at rapid degradation conditions. However, the degradation rate of both types of scaffolds was at comparable values for both degradation conditions until the last incubation day (i.e. 21st and 56th) in which the Mv was significantly higher for the fiber reinforced scaffolds. The bending stiffness for all types of scaffolds examined under the normal degradation conditions decreased between 60 and 75% over the 56 day period. However, at rapid degradation conditions the pure PLA and low content fiber composites (up to 10%) exhibited a ~ 90% decrease in bending stiffness. Increasing, however, the fiber content to 20% resulted in a 65% retention of the initial value . This suggests that at the initial stages of implantation the role of Mg-based fibers in the mechanical stability of the scaffold is not that crucial, however, as bone regeneration requires more than a few weeks it is important to use such fiber reinforcement to assure that the implant will be able to withstand the skeletal forces until sufficient osteogenessis has occurred. The morphology of the scaffold after degradation at normal and accelerated rates is shown in Fig. 4.8.
Fig. 4.8 Changes in the morphology of the Mg-based alloy fibers after immersion in Sorensen’s phosphate-buffered solution for (a) 56 days at 37 °C and (b) 21 days at 50 °C (Reproduced with permission from Ref. )
The degradation process proposed for the PLA fiber composites is as follows: first the PLA reacts with water which initiates the hydrolysis of the PLA and forms acidic carboxyl end groups . As the PLA begins to degrade the water reacts with the surface of the fibers (i.e. the MAO coating) fracturing it and penetrating further into the Mg-alloy fiber. During degradation of the coating, OH- and Mg2+ are dissolved and released into the immersion fluid, giving rise to a higher mass loss with an increase in % fiber in the composites. The release of OH- is desired since they diffuse into the PLA and react with the acidic hydrogen ions from the acidic carboxyl end groups that form during degradation of the PLA. This allows for a steady pH to be attained throughout implantation, which allows for a significantly lower and controlled degradation rate of PLA but also minimizes bone resorption, which can result from decreasing pH values [45, 46]. Therefore, the present fiber PLA composites have a long term stability that is significantly enhanced over pure PLA scaffolds.