Biomedical Engineering Reference
In-Depth Information
hydrophilic groups on the backbone. This refractive index for (PLL/HA) films is of the
same order of magnitude of that found via ellipsometry (Burke and Barrett 2003) for wet
films (1.35), and has to be compared to the refractive index for dried films (1.56). This indi-
cates that the film swells by about 830% (initial conditions for film assembly were pH 9 and
0.1 M NaCl). The high swelling capacities of polysaccharides, and in particular for hyaluro-
nan (Lapcik et al. 1998) renders the buildup of much thicker films possible—up to several
hundred nanometers (Richert et al. 2004c) or even several micrometers after deposition of
20 to 30 layer pairs (Picart et al. 2002). Because water content is an important parameter in
film structure, the temperature of film buildup or of posttreatment storage can also play
a role in the internal secondary structure of the film. This was nicely demonstrated by
Boulmedais et al. (2003) for (PLL/PGA) films heated up to 89°C.
These polysaccharide-based films were often, if not always, found to inhibit cell attach-
ment (Elbert et al. 1999; Richert et al. 2004a; Richert et al. 2004c), except when films were
stiffened by covalent cross-linking (Richert et al. 2004). Therefore, a trend that seems to
emerge from all these cell lines and primary cell studies is that nanometer-thin and dense
films formed by few layer pairs, which are also very stiff, are more favorable for cellular
adhesion than thick and highly hydrated films that are much softer (Picart et al. 2007).
A detailed study of the hydration and swelling properties of (PLL/HA) films indicates
that the most important parameters are (1) the assembly pH (that can be varied from 5
to 9 for these particular films) and ionic strength and (2) the swelling medium (Burke
and Barrett 2003). Thus, depending on the combination of these parameters, very differ-
ent film properties can be achieved. Polysaccharides such as HA have a random coil confor-
mation but can form hydrogen bonds with water (Hammond 1999). They can also exhibit
hydrophobic interactions (Laurent 1998), which are influenced by ionic strength. Very
interestingly, the p
K
a
of polyelectrolytes in the film demonstrates that both PLL and HA
experience a significant shift in their p
K
a(apparent)
values upon adsorption, compared to the
accepted values (in dilute solution) of 9.36 ± 0.08 and 3.08 ± 0.03, respectively, in the presence
of 1.0 mM NaCl. The p
K
a(apparent)
values of both PLL and HA remained relatively constant
after the first three to four deposited layers (at pH = 7, it is 4.8 for HA and 6.8 for PLL). This
decrease in the acid strength of HA and base strength of PLL is similar to that reported
for other polyelectrolyte pairs (Boulmedais et al. 2002). It has been previously speculated
and experimentally shown that the charge on the multilayer film surface strongly influ-
ences the acid-base equilibria of adsorbing polyelectrolyte chains (Shiratori and Rubner
2000). According to Barret et al., for (PLL/HA) multilayer films, the overall trends in the
p
K
a(apparent)
shifts upon adsorption are influenced by the ability of both of these polymers
to adopt some degree of secondary conformational order with changes in the local pH
and ionic strength environment (Burke and Barrett 2003, 2005; Turner et al. 1988; Yasui
and Keigerling 1986). In the intermediate pH range, HA is known to have some degree of
chain stiffening in solution due to local hydrogen bonded regions, whereas PLL chains
are reported to experience a random coil to
α
-helix transition at pH = 10.5 (Yasui and
Keigerling 1986). The same authors also investigated the swelling of (PAH/HA) films and
found that these films exhibit a high dependence of swelling on the assembly solution pH.
The swelling ratio varied between 2 at physiological pH = 7 to more than 8 at very acidic
pH = 2 and was more pronounced than that at basic pH = 10 (swelling ratio of about 5).
Influence of Molecular Weight on Film Buildup
Few studies have systematically investigated the influence of polymer molecular weight
on the physical properties of PEM, either for synthetic polyelectrolytes (Sui et al. 2003) or
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