Blood interface biomaterials - Biomaterials and Devices for the Circulatory System - page 39

on the coating technology and blood compatibility assessment for DLC-
modifi ed polymers.

2.5 Microdomain structured and bioactive surfaces
2.5.1 Microdomain structured surfaces

It is known that all of the cells and tissues in the living organism are built
up with a microdomain structured surface (Ishihara et al. , 1993). The normal
vascular endothelium, which possesses ideal antithrombogenic properties,
has a microphase separated structure composed of hydrophilic and hydro-
phobic microdomains (Sawyer et al. , 1964). In order to achieve a blood-
compatible surface, mimicking these biosurfaces, many synthetic polymers
with microdomain structured surfaces have been designed, mainly through
segmented block and graft copolymerisation.
Polyurethanes are the most widely investigated biomaterials with
microphase separated surface structures (Lelah & Cooper, 1986). The rel-
atively good blood compatibility was suggested by Lyman et al. (1975)
who assumed it was because of the similar domain size as the size of
globular proteins. In addition to the common polyether-urethane and
polyester-urethane, Yu et al. (1985) developed polydimethylsiloxane-
polyurethane elastomers and they have been found to have favourable
blood-contacting properties compared with a polyetherurethane (Lim
et al. , 1994).
Block copolymers with hydrophilic and hydrophobic segments have been
reported to be antithrombogenic. Okano et al. (1978) synthesised a triblock
(A-B-A) copolymer consisting of HEMA (A) and styrene (B), forming a
typical domain structure. The antithrombogenicity of these A-B-A triblock
copolymers was proposed to have arisen from their microphase separation,
which in turn affects protein adsorption and thereby infl uences platelet
adhesion and activation (Okano et al. , 1981).
By grafting a hydrophobic group to a hydrophilic polymer, it is also
possible to produce a microdomain structured surface, exhibiting blood
compatibility (Mathew et al. , 1992). Grainger et al. (1990) reported on
poly(dimethylsiloxane)-poly(ethylene oxide)-heparin block copolymers,
which possess a microphase separated surface, as well as exhibiting anti-
thrombogenic activity.
Kawahito et al. (1995) synthesised a new microdomain structured copo-
lymer called fl uorine-acryl-styrene-urethane-silicone (FASUS). This new
copolymer may be effective in preventing thrombus formation in vitro , ex
vivo and in clinical situations.
From the above-mentioned examples, it is clearly shown that equilib-
rium between hydrophilicity and hydrophobicity is necessary to build a

￿ ￿ ￿ ￿ ￿