Biomedical Engineering Reference
In-Depth Information
to degeneration mediated by the deposition of calcium containing salts. Calcifica-
tion or mineralization of a tissue can be a normal or abnormal process. Nearly 99%
of calcium entering the body is deposited in bones and teeth. The remaining 1%
of calcium is dissolved in the blood. When a disorder affects the balance between
calcium and other minerals or hormones, calcium can be deposited in other parts of
the body, such as arteries, kidneys, lungs, brain, or prosthetic materials, which re-
sults in tissue hardening. The failure of certain implants is frequently caused by the
deposition of calcium phosphate or other calcium-containing compounds. These
deposits increase with time causing the failure of bioprosthetic device [3].
Calcification may occur on the surface of an implant (extrinsic calcification),
where it is often associated with attached tissue or cells, or within structural com-
ponents (intrinsic calcification). Pathological calcification can be either
dystrophic
(deposition of calcium salts in damaged or diseased tissues or biomaterials in indi-
viduals with normal calcium metabolism) or
metastatic
(deposition of calcium salts
in previously normal tissues, as a result of deranged mineral metabolism such as el-
evated blood calcium level). In general, the determinants of biomaterial mineraliza-
tion includes factors related to both the host metabolism (in presence of abnormal
mineral metabolism, calcification associated with biomaterials or injured tissues is
enhanced) and implant structure and chemistry.
6.3.4 SurfaceRoughness
Manufacturing plays an important role on the surface finish of various devices.
The finest irregularities of a surface generally results from a particular production
process or material condition. Imperfections at the surface lead to variation in the
surface topography, which influences the surface interactions. Surface texture is the
combination of fairly short deviations of a surface from the nominal surface. To
avoid tissue damage by disturbing fluid flow, many applications including materials
for orthopedic implants and contact lenses need a smooth surface. If cell-surface in-
teractions are desired, as in polymer scaffolds useful in tissue regeneration, surface
roughness enhances the cell in-growth and tissue integration within the implants.
In such cases, careful attention must be paid to the thrombogenicity of the inner
surface. A number of defects could occur due to the quality of the raw material used
during the process or during the finishing stages such as cleaning the edges so that
no dross or burrs are attached. More uniform distribution of plastic deformation
during tube drawing is critical to achieve better mechanical properties. Better con-
trolled dimensional tolerances and surface texture are important to ensure smooth
cutting. Minimizing drawing contaminations and more effective surface finishing
techniques for removing surface contaminations and oxides are paramount to elim-
inate corrosion concerns and to improve the biocompatibility of the final devices.
Surface texture includes waviness, lay, and roughness. Waviness includes the
more widely spaced deviations of a surface from its nominal shape. Lay refers to
the predominant direction of the surface texture. Lay is determined by the par-
ticular production method and geometry used. Turning, milling, drilling, grinding,
and other machining processes usually produce a surface that has lay (striations
or peaks and valleys in the direction that the tool was drawn across the surface).
Other processes such as sand casting and grit blasting produce surfaces with no
