Coating of Material Surfaces with Layer-by- Layer Assembled Polyelectrolyte Films - Biological and Biomedical Coatings: Processing and Characterization - page 338

(a)

2000

1500

(b)

1000

500

0

0 1 2 3 4 5 6 7 8

Number of layer pairs

FIGURE 8.3
(a) QCM frequency shifts −Δ f /ν for each layer as a function of number of layer pair n deposited on SiO 2 substrate.
Data are given for fundamental frequency at 5 MHz. Black line corresponds to fit of data by an exponential law.
(b) Z observation of a (PLL/HA) 19 -PLL 20 multilayer film by CLSM. Vertical section through film containing two
labeled PLL 19 FITC (green) and HA 19 TR layers. Image size is 26.2 × 8.4 μm. Supporting glass slide is indicated with
a white line. Green fluorescence (corresponding to PLL FITC ) is visible over a total thickness of about 4 μm (white
arrow). (Adapted from Picart et al., Proc. Natl. Acad. Sci. U S A , 99, 12531-12535, 2002.)

growth becomes dominant when NaCl concentrations increase (McAloney et al. 2001) or
when temperature is increased (Salomaki et al. 2005). Interestingly, isothermal titration
microcalorimetry investigations indicate that the linear growth regime is associated with
exothermic complexation, whereas the exponential growth regime relates to endothermic
complexation (Laugel et al. 2006).

Film Hydration and Swellability: Sensitivity to External
Parameters Such as pH and Ionic Strength

Film hydration can be estimated by measuring the film refractive index using techniques
such as optical waveguide lightmode spectroscopy (Picart et al. 2001) or ellipsometry
(by measuring respectively dry and hydrated film thickness) (Burke and Barrett 2003).
Refractive index of synthetic PEM films, such as (PSS/PAH) films, was measured in situ
by OWLS to be approximately 1.5 under physiological conditions (Picart et al. 2001). This
indicates that these films are relatively dense and contain only about 25% of water (a simple
approximation of the water content is based on the following formula: n PEM = 1.3340 × a + (1 − a ) ×
1.56, where 1.334 is the refractive index of a 0.15 M NaCl solution, 1.56 is the refractive index
of a pure polymer film (Burke and Barrett 2003), and a being the fraction of water). Similar
measurements have been done using PLL, poly(d-lysine) (PDL), or even chitosan as polyca-
tion in combination with polyanions such as gelatin (Ai et al. 2003), poly(l-glutamic) acid
(PGA) (Tryoen-Toth et al. 2002), or hyaluronan (Richert et al. 2004c). In general, films made
of polypeptides and polysaccharides in comparable ionic strength conditions are more
hydrated than films made of synthetic polyelectrolytes such as (PSS/PAH). This obser-
vation is based on refractive indices that are ≈1.36-1.38 for polysaccharide films (Picart
et al. 2001; Richert et al. 2004c) and ≈1.42 for (PGA/PLL) films (Lavalle et al. 2002), which
would correspond to water contents ranging respectively from 95% to 60%. Of note, the
majority of synthetic polyelectrolytes have hydrophobic chain backbones, which determine
film properties before complex formation. On the other hand, natural polyelectrolytes have