Bioceramics fulfil a unique function as biomedical materials. The development of biomaterials and manufacturing techniques has broadened the diversity of applications within the human body.
Behaviour of Ceramics in the Human Body
Ceramics employed within the body can fall into all three biomaterial classifications i.e. Inert, resorbable and active, meaning they can either remain unchanged, dissolve or actively take part in physiological processes.
What Forms Can Bioceramics Take?
Bioceramics are available as:
• Thin layers or coatings on a metallic implant
• Porous networks
• Composites with a polymer component
• Large well polished surfaces
What Materials are Classified as Bioceramics?
Materials that can be classified as bioceramics include:
• Calcium phosphates
• Silica based glasses or glass ceramics,
• Pyrolytic carbons
Some of these are reviewed in more detail below.
What Functions Do Bioceramics Fulfil?
Bioceramics satisfy needs as diverse as low co-efficients of friction for lubricating surfaces in joint prostheses, surfaces on heart valves that avoid blood clotting, materials that stimulate bone growth and those that can harness radioactive species for therapeutic treatments.
Calcium Phosphate Bioceramics
There are several calcium phosphate ceramics that are considered biocompatible. Of these, most are resorbable and will dissolve when exposed to physiological environments. Some of these materials include, in order of solubility:
Tetracalcium Phosphate (Ca4P2O9) > Amorphous calcium Phosphate > alpha-Tricalcium Phosphate (Ca3(PO4)2) > beta-Tricalcium Phosphate (Ca3(PO4) 2) >> Hydroxyapatite (Ca10(PO4)6(OH)2)
Unlike the other calcium phosphates, hydroxyapatite does not break down under physiological conditions. In fact, it is thermodynamically stable at physiological pH and actively takes part in bone bonding, forming strong chemical bonds with surrounding bone. This property has been exploited for rapid bone repair after major trauma or surgery.
While its mechanical properties have been found to be unsuitable for load-bearing applications such as orthopaedics, it is used as a coating on materials such as titanium and titanium alloys, where it can contribute its 'bioactive' properties, while the metallic component bears the load. Such coatings are applied by plasma spraying. However, careful control of processing parameters is necessary to prevent thermal decomposition of hydroxyapatite into other soluble calcium phosphates due to the high processing temperatures.
Alumina and Zirconia
Alumina and Zirconia are known for their general chemical inertness and hardness. These properties are exploited for implant purposes, where it is used as an articulating surface in hip and knee joints. Its ability to be polished to a high surface finish make it an ideal candidate for this wear application, where it operates against materials such as ultra high molecular weight polyethylene (UHMWPE).
Porous alumina has also been used as a bone spacer, where sections of bone have had to be removed due to disease. In this application, it acts as a scaffold for bone ingrowth.
Single crystal alumina or sapphire has also been used in dental applications, although its use in this application is declining with the advent of more advanced materials such as resin-based composites.
Pyrolytic carbon is commonly used in artificial heart valves and has been the most popular material for this application for the last 30 years. Properties that make this material suitable for this application include, good strength, wear, resistance and durability, and most importantly, thromboresistance, or the ability to resist blood clotting.
Pyrolytic carbon is also used for small orthopaedic joints such as fingers and spinal inserts.