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Materials Science in Additive Manufacturing Mouthguards: Disinfection versus properties changes

impact of disinfection treatments on these properties was widely used mouthguard material (EVA), the 4 mm EVA
assessed. specimens exhibited lower average values of both σ and
f
Since mouthguard materials must absorb, dissipate, E than the 2 mm 3D-printed specimens assessed in this
f
and redistribute impact energy across a broader area, they study.
require sufficient rigidity under load. As reported by 3.3.2. Charpy impact test
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Cummins and Spears, stiffer materials are generally more 5
51
effective at energy dissipation and stress redistribution. According to the ASTM F697-16, mouthguards must be
Accordingly, a higher E value indicates a stiffer material. fabricated from resilient materials capable of absorbing,
f dissipating, and redistributing impact energy to prevent
Overall, specimens incorporating HIPS exhibited higher injuries to the teeth and surrounding oral structures.
E values than those made with PMMA, aligning with Accordingly, the samples were subjected to transverse
f
literature findings that report superior stiffness in HIPS impact testing, and the results are presented in Figure 8.
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monomaterial samples. The flexural properties of control Among the tested configurations, those incorporating
11
specimens (dry and pre-disinfected) were broadly similar, HIPS exhibited the highest values for both impact strength
likely due to minimal mass change during the aging process. (Figure 8A) and absorbed energy (Figure 8B). These
Concerning the effects of disinfection protocols on the findings are consistent with previous studies, 11,17 which
mechanical properties of 3D-printed multi-material parts, have demonstrated that HIPS possesses superior impact
both disinfection methods resulted in lower E values resilience compared to PMMA.
f
compared to control samples (dry and pre-disinfected). Regarding the influence of multi-material configuration
This reduction in mechanical characteristics was observed (bi- or tri-layer) and disinfection treatment on the
across both PMMA- and HIPS-based configurations. transverse impact properties of HIPS- and PMMA-based
In the case of chemical disinfection, two factors 3D-printed parts, the results generally show no statistically
contribute to the decrease in flexural modulus: significant differences. However, notable exceptions
(i) Chemical interactions between the constituents of the include:
Polident solution and the polymer chains, which may • Bi-layered HIPS configurations, where a difference was
reduce the average molecular weight and compromise observed between the dry-state condition (20.4 ± 0.6
-2
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mechanical integrity. kJ·m ) and the post-Polident group (23.5 ± 2.5 kJ·m ),
(ii) The presence of water is used to dissolve the cleaning • Tri-layered HIPS configurations between disinfection
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tablet, which acts as a plasticizing agent and softens conditions (21.1 ± 8.0 kJ·m for post-UVC vs. 36.0 ±
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the polymer structure. 2.3 kJ·m for post-Polident), and
• Tri-layered PMMA configurations between
Meanwhile, UVC disinfection had the most pronounced disinfection groups (18.2 ± 1.2 kJ·m for post-UVC
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adverse effect on the flexural modulus. The high-energy vs. 26.3 ± 5.8 kJ·m for post-Polident).
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photons emitted during UVC exposure can cleave chemical
bonds within the polymer chains, initiating a degradation These findings suggest that the disinfection protocol
process. This degradation process involves the formation can influence the transverse impact properties of
of free radicals, which can react to form new chemical 3D-printed parts, particularly in tri-layered configurations.
bonds, thereby altering the polymer’s molecular structure Nevertheless, caution is warranted when interpreting these
and reducing mechanical stiffness. 17,52 Consequently, it can results due to the high standard deviations, which limit the
be surmised that disinfection adversely affects the rigidity statistical reliability of the conclusions.
of mouthguards, potentially impairing their ability to An inspection of the damaged specimens following the
effectively dissipate and redistribute impact forces. Charpy impact test (Figure 9) revealed superior interfacial
Moreover, across both material types, the tri-layered adhesion in tri-layer configurations compared to
configuration exhibited higher E values compared to the bi-layered ones. Clear separation at the material interface
f
bi-layered configuration, indicating superior impact energy was frequently observed in the bi-layered specimens,
dissipation due to greater overall rigidity. This is attributed which may explain the greater variability within these
groups. As evidenced in the literature, premature cooling
to the higher relative content of TPU—a polymer with of the first printed layer can impede proper adhesion
inherently low E —in the bi-layered specimens compared with the subsequently deposited layer. 53,54 In bi-layered
17
f
to the tri-layered ones.
configurations, TPU is printed directly onto the printer
Finally, when comparing the materials and multi- platform. In contrast, tri-layer configurations involve the
material configurations under investigation with the most deposition of a rigid polymer (PMMA or HIPS) atop an

Volume 4 Issue 2 (2025) 9 doi: 10.36922/MSAM025130018