2.1 Introduction

Synthetic materials are finding increasing applications in medicine and allied fieids and, in particular, the role of plastics is becoming more important. There are also many instances where different materials, either singly or in combination, are uniquely suited to solve a particular medical problem. A typical example is the pacemaker, an implantable battery-powered device whose hction is to stimulate and regulate the patient's heart rhythm under a wide range of conditions.

This, and other devices, will be considered in more detail in subsequent chapters, but it is sufficient here to understand that a modern pacemaker contains in its makeup a mix of plastics, rubbers and metals, which have to contend with two quite different sets of problems:

• Mechanical ie robustness, fatigue-resistance. lightness

• Chemical ie acceptable to the body and impervious to its fluids

The spectrum of materials available for use in medicine is made up of three main classes: metals, ceramics and polymers (plastics and rubbers), each with its own characteristic range of properties. In order, therefore, to be able to decide which material to use, and to derive the maximum benefit from it, it is essential that its place in the spectrum is thoroughly understood what it can and, equally important, what it cannot do.

2.2 Structure Property Relationships in Materials

All materials are made up from assemblies of atoms, and there are two principal factors which control the properties of the material:

• The nature and variety of the atoms (building bricks)

• The arrangement of the atoms in...