Biomedical Engineering Reference
In-Depth Information
bonded reactive functionalities suitable for polymerization or grafting them to polymer chains of the
host matrix. The nonreactive organic groups are chosen to modify the solubility and compatibility of
the POSS segments with the various polymeric segments. Incorporation of the POSS into a polymer
matrix can result in significant improvements in a variety of physical and mechanical properties due
to the reinforcement at the molecular level and the inorganic framework's ceramic-like properties.
The most common method of synthesis of silsesquioxanes involves the condensation of reaction
of alkyl silanes containing three hydrolysable functionalities. The reactions for the synthesis of POSS
compounds may be divided into two major groups depending on the nature of the starting materials
[12]
. Monomers of the general formula RSiX
3
(X
alkyl or alkoxide groups) react under appropriate
conditions by controlled hydrolysis and condensation giving rise to Si-O-Si bonds with the subse-
quent formation of polyhedral cage framework.
n
RSiX
+
1 5 H O
.
n
→
(
RSiO
)
+
3 HX
n
,
The second major class of reactions involves the manipulation of the substituents of the silicon atom
without affecting the silicon-oxygen skeleton of the molecules. A large number of substituents have
been appended to the silicon-oxygen cages. Such substituents include alcohols, phenols, alkoxysilanes,
chlorosilanes, epoxides, esters, fluoroalkyls, halides, isocyanates, acrylates, and methacrylates
[13]
.
3
2
1 5
n
2.2.4
Synthesis of Polymer-Templated Nanoparticles
Supramolecular assemblies of polymers that are ordered and structured in their alignment have been
utilized to create an environment for the synthesis of ordered materials on a nanoscale regime. These
environments can also be used to template the materials synthesized within them. Examples of suit-
able self-assembly molecules are natural and synthetic polypeptides, lipids, synthetic functional poly-
mers, and surfactants. High-molecular-weight polyacrylic acid has been used to synthesize needle-like
nanoapatite crystals with diameters smaller than 100 nm
[14]
. Naturally occurring polymers such as
chitosan and collagen have also been used as porous scaffold to precipitate nano-sized hydroxyapatite
crystals in situ
[15,16]
. Moreover, surfactant molecules, because of their tendency to form micelles,
can serve as templates for the synthesis of nanoparticles. Thus, poly(methylmethacrylate)-grafted nan-
oclays were prepared through the free radical polymerization of the monomer in the presence of sur-
factants. These nanoparticles can be used to reinforce dental adhesives
[17]
.
2.3
EXAMPLES OF DENTAL MATERIALS USING NANOPARTICLES
2.3.1
Nanocomposites Containing Oxide Nanoparticles
To date oxide nanoparticles have been the most prevalent types of nanomaterials used in dentistry. This
section will focus on the use of nanoparticles in dental composite filling materials. At present, there are
two distinct types of dental nanocomposites available which are nanofills and nanohybrids
[18,19]
.
1.
Nanofills—these contain nanometer-sized particles (1-100 nm) throughout the resin matrix, with
no other large primary particles being included.
2.
Nanohybrids—these consist of large particles (0.4-5 μm) with added nanometer-sized particles.
One of the most significant contributions in dentistry has been the development of resin-based
composite technology containing inorganic oxides or glass fillers. Adhesively bonded composites
