Materials for Biomedical Engineering. Mohamed N. Rahaman

       In recent years, there has been a radical shift in the approach to designing and selecting biomaterials, in which biological sciences are now playing a significant role, often comparable to materials science and engineering. Instead of the previous approach in which biomaterials were normally selected from durable materials available off the shelf to serve a mainly mechanical function, biomaterials are now being designed to be degradable or bioactive, to deliver biomolecules at targeted sites in the body, and to elicit specific responses from cells and tissues to stimulate the body to heal itself;

       Biomaterials have become a truly interdisciplinary (multidisciplinary) field that serves an important role in society as an academic field, as a research area for developing new or improved devices, and as a manufacturing industry.

Problems

      1 1.1 Distinguish between the terms degradable and bioactive as used to describe biomaterials.

      2 1.2 Discuss the materials design characteristics required for intravascular stents and how the design of the intravascular stent has evolved over the last few decades.

      3 1.3 Discuss the materials design and selection issues for the femoral stem of an artificial hip joint.

      4 1.4 Bearing couples in artificial hip joints are composed of (i) a metal ball articulating against a polyethylene liner (metal‐on‐polyethylene) or (ii) a ceramic ball articulating against a ceramic liner (ceramic‐on‐ceramic). Compare the engineering and biological issues related to the use of these two bearing couples.

      5 1.5 Describe the major ways in which the so‐called contemporary biomaterials differ from those previously used.

References

      1 Langer, R. and Vacanti, J.P. (1993). Tissue engineering. Science 260: 920–926.

      2 Rahaman, M.N., Yao, A., Bal, B.S. et al. (2007). Ceramics for prosthetic hip and knee joint replacement. Journal of the American Ceramic Society 90: 1965–1988.

      3 Ratner, B.D. (2013). A history of biomaterials. In: Biomaterials Science: An Introduction to Materials in Medicine, 3e (eds. B.D. Ratner, A.S. Hofmann, F.J. Schoen and J.E. Lemons), xli–liii. New York: Elsevier.

      4 Williams, D.F. (1999). The Williams Dictionary of Biomaterials. Liverpool, UK: Liverpool University Press.

      5 Williams, D. (2014). Essential Biomaterials Science. Cambridge University Press; Chapter 1.

Part II Materials Science of Biomaterials

2 Atomic Structure and Bonding

      2.1 Introduction

      A guiding principle in materials science is that the structure of a material controls its properties that, in turn, determine its applications. Consequently, an understanding of the structure of materials is crucial to understanding how their properties arise and how to create materials with optimal properties for specific applications. As biomaterials are typically three‐dimensional solids composed of an enormous number of atoms, we are concerned with their structural characteristics at different levels or length scales. In general, two types of structures have the most significant effect on the properties of solids.

      The first type of structure is at the atomic level. Atomic structure refers to the type of bonding between the atoms to form molecules. In this level of structure, we also include the way in which the atoms pack together to form a controllable structure. If the atoms pack together in a regularly repeating, ordered manner, the structure is described as crystalline. On the other hand, if the atoms pack together in a random non‐ordered manner, the structure is said to be amorphous.

      The second type of structure is at a larger scale, referred to as the microstructure. This refers to the nature, quantity, and distribution of the structural components or phases in the solid. A phase is a region of material with uniform physical and chemical properties. Water, for example is composed of a single phase whereas a mixture of ice and water consists of two phases, a solid and a liquid. A pure metal such as titanium is composed of a single phase. On the other hand, at room temperature, the titanium alloy Ti6Al4V is composed of two solid phases. A porous Ti6Al4V implant is composed of three phases, two solid phases and porosity that is considered a gaseous phase.

Table 2.1 shows the main types of structure in materials and their scales. The atomic structure determines the intrinsic properties of the material (Section 1.4), such as the melting point, coefficient of thermal expansion, whether the material is strong or weak, ductile or brittle, electrically conducting or not, and so on. In comparison, many of the properties crucial to the engineering or technological applications of a material are strongly dependent on its microstructure. Most ceramics, for example, are intrinsically strong due to their atomic structure. On the other hand, the actual measured strength relevant to their design for specific engineering applications is strongly dependent on their microstructure, such as the presence of pores within the solid phase, the size of crystals that make up the solid phase, and the presence of microcracks in the solid phase.

Table 2.1 Scales of structure in solids.

Structure	Scale (m)
Atomic structure	10−10
Crystal or amorphous structure	10−9
Nanostructure	10−9–10−7
Microstructure	10−7–10−4
Macrostructure	>10−4

      The scale of the crystalline or amorphous structure represents the level of the smallest controllable structure in which the atoms are packed to give a solid structure. When present, structural features at the nanoscale (referred to as the nanostructure) often lead to a significant improvement in the engineering properties of a material and can modulate cell and tissue response to the material in vivo. In situations where it is not emphasized, the nanostructure is often included within the more general and widely used term microstructure. The macrostructure, such as the external shape or geometry of the material, is often incorporated within the design criteria for the intended application while flaws at the macroscopic level, such as large observable cracks, should be eliminated by proper processing methods.

      In this chapter, we will discuss the atomic structure of materials, focusing on the bonding between atoms and molecules. Atomic packing resulting in either crystalline or amorphous structures will be discussed in the next chapter along with the microstructure. Together, these two chapters provide the fundamentals that underlie how the properties of materials are dependent on their structure and how this structure can be modified or manipulated to produce desirable properties for the intended application.

2.2 Interatomic Forces and Bonding Energies

The strength of the bond between two atoms can be described in terms of a force between the atoms which binds them together. A simple way of visualizing this force is in terms of a tiny spring that links one atom to the other. However, unlike a mechanical force due to a spring, the force between two atoms is more complex in nature and depends on the particular type of bonding between the atoms. Interatomic forces refer to these forces between atoms. Intermolecular forces refer to forces between molecules whereas intramolecular