3D printing of thermoplastic PPSU

3D printing and the influence of silver-coated zeolite fillers on the chemical and mechanical properties of PPSU restorations: Insights into a ZIM-funded research project between the RPTU in Kaiserslautern, Apium Additive Technologies, and the Materials Science of Dental Prosthetics at the Ludwig Maximilian University Munich.

Polyphenylene sulfone (PPSU) is a thermoplastic material that is newly used in dentistry. Initial studies have shown promising results for the mechanical properties of this material, highlighting its potential for the fabrication of removable and fixed dental prostheses. 

PPSU in 3D printing

A key feature that has fueled research efforts in this area is the ability to process PPSU using 3D printing. To produce a 3D-printed object from PPSU, the base material must be processed in two steps. 

1. First, the granules are extruded into filaments, which requires strict adherence to temperature settings and a carefully dried material beforehand. 
2. Secondly, the filament is Fused Filament Fabrication (FFF), which is carefully heated and extruded to form a 3D structure layer by layer. In this context, the printing parameters are crucial. The interactions between the temperature of the print bed, the print chamber, and the print nozzle, as well as the print speed and layer thickness, must be carefully coordinated to ensure cohesion between the various layers and high precision of the printed object. At the same time, a certain economic efficiency must be taken into account, which in the future will translate into minimal waiting times for the dental technician, dentist, and patient.

PPSU: Optimization of material properties

Recently, research has focused on modifying the composition of PPSU to achieve antimicrobial activity and reduce plaque adhesion. For this purpose, PPSU can be doped with silver-containing zeolite fillers. Silver has been known as an antimicrobial agent for centuries and is successfully used in many different applications, from medical devices to cosmetics. The zeolite, which consists of microporous aluminosilicates composed of SiO4/2 and AlO4/2 tetrahedra, is used as a support material, allowing the doped PPSU to release the antimicrobial silver over time.

The intended use as a dental prosthesis material requires that PPSU

• sufficient surface properties,
• high mechanical strength and
• has successful adhesion to other dental materials.

To function successfully as a bioactive material, PPSU should release antimicrobial substances over a longer period of time. qualitatively Surface characterization SEM and EDX analyses can be used. The determination of the Martens parameters of PPSU can be used for a quantitative surface characterization be considered. The investigation of the flexural strength enables the assessment of the mechanical performance of PPSU, while determining the shear strength on a veneering and luting composite allows conclusions to be drawn about the success of veneering or luting PPSU restorations. By examining the Leaching properties The antimicrobial behavior can be determined, as this property is often related to the amount of silver released from the material. To allow an approximation of the in vitro test setups to the clinical situation, corresponding Aging simulations included.

Silver has been known as an antimicrobial agent for centuries and is successfully used in many different applications, from medical devices to cosmetics. Zeolite, which consists of microporous aluminosilicates composed of SiO4/2 and AlO4/2 tetrahedra, is used as a support material, allowing the doped PPSU to release the antimicrobial silver over time.

The following null hypotheses were examined:

• the material composition has no influence on the surface properties of PPSU, 
• neither the material composition nor (3) aging have an influence on the Martens parameters and the flexural strength, 
• the adhesive systems have no influence on the shear strength to a veneering or luting composite and 
• neither the material composition nor 
• The release time of the silver ions has an influence on the leaching properties of PPSU.

material and methods

Unfilled PPSU1 specimens and PPSU2 specimens filled with antimicrobial silver-coated zeolites were fabricated from granulate GR or filament FI or 3D printed (Fig. 1).

Scanning electron microscopy and X-ray spectroscopy were performed. Martens hardness, elastic indentation modulus, and flexural strength were determined initially and after aging. The shear strength to veneering and luting composites after conditioning with seven adhesive systems was examined after aging. Silver leaching was tested after 7, 1, 3, 7, 14, 21, and 28 days. For statistical evaluation, analyses of variance, Kolmogorov-Smirnov, Kruskal-Wallis, Mann-Whitney U, unpaired t-tests, and Weibull modulus were performed (p < 42).

Results

The zeolites were homogeneously distributed (Fig. 2). PPSU1-GR and PPSU1-FI exhibited the highest Martens parameters and elastic indentation modulus, followed by PPSU2-GR, PPSU1-3D, and PPSU2-3D. PPSU2-FI exhibited the lowest Martens parameters and elastic indentation modulus and showed micropitting. Aging showed lower Martens parameters and elastic indentation modulus for PPSU1 and no effect on PPSU2, while flexural strength increased (PPSU1) or decreased (PPSU2). PPSU2-3D exhibited lower flexural strength than PPSU1-3D. With the exception of the PR adhesive system, high shear strengths were observed for the luting (7,0–16,2 ​​MPa) and veneering composite (11,8–22,2 MPa). PPSU2-3D showed the highest silver release (9,6%), with all compositions releasing silver over 42 days.

Conclusion

During the six-week study period, antimicrobial silver ions were released from the filled PPSU. The high shear strength between PPSU and the veneering/luting composite confirmed the feasibility of esthetic veneering and luting of filled PPSU. To achieve the same mechanical properties as unfilled PPSU, the processing parameters of filled PPSU must be refined. This study provides proof of principle that PPSU can be successfully doped with silver-coated zeolites. The combination of 3D printing with an antimicrobial thermoplastic represents a major opportunity in the field of prosthetic dentistry. Potential applications include clasps for removable dentures, temporary or fixed prostheses, and implant abutments.