Biomedical Engineering Reference
In-Depth Information
weight polyethylene (UHMWPE) are well-known polymers. Of these, titanium
and its alloys are generally considered to be good blood compatible alloys
(Muraleedharan et al., 1995).
Titanium owing to the presence of non-porous titanium oxide layer presents
a ceramic surface to blood. Though titanium is one of the most chemically reac-
tive of all metals, it is also one of the most corrosion-resistant because of the
lower reactivity of this oxide fi lm. The biocompatibility and specifi cally blood
compatibility of titanium and titanium alloys result from the chemical stability,
repassivation ability, and structure of this titanium oxide fi lm that is typically only
a few nanometers thick. The fi lm is normally composed of a titanium/titanium-
oxide composite close to the bulk material and a titanium-oxide fi lm that could be
a few nanometers thick. The surface of the titanium-oxide fi lm may have hydro-
xide and adsorbed water bond with titanium ions, which forms an extremely
blood-compatible interface since it allows adsorption of proteins when exposed
to blood. Even when any physical damage occurs to this coating, the interface is
rebuild suffi ciently fast by minimizing the chance for platelet adhesion. This
ability of the coating to repassivate with suffi cient speed could be attributed to
the blood compatibility of titanium alloys.
Most often, metallic and polymer surfaces are coated with special coatings to
enhance their blood compatibility. Much of the efforts in biomaterials research
over the past 50 years have been directed toward the development of coatings
that do not react with platelets and coagulation factors. Mainly three categories
of coatings are employed for enhancing blood compatibility, that is, coating of
pharmacological agents (heparin and heparin-like agents), anti-adhering coat-
ings, and inert coatings.
21.4.1 Heparin and Heparin-Like Coatings
Heparin-based coatings have demonstrated substantial improvement in the
performance of a variety of blood contacting medical devices. Heparin is a
pharmaceutical that has been used clinically for decades as an intravenous
anticoagulant to treat inherent clotting disorders and to prevent blood clot for-
mation during surgery and interventional procedures. Heparin molecules are
polysaccharides with a unique chemical structure that gives them specifi c biologi-
cal activity. When heparin is immobilized on the surface of a medical device
material, it improves the performance of the material when it is exposed to blood
in several ways:
It provides local catalytic activity to inhibit several enzymes critical to the
formation of fi brin.
It reduces the adsorption of blood proteins.
It reduces the adhesion and activation of platelets.
Alternatives to the heparin approach have also been developed for pre-
venting surface-induced thrombus formation. In these approaches, synthetic,
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