Biomedical Engineering Reference
In-Depth Information
2.
Balancing thromboresistance and haemostatic functions
. Under normal
conditions, EC reserves anti-thrombogenic and thrombolytic functions
to maintain normal blood rheology. For this purpose, multiple path-
ways are involved: (i) synthesizing anti-coagulants like anti-thrombine III
[AT-III], thrombomodulin [TM], heparin-like polysaccharides, and (ex-
ternal) tissue factor pathway inhibitor [TFPI]; (ii) producing thrombolytic
promoters like tissue plasminogen activator [t-PA] and u-PA; (iii) re-
leasing platelet adhesion and aggregation inhibitors like prostaglandin
(6-keto-PGE
1
) and 13-hydroxy stearic diene acid; and (iv) secreting vas-
cular smooth muscle relaxation factors like prostacyclin and endothelial-
derived relaxing factor [EDRF]. On the other hand, the haemostatic func-
tion of EC acts as a protecting reaction that maintains vascular integrity
and endothelium continuity under abnormal conditions like trauma or
pathological injury. The corresponding events include generation of ther-
apeutic proteins and the release of EC-derived factors to encourage the
constriction of vascular smooth muscles, both of which promote the re-
covery of vascular defects.
3.
Resisting vascular wall cell migration and hyperplasia
.VascularECcan
be induced to resist SMC invasion. For this mechanism, besides the func-
tions of EC-produced multiple factors, EC itself is also capable of migra-
tion with the cue of migrating factors secreted by platelets or leukocytes
and the shear stress by blood flow. EC migration usually accompanies pro-
liferation, which frequently relates to new vascular formation or defective
vascular regeneration [100, 101].
The conclusion drawn from the above-discussed features of native endothe-
lium is that—where feasible—construction of host endothelium on the sur-
face of synthetic implants—i.e. engineered endothelialization—may present
a promising pathway for fundamentally resolving the biocompatibility and
bio-functionality of artificial cardiovascular transplantation. This proposed
biocompatibility is, however, not simply thromboresistance (or blood com-
patibility) plus blood-cell compatibility, but a real bioactive, self-renewable
and specifically functional alliance of tissues and synthetics.
1.4.2
Engineered Endothelialization for Artificial Implants
Over the five decades following invention and application in the clinic of the
first artificial vascular graft in the 1950s investigators initially endeavored
to develop implantable products using native biological derivatives or syn-
thetic materials that intrinsically possess outstanding mechanical and biolog-
ical properties without further modification. The typical synthetic products
are made of poly(ethylene terephthalate) [PET] or expanded poly(tetraflouro
ethylene) [ePTFE]. They are useful for inducing vascular endothelialization,
