Biomedical Engineering Reference
In-Depth Information
•
maintenance of body temperature;
•
protection against foreign substances (complement system);
•
clot formation preventing bleeding after injury (haemostasis/coagulation).
The last two processes are the most important in the material tissue interaction,
since they dramatically infl uence the safe and appropriate function of a blood-
contacting device. The activation of the complement system is associated with
infl ammation surrounding the implant, while the activation of the coagulation
cascade leads to thrombus formation and all related pathologies.
The interaction of biomaterials with blood constituents leads to many specifi c
reactions. The extent and direction of these reactions determine the blood com-
patibility of the material. The prominent pathways of blood-material interaction
could be classifi ed as:
1. water adsorption
2. protein deposition and adsorption
3. blood cell - material interactions
4. haemostasis and coagulation
5. complement activation
21.3.1 Water Adsorption
The fi rst event on biomaterial in contact with blood is the adsorption of inorganic
ions and water molecules. On contact with water, the superfi cial structure of
hydrophobic materials is not changed, but an intermediary layer of water of a few
molecules thickness, having a more ordered structure than in pure liquid water, is
formed. On the other hand, the surface of hydrophilic materials is greatly modi-
fi ed by water. Water molecules become enmeshed in the macromolecular network
and break certain interactions between the polymer chains (Vogler, 1998).
21.3.2 Protein Adsorption
Another early event in blood material interaction is the rapid and selective
adsorption of proteins by a multi stage process. The proteins may bind to a surface
via three possible mechanisms:
•
electrostatic bond formation between charged groups on the protein and
oppositely charged surface sites (on ceramics surfaces, for instance),
•
hydrogen bonding (on relative polar substrates) and,
•
hydrophobic interactions (on low polar polymers), defi ned as the interac-
tion of non-polar groups in aqueous media.
Adsorption of protein molecules is relatively rapid, but variable, and as much as
75% may get completed in the fi rst fi ve minutes, reaching a maximum in about
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