Preparation and Performance Evaluation of Duotone 3D-Printed Polyetheretherketone As Oral Prosthetic Materials: a Proof-Of-Concept Study

polymers

Article Preparation and Performance Evaluation of Duotone 3D-Printed Polyetheretherketone as Oral Prosthetic Materials: A Proof-of-Concept Study

Ling Ding 1,†, Wei Lu 1,†, Jiaqi Zhang 1, Chuncheng Yang 2 and Guofeng Wu 1,3,*

1 Department of Prosthodontics, Nanjing Stomatological Hospital, Medical School of Nanjing University, Nanjing 210000, China; [email protected] (L.D.); [email protected] (W.L.); [email protected] (J.Z.) 2 State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710000, China; [email protected] 3 Digital Engineering Center of Stomatology and Department of Prosthodontics, Nanjing Stomatological Hospital, Medical School of Nanjing University, Nanjing 210000, China * Correspondence: [email protected] † L.D. and W.L. contributed equally to the research and should be regarded as joint ﬁrst authors.

Abstract: Literature has reported the successful use of 3D printed polyetheretherketone (PEEK) to fabricate human body implants and oral prostheses. However, the current 3D printed PEEK (brown color) cannot mimic the vivid color of oral tissues and thus cannot meet the esthetical need for dental application. Therefore, titanium dioxide (TiO ) and ferric oxide (Fe O ) were incorporated into PEEK 2 2 3 to prepare a series of tooth-color and gingival-color PEEK composites in this study. Through color measurements and mechanical tests, the color value and mechanical performance of the 3D printed Citation: Ding, L.; Lu, W.; Zhang, J.; PEEK composites were evaluated. In addition, duotone PEEK specimens were printed by a double Yang, C.; Wu, G. Preparation and Performance Evaluation of Duotone nozzle with an interface between tooth-color and gingival-color parts. The mechanical performance 3D-Printed Polyetheretherketone as of duotone PEEK with two different interfaces (horizontal and vertical) was investigated. With the Oral Prosthetic Materials: A addition of TiO2 and Fe2O3, the colors of 3D printed PEEK composites become closer to that of dental Proof-of-Concept Study. Polymers shade guides. 3D printed PEEK composites generally demonstrated superior tensile and flexural 2021, 13, 1949. https://doi.org/ properties and hence have great potential in the dental application. In addition, duotone 3D printed 10.3390/polym13121949 PEEK with a horizontal interfacial orientation presented better mechanical performance than that with a vertical one. Academic Editors: Meng-Yi Bai and Dimitrios Bikiaris Keywords: polyetheretherketone (PEEK); mechanical performance; color measurement; dual-color 3D printing; prosthodontics Received: 28 April 2021 Accepted: 8 June 2021 Published: 11 June 2021 1. Introduction Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Polyetheretherketone (PEEK) is a high-performance semi-crystalline polymer with published maps and institutional affil- excellent biocompatibility and great processability [1]. PEEK possesses great potential iations. 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].

Copyright: © 2021 by the authors. Despite the disadvantage of comprised esthetics, PEEK has great potential for further Licensee MDPI, Basel, Switzerland. modification of various properties [5–8]. Much research has been performed to investigate This article is an open access article the modified PEEK materials for enhanced performance [9]. By using compounding and distributed under the terms and injection molding [10], Ma et al. reported preparation of HA/PEEK composites and the conditions of the Creative Commons enhanced osteogenesis acquired. Han et al. investigated the carbon fiber reinforced PEEK Attribution (CC BY) license (https:// (CFR-PEEK) composite fabricated by fused deposition modeling (FDM), and CFR-PEEK creativecommons.org/licenses/by/ revealed better mechanical strengths than the printed pure PEEK [8]. Another research 4.0/).

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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 [12,14–16]. 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 (TiO2) and ferric oxide (Fe2O3) 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 [19–21]. 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.

2. Materials and Methods 2.1. Materials Preparation The PEEK powder (VICTREX, Lancashire, UK) was mixed with nano-titanium dioxide (TiO2) and ferric oxide (Fe2O3) (YIPIN Bio-Tech Co., Ltd., Ningbo, China) in this study to prepare a series of white and pink PEEK composites which imitate the colors of tooth and gingiva, respectively. The PEEK composites were mixed (mix proportions shown in Table 1) by a V-type mixer at 50 rpm for 2 min and dried in an oven at 120 ◦C for 3 h before use.

Table 1. The mix proportions of PEEK composites.

Description PEEK (wt.%) TiO2 (wt.%) Fe2O3 (wt.%) PEEK-D1 80% 20% 0% PEEK-D2 90% 10% 0% PEEK-D3 95% 5% 0% PEEK-G1 79% 20% 1% PEEK-G2 89% 10% 1% PEEK-G3 99% 0% 1% Polymers 2021, 13, 1949 3 of 13

The mixtures were then processed separately by a twin-screw extruder (YTG-20, Shannxi Jugao-AM Technology Co., Ltd., Xi’an, China) to produce continuous filaments. The filaments (1.7 mm diameter) were cooled to 50 ◦C and rolled onto a reel throughout the extrusion process. Subsequently, all specimens were printed using the white (D1, D2, D3) and pink filaments (G1, G2, G3), and the printing parameters were optimized (Table2).

Table 2. Dual-nozzle 3D printing parameters.

Description Value Nozzle diameter 0.4 mm Nozzle temperature 420 ◦C Nozzle pitch 87.5 mm Bead width 0.36 mm Layer thickness 0.1–0.2 mm Printing speed 40 mm/s Raster angle ±45◦ Ambient temperature RT z-axis layer 0.2 mm Inﬁll percentage 100%

2.2. Color Evaluation A Chroma Meter (TS7X, 3nh, Shenzhen, China) with the CIELAB color system was used to evaluate the color of the 3D-printed PEEK. The CIELAB system is three-dimensional, where a* axis is relative to the green (−) to red (+) opponent colors, b* axis represents the blue (−) to yellow (+) opponents, and L* axis measures relative white (100) to black (0) color. The disc-shaped specimens (5 per group) were printed using the prepared ﬁlaments separately with a diameter of 15 mm and a thickness of 3 mm. Pure PEEK specimens were printed using Jugao-MT45 PEEK ﬁlaments from the same company. All specimens were polished by hand with 1500, 2400, and 3000 grit sandpapers to smoothen the surface (STARCKE, Melle, Germany). The color measurements were performed with a white as well as a black background and repeated for each sample to measure the L, a*, and b* values (Table1, Table2). The white PEEK specimens (D1, D2, D3) were compared to the VITA Classical shade guide (Vita Zahnfabrik, Bad Säckingen, Germany) and pink specimens (G1, G2, G3) were compared to the Shofu gingiva shade guides (Shofu Dental Corp., Fukuoka, Japan) and IPS ceramic gingiva shade guide (Ivoclar Vivadent, Schaan, Liechtenstein) (Figure1). The color difference ( ∆E*) was observed and calculated according to the classical CIE76 formula [22]:

1 ∆E* = [(∆L*2) + (∆a*2) + (∆b*2)] (1) 2

2.3. Mechanical Evaluation Mechanical properties of the 3D-printed PEEK were evaluated by tensile and ﬂexural tests using a testing machine (EXCEED E44, MTS, Eden Prairie, MN, USA) following the manufacturer’s instruction. The specimens were designed using 3D computer-aided design (CAD) software (Dassault Systèmes SOLIDWORKS Corp., Waltham, MA, USA) according to ISO 527-2:2012 and ISO 604:2002 (Table3)[ 23,24]. Standard specimens were printed using each group of the PEEK ﬁlaments separately (Group D1–D3, G1–G3).

Table 3. Mechanical tests and standard specimens.

Tests Size (mm) ISO Tensile test 90.00 × 10.00 × 4.00 ISO 527-2:2012 [23] Flexural test 80.00 × 10.00 × 4.00 ISO 178:2019 [24] Polymers 2021, 13, x FOR PEER REVIEW 4 of 13

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Figure 1. Dental shade guides. (a) VITA classical shade guide (Germany); (b) IPS e.max Ceram Gin- giva Shade (Lichtenstein); (c) Shofu gingival shade guide (Japan).

2.3. Mechanical Evaluation Mechanical properties of the 3D-printed PEEK were evaluated by tensile and flexural tests using a testing machine (EXCEED E44, MTS, Eden Prairie, MN, USA) following the manufacturer's instruction. The specimens were designed using 3D computer-aided design (CAD) software (Dassault Systèmes SOLIDWORKS Corp., Waltham, MA, USA) according to ISO 527-2:2012 and ISO 604:2002 (Table 3) [23,24]. Standard specimens were printed using each group of the PEEK filaments separately (Group D1–D3, G1–G3).

Table 3. Mechanical tests and standard specimens.

Tests Size (mm) ISO TensileFigureFigure tes 1.1.t Dental shade shade guides. guides. 90.00 (×a ()10.00a VITA) VITA × classical 4.00 classical shade shade guide ISOguide (Germany); 527-2:2012 (Germany); ( b[23]) (IPSb ) IPSe.max e.max Ceram Ceram Gin- FlexuralGingivagiva Shadetest Shade (Lichtenstein); (Lichtenstein); 80.00 (c) (Shofuc ×) Shofu10.00 gingival gingival× 4.00 shade shade guide guide (Japan). ISO (Japan). 178:2019 [24] 2.3. Mechanical Evaluation In addition, Induotone addition, specimens duotone specimensconsisting consistingof white and of white pink and parts pink were parts designed were designed with with horizontalhorizontal andMechanical vertical and verticalinterfacial properties interfacial orientations of the 3D-printed orientations (Figure PEEK 2), (Figure and were these2), evaluated and different these by different interfa- tensile and interfacial flexural orientations could affect the tensile and flexural properties of the 3D-printed PEEK. Thus, cial orientationstests could using affect a testing the tensilemachine and (EXCEED flexural E44,properties MTS, Eden of the Prairie, 3D-printed MN, USA)PEEK. following the duotone tensile and flexural specimens (Group XY and Group Z) were printed with PEEK- Thus, duotonemanufacturer's tensile and flexural instruction. specimens The specimens(Group XY wereand Group designed Z) were using printed 3D computer-aided with de- D1 and PEEK-G1 filaments by double nozzle simultaneously [13], which is illustrated in PEEK-D1 andsign PEEK-G1 (CAD) filamentssoftware (Dassaultby double Systèmes nozzle simultaneously SOLIDWORKS [13], Corp., which Waltham, is illus- MA, USA) ac- Figure3. trated in Figurecording 3. to ISO 527-2:2012 and ISO 604:2002 (Table 3) [23,24]. Standard specimens were printed using each group of the PEEK filaments separately (Group D1–D3, G1–G3).

Table 3. Mechanical tests and standard specimens.

Tests Size (mm) ISO Tensile test 90.00 × 10.00 × 4.00 ISO 527-2:2012 [23] Flexural test 80.00 × 10.00 × 4.00 ISO 178:2019 [24]

In addition, duotone specimens consisting of white and pink parts were designed with horizontal and vertical interfacial orientations (Figure 2), and these different interfacial orientations could affect the tensile and flexural properties of the 3D-printed PEEK. Thus, duotone tensile and flexural specimens (Group XY and Group Z) were printed with Figure 2. InterfacialFigure orientations 2. InterfacialPEEK-D1 of orientations duotone and specimens.PEEK-G1 of duotone (filamentsa specimens.) vertical interfaceby (a )double vertical between nozzleinterface white simultaneously between and pink white parts; and [13], ( bpink) horizontalwhich is illus- interface betweenparts; white (b) horizontal andtrated pink parts.interface in Figure between 3. white and pink parts.

Figure 2. Interfacial orientations of duotone specimens. (a) vertical interface between white and pink parts; (b) horizontal interface between white and pink parts.

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FigureFigureFigure 3. 3.3. IllustrationIllustrationIllustration ofof of dual-nozzledual-nozzle dual-nozzle prpr printingintinginting ofof ofthethe the duotoneduotone duotone specimenspecimen specimensss usingusing using whitewhite white andand pink andpink pinkpoly-polyetheretherketone (PEEK) filaments, simultaneously. polyetheretherketoneetheretherketone (PEEK) (PEEK) filaments, ﬁlaments, simultaneously. simultaneously.

TheTheThe specimens specimensspecimens were werewere polished polishedpolished and andand dried drieddried prior priorprior to toto testing, testing,testing, and andand 5 55 samples samplessamples were werewere selected selectedselected forforfor each eacheach groupgroupgroup (Group (Group D1–D3, D1–D3, G1–G3,G1–G3, G1–G3, XY,XY, XY, andand and Z).Z). Z). TensileTensile Tensile teststests tests werewere were performedperformed performed usingusing using anan anMTSMTS MTS testingtesting testing machinemachine machine accordingaccording according toto ISO toISO ISO 527-2:1993.527-2:1993. 527-2:1993. Dumb-bellDumb-bell Dumb-bell specimensspecimens specimens withwith with 9090 mmmm 90 mm testtest testlengthlength length andand and 44 mmmm 4 mm thicknessthickness thickness werewere were tested,tested, tested, andand and thethe the spanspan span waswas was 60 60 mm.mm. FlexuralFlexuralFlexural teststeststests were werewere performedperformedperformed using usingusing an anan MTS MTSMTS testing testingtesting machine machinemachine according accordingaccording to ISO toto ISO 604:2002.ISO 604:2002.604:2002. Rectangular RectangularRectangular specimens speci-speci- withmensmens 80 withwith mm 8080 test mmmm length testtest lengthlength and 4 mmandand 4 thickness4 mmmm thicknessthickness were tested, werewere tested,tested, and the andand span thethe wasspanspan 69 waswas mm. 6969 Themm.mm. ◦ testsTheThe tests weretests werewere performed performedperformed at 25 atatC 2525 at °C°C constant atat constantconstant speeds speedsspeeds according accordinaccordin to ISOgg toto standards, ISOISO standards,standards, respectively. respec-respec- Figuretively.tively.4 Figure Figureshows 44 the showsshows tensile thethe and tensiletensile flexural andand flexural specimensflexural specimensspecimens from each fromfrom category eacheach categorycategory described describeddescribed in Table inin 4TableTable, including 4,4, includingincluding white flexuralwhitewhite flexuralflexural specimens, specimens,specimens, pink tensile pinkpink tensiletensile specimens, specimens,specimens, and duotone andand duotoneduotone specimens. speci-speci- Figuremens.mens. 5 FigureFigure shows 55 the showsshows test thethe equipment. testtest equipment.equipment. The tensile TheThe tensiletensile and flexural andand flexuralflexural properties propertiesproperties of the of specimensof thethe speci-speci- weremensmens obtained werewere obtainedobtained from the fromfrom stress–strain thethe stress–strainstress–strain curves and curvescurves compared andand comparedcompared with that withwith of PMMA thatthat ofof (HUGE, PMMAPMMA Rizhao,(HUGE,(HUGE, China) Rizhao,Rizhao, by China) molding.China) bybyData molding.molding. for tensile DataData and forfor flexuraltensiletensile andand strength flexuralflexural and strengthstrength modulus andand are reportedmodulusmodulus asareare the reportedreported mean ± asasstandard thethe meanmean deviation ±± standardstandard (n = deviationdeviation 5) and analyzed ((nn == 5)5) andand with analyzedanalyzed one-way withwith ANOVA one-wayone-way for multiple ANOVAANOVA comparisonsforfor multiplemultiple comparisons usingcomparisons statistical usingusing software statisticalstatistical (IBM softwaresoftware SPSS25.0, (IBM(IBM IBM SPSSSPSS Corp, 25.0,25.0, Armonk,IBMIBM Corp,Corp, NY, Armonk,Armonk, USA) (aNY,NY, = 0.05). USA)USA) (a(a == 0.05).0.05).

Figure 4. Standard test specimens. (a) Pink tensile specimens; (b) white flexural specimens; (c) du- FigureFigure 4.4. Standard test test specimens. specimens. (a ()a )Pink Pink tensile tensile specimens; specimens; (b) ( bwhite) white flexural ﬂexural specimens; specimens; (c) du- (c) otone tensile specimens with vertical interfacial orientations; (d) duotone flexural specimens with duotoneotone tensile tensile specimens specimens with with vertical vertical interfacial interfacial orientations; orientations; ( d) duotone ﬂexuralflexural specimensspecimens with with horizontalhorizontal interfacialinterfacial orientations.orientations. horizontal interfacial orientations.

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Table 4. Groups in the mechanical tests and the ﬁlaments used. Table 4. GroupsTable in the 4. mechanical Groups in thetests mechanical and the filaments tests and used. the filaments used. Category Groups Filaments Filaments Category Category Groups GroupsFilaments FilamentsFilaments Filaments Group D1 GroupGroup D1 D1 PEEK-D1 PEEK-D1 PEEK-D1 / / / White Group D2 PEEK-D2 / White White Group D2 GroupGroup D3 D2 PEEK-D2 PEEK-D3 PEEK-D2 / / / Group D3 Group D3 PEEK-D3 PEEK-D3 / / Group G1 / PEEK-G1 Pink Group G1 GroupGroup G2 G1 / / / PEEK-G1 PEEK-G2 PEEK-G1 Pink Pink Group G2 GroupGroup G3 G2 / / / PEEK-G2 PEEK-G3 PEEK-G2 Group G3 GroupGroup XY G3 / PEEK-D1 / PEEK-G3 PEEK-G1 PEEK-G3 Duotone Group XY GroupGroup Z XY PEEK-D1 PEEK-D1 PEEK-D1 PEEK-G1 PEEK-G1 PEEK-G1 Duotone Duotone Group Z Group Z PEEK-D1 PEEK-D1 PEEK-G1 PEEK-G1

Figure 5. MechanicalFigureFigure testing 5. Mechanical 5. Mechanical using an testing MTS testing testing using using anmachine. anMTS MTS testing testing machine. machine.

3. Results 3.3. ResultsResults 3.1. Filament Preparation3.1.3.1. FilamentFilament PreparationPreparation Nano TiO and Fe O are incorporated as functional fillers into pure PEEK by blending, Nano TiO2 andNano Fe2O TiO3 are22 andincorporated Fe2 2O33 are asincorporated functional fillersas functional into pure fillers PEEK into by pure blend- PEEK by blending, and throughanding, Fused throughand through Filament Fused Fused Fabrication Filament Filament Fabrication (FFF), Fabrication a series (FFF), (FFF),of dual-color a series a series of filamentsof dual-color dual-color with filaments filaments a with witha a diameter of 1.7diameterdiameter mm were ofof 1.71.7fabricated. mmmm werewere The fabricated.fabricated. filaments The Theare filamentsdividedfilaments into areare dividedtwodivided categories intointo twotwo with categoriescategories withwith three tooth-likethreethree colors tooth-liketooth-like (PEEK D1-D3) colors (PEEK and thr D1-D3) D1-D3)ee gingiva-like and and thr threeee colors gingiva-like gingiva-like (PEEK G1–G3),colors colors (PEEK (PEEKeach asG1–G3), G1–G3), each each as described in Tableasdescribed described 1. Six in groups inTable Table of1. 1filamentsSix. Six groups groups were of filaments of rolled filaments onto were werethe rolled reels, rolled onto respectively onto the thereels, reels, (Fig- respectively respectively (Fig- ure 6), and ready(Figureure for6), 6 anduse), and inready 3D ready printing.for for use use in 3D in 3D printing. printing.

Figure 6. A seriesFigureFigure of dual-color 6.6. AA seriesseries PEEK ofof dual-colordual-color filaments. PEEKPEEK From filaments.filaments. left to right FromFrom: PEEKleft left to filamentstoright right:: PEEK PEEK with filaments tooth-likefilaments with with tooth-like tooth-like colors (PEEK D1,colorscolors D2, (PEEKD3),(PEEK pure D1,D1, PEEK D2,D2, D3),D3), filaments purepure PEEKPEEK for contrast, filamentsfilaments and forfor PEEK contrast,contrast, filaments andand PEEK PEEK with filaments gingiva-likefilaments with with gingiva-like gingiva-like colors (PEEK G1,colors G2, G3).(PEEK G1, G2, G3). colors (PEEK G1, G2, G3).

3.2. Color Analysis3.2.3.2. Color Color AnalysisAnalysis 3.2.1. Results 3.2.1.of3.2.1. Color ResultsResults Changes ofof ColorColor ChangesChanges

With additionWithWith of TiO additionaddition2, the color ofof TiOTiO of2 3D2,, thethe printed colorcolor of ofPEEK 3D3D printed printedcould be PEEKPEEK altered couldcould from bebe brown alteredaltered to fromfrom brownbrown to to toothlike colorstoothliketoothlike (Figure colors colors7) and (Figure becomes7 7)) andcloserand becomesbecomes to the dental closer shade to the the guide. dental dental With shade shade addition guide. guide. With of With addition addition of TiO2 and/or FeofTiO2O TiO32, and/or2theand/or color Fe Fe2ofO23 ,O3D the3, printed the color color of PEEK of3D 3D printed could printed bePEEK PEEK altered could could from be be brownaltered altered tofrom from pink brown toto pinkpink (Figure 8) and(Figure(Figure become8 )8) andcloser and become become to the closergingiva closer to toshade the the gingiva gingiva guide. shade shade guide. guide.

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2 FigureFigure 7.7.3D 3D printedprinted PEEKPEEK withwith tooth-like tooth-like colors. colors. ( a(a)) pure pure PEEK PEEK for for contrast; contrast; ( b(b)) D1 D1 (5% (5% TiO TiO2/PEEK);/PEEK); Figure 7. 3D printed PEEK with tooth-like colors. (a) pure PEEK for contrast; (b) D1 (5% TiO2/PEEK); (c(c)) D2 D2 (10% (10% TiO TiO2/PEEK);/PEEK); (d) D3 (20% TiO 2/PEEK). (c) D2 (10% TiO22/PEEK); (d) D3 (20% TiO2/PEEK).

Figure 8. 3D printed PEEK with gingiva-like colors. (a) pure PEEK for contrast; (b) G1 (20% TiO2/1% Figure 8. 3D printed PEEK with gingiva-like colors. (a) pure PEEK for contrast; (b) G1 (20% TiO2/1% FigureFe2O3/PEEK); 8. 3D printed (c) G2 PEEK(10% TiO with2/1% gingiva-like Fe2O3/PEEK); colors. (d ()a G3) pure (1% PEEK Fe2O3 for/PEEK). contrast; (b) G1 (20% TiO2/1% Fe2O3/PEEK); (c) G2 (10% TiO2/1% Fe2O3/PEEK); (d) G3 (1% Fe2O3/PEEK). Fe2O3/PEEK); (c) G2 (10% TiO2/1% Fe2O3/PEEK); (d) G3 (1% Fe2O3/PEEK). 3.2.2. Color Coordinates and Color Differences 3.2.2.3.2.2. ColorColor CoordinatesCoordinates andand ColorColor DifferencesDifferences Table 5 shows the color coordinates of white PEEK specimens (D1–D3), VITA A1-A3 TableTable5 5shows shows thethe colorcolor coordinates of wh whiteite PEEK PEEK specimens specimens (D1–D3), (D1–D3), VITA VITA A1-A3 A1- shade, and pure PEEK. Table 6 shows the color coordinates of pink PEEK specimens (G1– A3shade, shade, and and pure pure PEEK. PEEK. Table Table 6 shows6 shows the thecolor color coordinates coordinates of pink of pink PEEK PEEK specimens specimens (G1– G3), gingiva shade guides, and pure PEEK for contrast. The color difference was calcu- (G1–G3),G3), gingiva gingiva shade shade guides, guides, and pure and purePEEK PEEK for contrast. for contrast. The color The colordifference difference was calcu- was lated based on the color coordinates of each category, which were consistent with visual calculatedlated based based on the on color the colorcoordinates coordinates of each of category, each category, which which were consistent were consistent with visual with evaluation. Figure 9 shows the results of the color difference between D1–D3 and VITA visualevaluation. evaluation. FigureFigure 9 shows9 shows the results the results of the of color the colordifference difference between between D1–D3 D1–D3 and VITA and A1, and the ΔE* values varied from 5.87 (D1) to 7.92 (D3) which were smaller than the VITAA1, and A1, the and Δ theE* values∆E* values varied varied from from 5.87 5.87(D1) (D1)to 7.92 to 7.92(D3) (D3) which which were were smaller smaller than than the 17.36 of pure PEEK. Figure 10 shows the results of the color difference between G1–G3 the17.36 17.36 of pure of pure PEEK. PEEK. Figure Figure 10 10 shows shows the the results results of of the the color color difference betweenbetween G1–G3G1–G3 and Shofu G1. The ΔE* values varied from 14.14 (G1) to 10.94 (G3) and were smaller than andand ShofuShofu G1.G1. TheThe ∆ΔE*E* valuesvalues variedvaried fromfrom 14.1414.14 (G1)(G1) toto 10.9410.94 (G3)(G3) andand werewere smallersmaller thanthan the 19.79 of pure PEEK. When compared to ceramic gingiva shade, G1 and G2 were close thethe 19.7919.79 ofof purepure PEEK.PEEK. WhenWhen comparedcompared toto ceramicceramic gingivagingiva shade,shade, G1G1 andand G2G2 werewere close close to Ceram-GZL, and G3 was close to Ceram-G4 with ΔE* values of 8.73, 5.85, and 7.73. toto Ceram-GZL,Ceram-GZL, andand G3G3 waswas closeclose toto Ceram-G4Ceram-G4 withwith∆ ΔE*E* valuesvalues ofof 8.73,8.73, 5.85,5.85, andand 7.73.7.73.

TableTable 5. 5. Color coordinates of VITA A1–A3 and white PEEK specimens. Table 5.Color Color coordinates coordinates of of VITA VITA A1–A3 A1–A3 and and white white PEEK PEEK specimens. specimens. Groups L* a* b* Groups L*L* a* a* b* b* VITA-A1 79.57 −1.61 13.05 VITA-A1 79.5779.57 −−1.61 13.0513.05 VITA-A2 76.04 −0.08 16.73 VITA-A2 76.0476.04 −−0.08 16.7316.73 VITA-A3 75.3675.36 1.361.36 19.6119.61 VITA-A3 75.36 1.36 19.61 D1D1 86.34 86.34 1.281.28 10.1410.14 D1 86.34 1.28 10.14 D2D2 84.9184.91 0.980.98 9.709.70 D3D2 83.2984.91 1.790.98 10.059.70 D3 83.29 1.79 10.05 PEEKD3 63.42 83.29 4.171.79 15.7010.05 PEEK 63.42 4.17 15.70 PEEK 63.42 4.17 15.70 TableTable 6. 6.Color Color coordinates coordinates of of gingiva gingiva shade shade guide guide and and pink pink PEEK PEEK specimens. specimens. Table 6. Color coordinates of gingiva shade guide and pink PEEK specimens. GroupsGroups L*L* a* a* b* b* Groups L* a* b* Shofu-GL 52.8352.83 13.48 13.48 1.81 1.81 Shofu-GL 52.83 13.48 1.81 Shofu-GM 50.5650.56 13.74 13.74 3.06 3.06 Shofu-GM 50.56 13.74 3.06 Shofu-GD 44.1444.14 12.21 12.21 3.55 3.55 Ceram-GZLShofu-GD 61.9544.14 18.78 12.21 15.83 3.55 Ceram-GZL 61.95 18.78 15.83 Ceram-GZLCeram-G4 51.5461.95 17.41 18.78 11.28 15.83 Ceram-G4 51.54 17.41 11.28 Ceram-G4G1 65.1851.54 16.11 17.41 11.288.17 G2G1 61.0165.18 18.8316.11 10.068.17 G1 65.18 16.11 8.17 G3G2 45.2661.01 18.7018.83 10.06 7.73 G2 61.01 18.83 10.06 PEEKG3 63.4245.26 18.70 4.17 15.707.73 G3 45.26 18.70 7.73

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Polymers 2021, 13, 1949 8 of 13 PEEKPEEK 63.4263.42 4.174.17 15.7015.70

Figure 9. The color difference between PEEK specimens and dental shade. FigureFigure 9.9. The colorcolor differencedifference betweenbetween PEEKPEEK specimensspecimens andand dentaldental shade.shade.

FigureFigure 10. 10. TheThe color color difference difference between between PEEK PEEK specimens specimens and and gingiva gingiva shade. shade. Figure 10. The color difference between PEEK specimens and gingiva shade.

3.3.3.3. Mechanical Mechanical Properties Properties 3.3.1.3.3. Mechanical Tensile Performance Properties 3.3.1. Tensile Performance 3.3.1.The Tensile mechanical Performance properties of the 3D printed PEEK were characterized using tensile The mechanical properties of the 3D printed PEEK were characterized using tensile and ﬂexuralThe mechanical mechanical properties performance, of the 3D which printed was PEEK generally were better characterized compared using to PMMA. tensile and flexural mechanical performance, which was generally better compared to PMMA. Tensileand flexural strengths mechanical of the PEEK performance, specimens which ranged was between generally 62.74 better and compared 94.17 MPa to (FigurePMMA. Tensile11Tensile). Group strengths strengths D3 had of of the thethe PEEK lowestPEEK specimens specimens strength rang of rang 62.74eded between MPa,between while 62.74 62.74 D2 and and had 94.17 94.17 higher MPa MPa strength (Figure (Figure than11). 11). Group D3 had the lowest strength of 62.74 MPa, while D2 had higher strength than ex- expected.Group D3 Grouphad the G1–G3 lowest that strength contained of 62.74 1 wt.% MP Fea, 2whileO3 exhibited D2 had superiorhigher strength tensile strengththan expected. Group G1–G3 that contained 1 wt.% Fe2O3 exhibited superior tensile strength with withpected. no Group statistical G1–G3 difference that contained observed. 1 wt.% Group Fe Z2O was3 exhibited not statistically superior differenttensile strength from other with nosuperiorno statistical statistical groups difference difference (D1–D2, observed. observed. G1–G3). Group GroupGroup Z XYZ wa wa hads snot not a signiﬁcantlystatistically statistically different lower different strength from from other thanother supe- Group supe- riorZrior and groups groups is likely (D1–D2, (D1–D2, a consequence G1–G3). G1–G3). Group ofGroup the XY vertical XY had had a interface asignificantly significantly between lower lower the strength pinkstrength and than whitethan Group Group parts. Z Z andand is is likely likely a aconsequence consequence of of the the vertical vertical interface interface between between the the pink pink and and white white parts. parts.

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Figure 11. Mean and standard deviations of tensile strength of 3D-printed PEEK. *p < 0.05. FigureFigure 11.11. MeanMean and and standard standard deviationsdeviations ofof tensiletensile strengthstrength ofof 3D-printed3D-printed PEEK.PEEK. **pp << 0.05.0.05. Tensile moduli of the PEEK specimens ranged between 2727 and 4751 MPa (Figure TensileTensile modulimoduli of of the the PEEK PEEK specimens specimens ranged ranged between between 2727 2727 and and 4751 4751 MPa MPa (Figure (Figure 12). 12). GroupTensile G1 moduli had the of thehighest PEEK tensile specimens modu rangedlus of 4750.93 between ± 153.332727 and MPa 4751 and MPa pink (Figure speci- Group12). Group G1 had G1 thehad highest the highest tensile tensile modulus modu oflus 4750.93 of 4750.93± 153.33 ± 153.33 MPa MPa and pinkand pink specimens speci- 12).mens Group that contained G1 had the more highest fillers tensile exhibited modu hilusgher of tensile 4750.93 modulus. ± 153.33 GroupMPa and D3 pinkand G3speci- ex- thatmens contained that contained more fillersmore exhibitedfillers exhibited higher hi tensilegher tensile modulus. modulus. Group Group D3 and D3 G3 and exhibited G3 ex- menshibited that inferior contained tensile more modulus fillers exhibitedcompared hi togher the tensileremaining modulus. groups Group and isD3 likely and G3due ex- to inferiorhibited tensileinferior modulus tensile modulus compared compared to the remaining to the remaining groups and groups is likely and due is likely to the due much to hibitedthe much inferior lower tensilecontent modulus of fillers. comparedThe tensile to modulus the remaining did not groupssignificantly and is differ likely between due to lowerthe much content lower of content fillers. of The fillers. tensile The modulus tensile modulus did not significantlydid not significantly differ between differ between Group theGroup much D1 lower (20 wt.%) content and of fillers.D2 (10 The wt.%). tensil Groupe modulus XY was did notnot significantly statistically differdifferent between from D1Group (20 wt.%)D1 (20 and wt.%) D2 (10and wt.%). D2 (10 Group wt.%). XY Group was notXY statisticallywas not statistically different fromdifferent Group from Z, Group Z,D1 although (20 wt.%) the and interfacial D2 (10 orientationswt.%). Group were XY different. was not statistically different from GroupalthoughGroup Z,Z, thealthoughalthough interfacial thethe interfacialinterfacial orientations orientationsorientations were different. werewere different.different.

Figure 12. Mean and standard deviations of tensile modulus of 3D-printed PEEK. * p < 0.05. FigureFigure 12.12. MeanMean and and standard standard deviationsdeviations ofof tetensiletensilensile modulusmodulus ofof 3D-printed3D-printed PEEK.PEEK. ** p p << 0.05.0.05. 3.3.2. Flexural Performance 3.3.2.3.3.2. FlexuralFlexural PerformancePerformance Flexural strengthsstrengths of of the the PEEK PEEK specimens specimen rangeds ranged between between 109 109 and and 164.8 164.8 MPa MPa(Figure (Figure 13) FlexuralFlexural strengthsstrengths ofof thethe PEEKPEEK specimenspecimenss rangedranged betweenbetween 109109 andand 164.8164.8 MPaMPa (Figure(Figure and13) and were were significantly significantly higher higher compared compared to that to of that PMMA. of PMMA. Group XYGroup had XY the had lowest the flexural lowest 13)13) andand werewere significantlysignificantly higherhigher comparedcompared toto thatthat ofof PMMA.PMMA. GroupGroup XYXY hadhad thethe lowestlowest strengthflexural strength of 109.10 of± 109.103.61 MPa, ± 3.61 and MPa, no statisticaland no statistical difference difference was found was in found the remaining in the re- flexuralflexural strengthstrength ofof 109.10109.10 ±± 3.613.61 MPa,MPa, andand nono statisticalstatistical differencedifference waswas foundfound inin thethe re-re- groups.maining groups. A possible A possible reason reason could becould that be flexural that flexural strengths strengths were were more more affected affected by the by mainingmaining groups.groups. AA possiblepossible reasonreason couldcould bebe thatthat flexuralflexural strengthsstrengths werewere moremore affectedaffected byby interfacialthe interfacial orientations, orientations, rather rather than th thean differentthe different content content of fillers. of fillers. thethe interfacialinterfacial orientations,orientations, ratherrather ththanan thethe differentdifferent contentcontent ofof fillers.fillers.

Figure 13. Mean and standard deviations of ﬂexural strength of 3D-printed PEEK. * p < 0.05.

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Figure 13. Mean and standard deviations of flexural strength of 3D-printed PEEK. * p < 0.05.

TheThe flexuralflexural modulimoduli ofof thethe PEEKPEEK specimensspecimens rangedranged betweenbetween 41724172 andand 57405740 MPa,MPa, whichwhich werewere significantlysignificantly higherhigher comparedcompared toto thatthat ofof PMMA.PMMA. (Figure(Figure 1414).). GroupGroup D1D1 hadhad thethe highesthighest flexuralflexural modulusmodulus ofof 5740.205740.20± ± 215.93215.93 MPaMPa andand waswas notnot statisticallystatistically differentdifferent fromfrom GroupGroup G1,G1, XY,XY, and and Z, Z, which which also also contained contained 20%wt 20%wt TiO TiO22.. GroupGroup D2D2 andand D3D3 exhibitedexhibited lowerlower flexuralflexural modulus modulus compared compared to to D1 D1 possibly possibly because because of theof lowerthe lower content content of TiO of2 ,TiO and2, noand significant no significant difference difference was observedwas observed between between Group Group D2 (10wt.%) D2 (10wt.%) and D3and (5wt.%). D3 (5wt.%). The pinkThe pink specimens specimens that containedthat contained higher higher content cont ofent fillers of fillers had ahad higher a higher flexural flexural modulus modulus (G1 >(G1 G2 > > G2 G3). > G3).

FigureFigure 14.14. MeanMean andand standardstandard deviationsdeviations ofof ﬂexuralflexural modulusmodulus ofof 3D-printed3D-printed PEEK.PEEK. **p p< < 0.05.0.05.

4.4. DiscussionDiscussion RemovableRemovable dentaldental prosthesesprostheses restorerestore hardhard andand softsoft tissuestissues andand couldcould consistconsist ofof tooth- colorcolor andand gingiva-colorgingiva-color partsparts toto ensureensure bothboth functionfunction andand estheticsesthetics [[2,14].2,14]. Currently,Currently, onlyonly aa few commercial PEEK PEEK materials materials are are available available for for molding molding and and milling, milling, which which could could not notmeet meet the need the need for esthetical for esthetical dental dental restoration restoration as well asas well3D dental as 3D printing dental [12]. printing 3D print- [12]. 3D printing is a kind of rapid formation (RP) technology, and it allows a customized ing is a kind of rapid formation (RP) technology, and it allows a customized optimization optimization of parameters, which can be essential for the dental industries [2]. PEEK, as a of parameters, which can be essential for the dental industries [2]. PEEK, as a thermo- thermoplastic biopolymer, possesses great thermal properties and biocompatibility, which plastic biopolymer, possesses great thermal properties and biocompatibility, which could could be suitable for 3D dental printing [12]. This study presented here is an early attempt be suitable for 3D dental printing [12]. This study presented here is an early attempt to to develop dual-color PEEK filaments for fabricating dental prostheses that consists of develop dual-color PEEK filaments for fabricating dental prostheses that consists of tooth- tooth-color and gingiva-color parts. In this preliminary study, functional fillers (nano TiO2 color and gingiva-color parts. In this preliminary study, functional fillers (nano TiO2 and and Fe2O3) are incorporated into pure PEEK to change the brown color of 3DP PEEK to Fe2O3) are incorporated into pure PEEK to change the brown color of 3DP PEEK to tooth- tooth-like and gingiva-like colors through Fused Filament Fabrication (FFF) [9]. Based on like and gingiva-like colors through Fused Filament Fabrication (FFF) [9]. Based on the the filaments, duotone PEEK specimens have been successfully printed using dual-nozzle filaments, duotone PEEK specimens have been successfully printed using dual-nozzle printing technology, which could provide technical support for future dual-color dental printing technology, which could provide technical support for future dual-color dental printing. One-piece fabrication can eliminate the interface between different parts and offersprinting. great One-piece efficiency fabrication and more can comfort eliminate for patients the interface compared between to traditional different parts procedures. and of- Lifers et great al. reported efficiency the and one-piece more comfort fabrication for pati of removableents compared partial to traditional dentures using procedures. PEEK by Li milling,et al. reported which the showed one-piece satisfying fabrication fits [25 of]. re Thismovable study partial indicates dentures the promising using PEEK application by mill- ofing, one-piece which showed printing satisfying using dual-color fits [25]. PEEK, This study which indicates reduces materialthe promising waste application and provides of improvedone-piece estheticsprinting using compared dual-color to the one-piecePEEK, which milling. reduces However, material long-term waste and data provides for the dual-colorimproved PEEKesthetics are compared not yet available, to the one-piece and continued milling. observation However, islong-term necessary data to further for the verifydual-color the clinical PEEK are outcomes. not yet Moreover,available, and the contentcontinued of fillersobservation and printing is necessary parameters to further can beverify flexibly the clinical adjusted outcomes. and thus Moreover, the properties the co ofntent the printedof fillers prostheses and printing can parameters be tailored can by furtherbe flexibly studies. adjusted The results and thus of the the color properties evaluation of the revealed printed that prostheses the novel can PEEK be compositestailored by developedfurther studies. in this The study results were of closer the color to dental evaluation shade revealed guides compared that the novel to pure PEEK 3DP compo- PEEK, whichsites developed provides improvedin this study esthetics were forcloser dental to dental application. shade guides This proof compared of concept to pure showed 3DP thatPEEK, color which modification provides of improved 3DP PEEK esthetics by blending for dental and FFF application. can be effective This [8proof,9,20 ],of although concept theshowed colors that obtained color modification in this study of are3DP still PEEK limited by blending compared and toFFF the can dental be effective shade [8,9,20], guides. Morealthough research the colors is required obtained to provide in this morestudy color are still options limited for 3DPcompared PEEK withto the greater dental variety shade andguides. different More content research of fillersis required and further to provide improve more the color esthetics options in the for future. 3DP PEEK with A series of PEEK specimens with different colors have been successfully 3D printed with optimized parameters, which showed that these novel PEEK composites developed

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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 en- hancements obtained with the addition of TiO2 and Fe2O3 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 Fe2O3, higher than that of groups with TiO2 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.% TiO2) 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. Polymers 2021, 13, 1949 12 of 13

Author Contributions: Conceptualization, G.W. and L.D.; methodology, G.W.; software, J.Z.; vali- dation, C.Y., L.D. and W.L.; formal analysis, L.D.; investigation, W.L.; resources, C.Y.; data curation, J.Z.; writing—original draft preparation, L.D.; writing—review and editing, W.L.; visualization, J.Z.; supervision, C.Y.; project administration, G.W.; funding acquisition, G.W. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Jiangsu Provincial Key Research and Development Program, Grant No. BE2019622, and the open project of Research Center of Engineering and Technology for Digital Dentistry, Ministry of Health, Grant No. PKUSS20200501. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The authors would like to thank the Jugao-AM Technology Co., Ltd. for their 3D printing support. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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