Production methods and characterization of porous Mg and Mg alloys for biomedical applications

Magnesium (Mg) and some of its alloys are attracting increasing interest as promising biodegradable materials for medical applications due to their analogous mechanical properties in relation to bone tissues, functional roles in the human body, decent biocompatibility, and higher strength to weight ratio than other metallic, polymeric, and ceramic biomaterials. Porous Mg and Mg alloys can be used as bone substitutes with bone-mimicking characteristics. Since the structures of bone tissue and porous Mg materials are similar, host bone cells can grow into the pores of an Mg scaffold and form new bone tissue. Moreover, the gradual biodegradation of porous Mg and Mg alloys after implantation, followed by the absorption of Mg ions in the body, eliminates the necessity of removing the implant via a subsequent surgical procedure, which makes Mg scaffolds more favorable than other porous metallic biomaterials. Different production methods have been developed in the fabrication of porous Mg and Mg alloys, which can be classified into two categories: the melt processing method and the solid processing method, according to the state of materials that are being processed. Casting, metal–gas eutectic solidification, infiltration, and negative salt-pattern molding processes are examples of melt processing methods in which a blowing agent is introduced into the molten metal followed by stirring, or by infiltration of a bed of hollow spheres, or by the introduction of a gas or even a vacuum into Mg melt prior to solidification. Powder metallurgy with a blowing agent or space holder can be considered as a solid processing method. There are also other methods in which Mg alloy chips, strips, or fibers are used as a starting material, and a porous structure is formed through one or both of the melt and solid processing methods. Some porous Mg and Mg alloys have also been made through a combination of metallurgical and mechanical processes. The technological advancements as well as the increase in understanding of porous structures and properties cause many methods to be utilized to produce porous structures. However, some methods are not economically viable. Some methods produce a porous structure that is not suitable for implant applications due to the nonbiocompatible residues in the final product or inappropriate pore size and porosity. Therefore it is crucial to choose a production method that is suitable to produce porous Mg and Mg alloys with a controlled pore size, shape, and porosity that are compatible with medical applications. This chapter elucidates a variety of manufacturing methods for porous Mg and Mg alloys. The macro- and microstructures, mechanical and corrosion properties, and in vitro and in vivo biocompatibility of different porous Mg and its alloys are highlighted. We provide insights for a comprehensive understanding of the characteristics of porous Mg and Mg alloy produced through different methods in order to enable the fabrication of Mg scaffolds with improved mechanical and biological properties. Furthermore, the manufacturing methods for producing Mg scaffolds are technologically, scientifically, and technically advanced and economical. Future research directions in the field of porous Mg alloy biomaterials are also provided.