Biomedical Engineering Reference
In-Depth Information
for natural polyelectrolytes. One of the reasons is the difficulty in obtaining monodis-
persed polymers, especially for natural polyelectrolytes that have very diverse production
methods and are subjected to natural variation. Kujawa et al. (2005) found that CHI/HA
film thickness increased when the
M
W
of these polysaccharides was higher. However, this
effect was only attributed to a difference in film growth onset and not to actual differences
in mass deposited per layer. Sun et al. (2007) used the (PAH/PAA) synthetic film model to
underline the importance of molecular weight in the buildup process. With lower molecu-
lar weight PAA, they observed that growth approached that of exponentially growing
systems. The use of low-molecular-weight PAA instead of a higher MW (the one com-
monly used and commercially available) made it possible to diffuse it within these films,
resulting in thicker and exponentially growing films. Such differences in diffusion were
also observed by Porcel et al. (2007), who studied MW changes in the PLL/HA system.
Changes in PLL and HA molecular weight did not significantly affect the mass deposited
per layer, but the use of high MW PLL did restrain PLL diffusion within the upper part of
the film. Most of the investigations that focused on the influence of polymer length essen-
tially studied the influence on the film growth curve and thickness, with other proper-
ties (e.g., mechanical) rarely investigated (Lingstrom and Wagberg 2008). Overall, both the
nature of diffusing species and its length are important.
Mechanical Properties
In recent years, designing PEM with adjustable mechanical properties has become a major
challenge for applications in chemistry, physics, and biology. The characterization of their
viscoelastic properties is thus crucial, and several methods that are specific to thin films
have been employed. A common one is to perform nanoindentation experiments via atomic
force microscopy (AFM), possibly using a colloidal probe as indenter (Dimitriadis et al.
2002; Richert et al. 2004b). Several other methods, relying on different physical principles,
are well suited to characterizing thin films, some of them in a liquid state. These include
quartz crystal microbalance (Salomaki et al. 2004; Salomaki and Kankare 2007), piezorhe-
ometry (Collin et al. 2004), or bulging tests (Lin et al. 2007). The stiffness of PEM can be
modulated from a few kPa to several GPa depending on the structural properties of the
polyelectrolytes, and also on the degree of cross-linking inside the film. In fact, both ionic
cross-links, affected mostly by pH and ionic strength, and covalent cross-links induced
by chemicals or photoactivation will impact film stiffness. It can also be observed that the
stiffness of a native multilayer film is also related to the buildup regime in the LBL process.
Films that grow exponentially are generally considered to be softer than those with a lin-
ear buildup (Collin et al. 2004). For instance, PLL/HA native films are rather viscous and
Young's modulus (
E
0
) for cross-linked films reaches a maximum of 500 kPa, whereas PSS/
PAH microcapsules in water are 300 MPa (Picart et al. 2007).
A first means of modulation consists in modifying the film's internal structure by using
polyelectrolytes with different conformations such as carrageenans. Schoeler et al. (2006)
thus characterized the stiffness of PEM containing PAH as the polycation and two dif-
ferent anionic sulfated polysaccharides:
ι
-carrageenan, which forms helical structures,
and
λ
-carrageenan, which has a random coil conformation. Using AFM indentation, they
found that films prepared with
ι
-carrageenan were about three times stiffer than those
with
λ
-carrageenan, highlighting the strong influence of polyelectrolyte structures on
the film's rigidity (Schoeler et al. 2006; Schönhoff et al. 2007). In a similar manner, graft-
ing phospholipid (Kujawa et al. 2007) or sugar molecules, such as lactose or mannose
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