186 K.Yu,G.GaoandJ.N.Kizhakkedathu
Figure 8.13. Synthesis of Tethered OEG
n
-Containing Polymer Brushes and |-OEG
n
Assem-
blies on SiO
X
.Images adapted from Ref. 84.
3
O
O
O
-
O
N
+
SKRVSKRU\OFKROLQH
6
-
O
N
+
&
O
N
+
-
O
O
O
VXOIREHWDLQH
FDUER[\EHWDLQH
Figure 8.14. Chemical structure of phosphorylcholine (a), sulfobetaine (b) and cabocybe-
taine (c).
for their resistance to protein adhesion.
100,101
Zwitterionic structures similar to
phosphorylcholine such as phosphobetaine, sulfobetaine, and carboxybetaine
(Fig. 8.14) were also ideal for developing non-fouling surfaces when the surface
density and chain length of zwitterionic groups were well controlled.
102108
We will discuss two important modification strategies (a) self assembled
monolayers containing zwitterionic groups
93,94
and (b) surface initiated polymer-
ization of a zwitterionic group containing monomers
97,102104,106108
in the follow-
ing section. The physical adsorption of zwitterionic group based polymers was
also used as a method to make non-fouling surfaces
105
but will not be discussed
here.
8.3.1 Self-assembled Monolayers Containing Zwitterionic Groups
Holmlin and Whitesides fabricated SAMs terminated with charged groups on the
gold surface to evaluate their ability to resist the nonspecific adsorption of proteins
from aqueous buffer solutions.
93
Single-component SAMs with all positive groups
(trimethylammonium group) and all negative groups (sulfonate group) adsorbed
nearly full monolayers of fibrinogen and lysozyme. SAMs formed from a 1:1
mixture of thiols terminated with a negatively charged group and a positively
SO13997_text.indd 196SO13997_text.indd 196 14/02/2011 11:23 AM14/02/2011 11:23 AM
Polymer-Based Biocompatible Surface Coatings 187
Figure 8.15. Single-component SAMs with single charges (SAMs of 1 or 2), zwitterionic
mixed SAMs (SAM of 1 and 2 in a 1:1 ratio), and single-component zwitterionic SAMs
(SAMs of 3 or 4). Images adapted from Ref. 93.
charged group adsorbed <1 % of a monolayer of protein (Fig. 8.15). Single-
component zwitterionic SAMs composed of thiols terminated with N, N-dimethyl-
amino-propane-1-sulfonic acid (-N
+
(CH
3
)
2
CH
2
CH
2
CH
2
SO
3
) groups were sub-
stantially more effective in resisting the adsorption of fibrinogen and lysozyme
from buffer solutions at physiological ionic strength and pH. These surfaces
had better non-fouling properties than the single-component zwitterionic SAMs
composed of thiols terminating in phosphoric acid 2-trimethylamino-ethyl ester
(-OP(O)
2
OCH
2
CH
2
N
+
(CH
3
)
3
) (PC). The ability of zwitterionic surfaces to resist
protein adsorption supports the notion that one of the essential characteristics in
the design of a bio-inert surface was the electrical neutrality of the surface.
Chen and Jiang synthesized PC thiols using a modified synthetic route and
formed the zwitterionic phosphorylcholine (PC) self-assembled monolayers on
Au(111) (Fig. 8.22).
94
It was demonstrated that zwitterionic PC SAMs were highly
resistant to protein adsorption. Balanced charge and minimized dipole were the
two key factors for their non-fouling behavior. The experimental results showed
that PC SAMs had very low protein adsorption when the nitrogen/phosphorus
Figure 8.16. Synthesis route of 11-mercaptoundecylphosphorylcholine. Images adapted
from Ref. 94.
SO13997_text.indd 195SO13997_text.indd 195 26/01/2011 3:50 PM26/01/2011 3:50 PM

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