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.

Heat Transfer-Based Non-isothermal Healing Model for the Interfacial Bonding Strength of Fused Filament Fabricated Polyetheretherketone

Fused Filament Fabrication (FFF) as an Additive Manufacturing (AM) method for Polyetheretherketone (PEEK) has established a promising future for medical applications so far, however interlayer delamination as a failure mechanism for FFF implants has raised critical concerns. A one-dimensional (1D) heat transfer model (HTM) was developed to compute the layer and interlayer temperatures by considering the nature of 3D printing for FFF PEEK builds. The HTM was then coupled with a non-isothermal healing model to predict the interlayer strength through thickness of a FFF PEEK part. We then conducted a parametric study of the primary temperature effects of the FFF system, including the print bed, nozzle, and chamber temperatures, on layer healing. The heat transfer component of the model for the FFF PEEK layer healing assessment was validated separately. An idealized PEEK cube design (10x10x10 mm3) was used for model development and 3D printed in commercially available industrial and medical FFF machines. During the printing and cooling processes of FFF, thermal videos were recorded in both printers using a calibrated infrared camera. Thermal images were then processed to obtain time-dependent layer temperature profiles of FFF PEEK prints. Both the theoretical model and experiments confirmed that the upper layers in reference to the print bed exhibited higher temperatures, thus higher healing degrees than the lower layers. Increasing the print bed temperature increased the healing of the layers allowing more layers to heal 100%. The nozzle temperature showed the most significant effect on the layer healing, and under certain nozzle temperature, none of the layers healed adequately. Although environment temperature had less impact on the lower layers closer to the print bed, 100% healed layer number increased when the chamber temperature increased. The model predictions were in good agreement with the experimental data, particularly for the mid-part of FFF PEEK cubes printed in both FFF machines.

Structure, properties, and bioactivity of 3D printed PAEKs for implant applications: A systematic review

Additive manufacturing (AM) of high temperature polymers, specifically polyaryletherketones (PAEK), is gaining significant attention for medical implant applications. As 3D printing systems evolve toward point of care manufacturing, research on this topic continues to expand. Specific regulatory guidance is being developed for the safe management of 3D printing systems in a hospital environment. PAEK implants can benefit from many advantages of AM such as design freedom, material and antibacterial drug incorporation, and enhanced bioactivity provided by cancellous bone-like porous designs. In addition to AM PAEK bioactivity, the biomechanical strength of 3D printed implants is crucial to their performance and thus widely studied. In this review, we discuss the printing conditions that have been investigated so far for additively manufactured PAEK implant applications. The effect of processing parameters on the biomechanical strength of implants is summarized, and the bioactivity of PAEKs, along with material and drug incorporation, is also covered in detail. Finally, the therapeutic areas in which 3D printed PAEK implants are investigated and utilized are reviewed.

A systematic review of preclinical in vivo testing of 3D printed porous Ti6Al4V for orthopedic applications, part I: Animal models and bone ingrowth outcome measures

For Ti6Al4V orthopedic and spinal implants, osseointegration is often achieved using complex porous geometries created via additive manufacturing (AM). While AM porous titanium (pTi) has shown clinical success, concerns regarding metallic implants have spurred interest in alternative AM biomaterials for osseointegration. Insights regarding the evaluation of these new materials may be supported by better understanding the role of preclinical testing for AM pTi. We therefore asked: (a) What animal models have been most commonly used to evaluate AM porous Ti6Al4V for orthopedic bone ingrowth; (b) What were the primary reported quantitative outcome measures for these models; and (c) What were the bone ingrowth outcomes associated with the most frequently used models? We performed a systematic literature search and identified 58 articles meeting our inclusion criteria. We found that AM pTi was evaluated most often using rabbit and sheep femoral condyle defect (FCD) models. Additional ingrowth models including transcortical and segmental defects, spinal fusions, and calvarial defects were also used with various animals based on the study goals. Quantitative outcome measures determined via histomorphometry including ''bone ingrowth'' (range: 3.92-53.4% for rabbit/sheep FCD) and bone-implant contact (range: 9.9-59.7% for rabbit/sheep FCD) were the most common. Studies also used 3D imaging to report outcomes such as bone volume fraction (BV/TV, range: 4.4-61.1% for rabbit/sheep FCD), and push-out testing for outcomes such as maximum removal force (range: 46.6-3092 N for rabbit/sheep FCD). Though there were many commonalities among the study methods, we also found significant heterogeneity in the outcome terms and definitions. The considerable diversity in testing and reporting may no longer be necessary considering the reported success of AM pTi across all model types and the ample literature supporting the rabbit and sheep as suitable small and large animal models, respectively. Ultimately, more standardized animal models and reporting of bone ingrowth for porous AM materials will be useful for future studies.

Thermal Localization Improves the Interlayer Adhesion and Structural Integrity of 3D printed PEEK Lumbar Spinal Cages

Additive manufacturing (AM) is a potential application for polyetheretherketone (PEEK) spinal interbody fusion cages, which were introduced as an alternative to titanium cages because of their biocompatibility, radiolucency and strength. However, AM of PEEK is challenging due to high melting temperature and thermal gradient. Although fused filament fabrication (FFF) techniques have been shown to 3D print PEEK, layer delamination was identified in PEEK cages printed with a first generation FFF PEEK printer [1]. A standard cage design [2] was 3D printed with a second generation FFF PEEK printer. The effect of changing layer cooling time on FFF cages' mechanical strength was investigated by varying nozzle sizes (0.2 mm and 0.4 mm), print speeds (1500 and 2500 mm/min), and the number of cages printed in a single build (1, 4 and 8). To calculate the porosity percentage, FFF cages were micro-CT scanned prior to destructive testing. Mechanical tests were then conducted on FFF cages according to ASTM F2077 [2]. Although altering the cooling time of a layer was not able to change the failure mechanism of FFF cages, it was able to improve cages' mechanical strength. Printing a single cage per build caused a higher ultimate load than printing multiple cages per build. Regardless of the cage number printed per build, cages printed with bigger nozzle diameter achieved higher ultimate load compared to cages printed with smaller nozzle diameter. Printing with a bigger nozzle diameter resulted in less porosity, which might have an additional affect on the interlayer delamination failure mechanism.

Acoustic Parameters for Optimal Ultrasound-Triggered Release from Novel Spinal Hardware Devices

Post-operative infection is a catastrophic complication of spinal fusion surgery, with rates as high as 10%, and existing preventative measures (i.e., peri-operative antibiotics) are only partially successful. To combat this clinical problem, we have designed a drug delivery system around polyether ether ketone clips to be used for prophylactic post-surgical release of antibiotics upon application of ultrasound. The overall hypothesis is that antimicrobial release from this system will aggressively combat post-surgical bacterial survival. This study investigated a set of acoustic parameters optimized for in vitro ultrasound-triggered coating rupture and subsequent release of encapsulated prophylactic antibiotics. We determined that a transducer frequency of 1.7 MHz produced the most consistent burst release and that, at this frequency, a pulse repetition frequency of 6.4 kHz and acoustic output power of 100% (3.41 MPa) produced the greatest release, representing an important proof of principle and the basis for continued development of this novel drug delivery system.

Structure-Property Relationships for 3D printed PEEK Intervertebral Lumbar Cages Produced using Fused Filament Fabrication

Recent advances in additive manufacturing technology now enable fused filament fabrication (FFF) of Polyetheretherketone (PEEK). A standardized lumbar fusion cage design was 3D printed with different speeds of the print head nozzle to investigate whether 3D printed PEEK cages exhibit sufficient material properties for lumbar fusion applications. It was observed that the compressive and shear strength of the 3D printed cages were 63-71% of the machined cages, whereas the torsion strength was 92%. Printing speed is an important printing parameter for 3D printed PEEK, which resulted in up to 20% porosity at the highest speed of 3000 mm/min, leading to reduced cage strength. Printing speeds below 1500 mm/min can be chosen as the optimal printing speed for this printer to reduce the printing time while maintaining strength. The crystallinity of printed PEEK did not differ significantly from as-machined PEEK cages from extruded rods, indicating that the processing provides similar microstructure.