Blood interface biomaterials - Biomaterials and Devices for the Circulatory System - page 38

with a PA hydrogel. The hydrogel-modifi ed surface formed showed excel-
lent blood compatibility.

2.4.2 Increase in hydrophobicity

On the basis of the classical recognition of the potential benefi t of a hydro-
phobic surface in minimising blood response, which was reported by Bordet
and Gengou (1903), hydrophobic polymer surfaces for blood-contacting
applications have been developed. The surfaces of most interest are sili-
cones, PTFE and diamond-like surfaces.
In general, hydrophobic surfaces possess relatively low surface free ener-
gies. It was proposed that the clotting time for human blood increased
linearly with the logarithm of the critical surface tension (Lyman et al. ,
1965) and this suggested that the minimisation of surface energy is a valid
approach to obtain non-clotting surfaces. In practice, the lowest critical
surface tension is that obtained from a PTFE surface, which was found to
exhibit less adhesion and shape change of platelets when it was grafted on
to a PU surface (Han et al. , 1992). The results indicate that this type of
hydrophobic surface is highly blood compatible. It is also interesting to fi nd
that the enhanced blood compatibility of strongly hydrophobic PU-PFDA
(perfl uorodecanoic acid) was equivalent to hydrophilic PU-PEO.
The mechanism of blood compatibility of a hydrophobic surface has also
been extensively investigated. It is generally accepted that proteins and
other coagulation factors adsorb considerably on such a surface. However,
the preferential adsorption of albumin from plasma can protect soluble
thrombin from inactivation by the hydrophobic surface and retained less
fi brinogen than when the PTFE was incubated with a pure fi brinogen solu-
tion of the same concentration (Schlosser et al. , 1993). Similarly, expanded
PTFE (e-PTFE) generally seems to exhibit better blood compatibility com-
pared with other materials that were used in the past for partial replace-
ment of blood vessels in vascular surgery (Silver & Doillon, 1989).
Diamond-like coatings (DLCs) have been exploited to build a highly
hydrophobic surface with a lowest coeffi cient of friction value for biomedi-
cal applications. The coating can be achieved via a technology called chemi-
cal vapour deposition (Oleary et al. , 1995).
Protein adsorption on carbon surfaces has been studied. It was found
there was no preference on adsorption of proteins but there were a very
high rate and high concentration of adsorption. This implies that the carbon
surface accomplishes its blood compatibility through a passivating fi lm of
strongly adsorbed bland proteins, which do not interact with platelets or
participate in blood coagulation (Feng & Andrade, 1995).
Although it is predicted for the future that the best coating material
would be DLC or a crystalline diamond coating, little work has been done

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