The Mechanical, Thermal, and Biological Properties of Materials Intended for Dental Implants: A Comparison of Three Types of Poly(aryl-ether-ketones) (PEEK and PEKK)

Regarding the dynamic development of 3D printing technology, as well as its application in a growing part of industries, i.e., in the automotive industry, construction industry, medical industry, etc., there is a notable opportunity for its application in producing dental implants, which presents a promising alternative to traditional implant manufacturing methods. The medical industry is very restrictive regarding the applied materials, and it is necessary to use materials that exhibit very good mechanical and thermal parameters, show clinical indifference and biocompatibility, are non-allergenic and non-cancerous, and are likely to sterilize. Such materials are poly(aryl-ether-ketone)s (PAEK)s, mainly poly(ether-ether-ketone) (PEEK) and poly(ether-ketone-ketone) (PEKK), that are found to be high-performance polymers and can be defined as materials that retain their functionality even in extreme conditions. In the present paper, two types of PEEKs and PEKK were compared regarding their structural, mechanical, and thermal properties along with the biological activity toward selected strains. The tested samples were obtained with Fused Deposition Modeling (FDM) technology. The PEKK, after heat treatment, exhibits the most promising mechanical properties as well as less bacterial adhesion on its surface when compared to both PEEKs. Consequently, among the evaluated materials, PEKK after heat treatment stands out as the optimal selection for a dental prosthesis.

Taguchi optimization of 3D printed short carbon fiber polyetherketoneketone (CFR PEKK)

In this study, the Taguchi method was utilized to optimize fused filament fabrication (FFF) additive manufacturing with the goal of maximizing the flexural strength of 3D printed polyaryletherketone specimens. We analyzed 3D printed (3DP) carbon fiber reinforced poly-etherketoneketone (CFR PEKK), 3D printed and pressed (3DP + P) CFR PEKK, and injection molded medical grade polyetheretherketone (PEEK) as a control. Fracture surfaces were analyzed via scanning electron microscopy (SEM). The parameters that were varied in the optimization included nozzle diameter, layer height, print speed, raster angle, and nozzle temperature. We analyzed the flexural strength and flexural modulus determined from 3-point bending (ASTM D790). Using Taguchi optimization, the signal to noise ratio (SNR) was calculated to determine the relationship between the input parameters and flexural strength and to determine optimal print settings. Results were confirmed with analysis of variance (ANOVA). The raster angle and layer height were determined to have the greatest impact on the flexural strength of specimens printed in the FFF process for 3DP CFR PEKK. The optimized printing parameters were found to be 0/90 Raster Angle, 0.25 mm layer height, 0.8 mm Nozzle Diameter, 375 °C nozzle temperature, and 1100 mm/min print speed. The optimized 3DP CFR PEKK test samples had a flexural strength of 111.3 ± 5.3 MPa and a flexural modulus of 3.5 GPa. 3DP + P CFR PEKK samples had a flexural strength of 257.2 ± 17.8 MPa and a flexural modulus of 8.2 GPa. Statistical comparisons between means demonstrated that pressing significantly improves both flexural strength and flexural modulus of 3DP CFR PEKK. The results of this study support the hypothesis that post consolidation of 3DP specimens improves mechanical properties. Post-processing composites via pressing may allow greater design freedom within the 3DP process while improving mechanical properties.

Micro-CT Marginal and Internal Fit Evaluation of CAD/CAM High-Performance Polymer Onlay Restorations

(1) Background: The use of high-performance polymers for fixed restorations requires additional studies regarding their adaptability and processing with CAD/CAM technology. This in vitro study aims to assess the marginal and internal fit of PEEK and PEKK materials using microcomputed tomography. (2) Methods: Twenty-four (n = 8) MOD onlays made of PEKK (Pekkton ivory), unmodified PEEK (Juvora medical), and modified PEEK (BioHPP) were investigated. A typodont mandibular left first molar was scanned to achieve 24 resin, 3D printed abutment teeth. The onlays were fabricated with a five-axis milling machine, and after cementation of the specimens, the marginal (MG) and internal gaps (IG) were evaluated at twelve points in the mesio-distal section and thirteen points in the bucco-lingual section using microcomputed tomography. For statistical data analysis, Wilcoxon signed-rank/paired Student t-Test, Mann-Whitney/unpaired Student t-Test, and one-way ANOVA test were applied. (3) Results: Significant differences (p < 0.05; α = 0.05) were reported between the MG and IG for each material for all three polymers and also among two materials in terms of the MG and IG (except Juvora-BioHPP). The highest IG values were recorded in angular areas (axio-gingival line angle) in the mesio-distal section for all the polymers. (4) Conclusions: For all the materials, MG < IG. The type of polymer influenced the adaptability; the lowest marginal and internal gap mean values were recorded for BioHPP. The analyzed polymer used for onlays are clinically acceptable in terms of adaptability.

Bacterial Adhesion on Dental Polymers as a Function of Manufacturing Techniques

The microbiological behavior of dental polymer materials is crucial to secure the clinical success of dental restorations. Here, the manufacturing process and the machining can play a decisive role. This study investigated the bacterial adhesion on dental polymers as a function of manufacturing techniques (additive/subtractive) and different polishing protocols. Specimens were made from polyaryletherketone (PEEK, PEKK, and AKP), resin-based CAD/CAM materials (composite and PMMA), and printed methacrylate (MA)-based materials. Surface roughness (R_z; R_a) was determined using a laser scanning microscope, and SFE/contact angles were measured using the sessile drop method. After salivary pellicle formation, in vitro biofilm formation was initiated by exposing the specimens to suspensions of Streptococcus mutans (S. mutans) and Streptococcus sanguinis (S. sanguinis). Adherent bacteria were quantified using a fluorometric assay. One-way ANOVA analysis found significant influences (p < 0.001) for the individual parameters (treatment and material) and their combinations for both types of bacteria. Stronger polishing led to significantly (p < 0.001) less adhesion of S. sanguinis (Pearson correlation PC = -0.240) and S. mutans (PC = -0.206). A highly significant (p = 0.010, PC = 0.135) correlation between S. sanguinis adhesion and R_z was identified. Post hoc analysis revealed significant higher bacterial adhesion for vertically printed MA specimens compared to horizontally printed specimens. Furthermore, significant higher adhesion of S. sanguinis on pressed PEEK was revealed comparing to the other manufacturing methods (milling, injection molding, and 3D printing). The milled PAEK samples showed similar bacterial adhesion. In general, the resin-based materials, composites, and PAEKs showed different bacterial adhesion. Fabrication methods were shown to play a critical role; the pressed PEEK showed the highest initial accumulations. Horizontal DLP fabrication reduced bacterial adhesion. Roughness < 10 µm or polishing appear to be essential for reducing bacterial adhesion.

The impact of non-thermal plasma on the adhesion of polyetherketoneketone (PEKK) to a veneering composite system

The PEKK material can be used in prosthodontics for framework manufacturing and is commonly laminated with veneering composites to achieve a better esthetics. Various surface treatment methods including sandblasting, etching, laser and cold plasma treatments were reported to enhance the adhesive properties of dental polymers. Both tensile and shear bond test were employed to quantify the bond strength between PEKK and veneering composites. The present in vitro study aims to evaluate the influence of acetylene, argon, air, nitrogen and oxygen plasma on the shear bond strength between PEKK and one veneering composite. Firstly, to determine which bond test type should be applied, n = 40 PEKK specimens were treated with argon plasma. Both shear and tensile bond tests were performed and compared to the control group (n = 40). In shear bond testing, values were 8.14 ± 1.70 MPa for Argon plasma while 5.83 ± 1.42 MPa for control group. In tensile bond testing, Argon plasma 1.50 ± 0.51 MPa while control group 0.58 ± 0.50 MPa. Afterwards n = 160 PEKK specimens were treated with rocatec sandblasting (n = 20), adhesive (n = 20), acetylene (n = 20), argon (n = 20), air (n = 20), nitrogen (n = 20), oxygen (n = 20) plasma types and compared to the untreated control group (n = 20) using shear bond strength test (SBS). Additionally surface roughness and scanning electron microscopy analyses were performed. The following SBS values were revealed: 10.22 ± 1.06 MPa for rocatec; 9.89 ± 3.08 MPa for acetylene, 9.16 ± 1.48 MPa for adhesive, 7.54 ± 1.52 MPa for argon, 7.09 ± 1.99 MPa for air, 7.03 ± 1.48 MPa for nitrogen, 5.69 ± 1.59 MPa for oxygen plasma types and 4.71 ± 1.54 MPa for the control group. All groups, except control group, showed SBS over 5 MPa, which means that they are suitable for the clinical application, according to ISO 10477. Acetylene showed the highest SBS among all plasma types (p < 0.0001), which was on a level of rocatec sandblasting group. Rocatec and acetylene groups demonstrated Ra values significantly different to the reference group (p < 0.0001). Plasma treatment especially with acetylene gas can be an effective more convenient surface treatment method for strengthening the bond strength between PEKK and veneering composites than traditional sandblasting/adhesive treatment.

Anion Exchange Membranes Prepared from Quaternized Polyepichlorohydrin Cross-Linked with 1-(3-aminopropyl)imidazole Grafted Poly(arylene ether ketone) for Enhancement of Toughness and Conductivity

A novel anion exchange membrane was synthesized via crosslinking of the quaternized polyepichlorohydrin (QPECH) by 1-(3-aminopropyl) imidazole grafted poly(arylene ether ketone) (PAEK-API). While the QPECH provided an excellent ion conductive property, the rigid rod-structured PAEK-API played a reinforcing role, along with providing the high conductivity associated with the pendant API group. The chemical structure of QPECH/PAEK-API membranes was identified by ¹H nuclear magnetic resonace spectroscopy. A variety of membrane properties, such as anion conductivity, water uptake, length swelling percentage, and thermal, mechanical and chemical stability, were investigated. The QPECH/PAEK-API1 membrane showed quite high hydroxide ion conductivity, from 0.022 S cm⁻¹ (30 °C) to 0.033 S cm⁻¹ (80 °C), and excellent mechanical strength, associated with the low water uptake of less than 40%, even at 80 °C. Such high conductivity at relatively low water uptake is attributed to the concentrated cationic groups, in a cross-linked structure, facilitating feasible ion transport. Further, the QPECH/PAEK-API membranes showed thermal stability up to 250 °C, and chemical stability for 30 days in a 4 NaOH solution, without significant loss of ion exchange capacity.

Comparison of various 3D printed and milled PAEK materials: Effect of printing direction and artificial aging on Martens parameters

OBJECTIVES

The aim of this study was to investigate the effect of artificial aging on the Martens parameters of different 3D printed and milled polyaryletherketon (PAEK) materials.

METHODS

In total 120 specimens of 4 different polyetheretherketon (PEEK) materials (Essentium PEEK, KetaSpire PEEK MS-NT1, VICTREX PEEK 450G and VESTAKEEP i4 G) were additively manufactured via fused layer manufacturing (FLM) in either horizontal or vertical directions (n=15 per group). 75 specimens were milled out of prefabricated PAEK blanks from the materials breCAM.BioHPP, Dentokeep, JUVORA Dental Disc 2 and Ultaire AKP (=15 per group). Martens hardness (HM), indentation hardness (H_IT) and indentation modulus (E_IT) were determined initially and longitudinally after thermocycling (5-55°C, 10,000x) and autoclaving (134°C, 2bar). In each case, the surface topography of the specimens was examined for modifications using a light microscope. Data were analysed with Kolmogorov-Smirnov test, univariate ANOVA followed by post-hoc Scheffé test with partial eta squared (η_p²), Kruskal-Wallis-, Mann-Whitney-U-, Friedman- and Wilcoxon-Test. A value of p<0.05 was considered as significant.

RESULTS

Milled specimens showed higher Martens parameters than printed ones (p<0.001). Artificial aging had a negative effect on the measured parameters (p<0.001). Horizontally printed specimens presented higher Martens parameters than vertically printed ones, regardless of material and aging process (p<0.001). Essentium PEEK and breCAM.BioHPP showed the highest and VICTREX PEEK 450G as well as Ultaire AKP the lowest values of all investigated PAEK materials initially, after thermocycling and after autoclaving (p<0.001). Microscopic examinations showed that artificial aging did not cause any major modifications of the materials.

SIGNIFICANCE

Additively manufactured PEEK materials showed lower Martens parameters than milled ones, whereas horizontally printed specimens presented higher values than vertically printed ones. Artificial aging had a negative effect on the Martens parameters, but not on the surface topography.