Biomedical Engineering Reference
In-Depth Information
substrate are probably important constraints. Therefore, particularly rapid methods such
as spraying are being further developed (Izquierdo et al. 2005). Spray depositing was found
to be effective even under conditions for which dipping failed to produce homogeneous
films (e.g., extremely short contact times). Moreover, it was found that the rinsing step
could be skipped, making it possible to speed up the whole buildup process.
Regarding anchorage to the underlying substrate, a recent development reported by Lee et al.
(2008) paves the way to better anchoring of the film and could be applied in the case of par-
ticularly hydrophobic surfaces such as poly(tetrafluoroethylene) (PTFE) and poly(ethylene)
(PE), which often require priming methods. These authors showed that mussel adhesive-
inspired polymers containing catechol and amine functional groups, such as dopamine, can
be grafted to various polyelectrolytes that allow the adsorption of polyelectrolytes and LbL
buildup on various polymeric surfaces such as PTFE, polyethyleneterephthalate, and poly-
carbonate. These catecholamine polymers could thus be used as universal surface primers
for facilitating LbL assembly on metal, oxide, and polymer substrates.
Overall, the numerous possibilities for depositing methods and the recent development
of reinforced polyelectrolyte/surface anchoring strengthen the notion that films can be
efficiently and reproducibly coated onto any kind of substrate.
Growth Mode: Linear versus Exponential
The first investigated polyelectrolyte systems were described by Decher et al. (1992). These
systems showed linear growth of both mass and film thickness with the number of layers
deposited. Poly(styrene sulfonate)/poly(allylamine hydrochloride) (PSS/PAH) films are
one of the most prominent examples of linearly growing systems (Caruso et al. 1997; Picart
et al. 2001a). These films have a stratified structure, each polyelectrolyte layer interpen-
etrating only its neighboring layers.
Films that grow exponentially have been described more recently by Elbert et al. (1999)
and Picart et al. (2001b) for poly(l-lysine)/alginate (PLL/ALG) and PLL/hyaluronan (PLL/
HA) films. Initially, this type of growth was mostly observed in films based on polyamino-
acids and polysaccharides (Lavalle et al. 2002; Richert et al. 2004c; Tezcaner et al. 2006) before
being found to be much more commonly encountered. Either film roughness (McAloney
et al. 2001) or polyelectrolyte diffusion in and out of the film of one of the polyelectrolytes
(Picart et al. 2002) was found to be at the origin of this growth, with the latter mechanism
being increasingly recognized as a key feature in film growth. Even films made of syn-
thetic polyelectrolytes such as polyacrylic acid (PAA) can exhibit an exponential growth
(Sun et al. 2007; Zacharia et al. 2007). Exponential growth can be evidenced by QCM-D (Fig-
ure 8.3a). In addition, polyelectrolyte diffusion can be easily visualized via confocal laser
scanning microscopy (CLSM) when the films are thick enough (at least 1 μm) (Figure 8.3b).
This was shown by observing PLL
FITC
(Picart et al. 2002), chitosan (CHI)
FITC
(Richert et al.
2004c), and end-labeled PAA (Sun et al. 2007) diffusion. A mechanism for this diffusion has
also been proposed (Lavalle et al. 2004). According to a theoretical model by Salomaki and
Kankare (2007), PLL/HA and CHI/HA films should always grow exponentially. Recently,
the exponential growth mechanism has seen some significant developments. Films con-
taining inorganic sheets can thus also show exponential growth (Podsiadlo et al. 2008).
In addition, growth can be amplified by pH leading to even thicker films obtained in a
very limited number of depositing cycles (Ji et al. 2006). Better understanding of the dif-
ferent growth mechanisms is also emerging (von Klitzing 2006). For instance, Porcel et al.
(2006) showed that a transition from exponential to linear growth occurs at a certain level
in film buildup. It also appears that even for synthetic polyelectrolyte films, exponential
Search WWH ::
Custom Search