Biomedical Engineering Reference
In-Depth Information
non-biological molecules are used to create surfaces with improved blood
compatibility. Passivation with hydrophilic molecules to mask the underlying
thrombogenic surface from the blood is one method. The passivated surface
reduces or prevents the adhesion of thrombogenic cells and proteins onto the
underlying substrate or material, thereby preventing surface-induced blood
clotting. Another approach involves coatings that actively recruit and bind native
albumin from the patient's own blood onto the device surface. This albumin-
binding coating acquires a thin, self-regenerating, absorbed albumin layer on the
surface. The albumin covered surface minimizes and prevents the adhesion of
unwanted thrombogenic cells and proteins.
21.4.2 Anti-Adherent Coatings
For applications requiring short term blood compatibility, it is important only that
the device repel platelets, proteins, or cells. It may not be necessary to provide
heparin or heparin-like coatings on the surface, since the attachment of blood
components must be prevented for a limited time only, and the risk of introducing
emboli into the blood stream is minimal. In addition, the patient usually receives
systemic anticoagulants during such procedures, further reducing the need for a
bioactive coating on the device.
Most of the anti-adherent coatings employed make use of hybrid polymer
systems such as hydrogels and hence provide the additional feature of reducing
friction on the device surface. Products that can benefi t from the use of these anti-
adherent coatings are catheters, such as percutaneous transluminal coronary
angioplasty (PTCA) catheters, guide catheters, angiography catheters, dilators,
introducers, and drug-infusion catheters.
21.4.3 Inert Coatings
Most of the blood-compatible coatings discussed in earlier sections are meant for
devices that come in contact with blood for a short span of a few hours to a few
weeks. But when it comes to long-term implants, these techniques do not produce
very encouraging results. Coatings incorporating pharmacological agents like
heparin have limited service life because the activity of the pharmacological
agent diminishes with time. Hydrogel coatings can get metabolized by enzymes
over time and hence become ineffective in long duration implants.
The best strategy in long-term implant application is to make the substrate
itself suffi ciently blood compatible or to impart coatings that will stay intact for
the life of the implant. The use of ceramic coatings, especially on metallic sub-
strates, becomes more signifi cant in this situation.
Diamond-like carbon (DLC) and Titanium nitride (TiN) are two ceramic
materials which offer tremendous potential for use as blood-compatible coat-
ings for long-term applications. Both DLC and TiN exhibit similar properties
such as high hardness, low frictional coeffi cient, high wear and corrosion resis-
tance, chemical inertness, high electrical resistivity, infrared-transparency,
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