Biomedical Engineering Reference
In-Depth Information
Thus, the flow conditions (shear rates, turbulence, secondary flows, etc.), dura-
tion of contact, size of the contact surface area, and actual placement site in the
cardiovascular system are very important parameters to be considered, in addition
to the surface finish due to fabrication and sterilization effects.
It is generally accepted that the first step of the interaction process between
blood and artificial surfaces is the adsorption of plasma proteins. Subsequent events
involve platelet and leukocyte adhesion and aggregation, and activation of the
coagulation system, as well as the fibrinolysis and complement systems, resulting
eventually in the formation of a
thrombus
on the artificial (e.g., polymer) surface.
More specifically, the nature of the artificial surface, under specific hemodynamic
conditions, may promote:
Specific plasma protein
adsorption
with resultant activation, denaturation and/or
desorption.
Platelet
adhesion
and
activation
with release of active substances that are impor-
tant in platelet
aggregation
leading to thrombus formation.
Hemolysis.
Leukocyte adhesion,
spreading
and activation, with further involvement of the
inflammatory and complement systems.
Interaction with other blood components, such as lipoproteins, trace plasma pro-
teins,
inhibitors
of activated plasma proteins, and adhesive proteins (fibronectin,
thrombospondin).
It should be noted that each of the above systems is quite complex in its own
interaction with foreign surfaces and, as all the plasmic components, proteins and
cells, interact among themselves, the complexity of the events becomes enormous.
It should once more be emphasized that here we deal only with the complex inter-
actions during the
initial events
at the
interface
and not the long-term effects at the
site of implantation or elsewhere in the body.
It is almost axiomatic that when protein solutions are put into contact with solid
surfaces, adsorption occurs. However as there are many different plasmic proteins
with great variations in size, concentration, and purpose, it is logical to ask: What is
the composition of the protein layer that is laid down on different surfaces in contact
with blood? Is there a preference for specific proteins to be absorbed depending
on the type of surface? Does the composition on a particular surface change as a
function of time? And what happens to the proteins that get adsorbed: do they simply
bind to the surface, do they undergo (after binding) different types of transformation,
for example denaturation, changes in their biological activities...?
Among the general conclusions that have come out of protein adsorption studies
are the following [
395
]:
Most proteins are adsorbed on most surfaces.
The relative amounts of the different proteins vary from surface to surface.
Many proteins (when whole blood is used) become degraded. This may be due
to proteolytic enzymes derived from damaged cells.
