Recent studies have shown great potential of Mg matrix composites for biodegradable orthopedic devices. However, the poor structural integrity of these composites, which results in excessive localized corrosion and premature mechanical failure, has hindered their widespread applications. In this research, an in-situ Powder Metallurgy (PM) method was used to fabricate a novel biodegradable Mg-bredigite composite and to achieve enhanced chemical interfacial locking between the constituents by triggering a solid-state thermochemical reaction between Mg and bredigite particles. The reaction resulted in a highly densified and integrated microstructure, which prevented corrosion pits from propagating when the composite was immersed in a physiological solution. In addition, chemical interlocking between the constituents prohibited interparticle fracture and subsequent surface delamination during compression testing, enabling the composite to withstand larger plastic deformation before mechanical failure. Furthermore, the composite was proven to be biocompatible and capable of maintaining its ultimate compressive strength in the strength range of cortical bone after 25-day immersion in DMEM. The research provided the necessary information to guide further research towards the development of a next generation of biodegradable Mg matrix composites with enhanced chemical interlocking.@en

When a bone is fractured, it loses its structural integrity which makes it unable to bear any mechanical load. Therefore, a broken bone must be supported until it regains its strength to handle the body's movement and weight. A surgical procedure is needed to set a fractured bone. This procedure often involves repositioning the bone fragments into their natural position and then, attaching them together using internal fixation devices such as plates and screws. These fixation devices restore load-beanng capacity to bone, allowing the fractured bone to be healed by the primary bone healing mechanism. To date, implants used for internal fixadon are usually made from titanium and stainless steel, which are strong but, notorious for triggering adverse reactions such allergic responses caused by implant erosion in patients. Therefore, permanent fixtures should be removed from the body after the fractured bone heals sufficiently, which imposes another invasive surgery on the patient. The advent of biodegradable magnesium-based composites about two decades ago was an attempt to address the clinical complications regarding the permanent fixtures. However, magnesium-based composites are still in their infancy, and a have a lot to achieve before being considered as fully functional materials for bone fixation purposes. Currently, there are two major issues with magnesium composites. Firstly, most of the magnesium-based composites made to date lack sufficient mechanical integrity, making them unsuitable for load-bearing applications. The second, and the most important, issue would be the rapid degradation of magnesium when exposed to physiological solutions, causing pre-mature mechanical failure before the patient fully recovers. The main aim of this thesis is to provide the necessary background and technical information to address these issues, and to be a reliable platform for future researches on the subject to build upon. @en

The present research was aimed at developing magnesium-matrix composites that could allow effective control over their physiochemical and mechanical responses when in contact with physiological solutions. A biodegradable, bioactive ceramic - bredigite was chosen as the reinforcing phase in the composites, based on the hypothesis that the silicon- and magnesium-containing ceramic could protect magnesium from fast corrosion and at the same time stimulate cell proliferation. Methods to prepare composites with integrated microstructures - a prerequisite to achieve controlled biodegradation were developed. A systematic experimental approach was taken in order to elucidate the in vitro biodegradation mechanisms and kinetics of the composites. It was found that the composites with 20–40% homogenously dispersed bredigite particles, prepared from powders, could indeed significantly decrease the degradation rate of magnesium by up to 24 times. Slow degradation of the composites resulted in the retention of the mechanical integrity of the composites within the strength range of cortical bone after 12 days of immersion in a cell culture medium. Cell attachment, cytotoxicity and bioactivity tests confirmed the stimulatory effects of bredigite embedded in the composites on the attachment, viability and differentiation of bone marrow stromal cells. Thus, the multiple benefits of adding bredigite to magnesium in enhancing degradation behavior, mechanical properties, biocompatibility and bioactivity were obtained. The results from this research showed the excellent potential of the bredigite-containing composites for bone implant applications, thus warranting further in vitro and in vivo research.@en

Mineral trioxide aggregate (MTA) has been used as root-end filling material in dentistry due to its excellent sealing ability as well as biocompatibility. However, some drawbacks of MTA cements, including low cohesive property and long setting time, have limited their widespread applications. In this research, we evaluated the effect of three setting solutions, namely distilled water, malic acid, and chitosan solution on the mechanical, handling, and setting properties of the cements. The chitosan-containing cements showed superior setting time and handling properties over the water- and malic-acid-containing cements. The mechanical strength of water- and chitosan-containing cements was almost similar but significantly improved as compared to malic-acid-containing cements. Our results from morphology and chemical analysis showed that the chitosan-containing MTA cements improved the hydroxyapatite-formation ability as compared to those which were made by water and malic acid. It was concluded that the chitosan-containing MTA cements have an excellent potential to be acceptable as root-end filling materials.@en

In our previous study, we developed Mg-matrix composites with bredigite as the reinforcing phase and achieved improved degradation resistance in comparison with Mg. However, the effects of materials processing method and process parameters on the mechanical behavior of the composites before and during degradation were still unknown. This research was aimed at determining the mechanical properties of Mg-bredigite composites prior to and during degradation. It was found that by optimizing the process parameters of Pressure Assisted Sintering (PAS), low-porosity Mg-bredigite composites with strong interfaces between homogeneously distributed bredigite particles and the Mg matrix could be fabricated. By reinforcing Mg with 20 vol% bredigite particles, the ultimate compressive strength and ductility of Mg increased by 67% and 111%, respectively. The in vitro degradation rate of the Mg-20% bredigite composite in a cell culture medium was 24 times lower than that of monolithic Mg. As a result of retarded degradation, the mechanical properties of the composite after 12 days of immersion in the cell culture medium were comparable to those of cortical bone. The encouraging results of this research warrant further investigations on the in vivo degradation behavior and mechanical properties of the composites.@en

In our previous study, we developed Mg-matrix composites with bredigite as the reinforcing phase and achieved improved degradation resistance in comparison with Mg. However, the effects of materials processing method and process parameters on the mechanical behavior of the composites before and during degradation were still unknown. This research was aimed at determining the mechanical properties of Mg-bredigite composites prior to and during degradation. It was found that by optimizing the process parameters of Pressure Assisted Sintering (PAS), low-porosity Mg-bredigite composites with strong interfaces between homogeneously distributed bredigite particles and the Mg matrix could be fabricated. By reinforcing Mg with 20 vol% bredigite particles, the ultimate compressive strength and ductility of Mg increased by 67% and 111%, respectively. The in vitro degradation rate of the Mg-20% bredigite composite in a cell culture medium was 24 times lower than that of monolithic Mg. As a result of retarded degradation, the mechanical properties of the composite after 12 days of immersion in the cell culture medium were comparable to those of cortical bone. The encouraging results of this research warrant further investigations on the in vivo degradation behavior and mechanical properties of the composites.@en

In our previous study, we developed Mg-matrix composites with bredigite as the reinforcing phase and achieved improved degradation resistance in comparison with Mg. However, the effects of materials processing method and process parameters on the mechanical behavior of the composites before and during degradation were still unknown. This research was aimed at determining the mechanical properties of Mg-bredigite composites prior to and during degradation. It was found that by optimizing the process parameters of Pressure Assisted Sintering (PAS), low-porosity Mg-bredigite composites with strong interfaces between homogeneously distributed bredigite particles and the Mg matrix could be fabricated. By reinforcing Mg with 20 vol% bredigite particles, the ultimate compressive strength and ductility of Mg increased by 67% and 111%, respectively. The in vitro degradation rate of the Mg-20% bredigite composite in a cell culture medium was 24 times lower than that of monolithic Mg. As a result of retarded degradation, the mechanical properties of the composite after 12 days of immersion in the cell culture medium were comparable to those of cortical bone. The encouraging results of this research warrant further investigations on the in vivo degradation behavior and mechanical properties of the composites.@en

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. @en