54 Sara Llamas, Laura Fernández-Peña, Ana Mateos-Maroto et al.
interactions [138-159]. Another advantage of the LbL method in relation to
other polyelectrolyte assembly approaches is the possibility to use a wide
range of substrates with different nature as templates [160, 161]: flat
macroscopic substrates, colloidal particles, fluid interfaces, liposomes or
vesicles, or even cells. Therefore, the LbL methodology allows manufacturing
PEMs onto surfaces presenting almost any shape and geometry, thus it is
possible the fabrication of materials with a wide range of characteristics and
properties [160, 161].
In recent years the LbL materials has been extensively developed for their
application in several scientific and technological fields, including [134, 162-
172]: optical devices, contact lenses, conductive layers, permselective
membranes,^ sensors, light-emitting thin films, electrochromic films, non-linear
optical devices, nanocapsules, and self-healing coatings. Furthermore, an
important interest is starting to be paid on the development of new applications
for the LbL systems in biomedical applications, e.g., tissue engineering,
fabrication of medical devices or nanocapsules for drug delivery [173, 174]. It
is also worth mentioning that the development of microrreactors or biomimetic
systems with multiscale organization based on LbL capsules are fields
with growing interest [175-181]. Thus, the fabrication of PEMs is a
multidisciplinary challenge involving many research areas.
5.1. Assembly Methodologies
The most extended methodology for the fabrication of LbL multilayers is
the dipping (see Figure 5) [136, 182], which is based on the self assembly of
the PEMs by alternate inmersion of flat macroscopic templates into solutions
of oppositely charged polyelectrolytes. Between the adsorption steps of two
adjacent polyelectrolyte layers a rinsing step is included. Thus, it is possible to
remove the polymer chains that are not strongly attached to the PEMs.
Therefore, the formation of bulk complexes is avoided during the consecutive
adsorption of the layers. The drying of the layers between the alternate
deposition of the layers can be, in some particular cases, recommendable
[183].
The main limitations of the dipping on industrial applications are the
time necessary for manufacturing the supramolecular architectures and the
difficulties associated with its scaling-up. This has led to the development of
new methodological approaches [183] to fabricate LbL films, being the most
extended: the spin-coating (Figure 6) [184-186] and the spraying (Figure 7)