178 K.Yu,G.GaoandJ.N.Kizhakkedathu
Figure 8.2. High-resolution S2p X-ray photoelectron spectra (in counts per second).
These spectra are for CH
7SH (1-octadecanethiol), HOCH
SH (mercapto-1-
undecanol),and HOOC(CH
SH (11-mercaptoundecanoic acid) exposed to ambient en-
vironmental conditions (fresh, 5 days, 11 days) and PEG-bl-PPS-bl-PEG (fresh, 5 days,
41 days and exposed to ultraviolet and ozone).Spectra comprise doublet structures due to
the presence of the S
and S
peaks. In less than 2 weeks, thiols show significant
oxidation, whereas the PEG-bl-PPS-bl-PEG only shows nominal oxidation near 6 weeks.
The sample treated with ultraviolet and ozone demonstrates shifted oxidized S
spectra as
positive control. Inset indicates chemical structure for PEG-bl-PPS-bl-PEG. Images adapted
from Ref. 65.
Bearinger et al. explored an alternative structure to alkanethiolates, namely
block copolymers comprising sulphides in a chemisorbing block (PEG
) where PPS = polypropylene sulfide, in an expectation of
better resistance to oxidative instability.
These materials offer a more robust
approach to passivate gold surfaces against protein adsorption and cell adhesion.
The copolymer PEG
chemisorbed to form a 2.2 nm dense
monolayer of 226 ± 26 ng cm
. The copolymeric monolayer was much more stable
to oxidation than commonly used alkanethiolates (Fig. 8.2). Protein adsorption
was greatly reduced (>95%), even when exposed to whole blood serum (>55 mg
protein concentration). The cell adhesion was also reduced over long culture
durations (> 97%) (Fig. 8.3). These results such that polysulphides containing
copolymers are attractive alternatives to alkanethiolates with greater stability and
allows greater flexibility in the design.
8.2.3 Covalent Grafting of Poly (ethyelene glycol)
Compared to oligo(ethylene glycol) SAMs, grafting of preformed poly(ethylene
glycol) (PEG) provided more robust films and versatile architecture, and chemistry
for the development of non-fouling surfaces. Several approaches have been
reported for grafting PEG chains on solid surfaces such as metal, silicon, plastics
to improve the biocompatibility relying on different coupling chemistry or tech-
Unsworth and Brash used chain-end thiolated PEO assemblies to investigate
the effect of PEO chain density on protein adsorption.
Their study showed that
resistance to fibrinogen adsorption for 750 and 2000 MW (molecular weight) PEO
layers increased with graft density of PEO chains and the least protein adsorption
was observed at 0.5 chains/nm
. However, the protein adsorption increased
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Polymer-Based Biocompatible Surface Coatings 179
Figure 8.3. Surface plasmon resonance measurements. a, Intensity of reflected light
against angle of incidence before and after deposition of PEG-bl-PPS-bl-PEG in methanol
on a gold surface. Polymer adsorption induces a shift of the resonance angle,which is
proportional to adsorbed mass. b, Shift of resonance angle observed in methanol as a
function of time by monitoring the increase of reflectivity at about 1
gbelow the resonance
angle.c,Intensity of reflected light against time for serum addition, followed by subse-
quent buffer wash, on PEG-bl-PPS-bl-PEG-coated gold surface.d , Intensity of reflected light
against time for serum addition, followed by subsequent buffer wash, on gold surface. The
chain density and architecture are two important factors which influence the anti-fouling
properties. Images adapted from Ref. 65.
when the graft density was increased beyond 0.5 chains/nm
. For 5000 MW PEO,
the optimal chain density was found to be 0.5 chains/nm
.Brashet al. suggested
that at high chain density, the chemisorbed PEO was dehydrated giving a surface
that was no longer protein resistant.
Sofia et al. tested surfaces covalently grafted with poly(ethylene oxide) poly-
mers with different architecture, in linear and star form, on silicon membrane to
determine their ability to prevent protein adsorption.
Modified surfaces were
exposed to solutions of each of three proteins: cytochrome-c, albumin, and fi-
bronectin. It was found that protein adsorption decreased almost exponentially
with graft density of chains and it reached almost negligible adsorption at the
highest grafting densities. In contrast to surfaces grafted with linear PEG, star
PEG-grafted surfaces can prevent adsorption of larger proteins even though open
spaces existed between bound molecules. The adsorption was prevented as long
as the open spaces were smaller than the effective size of the protein. They
also proposed a physical model of the surface to explain the protein adsorption
resistance in terms of the spacing and degree of overlap of grafted PEO chains
which lead to steric repulsion between surface and protein. In the case of linear
PEG surfaces, a complete coverage of the surfaces with grafted polymer (chains
were roughly half-overlapping (i.e., L R
linear; Rg = radius of gyration, L-
distance between the grafting point)) was important for the prevention of protein
adsorption. Since the star PEO-grafted surfaces have much more polymer segment
concentration than linear chains of equivalent molecular weight or equivalent size
(Fig. 8.4), they prevented the adsorption of proteins to surface more efficiently.
Zhu et al. reported a strategy for the formation of uniform and ultra-flat
PEG monolayers on the atomically flat Si(111) surface with high grafting density
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