Biomedical Engineering Reference
In-Depth Information
Although the surface coating PEO-PPO surfactant might be an effective
approach to achieve a blood-compatible surface, the possible leaching or
desorption of the surfactant may reduce the long-term effectiveness. To
overcome this problem, a technique using the combination of physical
adsorption and covalent immobilisation has been developed (Terlingen
et al.
, 1992).
Sheu
et al.
(1993) applied this concept successfully to prepare non-fouling
surfaces on biomaterials. They developed a novel radio-frequency glow
discharge (RFGD) process, in which the surfactant is fi rst 'anchored' on the
polymer surface via physical adsorption from a solvent which swells the
substrate polymer. Next, the solvent is evaporated and the adsorbed sur-
factant is bound covalently to the surface by a gas discharge treatment,
which causes 'crosslinking' of the surfactant with the polymer molecules in
the surface. The modifi ed surfaces exhibit a signifi cant reduction in fi brino-
gen adsorption.
As previously stated, a material that is surface active in a condensed
phase will have a higher concentration in the surface than it has in the bulk.
In fact, even in metal alloys, the component of lowest surface tension will
enrich the air-facing surface if suffi cient time is available for that compo-
nent to diffuse to the surface (Somorjai, 1981). For instance, lubricants have
been added to avoid excessive sticking on the processing mill to give good
mould-release surface properties for PVC processing. PEO surfactants are
such surface-active agents, which have been reported as antifogging agents
added in small amounts (2 to 5 phr (per hundred parts of resin)) to the
PVC formulation to modify the surface wettability. The modifi ed surface
causes the condensed water to wet the surface and leaves the fi lm cleaner.
In theory, the best way to achieve a hydrophilic surface is by a covalent
bonding process, but such methods are generally diffi cult and too costly for
commercialisation. An alternative has been reported which involves melt
blending of the water-soluble polymer into the base polymer, accompanied
by shear processing to drive the water-soluble polymer towards the surface.
This technique was considered much easier and more economical than
covalent bonding, provided that the resulting surface modifi cation can be
made suffi ciently permanent. PEO, PVA and poly(
n
-vinyl pyrolidone)
(PNVP) have been selected for surface modifi cation to obtain low protein
adsorption biomaterials (Ding
et al.
, 1996).
Zhao & Courtney (2007) utilised the combination of cyclodextrins, with
and without the addition of PEO or Pluronic F68, for incorporation into
medical grade DEHP plasticised PVC compound on a two-roll mill. PVC
compound without addition served as a control. Flat sheets were obtained
by moulding, Fibrinogen adsorption was measured and surface characteri-
sation was performed by ATR-FTIR analysis. The conclusions were as
follows:
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