Surface Coating and Modification of Metallic Biomaterials

Cuie Wen

• Publisher: Woodhead Publishing
• Year: 2015
• Print ISBN: 9781782423034
• eBook ISBN: 9781782423164
• Topic: Technology & Engineering
• Subtopic: Medical Technology & Supplies
• Index: Technology & Engineering

Despite advances in alternative materials, metals are still the biomaterial of choice for a number of clinical applications such as dental, orthopedic and cardiac implants. However, there are a number of intrinsic problems associated with implanting metal in the biological environment, such as wear, corrosion, biocompatibility and toxicity, which must be addressed. Modern technology has enabled scientists to modify metal surfaces or apply special coatings to metals to improve their performance safety. Surface Coating and Modification of Metallic Biomaterials will discuss the most important modification techniques and coatings for metals, first covering the fundamentals of metals as a biomaterial and then exploring surface modification techniques and coatings.- An expansive overview of surface modification techniques for biomedical use- In-depth exploration of issues arising from metal biomaterial use- Includes examples of applications in a clinical setting

Part One

Fundamentals of surface coating and modification of metallic biomaterials

Outline

1. Introduction to surface coating and modification for metallic biomaterials

2. Surface properties and characterization of metallic biomaterials

1

Introduction to surface coating and modification for metallic biomaterials

A. Nouri¹, and C. Wen^{2 1}Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, ON, Canada ²School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Bundoora, VIC, Australia

Abstract

Biomaterials are invariably in contact with living tissues. Thus, interactions between the surface of a synthetic material and biological environment must be well understood. This chapter reviews the benefits and challenges associated with surface modification of the metals in biomedical applications. The chapter elaborates how the surface characteristics of metallic biomaterials, such as surface chemistry, topography, surface charge, and wettability, influence the protein adsorption and subsequent cell behavior in terms of adhesion, proliferation, and differentiation at the biomaterial–tissue interface. The chapter also highlights various techniques required for surface modification and coating of metallic biomaterials, including physicochemical and biochemical surface treatments and calcium phosphate and oxide coatings.

Keywords

Cell−material interaction; Metallic biomaterials; Protein adsorption; Surface modification; Surface properties

1.1. Introduction

The demand for safe and effective materials is greatly increasing in biomedical engineering due to the annual increase of the world’s population, the growing number of older people, and the high functional demands of younger people. The fundamental requirement of a biomaterial is that the material and the surrounding physiological environment should coexist without having any undesirable effect on one another. Because the surface is the interface where the biomaterials meet and interact with the biological environment (i.e., bone, soft tissue, blood), the surface properties are the major factors that ultimately determine the rejection or acceptance of a biomaterial in the body. The biological events that regulate host responses to materials such as protein adsorption and cell adhesion occur at the biomaterial–tissue interface and are modulated by the physicochemical properties of the material (Anderson, 2001).

Although the bulk properties of the biomaterial are critical determinants of the biological performance of the material, it is rare that a biomaterial with suitable bulk properties also possesses appropriate surface characteristics for clinical applications, and very few surfaces are truly biocompatible. In these cases, a biocompatible modified layer with suitable wear and corrosion resistance would mitigate the above problems. In addition, despite the fact that the mechanical properties of biomaterials are dictated by the bulk properties, tissue–biomaterial interactions are governed by surface properties. These interactions have been hypothesized to occur within a narrow zone of less than 1 nm (Ong and Lucas, 1998).

Because the top layer of surface atoms are those that are in immediate contact with the other phases (i.e., gas, liquid, or solid), this top layer of surface atoms could be regarded as the surface. On the other hand, the structure and chemistry of that top layer of atoms or molecules are significantly determined by the atoms or molecules immediately below. This implies that the surface could be the top 2–10 atomic or molecular layers (say, 0.5–3 nm) (Vickerman, 2009). Atoms at the surface of metallic materials are considered partly reactive to the environment because atomic configuration terminates at the surface. Due to high surface energy, a single molecular layer readily forms on the solid surface where gas molecules are adsorbed at 1 Pa in 10⁻⁴ s. For example, in the presence of oxygen atoms, oxygen and metal atoms chemically bond together to form an oxide layer. This means that the surface composition of a metal is different from its bulk composition in the order of nanometers (Hanawa, 2004).

Surface treatment of biomaterials offers the ability to improve material and biological responses through changes in a material’s surface chemistry, topography, energy, and charge, while still maintaining the bulk properties of the implant. Surface modifications can broadly be classified into three categories: (1) addition of materials of desirable functions to the surface; (2) conversion of the existing surface into more desirable compositions and/or topographies; and (3) removal of material from the existing surface to create specific topographies (Duan and Wang, 2006).

Surface modification of biomaterials can yield a more profitable return in a much shorter time compared to the painstaking and laborious processes of inventing novel materials. With surface modification, chemical and mechanical durability and tissue compatibility of a surface layer would be improved. From an economical point of view, surface modification is considered an inexpensive process because only the surface layer needs to be modified. In other words, with surface modification, the key physical properties of a biomaterial can be retained while only the outermost surface is modified to tailor to the biointeractions. The chief purpose of surface modification is to improve corrosion resistance, wear resistance, antibacterial property, bioadhesion (bone ingrowth), and biocompatibility, while other important requirements such as adequate mechanical strength and processability are governed by the bulk material properties. Surface modifications should provide distinct properties of interaction with biomolecules or cells of the biological environment. These would promote, for example, the adaptation or in growth of cells onto the surface of fixation elements of artificial joints, or prevention of cellular interaction with the surface to inhibit endothelial cell proliferation to provide cardiovascular devices with a suitable blood compatible surface (Thull, 2010).

There is no universal technique for the surface modification that can be applied to all biomaterials, and variations exist depending on the application and the type of materials. For instance, in bone implant materials, rapid bone conductivity is required; materials of cardiac stents have to be structured to avoid cell proliferation provoking restenosis; in cardiovascular devices, blood compatibility or antithrombogenecity is required; in dental implants, soft-tissue compatibility is required to prevent bacteria invasion from the crevice, which is between the dental implant and gingival epithelium (Hanawa, 2004).

The choice of a suitable method is dependent on many factors, including the substrate material, component design and geometry, cost, and the end applications in which two aspects of the surface engineering process, coating thickness and process temperature, are often highlighted.

1.1.1. Reasons for surface modification

The environment inside the body is mechanically, chemically, and electrically active, and the interface between an implanted biomaterial and the biological environment (such as hard and soft tissue, blood, body liquid, or saliva) is the location of a variety of dynamic biochemical processes and reactions. The biological response from living tissues to the biomaterials is reliant on the surface properties such as chemical composition, cleanliness, topography, surface energy, corrosion resistance, and the tendency to denaturalize neighboring proteins.

Unfavorable conditions, e.g., premature loading that leads to micromotion, will disrupt the newly forming tissue and lead to the formation of a fibrous capsule (Puleo and Nanci, 1999). The interface of such fibrous capsule is mechanically weak and is susceptible to micromotion, which results in premature loosening. The surface properties can be selectively modified to enhance biological and/or tribological performance of the implants for a variety of orthopedic and dental applications. To this end, the development of novel surfaces has been the principle driving force behind many studies to promote bone formation and firm implant−bone attachment by reducing the implant micromotion and minimizing fibrous tissue formation around the implant. The fast fixation of an implant to hard tissue (or the early implant−bone integration) allows early physiologic loading and thus prevents the bone from disuse atrophy (Duan and Wang, 2006). It also reduces the hospitalization time and cost and improves the quality of life for patients.

When considering the surface modification for a blood-contacting material or device, scientists and engineers should develop materials that suppress the adsorption of blood components (such as proteins, lipids, or calcium) onto the surface of the material and the adhesion of blood cells or components of blood cells onto the surface of the material (Dee et al., 2002).

1.1.2. Pros and cons of metallic biomaterials

Metals are inorg...

Table of contents

• Cover image
• Title page
• Table of Contents
• Related titles
• Copyright
• List of contributors
• Woodhead Publishing Series in Biomaterials
• Part One. Fundamentals of surface coating and modification of metallic biomaterials
• Part Two. Surface-modification techniques for metallic biomaterials
• Part Three. Applications of surface modification and coatings
• Index