| Orthopaedic problems are becoming a major issue for our health care, which greatly stimulates the development of novel implantable materials, for which magnesium (Mg) and its alloys are most appealing. Compared to traditional metal-based implants, biomedical Mgbased metals demonstrate outstanding advantages, such as high cytocompatibility, favorable mechanical strength, and good biodegradability in physiological conditions. Moreover, recent studies have revealed that the release of magnesium ions (Mg2+) during implantation would promote the growth of new bone tissue. However, low corrosion resistance, fast hydrogen release, and rapid pH increase around the implants have greatly obstructed their broad clinical application. To alleviate these limitations, surface modification via polymer coating, which can simultaneously control Mg degradation while improving its biocompatibility, is receiving increasing attention. However, many previous studies reported only limited success in producing long-lasting coatings on Mg-based implants. The main issue is related to polymer coatings induced by solvent evaporation that typically lead to a highly porous surface and densely packed bulk, which in turn make it hard to control the release of Mg2+ and suffer from penetration of water and membrane bursts. Consequently, the formation of a favorable asymmetric coating on Mg substrate resulting from a conventional method remains a problem for the development of magnesium-based implants for broad clinical applications.;This thesis mainly aims to investigate a novel strategy for the development of novel polymer coatings that can improve the corrosion resistance of magnesium alloys, which were developed for orthopaedic applications. In particular, we combined the traditional dip-coating method with the wet-phase inversion process to control the formation of various coatings, thus improving the corrosion resistance of magnesium implants, while regulating the release of Mg 2+ for stimulating bone growth.;Chapter 1 provides a brief review of the biomaterial development and Mg-based implants. Both advantages and disadvantages of Mg-based implants are described, respectively. Since significant efforts have been invested to improve the corrosion resistance of Mg-based implants, a systematic review of these strategies, especially polymer coatings, is subsequently presented, and their results and limitations have been summarized and compared. To better illustrate our work, a brief introduction of wet-phase inversion is also presented in this chapter.;Secondly, the preparation of polymer coatings on pure magnesium rods via the wetphase inversion process induced by different organic liquids is described. In this section, we used four different types of organic liquids as coagulation baths to control the phase separation process of the poly(L-lactic acid) (PLLA) solution. The results showed that the wet-phase inversion process could effectively control coating morphologies, compared to the traditional dip-coating method. In vitro results demonstrated that polymer coatings with dense surface and porous inner structure could provide better protection to the Mg substrates. Furthermore, these in vivo results also showed best cytocompatibility and corrosion resistance of coated samples via the wet-phase inversion process. Based on results of both in vitro and in vivo experiments, a new process for the corrosion mechanism of the magnesium implants during immersion has been proposed.;Thirdly, the formation of PLLA coatings on pure magnesium rods via wet-phase inversion process induced by different organic mixtures is presented. A series of ethanol and hexane mixtures were chosen as coagulation baths, and the results showed that using different organic mixtures would also vary morphologies of the coatings. The in vitro results demonstrated that different polymer coatings with various surface and inner structures could provide different protection capabilities to the Mg substrates, which would further control the releasing rate of Mg2+. Furthermore, we studied the influence of concentration of the PLLA solution on the corrosion resistance of the Mg substrates in this section. This illustrated that these strategies offer a promising method to fabricate different coatings to meet various requirements.;Fourthly, the preparation of PLLA coating with incorporated magnesium salts on magnesium alloys via wet-phase inversion process induced by different organic mixtures is presented. The results demonstrated that both the coagulation bath and magnesium salt affected the morphologies of the resulting coatings. The in vitro results showed that not only the existence of the Mg salts, but also their concentrations would have an important influence on the protection capability of the hybrid coatings. Furthermore, the in vitro results indicated that the release of Mg2+ could be regulated by simultaneously controlling the corrosion of the Mg substrate and the dissolution of Mg salt, which would further affect the cell activity during implantation. This provides a new strategy for the preparation of multi-functional coatings, which could both enhance the corrosion resistance of the Mg substrate, while releasing beneficial drugs at the same time.;Since this novel method is not suitable for irregularly shaped substrates, we present the preparation of biomedical polymer coatings on pure magnesium rods from colloidal particles through direct electrophoretic deposition in the last section. Different types of bio-degradable particles (poly(lactic- co-glycolic acid) (PLGA) and poly(dopamine) (PDA)) have been prepared via different methods, including self-organized precipitation, solvent-evaporation, and self-polymerization. It could be demonstrated that all these colloidal particles could be uniformly electrodeposited onto pure magnesium rods. The in vitro results also showed that the deposition of colloidal particles could enhance the corrosion resistance of Mg substrates; however, particles with higher hydrophilicity would result in inferior protection capacity. |
