Coating of Material Surfaces with Layer-by- Layer Assembled Polyelectrolyte Films - Biological and Biomedical Coatings: Processing and Characterization - page 354

Bioactive Proteins and Growth Factors
Since PEM films were first developed, proteins have been employed as building blocks in
LbL films. Thus, multicomponent LbL films containing proteins have been assembled via
electrostatic interactions (Lvov et al. 1995; Onda et al. 1996; Lvov et al. 1998). One of the first
pieces of evidence that protein embedded in PEM films retain their bioactivity came from
Jessel et al. (2003). In their study, protein A was embedded at different levels in (PLL/PGA)
films and was found to induce a time-dependent expression of TNF- α in THP-1 phagocytic
cells. Interestingly, the cells were shown to come into contact with the protein by local deg-
radation of the films. Notably, the cells were able to degrade the PLL but not poly(d-lysine),
which forms a barrier between the cells and protein A. In this case, TNF- α production was
significantly reduced.
Nonphagocytic cells such as neurons have also been shown to respond to protein
embedded in a multilayer architecture. In the study by Vodouhe et al. (2005), multilayer
films (mostly ending by PSS) were functionalized with a growth factor, brain derived
neurotrophic factor (BDNF) or a chemorepulsive protein, semaphorin 3A (Sema3A). The
quantitative amount of protein adsorbed was estimated by optical waveguide lightmode
spectroscopy. The authors showed that the embedded proteins were stable in the multi-
layer architecture and that the protein was not released in the culture medium after 2 days
in culture (or at least, the release level was below the detection level). Very interestingly,
BDNF induced an increased neuronal activity and an increased neurite length, whereas
Sema3A induced a decreased activity and neurite length. Thus, the structure of the films
could be correlated to their effective biological activity.
Ma et al. introduced a new class of bioactive films using directly growth factors (acidic
or basic fibroblasts growth factors, aFGF or bFGF, respectively) as building blocks, either
mixed with heparin and deposited alternately with PEI, or directly used as polycation
and deposited with chondroitin sulfate A (Ma et al. 2007; Mao et al. 2005). An enhanced
secretion of collagen type I and interleukin 6 (IL-6) by fibroblasts seeded on the five layer
pairs of (aFGF/HEP)/PEI was also observed by immunohistochemistry. When bFGF was
directly built in multilayer films with CSA, the films containing bFGF had an improved
bioactivity. In vitro incubation of the CSA/bFGF multilayers in PBS showed that about 30%
of the incorporated bFGF was released within 8 days. The fact that growth factors retained
their biological activity is extremely interesting for biomedical applications.
Using films made of (PLL/CSA), Tezcaner et al. (2006) prepared functionalized multilay-
ers by adsorbing bFGF or the insoluble fraction of the intercellular photoreceptor matrix
(IPM) on or within the PLL/CSA PEMs. They showed that bFGF and IPM adsorption on
top of the (PLL/CSA) 10 /PLL polyelectrolyte films increased the number of photoreceptor
cells attached, and in particular bFGF adsorbed on the top led to a statistically signifi-
cant increase in photoreceptor cell survival at day 7. Recent developments include the use
of films containing growth factors, such as bone morphogenetic protein 2 (BMP2) and
transforming growth factor 1 (TGF β 1) for inducing the specific differentiation of embry-
onic stem cells to form bone tissue (Dierich et al. 2007). The authors used monocarboxylic
β -cyclodextrins to favor the insertion of both growth factors and showed that both were
required for inducing an effective differentiation. However, little is known about the exact
amount of protein adsorbed or about the interaction mechanism of the growth factors and
the embryonic-like bodies.
Recently, the vascular endothelial growth factor was adsorbed onto (PLL/PGA) and
(PSS/PAH) films deposited on porous titanium implants (Muller et al. 2008). The (PAH/
PSS) 4 architecture was selected to functionalize porous titanium, both for its high efficiency