Bioresorption and Degradation of Biomaterials - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 320

and has therefore been studied extensively in metallic biomaterials [ 30 , 31 ].
This corrosion is facilitated by anions, cations, biological macromolecules and
mechanical load, which are available in abundance in the physiological environment
into which biomaterials are implanted [ 32 ]. On the side of the implant, geometric,
metallurgical and surface properties play a role [ 30 ]. Degradation may result from
electrochemical dissolution phenomena, wear, or a synergistic combination of the two
[ 30 ]. The electrochemical degradation can be generalized, affecting the whole surface,
localized at areas that are shielded from tissue fluids (crevice corrosion), or random
(pitting corrosion) [ 30 ]. Examples of interactions between electrochemical and
mechanical influences include stress corrosion cracking, corrosion fatigue and fretting
corrosion, which may cause premature structural failure and accelerated release of
degradation products [ 30 ]. These degradation products include wear debris, colloidal
organometallic complexes (specifically or non-specifically bound), free metallic ions,
inorganic metal salts or oxides, and precipitated organometallic storage forms [ 33 ].
These products can form complexes that sometimes elicit hypersensitivity responses
[ 33 ]. There are also reports indicating possible side effects due to release and accu-
mulation of metal ions into the surrounding tissues [ 34 ]. Thus the current aim of
researchers and engineers is to control the rate of degradation/corrosion by developing
novel alloys while minimizing the effects of the products of degradation.
2.1.1 Iron
Cardiovascular diseases account for the majority of the deaths worldwide, and the
numbers are only expected to grow in the coming years [ 35 ]. This has led to sizeable
resources being allocated to research in developing better arterial stents, which help
combat coronary heart diseases. It is well know that iron is an essential element in the
human body. For this reason iron is thought to offer acceptable biocompatibility
when used in degradable materials investigated for stent applications [ 36 ]. Iron has
good mechanical properties and a relatively low degradation rate. Its biodegradation
leads to oxidation of iron into ferrous and ferric ions, which then dissolve in bio-
logical fluids [ 37 ]. Iron stents can be fabricated by casting or using electroformed
iron (E-Fe), which has a faster degradation speed with a uniform degradation
mechanism. E-Fe also inhibits cell proliferation of smooth muscle cells without
decreases in metabolic activity [ 36 ]. This may be due to the increased degradation
speed, probably leading to higher concentrations of ferrous ions around the cells,
which has been shown to reduce the proliferation of smooth muscle cells under in
vitro conditions [ 37 ]. For application as stent material, this may be an additional
benefit, as it may inhibit neointimal hyperplasia and in-stent restenosis [ 37 ].
2.1.2 Magnesium
The use of magnesium in biological systems dates back to 1906 when it found
application as fracture fixation plates [ 38 ]. Thus when novel applications for
biodegradable implants were developed recently, magnesium was an obvious