Biomedical Engineering Reference
In-Depth Information
static mechanisms include platelet activation, coagulation, and fi brinolysis. The
processes of coagulation and fi brinolysis represent a dynamic balance involving
the formation and removal of fi brin. If the balance of these processes is upset,
either excessive bleeding or thrombosis may occur. The abnormal thrombus
formation and related cardiovascular disorders is one of the major challenges in
blood - compatible materials design.
21.3.5 Complement Activation
The complement system consists of more than twenty plasma proteins that func-
tion either as enzymes or as binding proteins. Complement activation is initiated
through different pathways and all of them contain an initial enzyme that cataly-
ses the formation of the assembly of the terminal complement complex. Various
complement products (such as C3b, C4b and iC3b) bind to material surfaces,
which facilitate their uptake by infl ammatory cells. Complement activation also
releases anaphylatoxins which are humoral messengers that bind to specifi c
receptors on neutrophils, monocytes, macrophages, mast cells and smooth muscle
cells. They produce smooth muscle contraction, mast cell histamine release, affect
platelet aggregation, and act as mediators of the local infl ammatory process
(Hakim, 1992 ).
21.4 CARDIOVASCULAR BIOMATERIALS
In the study of blood material interactions, a strong contrast exists between
natural blood contacting surfaces and artifi cial material surfaces. The entire blood
contacting natural surfaces in human body are lined with the endothelial cells.
The most important function of endothelium is to offer a blood-compatible
surface to fl owing blood. The platelets, erythrocytes, and leucocytes do not adhere
to the endothelium. With the help of variety of mechanisms, endothelium pre-
vents adhesion of cells and activation of clotting factors and ensures the removal
of thrombus formed on it (Courtney et al., 1998).
The artifi cial surfaces can neither perform an active role similar to that
achieved by the endothelium through the synthesis and release of specifi c sub-
stances, nor provide a surface that blocks the adhesion of proteins and cellular
elements from blood. The inability of artifi cial surfaces to perform an active role
means that the application of such surfaces may require simultaneous therapy
with anticoagulants, platelet aggregation inhibitors, and/or plasminogen activa-
tors. This calls for anti platelet and anticoagulant therapy to be employed with
most of the cardiovascular implant devices.
Only very few industrial materials are known to be employed in cardiovascu-
lar devices. Certain grades of stainless steels (SS 316L, SS316LVM), cobalt based
alloys (Stellite 21, Haynes-25) and titanium alloys (Commercially pure titanium,
Ti6Al4V) are widely-used metallic cardiovascular biomaterials. Polyethylene
terepththalate (PET), Polytetrafl ouroethylene (PTFE) and ultra high molecular
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