Biomedical Engineering Reference
In-Depth Information
one hour time period. The surface concentration may depend on the type of
protein adsorbed, but could be of the order of 0.1-1.0
g/cm
2
.
In general it has been found that preferential or selective adsorption occurs
so that certain proteins may be enriched in the surface and vice versa. Surface
chemistry, time of adsorption, and protein type are major factors in determining
the composition of the adsorbed layer. Small proteins, by virtue of their higher
diffusion coeffi cients, will initially adsorb faster than large proteins during com-
petitive adsorption (Ramsden, 1995).
Proteins also undergo conformational changes during the adsorption process.
These conformational changes may have a positive consequence (surface pass-
ivation with an irreversibly adsorbed protein monolayer) or negative conse-
quence (initiation of blood-enzyme systems, in particular the activation of the
coagulation and complement system due to the desorption of the surface acti-
vated protein components (Eloy et al., 1990).
μ
21.3.3 Blood Cell Interactions
Cells adhesion occurs at a later stage and is mediated by the proteins initially
adsorbed on the surface. Platelets, amongst the other cells present in blood, play
the more important role in blood-material interactions. Hemodynamic conditions
are of major importance in the adhesion of platelets to material surface and in
determining localization, growth, and fragmentation of thrombi. Platelet adhe-
sion to a surface is governed by two independent mechanisms: the transport of
platelets to the surface, which depends on the fl ow conditions, and the reaction of
platelets with the surface, which depends on the nature of the surface and the
adsorbed proteins (Hanson, 2004).
Red cells can bind weakly to some non-endothelial materials' without spread-
ing. They signifi cantly contribute to blood surface interactions in several ways.
Collision of red cells with other blood cells and plasma proteins reduces their
adsorption on the materials' surfaces. Leucocytes, particularly neutrophils and
monocytes, have a strong tendency to adhere to surfaces. As a result of leukocyte
adhesion, several reactions are initiated. These include platelet-platelet and
platelet-leukocyte interactions, the detachment of adherent thrombi by the action
of leukocyte proteases, the detachment of adherent platelets and adsorbed pro-
teins by leucocytes, and the release of leukocyte products that may rise to both
local and systemic reactions.
21.3.4 Haemostasis and Coagulation
Several distinct but interrelated thrombotic and anti-thrombotic systems exist to
prevent the formation of intravascular clots that are expected in response to vas-
cular trauma. Haemostasis is the sum of these mechanisms and serves to limit
blood loss following injury. Once regulation is initiated, the same mechanisms
fi rst combine to localize the clot at the site of injury, then to terminate coagula-
tion, and fi nally to remove the clot once it has served its purpose. These haemo-
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