Biomedical Engineering Reference
In-Depth Information
containing aluminium (2-10 wt%) and minor amounts of zinc and manganese
(az31, az91) or with addition of rare earth elements (We43, Lae442)
have been investigated.
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achievements with new alloys developed for
orthopaedic applications have been recently reviewed.
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Biodegradable
magnesium alloys provide a twofold higher tensile strength and fourfold
higher Young's modulus than degradable polymeric implant materials that
are used, like polyglycolic acid (PGa) and polylactic acid (PLa). surface
modifications by alkali-heat treatment or non-toxic coatings can also be
applied to improve the corrosion resistance.
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6.5.5 Bioactive materials
The conventional metallic alloys used for bone replacement are appreciated
for their good mechanical properties and reasonable biocompatibility. after
implantation into the living body, they become spontaneously encapsulated,
without direct contact with bone, by a fibrous tissue with a thickness that
is proportional to the amount and toxicity of the dissolution products. This
granulomatous hypertrophic tissue allows diffusion of ions and microparticles
and impairs the mechanical and biological stability of the interface. It is
interesting to note that bioinert materials, as titanium or its alloys, may
elicit a minimal fibrous encapsulation, whereas biotolerant materials, like
stainless steel and CoCr alloys, give rise to a thicker fibrous capsule (up to
about 2 mm thick).
encapsulation of the implant is not observed in the case of some bioactive
ceramics, used to repair bones, which promote an increase in the formation
of new mineralized bone although their low mechanical properties limit
their use in loading applications. Therefore, the search for biomaterials that
facilitate the osseointegration is one of the key objectives in the development
of a new generation of dental and orthopaedic implants. Continuous efforts
have been addressed by developing surface modifications or coating by using
chemical or biological methods that are out of the scope of this chapter.
6.5.6 Metallic scaffolds
The first generation of modern biomaterials (mid-20th century) selected for
medical applications were high-performance industrial materials originally
developed for airplane components. The goal was to achieve a minimal
response from the host tissue; therefore, they were intended to be bioinert.
In the 1980s, the trend was to induce a controlled reaction with the tissues,
which yielded the second generation of biomaterials which were considered
to be bioactives. The third generation of biomaterials is intended to regenerate
the tissue rather than its replacement. a key concept is the use of scaffolds,
in which cells could be seeded, proliferated and differentiated
in vitro
