Biomedical Engineering Reference
In-Depth Information
the bottom of the lid. The chromium is cathodic to the steel and provides another source of
galvanic corrosion. (5) There are potholes in the roads. This causes bouncing of the lid against
the frame. This can chip the paint and expose the unprotected metal, or it may be a cause of
fretting corrosion.
Bolting on a zinc block would not help except for corrosion of metal in the same electrolyte
pool. A piece of zinc in the crevice would help slow the corrosion of the crevice. If the car fell into
the ocean, then the zinc block would protect the whole car.
EXAMPLE PROBLEM 5.7
Your grandmother has a stainless steel total hip. Now she needs the other hip replaced, and the
doctor wants to use one made of a cobalt chromium alloy. Is that a problem for corrosion?
Solution
No. One of the four essential elements for galvanic corrosion is missing. There is an anode: the
stainless steel. There is a cathode: the cobalt alloy. There is an electrolyte: the salt water of the
body. However, there is no electrical connection, so there is no problem. If she were to fall and
fracture her pelvis, and the break was repaired with an external fixator, then there might be an
electrical connection and a problem. However, these alloys are so corrosion resistant and similar
electrochemically that there is probably no need to worry.
5.4.4 Bacterial Colonization and Medical Biofilms
Just as microbial biofilms form on devices and hardware used in aquatic ecosystems,
wastewater treatment systems, and marine vessels and structures, they also have been
observed on most, if not all, medical devices implanted into humans. This includes pros-
thetic heart valves, orthopedic implants, intravascular catheters, artificial hearts, cardiac
pacemakers, urinary catheters, and intrauterine contraceptive devices. Regardless of the
technological sophistication of the biomedical implant—for example, a catheter versus
a stem cell-containing biomaterial scaffold—all medical devices or tissue engineering
constructs are susceptible to microbial colonization and infection.
As already mentioned, immediately upon implantation and contact with blood, the
surface of the biomedical implant is coated or “fouled” with a film consisting of proteins
and glycoproteins, such as fibronectin, vitronectin, fibrinogen, albumin, and immunoglobu-
lins. This film enables attachment of cells
bacteria through receptors on the surface
of the colonizing bacteria. Within minutes of attachment, the bacteria cells start producing
bacteria-signaling biomolecules that lead to an orchestrated activity of the bacterial colony,
called quorum sensing. The attached bacteria also secrete a mostly insoluble gelatinous
polymer substance, thereby forming a three-dimensional cell:polymer matrix known as a
biofilm. The bacteria thrive in this protected environment. Biofilms are found throughout
the healthy body in places like the lung and intestinal mucus layer, and fortunately, most
of the bacteria are not dangerous to the host.
and also
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