Biomedical Engineering Reference
In-Depth Information
of stainless steel guide wires coated with a-C:H and a-C:H(F) was studied [196]. It was
noted that a-C:H and a-C:H(F) improved smoothness, uniformity, and reduced polishing
scars. Both films were found to improve the lubrication behavior by 30% when compared
to uncoated guide wires. Fluorine incorporation in a-C:H also significantly reduced the
thrombogenicity.
Some studies were done on the coating of contact lenses by carbon films to improve their
ophthalmological applications [197,198]. Ion-assisted deposited a-C:H on contact lenses
was found to adhere well to the lens materials, enhance stability, provide UV protection,
and strengthen the lens life significantly [197]. The coating also minimized antibacterial
activity and inflammatory processes and improved the chemical resistance to the steriliza-
tion and storage solution of contact lenses. Hydrogenated a-C coatings on contact lenses
increased the refractive index, thereby reducing the thickness of contact lens [198]. The
coatings of thickness in the range 20-200 nm have good transmission in the visible region,
but minimize UV transmission by about 40-50%. Contact lens casings coated with a-C:H
also showed significant surface integrity on exposure to saline solution, prevented the
formation of microbial contamination, and proved to be a safe custodian of the contact
lenses.
Summary
Amorphous carbon, renowned for its superior mechanical properties, is currently in the
spotlight once again for its potential to be utilized in biomedical applications. Numerous
studies in vitro, in vivo, and clinical have been done to prove the values of this material.
However, there are multitudes of conflicting issues regarding its biological properties. As
such, this class of “biomaterial” is complicated to characterize. Even so, some films are
already utilized in commercial products.
The results on blood compatibility are very promising. The material is able to reduce
blood platelet adhesion and activation to prevent thrombosis. In general, the property can
be improved further if the material has a higher sp
3
to sp
2
fraction, becoming more hydro-
phobic. But studies have also shown hydrophobicity is not alone in determining the hemo-
compatibility. Surface energetics can affect the adhesion of proteins and hence the amount
of adherent platelets. A surface is favorable when it can attract more albumin than fibrino-
gen. Results have also confirmed that material surface properties can be moderated by dop-
ing and surface treatment. There are already a couple of commercial biomedical implants
utilizing this material in blood contacting applications (e.g., stents and heart valves).
Amorphous carbon has also proven to be noncytotoxic to a vast number of living bio-
logical cells, including the three cells (macrophages, fibroblasts, and osteoblasts) found in
periprosthetic tissues and endothelial (found in blood intravascular inner wall linings).
Doping and surface treatment have also helped to improve the material-cell interactions.
Both hydrophobic and hydrophilic films have been proven to increase cellular prolifera-
tion and activity. Again, the surface energetics govern these interactions in addition to
wettability. A number of experiments done on animal models have also shown minimal
or no adverse effects.
The latest interesting studies are on the antibacterial properties of amorphous carbon. A
number of bacteria have been used for these studies, and most results have shown reduced
microbe adhesion and activity on the material. A hydrophobic surface is preferred, but
