Biomedical Engineering Reference
In-Depth Information
this leads to precipitation of oxide particles. Although this is not a problem in classical ceramic pro-
cessing it becomes detrimental to creating stable nanoparticles needed to control the nanostructure
and provide the desired properties. By controlling the sol-gel process small clusters of controlled
sizes can be obtained. However, it is still important to prevent agglomeration of the nanoclusters
beyond a critical length. The best way to prevent undue agglomeration and control the size of the
nanoclusters is to chemically modify their surfaces. Since the nano-sized building blocks have to be
incorporated into a host matrix to provide a desired end-product it is convenient to functionalize the
surface of the inorganic oxide nanoparticle or nanocluster with reactive organic group that can be
later tethered to the organic host matrix by strong covalent or ionic bonding.
The sol-gel method has been used with much success in designing and manufacturing nanopar-
ticles for dental composites. Nanoparticles of silica and zirconia derived by the sol-gel method are
used in the commercial nanofill composites, glass ionomers, and adhesives. The zirconia nanopar-
ticles provide radiopacity as well as serve as reinforcement for the matrix without sacrificing the
optical properties. A more detailed description is given in Sections 2.3 and 2.4. An aqueous sol-gel
method was also used for the synthesis of nanostructured SiO
2
-BaO powder for use in dental com-
posite resins
[9]
. Tantalum oxide nanoparticles prepared by this technique have been used for impart-
ing radiopacity to dental composites
[10]
.
2.2.2.1 Functionalization of Oxide Nanoparticles
There are two strategies for the functionalization of the surface of nanoparticles. In one method,
the organic group can be grafted after the isolation of the nanoparticle or nanocluster. In the second
method, the organic group can be introduced in situ during the synthesis of the nanoparticle. The
most common type post-synthesis modification involves the use of silane coupling agents. The ter-
minal O
or OH groups at the surface of the oxide nanoparticles are reacted with silicon halides or
alkoxides with organic functionality. Often the organic functional group contains a reactive group that
can be used for anchoring the derivatized particle to the host matrix. Common reactive groups are
vinyl, acryloyl, methacryloyl, epoxy, and amino groups.
In another method of functionalization and stabilization of the nanoparticles the groups at the sur-
face of preformed clusters are exchanged with treatment agents within solution. Surface oxide ions
can be exchanged with multidentate ions to prevent agglomeration. Electrostatic interaction between
a tin oxide nanocluster and methacrylic acid was used to attach a polymerizable group
[11]
.
2.2.3
Synthesis of Silsesquioxane Nanoparticles
The term silsesquioxane refers to all structures with the empirical formula RSiO
1.5
, where R is hydro-
gen or any alkyl, alkylene, aryl, or arylene group or organofunctional derivatives of the aforemen-
tioned groups. The silsesquioxanes include random structures, ladder structures, cage structures, or
partial cage structures. The oxygen to silicon ratio of 1.5 in these materials is intermediate that of
silica (RSiO
2
) and silicones (R
2
SiO) thus making them interesting hybrid compositions with unique
physical properties. They are known to exist in the form of polycyclic oligomers and polymers. The
polyhedral oligomeric silsesquioxane structures are often referred to as POSS materials. The majority
of POSS compounds have highly symmetrical, fully condensed silicone-oxygen frameworks with as
equivalent oxygen functionality on each silicone atom. POSS compounds are physically large with
respect to polymer dimensions and fall in the nanoscale range. A POSS molecule contains covalently
