Introduction and bio-requirements

12.2.1 Introduction and bio-requirements

Biomaterials are materials used in medicine and dentistry that are intended to come in contact with living tissue. The familiar tooth filling is where most humans first encounter biomaterials, but increas- ingly many people now rely on more critical implants such as joint replacements, particularly hips, and cardiovascular repairs. Undoubtedly, these biomaterial implants improve the quality of life for an increasing number of people each year, not just for an ageing population with greater life expectancy, but for younger people with heart problems, injuries or inherited diseases.

The requirements for a biomaterial are extremely demanding. Replacement or repair of a body feature, tissue, organ or function often necessitates the material used to have specialized mechanical, physical and chemical properties. However, the very first requirement is biocompatibility with the human body, i.e. the ability of the material to perform with an appropriate host response. Unfor- tunately, no material is universally biocompatible, since a material may be biocompatible in one application but not with another. Biocompatibility is therefore application specific.

For the successful use of the biomaterial, consideration has to be given to the appropriate material selection, engineering design and manufacturing process. While proper design and manufacture is essential, it is particularly important to select the correct material to provide the appropriate properties as well as being biocompatible, recognizing that the combined influence of mechanical and chemical factors can be quite serious, e.g. causing fatigue, corrosion fatigue, stress corrosion, wear, fracture. It is also important to recognize that the biological environment is not constant, and that oxygen levels, availability of free radicals and cellular activity will vary. Corrosion and degradation can lead to loss of integrity of the implant and, of course, release ions into the body, often setting up an allergic reaction.

584 Physical Metallurgy and Advanced Materials Table 12.1 Composition of orthopedic implant alloys (wt%); from Bonfield, 1997.

Element Cobalt-based alloys

Titanium alloys ASTM

Stainless steel

Commercial Ti–6Al–4V F75

ASTM

ASTM F563 ASTM

ASTM

purity cast

F90

isostatically F138/A

F138/9B

titanium Co

0.25 max Mo

Fe 0.75 max

Balance Balance Al

0.08 max Mn

C 0.35 max 0.05–0.15

0.03 max 0.025 max – – S

0.01–0.015 0.012 max N

0.05 max

Biomaterial applications make use of all classes of material, metals, ceramics, polymers and composites, divided roughly into three user types. These are (i) inert or relatively inert with minimal host response, (ii) bioactive, which actually stimulates bonding to the surrounding tissue, and (iii) biodegradable, which resorb in the body over a period of time. Metals are generally chosen for their inert qualities, whereas ceramics and polymers may offer bioactivity or resorption.

The most common metallic materials used are austenitic stainless steels, cobalt–chromium alloys or titanium; typical compositions are shown in Table 12.1. Recently, titanium alloys, particularly Ti–6Al–4V, have been introduced because of their corrosion resistance, strength and elastic modulus

(see Table 12.2), but poor tribology can still be a problem. It is also favored for its superior biocompati- bility and, unlike Co–Cr or stainless steel, does not cause hypersensitivity. Of the ceramics, aluminum oxide, calcium phosphate, apatite, carbon/graphite and bioglass are in use mainly for their inertness, good wear characteristics, high compressive strength and in some cases bioactivity. Their poor tensile properties and fracture toughness are design limitations. Polymers are widely used, both alone and in combination with ceramics or metals. These include: polymethyl methacrylate (PMMA) for cement and lenses; polyethylene for orthopedics; polyurethane as blood contact material, e.g. vascular tubing, cardiovascular devices, catheters; polysiloxanes in plastic surgery, maxillofacial and cardiovascular surgery; polyesters and polyamides in wound closure management. Composites such as ultra-high- molecular-weight polyethylene reinforced with either carbon fibers or the ceramic hydroxyapatite are increasingly being considered for applications involving high contact stress and wear resistance.