Biomedical Engineering Reference
In-Depth Information
have the advantage of conserving sound tooth structure with the potential of tooth reinforcement,
while at the same time providing cosmetically acceptable restorations. However, for many years
none of the composite materials has been able to meet all the requirements of both posterior and
anterior restorations
[20]
. Prior to the advent of nanotechnology in dental materials a class of com-
posites called “microfills” were used for anterior applications because of their high initial gloss and
superior polish retention. Unfortunately microfill composites are not suitable for high stress bearing
areas (e.g., Class I, II, and IV restorations) of the dentition. On the other hand, the “hybrid” compos-
ite materials containing blends of micron and submicron fillers were of sufficient strength and wear
resistance to be used for posterior teeth but did not have the long-term polish retention desirable for
anterior teeth
[21]
. The use of nanoparticles addresses the aforementioned difficulty by combining
high mechanical strength with long-term polish retention in one material.
2.3.1.1 Nanofill Composites
As mentioned earlier the nanofills are dental composites in which all the fillers are in the 1-100 nm
range. Two types of nanoparticles have been synthesized and utilized for preparing the nanofill den-
tal composites
[22]
. The first of these is the most common and are nanomeric particles which are
essentially monodispersed non-aggregated and non-agglomerated particles of silica. The surface of
the nanoparticles are treated with silane coupling agents (e.g., 3-methacryloxypropyl-trimethoxysi-
lane, MPTS) using a proprietary method. MPTS is a bifunctional material, also known as a coupling
agent. It contains a silica ester function on one end for bonding to the inorganic surface and a meth-
acrylate group on the other end to make the filler compatible with the resin before curing to prevent
any agglomeration or aggregation. MPTS also allows for chemical bonding of the nanomers to the
resin matrix during curing. Nanomers usually start from sols and hence have a very high monodis-
persity (i.e., narrow size distribution). Because of this if nanomeric particles alone are used to make
highly filled composites the rheological properties are rather poor. Attempts have been made to mix
nanomers of different sizes but with little improvement in handling without sacrificing the optical
properties.
To overcome the disadvantage of using nanomeric particles only and to avoid the disadvantages
of nanohybrid composites (see Section 2.3.1.2) researchers at 3M Company have designed a second
type of nanofillers that are called nanoclusters. The nanoclusters are made by lightly sintering nano-
meric oxides to form clusters of a controlled particle size distribution so as to provide composites
with good rheological properties
[22]
. Nanoclusters from silica sols only
[23]
as well as from mixed
oxides of silica and zirconia
[24]
have been synthesized. The primary particle size of the nanomers
used to prepare the clusters range from 5 to 75 nm while the clusters have a wider distribution rang-
ing from 100 nm to submicron level and have an average size of 0.6 μs.
Figure 2.1
shows a Scanning
electron micrograph (SEM) image of a nanocluster of silica in the composite 3M™ ESPE™ Filtek™
Supreme Plus
1
after the resin matrix was removed by washing with acetone. In this material, the sur-
face of the nanoclusters are treated with MPTS to provide compatibility and chemical bonding with
the resin system. The differences in particle architecture of nanomers, nanoclusters, and conventional
microhybrid fillers are readily apparent from transmission electron micrographs (TEMs) of com-
posites prepared from these fillers.
Figure 2.2A
shows a nanocomposite filled with 75 nm diameter
1
3M™ ESPE™ Filtek™ Supreme Plus Universal Restorative System, 3M ESPE Dental Products, 3M Center, St. Paul MN,
55144-1000.
