Assessment of thermal and flexural properties of 3D-printed denture base after addition of YSZ nanoparticles

Abstract

Integrating nanoparticles into 3D-printed denture base resin has been demonstrated to attain innovative biomedical and physicomechanical characteristics, paving the way for a new generation of advanced nanocomposites. Whereas additive manufacturing technology presents a significant development in prosthodontics, delivering efficiency, cost-effectiveness, and a helpful approach for creating high-quality prostheses, achieving superior mechanics in 3D-printed denture resins remains challenging despite good biocompatibility and aesthetics. Therefore, the present study aimed to evaluate the effect of incorporating varying concentrations of yttria-stabilized zirconia nanoparticles (YSZ NPs) (1%, 3%, 5%, and 7%) into 3D-printed denture base resin on the thermal and flexural properties. A total of one hundred specimens were made up in dimensions per particular test and divided into five groups. A thermal Constants Analyzer TPS 500R, three-point bending test, and field emission scanning electron microscopy (FE-SEM) were employed to measure thermal conductivity, flexural strength, and the specimens’ microstructure, respectively. The study results verified that adding YSZ NPs to a 3D-printed denture resin significantly enhanced its thermal and flexural properties (p < 0.05) compared to unmodified resin. These findings demonstrate the potential to develop a novel 3D-printed nanocomposite denture base with enhanced material performance. The need for further search for alternative denture base materials and the expedition for materials that combine desirable aesthetics, ease of processing, and enhanced physical and mechanical properties continue active research areas in the contemporary dentistry field.

Keywords

3D-printed denture base resin additive manufacturing digital dentures flexural strength rapid prototyping thermal conductivity testing YSZ nanoparticles

Introduction

Despite its widespread use in denture base fabrication,^1,2 PMMA’s inherent limitations, including low thermal conductivity and suboptimal mechanical properties, continue to pose challenges.^3,4 Furthermore, while 3D-printing offers advantages in fabrication, current additive manufacturing resins often exhibit diminished performance compared to conventional PMMA, particularly in flexural strength.^5–7 These shortcomings necessitate the development of improved denture base materials that combine the benefits of digital manufacturing with enhanced functional properties.^8,9

While the potential of nanoparticle reinforcement is recognized,^10–13 the specific influence of yttria-stabilized zirconia (YSZ) nanofiller incorporation on the thermal conductivity and flexural strength of 3D-printed acrylic denture base resins remains largely unexplored. Due to its biocompatibility and favorable mechanical properties, YSZ holds particular promise for dental applications.¹⁴

Therefore, this study investigates the effects of incorporating 3 mol.% YSZ nanoparticles of a 3D-printed acrylic denture base resin by measuring thermal conductivity, assessing flexural strength, and comparing the properties of the nanocomposite with the base resin. Numerous studies have explored the incorporation of nanoparticles, such as silver, silicon carbide, and carbon nanotubes, to enhance the properties of acrylic resins.^3,15,16 Similarly, research has shown improvements in flexural strength by adding silicon dioxide and zirconia oxide nanoparticles in 3D-printed resins.^6,17

This study hypothesizes that the incorporation of 3 mol.% YSZ nanoparticles will not significantly influence the thermal conductivity or flexural strength of the 3D-printing material. By addressing this gap, this study contributes to the development of next-generation nanocomposite denture base materials with enhanced performance and functionality. This would ultimately improve patient outcomes and potentially address the long-standing challenge of balancing patient comfort with denture longevity.

Materials and methods

Using the World Health Organization formulas, the research sample size was measured using power analysis, yielding an 80% study power, a significance level of 5%, and a marginal error of 5%.

Preparation of nanocomposites mixture

3D-printed denture base resin (FREEPRINT® denture) was used in this study, modified with yttria-stabilized zirconia nanopowder (99.95% purity, US Research Nanomaterials, Inc., USA) as the inorganic filler agent for fabricating the nanocomposite denture base specimens, with an average size of 31 nm as revealed by field emission scanning electron microscopy (FE-SEM) (Figure 1).

Figure 1.

FE-SEM image of YSZ NPs powder at field view of 500 nm.

YSZ NPs were added in various concentrations (1 wt.%, 3 wt.%, 5 wt.%, and 7 wt.%) to 3D-printed resin, while one group remained without NPs addition (control group). This concentration range was selected to optimize property enhancement while mitigating potential adverse effects. This approach, informed by previous studies, ensured a systematic and informed investigation.^18,19 100 specimens were printed, 50 (n = 10) for thermal conductivity and 50 (n = 10) for flexural strength. Precisely weighed amounts of NPs were added to separate containers containing the base resin at the designated concentrations. The mixtures were thoroughly mixed and stirred, then printed.^19,20

Preparations of specimens

The specimen’s dimensions were 64 × 10 × 3.3 ± 0.2 mm for the flexural strength test according to the ISO 20795-1: 2013 standard, while for the thermal conductivity test, were 40 × 7 mm according to the test machine specification.²¹

The specimens were designed using an open-source CAD program. They were saved as an STL file and printed by Asiga MAX UV. Each layer was printed with an orientation of 0° and a 50 µm layer thickness.²² Afterward, Optik Otoflash G171 was used for the post-curing process; according to the manufacturer’s guidance, 2 × 2000 flashes under inert gas turned components after 2000 flashes. Finally, specimens were cleaned with isopropyl alcohol (99.9%), trimmed of excess resin, and then finished and polished, as shown in Figure 2.²³

Figure 2.

Fabrication process for 3D-printed specimens modified with yttria-stabilized zirconia nanoparticles (YSZ NPs).

Flexural strength test

Flexural strength was determined using a three-point bending test (Instron H50 KT Tinius Olsen) until fracture. The calculation followed the standard formula: S = 3FL/2BH,² where S (MPa), F (N), L (mm), B (mm), and H (mm) represent flexural strength, the breaking load, the support span, specimen width, and specimen thickness respectively.

Thermal conductivity test

Utilizing a hot disc thermal conductivity testing apparatus (Thermal constants analyzer TPS 500), the transient plane source (TPS) method assessed the specimens’ thermal conductivity.¹⁶ The instrument was interfaced with a computer program to control the experiment. The software automatically configured the TPS method upon selection of the experiment type.

Surface morphology (FE-SEM)

A field emission scanning electron microscope (FEI, ISPECT F50) was utilized to detect the broken surfaces of the test specimens and assess how evenly dispersed YSZ NPs were within the resin matrix.

Statistical analysis

Data were analyzed using SPSS version 25. The Shapiro-Wilk test confirmed the data’s normal distribution. One-way ANOVA was performed to compare group means. Tukey’s HSD post hoc test was used to determine specific group differences. A p-value of 0.05 was used to determine statistical significance.

Results

FE-SEM images showed apparent differences in the microstructure between the control group and the modified groups. The nanoparticles appeared well-distributed within the resin matrix for the 1 wt.% and 3 wt.% modified groups, although some clustering was observed on the fractured surfaces of the 3% modified specimens (Figure 3).

Figure 3.

Scanning electron micrographs illustrating the surface morphology of the specimens at varying yttria-stabilized zirconia (YSZ) nanoparticle concentrations: (a) 0 wt.%, (b) 1 wt.%, and (c) 3 wt.%. Scale bars = 100 μm.

Thermal conductivity results are shown in Figure 4(A) and Table 1. The 7 wt.% group demonstrated the highest mean thermal conductivity (0.284 W/m∙K), while the control group exhibited the lowest (0.208 W/m∙K). Tukey’s HSD posthoc test revealed a statistically significant difference between the experimental groups (p < 0.001), and no statistically significant difference was observed between the 1 and 3 wt.% groups (p = 0.461).

Figure 4.

Effect of yttria-stabilized zirconia nanoparticle (YSZ NPs) addition (1, 3, 5, and 7 wt.%) on the (A) thermal conductivity and (B) flexural strength of 3D-printed denture base materials. (A) Thermal conductivity increased with increasing YSZ NP concentration, rising from 0.21 W/m·K in the control to 0.28 W/m·K at 7 wt.%. (B) Flexural strength peaked at 135 MPa at one wt.% YSZ NPs, followed by a decrease to 82.83 MPa at seven wt.% YSZ NPs.

Table 1.

Descriptive statistics of the thermal conductivity and flexural strength values include an F test by ANOVA.

Properties	Groups	Mean ± SD	Minimum	Maximum	F test	p- value
Thermal conductivity	Control	.208 ± 0.12	0.184	0.225	206.844	0.000
	1 wt.%	.221 ± 0.003	0.215	0.225
	3 wt.%	.226 ± 0.003	0.222	0.230
	5 wt.%	.253 ± 0.007	0.241	0.261
	7 wt.%	.284 ± 0.003	0.280	0.291
Flexural strength	Control	113.96 ± 2.69	110	118	918.586	0.000
	1 wt.%	135.13 ± 1.49	132	137
	3 wt.%	133.17 ± 2.10	130	136
	5 wt.%	90.45 ± 3.09	86	95
	7 wt.%	82.83 ± 1.34	80.30	85

Flexural strength results are shown in Figure 4 (B) and Table 1. The 1 wt.% group exhibited the highest mean flexural strength (82.825 MPa), while the 7 wt.% group exhibited the lowest (135.03 MPa). The Tukey’s HSD posthoc test revealed significant differences (p < 0.001) between the control group and all other groups (1, 3, 5, and 7 wt.%) and no statistically significant difference between the 1 wt.% and 3 wt.% groups (p = 0.357).

Discussion

This investigation evaluated the impact of incorporating YSZ NPs into denture base resin on its thermal conductivity and flexural strength. Statistical analysis revealed a significant (p < 0.05) deviation in these properties between the YSZ NP-modified resin and the control group, except flexural strength at higher nanoparticle concentrations (5 and 7 wt.%).

Previous studies have demonstrated a concerted effort to enhance the properties of denture materials.^24–26 However, a significant limitation of conventional acrylic dentures remains their inherently low thermal conductivity.¹⁵ While research has investigated the impact of reinforcement materials on various denture properties, their influence on thermal behavior has received limited attention.

Incorporating YSZ nanoparticles yielded a substantial improvement in thermal conductivity, which is consistent with previous research. Wu et al.²⁷ demonstrated similar enhancements using carbon-based thermally conductive fillers within a polymer matrix. This observation is further supported by Al-Judy et al.’s¹⁶ work on silicon carbide nanoparticle reinforcement in PMMA acrylic denture base resin revealed a concentration-dependent increase in thermal conductivity—furthermore, studies by Abd Alwahab et al.²⁸ and Ghafari et al.¹⁵ We have also reported enhanced thermal conductivity in PMMA denture base materials incorporating various nanoparticles, including ZnO, ZrO₂, and silver.

This enhancement could be attributed to the nanoparticles acting as effective thermal conduits within the polymer matrix. Given the polymer’s inherently low thermal conductivity, incorporating these nanoparticles bridges the intermolecular gaps, facilitating more efficient heat transfer.²⁸

The incorporation of YSZ NPs into 3D-printed denture resin resulted in a significant enhancement in flexural strength compared to the control group (p < 0.05). Indeed, the modified groups exceeded the ISO 1567.²⁹ The requirement of 65 MPa for flexural strength potentially broadens their clinical applicability.

A 1 wt.% YSZ nanoparticle concentration yielded the most significant property enhancement, consistent with Chen et al.’s findings on TiO₂ and PEEK nanoparticles in 3D-printed DBMs.³⁰ However, increasing the YSZ NPs concentration beyond this point led to decreased flexural strength; this trend aligns with the findings reported by Gad et al. for SiO₂ nanoparticle reinforcement in 3D-printed resin, where flexural strength initially increased at a 0.5% concentration but subsequently diminished with higher nanoparticle loadings.⁶ This study’s findings contrast with those of Cevik et al.,⁹ who reported increased mechanical properties in acrylic resin with higher filler concentrations.

The observed improvements in flexural strength may be attributed to several factors, including printing orientation and layer thickness. The 0° orientation employed in this study demonstrated superior flexural strength compared to 45° and 90° orientations, consistent with previous reports.^31–33 Furthermore, the 50-µm layer thickness utilized herein correlates with increased flexural strength, corroborating Perea-Lowery et al.’s.⁵ They observed higher strength at 50 µm compared to 100 µm and reported a trend of increasing strength with decreasing layer thickness.

Post-curing also significantly enhances flexural strength. Specifically, post-curing under an inert nitrogen atmosphere mitigates the detrimental effects of the oxygen inhibition layer.⁵ Additionally, the curing unit temperature, a factor known to influence strength directly, contributes to the observed enhancements.³⁴

The improvement in flexural strength observed across the experimental groups could also be attributed to the incorporated nanoparticles’ multiple reinforcing mechanisms. These nanoparticles likely act as fillers, mitigating micro-void formation and hindering crack initiation within the resin matrix. Furthermore, their uniform dispersion facilitates interaction with the polymer chains, restricting chain mobility and enhancing the material’s flexural strength.³⁵

FE-SEM analysis revealed a critical observation: the group incorporating one wt.% YSZ NPs exhibited a more homogenous dispersion within the polymer matrix than groups with higher NP concentrations. This finding suggests that achieving a uniform distribution of nanoparticles within the resin matrix may be crucial for optimizing the flexural strength of 3D-printed denture composites, corroborating previous observations.^4,18

This study uniquely investigates YSZ nanoparticle reinforcement in 3D-printed PMMA denture base resin, optimizing concentration to enhance thermal conductivity for patient comfort and improve flexural strength for durability—a previously unexplored balance in this context. Our findings reveal a novel relationship between YSZ concentration and these combined properties, informing the development of improved 3D-printed dentures.

Further research, encompassing both in vitro and in vivo investigations, is essential to ensure the safety and efficacy of YSZ NP-modified resins before widespread clinical application. Of particular concern is the potential for cytotoxicity arising from the leaching of nanofillers from the material, which could adversely affect surrounding tissues. The current research focused on a specific 3D-printed denture resin, a single YSZ nanoparticle variant, and a particular printing orientation. Future research endeavors should expand upon these findings by examining the effects of YSZ nanoparticles on a broader spectrum of denture base resins to gain a more comprehensive understanding. Furthermore, evaluating the influence of various nanoparticle types, concentrations, and printing orientations on these composites’ mechanical and thermal properties would be highly valuable.

Clinical significance

This study suggests that incorporating YSZ nanoparticles into denture base resin presents several potential clinical benefits. These include improved patient comfort through enhanced thermal conductivity, subsequent heat dissipation, and increased flexural strength for improved durability and fracture resistance. Specifically, incorporating YSZ nanoparticles into the palatal region of dentures may offer a clinically relevant strategy for enhancing thermal conductivity without sacrificing strength or adding weight.

Conclusions

The following conclusions are drawn within the scope of this study and acknowledging limitations related to long-term biocompatibility and cost-effectiveness. Incorporating YSZ nanoparticles into 3D-printed denture base material demonstrably influenced its thermal and mechanical properties. Specifically, thermal conductivity increased consistently with increasing YSZ NP content. Conversely, the flexural strength response was more complex. A statistically significant increase in flexural strength was observed at a one wt.% YSZ NPs concentration relative to the control. However, subsequent increases in YSZ NP concentration (3 wt.%, 5 wt.%, and 7 wt.%) resulted in a progressive reduction in flexural strength, highlighting the critical importance of optimizing YSZ NP concentration to achieve desired material properties.

Footnotes

Acknowledgments

The authors appreciate Ibn Sina University of Medical and Pharmaceutical Sciences (Ibn Sina University for Medical and Pharmaceutical Sciences) and the University of Baghdad () for their encouragement in the current work.

ORCID iD

Hadeel Fikrat Majeed

Statements and declarations

Author contributions

Conceptualization, H.F.M and T.I.H; methodology, H.F.M and T.I.H; software, H.F.M; validation, T.I.H; formal analysis, H.F.M; investigation, H.F.M; data curation, T.I.H; writing—original draft preparation, H.F.M; writing—review and editing, T.I.H; visualization, T.I.H; project administration, H.F.M. All listed authors have confirmed the authorship of this manuscript.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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