FFF implementation in the medical sector using thermoplastic biopolymers looks promising. Devices such as surgical instruments or guides for single-use can be made custom and the simplified manufacturing process allows for easy handling by the practitioner.21. Biopolymers can also have positive impacts on the environment when they are made from renewable feedstocks. PLA, PBAT and PHA can also be biodegradable under certain conditions. This allows them to counter uncontrolled littering as well as landfill.22. FFF faces some challenges in professional medical settings. The printed objects should have stable dimensions and be autoclavable using a suitable protocol. This study evaluated the dimensional accuracy and annealing of various biocopolyester combinations after extrusion-based, annealing and steam sterilization.
As the test specimen, a square specimen was chosen, as it is more susceptible to warpage. The longitudinal orientation of the hair strands enhances the contraction of straight sections. This increases the pulling force at the corners. This design was used in studies that evaluated the warpage of FFF printable polyester polypropylene.20,23The side length was shortened in this study to make it more similar to a surgical guide. One corner was previously fixed with a force, and the height of the opposite corner was measured. This allowed us to quantify the warpage.20. To achieve greater accuracy in measurement, the present study measured warpage after digitization. From this volume, the original STL data was subtracted.
All the materials evaluated were biocopolyester-blends containing PLA, PHA, and PBAT. The composition of the materials and specifically the amount of melt stabilizers, nucleating agents and fillers had a significant impact on the accuracy of specimens after printing. While there was no visible warpage between groups that were printed with the same standard parameters, significant differences could be observed. Material D, which was based on a different basis composition compared to materials A–C, showed the highest warpage after sterilization.
The mechanical properties of extrusion printed parts can be improved with thermal annealing. Commonly, annealing is performed approximately 20 °C below the polymer melt temperature, where amorphous and poorly organized crystalline regions can reorganize24. This is often a result of a reduction in stress.25This is done with higher thermal and mechanical stability26. Annealing can also reduce the impact of changing printing conditions on dimensional accuracy20. The annealing process must be prolonged at significantly lower temperatures. This results in limitations that can be used for practical purposes. The thermal post-processing results of the square printed specimens showed different effects on their dimensional accuracy. Materials A and B, which contained a higher amount of mineral fillers (21–32 wt%) as well as nucleating agents, showed no significant dimensional change due to annealing. The steam sterilization that followed did not cause significant warpage. The small warpage from both of these processing steps resulted in significant specimen warpage. The overall deviation was however significantly lower than those sterilized with no prior annealing.
Mineral fillers are responsible for the fact that annealing did not result in significantly higher warpage in test specimens in groups B and A. Increased nucleation rates during the crystallization process of polymers can be attributed to nucleating agents. This will lead to faster crystallization. This is done by lowering the surface energy barrier to nucleation and allowing polymers crystallize at higher temperatures.27. DSC analyses did not detect any influence of the nucleating agents on the total crystallinity and polymer portions of the samples. The nucleating agent can be used to reduce the size and tendency to warp of the crystallites. Further research is needed to investigate this issue. Material R with lower filler content had high warpage upon annealing. This could indicate that higher filler contents in combination with nucleating agent might be beneficial to minimize warpage during the annealing process. Group R annealed showed a greater increase in warpage than samples that had been steam sterilized directly after printing, suggesting no benefit to thermal annealing.
The increase in crystallization correlates qualitatively to warpage caused by sterilization. Due to the increased temperatures of 134 °C during sterilization, crystallization of the PLA occurred. Warpage resulted from the associated volume shrinkage. Warpage was especially evident in filler-free groups D and C. The presence of crystalline PLA in sterilized specimens was more apparent in those without fillers. A growing number of mineral fillers counteracted the post-crystallization and stiffened the material at high temperatures. The fillers had a positive effect on warpage. However, they also caused an accelerated aging of the material or degrading it at higher temperatures during sterilization. This could limit repeat sterilization. For other polymers, the significant effect of filler contents on dimensional stability was also described.28. Additional fillers are beneficial for materials with a higher crystalline content such as polyolefins.23. Smaller fillers may be more effective in decreasing shrinkage and warpage. PLA-based biopolymers’ material properties are also affected by the filler sizes, geometries and types.29. After PLA-based filaments are solidified, it was discovered that adding fillers can increase flexural stiffness or improve dimensional stability.9. It was demonstrated that increased thermal stability of PLA can be achieved by adding calcium carbonate to materials A and B.30Particularly when combined with talc31. The use of fillers may also have drawbacks such as reduced strength, brittleness and increased density. Plastic processing machines can also be subject to greater wear due to the use of mineral fillers.9. Mineral fillers could limit the biodegradability and biodegradability of the material being evaluated. Natural fibers (e.g. This is why natural fibers, e.g. from pulp, are being used in biodegradable materials.
PLA has a disadvantage: the Vicat melting temperature/softening temperature is lower than thermoplastics that have a higher crystal content. PLA’s melting point is very close to the melting point. It was however demonstrated that the composition can increase the softening temperatures by adding other biopolymers or additives. In the case of the examined reference material, the Vicat softening temperature was around 160 °C, which was significantly higher than the typical Vicat softening temperature starting at approximately 60 °C for PLA32. However, these biocopolyesters showed greater resistance to higher temperatures than polymers with a higher crystal content.33. In the present study, a steam sterilization protocol with a temperature of 134 °C was selected, representing a frequently applied protocol in dental offices. Similar studies investigating the capability to sterilize various 3D printable materials often used solely a temperature of 121 °C34,35. In previous studies, the reference material R was found to have acceptable dimensional stability under steam sterilization at 121 °C17,36. Thus, it can be assumed that sterilizing the printed biocopolyester specimens at a temperature of 121 °C would have amounted to a reduced warpage, whereas H2O2 Plasma sterilization may be another option. It is not available and it can be expensive, which could compromise the goal of low-budget sterilization.37.
The dimensional accuracy of FFF-printed parts can be affected by many factors, including the material composition (fillers and nucleating agents), as well as the printing parameters. On the basis of the dimension accuracy, there were significant differences in the nozzle temperature, outershells and infill. The reduced number and stability of printed parts was evident by increased warpage during sterilization. Dong et. al. found that a higher number of shells is beneficial for mechanical strengths. PLA38. Although the present study revealed a reduced deviation of printed samples at 230 °C compared to the original file, further research is necessary to evaluate the influence of high nozzle temperatures on the mechanical parameters such as strength and stiffness39. You might find solutions to many of the drawbacks using the different printing parameters. This can often require trial and error.9. Even with the same parameters, other factors can influence the outcome, especially if a low-budget workflow goal is pursued. This study was designed to evaluate an affordable and easy workflow. We used a desktop extrusion-based printer that didn’t require any additional equipment. Because the printer doesn’t have a chamber, ambient conditions such a room temperature or humidity might have had an effect on the printing quality.8. It was done with extreme care to ensure that the filaments were kept in a dry, cool place. However, slight variations in humidity could have caused differences in the dimensional accuracy of two cohorts that were printed using the same printing parameters.
Dimensional printability is also affected by the geometry of the object. Although square specimens are more susceptible to warpage and have significant differences, surgical guides show fewer differences. Only group B had a significantly higher warpage than the other groups after sterilization and annealing. The heatmaps revealed that even after steam sterilization, most regions only showed deviations of up to 0.2 mm. Preclinical research showed no differences between SLA-printed surgical guides and printed surgical guides made from material R when placing implants.17. Because they are thicker and stiffer than squares, the result of surgical guides showing less distortion is possible. The stiff areas of the molten plasticmer can resist deformation and shrink further, resulting in less distortion. The stress must be relieved by the last strand that is deposited. It is similar to the warpage tendency in squares that the highest level of warpage in surgical guides was found in the molar area. This is because the shrinkage of polymer paralleled the length of the longest printed line.20. However, the clinical application must determine if slight distortions have any effect on the application. These surgical guides are made from novel biocopolyester mixtures. The implant position after placement should be compared with the plan position in vitro.17. The biocompatibility must also be verified. A recent study found that the used reference material was uncytotoxic to oral cells. It may also be more biocompatible than a popular photopolymer for surgical guidebooks.40. In different studies, it has been shown that vat polymerization light-curing compounds can cause cytotoxic reactions.41,42. The promising results for R’s biocompatibility suggest that similar results can be expected for other extrusion materials. These will need to be evaluated in further research.