Biomedical Engineering Reference
In-Depth Information
surfactant will be spherical. However, shape may be controlled by using templates such as anodic
aluminum oxide (AAO). AAO has been employed in the manufacturing of nanoparticles as a tem-
plate due to the fact that the pore size and length can be easily controlled and can be used to syn-
thesize various shapes. Metal and polymer nanorods, wires, and tubes can be created by generating
an AAO template followed by selectively removing the template
[33,34]
. Recently, Huang and cow-
orkers
[35]
demonstrated that the shape of MSNs could be controlled by changing the surfactant and
ammonia concentration, which was proportional to increased length and thickness, respectively.
Nonmesoporous silica-based nanomaterials can also be manufactured in various shapes in
addition to size including rods, fibers, and even cubes
[36
38]
. Shape can influence the cellular
response to the nanoparticle such as uptake efficiency and potential toxicity
[39,40]
. A study investi-
gating cellular uptake efficiency among various size and shape nanomaterials demonstrated that
50 nm spherical gold nanoparticles have the highest uptake efficiency and that short rods were more
effectively internalized relative to long
[41]
. At present the spherical particles have been demon-
strated to be mostly biocompatible regardless of composition and are usually taken up by cells
through an organized and energy-dependent endocytosis
[42]
. Shape also effects biodistribution and
clearance in vivo. MSNs as short rods (aspect ratio
B
1.5, length 185
6
22 nm) were mainly detected
in the liver; however, long rods (aspect ratio
65 nm) were detected preferentially in
the spleen. The clearance of the short rods was faster than the long rods
[35]
. The above studies have
been performed in biological systems; however, the somewhat unique applications of nanotechnol-
ogy to dentistry may allow for the use of shapes not compatible with biological systems. For
example, the rod shape may be more useful in dental applications as an antibacterial although, to
date, the effect of shape on various dental applications has not been completely explored.
B
5, length 720
6
4.4.3
Surface properties and modifications
One of the most important features of a nanoparticle in terms of mechanical and biological proper-
ties is surface charge. Changes in surface charge lead to differences in dispersibility in aqueous and
organic solutions, the ability to interact with and translocate cell membranes, and the potential reac-
tivity with numerous proteins, enzymes, and surfaces. The sol
gel process results in a terminal OH
group (Si
OH or silanol group) on the surface which is relatively easily coupled with silane con-
taining compounds by condensation. The silanol group gives a hydrophilic character to the nano-
particles as shown in
Figure 4.5
.
The ability to modify the surface of nanoparticles is an important advantage in selective target-
ing of specific cell populations and organs. Modifications involve surface decoration of nanoparti-
cles using targeting proteins such as antibodies or ligands recognized by specific cell surface
receptors. Examples of modifications include the addition of avidin, avidin antibody conjugates,
and direct
immunoglobulin conjugation. In order to link biomolecules for specific targeting,
avidin
antibody coupling reactions can be employed through the surface
modification with APS to generate amine groups. There are potentially infinite numbers of different
molecules that could be coupled to the surface of nanoparticles in order to target them to specific
cell populations, as would be important in the rational design of novel nanoparticles for therapeutic
applications (
Figure 4.6
). Changes to the surface of nanoparticles are likely to also change impor-
tant properties such as the ability to enter cells and location to specific organelles ultimately
biotin couple or antigen
