In vitro degradation of magnesium metal matrix composites containing bredigite

Abstract

Ti is currently the most popular material for bone fracture fixation devices. One of the disadvantages of using Ti and other metals is that they are too strong and too stiff, causing the stress-shielding effect. Often, a second invasive surgery is needed to remove the implant after the healing process is completed. Biodegradable plates and screws can eliminate the need for implant removal operations. In recent years, much attention has been paid to developing Mg and its alloys for orthopedic applications. These materials possess densities and elastic moduli closer to those of the human bone than other metallic biomaterials for permanent implants. However, Mg corrodes too rapidly in physiological environments, which has halted the advances towards its clinical applications. Moreover, Mg lacks bioactivity to promote cell growth and speed up the healing process. The degradation rate of Mg can be reduced and its bioactivity enhanced by adding a bioactive ceramic agent, e.g., bridigite Ca7Mg(SiO4)4 with proven bioactivity, to Mg to form Magnesium Metal Matrix Composites (Mg-MMCs). With powder metallurgy (P/M) techniques, the maximum amount of bioceramic addition has been limited to 15 vol.%, above which ceramic particles tend to form agglomerates, negatively affecting mechanical properties [1, 2]. The present study aimed at exploring the possibility of adding bredigite particles up to 40 vol.% to a Mg powder and determine the benefits in terms of the reduction in degradation rate and formation of bone-like apatite (Ca-P-containing compounds) on composite surface. Mg-MMCs with 0, 10, 20, 30 and 40 vol.% of bredigite were prepared from powder mixtures using a vacuum hot press. Bredigite particles were uniformly distributed in all the Mg-MMCs. No structural disintegrity could be observed under optical microscope, as shown in Fig. 1. The degradation rates of Mg-MMCs were determined by measuring the amount of evolving H2 during immersion tests in the DMEM cell culture medium (Dulbecco's Modified Eagle's Medium) for up to 24 h and the amounts of ions released or lost using an inductively coupled plasma atomic emission spectroscopy (ICP-AES). Fig. 2 compares the amounts of H2 after 24 h immersion in DMEM. Clearly, H2 evolution decreased with increasing volume fraction of bredigite, confirming the benefits from adding bredigite to Mg. Mg-40Br exhibited the lowest amount of H2, corresponding to the lowest rate of degradation. Fig. 3 shows the amounts of ions (Mg, Si, Ca and P) released from samples to DMEM or lost from DMEM over time. All the samples released increasing amounts of Mg over time, indicating gradual degradation. Mg-40Br consistently released the least amounts of Mg, confirming the results of H2 measurement. The substantial differences in Mg release between Mg-0Br and Mg-20Br demonstrated the effect of bredigite in slowing down the degradation of Mg. Ca and P should be treated together since they form Ca-P-containing precipitates on sample surface. The losses of Ca and P in DMEM over time imply the deposition of Ca-P compounds on sample surface. A maximum amount of Ca was lost to pure Mg after 24 h, while P losses were very similar between all the samples. Considering the fact that bredigite contained about 42 wt.% of Ca, the loss of Ca in DMEM leading to Ca-P deposition must have been counteracted by Ca release from the composites as a result of bredigite degradation. In conclusion, Mg-MMCs with large volume fractions of bredigite were successfully made using the (P/M) technique. The in vitro degradation tests in DMEM showed decreasing amounts of H2 with increasing volume fraction of bredigite, confirming the beneficial effect of bredigite in slowing down the degradation of Mg. After 24 h, the amount of free Ca in DMEM was larger for the composites with larger fractions of bredigite, suggesting the release of Ca ions to compensate for the loss of Ca for Ca-P precipitation on composite surface.