Biomedical Engineering Reference
In-Depth Information
4.1
Introduction
Nanoparticles, engineered or naturally occurring, are broadly defined as compounds that exist on a
scale of
100 nm. Advances in nanotechnology over the last decade have raised exciting possi-
bilities for the application of nanomaterials in medicine. Nanoparticles can be grouped into three
general categories: (i) environmental, such as those generated from forest fires and volcanoes, (ii)
nonengineered, which often represent by-products of human activity including those produced by
power plants and incinerators, and (iii) engineered, such as those being developed for diagnostic
imaging and as vehicles for drug delivery. Engineered materials can be synthesized as pure parti-
cles or composites and in various shapes and sizes as well as surface features which can addition-
ally be conjugated to different bioactive molecules, creating an almost infinite number of variations
with an unlimited potential for biological applications
[1]
.
Engineered nanomaterials can be organized into five general types: (i) metal-based metal oxides
(e.g., TiO
2
) with biocompatibility often increased by the use of incorporating a silica coating or shell,
(ii) semiconductor nanocrystal-quantum dots which are also metal based and have silica shells to pro-
vide some degree of biocompatibility, (iii) silica-based, which are composed entirely of silica and
are considered highly biologically compatible materials, (iv) carbon-based, which are composed
entirely of carbon and come in various shapes such as hollow spheres and tubes, and (v) dendrimers,
which are three-dimensional polymer structures that can be used for drug and gene delivery
[2]
(
Table 4.1
). Within each category, size, shape, and surface alterations such as charge and biologically
compatible/active coatings play key roles in determining the physicochemical properties of nanopar-
ticles
[3]
. Nanosized materials can have very different biological effects from the bulk forms based
on an increased surface-to-mass ratio. Although the application of nanomaterials to medicine (nano-
medicine) holds great promise, current biomedical applications are still in their infancy. The unique
combination of semistructured extracellular matrix, biomechanical properties, and active remodeling
makes dentin and bone unique tissues particularly suited to nanomedicine
[4]
.
1
B
4.2
Silica nanoparticles
Silica represents a well-suited material for biomedical applications and in particular to dentistry.
Chemically, silica is an oxide of silicon (silicon dioxide, SiO
2
). Silica in the form of orthosilicic acid is
the form predominantly absorbed by humans and is found in numerous tissues including bone, tendons,
aorta, liver, and kidney. Dietary silica is generally presumed safe in humans and no adverse effects are
observed in rodents at doses as high as 50,000 ppm
[5]
. Silica is used extensively as a food additive,
used as an anticaking agent, as a means to clarify beverages, to control viscosity, as an antifoaming
agent, dough modifier, and as an inactive filler in drugs and vitamins
[6]
.Furthermore,theFDAclassi-
fies silica as a “generally regarded as safe” (GRAS) agent, making it an ideal candidate for biomedical
applications. Being the second most prevalent element after oxygen
[5]
, silica is abundant and cheap.
Silica has long been used for dental applications, mainly as a component of fillers because of its physi-
cal and optical properties as well as compatibility in composites
[7]
. The emergence of nanotechnology
has provided new opportunities to package and deliver certain elements at the nanoscale, with the intent
of enhancing biological effects or properties.
