Synthesis and characterization of non shrinking nanocomposites for dental applications 2

1
CHAPTER 1


Introduction

1.1 Composite Resins – An Alternative to Dental
Amalgam
Before the introduction of polymeric dental composites, dental amalgam
was the material of choice for stress bearing dental fillings.
1
This material was
inexpensive and had the advantages of high strength, excellent wear resistance,
ease of application, good adaptability, ease of manipulation and finishing.
1-5

Despite its excellent clinical record, dental amalgam has several disadvantages.
These include the need for removal of sound tooth structure for retention, inability
to bond to tooth surface and susceptibility to corrosion. The use of amalgam has
also been subjected to more and more controversies due to the fear of mercury
toxicity. During the placement and removal of amalgam restorations, small
amounts of mercury vapors are released leading to health and environmental
concerns.
6,7
Although dental healthcare workers generally do not show signs of
mercury toxicity, the mercury body burden of dental personnel was found to be
slightly higher than the non-exposed group. Dental healthcare workers with high
occupational exposure to mercury vapors were also found to be less fertile than
unexposed groups.
6

While issues of mercury toxicity by amalgam restorations are
still being debated, the Swedish government has proposed for the elimination of
amalgam as a dental restorative material since 1997,
8
due to environmental
concerns especially the waste management of amalgam.
9


2
While the risks associated with mercury in dental amalgam are debatable,
it is interesting to note that the use of amalgam as a restorative material has
declined rapidly in the last half-decade due to aesthetic reasons. Amalgam is
aesthetically unattractive, metallic in color and does not resemble the physical
characteristics of tooth structure. The release of metallic ions from the amalgam
restoration can also discolor the neighboring tooth structure.
10
Thus, with increase
in aesthetic demands by patients and clinicians, tooth-colored composite resin
restorations were viewed as an attractive alternative to amalgam restorations.

Dental composites which consists of monomer resins, ceramic fillers,
coupling agents and initiator/catalyst systems for polymerization were first
developed in the early 1960s
11,12
as aesthetic alternatives for tooth restorations.
One of the major improvements in resin-based composite has resulted from
increased filler loading along with variation in distribution, size, shape and
composition. This modification to the filler component brings improvements in
wear resistance, color stability, strength, radiopacity and degree of conversion of

dental composites and thus the overall improvement in clinical performance of
these materials. However, despite vast improvements in composite materials and
their mechanical properties, present day composite resins still have shortcomings
limiting their application. Inadequate resistance to wear (loss of anatomic form)
under masticatory attrition, fracture of the restorations, discoloration, marginal
adaptation, secondary caries and marginal leakage due to polymerization
shrinkage are some of the factors limiting the longevity of composite resins.
4,13-16



3
Commercial dental composites exhibit 2-14 % volumetric shrinkage
during the polymerization process.
17-20
When composites shrink, stresses are
generated at the composite/tooth interface. These shrinkage stresses can cause
marginal openings if the bonding system is unable to withstand the polymerization
forces and thus lead to leakage and ultimately caries. Despite the dramatic
improvements in the formulation of newer generation bonding agents with
enhanced marginal adaptation and bond strengths, a perfect marginal seal is still
not achievable. Clinical studies carried out for resin-based composite restorations
for Class I and II cavities for a period of 3 to 6 years have also shown that
secondary and/or recurrent caries were the main reasons for restoration
failure
4,21,22
and polymerization shrinkage has been cited as one of the most
significant factors influencing the seal between tooth structure and polymer-based
restorative materials. Thus, the major and most significant drawback of
composite-based resins is the shrinkage during the polymerization process. This

remains one of the greatest challenges in composite resin technology and the
ultimate solution to polymerization shrinkage is to develop “non-shrinking” resins.

1.2 Research Objectives
With the development of dental monomers with reduced polymerization
shrinkage and stress becoming a major focus of dental biomaterials research, we
aim to develop novel low/non-shrinking nanocomposites based on polyhedral
silsesquioxanes (SSQ) for dental applications. The objectives of this research were
to:
(a) Design and develop low/non-shrinking SSQ-based nanocomposites with
methacrylate and epoxy functional groups.

4
(b) Synthesize and characterize the SSQ neat resins for their chemical,
thermal, physical and mechanical properties.
(c) Study the effects of mixing SSQ-based nanocomposites with existing
dental monomers in different compositions for improvement in physico-
mechanical properties.
(d) Develop and characterize promising experimental nanocomposites by
mixing SSQ-based nanocomposites and/or dental monomers with ceramic
fillers.

5
CHAPTER 2


Literature Review

2.1 Chemically Cured Composite Resins
Chemically cured (self or auto cured) dental composite resins were first
developed in the late 1950s. They were found to be insoluble, aesthetic,
insensitive to dehydration, inexpensive and easy to manipulate. Curing of the
composites was initiated by mixing two pastes that brought together the initiator,
dibenzoyl peroxide, and the activator, tertiary amines such as N,N-di-(2-
hydroxyethyl)-p-toluidine (DHEPT) or N,N-dimethy-p-toluidine (DMPT), in order
to initiate the polymerization reaction (Figure 2.1).
23
Curing of the composite
ensures uniform polymerization throughout the bulk of restorative material.
However, the materials were found to be only partially successful and are not
commonly used today due to issues such as poor activator systems, poor wear
resistance, high polymerization shrinkage and mis-matched coefficient of thermal

expansion. These adverse physical properties prevented chemically cured
composites from being the material of choice for clinicians. The lack of wear
resistance prevented them from preserving restoration contour in areas subject to
abrasion or attrition. They were not meant for use in high-stress areas due to low
strength of the material which tended to flow under load. Their high
polymerization shrinkage and coefficient of thermal expansion led to
microleakage and discoloration at the margins due to percolation.
24
Clinicians
were also constrained by the polymerization setting time when placing and

6
shaping the restoration. In addition, clinical studies showed that self-cured
composites undergo more darkening than photo-cured composites over time.
25

Thus, the aforementioned limitations of self-cured composites have led to the
development of light-activated composites that offer the advantages of controlled
working time and the elimination of time consuming mixing procedures that often
introduce unwanted porosities to the restorations. When compared to the
chemically cured composites, light-activated composites demonstrated greater
strength, fracture toughness, better shade selection, color stability and higher
surface polymerization conversion rates.


O
O
O
O
O

O
N
OH
HO
DHEPT
+
N
OH
HO
O
O
benzoyloxy radical
b
enzoate ion
Dibenzoyl peroxide

Figure 2.1. Chemical activation of dibenzoyl peroxide to produce free radical for
polymerization.



2.2 Light-activated Composite Resins
The beginning of modern restorative dentistry was marked by the
discovery of Bowen’s Bis-GMA (2,2-bis[4-(2-hydroxy-3-

7
methacryloxypropoxy)phenyl]-propane)/ inorganic particle formulations in the
early 1960s (Figure 2.2).
11,12
The introduction of this composite-based resin

technology to restorative dentistry was one of the most significant contributions to
dentistry in the last century. Applications for this new polymer include anterior
and posterior composite resin restorations, indirect inlays/onlays, pit and fissure
sealants and more wear-resistant denture teeth.
26



OO
OO
O
O
OH
OH

Figure 2.2. Chemical structure of Bis-GMA monomers.


Composite materials refer to a mixture of two or more distinctly different
materials with properties that are superior or intermediate to those of the
individual constituents. Dental composites are complex, tooth-colored filling
materials composed of synthetic polymers, inorganic particulate fillers, initiators
and activators that promote light-activated polymerization of the organic matrix to
form cross-linked polymer networks, and silane coupling agents which bond the
reinforcing fillers to the polymer matrix. Further additives such as stabilizers and
pigments are also included. Each component of the composite is crucial for the
success of the final dental restoration.
23

Light-activated composite resins undergo free radical polymerization by
irradiation with blue light in the wavelength range of 410 - 500 nm. Light in this
region is most effectively absorbed by an α-diketone photoinitiator, usually

8
camphorquinone (CQ), and creates an excited state that reacts with an amine
reducing agent such as N,N-dimethylaminoethyl methacrylate (DMAEMA) or
ethyl p-dimethylaminobenzoate (DMAB) to produce free radicals that initiate the
cross-linking polymerization (Figure 2.3).
27,28
The absorption spectrum of CQ lies
in the 450 - 500 nm wavelength range, with peak absorption at 470 nm.
29,30



camphorquinone
O
O
H
3
C
H
3
C
CH
3
O
O
N

DMAEMA
hv
O
OH
H
3
C
H
3
C
CH
3
O
O
N

Free radical
initiators

Figure 2.3. Light activation mechanism.

2.3 Organic Matrix
The current organic matrix used in dental composites is based on
methacrylate chemistry with cross-linking dimethacrylate being most universal.
Approximately eighty to ninety percent of commercial dental composites use Bis-
GMA monomer as their organic matrix.
31,32
Other base monomers used in present
commercial composites include triethyleneglycol dimethacrylate (TEGDMA),
urethane dimethacrylate (UDMA), ethoxylated bisphenol-A-dimethacrylate (Bis-

EMA), decanediol dimethacrylate (D
3
MA) bis(methacryloyloxymethyl)
tricyclodecane and urethanetetramethacrylate (UTMA). The chemical structures
for some of these monomers are shown in Figure 2.4.

9
The most commonly used organic matrix, Bis-GMA has a very high
viscosity due to the hydrogen bonding interactions that occur between the
hydroxyl groups on the monomer molecules. Therefore, Bis-GMA must be diluted
with more fluid monomers to provide the proper viscosity for use in dental
composites.
23
TEGDMA which is less viscous and has excellent copolymerization
characteristics is frequently used as the diluent monomer for UDMA and
BisGMA-based composites to produce a fluid resin that can be maximally filled
with inorganic filler particles. Optimal properties are produced when TEGDMA is
used in a 1:1 ratio with Bis-GMA.
33
Some other diluents include ethylene- and
hexamethylene-glycoldimethacrylate and benzyl methacrylate.
34
TEGDMA has
also been replaced with UDMA and BisEMA in several products to reduce
shrinkage, aging and environmental effects.
35
UDMA and BisEMA have higher
molecular weights and fewer double bonds per unit of weight when compared to
TEGDMA that generally results in lower shrinkage.

O
O
O
O
O
O
TEGDMA
O
O N
N O
O
O
O
O
O
H
H
O
O
O
O
O
O
Bis-EMA
O
O
O
O
UDMA
D

3
MA

Figure 2.4. Chemical structures of common base monomers used in dental
composites.

10
2.4 Inorganic Fillers
The use of resin matrix by itself is not a suitable restorative material as it
demonstrates unsuitable physico-mechanical properties. Addition of inorganic
fillers is often needed to strengthen mechanical properties, provide radiopacity
and reduce thermal expansion, polymerization shrinkage and water sorption. In
general, the physico-mechanical properties of composites are improved in direct
relationship to the amount of filler added. Fine powders of crystalline or non-
crystalline silica or silicates are normally used as fillers. The type and size of filler
material used has been employed as a basis for classification of modern dental
composites (Table 2.1).
36



Table 2.1. Classification of dental composites by filler particle size.
36

Type
Filler Size
(μm)
Heterogeneous Hybrid Homogeneous
Megafill 0.5 - 2 mm √
Macrofill 10 – 100 √

Midifill 1 – 10 √ √ √
Minifill 0.1 – 1 √ √ √
Microfill 0.01 – 0.1 √ √
Nanofill 0.005 – 0.01 √ √


One of the most significant improvements in the evolution of commercial
composites has been the modifications to the filler. Optimization of filler particle

11
size and filler-loading improved the wear resistance of the early composite resins.
Modern composite systems contain fillers such as quartz, colloidal silica and silica
glass containing barium, strontium and zirconium. These fillers increase strength
and modulus of elasticity and reduce the polymerization shrinkage, the coefficient
of thermal expansion and water sorption.
37
The type of fillers and improvements
related to nanofilled composites will be discussed in section 2.8.

2.5 Silane Coupling Agent
Formation of a strong covalent bond between the inorganic filler particles
and organic matrix is essential for obtaining good mechanical properties in dental
composites.
38
Failure of the filler-matrix interface will result in fracture and
subsequent disintegration of the composite as a result of uneven distribution of
stresses developed under load throughout the material. Bonding of these two
phases is achieved by coating the fillers with a silane coupling agent that has
functional groups to chemically link the filler and the matrix. A typical coupling
agent used is γ-methacryloxypropyltrimethoxysilane (γ-MPTS) (Figure 2.5). One

end of the molecules can be bonded to the hydroxyl groups of the silica particles
with the other end capable of copolymerizing into the polymer matrix.

OSi
OCH
3
OCH
3
OCH
3
O

Figure 2.5. Structure of MPTS, a typical silane coupling agent used in dental
composites.


12
2.6 Limitations of Current Dental Composites
The development of light-activated composite materials in the 1970s
heralded a period of rapid progress in the field of tooth-colored restorations. One
of the most obvious changes in dental practice during the 1970’s was the way in
which composites became the most popular material for aesthetic anterior
restorations.
39
However, despite vast improvements in composite materials,
present day composite resins still have shortcomings limiting their application. As
mentioned in Chapter 1, inadequate resistance to wear under masticatory attrition,
fracture of the restorations, incomplete conversion and cross-linking, undesirable
water sorption, marginal adaptation, secondary caries and marginal leakage due to
polymerization shrinkage are often cited as being the main problems of composite

resins.
13,15,40


2.6.1 Polymerization shrinkage
Despite improvements in components and characteristics of composite
materials, polymerization shrinkage still remains a clinically significant
problem.
41-44
Dental composites exhibit the inherent problem of 2-14 %
volumetric shrinkage during polymerization processes
17,19,45,46
and are affected by
factors such as constituents of the resin-based composite material, configuration
of the cavity preparation, spectral distribution and power of the visible light-
curing unit, and clinicians technique.
47
The total shrinkage of composite materials
can be divided into pre-gel and post-gel phases. During the pre-gel polymerization,
the composite is able to flow and stresses within the structure are relieved.
48
After
gelation, viscosity increases significantly and stresses due to shrinkage cannot be
compensated. Post-gel polymerization thus results in significant stresses in the

13
surrounding tooth structure and composite tooth bond
49
that may lead to bond
failure, microleakage, post-operative sensitivity and recurrent caries. These

stresses could also result in deformation of the surrounding tooth structure if the
composite-tooth bond is strong, predisposing the tooth to fracture.
50


As previously mentioned, the stress associated with the curing contraction
is one of the most significant problems for current materials, as it adversely affects
the seal at the cavosurface margin and causes occurrence of secondary caries.
51

When bonding of the adhesive to the tooth structure is inadequate, composite
shrinks and pulls away from the cavity walls, forming an opening. This opening at
the restoration margins causes clinical problems such as microleakage, straining,
sensitivity, and/or recurrent caries. However, when the bonding to tooth structure
is strong enough, polymerization stress is applied to the tooth as composites
shrink. This causes fractured cusps, movement of cusps, and/or postoperative
sensitivity.
47


While Bis-GMA, TEGDMA and UDMA composite resin systems exhibit
significant volume shrinkage on curing
48,52,53
, water sorption by polymer network
contributes to stress reduction. However, its effect is minimized as water uptake
by composite resins takes place at a much slower rate, requiring hours to reach
saturation.
54
In addition, water sorption has also been found to weaken the resin
matrix and to cause filler/matrix debonding and hydrolytic degradation of the

fillers with a subsequent reduction in mechanical properties and wear resistance.
55-
57
The effect of water sorption can be reduced by the use of more hydrophobic

14
monomers, such as BisEMA, which do not contain unreacted hydroxyl groups on
the main polymer chain.
34


Besides, differences in monomer chemistry, various degrees of final
polymerization, filler types and filler concentrations, the amount of stress
generated is also dependent on the configuration of the cavity preparation.
Configuration factor, commonly known as the C-factor, is defined as the ratio of
the bounded area of the restoration to the unbounded area.
49
The higher the C-
factor, the greater the stress on the bonded surfaces.
58
Since composite flow is
more likely to occur from the free surfaces of the specimen, a higher proportion of
free composite surface would correspond to a smaller restriction to shrinkage,
thereby reducing stress. When the free surface is reduced, the ability to flow and
compensate for shrinkage is restricted by the bonded surfaces thus, increasing
stress. As cavity preparations present a much more complex geometry with
heterogeneous stress distribution
59
, the application of the C-factor concept to
clinical practice must be performed carefully.

The effect of post-gel shrinkage and stress can also be minimized by
clinical techniques such as incremental layering of the composite during
placement
60
and application of a low elastic modulus liner between the tooth and
shrinking composite restorative.
61
A recent method to minimize polymerization
shrinkage without affecting the degree of conversion in light-activated composites
is to reduce the viscosity during setting by means of controlled polymerization.
This can be achieved by application of short pulses of energy (pulse activation),
delays between exposures (pulse delay) or pre-polymerization at low-intensity

15
light followed by a final cure at high intensity (soft-start techniques). While some
studies have shown that these polymerization modes resulted in lower shrinkage,
smaller marginal gap, increased marginal integrity and improved material
properties
62-64
, others have found no significant difference in shrinkage when
compared to continuous cure modes.
65-68
Thus, one of the greatest challenges and
the ultimate solution to polymerization shrinkage is to develop expanding, low-
shrinking or non-shrinking resins.

2.7 New Resin Technology
While shrinkage stresses can be reduced by increasing filler loading, the
ultimate solution to polymerization shrinkage is to develop “non-shrinking” resins.

Although earlier efforts to synthesize such resins were not successful, several
developments in the last decade are more encouraging.

2.7.1 Ring-opening Monomers
In 1992, Stansbury
69
synthesized spiro-orthocarbonate monomers (SOCs)
which expand during polymerization through a double-ring opening process.
These monomers contain methylene groups capable of free radical polymerization,
making them useful as additives to dimethacrylates (Figure 2.6). However, the
expanding SOCs synthesized were found to have low reactivity for free radical
additions. In order to enhance the reactivity of SOC monomers, SOC-substituted
methacrylates
70
were synthesized (Figure 2.7). These monomers resulted in nearly
complete ring-opening of the SOC when polymerized in dilute solutions. However,

16
less ring-opening was obtained when the resin was cured in bulk and the
composites had about 1 % shrinkage.

O
O
O
O
O
O
O
O
Ph

Figure 2.6. Chemical structures of SOCs containing methylene groups.


O
O
O
O
O
O
O
O
O
O
O
O

Figure 2.7. Chemical structures of SOC-substituted methacrylate.

The synthesis of six-membered SOCs co-polymerized with epoxy
functional groups via cationic UV photo-initiation has also been reported.
71,72
This
alicyclic SOCs (trans/trans-2,3,8,9-di(tetramethylene)-1,5,7,11-tetraoxaspiro[5.5]
undecane) (Figure 2.8) containing four rings attached to a central spiro carbon was
polymerized in the presence of cationic initiator (4-octyloxyphenyl)-
phenyliodonium hexafluoroantimonate, with chlorothioxanthone as a sensitizer.
The neat SOC monomers resulted in an expansion of 3.5 vol%. The mixing of 5%
of the SOC monomers in an epoxy base produced a resin with substantial tensile
strength and modulus, acceptable water sorption and solubility, and a slight

expansion. Increased concentrations of the SOC produced greater expansion and
slightly stronger polymers, but high water sorption and solubility due to
incomplete reaction of the SOC. While results of these studies are promising, no

17
commercial materials are available to date. This may be due to the high cost of the
monomer.

O
O
O
O

Figure 2.8. Cationic polymerizable SOC.

Miyazaki et al.
73
have also reported on the development of acrylate and
methacrylate containing spiro ortho esthers that were capable of being
polymerized by heat, ionic and free radical initiators. Though all common
methods of curing were possible, the resultant polymers were weak and the
reduction in shrinkage was not clinically significant.

Besides SOCs, cationic photopolymerizable epoxy monomers, in
particular cross-linking cycloaliphatic epoxy compounds, were of great interest.
These epoxy resins demonstrate significantly lower shrinkage than dental
methacrylate resins and were reactive enough to be cured by cationic
photopolymerization in an acceptable time frame using visible light-curing units.
One example in the application of this type of epoxy resins is that of epoxy-polyol
mixtures. While water sorption for these new resins was found to be slightly

higher than that of traditional dimethacrylate, owing to the hydrophilic nature of
the polyols, polymerization shrinkage was found to be significantly lower with
comparable strength and stiffness.
74
Recent work from 3M has also seen an
experimental composite based on a mixture of two oxirane monomers and a small

18
amount of polyol (pTHF) (Figure 2.9) set by a cationic-initiated, light-activated
reaction.
75
This composite resulted in a volumetric shrinkage of 1.6% at 24 hours
and was found to be significantly less than that of a conventional posterior
composite, Z250.
76


O
O
OO
OO
OO
H
O
O
O
O
H

Figure 2.9. Chemical structures of cycloaliphatic diepoxide and polyol.

In addition to epoxy resins, ring opening monomers such as oxetanes
77,78
,
cyclic acetals
79,80
, cyclic allyl sulfides
81
and vinylcyclopropanes
82,83
have also
been evaluated for dental applications. However, none of them were found to be
promising. While oxetanes demonstrated a higher basicity with polymerization
reactivity substantially affected by the type of atmosphere used
78
, cyclic acetals,
cyclic allyl sulfides and vinylcyclopropanes compounds were found to be either
unstable, have low reactivity, exhibited glass transition temperature (T
g
) that were
unacceptable for dental applications or resulted in polymers that have high
flexibility.

2.7.2 Liquid Crystalline Monomers
As the search for low-shrinking polymers continue, liquid crystalline or
branched cross-linkers have also been synthesized and evaluated for their

19
usefulness as low-shrinking monomers. While these pre-ordered monomers have
the advantages of low viscosity, high degree of conversion and low shrinkage

properties when compared to that of their corresponding linear monomers, they
were found to melt at temperatures higher than 80
o
C, resulting in complicated
curing conditions. In order to overcome this problem, several new liquid
crystalline dimethacrylates
84-86
(Figure 2.10) and/or branched liquid crystalline
bismethacrylates
87,88
(Figure 2.11) monomers with polymerization shrinkage
ranging from 1.3 – 2.5 vol% have also been synthesized. The syntheses of these
monomers with a decrease in transformation temperature were achieved by
modifying the spacer length, varying the mesogenic group and introducing
suitable substituents in the mesogenic group. While these monomers have the
potential to be used as matrix monomers for dental composites due to their low
polymerization shrinkage, low viscosity and high monomer conversion, they were
expensive to synthesize and have low mechanical properties as a result of the
more flexible polymer network.


OC
O
O(CH
2
)
6
O
OC
O

O(H
2
C)
6
O
O
O

Figure 2.10. One example of near room temperature liquid crystalline
dimethacrylate.



20
OOOO
O
R
O
O
R
O
R =
CNO
OO


Figure 2.11. Branched liquid crystalline bismethacrylates.


2.7.3 Branched and Dendritic Monomers

Besides development in liquid crystalline monomers, highly branched non-
liquid crystalline
89-92
and dendritic monomers
93,94
have also been synthesized and
evaluated for dental composites. These monomers were found to have the
advantages of low polymerization shrinkage, viscosity and can be incorporated
into formed polymer network efficiently. However, due to their high flexibility of
the formed polymer networks, these monomers often result in poor mechanical
properties. Thus, for successful application in dentistry, these monomers must
produce networks with improved mechanical properties.

2.7.4 Ormocers
Ormocers (organically modified ceramics), which refers to an inorganic-
organic hybrid dental materials is another type of dental materials developed with
the aim of reducing polymerization shrinkage, improving marginal adaptation,
abrasion resistance and biocompatibility. The ormocers which have an inorganic
backbone based on SiO
2
are functionalized with polymerizable organic units such
as dimethacrylate. They are usually synthesized from an alkoxy silane

21
functionalized with a polymerizable group, followed by hydrolysis and
condensation which led to an oligomeric Si-O-Si nanostructure (Figure 2.12).
95

However, due to its high viscosity, a diluent, TEGDMA, is often needed. Despite
comparable marginal adaptation, a recent study conducted on low shrinkage

composites showed that a considerable amount of polymerization shrinkage is still
present with this class of materials.
15



O
R
O
Si(RO
3
)
n
H
+
/ H
2
O
O
R
O
Si
O
Si
O
Si
R
O
O
O

O
R
O
O
O
O
O

Figure 2.12. Synthesis of SiO
2
nanostructures.

Although these polymers are promising, problems balancing mechanical
properties, water sorption, solubility, curing times and expansion still exist.
Recent advances of dental composites for reduced shrinkage with good
mechanical properties and enhanced clinical performances have been made in the
area of nanotechnology for the development of dental nanocomposites.

2.8 Nanotechnology with Dental Composites
Nanotechnology, also known as nanoscience or molecular engineering, is
defined as the creation of functional materials and structures with a characteristic
dimension in the range of 0.1-100 nanometers by different physical or chemical
techniques.
96
This new technology that has become an important discipline in
science and technology over the past ten years has shown promise in potential

22
applications areas such as aerospace, computers, telecommunications,
microelectronics, biomedical, dental adhesives and dental composites. This

technology has also allowed for tougher, lighter, uncontaminated and more precise
materials to be developed. These great advances in nanotechnology have also
resulted in the development of several dental nanocomposites with enhanced
properties. In composite resin technology, particle size and concentration within
the matrix is responsible for the polishability, wear and fracture resistance.

Dental filler particles are divided into groups according to their size as
macrofiller, midifiller, minifiller and microfiller while megafill composites refer
to the addition of large glass beta-quartz inserts for protection against wear (Table
2.1). Macrofill composites, involved milling of large crystalline quartz and
various borosilicate or lithium aluminosilicate glasses into various particle sizes
ranging from 10 to 100 μm, and are not commonly used today due to esthetic
concerns.
97
These macrofillers which are hard and more resistant to wear when
compared to the polymer matrix resulted in rough and less enamel-like restoration
during abrasion.

Microfill composites, which contain amorphous silica with an average
particle size of 0.04 μm and a range of 0.01 to 0.1 μm, were developed to
overcome polishing and esthetic requirements on the anterior restorations.
However, the large surface area of microfillers, lead to high viscosity formulations
that are unusable and are the least highly filled composites.
36
The lower
concentration of fillers thus results in poorer mechanical properties such as lower
strength and stiffness when compared to the macrofills.
40,98-100
A high percentage

23
of material fractures were also observed in clinical trials when microfilled
composites are placed in high stress areas.
101,102
Based on many studies reported
on the correlation between mechanical properties and filler volume
103
, the current
trend of composites is towards minimizing filler size and maximizing filler
loading in an attempt to satisfy all the requirements for dental composites. As
there are currently no composite materials available to satisfy both the functional
needs of a posterior Class I and II restoration and the superior esthetic
requirements for anterior restorations
104
, the development of nanofillers and
finally nanocomposites may be the solution.

Nanofillers, which can be prepared by various techniques such as flame
pyrolysis, flame spray pyrolysis and sol-gel processes, have particle sizes smaller
than microfillers. The extremely small filler particles have dimensions below the
wavelengths of visible light and resulted in an inability to scatter or absorb visible
light. Thus nanofillers are virtually invisible and offer the advantage of optical
properties improvement.
105
In addition, due to small particle sizes, nanofillers are
also capable of increasing the overall filler level by fitting into spaces between
other particles in a composite. Nanofillers theoretically should allow an overall
filler level of 90-95% by weight. This increase in filler level will also significantly
reduce the effect of polymerization shrinkage and dramatically improve physical
properties.

36
In addition, composites containing nanofillers resulted in smoother
surfaces with their ease of polishability
106
, increased abrasion resistance and
surface hardness.
107



24
The recent introduction of dental nanocomposites requires the effort of
incorporating nanotechnology into the direct composite resin materials. Filtek
Supreme (3M ESPE, St. Paul, MN) is one such example. This advanced
restorative system makes use of a synthetic chemical process to develop building
blocks on a molecular scale. The nanocomposite is composed of nanomeric
particles and nanoclusters (Figure 2.13).
105
Nanomers are monodisperse non-
agglomerated and non-aggregated silica particles of 20-75 nm in dimension.
Nanocluster fillers are loosely bound agglomerates of nano-sized particles. The
agglomerates act as an individual component that allow for higher filler loading
and higher strength.


Figure 2.13. Transmission electron micrographs of a hybrid, nanomer and
nanocluster (Courtesy of 3M ESPE).


Several research studies have also demonstrated that nanofilled composites

have the capability of achieving a smoother surface with high translucency, high
polish and polish retention comparable to those of microfills while maintaining
physical properties and wear resistance equivalent to those of several hybrid
composites.
105,108
Mechanical properties such as fracture resistance, compressive
and diametral strengths of the nanocomposite were found to be equivalent or
higher than those of the other commercial composites tested.
105
In a case report,

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Davis
109
has also maintained that the nanocomposite (Filtek Supreme) used for
restoration has exhibited excellent handling properties when compared to other
composites.

Premise (Kerr/Sybron, Orange, CA), has developed a new approach to
increase ceramic loadings using a distribution of trimodal particle sizes. This
trimodal approach was realized by integrating non-agglomerated discrete silica
nanoparticles of 20 nm in dimension, prepolymerized filler (PPF) of 30-50 μm and
barium glass filler of 0.4 μm in average size (proprietary Point 4 filler technology)
into a resin matrix. The silica nanofillers together with PPF and barium glass
fillers allow for increased filler loading of 69% by volume and/or 84% by weight.
These discrete unassociated nanoparticles that are well dispersed in the matrix on
a nanoscale level allow for reduced viscosity in the resin matrix and thus resulted
in increased hardness, abrasion resistance, fracture resistance, improved
polishability and reduced polymerization shrinkage and shrinkage stress.
110-112

Another area of special interest with intense research and development has
been in the area of polymerizable silica organosols. These silica organosols
contained discrete, non-agglomerated nanoparticles that are capable of being
modified by organic silanes or other organic molecules. Organosols can be
prepared by Stöber’s method (Figure 2.14), which is a sol-gel process using
alkoxysilanes as precursors
113,114
followed by solvent exchange for introducing a
polymerizable particle periphery.
113,115,116