Bioresorption and Degradation of Biomaterials - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 321

choice for evaluation [ 39 ]. The modulus of elasticity of magnesium is closer to
bone than the elastic modulus of commonly used metal implants like titanium or
cobalt-chrome-molybdenum-alloys. This property of magnesium makes it inter-
esting for orthopedic applications. The corrosion rate of pure magnesium is
however too high for application as an implant material [ 40 ]. Studies have shown
that corrosion as well as mechanical properties can be positively influenced when
magnesium is alloyed with rare earth metals [ 40 - 44 ], The properties of these
magnesium alloys may help develop implants that can act as a scaffold on which
new bone can grow, as well as fixtures to hold together bone long enough to allow
natural healing to take place [ 41 ]. Magnesium has also recently been investigated
as a possible material for intravascular stent applications [ 41 , 45 ]. It has been
shown that hemolysis and platelet adhesion can be positively influences by certain
alloys [ 45 ].
2.1.3 Tungsten
Degradable coils made of tungsten were used in a series of pediatric patients [ 46 ]
for the occlusion of pathological vessels. On follow-up, fluoroscopic analysis
showed a decreased radio-opacity indicating degradation of the coils. Corre-
spondingly, there was a marked increase in serum levels of tungsten and the
previously occluded vessels were recanalized. Although in vitro analysis of the
tungsten coils, which interestingly was performed after the clinical application,
showed a slow degradation and low toxicity of tungsten [ 47 ], its further applica-
tion for this indication is not recommended [ 46 ] by the authors.
2.2 Ceramics and Glasses
When considering ceramics, glasses and glass-ceramics as biomaterials, a wide
range of characteristics are observed especially in terms of stability in physiological
environments [ 1 ]. They can be categorized into three groups based on their surface
reactivity: essentially inert materials, soluble materials, and intermediate materials
with limited or controlled surface reactivity [ 48 ]. Ceramics, typically alumina [ 49 ]
and certain hydroxyapatites (HA), are biologically inert, especially dense calcium
hydroxyapatite [ 48 ], the naturally occurring mineral phase of bones and teeth [ 1 ].
Close relatives of these HA such as tri-calcium phosphate (TCP) or calcium-alkali-
orthophosphates are biodegradable [ 50 - 53 ]. Different factors have been determined
that influence the degradation behavior of ceramic biomaterials. These include:
physical forms (degrees of micro- and macro-porosity; density); composition (TCP
vs. HA; glass composites vs. HA ceramic) and crystallinity (e.g. coralline HA vs.
HA ceramic) [ 54 ]. In materials made from powders, disintegration is mainly
governed by the solubility product of the necks connecting the powder particles
after crystallization [ 55 ]. Consequently, when the necks are dissolved, the materials