Biomedical Engineering Reference
In-Depth Information
There is always a concern that the corrosion of metals and alloys that the
implant is made of could occur in the wet, salty surroundings of the human body,
which could release toxins into the systemic circulation. A large number of metals
and alloys, including aluminum, copper, zinc, iron and carbon steels, silver, nickel,
and magnesium are discarded for usage as they are too reactive in the body. When
stainless steel was introduced into general engineering as a new corrosion-resistant
material, it was soon utilized in surgery. However, many implant metals corrode in
vivo and release varying amounts of ions into the local tissues and corrosion prod-
ucts are formed that might be systemically transported to distant sites and accu-
mulate in the organs. For example, when implanted, Co-Cr-Mo alloys release Co,
Cr, and Mo ions to host tissues, and over time the level of metal ions may become
significant to cause toxicity problems. The local and systemic consequences of such
release include metabolic, immunological, and carcinogenic effects. Released metal
ions may contribute to inflammatory and hypersensitivity reactions, changes in tis-
sue remodeling processes that lead to aseptic loosening of the implant. Also, cobalt
shows signs of causing anemia by inhibiting iron from being absorbed into the
blood stream. Ulcers and central nervous system disturbances have been detected
as a result of chromium. Aluminum present in some implant materials may cause
epileptic effects.
Surface treatment techniques to form a passivating oxide layer are used to
minimize corrosive effects. The oxide layers at the surface of CoCr alloys and stain-
less steel provide chemical stability and thus good protection from further corro-
sion. Presence of oxide layers show protective effects with minimal release of ionic
or corrosion byproduct residue into the surrounding tissue. However, the periodic
removal and reformation of the passive oxide film under fretting conditions can
lead to a significant increase in corrosion and in the rate of formation of wear
fragments. To reduce the amount of corrosion, better quality materials such as
titanium, fiber-reinforced composites, and ceramics are selected.
Polymers are generally stable in the physiological environment. However, in
some cases it is the additives added to improve the property of the polymer that
cause concerns. For example, additives (plasticizers, stabilizers) added while manu-
facturing polyvinyl chloride tubing could leach and cause toxic effects. Another
factor is the use of degradable polymers in tissue engineering and controlled drug
delivery. The problem is to formulate materials that degrade in a controlled manner
with tissue-acceptable degradation products. The chemical resistance and physi-
cal property of polymers are based on the type and arrangement of atoms in the
polymer chain. In general, polymers that are hydrophilic and contain hydrolyzable
linkages are most likely to suffer from degradation. On the other hand, polymers
that are hydrophobic or do not contain hydrolysis-susceptible bonds are much less
prone to environmental attack. Polymers containing carbon-fluorine bond such as
poly(tetra fluoro ethylene) have a much wider range of chemical resistance than
polymers containing carbon-hydrogen bonds such as poly(ethylene). However, hy-
drophobic polymers with no reactive sites may degrade slightly by the action of
many enzymes present in the body. Degradation of the polymer could result in
swelling, softening, discoloration, or delamination in the case of composites.
Degeneration of an implant could occur due to indirect interactions with the
body. For example, glutaraldehyde cross-linked bioprosthetic heart valves fail due
