Biomedical Engineering Reference
In-Depth Information
Heparin
molecule
Anticoagulant
active
sequence
Covalent
bond
Sulphate/
sulfonate
groups
Hydrated
PEO
chain
Functional layer
Priming layer
Artificial surface
FIGURE 10.7
Generalized schema of the construction of a Trillium Biosurface, as depicted on the Medtronic Corporation
Web site.
The Carmeda coating has also been applied to vascular grafts. The Gore Viabahn
Endoprosthesis (W.L. Gore & Associates) is a Carmeda-coated stent graft and is approved
for use in the treatment of lower limb artery disease. This is a PTFE-based graft that has
been surface modified using an approach similar to that mentioned above for PE. The
PROPATEN
®
graft has been approved by the FDA in 2006 and can be used in cardiovascu-
lar disease generally. It has been proven superior to noncoated synthetic grafts in clinical
studies.
The same coating has also been applied to ventricularly assist devices successfully. The
device is marketed by Berlin Heart GmbH, under the name of Excor
®
and Incor
®
and is
being readied for approval in the US.
Taken together, the Hepacoat stent (although it really addresses only sub-acute thrombo-
sis, or SAT) and the LVADs
(left ventricular assist device) would establish the first success-
ful long-term usefulness of the heparinization approach.
The DurafloII coating (Baxter) relies on initial sulfation of the surface to generate negative
charges, followed by attachment or coating with a positively charged surfactant (contain-
ing quarternary ammonium groups). These positively charged molecules then ionically
bind to heparin, preserving their bioactivity substantially. This coating has also been suc-
cessfully applied to oxygenator surfaces.
These are the true success stories of nonthrombogenic surface generation. Longer-term
blood-contacting devices such as stents and heart valves may require a different solu-
tion, namely, the generation of a functioning endothelium. Those attempts are described
next.
Endothelialization of Biomaterial Surfaces
If the blood-contacting surface of an implant can be completely covered by endothelial
cells, the blood then treats the device the same way as it does a blood vessel: no platelets
adhere and cause clotting. Part of the reason for this is the negative charge on the endothe-
lial surface that seems to repel platelets. Simply making a surface negative helps, but does
not provide long-term protection. Thus, in the following sections, we describe the various
approaches to accelerating endothelialization of surfaces.
