Page 46 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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

186 Biomaterials, Synthetic Synthesis, Fabrication, and Applications

A range of carbons with different structure are used implant and tissue. In all cases interfacial dissolution of
in the production of artificial heart valves and orthope- the ceramic phase in the saline environment of the body
dic applications. Glassy carbons (showing different de- leads to modifications in surface chemistry which affect
grees of short-range order) and pyrolytic carbons are both the precipitation of the calcium phosphate phase prior
used. Glassy carbon manufactured by pyrolysis of a semi- to cell growth and the more general incorporation of the
coke retains some porosity and has relatively good wear implant. Four major categories of materials have been
resistance. Silicon carbide/carbon composites are pre- developed. These include: Dense hydroxy(l)apatite (HA)
pared by impregnating a porous carbon with liquid silicon. ceramics, Bioactive glasses, Bioactive glass–ceramics,
Carbon-reinforced carbons and other reinforced carbons Bioactive composites.
are made by impregnating organized filaments with a car- Bioactive glasses are conventionally prepared by the
bon filler, compressing, heat-treating, and carbonizing or traditional methods of mixing particles of the oxides or
graphitizing. Composites such as CFSiC admixed with carbonates and then melting and homogenizing at tem-
◦
SiC lead to materials with Young’s moduli close to that peratures of 1250–1400 C. The molten glass is cast into
of bone. They exhibit biological and mechanical stabil- steel or graphite molds to make bulk implants. A final
ity and they are being investigated as new candidates for grind and polish are often required. Powdered materi-
hip joint replacement rather than the metal or oxide phase als are produced by grinding and sieving the ceramic
conventionally used. to achieve the desired particle size characteristics. The
The other ceramic widely used are phosphate salts of chemical components of bioactive glasses include CaO,
calcium, with the chosen phase usually being hydroxyap- P 2 O 5 ,Na 2 O, and SiO 2 . The bonding to bone has been
atite. This material is conventionally prepared by thermal associated with the formation of hydroxyapatite on the
◦
methods at temperatures well in excess of 1000 C. As a re- surface of the implant. Although a range of composi-
sult of their preparation at high temperatures, the salts are tions can be used (up to 60% silica), an even narrower
carbonate free and are made up of much larger and more range of compositions are found to bond to soft tissues.
perfect crystals than those found in biological apatite min- A characteristic of the soft-tissue bonding compositions
erals including bone. The imperfect crystalline structure is the very rapid rate of hydroxyapatite formation. This
of bone mineral leads to the natural material being solu- has previously been attributed to the presence of Na 2 O
ble and reactive with respect to body fluids. In contrast, or other alkali cations in the glass composition which
the synthetic materials are much less reactive than those increases the solution pH at the implant-tissue interface
found in living tissue and problems with biocompatibility and thereby enhances the precipitation and crystallisation
can arise. of hydroxyapatite. The rate of hydroxyapatite formation
In all cases, solid nonporous implants do not allow for has also been shown to be strongly dependent on the ra-
biofilm or cell attachment at any site other than the bulk tio of SiO 2 , the glass network former to Na 2 O, the net-
surface. If porous implants of the previously mentioned work modifier in the glass. When the glass contains over
materials can be made interfacial stability between the 60% SiO 2 or more, bonding to tissues is no longer ob-
implant and tissue will increase as cells will migrate into served. The solubility and chemistry (including diffusion
the structure. For example, bone will grow in pores greater of Na ions, for example, by the addition of La 2 O 3 )ofthe
+
than 100 µm in diameter and a blood supply can be main- glass phase can be modified by the incorporation of other
tained throughout a material with such porosity. However, phases.
such materials show reduced strength and toughness. A Problems which associated with the conventional high
compromise is the application of porous ceramic coatings temperature method of production arise from:
to metals as in-growth of, for example, bone can occur
3+
at the porous interface with the mechanical load being 1. Highly charged impurities such as Al ,Zr ,Sb ,
3+
4+
4+
carried by the bulk–metal substrate. Problems with such Ti ,Ta 5+ etc., which can be picked up at any stage
implants usually arise from any incompatibilities between of the preparation process. The incorporation of
the metal substrate and ceramic film rather than between impurity ions leads to dramatic reductions in
the ceramic and the natural tissue which overgrows the bioactivity.
implant. 2. Processing steps such as grinding, polishing etc. all
expose the bioactive powder to potential
contaminants.
2. Bioactive Ceramics
3. There is a compositional limitation on materials
Bioactiveceramicsaredefinedasthosewhicharenontoxic prepared by the conventional high temperature
and biologically active and that favor the development methods due to the extremely high equilibrium
◦
of an interfacial bond, 0.01 to 200 µm thick between liquidus temperature of SiO 2 , 1713 C, and the