Biomedical Engineering Reference
In-Depth Information
with the complement system as well as the coagulation and the fibrinolytic sys-
tem. For example, stimulated monocytes express
tissue factor
, which can initiate the
extrinsic coagulation cascade, while neutrophils may contribute to clot dissolution
by releasing fibrinolytic enzymes (for example, elastase) [
394
].
Platelets
(or thrombocytes) are the smallest class of blood cells. The nonactivated
platelet is disc shaped, 2-4 m in diameter, 10 m
3
in volume. There are 2-4
10
5
platelets mm
3
in the human circulating blood. These cells are not nucleated but
contain granules of certain substances in their cytoplasm and certain receptors in
their membrane that make them very important for the process of arresting a bleed-
ing by forming a platelet plug and catalyzing coagulation reactions. In particular,
there are glycoproteins on the platelet membrane that are responsible for
platelet
adhesion
(i.e., platelet adhering to a surface) or
platelet aggregation
(i.e., platelet
adhering to another platelet). The phospholipids on the membrane accelerate coag-
ulation reactions. Granules inside platelets contain the specific platelet proteins,
platelet factor 4
and ˇ
-thromboglobulin
, proteins found in the plasma (
fibrinogen,
albumin, fibronectin, factors V and VIII
), ADP, calcium ions,
serotonin
and lysing
enzymes. Platelets are extremely sensitive cells and only a slight stimulation is suf-
ficient to make them respond in different ways: they become sticky, they change
shape with many pseudopods formed and they release different contents in response
to different stimuli. In terms of their surface, interactions of great importance are
the three processes: platelet adhesion, aggregation and activated release reaction.
If one separates the cells from the blood, the remaining fluid is called
plasma
(and
if you still separate from plasma the fibrinogen you have a more clear solution, the
serum
). Plasma contains hundreds of proteins with a total mass of 70-80 g l
1
.In
the following table (Table
12.1
), the most important, as far as we know, proteins are
listed, with their name, molecular weight and functional category they belong to.
To summarize:
A brief summary of the blood constituents, cells and proteins,
which participate in blood-material interactions was presented. The genetically pro-
grammed steady-state action of each blood element in the physiological (or even in
certain stabilized pathological) range will be disrupted at the moment that the closed
circulatory system is interfered with. When inserting a catheter, or lead the blood
into extracorporeal circulation or fit the organism with a blood contacting artificial
device, an alarm sounds and a series of actions by the blood elements start. The
extent of blood element recruitment to deal with the new alarming situation, the race
for the intrusion site, the competition or synergy of the various elements depend on
the
material
surface they encounter and on the
device
as a structure.
The
material
, with biomolecules (such as albumin or heparin) grafted on its sur-
face or not, should have the biomechanical properties necessary for the specific
application and should be able to be processed to forms, shapes, and sizes appropri-
ate to its final use. Of primary importance are of course its surface properties, as it
is the surface that contacts the elements of blood and will interact dynamically with
them. The surface properties are important for the initial contact with the blood so
as to facilitate or hinder the initial chemical contact. Therefore
hydrophobicity
or
hydrophilicity
,
surface charge
,
polarity
, heterogeneity in the distribution of reactive
chemical groups
(domains)
,
mobility
of the surface molecules,
smoothness
,etc.may
