Sintering Process Optimization for 3YSZ Ceramic 3D-Printed Objects Manufactured by Stereolithography

The physical-mechanical properties of 3D-printed versus conventional milled zirconia for dental clinical applications: A systematic review with meta-analysis

AIM OF STUDY

This systematic review aimed to compare the physical-mechanical properties of 3D-printed (additively manufactured (AM)) zirconia compared to conventionally milled (subtractive manufactured: SM) zirconia specimens.

MATERIALS AND METHODS

A thorough search of Internet databases was conducted up to September 2023. The search retrieved studies that evaluated AM zirconia specimens and restorations regarding the physical-mechanical properties and mechanical behavior of zirconia. The main topic focused on 3Y-TZP. However, records of 4YSZ and 5YSZ were also included to gather more comprehensive evidence on additively manufactured zirconia ceramic. The quality of studies was assessed using the ROB2 tool, Newcastle Ottawa scale, and the Modified Consort Statement. Of 1736 records, 57 were assessed for eligibility, and 38 records were included in this review, only two clinical trials meet the inclusion criteria and 36 records were laboratory studies. There were no signs of mechanical complications and wear to antagonists with short-term clinical observation. SM thin specimens ≤1.5 mm showed statistically significant higher flexural strength than AM zirconia (p ≤ 0.01), while thicker specimens showed comparable outcomes (p > 0.5). The fracture resistance of dental restorations was dependent on the aging protocol, restoration type, and thickness. The bond strength of veneering ceramic to zirconia core was comparable.

CONCLUSIONS

The results pooled from two short-term clinical trials showed no signs of mechanical or biological complications of additively manufactured 3Y-TZP zirconia crowns. The flexural strength might depend on the specimens' thickness, but it showed promising results to be used in clinical applications, taking into account the printing technique and orientation, material composition (yttria content), solid loading, and sintering parameters. 3D-printed restorations fracture resistance improved when adhered to human teeth. The veneering ceramic bond was comparable to milled zirconia specimens. Long-term RCTs are recommended to confirm the mechanical behavior of 3D-printed restorations.

3D-printed versus conventionally milled zirconia for dental clinical applications: Trueness, precision, accuracy, biological and esthetic aspects

OBJECTIVES

This systematic review aimed to compare the clinical outcome, internal gap, trueness, precision, and biocompatibility of 3D-printed (AM) compared to milled (SM) zirconia restorations.

DATA SOURCE

A thorough search of Internet databases was conducted up to September 2023. The search retrieved studies compared AM zirconia to SM zirconia restorations regarding clinical outcome, fit, trueness, precision, and biocompatibility.

STUDY SELECTION

Of 1736 records, only 59 were screened for eligibility, and 22 records were included in this review. The quality of studies was assessed using the revised Cochrane risk-of-bias tool (ROB2), and the Modified Consort Statement. One clinical study exhibited a low risk of bias. All laboratory studies revealed some bias concerns. Short-term observation showed 100 % survival with no signs of periodontal complications. 3D-printed zirconia crowns showed statistically significant lower ΔE and a better match to adjacent teeth (p ≤ 0.5). The fit, trueness, and precision vary with the printing technique and the tooth surface.

CONCLUSIONS

3D-printed zirconia crowns provide better aesthetic color and contour match to adjacent natural teeth than milled crowns. Both 3D printing and milling result in crowns within the clinically acceptable internal and marginal fit. Except for nanoparticle jetting, the marginal gap of SM crowns was smaller than AM crowns, however, both were clinically acceptable. Laminate veneers might be more accurately produced by 3D printing. 3D-printed axial surface trueness was better than milled axial surfaces. Long-term RCTs are recommended to confirm the clinical applicability of 3D-printed restorations.

CLINICAL SIGNIFICANCE

Internal fit and gap, precision, and trueness are fundamental requirements for successful dental restorations. Both techniques produce restorations with clinically acceptable marginal and internal fit. Axial surfaces and narrow or constricted areas favored 3D-printed than conventionally milled zirconia.

Hydroxyapatite-Based Coatings on Silicon Wafers and Printed Zirconia

Dental surgery needs a biocompatible implant design that can ensure both osseointegration and soft tissue integration. This study aims to investigate the behavior of a hydroxyapatite-based coating, specifically designed to be deposited onto a zirconia substrate that was intentionally made porous through additive manufacturing for the purpose of reducing the cost of material. Layers were made via sol-gel dip coating by immersing the porous substrates into solutions of hydroxyapatite that were mixed with polyethyleneimine to improve the adhesion of hydroxyapatite to the substrate. The microstructure was determined by using X-ray diffraction, which showed the adhesion of hydroxyapatite; and atomic force microscopy was used to highlight the homogeneity of the coating repartition. Thermogravimetric analysis, differential scanning calorimetry, and Fourier transform infrared spectroscopy showed successful, selective removal of the polymer and a preserved hydroxyapatite coating. Finally, scanning electron microscopy pictures of the printed zirconia ceramics, which were obtained through the digital light processing additive manufacturing method, revealed that the mixed coating leads to a thicker, more uniform layer in comparison with a pure hydroxyapatite coating. Therefore, homogeneous coatings can be added to porous zirconia by combining polyethyleneimine with hydroxyapatite. This result has implications for improving global access to dental care.

The Effect of Sintering on Zirconia Manufactured via Suspension-Enclosing Projection Stereolithography for Dental Applications: An In Vitro Study

BACKGROUND

Zirconia is a widely used material in the dental industry due to its excellent mechanical and aesthetic properties. Recently, a new 3D printing process called suspension-enclosing projection stereolithography (SEPS) was introduced to fabricate zirconia dental restorations. However, the effect of the sintering time and temperature on the properties of zirconia produced via SEPS has not been fully investigated.

METHODS

Zirconia slurries were prepared with varying percentages of zirconia powders and 3D printing resins, and 5Y-TZP (5 mol% yttria-stabilized zirconia) (n = 40) and 3Y-TZP (3 mol% yttria-stabilized zirconia) (n = 40) bar specimens were fabricated via SEPS manufacturing. The specimens were sintered at different temperatures and dwell times, and their flexural strength, density, and phase composition were measured. The viscosity of the slurries was also measured. Statistical analysis was performed using Welch's ANOVA and Kruskal-Wallis tests to evaluate the impact of the sintering conditions.

RESULTS

Significant differences in flexural strength (p < 0.01) were observed between the 5Y-TZP samples, with those sintered at 1530 °C for 120 min showing an average strength of 268.34 ± 44.66 MPa, compared to 174.16 ± 42.29 MPa for those sintered at 1450 °C for 120 min. In terms of density, significant differences (p < 0.01) were noted for the 3Y-TZP specimens, with an average density of 6.66 ± 0.49 g/cm3 for samples sintered at 1530 °C for 120 min, versus 5.75 ± 0.55 g/cm3 for those sintered at 1530 °C for 10 min. X-ray diffraction confirmed the presence of a predominantly tetragonal phase in both materials.

CONCLUSIONS

Zirconia printed via SEPS manufacturing can be sintered at a higher temperature with shorter dwell times, thereby producing high density samples. Different sintering conditions can be used to fully sinter 3D-printed zirconia for potential dental applications.

Bilayer osteochondral graft in rabbit xenogeneic transplantation model comprising sintered 3D-printed bioceramic and human adipose-derived stem cells laden biohydrogel

Reconstruction of severe osteochondral defects in articular cartilage and subchondral trabecular bone remains a challenging problem. The well-integrated bilayer osteochondral graft design expects to be guided the chondrogenic and osteogenic differentiation for stem cells and provides a promising solution for osteochondral tissue repair in this study. The subchondral bone scaffold approach is based on the developed finer and denser 3D β-tricalcium phosphate (β-TCP) bioceramic scaffold process, which is made using a digital light processing (DLP) technology and the novel photocurable negative thermo-responsive (NTR) bioceramic slurry. Then, the concave-top disc sintered 3D-printed bioceramic incorporates the human adipose-derived stem cells (hADSCs) laden photo-cured hybrid biohydrogel (HG + 0.5AFnSi) comprised of hyaluronic acid methacryloyl (HAMA), gelatin methacryloyl (GelMA), and 0.5% (w/v) acrylate-functionalized nano-silica (AFnSi) crosslinker. The 3D β-TCP bioceramic compartment is used to provide essential mechanical support for cartilage regeneration in the long term and slow biodegradation. However, the apparent density and compressive strength of the 3D β-TCP bioceramics can be obtained for ~ 94.8% theoretical density and 11.38 ± 1.72 MPa, respectively. In addition, the in vivo results demonstrated that the hADSC + HG + 0.5AFnSi/3D β-TCP of the bilayer osteochondral graft showed a much better osteochondral defect repair outcome in a rabbit model. The other word, the subchondral bone scaffold of 3D β-TCP bioceramic could accelerate the bone formation and integration with the adjacent host cancellous tissue at 12 weeks after surgery. And then, a thicker cartilage layer with a smooth surface and uniformly aligned chondrocytes were observed by providing enough steady mechanical support of the 3D β-TCP bioceramic scaffold.

Manufacturing and characterization of a 3D printed lithium disilicate ceramic via robocasting: A pilot study

OBJECTIVE

The objective of this study was to manufacture and to evaluate the physico-mechanical properties of the Lithium disilicate (Li2O5Si2) ceramic structures fabricated using additive manufacturing (3D printing).

METHODS

Li2O5Si2 samples were divided into (n = 30/group): SM (subtractively manufactured) and AR (additive/robocasting). For the AR group, Li2O5Si2 powder was combined with ammonium polyacrylate, hydroxypropyl methylcellulose, and polyelectrolyte to create a colloidal gel, which was then used for printing. A digital CAD model of a disc was designed, and the G-code transferred to a custom built DIW 3D printer. The control group samples were prepared using pre-crystallized ceramic blocks, which were cut to obtain discs with same dimensions as the AR group. Disc-shaped specimens from both groups were crystallized at 840 °C. Mechanical properties were evaluated using biaxial flexural strength test (BFS) and Vickers hardness test. Representative fractographic images of the specimens were acquired using scanning electron microscopy (SEM) to analyze the fracture origin and crack propagation. Energy-dispersive X-ray spectroscopy (EDS) and attenuated total reflection Fourier transform infrared spectroscopy (FTIR-ATR) were used for chemical analysis, and X-ray diffractometry (XRD) was performed to analyze the crystalline phases.

RESULTS

AR group yielded lower values of BFS (120.02 MPa ±33.91) and hardness (4.07 GPa ±0.30), relative to the SM group, (325.09 MPa ±63.98) and (5.63 GPa ±0.14), respectively. For EDS analysis, AR and SM groups showed similar elemental composition. In FTIR-ATR analysis, higher peaks referring to the crystalline structure were found for SM group. XRD analysis indicated a decreased formation of Li2O5Si2 from Lithium metasilicate (Li2O-SiO2) in the AM group. SEM micrographs showed a more porous microstructure associated with the 3D printed samples.

SIGNIFICANCE

The viability of fabricating Li2O5Si2 ceramic constructs using the Robocasting technique was successful. However, the samples prepared using subtractive manufacturing presented higher mechanical properties compared to the 3D printed constructs. The difference in properties between the manufacturing may be correlated to the decreased formation of Li2O5Si2 crystals and higher degrees of porosity.

Development of Photo-Polymerization-Type 3D Printer for High-Viscosity Ceramic Resin Using CNN-Based Surface Defect Detection

Due to the high hardness and brittleness of ceramic materials, conventional cutting methods result in poor quality and machining difficulties. Additive manufacturing has also been tried in various ways, but it has many limitations. This study aims to propose a system to monitor surface defects that occur during the printing process based on high-viscosity composite resin that maximizes ceramic powder content in real time using image processing and convolutional neural network (CNN) algorithms. To do so, defects mainly observed on the surface were classified into four types by form: pore, minor, critical, and error, and the effect of each defect on the printed structure was tested. In order to improve the classification efficiency and accuracy of normal and defective states, preprocessing of images obtained based on cropping, dimensionality reduction, and RGB pixel standardization was performed. After training and testing the preprocessed images based on the DenseNet algorithm, a high classification accuracy of 98% was obtained. Additionally, for pore and minor defects, experiments confirmed that the defect surfaces can be improved through the reblading process. Therefore, this study presented a defect detection system as well as a feedback system for process modifications based on classified defects.

Fabrication of 3D Printed Ceramic Part Using Photo-Polymerization Process

Ceramics are high-strength and high-temperature resistant materials that are used in various functional parts. However, due to the high strength and brittleness properties, there are many difficulties in the fabrication of complex shapes. Therefore, there are many studies related to the fabrication of ceramic parts using 3D printing technology optimized for complex shapes. Among them, studies using photo-polymerization (PP) 3D printing technology with excellent dimensional accuracy and surface quality have received the most widespread attention. To secure the physical properties of sintered ceramic, the content and distribution of materials are important. This study suggests a novel 3D printing process based on a high-viscosity composite resin that maximizes the content of zirconia ceramics. For reliable printing, the developed 3D printers that can adjust the process environment were used. To minimize warpage and delamination, the divided micro square pattern images were irradiated in two separate intervals of 1.6 s each while maintaining the internal chamber temperature at 40 °C. This contributed to improved stability and density of the sintered structures. Ultimately, the ceramic parts with a Vickers hardness of 12.2 GPa and a relative density of over 95% were able to be fabricated based on a high-viscosity resin with 25,000 cps.

The Effect of Sintering Temperature on the Phase Composition, Microstructure, and Mechanical Properties of Yttria-Stabilized Zirconia

It is known that the yttria-stabilized zirconia (YSZ) material has superior thermal, mechanical, and electrical properties. This material is used for manufacturing products and components of air heaters, hydrogen reformers, cracking furnaces, fired heaters, etc. This work is aimed at searching for the optimal sintering mode of YSZ ceramics that provides a high crack growth resistance. Beam specimens of ZrO₂ ceramics doped with 6, 7, and 8 mol% Y₂O₃ (hereinafter: 6YSZ, 7YSZ, and 8YSZ) were prepared using a conventional sintering technique. Four sintering temperatures (1450 °C, 1500 °C, 1550 °C, and 1600 °C) were used for the 6YSZ series and two sintering temperatures (1550 °C and 1600 °C) were used for the 7YSZ and 8YSZ series. The series of sintered specimens were ground and polished to reach a good surface quality. Several mechanical tests of the materials were performed, namely, the microhardness test, fracture toughness test by the indentation method, and single-edge notch beam (SENB) test under three-point bending. Based on XRD analysis, the phase balance (percentages of tetragonal, cubic, and monoclinic ZrO₂ phases) of each composition was substantiated. The morphology of the fracture surfaces of specimens after both the fracture toughness tests was studied in relation to the mechanical behavior of the specimens and the microstructure of corresponding materials. SEM-EDX analysis was used for microstructural characterization. It was found that both the yttria percentage and sintering temperature affect the mechanical behavior of the ceramics. Optimal chemical composition and sintering temperature were determined for the studied series of ceramics. The maximum transformation toughening effect was revealed for ZrO₂-6 mol% Y₂O₃ ceramics during indentation. However, in the case of a SENB test, the maximum transformation toughening effect in the crack tip vicinity was found in ZrO₂-7 mol% Y₂O₃ ceramics. The conditions for obtaining YSZ ceramics with high fracture toughness are discussed.

Three-Dimensional Printing of Hydroxyapatite Composites for Biomedical Application

Hydroxyapatite (HA) and HA-based nanocomposites have been recognized as ideal biomaterials in hard tissue engineering because of their compositional similarity to bioapatite. However, the traditional HA-based nanocomposites fabrication techniques still limit the utilization of HA in bone, cartilage, dental, applications, and other fields. In recent years, three-dimensional (3D) printing has been shown to provide a fast, precise, controllable, and scalable fabrication approach for the synthesis of HA-based scaffolds. This review therefore explores available 3D printing technologies for the preparation of porous HA-based nanocomposites. In the present review, different 3D printed HA-based scaffolds composited with natural polymers and/or synthetic polymers are discussed. Furthermore, the desired properties of HA-based composites via 3D printing such as porosity, mechanical properties, biodegradability, and antibacterial properties are extensively explored. Lastly, the applications and the next generation of HA-based nanocomposites for tissue engineering are discussed.