Magnesium Metal Matrix Composites for Biomedical Applications

Introduction

Material science is a branch of science that investigates different materials for various applications like automobile, aerospace, and especially in medical implantations. Materials that are used for introduced into the human body environment are called biomaterials. Biomaterials have excellent mechanical properties and must be biocompatible, bio-adhesive, biofunctional, corrosion resistant, and osteoconductive (Gohil et al., 2012). The biomaterial must have tensile strength, ductility, and the ability to absorb strain energy. These properties have been satisfying biomedical applications such as joint replacements, bone plates, wires, screws, rods, dental implants, and cardiovascular stents (Katz et ah, 1980). The properties of biomaterials are briefly given in Table 14.1. Biomaterials are further classified into different categories such as metals, ceramics, polymers, and composites in ceramic-based biomaterials; the most commonly used materials are calcium phosphate materials because of their excellent non-toxicity, biocompatibility, and osteoconductive nature in the human body environment (Kalyan et ah, 2016).

Ceramic materials are bioactive, bioinert, biodegradable, and exhibit poor mechanical properties due to lack of hardness. These were limitedly developed as compared to metallic and polymeric materials. Polymeric-based biomaterials are most commonly used in bone tissue fixation applications as they can easily develop any type of complex shape and size. The mechanical and chemical properties may change to certain degrees during the sterilization process; surface properties may also be able to be easily modified. These are limited in applications because of their unsatisfactory mechanical properties, and toxic additives

TABLE 14.1

Biomaterial Properties

such as plasticizers and stabilizers that are used in the synthesis of polymers. Polymeric biomaterials can be harmful in healing tissue and also damage the human body’s metabolism (Ratner et ah, 2005).

Metallic implants are generally used to repair fractured tissues and give support to heal the tissue. The commercially available metallic implants like cobalt chromium, stainless steel, and titanium-based alloys and composites are used as metallic implant material to heal tissues (Wu et ah, 2013). These materials have outstanding mechanical properties such as hardness, load-bearing capacity, ductility, and high strength; additionally, biometalic implants can be produced in any type of complex shape and size. Mechanical properties and biocompatibility are very imperative factors for the implant materials. However, most metallic implants are not biodegradable, after tissue healing the implants may begin to corrode and damage the human body’s metabolism. This is due to chemical reactions, which release metal ions causing a toxic environment in the human body.

Magnesium composites are different when compared to other biomaterials because of their biocompatibility, biodegradability, and mechanical properties. Magnesium composites are used as cardiovascular stents and orthopedic implants because magnesium is one of the essential nutrients for human metabolism (Charyeva et ah, 2016). The biodegradable material completely dissolves In vivo and In vitro conditions after the tissue healing, so there is no need to remove the implant material from the human body. By implanting biodegradable material into the physiological environment, a nontoxic oxide layer may develop without causing any harm to the metabolism. Excess oxides may generate, which will be exerted in the urine system. The major components presented in the biodegradable implant material will not be metabolized in the human body and also show a suitable degradation rate to heal the tissue. The primary drawback of magnesium implants is their low corrosive stability in the physiological environment. Much research has undertaken the challenge to increase its corrosive resistance to heal the tissue.

Magnesium biodegradable biomaterial is classified as pure metal, alloys and metal matrix composites. Pure magnesium and its alloys were already introduced as implant materials and faced a problem in that those corrode quickly before healing the tissue. Developing of metal matrix composites was meant to reduce the degradation rate and increase the mechanical properties to withstand stress shielding effect before heal the tissue. Metal matrix composites used as implant material exhibit the properties of tensile strength, compressive strength, elastic modulus, and corrosive resistance by selecting appropriate reinforcement material. The composite may contain metal matrix and reinforcement to increase its mechanical properties as well as weight to strength ratio. The selection of metal matrix and reinforcement plays a very important role because it should be biodegradable, nontoxic, and biocompatible in the physiological environment.

Magnesium is very light material and has a density of 1.74-2.0 g/cc. Mostly magnesium implants were used as orthopedic implants due to their similarity in properties with human bones. Table 14.2 shows the mechanical properties of magnesium material to the human bone (Xu et al.. 2009).Mostly Mg-Ca, Mg-Sn, Mg-Zn,

TABLE 14.2

Mechanical Properties of Composites and Human Bone (Bommala et al., 2019)

Mg-Sr, Mg-Si, and Mg-Zr magnesium alloys are used in biomedical applications. Magnesium-based composites mostly using reinforcement materials of calcium- based ceramics, calcium phosphate particles (CPP). hydroxyapatite (HAP), and tricalcium phosphate (В-TCP) (Asgar et al., 2009).