Biomedical Engineering Reference
In-Depth Information
impurities were detected in some zirconia ceramics. Measured radioactivity
levels are low but more work needs to be done on this aspect.
the surface degradation of zirconia balls caused by the phase transformation
do not seem significant. However, zirconia femoral heads have a relatively
short history and more investigation is required. the mechanical integrity of
all ceramic components is dependent on manufacturing quality controls. in
general, ceramic particulate debris is chemically stable and biocompatible
and causes undesirable biological responses at high concentrations.
Other possible improvements in these almost bioinert ceramics could
come from the preparation of alumina/zirconia composites. Alternatively,
new advances could come from research into non-oxide bioinert ceramics,
such as nitrides and carbides like Si
3
n
4
or SiC.
7.2.3 Carbons
they are a group of compounds included in the category of almost bioinert
ceramics that can be made in many allotropic forms: graphite, diamond,
nanocrystalline glassy carbon and PyC. In addition, carbon fibres have been
also used in reinforced biomedical composites. Recently, new variations
of carbon, such as nanotubes and bucky-balls, were produced and their
potential clinical applications have also been explored. However, the most
common form of carbon used in implants is PyC which can be presented in
two forms: lti and Ulti.
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From the mid-1960s LTI-PyC received great attention in the biomaterials
industry, specifically for mechanical heart valves, because it is highly
thromboresistant and exhibits good biocompatibility with blood and soft
tissues. Moreover, lti carbon shows excellent durability, strength and
resistance to wear and fatigue. nowadays over 90% of mechanical heart-valve
prostheses utilize components manufactured from silicon alloyed LTI-PyC
as a coating on a polycrystalline substrate or as a monolithic material. Up to
20 wt% silicon is added to lti carbon to improve its mechanical properties
without significant changes in biocompatibility. To obtain more complicated
shapes and mechanical properties, an impermeable layer of Ulti carbon
can be obtained on different substrates by vapour deposition. On the other
hand, because of the low density and weakness of the glassy carbons they
are used as thick coatings mechanically reinforced by the substrate.
PyC is also used for small orthopaedic joints such as fingers and spinal
inserts. attempts to use carbon in other clinical applications, such as dental
implants, vascular, ligaments, tendons and so on, obtained low success rates.
Research to improve PyC is devoted towards eliminating the small amounts
of silicon carbide in the coatings, which are partially thrombogenic, and to
improve its mechanical properties. in addition, a better characterization of
the obtained materials is required.
