Biomedical Engineering Reference
In-Depth Information
kinetic coefficient of friction. When enlarging the distance between the bracket and the long axis of
the Instron system, thus creating a contact angle of 3.8 and simulating superior load applied onto
the wire, a decrease in both static and kinetic coefficients was noted at the coated wire. This
decline in static and kinetic coefficients was of 22% and 34%, respectively, with respect to the
uncoated wire. In an additional series of measurements made only on the Co
IF coated wire in a
contact angle of 5 , a static coefficient of 0.08 and kinetic friction coefficient of 0.061 were
observed. The trend of the frictional force reduction as the load rises sustained throughout the entire
series of experiments. The unique coating of IF-WS 2 NP embedded in cobalt matrix demonstrated
a significant friction reduction of the Ni
1
Ti alloy.
The reduction in the friction as a result of the Co/IF-WS 2 coatings can be attributed to the release
of minute amounts of IF-WS 2 NP which are impregnated in the cobalt coating. WS 2 and MoS 2 are
known to provide efficient lubrication even in bulk (platelets) form. These kind of compounds are
typified by the strong covalent bonds between sulfur and metal (tungsten) atoms within each plane
while only weak van der Waals forces exist between planes; thus, low interplanar shear strength
exists and permits sliding of two adjacent planes [31
34] . The WS 2 NP prevent asperity contact
between the bracket and wire surfaces. Their round shape suggests that a rolling friction scenario is
also possible in this case. Furthermore, elastic deformations of the NP augment their resilience and
diminish the energy dissipation associated with friction and wear under the load [16] . In addition,
the IF-WS 2 NP act as a protection against oxidation of the metal surface [37] ;hence,itcouldbe
suggested that the coating also reduces or even obstructs nickel (cobalt) release, which is a known
allergen, from the Ni
Ti wire.
13.5 Safety: toxicity and biocompatibility
The ongoing emergence and spread of nanoscale man-made materials for medical applications
aroused the awareness of the scientists, the regulatory agencies, and the public to their safety.
Safety, in terms of health-care use is determined by both nontoxic response and biocompatibility.
Briefly, toxicity is the degree to which the material can harm humans and animals. The local and
systemic responses of the host to the mode and the period of material exposure are evaluated [45] .
Biocompatibility has recently been defined as the ability of a biomaterial to perform its desired
function with respect to a medical therapy, without eliciting any undesirable local or systemic
effects in the recipient or beneficiary of that therapy, but generating the most appropriate benefi-
cial cellular or tissue response in that specific situation, and optimizing its clinically relevant
performance [46] .
The toxicity of the IF-WS 2 NP, manufactured by “NanoMaterials,” for industrial applications,
was tested at various modes of exposure by three authorized laboratories. The results showed no
apparent toxic effect of IF-WS 2 after oral administration, dermal application, and inhalation tests in
rats [47
49] . Toxicity tests, initially performed on animals, conducted in compliance with the
OECD (Organisation for Economic Co-operation and Development) Principles of Good Laboratory
Practice, are a prerequisite for further biocompatibility evaluations toward medical use. These tests
are done on the as-prepared NP while biocompatibility of the medical device will be evaluated
afterward in conjunction with all the other components comprising the NP coating of the device.
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