Biomedical Engineering Reference
In-Depth Information
(Schneider et al. 2006), onto one of the polyelectrolytes can significantly influence the
film's stiffness.
A second strategy for adjusting the mechanical properties of PEM films is to incorpo-
rate nanoparticles into the films. Inspired by the inorganic-organic composite material
of seashells and lamellar bone, Kotov et al. studied the buildup of composite multilayer
films containing cationic polyelectrolytes and anionic nanoparticles, such as carbon nano-
tubes (Tang et al. 2003; Gheith et al. 2005), montmorrillonite (Podsiadlo et al. 2007; Tang et
al. 2003), or metallic nanoparticles (Jiang et al. 2004; Koktysh et al. 2002; Ostrander et al.
2001; Park et al. 2005). Evaluation of the mechanical properties of these composite films
displayed up to 2 orders of magnitude more on Young's modulus when compared with the
pure polyelectrolyte (Srivastava and Kotov 2008).
Another method of stiffening “soft” PEM films consists in inserting “stronger” poly-
electrolytes. Caruso et al. and Schaaf et al. groups thus tailored PEM rigidity by mixing
polyanions, whose behavior usually differed considerably when considered individually
as film constituents (Ball et al. 2009; Cho et al. 2004; Hubsch et al. 2004). Although the
rigidity in these studies was not measured directly, in many cases, the growth regime
could be changed from exponential to linear by suitable blending, showing that most
likely the mechanical properties had also changed. Considerable stiffening of the film
was thus observed, for instance, by inserting “stiff” PSS/PAH layers on top of a “soft”
PLL/HA film (Francius et al. 2007) or between layers of CHI/HA (Salomaki and Kankare
2009).
One of the many attractive features of PEM is the extent to which their mechanical prop-
erties can be tailored by varying the conditions used to assemble the films. Because of
the pH-dependent dissociation of the weak acidic and alkaline functional groups on the
chains, films prepared from weak polyelectrolytes (only partially charged at moderate pH
near their p
K
) are strongly modulated by the pH and ionic strength environment. Mermut
et al. (2003) thus showed that Young's modulus of films made of PAH and an azobenzene-
containing polyelectrolyte was reduced from 6.5 to about 0.1 MPa when the assembly pH
increased from 5 to 9. Several other groups, in particular Rubner et al., investigated the
remarkable nanoscale control that can be exercised over the properties of (PAH/PAA)
films (i.e., stiffness, thickness, roughness, wettability, and swelling behavior), by vary-
ing the pH conditions used to assemble the films (Mendelsohn et al. 2003; Pavoor et al.
2004; Thompson et al. 2005). In brief, PAH/PAA films assembled at a relatively neutral pH
are significantly thinner and about 2 orders of magnitude stiffer than those assembled in
acidic conditions.
It is also possible to adjust the mechanical properties of PEMs by chemical means, for
instance, by creating covalent cross-links within the films. As an example, it is already
known that high temperature (130°C) can induce the formation of amide or imide bonds
within films (Dai et al. 2001). A protocol based on the carbodiimide chemistry for cross-
linking carboxyl groups with amine groups in “mild” conditions (room temperature, salt-
containing medium), thereby forming covalent amide bonds (also called peptide bonds),
was proposed by Richert et al. (2004a) and Schuetz and Caruso (2003). Of note, the carbo-
diimide used (1-ethyl-3-(3-dimethylaminopropyl) carbodiimide [EDC]) was a zero-length
cross-linker, which means that no additional molecule was inserted into the film. The cross-
linking results in the selective transformation of ionic (ammonium and carboxylate) cross-
links into covalent amide bonds (Figure 8.4). This versatile protocol can be applied to many
different types of polyelectrolyte films, provided that they possess carboxylic and amine
groups.
Search WWH ::
Custom Search