Biomedical Engineering Reference
In-Depth Information
R 2
N
O
O
(1)
NH -R 2 or R 3
+
R 1
C
C
R 1
C
O C
C
OH
N
R 3
N R 3 or R 2
(HA)
(EDC)
O
H
O
O
SO 3 Na
(2)
O
SO 3 Na R 2
N
R 1
C
NH -R 2 or R 3
R 1
C
+ HO
N
+
C
O
O N
O
C
R 3
N
N-R 3 or R 2
O
H
O
(Sulfo NHS)
O
O
SO 3 Na
O
O
SO 3 Na
(3)
R 1
C
R 1
C
+ HO
N
+ H 2 N R 4
O N
NH -R 4
(PLL)
O
O
O
O
O
SO 3 Na
SO 3 Na
+
O
C
R 1
(4)
R 1
C
H 2 O
HO
N
+
O N
OH
O
O
FIGURE 8.4
EDC and sulfo-NHS coupling scheme. EDC reacts with a carboxylic group and activates it (1). Activated complex
is conversed into an active ester with sulfo-NHS (2). Active ester reacts with primary amine to form an amide
bound (3). Unreacted sites are hydrolyzed to give a regeneration of carboxyls (4). (Reproduced with permission
from Richert et al., Biomacromolecules , 5, 284-294, 2004. Copyright American Chemical Society 2004.)
Cross-linking has major consequences on film structure and mechanical properties.
Using AFM nanoindentation, considerable stiffening of both PLL/HA and CHI/HA films
was evidenced and Young's modulus was much higher when compared to the native
(uncross-linked) films (Francius et al. 2006; Richert et al. 2004b; Schneider et al. 2007b).
Moreover, another consequence of cross-linking is to significantly improve their resis-
tance to the biodegradation of CHI/HA films, both in vitro and in vivo (Etienne et al.
2005b; Picart et al. 2005b). Tuning the EDC cross-linker concentration allows us to vary the
Young's modulus of (PLL/HA) films over 2 orders of magnitude (Figure 8.5) (Francius et
al. 2006). Of note, the percentage of decrease in the carboxylic peak, which can be precisely
quantified by FTIR spectroscopy (Crouzier and Picart 2009), is found to be related to the
film Young's modulus E 0 (Figure 8.6).
Using a different strategy, Li and Haynie (2004) investigated cross-linking and stabiliza-
tion of polypeptide PEMs by formation of disulfide bonds, which are reversible. Disulfide
bonds are involved in the structural stabilization of proteins. More recently, Such et al.
(2007) reported a new method for covalent cross-linking via click chemistry to facilitate
the LbL assembly of thin films.
 
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