Biomedical Engineering Reference
In-Depth Information
Other strategies rely on the use of antifungal peptides embedded in films (Etienne et
al. 2005a), possibly incorporated by means of an amphiphilic polyelectrolyte precomplex
with an hydrophobic peptide (Guyomard et al. 2008) or on Ag
+
ions as bactericides. In
a first approach, Lee et al. (2005) prepared hydrogen-bonded multilayers containing Ag
nanoparticles synthesized in situ and that were deposited on both planar surfaces and
magnetic colloidal particles. The duration of the sustained release of antibacterial Ag ions
from these coatings was prolonged by increasing the total supply of zero valent silver in
the films via multiple loading and reduction cycles. Later, Li et al. (2006) constructed thin
films with two distinct layered functional regions: a reservoir for the loading and release
of bactericidal chemicals and a nanoparticle surface cap with immobilized bactericides.
This resulted in dual-functional bactericidal coatings with both a chemical-releasing bacteria-
killing capacity and a contact bacteria-killing capacity. These dual-functional coatings
showed very high initial bacteria-killing efficiency due to the release of the Ag ions and
retained significant antibacterial activity after the depletion of the embedded Ag because
of the immobilized quaternary ammonium salts. Another strategy consists in loading sil-
ver ions into liposomes and subsequently embedding liposome aggregates into PLL/HA
films (Malcher et al. 2008). The strong bactericidal effect observed was attributed to the
diffusion of the silver ions out of the AgNO
3
coatings, leading to a significant bactericidal
concentration close to the membrane of the bacteria.
Film Biodegradability
Although, in principle, multilayer films can be built under extremely different conditions
in terms of pH and ionic strength, the final suspending medium may depend on the fore-
seen application. In particular, when cell culture studies or deposition on biomaterial sur-
faces are foreseen, it is then necessary that the films are stable in culture medium and
in physiological conditions. These requirements may greatly limit the range of possible
buildup conditions due to stability constraints. On the other hand, if the films are to be
used for a subsequent release of a film component itself or of a bioactive molecule (see
below), then stability is not a matter of importance, or at least is not as important as in the
first case.
It stems from the aforementioned properties of the natural-based multilayer films (weak
electrostatic charge, high hydration and swellability, secondary interactions) that these
softer films are also more fragile than synthetic ones. This is particularly true when the
films are built in a medium that has a different pH and/or ionic strength from a physi-
ological medium (ionic strength of about 0.15 M NaCl, neutral pH). Next, the films are
subjected to stresses upon medium change and can potentially be disrupted due to very
high internal stresses. Typical cases are films built at acidic pH such as COL/HA films or
CHI/HA films, for which COL and CHI are polycationic at acidic pH (4 for COL and less
than ~5.5 for CHI). Johansson et al. (2005) found that COL/HA films are not stable when
the pH is raised from 4 to 7. This could be explained by the protonation/deprotonation
process for the polyelectrolytes involved in the interaction. At pH 4.0, most acid functional-
ities are protonated, whereas they are deprotonated at pH 7. Regarding collagen, the num-
ber of negatively charged acids on collagen approaches the number of protonated amines
or the isoelectric point. The protonation/deprotonation processes induces the changes
in the three-dimensional structure of the polyelectrolytes, which affects the electrostatic
forces that existed between the polyelectrolyte layers. This dissolution was found to be
irreversible.
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