The effect of thermomechanical aging on the fracture resistance of additive and subtractive manufactured polyetheretherketone abutments.

OBJECTIVES: To evaluate the fracture resistance (FR) of polyetheretherketone (PEEK) abutments produced by additive and subtractive methods compared to milled zirconia abutments. METHODS: Custom abutments were designed on Ti-base abutments and produced from three different materials, namely additively manufactured PEEK (PEEK-AM), subtractively manufactured PEEK (PEEK-SM), and zirconia (N=60). PEEK-AM abutments were printed using PEEK filaments (VESTAKEEP®i4 3DF-T, Evonik Industries AG) on a M150 Medical 3D Printer (ORION AM) by fused filament fabrication (FFF). All surface treatments were carried out according to the manufacturer's instructions. All abutments were cemented on Ti-bases with hybrid abutment cement and then restored with milled zirconia crowns. Each subgroup was divided into non-aged and aged subgroups (n=10). The aged groups were subjected to thermomechanical aging (49 N, 5-55°C, 1.2 million cycles). FR tests were performed by using a universal testing machine. Data were statistically analyzed with one-way and two-way ANOVA and t-test. RESULTS: The survival rate of the specimens after aging was determined as 100%. It was found that both the material and aging had a significant effect on the FR (p<.001). There was a statistical difference among the fracture values of the groups (p<0.001). In both the aged and non-aged groups, PEEK-AM showed the statistically lowest FR, while the highest FR was seen in the zirconia group, which was significantly higher than the PEEK-SM (p<0.001). CONCLUSION: Hybrid abutments were successfully manufactured, and extrusion-based processed PEEK seems to be a good alternative to subtractive processed PEEK. However, since subtractive manufacturing still appears to be superior, further developments in additive manufacturing are needed to further improve the quality of 3D-printed PEEK parts, especially in terms of accuracy and bonding between adjacent layers. CLINICAL SIGNIFICANCE: Additively manufactured PEEK abutments have the potential to be an alternative for implant-supported restorations in the posterior region.

Dimensional Stability of Additively Manufactured Dentate Maxillary Diagnostic Casts in Biobased Model Resin.

This study aimed to evaluate the dimensional stability of maxillary diagnostic casts fabricated from a biobased model resin, which consists of 50% renewable raw materials for sustainable production, a model resin, and stone, over one month. A master maxillary stone cast was digitized with a laboratory scanner to generate a reference file. This master cast was also scanned with an intraoral scanner to additively manufacture casts with a biobased model resin (BAM) and a model resin (AM). Polyvinylsiloxane impressions of the master cast were also made and poured in type III stone (CV) (n = 8). The same laboratory scanner was used to digitize each model one day (T0), 1 week (T1), 2 weeks (T2), 3 weeks (T3), and 4 weeks (T4) after fabrication. Deviations from the reference file were calculated with an analysis software and analyzed with generalized linear model analysis (α = 0.05). The interaction between the material and the time point affected measured deviations (p < 0.001). Regardless of the time point, CV had the lowest and AM had the highest deviations (p < 0.001). BAM mostly had lower deviations at T0 and mostly had higher deviations at T4 (p ≤ 0.011). AM had the highest deviations at T4 and then at T3, whereas it had the lowest deviations at T0 (p ≤ 0.002). The measured deviations of CV increased after each time point (p < 0.001). BAM casts had deviations within the previously reported clinically acceptable thresholds over one month and had acceptable dimensional stability. Therefore, tested biobased resin may be a viable alternative for the sustainable manufacturing of maxillary diagnostic casts that are to be used clinically.

Positional accuracy of a single implant analog in additively manufactured casts in biobased model resin.

OBJECTIVES: To evaluate the positional accuracy of implant analogs in biobased model resin by comparing them to that of implant analogs in model resin casts and conventional analogs in dental stone casts. METHODS: Polyvinylsiloxane impressions of a partially edentulous mandibular model with a single implant were made and poured in type IV dental stone. The same model was also digitized with an intraoral scanner and additively manufactured implant casts were fabricated in biobased model resin (FotoDent biobased model) and model resin (FotoDent model 2 beige-opaque) (n = 8). All casts and the model were digitized with a laboratory scanner, and the scan files were imported into a 3-dimensional analysis software (Geomagic Control X). The linear deviations of 2 standardized points on the scan body used during digitization were automatically calculated on x-, y-, and z-axes. Average deviations were used to define precision, and 1-way analysis of variance and Tukey HSD tests were used for statistical analyses (α = 0.05). RESULTS: Biobased model resin led to higher deviations than dental stone (all axes, P ≤ 0.031) and model resin (y-axis, P = 0.015). Biobased model resin resulted in the lowest precision of implant analog position (P ≤ 0.049). The difference in the positional accuracy of implant analogs of model resin and stone casts was nonsignificant (P ≥ 0.196). CONCLUSIONS: Implant analogs in biobased model resin casts mostly had lower positional accuracy, whereas those in model resin and stone casts had similar positional accuracy. Regardless of the material, analogs deviated more towards mesial, while buccal deviations in additively manufactured casts and lingual deviations in stone casts were more prominent.

Impact of Multiple Reprocessing on Properties of Polyhydroxybutyrate and Polypropylene.

Biobased plastics have the potential to be sustainable, but to explore their circularity further, current end-of-life options need to be broadened. Mechanical recycling is one of the most accepted methods to bring back plastics into the loop. Polyhydroxybutyrates (PHBs) are biobased and biodegradable in nature with promising properties and varied applications in the market. This study focuses on their potential for mechanical recycling by multiple extrusion cycles (E1-E5) and multi-faceted characterization of the virgin (V) and reprocessed materials from E1 to E5. The behavior is compared to polypropylene (PP) as a reference with a similar property profile, which has also been reprocessed five times. The thermal properties of both series showed a stable melting point and thermal decomposition temperature from thermal analyses (differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA)). However, a steady increase in the degree of crystallinity was observed which could counterbalance the decrease in molecular weight due to repeated extrusion measured by gel permeation chromatography and resulted in similar values of tensile strength across the cycles. The strain at break was impacted after the first extrusion, but no significant change was observed thereafter; the same was observed for impact strength. Even in scanning electron microscopy (SEM) images, virgin and E5 samples appeared similar, showing the stability of morphological characteristics. Fourier transform infrared spectroscopy (FTIR) results revealed that no new groups are being formed even on repeated processing. The deviation between the PHB and PP series was more predominant in the melt mass flow rate (MFR) and rheology studies. There was a drastic drop in the MFR values in PHB from virgin to E5, whereas not much difference was observed for PP throughout the cycles. This observation was corroborated by frequency sweeps conducted with the parallel plate method. The viscosity dropped from virgin to E1 and E2, but from E3 to E5 it presented similar values. This was in contrast to PP, where all the samples from virgin to E5 had the same values of viscosity. This paper highlights the possibilities of mechanical recycling of PHB and explains why future work with the addition of virgin material and other additives is an area to be explored.

Influence of the Print Envelope Temperature on the Morphology and Tensile Properties of Thermoplastic Polyolefins Fabricated by Material Extrusion and Material Jetting Additive Manufacturing.

Additive manufacturing (AM) nowadays has become a supportive method of traditional manufacturing. In particular, the medical and healthcare industry can profit from these developments in terms of personalized design and batches ranging from one to five specimens overall. In terms of polymers, polyolefins are always an interesting topic due to their low prices, inert chemistry, and crystalline structure resulting in preferable mechanical properties. Their semi-crystalline nature has some advantages but are challenging for AM due to their shrinkage and warping, resulting in geometrical inaccuracies or even layer detaching during the process. To tackle these issues, process parameter optimization is vital, with one important parameter to be studied more in detail, the print envelope temperature. It is well known that higher print envelope temperatures lead to better layer adhesion overall, but this investigation focuses on the mechanical properties and resulting morphology of a semi-crystalline thermoplastic polyolefin. Further, two different AM technologies, namely material jetting (ARBURG plastic freeforming-APF) and filament-based material extrusion, were studied and compared in detail. It was shown that higher print envelope temperatures lead to more isotropic behavior based on an evenly distributed morphology but results in geometrical inaccuracies since the material is kept in a molten state during printing. This phenomenon especially could be seen in the stress and strain values at break at high elongations. Furthermore, a different crystal structure can be achieved by setting a specific temperature and printing time, also resulting in peak values of certain mechanical properties. In comparison, better results could be archived by the APF technology in terms of mechanical properties and homogeneous morphology. Nevertheless, real isotropic part behavior could not be managed which was shown by the specimen printed vertically. Hence, a sweet spot between geometrical and mechanical properties still has to be found.

Optimization and manufacture of polyetheretherketone patient specific cranial implants by material extrusion - A clinical perspective.

Polyetheretherketone (PEEK) is a high performing thermoplastic that has established itself as a 'gold-standard' material for cranial reconstruction. Traditionally, milled PEEK patient specific cranial implants (PSCIs) exhibit uniform levels of smoothness (excusing suture/drainage holes) to the touch (<1 µm) and homogenous coloration throughout. They also demonstrate predictable and repeatable levels of mechanical performance, as they are machined from isotropic material blocks. The combination of such factors inspires confidence from the surgeon and in turn, approval for implantation. However, manufacturing lead-times and affiliated costs to fabricate a PSCI are high. To simplify their production and reduce expenditure, hospitals are exploring the production of in-house PEEK PSCIs by material extrusion-based additive manufacturing. From a geometrical and morphological perspective, such implants have been produced with good-to-satisfactory clinical results. However, lack of clinical adoption persists. To determine the reasoning behind this, it was necessary to assess the benefits and limitations of current printed PEEK PSCIs in order to establish the status quo. Afterwards, a review on individual PEEK printing variables was performed in order to identify a combination of parameters that could enhance the aesthetics and performance of the PSCIs to that of milled implants/cranial bone. The findings from this review could be used as a baseline to help standardize the production of PEEK PSCIs by material extrusion in the hospital.

Processing Conditions of a Medical Grade Poly(Methyl Methacrylate) with the Arburg Plastic Freeforming Additive Manufacturing Process.

The Arburg Plastic Freeforming process (APF) is a unique additive manufacturing material jetting method. In APF, a thermoplastic material is supplied as pellets, melted and selectively deposited as droplets, enabling the use of commercial materials in their original shape instead of filaments. The medical industry could significantly benefit from the use of additive manufacturing for the onsite fabrication of customized medical aids and therapeutic devices in a fast and economical way. In the medical field, the utilized materials need to be certified for such applications and cannot be altered in any way to make them printable, because modifications annul the certification. Therefore, it is necessary to modify the processing conditions rather than the materials for successful printing. In this research, a medical-grade poly(methyl methacrylate) was analyzed. The deposition parameters were kept constant, while the drop aspect ratio, discharge rate, melt temperatures, and build chamber temperature were varied to obtain specimens with different geometrical accuracy. Once satisfactory geometrical accuracy was obtained, tensile properties of specimens printed individually or in batches of five were tested in two different orientations. It was found that parts printed individually with an XY orientation showed the highest tensile properties; however, there is still room for improvement by optimizing the processing conditions to maximize the mechanical strength of printed specimens.

Mechanical properties of polymeric implant materials produced by extrusion-based additive manufacturing.

The application of material extrusion-based additive manufacturing methods has recently become increasingly popular in the medical sector. Thereby, thermoplastic materials are likely to be used. However, thermoplastics are highly dependent on the temperature and loading rate in comparison to other material classes. Therefore, it is crucial to characterise these influences on the mechanical properties. On this account, dynamic mechanical analyses to investigate the application temperature range, and tensile tests at different crosshead speeds (103, 101, 10-1 and 10-3 mms-1) were performed on various 3D-printable polymers, namely polyetheretherketone (PEEK), polylactide (PLA), poly(methyl methacrylate) (PMMA), glycol-modified poly(ethylene terephthalate) (PETG), poly(vinylidene fluoride) (PVDF) and polypropylene (PP). It was found that the mechanical properties of PEEK, PLA, PMMA and PETG hardly depend on temperature changes inside the human body. PVDF and PP show a significant decrease in stiffness with increasing body temperatures. Additionally, the dependency of the stiffness on the strain-rate is increasing between PLA, PP, PEEK, PETG, PMMA and PVDF. Besides the mechanical integrity of these materials (strength, stiffness and its strain-rate and temperature dependency inside the body), the materials were further ranked considering their filling density as a measure of their processability. Hence, useful information for the selection of possible medical applications for each material and the design process of 3D-printed implants are provided.