Biomedical Engineering Reference
In-Depth Information
joint components, and vascular stents [254, 255]. Co-Cr is ot en chosen for
applications in which there may be sliding wear because of its high hardness
and wear resistance. h e development of nanostructured Co-Cr superal-
loys for medical applications of ers the prospect of higher strength and the
possibility of even higher wear resistance through the ef ects of high shear
on the morphology of precipitates and homogeneity of their distribution.
Improving wear resistance is particularly important in in Co-Cr alloys since
wear can create particulate debris. h e cobalt and chromium particles have
been shown to have some toxicological ef ects in humans [256, 257].
New medical applications of nanostructured Co-Cr alloys will appear
only as processes to nanostructure are developed and validated. Surface ori-
ented techniques or powder-based appear most promising. One technique
of particularly high potential is Surface Mechanical Attrition Treatment
(SMAT) [258, 259]. SMAT induces large deformation in surfaces by recur-
ring impact of hard spheres. h ese deformations result in the formation of
microstructures containing high densities of strengthening defects, includ-
ing twins and the intersecting twin systems, dislocation walls, microbands,
highly disoriented polygonal submicronic grains, and randomly oriented
nanograins [258]. h e SMAT technique has been successfully demon-
strated for cobalt [260] and other dii cult to deform metals [261-264].
h e application of SMAT for enhancing the biocompatibility of titanium
surfaces has been documented [265, 266].
1.2.4 MagnesiumAlloys
Magnesium is highly promising for medical applications because of its very
light weight and its ability to be bioabsorbed [267-272]. As the lightest of all
structural metals (excepting beryllium) magnesium enables light-weight-
ing of many medical structures, ranging from wheelchairs and stretchers
to surgical tools, to vascular stents, to orthopedic implants [273-279].
Magnesium is also among the most biocompatible of metals. It is essential
to human health, with approximately 35 grams distributed between bones,
tissues, and blood in an average size human adult [270, 280]. h e normal
level of magnesium in the blood is 1.7 to 2.2 mg/dL [254]. Magnesium is
necessary for many biochemical processes in our bodies, aiding in over 300
enzymatic reactions that help maintain normal functioning of muscles and
nerves and regulation of heart beat, blood sugar levels, and the immune
system [270]. h e normal daily demand by the human body for magne-
sium is about 375 mg/day [280].
Increasing investigation of magnesium as a biomaterial has been stimulated
by multiple trends. Magnesium alloys have become increasingly available
