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Biomaterials - An Introductory Overview

1.1 Introduction

Materials have been used to treat, replace, or augment tissues and organs in the human body since antiquity but their use and degree of sophistication have increased significantly over time, particularly over the last several decades. Advances in materials science, biological sciences, physical sciences, and engineering along with an evolution in medical treatment over the last several decades have led to the creation of biomaterials with more reproducible properties, better performance, and increased functionality. These advances have resulted in a considerable increase in the range of use and the efficacy of biomaterials. Nowadays, millions of lives are being improved or saved by the use of biomaterials in fracture fixation plates, implants for total hip and knee joint replacement, dental implants and restorations, heart valves, vascular grafts and stents, contact and intraocular lenses, skin substitutes, and wound healing materials, for example (Figure 1.1).

Figure 1.1 Examples of biomaterials in use for medical and dental applications. (a) Fracture fixation plate; (b) implant for total hip replacement; (c) implant for total knee replacement; (d) dental implant; (e) heart valve; (f) vascular graft; (g) intravascular stent; (h) intraocular lens; (i) degradable suture; (j) degradable screw for fracture fixation; (k) degradable polymer microsphere for delivery of therapeutics; (l) functional skin substitute.

The applications of biomaterials are many. Table 1.1 provides a list of selected applications and the types of materials used in these applications. Many of the biomaterials used in these applications were selected from durable, chemically inert materials that were available off the shelf, and they were designed to serve, mainly, a mechanical (or physical) function. The last few decades have seen a shift in emphasis in which the biological sciences are playing a role in the design of biomaterials of significance comparable to that of materials science. Biomaterials are now no longer designed to be chemically inert or to just serve a mechanical function. Instead, advances in biological sciences are being used to design biomaterials to regenerate tissues and organs and to direct the response of specific cells and tissues. In doing so, these biomaterials stimulate the body to heal itself. An example is the creation of functional skin substitutes to treat patients with severe burns (Figure 1.1l).

Table 1.1 Key applications of synthetic materials and modified natural materials in medicinea.

Source: Modified from Ratner (2013).

a Data compiled from many sources - these numbers should be considered rough estimates that are growing with changing markets and new technologies. Where only US numbers are available, world usage is estimated at 2.5 times the US usage.

The importance of biomaterials to society has been increasing significantly in the last several decades, both as an academic field, an area of research to develop new or improved devices, and as an industry (Figure 1.2). The number of biomedical engineering (or bioengineering) departments in academic institutions has increased rapidly over the last few decades. There are over 75 biomedical engineering (or bioengineering) departments in the United States alone. Biomaterials are an important area of teaching and research in these departments and they are often emphasized in engineering disciplines such as materials science and engineering, chemical engineering, and mechanical engineering.

Figure 1.2 Schematic showing the major components of the biomaterials field.

Outside of academic institutions, biomaterials are also an important area of research, development, and manufacturing in several industries, such as in the production of medical devices, dental restorations, and devices for drug delivery. The size of companies that manufacture biomaterials and devices for these applications cover a wide range, from small start-up companies to large established companies that are among the so-called Fortune 500 companies in the United States. The commercial market for devices that incorporate biomaterials is immense and it is projected to grow at a healthy rate. Estimates of the commercial market and the number of implantable medical devices that incorporate biomaterials annually are presented in Table 1.1.

1.2 Definition and Meaning of Common Terms

Several terms are commonly used in the biomaterials literature and throughout this book. This section provides a description of the meaning and definition of some of these common terms that are particularly relevant to this chapter.

Biomaterial

Our understanding of what a biomaterial is and, consequently, proposed definitions of the term biomaterial have evolved over time in response to advances in science and technology. An early use of the term and the beginnings of the biomaterials field emerged from symposia held in the United States around the late 1960s and early 1970s. One definition, endorsed by a consensus of experts in the field, is

• A biomaterial is a nonviable material used in a medical device, intended to interact with biological systems.

As biomaterials evolved, another definition was proposed:

• A biomaterial is a material intended to interface with biological systems to evaluate, treat, augment, or replace any tissue, organ or function in the body (Williams 1999).

While these early definitions reflected our understanding and use of biomaterials at that time, they are now too restrictive because of the evolution of biomaterials over the last two to three decades. Biological sciences, as noted earlier, are now playing an increasing role in designing biomaterials. At the same time,...