Corrosion of Metallic Biomaterials: a Review
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Review Corrosion of Metallic Biomaterials: A Review Noam Eliaz Department of Materials Science and Engineering, Tel-Aviv University, Ramat Aviv 6997801, Israel; [email protected]; Tel.: +972-3-6407384 Received: 17 December 2018; Accepted: 26 January 2019; Published: 28 January 2019 Abstract: Metallic biomaterials are used in medical devices in humans more than any other family of materials. The corrosion resistance of an implant material affects its functionality and durability and is a prime factor governing biocompatibility. The fundamental paradigm of metallic biomaterials, except biodegradable metals, has been “the more corrosion resistant, the more biocompatible.” The body environment is harsh and raises several challenges with respect to corrosion control. In this invited review paper, the body environment is analysed in detail and the possible effects of the corrosion of different biomaterials on biocompatibility are discussed. Then, the kinetics of corrosion, passivity, its breakdown and regeneration in vivo are conferred. Next, the mostly used metallic biomaterials and their corrosion performance are reviewed. These biomaterials include stainless steels, cobalt-chromium alloys, titanium and its alloys, Nitinol shape memory alloy, dental amalgams, gold, metallic glasses and biodegradable metals. Then, the principles of implant failure, retrieval and failure analysis are highlighted, followed by description of the most common corrosion processes in vivo. Finally, approaches to control the corrosion of metallic biomaterials are highlighted. Keywords: biomaterials; biocompatibility; corrosion; failure; titanium alloys; stainless steels; shape memory alloys; biodegradable metals; metallic glasses; body environment 1. Introduction Biomaterials are commonly defined as nonviable materials intended to interface with biological systems to evaluate, treat, augment or replace any tissue, organ or function of the body [1]. Before a new biomaterial is introduced to the market, various issues are considered, including its designated anatomic location, functional tissue structure and pathobiology, mechanical and other property requirements, toxicology, biocompatibility, the healing process, ethics, standardization and regulation [2]. Material, device and procedure standards are issued by international organizations, mainly the International Standards Organization (ISO) and The American Society for Testing and Materials (ASTM). To preclude ineffectually tested devices and materials from coming on market and to filter entities clearly unqualified to produce biomaterials, regulatory systems have been established by both the USA and the European Union. While the assessment in the USA is by a government agency (i.e., the U.S. Food & Drug Administration, FDA), in Europe it is by Notified Bodies (NBs). While the focus of the latter is primarily on proof of safety, the former puts significant emphasis also on effectiveness of a device. An essential European requirement for marketing is first obtaining a CE Marking. It should be noted that the FDA does not regulate the materials used in medical devices but rather the devices themselves. Biocompatibility is an essential requirement of a biomaterial. A biocompatible material performs with an appropriate host response (i.e., minimum disruption of normal body function) in a specific application [3]. Thus, the material causes no thrombogenic, toxic or allergic inflammatory response when it is placed in vivo. There are two key factors determining the biocompatibility of a material: Materials 2019, 12, 407; doi:10.3390/ma12030407 www.mdpi.com/journal/materials Materials 2019, 12, 407 2 of 92 the host reactions induced by the material, and the degradation of the material in the body environment. Often, both factors should be considered. Since about 4000 years ago, humans have been using artificial materials to repair fractured and diseased tissues and organs. In the early ages, the Greeks and Egyptians implanted wood and bones from animals in humans. The development of advanced biomaterials is related to the development of modern medicine and advanced materials. Only in 1546 was a synthetic material (gold plate) used to repair a cleft palate. Vanadium steel was developed in the early 1900s specifically for implants [4]. Its first application was bone fracture fixation plates introduced by Sherman and aimed at stabilizing bone fractures and accelerating their healing. Quickly, however, implant dysfunctionality due to corrosion, mechanical failure and poor biocompatibility was reported. In 1924, Zierold [5] reported the effect of various metals on the surrounding tissues. When inserted to bone, copper and nickel caused significant discoloration of the surrounding tissue, while iron and steel dissolved rapidly and aggravated tissue erosion. Although certain pure metals such as gold, silver and aluminium did not cause tissue discoloration, there were too soft for most medical devices. In 1926, the 18Cr–8Ni (wt%) stainless steel was first used in implants. This steel was both more corrosion resistant in vivo and stronger than the vanadium steel. Later that year, molybdenum was added to the steel to improve its corrosion resistance in chloride-containing water. This alloy is known as 316 stainless steel. In 1940, titanium and its alloys were first considered for orthopedic practice. These materials had been used in aircraft applications and showed excellent corrosion resistance in seawater. Therefore, good corrosion resistance in vivo could be anticipated. This was indeed observed after implant retrieval. In 1947, Maurice Down introduced a variety of orthopedic devices such as plates and screws made of titanium. In the 1950s, the 316L stainless steel was introduced. The carbon content in this alloy was reduced from 0.08 wt% to 0.03 wt% in order to improve the corrosion (sensitization) resistance and weldability compared to 316 stainless steel. In the 1960s, Sir John Charnley, the British orthopedic surgeon, introduced the first successful total hip replacement (THR) in patients suffering from osteoarthritis (OA). The damaged femoral head was removed and the hip replaced with a stainless steel ball and a high-density polyethylene (HDPE) socket. Methacrylate bone cement was used for implant fixation. This may be regarded as the beginning of modern orthopedics, in which the development of novel materials plays a central role. THR is one of the most successful and cost- effective operations in the whole of medicine. Charnley's operation has improved the quality of life of millions of humans by relieving pain of hips with arthritis and avascular necrosis, restoring mobility and correcting deformity. Nowadays, biomaterials are made of metals and alloys, ceramics, polymers and composites. Figure 1 illustrates some applications of metallic biomaterials. One example is the vascular stents made of stainless steel or shape memory alloy (SMA), sometimes coated with a polymer for drug eluting [6]. The global coronary stents market size was estimated at USD 9.3 billion (milliard) in 2016 and is expected to reach USD 15.2 billion by 2024. Another example is the use of calcium phosphate (CaP) bioceramics in the field of bone regeneration, both in orthopedics and in dentistry [7–32]. CaPs are common in the form of coatings on titanium implants, but they are also used as scaffolds, bone fillers and cements. Corrosion is an important factor in the design and selection of metals and alloys for service in vivo. Allergenic, toxic/cytotoxic or carcinogenic (e.g., Ni, Co, Cr, V, Al) species may be released to the body during corrosion processes. In addition, various corrosion mechanisms can lead to implant loosening and failure [33–39]. Therefore, biomaterials are often required to be tested for corrosion and/or solubility before they are approved by regulatory organizations. Hence, the corrosion behaviour of metallic implant materials has been widely studied, in the framework of quality assurance, implant retrieval analysis and failure analysis. The objective of this review paper is to introduce the reader with the fundamentals of biomaterials corrosion. First, the body environment is described in Section 2. As shown, this environment is harsh and puts several challenges with respect to corrosion control. Subsequently, the principles of biocompatibility are presented in Section 3, because this term is often used in corrosion- related reports. Next, Section 4 discusses the kinetics of corrosion, including passivity, its breakdown Materials 2019, 12, 407 3 of 92 and regeneration. Section 5 presents the major metals and alloys currently used in biomedical applications. A brief discussion of implant failure, retrieval and failure analysis is given in Section 6. Section 7 then reviews the most important corrosion mechanisms in vivo. Finally, Section 8 focuses on strategies for corrosion control in vivo. Figure 1. Application of metallic biomaterials as implants in different areas of the human body [39]. Reproduced with permission from Bentham Science Publishers. 2. The Body Environment The body environment and its effects on corrosion have been reviewed in many manuscripts (see, for example, [32,38–45]). The water content of the human body ranges from 40% to 60% of its total mass. Functionally, the total body water can be subdivided into two major fluid compartments, namely the extracellular and the intracellular fluids. Extracellular fluids (ECFs) consist of the plasma found in the blood vessels, the interstitial fluid that