Synthetic Materials in Medicine

1 Introduction

The use of synthetic materials in the body by medical and dental practitioners to provide repair and function has grown remarkably in the last 30-40 years, though the concept of using such artificial materials is very old. For example, Plaster of Paris was pioneered as bone-substitute material towards the end of the nineteenth century, and dental fillings, including amalgam, have been around for well over 150 years. The use of engineered structures fabricated from metals and polymers in orthopaedic surgery has a more recent history, however, beginning with Dr (later Sir) John Charnley's work on the replacement of arthritic hips in the early 1960s. This surgical repair technique, known as total hip arthroplasty, has seen particularly spectacular growth, and since Charnley's original cemented hip replacements there have been a variety of new materials and new designs for implantable devices, and these are now available not only for hips, but also for knees, toes and fingers.

Synthetic materials used in the body in this way are widely referred to as biomaterials. This use of the term appears to have originated in 1967 with the first 'International Biomaterials Symposium' at Clemson University, South Carolina, since which time it has been used extensively in this way. In many ways to apply the word biomaterials to synthetic materials is not very satisfactory since by analogy with, for example, the word biochemistry, it might be assumed to refer to materials of biological origin. However, within the field of implantable devices, the word biomaterial has been formally defined as a non-viable material used in a biomedical device intended to interact with biological systems. This definition was adopted at the Consensus Conference of the European Society for Biomaterials, held at Chester, UK, in March 1987, and has been widely accepted ever since. In fact, some sort of definition of this type was already implicit in the title of the organisation which ran the meeting, the European Society for Biomaterials, because the Society's object from the time it was established in 1976 was to promote the study the science of such synthetic materials. It was never primarily concerned with the science of natural substances, such as teeth or bones. The current definition was also implicit in the title of the scientific journal Biomaterials, which was first published in 1980. Whatever the rights and wrongs of the etymology, by usage the term biomaterial has now clearly come to mean a synthetic material with a biological destination rather than a biological origin.

There is a further caveat with the term, in that it is usually applied to materials designed to reside within the body for some considerable time. Thus, materials used to fabricate devices used only in surgery, ranging in sophistication from sensors to catheters, are not usually regarded as biomaterials. They may interact with a biological system, the body, but such interaction is usually relatively brief. Sutures, too, are not usually regarded as biomaterials for a similar reason. On the other hand, degradable polymers of the type used in sutures are finding increasingly novel uses in medicine, for example as temporary scaffolds and supports for bone immobilisation. These enable the body's own repair mechanisms time to bring about complete healing without premature loading and potential failure and under these circumstances, the polymers become biomaterials, because their interaction with the body must continue for a considerable time.

The field of biomaterials science encompasses all classes of material, i.e. polymers, ceramics, glasses and metals, and a wide range of branches of surgery: dental, ophthalmic, orthopaedic, cardiovascular and so on. The key requirement of any material or combination of materials used in the body is that, in addition to providing mechanical support or repair, it should be biocompatible. The subject of biocompatibility is covered in detail in Chapter 6, but at this stage we should note its definition. This is the ability of a material to perform with appropriate host response in a specific application. As stated in this definition, biocompatibility is not a property of a material per se; the material needs to elicit an appropriate response, and whether such a response is appropriate will depend on the site in the body at which it has been placed. A material which shows excellent biocompatibility, for example, in contact with bone would not necessarily show good biocompatibility when used in a blood-contacting device, such as an artificial heart valve. Thus the location within the body is as important in determining whether a material is biocompatible as the composition of the material.

The property of biocompatibility is distinct from that of inertness, which would imply a complete absence of response from the body. At one stage, it was thought that inertness was a desirable property, but nowadays inertness is not thought possible. Even materials which seem inert in most technical applications, such as polytetrafluoroethylene, PTFE, prove to be highly active when placed within the body. PTFE was once used to fabricate the acetabular cups used in experimental hip replacement surgery. When used in conjunction with a metal femoral head, it proved to have extremely poor wear characteristics, leading to build-up of high local concentrations of particulate wear debris. This wear debris provoked extreme adverse reactions in patients, leading to severe swelling and general discomfort. Consequently, the use of PTFE for this purpose was abandoned.

Because of experiences of this type, there has been a shift in thinking and the emphasis nowadays is on materials that will elicit a response from the body that is appropriate. This may be, as in the case of titanium implants, anchorage without formation of fibrous capsule. Although it desplays this desirable feature, titanium is by no means inert the human body. It may undergo corrosion, and this can be so severe that the tissues close to the implant become darkened by the build up of titanium within the cells. Despite the potential for such adverse effects, in general the presence of titanium is well tolerated by the body, and the use of titanium for the fabrication of implants is a current feature of many branches of surgery.

The successful use of biomaterials presents numerous challenges. A major one is the issue of maintenance, and in particular that most devices are implanted well into the body and therefore not freely available for inspection or repair. An artificial hip joint, for example, is completely inaccessible, except by major surgery, and hence cannot be routinely serviced. The body is a hostile environment, despite its sensitivity, and it provides very severe service conditions. In no other field of technology are manufactured items expected to function without maintenance for so long in comparably demanding conditions.

Life expectancy in the wealthier parts of...