Background
Biomaterials improve the quality of life for an ever increasing number of people each year. The range of applications is vast and includes such things as joint and limb replacements, artificial arteries and skin, contact lenses, and dentures. While the implementation of these materials may be used for medical reasons such as the replacement of diseased tissues required to extend life expectancies, other reasons may include purely for aesthetic ones including breast implants. This increasing demand arises from an ageing population with higher quality of life expectations. The biomaterials community is producing new and improved implant materials and techniques to meet this demand, but also to aid the treatment of younger patients where the necessary properties are even more demanding. A counter force to this technological push is the increasing level of regulation and the threat of litigation. To meet these conflicting needs it is necessary to have reliable methods of characterisation of the material and material/host tissue interactions.
Biomaterial Classifications
Biomedical materials can be divided roughly into three main types governed by the tissue response. In broad terms, inert (more strictly, nearly inert) materials illicit no or minimal tissue response. Active materials encourage bonding to surrounding tissue with, for example, new bone growth being stimulated. Degradable, or resorbable materials are incorporated into the surrounding tissue, or may even dissolve completely over a period of time. Metals are typically inert, ceramics may be inert, active or resorbable and polymers may be inert or resorbable.
Some examples of biomaterials are provided in table 1.
Table 1. Some accepted biomaterials
|
Metals |
Ceramics |
Polymers |
|
316L stainless steel Co-Cr Alloys Titanium Ti6Al4V |
Alumina Zirconia Carbon Hydroxyapatite |
Ultra high molecular weight polyethylene Polyurethane |
Key Properties
The main property required of a biomaterial is that it does not illicit an adverse reaction when placed into service.
The range of applications for biomaterials is large. The number of different biomaterials is also significant. However, in general:
- Metallic biomaterials are used for load bearing applications and must have sufficient fatigue strength to endure the rigors of daily activity eg walking, chewing etc.
- Ceramic biomaterials are generally used for their hardness and wear resistance for applications such as articulating surfaces in joints and in teeth as well as bone bonding surfaces in implants.
- Polymeric materials are usually used for their flexibility and stability, but have also been used for low friction articulating surfaces.
Applications
The following sections provide some detail on biomaterials applications. For more details, refer to particular materials or applications.
Metallic, ceramic and polymeric biomaterials are used in orthopaedic applications. Metallic materials are normally used for load bearing members such as pins and plates and femoral stems etc. Ceramics such as alumina and zirconia are used for wear applications in joint replacements, while hydroxyapatite is used for bone bonding applications to assist implant integration. Polymers such as ultra high molecular weight polyethylene are used as articulating surfaces against ceramic components in joint replacements.
Porous alumina has also been used as a bone spacer to replace large sections of bone which have had to be removed due to disease.
Metallic biomaterials have been used as pins for anchoring tooth implants and as parts of orthodontic devices. Ceramics have found uses as tooth implants including alumina and dental porcelains. Hydroxyapatite has been used for coatings on metallic pins and to fill large bone voids resulting from disease or trauma. Polymers, have are also orthodontic devices such as plates and dentures
Many different biomaterials are used in cardiovascular applications depending on the specific application and the design. For instance, carbon in heart valves and polyurethanes for pace maker leads
Materials such as silicones have been used in cosmetic surgery for applications such as breast augmentation.
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