Polymer-Based Biocompatible Surface Coatings 175
8.2.1.2 Electrostatic Adsorption
In this method, electrostatic adsorption of charged polymers on oppositely
charged surface was used for creating a non-fouling surface layers on metallic
surfaces.
4648
Hubbell and Spencer developed a class of copolymers based on
poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG), which was found to sponta-
neously adsorb from aqueous solution onto several metal oxide surfaces, pro-
viding a high degree of resistance to protein adsorption.
4647
The adsorbed co-
polymer layer formed a comb-like structure on the surface, with positively charged
primary amine groups of the PLL bound to the negatively charged metal oxide
surface due to electrostatic interaction, while the hydrophilic and uncharged PEG
side chains were exposed to the solution phase. The resulting adsorbed layers were
highly effective in reducing the protein adsorption from both individual protein
solutions and blood serum. The amount of protein adsorbed from human serum
on the modified surfaces was consistently below 1–2 ng cm
2
. Fibrinogen adsorp-
tion was reduced by 96-98% in comparison to the unmodified oxide surfaces.
8.2.2 Chemisorption of PEG Containing Thiol or Sulfide Groups
The spontaneous assembly of thiol or sulfide containing molecules on noble metal
surfaces is a good method for surface modification.
49
This method has been
extensivelyusedinthemodicationofgoldorgoldcoatedsurfaces.Onegreatad-
vantage of this method was the ease of preparation of surfaces; alkanethiols readily
assemble when gold surface was exposed to alkanethiols either in solution or
vapor phase.
4950
Self assembled monolayers (SAMs) of alkanethiols, terminated
with short oligomers of the ethylene glycol groups, (HS(CH
2
)
11
-(OCH
2
CH
2
)
n
OH),
were developed to resist the adsorption of several model proteins.
5159
Prime and Whitesides prepared monolayers from alkanethiols terminated
with short oligomers of the ethylene glycol group (HS(CH
2
)
11
(OCH
2
-CH
2
)
n
OH;
n= 3-7) and studied their protein exclusion behavior
51
They found that these
monolayers appeared to provide a general solution for controlling non-specific
adsorption of proteins with a range of molecular weights and isoelectric points
(pI), and under a wide range of solution conditions
5253
For a better understanding of the factors involved in the protein resistance
of oligoether SAMs, Herrwerth and Grunze synthesized a series of oligoether-
terminated alkanethiols with different oligoether backbones, different chain
lengths and different alkyl terminals.
5456
Their studies suggested that the pene-
tration of water molecules to the interior of the SAMs was a necessary prerequisite
for protein resistance. Three factors, including the hydrophilicity of the internal
units, packing density and the hydrophilicity of the terminal group should be
considered for making a non-fouling oligoether SAMs. It was demonstrated
that an interior hydrophilic structure was necessary as evident from the fact that
only the oligo(ethylene glycol) and oligo(trimethylene glycol) SAMs exhibited
protein resistance. Oligo(propylene glycol) SAMs did not show protein repelling
properties (Table 1). Another important factor was the packing density of SAMs.
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176 K.Yu,G.GaoandJ.N.Kizhakkedathu
Table 8. 1 AmountofadsorbedfibrinogenonSAMsongoldandsilver,mea-
sured by the ellipsometric thickness of the protein layer and normalized to the
amount of fibrinogen adsorbed on a monolayer of hexadecanethiol (C16SH) on
gold (=100%).
54
Protein Absorption[%] Protein Absorption[%]
Au Ag Au Ag
EG
1
OME(1) 22 57 EG
6
OMe(10) 0 35
EG
2
OH(2) 0 0 EG
6
OEt(11) 51 89
EG
2
OME(3) 0 62 EG
6
OPr(12) 69 100
EG
3
OH(4) 0 0 PRO
2
OMe(13) 52 87
EG
3
OME(5) 0 37 PRO
3
OMe(14) 49 80
EG
3
OEt(6) 60 88 PRO
4
OMe(15) 42 60
EG
3
Opr(7) 79 93 TRI
3
OH(16) 0 0
EG
3
OBu(8) 77 98 TRI
3
OMe(17) 0 52
EG
6
OH(9) 0 0
Figure 8.1. Amount of protein adsorption on a given oligoether SAM on gold normalized
to the amount of protein adsorbed on a monolayer of hexa-decanethiol on gold (100%)
versus advancing aqueous contact angle of the SAM. Symbols: red
,EG
2
OH; orange,
EG
3
OH; green+,EG
6
OH; blue,TRI
3
OH; blue ,EG
1
OMe; green,EG
2
OMe; light blue ,
EG
3
OMe; red ×,EG
6
OMe; blue ,TRI
3
OMe; red ×,PRO
2
OMe; blue,PRO
3
OMe; orange
+,PRO
4
OMe; purpl,EG
3
OEt; green,EG
6
OEt; blue,EG
3
OPr; green,EG
6
OPr; blue ,
EG
3
OBu. Images adapted from Ref. 54. For color reference, see page 270.
It was shown that SAMs with a relaxed lateral packing or with some defects or
disorder was required for high protein resistance. The methoxy-terminated SAMs
which form a highly ordered all-trans conformation lost non-fouling capacity due
to high packing densities. The terminal groups of the SAMs were shown to play
an equally important role (Fig. 8.1). For excellent protein resistance, the contact
angles should not exceed 70
.
54
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