الفهرس 
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Abstract
process of remineralization [13]. This demineralization is
manifested as white spot lesions (WSLs) and can lead to
further cavitation. Therefore, the occurrence of decalcification
and their appearance in the form of WSLs at the
time of debonding is one of the main concerns for both
the patient and the orthodontist [14].
Therefore, efforts were made to control or even prevent
the development of WSLs. Starting from maintaining
good oral hygiene measures that include teeth brushing
using fluoridated toothpaste and rinsing with fluoride
mouthwash [15, 16]. Moving to the application of varnishes
[17, 18], resin materials containing antibacterial
agent [19, 20] and using modified orthodontic elastomerics
[21]. Recently, attempts included the addition of nanoparticles
to orthodontic adhesives [22, 23], resin modified
glass ionomer cements [24, 25], elastomerics [26] and
coating of orthodontic brackets, wires and bands [27, 28].
Physical vapor deposition (PVD) is one of the coating
techniques that is characterized by its sustainability,
as coatings can be reproduced with more efficiency and
higher purity than when using other techniques [29].
Thermal evaporation, which is a type of PVD is considered
advantageous over magnetron sputtering in its better
control over the produced film thickness and higher
purity [30].
Nanoparticles are defined as insoluble materials of size
smaller than 100 nm [31]. Because of its small size, it has
a higher surface-to-volume ratio and a closer interaction
with microbial membranes, resulting in a larger surface
area of antimicrobial activity [32]. Several metals as silver,
copper, gold, titanium and zinc have been used since ages
to act as antimicrobial materials, where each of them has
different properties and range of activity [33, 34].
Using silver, silver ions and silver compounds have
been considered as antibacterial agents in biomedical
applications [35]. In dental applications, nanoparticles of
silver were proven to be an effective antimicrobial component
when added to dental resin composites, and also
when coated on orthodontic brackets and wires [22, 36,
37].
Zinc oxide nanoparticles (ZnO) was proven to be a
good antibacterial agent [38]. Also, on coating orthodontic
wires with ZnO, it was found to have a good antibacterial
activity [39]. Although, the nanoparticles of silver
(Ag) have displayed higher antimicrobial activity than
ZnO nanoparticles [40], several studies have shown that
Ag nanoparticles are cytotoxic and genotoxic to human
cells [41, 42]. However, a composite of Ag and ZnO nanoparticles
exhibited an improved antibacterial activity
against S. mutans [25].
Therefore, with the aim of benefiting from both ZnO
nanoparticles and Ag nanoparticles, while reducing their
individual cons which are the cytotoxicity of Ag nanoparticles
[41, 42], its higher cost than ZnO nanoparticles
[33], along with the reduced antibacterial effectiveness of
ZnO nanoparticles when compared to Ag nanoparticles
[40], a combination of Ag/ ZnO nanoparticles was used
in this study for coating of orthodontic brackets and its
antibacterial activity was compared to the antimicrobial
effect of Ag and ZnO nanoparticles coatings individually.
The null hypothesis was that the antibacterial effects of
the three types of coatings on the orthodontic stainlesssteel
brackets; Ag, ZnO and the combination of Ag/ ZnO
nanoparticles were not to be significantly different.
Methods
This study aimed at assessing the antibacterial effect of
three types of nanoparticles; Ag, ZnO and a combination
of both Ag and ZnO (Ag/ZnO) when applied as coatings
on orthodontic stainless-steel brackets through physical
vapor deposition, on two different strains of bacteria;
Streptococcus mutans and Lactobacillus acidophilus. This
evaluation was to be carried out immediately after coating
(T1) and after 3 months (T2) to see if the antibacterial
effect, if present, persisted.
The study was carried out at Faculty of Dentistry, Alexandria
University, the Egyptian Nanotechnology center,
Cairo University, El-Sheikh Zayed Campus and Faculty of
Science, Cairo University.
Sample grouping and preparation
The sample size was estimated based on assumptions of
alpha error to be equal 5% and study power 80% [37, 43,
44], a total of 48 brackets were to be included.
The brackets were divided into four groups, each constituting
12 brackets: control group (brackets as received
without modifications), Ag nanoparticles coated group,
ZnO nanoparticles coated group and Ag/ ZnO nanoparticles
coated group.
The brackets used were stainless steel “American orthodontics”
0.018’’ slot size brackets of lower premolars.
Before coating the brackets, ultrasonication was done at
Faculty of Dentistry, Alexandria University to remove any
adventitious macroscopic contamination [37]. Prior to
storage in an airtight container, the brackets were thoroughly
cleaned and sterilized using an autoclave.
Coating procedure
Physical vapor deposition was carried out using PROTOFLEX
1400 machine (USA; Figs. 1 and 2) at the Egyptian
Nanotechnology center, Cairo University, El-Sheikh
Zayed Campus. Thermal Evaporation was used in which
Ag and/ or ZnO were vaporized followed by their deposition
and coating of the surface of the orthodontic brackets.
First vacuum environment was achieved through