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Encyclopedia of Physical Science and Technology EN002G-62 May 19, 2001 19:27

174 Biomaterials, Synthetic Synthesis, Fabrication, and Applications

biomaterials has occurred in response to the growing num- Many of the materials used today in the clinical envi-
ber of patients afflicted with traumatic and nontraumatic ronment were not originally engineered for biomaterials
conditions. As the population grows older there is an in- applications. In crude terms, they became “biomaterials”
creased need for medical devices to replace damaged or when, by a series of trial-and-error experimentation, they
worn tissues. The market is a billion dollar per year market were implanted in the human body in a variety of forms
and requires the skills of clinicians, surgeons, engineers, and found to “work.” Clearly there were many other mate-
chemists, physicists and materials scientists to work coop- rials which did not ‘work’ causing at the least perhaps pain
eratively in the development of materials for clinical use. and discomfort to patients and at the worse unnecessary
There are five classes of biomaterials; metals, ceramics, suffering and death. Increase in litigation for problems
biological materials/polymers, synthetic polymers, and allegedly caused by biomaterials has caused some com-
composites. The choice of material to replace biological panies to remove products from sale in this area and may
tissue is largely governed by the physical properties of lead to a distinct shortage of available materials for device
both the natural tissue and the proposed replacement. In fabrication. For a variety of reasons, trial-and-error opti-
general, natural and synthetic polymers are used to replace mization is not the way forward for the production of the
skin, tendon, ligament, breast, eye, vascular systems, and next generation of materials to be used in the human body.
facial tissues, and metals, ceramics and composites are What is required is the systematic design of a wide
usedtoreplaceorreinforceboneanddentin.Replacements range of materials with specific functions for predefined
for these natural tissues clearly require materials of differ- medical use. The goal is to produce biomaterials that
ent strength. Table I shows the strength of the main groups smoothly integrate into living systems rather than fighting
of natural materials together with the synthetic counter- against the normal functioning of a living body. There are
parts used in the development of replacement materials. many factors to consider in order to understand how this
It is often the case that the strengths of the materials used might be accomplished. The factors include the structure
to replace natural components are stronger and/or stiffer of the original material to be replaced or augmented, its
which often leads to problems of compatibility both in re- physiology, anatomy, biochemistry and biomechanics in-
spect of mechanical behaviour of the implant within the cluding pathophysiological changes that have necessitated
host and in terms of the biologic response. the use of a substitute biomaterial. In addition, as devices
are often present in the body for considerable periods of
time then it is necessary to understand the natural degen-
TABLE I Physical Properties of Tissues and Materials Used
in Their Replacement erative processes of normal tissues, particularly in relation
to the biomaterial substitute. This latter area is at present
Ultimate strength Modulus very poorly understood. All of the above clearly impact on
Material (Mpa) (MPa)
the design and development of materials for clinical usage.
Natural materials Thus materials need to be developed with a clear under-
Soft tissue standing of the nature and extent of interactions between
Arterial wall 0.5–1.72 1.0 the device (whatever it is) and the surrounding tissue. It
Hyaline cartilage 1.3–1.8 0.4–19 cannot be emphasised too strongly the importance of bio-
Skin 2.5–16 6–40
Tendon/ligament 30–300 65–2500 compatibility in the development of the next generation
Hard tissue (bone) of materials for applications in a biological environment.
Cortical 30–211 16–20 (GPa) This chapter will describe the materials currently used
Cancellous (porous) 51–193 4.6–15 (GPa) as biomaterials and routes to their formation. It will de-
Synthetic materials scribe some aspects of the structural chemistry of natural
Polymers materials that are to be replaced or augmented and it will
Synthetic rubber 10–12 4
Glassy 25–100 1.6–2.6 (GPa) look at the way forward for the design of materials for use
Crystalline 22–40 (0.015–1) (GPa) in the medical environment in the 21st century.
Metal alloys
Steel 480–655 193 (Gpa)
Cobalt 655–1400 195 (GPa) II. ASPECTS OF THE STRUCTURAL
Platinum 152–485 147 (GPa) CHEMISTRY OF NATURAL MATERIALS
Titanium 550–680 100–105 (GPa)
Ceramics USED IN THE HUMAN BODY
Oxides 90–380 (Gpa) 160–4000 (GPa)
Hydroxylapatite 600 19 (GPa) Biological organisms make use of proteins, polysaccha-
Composites rides and combinations of these two types of molecule in
Fibers 0.9–4.5 (Gpa) 62–577 (GPa)
the polymeric phases that are found in a living organism
Matrices 41–106 0.3–3.1
together with simple calcium salts. Chemical composition