Cardiopulmonary bypass technologies - Biomaterials and Devices for the Circulatory System - page 144

mediated contact processes. The perfusion community is presented with a
myriad of surface treatment technology, all invariably claiming outstanding
performance in terms of minimizing the effects of bioincompatibility. The
approaches adopted by the manufacturers of devices for perfusion con-
sumption are varied, and range from 'conventional' heparin coating to the
so-called biomimetic treatments, which claim to represent artifi cial endo-
thelium; up to polymer-heparin composite materials.
A polymer surface can be altered by coating it with another polymer in
order to reduce the infl uence on blood components. A general objective is
a reduction of protein adsorption. Surface modifi cation is usually achieved
by dissolving the modifying polymer in an organic solvent, followed by dip-
coating. In chemical modifi cation, a particular chemical group on the surface
is altered by replacement of another. It is also possible to use graft poly-
merisation to increase the hydrophilicity. Plasma polymerisation and laser
plasma deposition are novel technologies. For attachment of macromole-
cules, there are numerous possibilities for the attachment of macromolecule
to a polymer surface. These cover the selection of bonding and the use of
spacer molecules. However, the infl uence on the surface is complex. Blend-
ing is a cost-effective method with importance for commercial use. Blending
can be used to achieve a hydrophilic/hydrophobic or natural/synthetic
polymer combination. 8
The general purpose of surface modifi cation is to improve selectively
features such as blood compatibility, strongly dependent on surface proper-
ties, while having little or no infl uence on bulk properties. The response of
blood to a biomaterial can be regarded as the involvement of protein
adsorption, platelet adhesion, release and aggregation, activation of intrin-
sic coagulation, participation of the fi brinolytic, complement and kallikrein-
kinin systems, and the interaction of cellular elements.
An examination of blood-biomaterial interaction indicates the objectives
for surface modifi cation could include alteration or reduction in protein
adsorption, provision of activity by the use of pharmaceutical agents, and
preparation of a surface resembling that cell membrane. However the com-
plexity and interactive nature of blood response to contact with an artifi cial
surface may ensure that surface modifi cation requires a compromise. This
is accomplished by an increase in negative charge that reduces the platelet
response but enhances the contact activation phase of the intrinsic coagula-
tion. An increase in positive charge could reduce complement activation,
but enhance platelet response; an increase in hydrophilicity by the introduc-
tion of hydroxyl groups could reduce the platelet response, but enhance
complement activation. 9
Surface modifi cation of extracorporeal circuits is not a new development
and was described as heparin surface coating in 1963, which led to its incor-
poration in tubing for thoracic aneurysms and liver transplant surgery. 10 In

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