REPOSITORY > RESULTS

Abstract

Vascular grafts made of polyethyleneterephthalate (PET) polymers have
successfully replaced large-diameter blood vessels, but the long-term performance
of small-diameter (< 6 mm) vascular grafts is still disappointing. The main problem
is insufficient biocompatibility of polymer surface with blood, which causes
complications after implementation; such as thrombosis or restenosis. These
complications immediately lead to an additional surgical procedure, which is
expensive and unpleasant (or sometimes even fatal) for the patient. Many efforts
have been done to improve surface biocompatibility of vascular grafts, mainly by
coating the surface with bioactive substances such as gelatin, albumin, collagen
and heparin. However, successful results have not yet been reported for smalldiameter
vascular grafts.
A promising way to modify surface properties of vascular grafts is by plasma
treatment, as this method enables modification of surface properties in terms of
surface roughness, surface chemistry, wettability and crystallinty, without changing
the bulk attributes. Because these surface properties play a key role in
biocompatibility of materials, the aim of the present work was to study effects of
plasma treatment on polymer surfaces and correlate them with proliferation of
endothelia cells and adhesion of platelets.
PET foils produced by different manufacturers and with different degree of
crystalline fraction were exposed to radiofrequency (RF) oxygen and nitrogen
plasma for different expouser times. By variation of discharge parameters (power,
discharge frequency, type of gas) and plasma parameters (density of neutrals and
ions, kinetic energy of electrons, gas temperature) it was possible to produce PET
foils with different surface properties.
In the first part of the work, the influence of plasma treatment on surface
parameters was conducted. It has been shown that already after short exposure
time (3 s) wettability of the surface increased, as the water contact angle changes
from 72 ° (untreated PET foil) to about 25 ° and 20 ° for nitrogen and oxygen
plasma treated surface, respectively. Formation of nitrogen and oxygen functional
groups was also achived shortly after plasma treatment and the concentration of
newly formed oxygen functional groups was shown to increase with plasma
treatment time. Plasma treatment also produces morphologycal changes of the
surface, as the PET surface exhibited sphere like structures after expouser to
nitrogen or oxygen plasma. The differences in morphology between nitrogen and
oxygen plasma treated surfaces were also observed, as oxygen plasma treated
surfaces exhibited more pronunced sphere like structures, which become elongated
after longer plasma treatment. Evidently the change in surface morphology also
affects the change in surface roughness, which increased with longer plasma
treatment time. However much higher surface roughness of PET foils treated in
oxygen plasma was observed in comparison to nitrogen plasma treated surfaces.
To some extent it was also possible to confirm that oxygen plasma treatment
increases crystallinity, as a higher degree of crystalline fraction was observed on
PET foil treated in oxygen plasma.
It has also been observed that PET foils with different initiall crystallinity may be
treated in plasma differently. Much faster surface heating of amorphous polymers
was observed during plasma treatment in comparison to semicrystalline polymers,
which can be assigned to different interactions of neutral atoms with the surface of
amorphous and semicrystalline polymers. Some changes in surface morphology
between PET polymers of different manufacturers were also observed, though all
plasma treated surfaces exhibited a similar sphere like formation. Similar
morphology was also observed on PET fibres (used for vascular grafts) after
oxygen plasma treatment.
In the second part of the work, the in vitro biological response of plasma treated
surfaces was studied. It has been shown that the most significant changes in
biological response were observed on oxygen plasma treated surfaces. These
surfaces enabled improved proliferation of endothelia cells and reduced adhesion of
platelets. This can mainly be attributed to newly formed oxygen functional groups,
which seem to have remarkable influence on adhesion of platelets. It has also been
shown that platelet adhesion is a function of polymer crystallinity, as much lower
platelet adhesion was observed on untreated semicrystalline polymers in
comparison to amorphous. Interestingly no correlation between platelet adhesion
and surface wettability was observed during this study, as there were no significant
differences in adhesion of platelets between highly hydrophobic or hydrophilic
surfaces.
According to the in vitro studies done on plasma treated PET foils, it has been
shown that oxygen plasma treatment is a promising technique to improve
proliferation of endothelia cells and to reduce adhesion of platelets. Thus oxygen
plasma could be a new method for improving hemocompatibile properties of
vascular grafts made of PET polymers.