Mesoporous silica- and silicon-based materials ... - Helda - Helsinki.fi

thus leaving the pores open for the loaded substance to be released. Coating with
polyelectrolyte multilayers can also be used to control the molecule release from the pores
either via pH or salt concentration changes of the release medium (Zhu et al., 2005). This
method is also based on the ionic interactions between the coating materials and the silica
surfaces. The pH responsivity of this system is similar to that of polycations, as the
coating is incompact at low pH-values and closes the pore openings at high pH-values.
Full blockage of the drug release is not achieved via these methods: a leakage of about
10% has been detected even at pH-values where the pores are closed. The polyelectrolyte
multilayers are also sensitive to salt concentrations (Zhu et al., 2005). High ionic strength
of 10 mM NaCl solution weakens the ionic interactions between the oppositely charged
layers and leaves the pore openings unsealed. At lower salt concentration (0.5 mM) the
electrostatic binding is not disturbed and the polyeletrolyte multilayers are able to cap the
pore openings. An interesting idea was the use of gluconic acid-modified insulin proteins
as caps in order to encapsulate cyclic adenosine monophosphate that induces insulin
secretion into boronic acid functionalized mesoporous silica (Zhao et al., 2009). The
release could be triggered by saccharides, such as glucose, providing a potential
application for the treatment of diabetes. Thermoresponsive polymer poly(Nisopropylacrylamide)
has also been incorporated into the silica structure to form hybrid
materials (Fu et al., 2007). The polymer changes its molecular chain conformation from
packed to loose when the temperature rises from room temperature to body temperature.
This structural change in the hybrid particles triggers the release of the loaded substance.
In addition, it has been shown that near-infrared radiation of mesoporous silica/gold
nanorods nanocomposite increases the temperature of the system and, consequently, the
release rate of the loaded substance (Al-Kady et al., 2011). The hyperthermia effect could
be applied as a controlled drug release mechanism in, e.g. cancer therapy applications
(Huang et al., 2006).
Controlled release of molecules from silica particles has also been achieved by
methods that utilize various triggers, although they might be less straightforwardly
applicable in human body. Coumarin derivatives that form dimers to block the pore
openings and react reversibly to UV light with different wavelengths, have been
successfully developed (Mal et al., 2003). When magnetic nanoparticles are used in pore
capping or otherwise incorporated to the particles, they can be directed to a site of interest,
from where the drug release is triggered (Giri et al., 2005; Huang et al., 2009). The
separation of the cap can be induced, for example, by cell-produced antioxidants, such as
dihydrolipoic acid, and also regulated by the trigger molecule concentration. Chemically
removable caps include also surface-derivatized cadmium sulphide nanoparticles that can
be removed from the pore openings by disulfide bond-reducing molecules (Lai et al.,
2003). Moreover, in this application the release rate of the molecule can be controlled by
the concentration of the trigger molecules. Other methods to control the release of
molecules from silica particles have also been studied for, e.g. electronically responsive
delivery (Batra et al., 2006) and ultrasound (Kim et al., 2006).
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