1. Introduction
Polyetheretherketone (PEEK) is a high-performance semi-crystalline polymer with excellent biocompatibility and great processability [
1]. PEEK possesses great potential as oral prosthetic materials given its lightweight and lower modulus (3–4 GPa), which makes it a suitable alternative for conventional Co-Cr alloy (230 GPa) and Ti (104 GPa) [
2]. In recent decades, PEEK has been widely used to fabricate crowns and frameworks for fixed and removable prostheses by using injection molding, milling, and 3D printing [
3,
4]. Despite the disadvantage of comprised esthetics, PEEK has great potential for further modification of various properties [
5,
6,
7,
8]. Much research has been performed to investigate the modified PEEK materials for enhanced performance [
9]. By using compounding and injection molding [
10], Ma et al. reported preparation of HA/PEEK composites and the enhanced osteogenesis acquired. Han et al. investigated the carbon fiber reinforced PEEK (CFR-PEEK) composite fabricated by fused deposition modeling (FDM), and CFR-PEEK revealed better mechanical strengths than the printed pure PEEK [
8]. Another research introduced electrostatically bonded PEEK composites with increased mechanical properties and osseointegration reported [
11].
3D printing has a great ability to process thermoplastics such as PEEK, with high production efficiency and low material waste compared to the traditional and subtractive techniques [
12]. Recent studies also proposed the dual-nozzle printing technology for 3D printing of different materials [
13], which could provide technical support for dual-color 3D printing of oral prosthetic materials with tooth-like and gingiva-like colors. Currently, the application of 3D printed PEEK is relatively few, and dual-color printing of PEEK has not been reported in dental applications. Several cases have reported the successful use of 3D printed PEEK to fabricate implants, artificial ribs, and removable prosthesis frameworks [
12,
14,
15,
16]. The limitation is that the 3D printed PEEK has a brown color, which resulted in comprised esthetics and hence affects its wide application [
17]. Chen et al. reported the fabrication of a speech aid prosthesis using titanium dioxide (TiO2)/PEEK framework with enhanced mechanical strength and improved esthetics. Removable dental prostheses consist of tooth-color and gingival-color parts to mimic both hard and soft oral tissues [
18]. The single-color PEEK material, however, cannot imitate the tooth and gingival colors at the same time [
14]. Apart from compromised esthetics, the PEEK framework requires subsequent laboratory procedures like casting and molding to make the definitive prosthesis. Conventional fabrication procedures can be time-consuming and technology demanding. Moreover, the binding interface between different parts cannot be eliminated compared to that of one-piece printing by a double nozzle.
Therefore, this study serves as a proof-of-concept and presents the preparation, dual-nozzle printing, and performance evaluation of the dual-color PEEK composites. A series of dual-color PEEK filaments were developed by incorporating various content of titanium dioxide (TiO
2) and ferric oxide (Fe
2O
3) into PEEK to alter the brown color to white and pink [
14]. Then, dual-color PEEK specimens were printed using a custom dual-nozzle printer (Surgeon Pro; Shaanxi Jugao-AM Technology Co., Ltd., Xi’an, China) with optimized printing parameters [
19,
20,
21]. Furthermore, duotone PEEK specimens with different interfacial orientations were printed using dual-color PEEK filaments by a double nozzle [
13]. All specimens were evaluated by observation and instrumental color measurements, and the color difference was calculated according to the classical CIE76 formula [
22]. In addition, mechanical performance of the standard test specimens was investigated using a mechanical testing machine (MTS). Their tensile and flexural properties were analyzed and compared with the frequently used dental polymethylmethacrylate (PMMA). In addition, duotone specimens with different interfacial orientations were compared based on their mechanical performance.
4. Discussion
Removable dental prostheses restore hard and soft tissues and could consist of tooth-color and gingiva-color parts to ensure both function and esthetics [
2,
14]. Currently, only a few commercial PEEK materials are available for molding and milling, which could not meet the need for esthetical dental restoration as well as 3D dental printing [
12]. 3D printing is a kind of rapid formation (RP) technology, and it allows a customized optimization of parameters, which can be essential for the dental industries [
2]. PEEK, as a thermoplastic biopolymer, possesses great thermal properties and biocompatibility, which could be suitable for 3D dental printing [
12]. This study presented here is an early attempt to develop dual-color PEEK filaments for fabricating dental prostheses that consists of tooth-color and gingiva-color parts. In this preliminary study, functional fillers (nano TiO
2 and Fe
2O
3) are incorporated into pure PEEK to change the brown color of 3DP PEEK to tooth-like and gingiva-like colors through Fused Filament Fabrication (FFF) [
9]. Based on the filaments, duotone PEEK specimens have been successfully printed using dual-nozzle printing technology, which could provide technical support for future dual-color dental printing. One-piece fabrication can eliminate the interface between different parts and offers great efficiency and more comfort for patients compared to traditional procedures. Li et al. reported the one-piece fabrication of removable partial dentures using PEEK by milling, which showed satisfying fits [
25]. This study indicates the promising application of one-piece printing using dual-color PEEK, which reduces material waste and provides improved esthetics compared to the one-piece milling. However, long-term data for the dual-color PEEK are not yet available, and continued observation is necessary to further verify the clinical outcomes. Moreover, the content of fillers and printing parameters can be flexibly adjusted and thus the properties of the printed prostheses can be tailored by further studies. The results of the color evaluation revealed that the novel PEEK composites developed in this study were closer to dental shade guides compared to pure 3DP PEEK, which provides improved esthetics for dental application. This proof of concept showed that color modification of 3DP PEEK by blending and FFF can be effective [
8,
9,
20], although the colors obtained in this study are still limited compared to the dental shade guides. More research is required to provide more color options for 3DP PEEK with greater variety and different content of fillers and further improve the esthetics in the future.
A series of PEEK specimens with different colors have been successfully 3D printed with optimized parameters, which showed that these novel PEEK composites developed in this study are printable. Literature has reported a range of parameters for printing pure PEEK [
19], and the best prints were obtained with a typical nozzle diameter of 0.4 mm and a layer thickness of 0.3 mm. Regarding the printing of other composite materials, a larger nozzle diameter (1 mm) was reported to ensure the flow rate from the nozzle [
26]. Considering the values used in the literature, the parameters were selected and optimized in this study. The nozzle diameter was set at 0.4 mm to ensure the quality of prints, and the flow from the nozzle was unobstructed with the nanofillers used in this study [
20,
27]. The layer thickness varied from 0.1–0.2 mm to improve printing precision and reduce the void formation between layers [
20]. In addition, a nozzle temperature of 420 °C, as well as a printing speed of 40 mm/s, was used considering the viscosity of the materials and the influence on the strength of prints [
28]. The key parameters employed in this study can be further investigated to optimize the printing process.
Apart from the printability, it is worth noting that the PEEK composites had superior mechanical properties compared to the pure PEEK in literature, which revealed the enhancements obtained with the addition of TiO
2 and Fe
2O
3 fillers in this study. Compared to rigid dental metals, the PEEK composites revealed closer tensile and flexural moduli to that of dentin, which exhibited great potential for dental application. The tensile strength reached around 90 MPa with incorporation of Fe
2O
3, higher than that of groups with TiO
2 addition only. The incorporation of the fillers also increased the flexural strength, which reached above 160 MPa. The composites with higher content of fillers (5–20 wt.% TiO
2) generally showed a higher modulus, and the highest tensile modulus of 4.75 Gpa and highest flexural modulus of 5.74 Gpa were obtained at 20 wt.%. As reported in the literature, the best mechanical performance was also reached at 20 wt.% when incorporating calcium sulfate into PEEK [
29]. Other studies suggested that PEEK/hydroxyapatite composites could be enhanced at 15–30 wt.%, and moving above 20–30 wt.% could result in decreased performance and poorer prints considering the viscosity of the composites [
9,
30]. Therefore, the content of fillers in this study was designed to range around 5–20 wt.% under these considerations for appropriate printing process and performance. In addition, duotone specimens with a horizontal interfacial orientation generally revealed better mechanical performance compared to that with a vertical interfacial orientation. More research is required to investigate higher incorporation levels of fillers and further optimize the printing process.
5. Conclusions
In this preliminary study, functional fillers were incorporated into the pure PEEK to improve its esthetics for 3D dental printing. The color and mechanical performance were investigated through color evaluation and mechanical tests. The conclusions are summarized as follows:
(1) With addition of nano TiO2 and/or Fe2O3, white and pink PEEK filaments were developed to imitate tooth and gingiva colors. Through visual evaluation and color measurements, the color differences between the developed 3DP PEEK composites and dental shade guides were smaller compared to the pure 3DP PEEK.
(2) The tensile and flexural performance of 3DP PEEK composites was generally better than that of dental PMMA. 3DP PEEK composites had tensile and flexural moduli close to that of dentin, which exhibited great potential for dental application.
(3) Duotone PEEK specimens were printed with G1 and D1 PEEK filaments by double nozzle simultaneously. The preliminary experiments are encouraging for application in dental prostheses that consist of tooth-color and gingiva-color parts. The interfacial orientations had a significant influence on the mechanical performance of duotone prints, and duotone specimens with a horizontal interfacial orientation generally revealed better mechanical performance compared to that with a vertical interfacial orientation.
(4) 3DP PEEK composites exhibited great potential for modification and for future application in dentistry.