Biomedical Engineering Reference
In-Depth Information
8.2.2.2
Biomimetic Siliceous Nanocapsules
Biomimetic siliceous nanocapsules are prepared through the hydrolysis and the condensation of a
neutral silicon alkoxide precursor by templating surfactants,
66,67
polymers,
68
and phospholipids.
69
A
variety of self-assembled organic molecules such as lyotropic lamellar phases and microemulsions
70
can be used as the template. In particular, siliceous liposome nanocapsules or “Liposil” can be syn-
thesized by using liposomes as templates (Figure 8.9).
48,49
Liposil, usually with a diameter of around
100 nm, comprises of a liposome encapsulated in a silica shell, with the silica wall assembled onto
the external surface of the liposome. These siliceous liposome nanocapsules exhibit good protec-
tion of the drug incorporated, excellent physical stability, which is desirable for long-term storage,
and high tolerance to low pH, making them promising drug carriers, especially for oral administra-
tion. Slow drug release rate can also be achieved using Liposil.
8.2.2.3 Porous Silicon
Porous silicon, which contains a network of interconnected pores within the crystalline silicon matrix,
is usually prepared by anodization of bulk silicon in hydrofl uoric acid solution (Figure 8.10). Since
the late 1990s, when the pioneering work of Canham and others demonstrated the biocompatibility
and the biodegradability of porous Si
in vitro
and
in vivo
,
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porous silicon has been under intensive
investigation for biomedical applications, especially as drug carriers. A versatile surface chemistry
and an easily adjustable morphology are the other properties of porous silicon that make it suitable
for controlled drug delivery. The surface of porous silicon can be conveniently modifi ed with organic
or biological molecules, for instance, antibodies.
82
Its pore size can be adjusted, ranging from a few
nanometers to a few microns, and its porosity can be easily tuned over 80%, providing a high portion
of free volume for drug loading. Swaan and coworkers performed
in vitro
experiments and showed
that porous Si particles can be used as effi cient delivery vehicles of insulin across intestinal epithelial
cells.
83
The drug permeation rate through the membrane was dramatically enhanced when delivered
through porous Si particles compared with conventional liquid formulations. Sailor and coworkers
demonstrated controlled drug loading and release of a steroid by engineering the surface chemistry
and the pore dimensions in porous silicon fi lms.
84
More recently, Vaccari and coworkers investigated
the time-dependent drug release of doxorubicin anticancer agent incorporated in the two-layered
porous silicon fi lm.
85
In addition to possessing the adjustable morphological and chemical properties, porous Si is
particularly attractive for constructing fl uorescent materials
86
and complex optical materials such
as Fabry-Pérot fi lms, photonic crystals, dielectric mirrors, and microcavities.
87-98
Both kinds of
(a)
(b)
100 nm
FIGURE 8.9
SEM micrograph (a) and TEM micrograph (b) of aggregated siliceous nanocapsules. (Reprinted
from Bégu, S. et al.,
Chem. Commun.
, 640, 2003.)
