Biomedical Engineering Reference
In-Depth Information
3.4
Hollow Silica Nanoparticles (HSN)
Hollow silica nanoparticles (Caruso et al. 1998 ) have also been used as drug carriers.
They possess much higher loading capacity for drug molecules than the plain ones,
but weaker interactions between the drug molecules and the silica matrix, making
the control of release more delicate. The two main methodologies for synthesizing
hollow silica nanoparticles are first, the template-based synthesis and second, the
dissolution-growth process of solid silica nanoparticles. The first method generally
utilises a solid nanoparticle composed of metal, metal oxide or polymer as a core
template. The silica shell is then grown on the template and the core is removed by
an etching process, dissolution or calcination. The second method is relatively
simple since the hollow cores are produced by etching the silica nanoparticles with
basis such as sodium hydroxide (NaOH) or sodium borohydride (NaBH 4 ), using
polyvinylpyrrolidone (PVP) molecule as a surface protective agent. Thus, the inner
part of the silica nanoparticle is selectively etched, leaving room for the drug
molecule to be encapsulated. The key factors that govern the drug-release kinetic
for hollow silica nanoparticles include cavity size, shell thickness and type of sur-
face functional groups (Yang et al. 2008 ). For instance, PEG-coated hollow silica
particles were found to release drug molecules at a much slower rate than the
hydroxyl- or amino- functionalized counterparts (Liu et al. 2007a ), probably due to
partial blockage of pores by PEG molecules at the surface of the nanoparticle.
Thicker shells also allow drug to be released more slowly and over a longer delay,
but the drug-loading capacity is concomitantly decreased due to reduced cavity
size. More generally, physical-chemical parameters such as temperature and pH can
also influence the drug release rate.
Porosity is also a factor of interest, since drugs could be incorporated in the inner
core and in the internal porosity of the shell. Li et al. developed a new method for
preparing such particles with a porous silica shell structure via the sol-gel route and
using inorganic CaCO 3 nanoparticles as templates (Li et al. 2004 ). When removed,
the inner core can also offer place to a drug model such as Brilliant Blue F (Fig. 7 ).
The first attempts made on such carriers exhibit typical sustained release without
any burst effect, since a slow release was observed for 1,140 min compared to only
10 min for usual SiO 2 nanoparticles. Liu et al. explored the in-vitro release of fluo-
rescein isothiocyanate (FITC), a drug model that can be easily detected by fluorescent
spectroscopy (Liu et al. 2007b ). They showed that the release time courses of FITC-
doped silica nanocapsules with very thin shells (in the range 3-10 nm) and large
cavities (ca. 70% of hollow core), ensuring the encapsulation of a great amount of
drug, are significantly different from the free FITC control, with a FITC release
peak at ca. 1.5 hr, highlighting the potential applications of such particles in drug
release. By the same method, Chen et al. carried out another CaCO 3 -templated
silica nanocapsules for the in-vitro cefradine delivery (an antibacterial agent) (Chen
et al. 2004 ). In this study, an interesting release profile, proceeding in three stages,
was explained by the authors as the drug release from the surface, the pore channels
in the wall, and the inside hollow part of the particles, respectively, markedly
encouraging the drug delivery application for such material.
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