Biomedical Engineering Reference
In-Depth Information
that elicit an immune response from macrophages. PLLA and PGA degrade in a time
period of six months to several years, depending on initial molecular weight and crystallin-
ity. Copolymers of the two typically degrade into fragments in a few months. The lactic acid
and glycolic acid fragments are eventually metabolized into carbon dioxide and water. Tri-
calcium phosphate ceramics degrade through a surface dissolution process into calcium
and phosphate salts, which are also present naturally in the body.
Biomaterial degradation may lead to chronic nonhealing wounds that are arrested at one
of the normal phases of wound healing. This may happen if a biomaterial degrades too
quickly and releases particulate matter that extends the inflammation stage. Persistent inflam-
mation leads to the formation of giant multinucleated cells that continue to attempt to remove
the offending material through secretion of acids and enzymes that also destroy healthy tis-
sue. They are the trademark of a foreign body response and may necessitate surgical removal
of the implanted device. If the healing passes through to the fibrous capsule formation stage,
there may still be complications. For example, a drug delivery implant may eventually no
longer function due to impaired drug release by the fibrous encapsulation in response to
the degrading drug delivery implant. Fibrous encapsulation of tissue engineering constructs
is considered a failure as well, because the host tissue is blocked from integrating with the
engineered tissue.
EXAMPLE PROBLEM 5.8
Is it possible to successfully pass through all four wound healing phases only to have the bio-
material degrade and lead to wound healing reversal? Explain your answer.
Solution
Yes. This has happened in some patients with total joint replacements. Total joint replacements
such as artificial hips typically consist of two metallic components that meet at a polymeric
bearing surface (typically UHMWPE). During bending of the joint, wear debris is produced as the
metal surface rubs against the softer polymeric surface. This leads to the recruitment of macro-
phages that identify the particles as foreign and attempt to remove them. Since the synthetic
particles cannot be degraded by cell enzymes, inflammation continues indefinitely. The excessive
production of inflammatory cytokines leads to resorption of the newly healed adjacent bone that
supports the implant, resulting in implant loosening. Fortunately, there have been improvements
in the processing of polyethylene so wear debris is no longer generated at the high rate observed
in some of the earliest used hip replacements. Ceramics on ceramic hip joints have been developed
that have better wear properties and are not susceptible to corrosion; however, ceramic hip replace-
ments are badly tolerated by elderly osteoporotic bone because the material is very hard.
5.4.6 Immunogenicity
Immunogenicity
is the tendency for an object to stimulate the immune response. Common
examples of immunogens are proteins in food that lead to allergies or inflammation and
pollen from grass or trees. Related to biomaterials, bacteria, small particulates of bioma-
terials, and cellular debris left behind after processing of allografts are also immunogens.
