Biomedical Engineering Reference
In-Depth Information
In this situation, it is important to work at low silica concentration to avoid gel
formation in the whole reaction volume. Thus, there should be a strong affinity of
the organic seed for silica, as showed for gelatin nanoparticles whose positive
charge attract negatively-charged silica precursors (Allouche et al.
2006
). However,
the silica condensation may lead to structural constraints on the templating particle
so that it is also important to control the kinetics of the silica shell formation. This
was clearly demonstrated for the design of so-called liposils consisting of silica-
coated liposomes (Bégu et al.
2007
) (Fig.
1
).
It is also possible to prepare homogenous nanocomposite particles. One possi-
bility is to introduce the polymer in the initial silica precursor solution within an
emulsion, as shown for poly-L-lactic acid (PLLA) or polyethylene glycol (PEG)
(Sertchook et al.
2007
; Zalzberg and Avnir
2008
) (Fig.
1
). Alternatively, the use of
spray-drying techniques was found very efficient to prepare bionanocomposite
associating silica and alginic acid, a polysaccharide, with a large variety of compo-
sition and size (Yang and Coradin
2008
). Compared to the previous hybrid struc-
ture, it is very important to have a limited interaction between the polymer and
silica to avoid precipitation during gelation.
A specific case of hybrid colloids is the family of mesostructured silica nanopar-
ticles. These particles consist of an organic templating mesophase, mainly self-
assembled surfactants or amphiphilic polymers, trapped in a silica network (Soler
Illia et al.
2002
). Noticeably, such systems are not much used as such but as col-
loidal precursors to ordered mesoporous nanoparticles, obtained after calcination or
template extraction (Fig.
1
). In fact, the formation of mesostructured materials is a
complex process which is mainly based on the simultaneous self-assembly of the
organic templating phase and the condensation of the inorganic phase. The chemical
routes to mesostructured silica nanoparticles are not significantly different from that
used for pure silica particle growth. In particular, many studies have been focusing
on the use of the spray-drying technique (Andersson et al.
2004
). This is due to the
fact that this process induces a fast evaporation step from precursors in solution to
solid particles. This step has a strong impact on the formation of the organic-inorganic
mesostructure. An improved understanding and control of this process, called
Evaporation-Induced Self-Assembly (Brinker et al.
1999
), has allowed to get access
to a wide variety of silica particles with different sizes, pore size and structure and
even to prepare mixed oxide phases in a single step (Boissiere et al.
2010
).
2.3
Silica Behavior in Biologically-Relevant Environment
2.3.1
Acellular Media
The typical preparation of silica nanoparticles results in an aqueous suspension at
relatively high silica concentration (0.1 M - 1 M), low ionic strength (below
0.01 M, to avoid aggregation) stored at room temperature or below. When put in
simulated (in vitro) or effective (in vivo) human body conditions, a strong change
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