Biomedical Engineering Reference
In-Depth Information
Due to the high melting point of most ceramics, which prevents them from being cast or
extruded, ceramic components are typically made from powdered stock. The powders are
formed by wet synthesis methods or by pulverizing raw materials. The ceramic powder
is either added to a liquid with binders to form a slurry that is cast in a mold or is dry
pressed to form “green ware.” The green ware must be finally sintered or fired to densify
the powders and remove the porosity between the powder particles. In weight-bearing
applications, the porosity must be nearly totally removed, or the residual porosity acts as
microcracks within the material and weakens it. In other applications, such as bone graft
substitutes or tissue engineering scaffolds, it is desirable to have large pores (
<
250-750
m) like those in trabecular or cancellous bone so cells can infiltrate the material and grow
new vital tissue. In this case, pores are typically created by using second phases, such as
polymer beads, that maintain pore space during the early processing steps and are then
burned out during the final sintering stage. More detailed descriptions of how porous scaf-
folds are formed are included later in this chapter.
m
EXAMPLE PROBLEM 5.2
What material is preferred for the acetabular cup of a hip implant? What design parameters
are utilized during the selection process? Use scientific, corporate, and patent websites to locate
information on this topic using keywords such as “ceramic” and “hip replacement.”
Solution
Acetabular cups are currently made with a metal support structure and a polyethylene cup;
however, problems with wear debris from the soft polyethylene have led to new products with
ceramic acetabular cups and femoral balls (alumina or zirconia). The cup must resist wear and
deformation and be a low-friction surface because it is in contact with the ball component of the
artificial joint. Ceramic materials generate less wear debris during use than the traditional metal
on plastic design. Thus, in theory, the risk that ceramic total hips will fail is low compared to
the traditional metal on plastic design, and they are now recommended for younger patients.
Since the ceramic components are fragile relative to the metallic components and not well toler-
ated by osteoporotic bone, both types of hip replacements are utilized today.
5.2.3 Polymers
Polymers are well suited for biomedical applications because of their diverse properties
and because they resemble natural materials such as protein-based extracellular matrices.
For example, by slightly varying the chemical bonding and structure, polymers can be
flexible or rigid, or can have low strength or high strength. Depending on the surface
modification, they are resistant to protein attachment or encourage protein attachment.
Polymers can be biodegradable or permanent, and they can be fabricated into complex
shapes by many methods. Some disadvantages of polymers are that they tend to have lower
strengths than metals or ceramics, and they tend to deform with time. Certain types of poly-
mers may deteriorate during sterilization and may degrade in the body catastrophically or
by release of toxic by-products. Thus, characterization before and after implantation is a
standard practice during the development of a new medical device made of a polymer.
