Biomedical Engineering Reference
In-Depth Information
showing great beneficial effect in terms of reducing friction
[13]
. However, these experiments
were conducted under dry conditions and the behavior of the DLC coatings in similar wet
environment should be evaluated.
These attempts and many others were successful to some extent but did not fully overcome the
problem. The improvements were mainly directed at the beginning of tooth movement where lower
friction was recorded, but the main impediment of the movement is encountered at the angles
where BI and NO are the components mainly responsible for the RS. Hence significant reduction
of friction at BI might reduce the excessive orthodontic force. Overcoming this inevitable frictional
force means using excessive orthodontic force than what is actually needed to move a tooth. Some
investigators found that 40
60% of the orthodontic force is aimed at overcoming the frictional
resistance.
The use of excessive orthodontic forces has several disadvantages especially on the anchor unit
usually desired to remain stable during treatment and it might increase the risk of root resorption.
It is assumed that the reduction in frictional resistance could enhance the alignment and the space
closure and therefore could lead to reduced treatment time. Thus, reducing frictional forces during
orthodontic tooth movement will significantly contribute to successful treatment outcome.
13.3
Materials considerations: fullerene-like nanoparticles
The search for material technologies may lead to significant reduction of friction coefficient in
orthodontic devices, self-lubricating coatings have been contemplated and a new technology for
applying these coatings on the wires was thus developed. To explain this technology, the concept
of inorganic fullerene-like (IF) nanoparticles (NP) and inorganic nanotubes (INT) is described
below.
13.3.1
Prelude: inorganic fullerene-like nanoparticles of WS
2
and MoS
2
Hollow closed-cage carbon structures, the fullerenes (C
60
) and carbon nanotubes are known for
some-time. Research into similar structures from other (inorganic) layered compounds started soon
after. Thus, IF NP and INT of tungsten disulfide (WS
2
) first and subsequently of molybdenum
disulfide (MoS
2
) were discovered in 1992
[14
16]
, and elicited considerable interest ever since in
this emerging field
[17
21]
. This observation is surprising in view of the fact that the chemical
bond is not stable beyond a few angstroms and hence structures with hollow spaces of a few nano-
meter and above were initially thought to be unfavorable. The formation of such hollow closed
cages can be attributed to the inherent instability of the planar nanostructures of layered
compounds. In graphite, the carbon atoms are bonded in flat sp
2
bonds forming a hexagonal
network (
Figure 13.3A
). The graphene sheets are stacked together via weak van der Waals forces.
In the case of MS
2
(
Figure 13.3B
), where M stands for a metal atom like molybdenum or tungsten,
the molecular sheet is made up of a layer of M atoms sandwiched between two outer sulfur layers.
Each M atom binds to six sulfur atoms forming a lattice with trigonal biprism (octahedral) coordi-
nation. In analogy to graphite, weak van der Waals forces are responsible for the stacking of the
S
M
S layers together. Therefore, these compounds are highly anisotropic with respect to many
