Peel bond strength between 3D printed tray materials and elastomers was studied.
The tray materials showed good chemical compatibility with the elastomer adhesives.
3D printed tray materials could provide clinically adequate bond strength.
Surface roughness generated by the AM technologies affected the bonding capacity.
Generation of 3D printed surface topographies was clarified.
The present study aimed to evaluate the bonding between three 3D printed custom tray materials and three elastomeric impression/adhesive systems using the peel test.
Test blocks were 3D printed by three different technologies using Dental LT, FREEPRINT tray, and polylactide (PLA) tray materials. The reference test blocks were conventionally fabricated with Zeta Tray LC, a light-curing resin. The surface topographies of the four tray materials were investigated by scanning electron microscopy (SEM) analyses and roughness measurements. The peel bond strength between the four tray materials and three impression/adhesive systems, vinylsiloxanether (VSXE), vinyl polysiloxane (VPS), and polyether (PE), was measured ( n = 12 per group). The peeling failure modes and rupture sites were identified microscopically.
The four tray materials featured different surface topographies. The peel bond strength was not significantly different with VSXE and PE, but PLA and the reference showed higher peel bond strength with VPS than the Dental LT and FREEPRINT tray ( p < 0.05). The rupture site of adhesive failure in all groups was partly at the adhesive-impression material interface and partly within the adhesive but never at the adhesive-tray material interface.
The 3D printed tray materials can achieve satisfactory chemical compatibility with the adhesives of VSXE, VPS, and PE. Surface topographies generated by the 3D printing technologies may affect bonding. Generally, 3D printed tray materials can provide clinically adequate bond strength with the elastomeric impression/adhesive systems. PLA is recommended for bonding with VPS when severe impression removal resistance is detected.
Compared to stock trays, custom trays provide prosthodontic dentistry with a more accurate solution for oral tissue recording . Since the polymerization-induced shrinkage of elastomeric impression material is proportional to the impression thickness , custom trays can ensure the accuracy of working models by maintaining a uniform thickness of the impression material . The precise fit of a custom tray to the dentition also reduces tissue distortion and the expense of impression materials during impression taking . In addition, streamline-designed custom trays minimize patient discomfort. However, the clinical use of custom trays has been limited by its fabrication process, which requires extra time and effort by dentists and dental technicians and often increases costs and the number of patient visits.
Presently, computer-aided design (CAD) and additive manufacturing (AM) have gained popularity in prosthodontic dentistry and have shown promise in facilitating the fabrication of custom trays . New tray materials have become commercially available for each of the prominent AM technologies (stereolithography (SLA), digital light processing (DLP), and fused filament fabrication (FFF)). To meet clinical needs, tray materials should be rigid and dimensionally stable but more importantly, they should provide sufficient retention for the impression material. Specifically, forces exerted during the impression removal may lead to debonding between the impression and the tray material, which can result in the deformation of the final impression, inaccuracy of the subsequent working model, and even failure of the prosthesis fit .
Bonding between conventional tray materials and elastomeric impression/adhesive systems has been thoroughly researched for decades. Both the surface chemistry and surface topography of the tray material have been shown to affect bonding. Studies have reported that the adhesive solvent of impression materials could slightly dissolve the outermost tray material surface , allowing the adhesive solute to penetrate the superficial molecular networks and be retained on the tray material surface . If chemical compatibility between the tray material and the adhesive cannot be achieved, bonding failure may occur at the adhesive-tray material interface. Maruo et al. and Payne et al. reported that the surface topography of the tray material significantly affected the bonding between the custom trays and the impression/adhesive systems. Payne et al. found that the bond strength of a light-curing resin with a vinyl polysiloxane (VPS) impression/adhesive system was significantly increased when the tray material surfaces were roughened with a tungsten carbide bur . Similarly, Davis et al. indicated that an acrylic resin adhered significantly better with a polysulfide impression/adhesive system after surface abrasion by silicon carbide paper .
At present, studies on the bonding behavior of elastomeric impression/adhesive systems to 3D printing tray materials are lacking. Therefore, the present study aimed to evaluate the bonding of three 3D printing custom tray materials printed by three AM technologies (SLA, DLP, and FFF) with three elastomeric impression/adhesive systems (vinylsiloxanether (VSXE), VPS, and polyether (PE)) using the peel test ( Table 1 ). The peel bond strength of the 3D printed custom tray materials was compared to that of a light-curing resin currently regarded as the conventional gold standard material for custom tray fabrication ( Table 1 ). It was hypothesized that the type of tray material has no influence on the peel bond strength.
|Tray material||Number of specimens||Corresponding AM technique||Component||Solidifying method||Flexural strength (MPa)||Manufacturer|
|Dental LT||36||SLA||(Meth)acrylate based||Blue light: 405 nm||≥50||Formlabs, Somerville, MA, USA|
|FREEPRINT tray||36||DLP||(Meth)acrylate based||UV light: 378–388 nm||>90||DETAX, Ettlingen, Germany|
|PLA||36||FFF||Polylactic Acid||Cooling below melting point (150 °C)||86–102||MakerBot Industries, Brooklyn, NY, USA|
|Zeta Tray LC||36||None (conventional method)||(Meth)acrylate based||UV light: 350–400 nm||79||Zhermack, Marl, Germany|
Materials and methods
CAD and AM of test blocks
The test block was designed using CAD software (OpenSCAD, 2015.03-2 Windows, www.openscad.org/ ) with a specific geometry to simulate the peeling action of the custom tray during the impression removal process . A cuboid handle with dimensions of 25.4 mm × 13 mm × 7 mm was connected at 45 degrees to the edge of a cuboid base with dimensions of 25.4 mm × 25.4 mm × 6 mm ( Fig. 1 a) . A 0.5 mm diameter hole was designed at the center of the handle ( Fig. 1 b). After the CAD design, the test block was saved in standard tessellation language (STL) format and transferred to the AM software for the print jobs.
A total of 108 test blocks were printed by SLA, DLP, and FFF 3D printers in the same printing direction ( Table 1 ). The Dental LT test blocks were printed by an SLA 3D printer (Form 2, Formlabs) with 100 μm layer thickness. The printing direction and layout are shown in Fig. 2 a . Twelve test blocks at a time were printed for 2 h and 20 min. After printing, the test blocks were washed in a post-cleaning device (Form Wash, Formlabs) with isopropyl alcohol (IPA, isopropanol 100%, SAV LP GmbH, Flintsbach am Inn, Germany) for 5 min, then cured in a post-curing device (Form Cure, Formlabs) at 80 °C for 20 min. The post-processed Dental LT test blocks ( Fig. 2 b) were then collected and stored in black plastic bags.
The FREEPRINT tray test blocks were printed by a DLP 3D printer (SOLFLEX 170, W2P Engineering, Vienna, Austria) using the Dental Gentle Vat Deflection Feedback System 3 mm build style. The test blocks were laid in the outermost positions of the platform and anchored to a supporting grid with 0.5 mm-wide supporting rods ( Fig. 3 a) . For each time, six test blocks were printed with a 100 μm layer thickness for about 53 min. After printing, the test blocks were ultrasonically cleaned with IPA twice for 3 min, then post-cured in a flash-light polymerization device (Otoflash G171, VOCO, Cuxhaven, Germany), where 2 × 2000 flashes were delivered under protective nitrogen. The post-processed FREEPRINT tray test blocks ( Fig. 3 b) were then collected and stored in black plastic bags.
An FFF 3D printer (Replicator+, MakerBot Industries) was used to print the PLA test blocks. A raft layer was created to support the test blocks with a 5 mm margin width ( Fig. 4 a) . The layer thickness and infill density were set to 100 μm and 100%, respectively. Twenty test blocks at a time were printed for 23 h. After printing, the PLA test blocks ( Fig. 4 b) were removed from the raft layer and collected for the peel test.
Fabrication of reference test blocks
The reference test blocks were fabricated using a 3D printed mold with extra space to accommodate the wax spacer. First, a 1.25 mm-thick base plate wax was adapted at the bottom of the mold ( Fig. 5 a) . A layer of conventional light-curing custom tray material (Zeta Tray LC, Zhermack, Marl, Germany) was adapted into the mold against the wax spacer ( Fig. 5 b). A UV-light device (Dentacolor XS, Kulzer, Hanau, Germany) was used to cure the first resin layer for 3 min. Additional light-curing resin was adapted onto the cured layer until the mold was fully filled ( Fig. 5 c). Another UV-light device (LML2000, Wilde, Walluf, Germany) was used to cure the subsequently adapted resin for 4 min under atmospheric pressure and another 4 min under vacuum. After light-curing, the reference test block was removed from the mold. A drilling machine (B 13S, Wörner, Denkendorf, Germany) equipped with a 4.5 mm drill bit was used to drill the centered hole of each reference test block. The quill travel was set to 49 mm and the drilling speed was set to 488 RPMs. After drilling, the reference test blocks were thoroughly cleaned using a steam jet cleaner (D-S 100 A, Harnisch + Rieth, Winterbach, Germany) and air-dried. The reference test blocks ( Fig. 5 d) were then collected and stored in black plastic bags.
Scanning electron microscopy
One sample of each tray material was sputtered by a sputter coater (SCD 050, BAL-TEC, Pfaeffikon, Switzerland) using Au-Pd alloy (SCD 050, BAL-TEC, Luebeck, Germany) with 61 mA current for 120 s to form a 20 nm thick Au-Pd coating. The sputtered samples were then observed by scanning electron microscopy (SEM) (Leo 1430, Zeiss, Jena, Germany). Representative images of the test surfaces were taken at 100X and 500X magnifications at an acceleration voltage of 10 kV.
Seven samples of each tray material were measured by a profilometer (S6P, Mahr, Goettingen, Germany) equipped with a stylus (2 μm, 90°). One hundred and twenty-one profiles with a length of 3 mm were recorded in a 3 mm × 3 mm area on the test surface. Next, the measured data were analyzed using roughness analysis software (MountainsMap Universal 7.3, Digital Surf, Besancon, France). After leveling the test surface, the profiles were filtered by a Robust Gaussian Filter (ISO 16610-71) with a cutoff value of 0.6 mm. Four 3D roughness parameters (arithmetic mean height (Sa), skewness (Ssk), core void volume (Vvc), and dales void volume (Vvv)) were calculated to evaluate the correlation between surface roughness and peel bond strength. Finally, 3D views of the test surfaces were reconstructed to visualize the surface topographies.
Peel bond strength test
The impression/adhesive systems investigated are shown in Table 2 . All adhesive drying times and impression set times given by the manufacturer’s instruction were respected in the execution of the test set-ups.
|Identium Heavy (5:1)||Flexitime Heavy Tray Dynamix||Impregum Penta H Duo Soft|
|Chemical component||Vinylsiloxanether (VSXE)||Vinyl polysiloxane (VPS)||Polyether (PE)|
|Impression type||Type 1, heavy body||Type 1, heavy body||Type 1, heavy body|
|Adhesive||Identium Adhesive||Universal Adhesive||Polyether Adhesive|
|Manufacturer, City, Country; LOT/Exp.MMYY||Kettenbach, Eschenburg, Germany;
|Kulzer, Hanau, Germany;
|3M Deutschland, Neuss, Germany;
|Tests per tray material||12||12||12|
|Adhesive drying time||5 min||2 min||1 min|
|Impression working time (23 °C)||2 min||2 min||2:30 min|
|Impression intraoral set time (37 °C)||2:30 min||2:30 min||3:30 min|
|Total set time||4:30 min||4:30 min||6 min|
The test blocks were cleaned by a steam jet machine (D-S 100 A, Harnisch + Rieth, Winterbach, Germany) and air-dried. A 3D printed carrier was bonded to each test block with molten adhesive wax (Supradent-Wachs, Oppermann, Bonn, Germany) to construct a test complex ( Fig. 6 a) . The adhesive was then smeared on the test surface and air-dried. The 3 mm-thick impression material was uniformly dispensed into an adaptor using an automatic mixing unit (Pentamix 2, 3M ESPE, Seefeld, Germany). Next, the test complex was placed onto the dispensed impression material at 23 °C for extraoral working time, then it was fixed using a custom-made loading device ( Fig. 6 b) in a 37 °C incubator (B 6200 (I), Heraeus, Hanau, Germany). A perpendicular weight of 1.4 kg was applied to the center of the test complex to mimic the compressive force during impression taking. Two lateral cubes of the carrier contacted the border of the adaptor, allowing reproducible impression depths for each test block. After impression setting, the carrier was removed and excessive impression material around the test block was cleared away with a sharp scalpel. The adaptor was then fixed in a universal testing machine (Z010, Zwick, Ulm, Germany). A tensile load at a crosshead speed of 300 mm/min was applied to the test block through a hook to produce a peeling force between the test block and the impression material ( Fig. 6 c). The peeling force and distance were dynamically recorded until the peeling was completed ( n = 12).
The width of each test surface was measured by a digital caliper (DIGI-MET, PREISSER, Gammertingen, Germany). The peel bond strength was calculated by Eq. (1) :