Keywords
clear aligners - PET-G - thermoplastic materials - staining substances - chemical–physical
characterization - micro-Raman spectroscopy.
Introduction
Current computer-aided design and manufacturing (CAD/CAM) techniques are widely used
in the manufacture of clear thermoplastic aligners for the treatment of various types
of dental malocclusion.[1]
[2]
[3]
[4] The biomechanical characteristics of commercial clear aligners are influenced by
the properties of the thermoplastic material used, the forming process,[2]
[5] and the precise aligner-teeth fitting.[6]
The thermoplastic polymers currently most used are based on polyurethanes (PU) or
polyesters such as polyethylene terephthalate glycol (PET-G).[7]
[8]
[9] These materials have good mechanical and physical properties, biocompatibility,
chemical stability, excellent aesthetic characteristics, good formability, and are
of low cost,[10] but also have various limitations such as low strength and poor wear resistance.[11] In particular, PET-G materials are structurally amorphous polymers that possess
an irregularly arranged molecular structure with poor packaging.[12] These polymers are transparent because they are passed through by visible light
and their degree of crystallization after thermoforming is negligible.[13]
However, the thermoforming could cause variations in the morphological and mechanical
properties of the polymer.[5] Furthermore, during their use, thermoplastic aligners are continuously subjected
to mechanical loads, temperature changes inside the oral cavity, and the influence
of saliva and food pigments.[14] These factors could lead to abrasions, surface alterations, color changes, and delamination
of the polymer.[14]
[15]
[16] Chemical cleaning methods may also alter the structural integrity of the thermoplastic
material. These degradations lead to a significant loss of mechanical strength and
transparency of the aligner, an essential requirement in an invisible orthodontic
treatment.[13]
[17]
Although clinicians recommend removing aligners before eating and drinking, patient
compliance is still insufficient. Therefore, it is necessary to simulate the intraoral
clinical conditions, even with the use of staining substances and agents such as nicotine
and cigarette smoke, to better evaluate any structural and/or optical changes of the
orthodontic aligners. The aim of this research is to characterize the PET-G-based
thermoplastic material used for Lineo aligners (Micerium Lab) after exposure to various
commonly used staining and disinfecting agents. In detail, micro-Raman spectroscopy
was used to identify any changes in the chemical composition with consequences on
the morphology and/or structure of the polymer in question.
Materials and Methods
Preparation of PET-G Samples
Four sheets of thermoplastic material in PET-G with a thickness of 0.8 mm were selected
for the study. Three of these slabs were thermoformed, while the fourth was not subjected
to hot molding techniques. A truncated pyramid-shaped mold was prepared and placed
in the thermoforming machine. A thermoforming quota of 20 mm and an angle of 11 degrees
was applied for less friction in extraction. The applied temperature is 160 to 165°C.
The models obtained after thermoforming were removed and the horizontal surface was
used to obtain samples with dimensions of 1 × 1.5 cm (n = 23; [Fig. 1]). A nonthermoformed sample of 1 × 1.5 cm was obtained from the fourth sheet, which
will be used to evaluate the influence of thermoforming on the PET-G material.
Fig. 1 Preparation of thermoformed polyethylene terephthalate glycol (PET-G) samples. (a) Truncated pyramid model used in the thermoforming phase. (b) Some of the thermoformed samples cut from the horizontal surface of the model obtained.
Treatment of PET-G Samples with Commonly Used Substances
It is essential to evaluate whether the intake of beverages or the use of certain
substances or detergents used in cleaning the aligners can influence the chemical–physical
characterization of the thermoplastic PET-G material under analysis. Twenty-two thermoformed
samples were used for this purpose. Each specimen was placed in a plastic container
containing 5 mL of one of the following solutions: (1) 6 g of coffee powder (Nescafé
Classic, Switzerland) was added to 200 mL of boiling deionized water; (2) sugar-free
tea (30 mg/150 mL, Earl Grey Twinings, London, England); (3) Coca-Cola (Coca-Cola
Company, Atlanta, Gorgia, United States); (4) undiluted red wine (San Crispino, Cantine
Ronco Romagna, Italy); (5) disinfectant based on colloidal silver; (6) the nicotine
solution was obtained by infusing cigarette filters into deionized water and then
filtered; (7) 120 mL of artificial saliva (Biotène Oral Balance, GlaxoSmithKline Consumer
Healthcare S.p.A.) were diluted in 480 mL of deionized water; (8) cigarette smoke
(a certain amount of smoke produced by cigarettes); (9) saliva and smoke; (10) solution
of saliva and nicotine in a 1:1 ratio; and (11) a solution of saliva and coffee in
a 1:1 ratio.
All specimens were maintained in immersion in a water bath at T = 37°C and were randomly
divided according to the immersion time in the chosen solvents (n = 11 up to 10 days; n = 11 up to 15 days). The samples were rapidly rinsed with deionized water (Milli-Q)
every 24 hours before being reimmersed in fresh solution.
As for the colloidal silver-based disinfectant, the two thermoformed samples, up to
10 and 15 days, were treated for a maximum of 3 hours a day to simulate the aligner
cleaning methods.
After 10 and 15 days, all the samples were washed by immersion in Milli-Q water, kept
in ultrasound for 5 minutes and adequately dried before being analyzed for the physicochemical
characterization ([Fig. 2]).
Fig. 2 Flowchart of the protocol adopted for the treatment of polyethylene terephthalate
glycol (PET-G) samples immersed in staining substances.
Micro-Raman Spectroscopy Analysis
Raman spectroscopy analysis was performed to investigate the chemical composition
of the samples in PET-G. The measurements were carried out with a micro-Raman spectrometer
(XploRA Plus, Horiba) using a laser source with a wavelength of 785 nm with a 100X
objective, 1,200 g/mm reticle, with 10-second acquisition time for five accumulations
in a range between 200 and 3,200/cm.
Results
Influence of the Thermoforming Process
The peaks characteristic of the vibrational modes of PET-G[18] are shown in [Table 1]. To assess the influence of the thermoforming process in the physicochemical characterization,
two PET-G samples were compared (n = 1 thermoformed and n = 1 nonthermoformed). The preliminary investigation shows that the structure of the
polymer used is not altered by the thermoforming process ([Fig. 3]). However, the morphological analysis shows that the surface of the thermoformed
sample is characterized by roughness, holes, and scratches in comparison with the
uniform and homogeneous surface of the one not subjected to thermoforming ([Fig. 4]).
Fig. 3 Raman spectra of non-thermoformed (blue) and thermoformed (green) polyethylene terephthalate glycol (PET-G) samples at 785 nm. The characteristic
peaks belong to the PET-G polymer and there is no change in the signals when heat
treatment is carried out.
Fig. 4 Morphological analysis under an optical microscope at 100x magnification. (a) Smooth and homogeneous non-thermoformed surface and (b) thermoformed surface with scratches and roughness.
Table 1
The assignment of observed micro-Raman bands of polyethylene terephthalate glycol
(PET-G)
|
Raman (per cm)
|
Assignment
|
|
3,082 w
|
CH stretching
|
|
3,068 w
|
CH stretching
|
|
3,000 w
|
CH stretching
|
|
2,960 w
|
CH2 stretching
|
|
1,730 m
|
C = O stretching
|
|
1,615 s
|
C = C stretching (ring)
|
|
1,450 w
|
CH deformation
|
|
1,412 vw
|
C–C stretching (ring)
|
|
1,370 vw
|
CH2 wagging
|
|
1,282 m
|
C–C stretching (ring), C–O stretching
|
|
1,270 m
|
C–C stretching (ring), C–O stretching
|
|
1,245 sh, w
|
C–C stretching (ring), C–O stretching
|
|
1,175 w
|
CH in plane bending (ring)
|
|
1,117 w
|
CH in plane bending (ring), C–O stretching
|
|
1,038 vw
|
C–C stretching (glycol)
|
|
890 vw
|
CH2 rocking
|
|
853 w
|
C–C stretching (ring breathing), C–O stretching
|
|
797 w
|
CH out of plane bending (ring)
|
|
633 s
|
CCC in plane bending (ring)
|
|
407 vw
|
Asymmetric ring torsion
|
|
272 w
|
C–C stretching (ring), CCC bending (ring)
|
Abbreviations: m, moderate; s, strong; sh, shoulder; w, weak; v, very.
Aging Treatments
From the analysis of the Raman spectra acquired for all the samples immersed in the
different substances (n = 22), no differences were found between those treated for 10 and 15 days ([Fig. 5]). Therefore, the results shown are comparable to the single solvent or agent used
regardless of the duration of the treatment itself.
Fig. 5 Raman spectra representative of the signals found on the surface of the samples subjected
to the different treatments for 10 (green) and 15 days (blue). Peaks found are those characteristics of the polyethylene terephthalate glycol
(PET-G) polymer and are stackable with each other.
The micro-Raman analysis shows that in all the samples the acquired spectra are those
of the PET-G polymer, whatever the agent used in the treatments at both 10 and 15
days. Only in the samples dipped in nicotine were areas of burns and morphological
changes of the polymer detected. Furthermore, in all the samples, different fluorescence
phenomena are found due to residues of substances accumulated in correspondence with
the asperities and scratches of the thermoformed material ([Figs. 6]
[7]
[8]
[9]
[10]
[11]
[12]). There are also fibrous residues of accidental origin attributable to cotton fibers
on the surface of the samples immersed in the saliva and coffee solution, at 10 and
15 days, and treated with colloidal silver disinfectant for 15 days ([Fig. 13]).
Fig. 6 (a,b) Morphological analysis of polyethylene terephthalate glycol (PET-G) samples immersed
in a coffee solution. There is presence of residues with a discrete diffuse fluorescence.
Fig. 7 (a,b) Morphological analysis of polyethylene terephthalate glycol (PET-G) samples immersed
in a sugar-free tea. There is presence of highly fluorescent tree structures.
Fig. 8 Morphological analysis of polyethylene terephthalate glycol (PET-G) samples immersed
in (a,b) Coca-Cola and (c,d) red wine. There are no structures and no fluorescence.
Fig. 9 Morphological analysis of polyethylene terephthalate glycol (PET-G) samples immersed
(a,b) in a silver colloidal and (c,d) in the solution of artificial saliva and coffee. Along with the usual structures,
large nonfluorescent cottonlike fibrous structures are present.
Fig. 10 Morphological analysis of polyethylene terephthalate glycol (PET-G) samples immersed
(a,b) in a pure nicotine and (c,d) in the solution of artificial saliva and nicotine. There is presence of carbon deposits
and rodlike structures, which cause less fluorescence.
Fig. 11 (a,b) Morphological analysis of polyethylene terephthalate glycol (PET-G) samples immersed
in an artificial saliva and saliva with cigarette smoke. There is presence of almost
regular circular spots without fibrous or rodlike structures.
Fig. 12 (a,b) Morphological analysis of polyethylene terephthalate glycol (PET-G) samples immersed
in cigarette smoke. There is presence of deposits with little fluorescence, without
fibrous structures or rods or circular spots.
Fig. 13 Raman spectra representative of some signal types found on the surface of the polyethylene
terephthalate glycol (PET-G) samples. The observed peaks correspond to those of cotton-fiber-like
foreign bodies found on the surface of samples immersed in saliva and coffee solution
(blue) and in those treated with colloidal silver (green) in comparison with a representative Raman spectrum of treated samples with other
substances (red).
Discussion
The qualitative properties of clear orthodontic aligners are strictly related not
only to the material used and the molding process for manufacturing but also to modifications
related to the oral environment. The thermoforming process applied to thermoplastic
materials involves a heating cycle followed by forming under vacuum or pressure, which
could cause morphological alterations of the polymer.[5] Furthermore, these materials in the form of aligners inserted inside the oral cavity
can also be subject to surface and color alterations due to factors such as humidity,
temperature variations, and coloring agents.[5]
[13]
[19] Despite clinicians' recommendations to wear aligners full-time except when eating,
drinking, or for oral hygiene maneuvers,[20] many patients take coloring agents (red wine, coffee, tea, Coca-Cola) or smoke with
their orthodontic devices.[21]
[22]
This in vitro study reports the chemical–physical characteristics of PET-G, widely used for the
fabrication of orthodontic aligners (Lineo, Micerium Lab), which has undergone various
aging treatments. Micro-Raman spectroscopy is a nondestructive chemical analysis technique
that provides detailed information on the chemical structure, phases, and polymorphy
and molecular interactions. It is integrated with an optical microscope, which, by
focusing the laser on the sample, allows the analysis of small regions.[23]
The originality of this work consists in the chemical–physical characterization of
the thermoplastic material that occurs after its exposure to substances for common
use for a prolonged time (15 days) equal to the period of use of a single aligner
during complex orthodontic treatment. This has great relevance in observing the structural
behavior of the polymer during intraoral aging.
Although the micro-Raman analysis did not reveal any changes in the chemical structure
of PET-G ([Fig. 3]), the thermoforming process used to manufacture the aligners caused evident surface
alterations of the polymer including roughness, holes, and scratches ([Fig. 4]). Also, Porojan et al[24] noted an increase, albeit insignificant, in the roughness of this material. In contrast,
the study by Zhang et al[25] showed a smoother PET-G surface after thermoforming.
Alterations in the physical properties of PET-G have also been reported after intraoral
exposure.[25] The micro-Raman analysis performed after immersion in different solvents for up
to 10 and 15 days has shown that the structure of PET-G is not altered and that the
fluorescence phenomena found are due only to the accumulation of residues of the substances
used ([Figs. 6]
[7]
[8]
[9]
[10]
[11]
[12]). The only morphological changes were detected in the nicotine-treated samples as
the interaction between the micro-Raman laser and the remaining carbon residues resulted
in local burn areas of the material. Furthermore, in the samples immersed in the solution
of saliva and coffee and in those treated with colloidal silver disinfectant, accidental
foreign bodies of a fibrous nature were detected, whose composition and characteristic
peaks are probably related to cotton fibers ([Fig. 13]).[26]
Transparency represents another important property possessed by thermoplastic materials.[27] Any color changes have been associated with the absorption or penetration of the
coloring substances that come into contact with the surface of the material within
the oral environment.[17]
[28]
[29] The surface characteristics of the thermoformed materials could accelerate the accumulation
of pigment with consequent loss of transparency.[30] Daniele et al[9] found alterations in the aesthetic characteristics of aligners made of PET-G and
PU, especially after immersion in coffee or red wine, with a consequent loss of transparency
due to impurities deposited on the surface. However, Porojan et al[24] have shown that color variations of the PET-G samples do not depend so much on the
surface roughness but on the penetration inside the material itself.
In the present study, no substantial variations in the colorimetric level were found;
however, the spectroscopic technique used in this study does not have the ability
to detect any changes of this type. Therefore, in future research, it would be appropriate
to evaluate transparency changes and colorimetric variations of PET-G with the length
of time used in this study (10 and 15 days) with more suitable instruments.
Conclusions
After thermoforming and aging treatments, there are no structural or color variations
on the PET-G material used for the manufacture of Lineo aligners produced by Micerium
Lab.
The heterogeneous morphological surface, which could suggest a loss of transparency
and aesthetic properties, depends on the intraoral exposure and on the contact with
coloring pigments deriving from beverages, nicotine, or cigarette smoke, which determine
the accumulation of deposits in the rough surface and scratches due to thermoforming.
Therefore, the surface characteristics of the PET-G samples found on the scanning
electron microscope (SEM) images after 10 and 15 days of immersion times are attributed
exclusively to the staining substances used.
