Determination of Shear Bond Strength between PEEK Composites and Veneering Composites for the Production of Dental Restorations

We studied the shear bond strength (SBS) of two PEEK composites (BioHPP, BioHPP plus) with three veneering composites: Visio.lign, SR Nexco and VITA VM LC, depending on the surface treatment: untreated, sandblasted with 110 μm Al2O3, sandblasted and cleaned ultrasonically in 80% ethanol, with or without adhesive Visio.link, with applied Visio.link and MKZ primer. For the BioHPP plus, differential scanning calorimetry (DSC) revealed a slightly lower glass transition temperature (Tg 150.4 ± 0.4 °C) and higher melting temperature (Tm 339.4 ± 0.6 °C) than those of BioHPP (Tg 151.3 ± 1.3 °C, Tm 338.7 ± 0.2 °C). The dynamical mechanical analysis (DMA) revealed a slightly higher storage modulus of BioHPP (E’ 4.258 ± 0.093 GPa) than of BioHPP plus (E′ 4.193 ± 0.09 GPa). The roughness was the highest for the untreated BioHPP plus, and the lowest for the polished BioHPP. The highest hydrophobicity was achieved on the sandblasted BioHPP plus, whereas the highest hydrophilicity was found on the untreated BioHPP. The highest SBSs were determined for BioHPP and Visio.lign, adhesive Visio.link (26.31 ± 4.17 MPa) or MKZ primer (25.59 ± 3.17 MPa), with VITA VM LC, MKZ primer and Visio.link (25.51 ± 1.94 MPa), and ultrasonically cleaned, with Visio.link (26.28 ± 2.94 MPa). For BioHPP plus, the highest SBS was determined for a sandblasted surface, cleaned ultrasonically, with the SR Nexco and Visio.link (23.39 ± 2.80 MPa).


Introduction
Polyetheretherketone (PEEK) is a member of the class of high performance polymers (HPP), with the common name polyaryletherketones (PAEK). PEEK is a semi-crystalline, polycyclic, aromatic polymer, where the aromatic rings are bonded with ether and ketone functional groups. The glass transition temperature (T g ) of PEEK is 143 • C and the melting temperature (T m ) is 343 • C [1][2][3][4]. PEEK became an interesting material for biomedical and dental applications because of its favourable features, such as high temperature resistance, biocompatibility and resistance to inorganic and organic chemicals. PEEK has a Young's modulus (E) of 4 GPa, [1,[5][6][7][8][9][10][11], which is much lower than that of titanium (102-110 GPa) [9,10], and very close to the human trabecular bone (1 GPa) [10]. The E of PEEK can be improved when using PEEK composites, and comes closer to the E of cortical bone (14)(15)(16)(17)(18) [1,9,10,12]. Immune reactions (type IV) can appear when using titanium alloys as a result of the possible release of titanium and metallic ions when the protective TiO 2 layer on titanium alloy implants breaks down as a consequence of the micromotions caused by cyclic loading and the acidic environment in the oral cavity. This is one of the reasons PEEK can also be used as an alternative to the titanium alloys used for artificial bone and orthopaedic spine or dental implants [13]. The limitation of PEEK is In dentistry, a PEEK composite with 20 wt% TiO 2 filler is used for abutments in dental implants, temporary abutments and frameworks for fixed or removable dental restorations. The term for a PEEK composite with 20 wt% TiO 2 is Biocompatible High Performance Polymer (BioHPP, REF: 54002030). A BioHPP with a semi-crystalline structure belongs to the group of technical polymers. The TiO 2 filler, in the form of fibres with a length of 0.3-0.5 µm, enables better mechanical properties than unfilled PEEK. The Young's modulus E of BioHPP is similar to the Young's modulus of PEEK. The material is on the market in a form for milling (the CAD/CAM technique) or in pellets/granulate for pressing [6,7,11,12,[23][24][25]34,[43][44][45].

BioHPP Plus
A newer type of PEEK composite (BioHPP plus) is also on the market, and has, according to the manufacturer (Bredent GmbH, Senden, Germany, REF: 54F2PP15), a higher Young's modulus than BioHPP. The higher modulus should positively influence the shear strength with veneering composites. BioHPP plus consists of 24 wt% inorganic fillers, for example TiO 2 and 1 wt% of an inorganic pigment. Pressed BioHPP plus specimens were manufactured using a For2press unit (Bredent GmbH).
2.1.3. Veneering Composites: Visio.lign, SR Nexco and VITA VM LC We used three veneering composites that differ in their polymer matrix and filler content: Visio.lign, SR Nexco and VITA VM LC (Table 1). According to the recommendation of the manufacturer of both PEEK composites, the Visio.lign composite system is the veneering composite of choice for BioHPP and BioHPP plus. In addition, veneering composite VITA VM LC with EDMA and TEGDMA matrix showed better tensile bond strengths for unfilled PEEK compared to an UDMA and EDMA veneering composite (GC Gradia) when the specimens were sandblasted with 50 µm Al 2 O 3 and adhesive Visio.link was applied. The results were lower when plasma was used as the surface treatment method of unfilled PEEK and PEEK composites with 20% TiO 2 . The results were higher for the PEEK composites with 20% TiO 2 and 1% pigment powder [5,39].  M8 14) to remove excess material and irregularities at the surface. The pressed specimens were manufactured according to the manufacturer's instructions. The cylinder-shaped specimens were 3D printed with a photopolymer resin (FotoDent cast resin) using Asiga Max UV, Puretone Ltd (Sydney, Australia). The 3D printed specimens were used to make a silicone mould, into which we poured dental wax (Thowax modelierwachs, beige, Yeti, LOT: 01111017). After pressing the specimens, they were sandblasted with 110 µm Al 2 O 3 (Chromokorund 110 µm, REF: 093904) and all excess material was removed.

BioHPP and BioHPP Plus Test Specimens for the SBS Test, Wettability and Surface Roughness Measurements
Overall, 273 specimens of both PEEK composites were manufactured for this study, in a cylindrical shape, with a diameter of 4.3 mm and height of 1 cm (Figure 1). The surface treatments are listed in Table 2. The specimens were arranged according to the framework material and surface treatment used ( Table 3). The first series, containing 147 BioHPP specimens, was milled out of blanks with the CAD/CAM milling machine. The second series, comprising 126 BioHPP plus specimens, was pressed out of pellets. The manufacturing of the BioHPP and BioHPP plus test specimens is described in Section 2.2.2.
at the surface. The pressed specimens were manufactured according to the manufacturer's instructions. The cylinder-shaped specimens were 3D printed with a photopolymer resin (FotoDent cast resin) using Asiga Max UV, Puretone Ltd (Sydney, Australia). The 3D printed specimens were used to make a silicone mould, into which we poured dental wax (Thowax modelierwachs, beige, Yeti, LOT: 01111017). After pressing the specimens, they were sandblasted with 110 µm Al2O3 (Chromokorund 110 µm, REF: 093904) and all excess material was removed.

BioHPP and BioHPP Plus Test Specimens for the SBS Test, Wettability and Surface Roughness Measurements
Overall, 273 specimens of both PEEK composites were manufactured for this study, in a cylindrical shape, with a diameter of 4.3 mm and height of 1 cm (Figure 1). The surface treatments are listed in Table 2. The specimens were arranged according to the framework material and surface treatment used ( Table 3). The first series, containing 147 BioHPP specimens, was milled out of blanks with the CAD/CAM milling machine. The second series, comprising 126 BioHPP plus specimens, was pressed out of pellets. The manufacturing of the BioHPP and BioHPP plus test specimens is described in Section 2.2.2.   The BioHPP specimens were classified into seven groups, each comprising 21 specimens, according to the surface treatment, whereas the BioHPP plus specimens were classified into six groups (we did not include the untreated group) ( Table 3).
Sandblasting was performed according to the manufacturer's instructions with 110 µm The groups were divided according to the surface treatments of BioHPP and BioHPP plus (mentioned further in the groups as the mark +) and the veneering composite used ( We also manufactured nine BioHPP and six BioHPP plus cylinder-shaped specimens for the wettability and roughness measurements. The nine BioHPP specimens were divided into three groups: untreated, sandblasted with 110 µm Al 2 O 3 and polished specimens. As untreated BioHPP plus specimens, we used specimens from which we had removed all casting material. Polishing of both PEEK composites was performed after removing all irregularities, first with rough burs, then with a polishing rubber disc, and last with Acrypole, REF 520

DSC Analysis
Thermal characterisation (the glass-transition temperature, T g , the melting temperature, T m , and the crystallisation temperature, T c ) was carried out using a differential scanning calorimeter, Mettler Toledo DSC 2 Star System. The specimens were dried before the analysis at 100 • C until all the water evaporated, using a Mettler Toledo HX024 moisture analyser. They were heated, subsequently cooled, and heated again in the temperature range of between 50 • C and 380 • C. The heating and cooling rates were 10 • C min −1 . All analyses were performed under a nitrogen atmosphere.
The degree of crystallinity (X c ) of both PEEK composites was calculated from the enthalpy of fusion (Equation (1)) [46]: where: ∆H f -is the enthalpy of fusion, ∆H f,c -is the enthalpy of fusion of the fully crystalline PEEK (130 Jg −1 ) [2], w-is the mass fraction of polymer in the PEEK composite

DMA Analysis
The viscoelastic properties (storage modulus, E , and loss modulus, E , damping factor, tan δ) and the glass transition temperature (T g ) of both PEEK composites were determined using a dynamical mechanical analyser DMA 8000, Perkin Elmer, with a dual cantilever, in accordance with ASTM D7028-07 (2015). The DMA was performed at a frequency of 1 Hz, an amplitude of 0.01 mm and a heating rate of 2 • C min −1 in the temperature range 30 • C-250 • C.

Wettability and Roughness Measurements
The wettability and roughness measurements were performed on untreated specimens, specimens sandblasted with 110 µm Al 2 O 3 and polished specimens. For the wettability measurements, we used a goniometer from DataPhysics, Goniometer OCA 35 (Filderstadt, Germany). Two test liquids were used: polar liquid (ultra-pure water, Millipore, Burlington, MA, USA) and nonpolar liquid (diiodomethane, Sigma-Aldrich, Burlington, MA, USA 99%), with a drop volume of 3 µL. The roughness measurements were performed using a contact profilometer, Mitutoyo SJ-210, to determine the mean roughness value (R a ) of the specimens at a scanning rate 0.25 mm s −1 , measuring track 2 mm and a distance between the tracks of 0.08 mm.
The surface free energies were calculated according to the Owen, Wendt, Rabel, Kaelble (OWRK) method (Equation (2)) using the computer program SCA-20 [47,48]: where γ lv -is the surface energy of the liquid, θ-is the contact angle between the test liquid and tested material, γ s d -is the disperse part of the surface free energy of the solid, γ l d -is the disperse part of the surface free energy of the test liquid, γ s p -is the polar part of the surface free energy of the solid and γ l p -is the polar part of the surface free energy of the test liquid.

Shear Bond Strength (SBS) Test
The loading of the specimens under shear stress is clinically more relevant than flexural or tensile stress, as shear stress occurs more commonly during chewing. In our case, we measured the shear strength between the framework material (BioHPP, BioHPP plus) and the three veneering composites with a crosshead speed of 0.01 m s −1 [12,17,23].
The SBS test was performed on a universal testing machine for the mechanical test, a Smitweld simulator-Tensile Test Unit 2002 (Nijmegen, The Netherlands), according to the standard ISO 10477: 2004 (E). A special mould, made of two parts and a hole through both parts, was inserted into the machine. The specimens were placed in the hole with a diameter of 4.3 mm and length of 1 cm. The interface between the substructure and veneering composite was in the middle of both mould parts.

Comparison of Stereomicroscope Images of the Interface Layer between the PEEK and Veneering Composites after the SBS Test
The analysis of the fracture surfaces was performed on a stereomicroscope Olympus SZX10 (Tokyo, Japan). For the analysis, we selected one random specimen from six groups: the specimens of the two groups that achieved the lowest SBS (AN and DVI+) and the specimens that achieved the highest SBS for BioHPP (group FV) were compared to group FV+ with the same surface treatment. In addition, group GN+, which achieved the highest SBS for BioHPP plus, was compared with group GN. We defined whether the failure was adhesive, cohesive or mixed. Adhesive failure, or delamination, is a failure along the interface surface between the PEEK framework and veneering composite resin, whereas cohesive failure occurs in the adhesive layer itself; a layer of adhesive remains on both surfaces, that is, the PEEK and the veneering composite resin. In a mixed failure, the adhesive and cohesive failure modes occur simultaneously [49,50].

Results
This section provides a summary of the DSC analysis, DMA analysis and the roughness, wettability and SBS measurements.

DSC Analysis
The glass transition temperatures of both PEEK composites differed by 0.9 • C, whereas the differences with the manufacturer's values were greater (Table 4, curves available in Supplementary materials). The T g of BioHPP (151.3 • C) was lower than the T g given by the manufacturer (155.8 • C), but higher than the T g of BioHPP plus (150.4 • C). This could be a consequence of the different manufacturing methods (CAD/CAM and pressing) and/or the composition of the PEEK composites. In addition, the DMA analysis showed a higher T g for BioHPP (169 • C) than for BioHPP plus (167 • C). However, the DMA and DSC values of T g differed because of the diverse measurement methods of both analyses [51]. The melting point, T m , of BioHPP (338.7 • C) was lower than the T m of BioHPP plus (339.4 • C) and the T m according to the manufacturer (345.1 • C). The degree of crystallinity (X c ) of BioHPP plus (27.9%) was lower than the degree of crystallinity of BioHPP (29.0%), and may affect the reduction in the T g value of BioHPP plus [20,52,53]. The first heating curve of BioHPP plus showed a cold crystallisation (T cc ) at 170.2 • C, which occurs simultaneously during the heating of quenched PEEK [54]. The temperature of crystallisation (T c ) of BioHPP (287.6 • C) could be lower than the T c of BioHPP plus (293.0 • C) because of the increased filler content in BioHPP plus (24 wt% inorganic filler and 1 wt% inorganic pigment) [55]. Table 4. The thermal transitions and enthalpy of fusion of the PEEK composites determined by DSC.

DMA Analysis
The viscoelastic properties of BioHPP and BioHPP plus were determined through DMA ( Table 5). The storage modulus (E ) of BioHPP is higher than the E of BioHPP plus. The results show that BioHPP plus is less rigid than BioHPP, despite the higher proportion of fillers, which may also be a consequence of the lower degree of crystallinity of BioHPP plus [54]. The loss modulus (E ) of BioHPP is lower than the E of BioHPP plus. The damping factors (tan δ) of BioHPP and BioHPP plus are the same, within a standard deviation value. The differences in the viscoelastic properties could be a consequence of the different manufacturing procedures, leading to some structural inhomogeneities that can occur during pressing, casting, heating and holding at a certain temperature, etc. [4,56].

Wettability and Roughness
The wettability and roughness tests were measured on the BioHPP and BioHPP plus specimens with various surface treatments (untreated, sandblasted specimens with 110 µm Al 2 O 3 and polished specimens). Table 6 represents the results of the wettability of BioHPP in water. The water contact angle of the BioHPP surface was θ > 90 • , except for the untreated and sandblasted surfaces, indicating a hydrophobic nature. On the contrary, the lowest contact angles and the most hydrophilic surfaces were those on the untreated BioHPP specimens, where the water contact angle was θ < 90 • . For all of the samples, the changes in the contact angle depend on the treatment of the specimen's surface, and were comparable to the results in the literature [26,30,37]. In addition, the changes in the BioHPP and BioHPP plus composites were evident. The surface free energy of the BioHPP and BioHPP plus specimens (Table 7) was calculated using the OWRK approach [47]. The results, in Table 7, show the main differences in the surface free energy, depending on the surface treatment of the BioHPP and BioHPP plus composites; the surfaces were either untreated, sandblasted or polished. In addition, sandblasting influenced the surface free energy of both BioHPP and BioHPP plus. Thus, it was calculated that the highest surface free energies were those on the sandblasted BioHPP plus specimens. When both the PEEK composites were polished, the specimens showed lower surface free energy than when untreated and sandblasted. The difference in the surface free energy of BioHPP and BioHPP plus could be the result of the different processing methods (CAD/CAM and pressing) and different surface geometries of the BioHPP and BioHPP plus specimens [10,57]. The results in Table 8 show the average roughness of BioHPP and BioHPP plus. The BioHPP specimens showed the lowest roughness. More specifically, the lowest roughness of the BioHPP specimens was measured on the polished surface; on the other hand, the highest roughness of the BioHPP was measured on the sandblasted specimens (Table 8), which agrees with the results reported in the literature [17,23,26,28,31,34,43]. In the case of BioHPP plus, the highest roughness was characteristic of an untreated surface, and the lowest roughness of a polished surface, comparable to BioHPP.

Shear Bond Strength (SBS)
In general, the BioHPP plus specimens showed lower SBS in comparison to the BioHPP specimens (Table 9 and Figure 2). The lowest SBS results of all the specimen groups are those of the BioHPP specimens, group A, with untreated surfaces. The highest SBSs were observed for the following BioHPP specimens: (a) Visio.lign veneering composite system after sandblasting and application of adhesive Visio.link (26. Two groups (AN and DVI+, Tables 3 and 9) did not reach the minimum required SBS of 5 MPa in the oral cavity, according to ISO 10,477 [23,28,43]. SBS higher than 10 MPa for PEEK used in veneering composite has been reported as clinically acceptable [8,26,29,35,39] and was not achieved in groups AV, AN, AVI, BN, CVI+, DN, DN+, DVI+ and EVI+. All the specimens in group D with a MKZ primer, with the exception of group DV, showed SBSs below 15 or 10 MPa. BioHPP with the veneering composite SR Nexco, as well as BioHPP plus with all the veneering composites, showed lower SBS with the use of Visio.link adhesive and a MKZ primer (group E) than specimens where only Visio.link adhesive was applied (group F). However, EV+ and EN+ showed SBSs higher than 10 MPa (15.70 MPa; 14.34 MPa). We believe that the lower SBS with the use of MKZ primer may imply impurities on the surfaces because we waited 30 s for the primer to dry. The specimens of group G, which were cleaned ultrasonically with 80% ethanol after sandblasting and then Visio.link adhesive was applied, showed a lower SBS on BioHPP with the Visio.lign composite system (20.31 MPa) compared to the BioHPP specimens of group G with the veneering composites SR Nexco (25.00 MPa) and VITA VM LC (26.28 MPa). However, the SBSs of all the G groups were much higher than the clinically acceptable shear bond strength (10 MPa). The lower SBS of the BioHPP plus in group G was achieved only with the use of the veneering composite VITA VM LC. Group C (sandblasting and ultrasonic cleaning with 80% ethanol) showed better SBS than group B (only sandblasting), the only exceptions being the BioHPP plus specimens with VITA VM LC. Table 9. Shear bond strength of BioHPP and BioHPP plus specimens with various surface treatments and three different veneering composites: Visio.lign (V), SR Nexco (N) and VITA VM LC (VI). A detailed description of individual groups is given in Tables 2 and 3.

SBS of BioHPP (MPa) SBS of BioHPP Plus (MPa)
Veneering Composites    Two groups (AN and DVI+, Tables 3 and 9) did not reach the minimum required SBS of 5 MPa in the oral cavity, according to ISO 10,477 [23,28,43]. SBS higher than 10 MPa for PEEK used in veneering composite has been reported as clinically acceptable [  The results show that the percentage value of standard deviations was the lowest in group EVI (7.6%), whereas the highest percentage value of standard deviations was in group AN (111%). When comparing the percentage of standard deviations for each group of veneering composites on BioHPP, the highest percentage was in the group A without surface preparation (AV, 53.1%; AN, 111%; AVI, 58.8%). This could be a consequence of the surface roughness and the surface that had the highest percent of standard deviation (45.8%). A higher percentage of standard deviations was seen also in groups DN (84.4%), BN (60.4%), EN (41.6%) and BVI (41.6%). This could also be a consequence of the surface roughness, due to irregularities on the surface.

SBS of BioHPP (MPa) SBS of BioHPP plus (MPa) Veneering Composites Visio Lign (V) SR Nexco (N) VITA VM LC (VI) Visio Lign (V+) SR Nexco (N+) VITA VM LC (VI+)
The shear modulus was calculated after the SBS test (Figure 3). We can conclude that the shear modulus results were quite in agreement with the SBS test results. The highest shear modulus was achieved by groups DV, EV and FV. Groups EVI and FVI, with two of the highest SBSs, had lower shear modulus results than groups DV and EV, but were still quite high compared to the other groups.
bond strength (10 MPa). The lower SBS of the BioHPP plus in group G was achieved only with the use of the veneering composite VITA VM LC. Group C (sandblasting and ultrasonic cleaning with 80% ethanol) showed better SBS than group B (only sandblasting), the only exceptions being the BioHPP plus specimens with VITA VM LC.
The results show that the percentage value of standard deviations was the lowest in group EVI (7.6%), whereas the highest percentage value of standard deviations was in group AN (111%). When comparing the percentage of standard deviations for each group of veneering composites on BioHPP, the highest percentage was in the group A without surface preparation (AV, 53.1%; AN, 111%; AVI, 58.8%). This could be a consequence of the surface roughness and the surface that had the highest percent of standard deviation (45.8%). A higher percentage of standard deviations was seen also in groups DN (84.4%), BN (60.4%), EN (41.6%) and BVI (41.6%). This could also be a consequence of the surface roughness, due to irregularities on the surface.
The shear modulus was calculated after the SBS test (Figure 3). We can conclude that the shear modulus results were quite in agreement with the SBS test results. The highest shear modulus was achieved by groups DV, EV and FV. Groups EVI and FVI, with two of the highest SBSs, had lower shear modulus results than groups DV and EV, but were still quite high compared to the other groups.  Figure 4 shows the stereomicroscope images of the fractured surfaces after the SBS test. From these images, we concluded whether the failure was adhesive, cohesive or  Table 3)

Optical Analysis of Fractured Surfaces with a Stereomicroscope
BioHPP plus  Figure 4 shows the stereomicroscope images of the fractured surfaces after the SBS test. From these images, we concluded whether the failure was adhesive, cohesive or mixed. None of the failures were completely cohesive, as only adhesive and mixed failures were observed (Table 10). Adhesive failures were observed for groups AN, DVI+ and FV+, whereas mixed failures occurred in groups GN, GN+ and FV. Group DVI+ also showed minor porosities, which occurred during veneering.   (Table 10). Adhesive failures were observed for groups AN, DVI+ and FV+, whereas mixed failures occurred in groups GN, GN+ and FV. Group DVI+ also showed minor porosities, which occurred during veneering.

Discussion
The SBS of the PEEK composites BioHPP and BioHPP plus with different veneering composites is influenced by many factors, such as the surface roughness and wettability, type of surface pretreatment and the use of adhesives and/or primers. Many studies have reported on the surface roughness, wettability and SBS of BioHPP with different veneering composites and cements, whereas, to the best of our knowledge, studies on BioHPP plus have not previously been conducted [23,26,28,30,34,43].
The contact angle with distilled water (Table 6) for untreated BioHPP (75.57 • ) was similar to the results of BioHPP reported in the literature (79.67 • ) [30]. The contact angle of sandblasted BioHPP was 89.75 • , which was higher than reported (84.83 • ) [30]. The contact angle with distilled water of sandblasted BioHPP plus (106.87 • ) was higher than of sandblasted BioHPP (89.75 • ), implying the higher hydrophilicity of sandblasted BioHPP. The surface free energy of sandblasted BioHPP plus was higher than that of BioHPP, which could be a consequence of the higher roughness of BioHPP plus. The polished specimens of both PEEK composites showed low surface free energy. However, the low surface free energy of polished specimens is desired because it decreases the possibility of bacteria adhering to the surface [58]. The untreated surfaces of BioHPP showed lower surface free energies than the unfilled PEEK (48.4 mN/m) [17], whereas BioHPP plus showed higher surface free energies than the unfilled PEEK, which indicates that the filler influences the surface free energies of both PEEK composites. Surface sandblasting increases the contact angle with distilled water. Nevertheless, our goal was to decrease the contact angle with distilled water and increase the surface hydrophilicity, which would positively influence the bond strength with veneering composites.
BioHPP plus also follows the trend of an increased contact angle with distilled water after sandblasting, although the roughness of the sandblasted surfaces was lower than that of the untreated BioHPP plus. The contact angle of the polished BioHPP plus surfaces was slightly lower, and the hydrophilicity was higher than that of the polished BioHPP surfaces. The polished BioHPP and BioHPP plus showed similar results in their surface free energies (40.93 mN/m and 38.80 mN/m, Table 7) to those reported in the literature (44.9 mN/m) [58].
The surface roughness of the BioHPP specimens differed depending on the type of pretreatment (Table 8): the highest surface roughness was found for sandblasted surfaces; those of the untreated BioHPP specimens were lower than the previous ones, whereas the lowest surface roughness was characteristic of the polished specimens. The surface roughness of the untreated BioHPP (0.568 µm,  [23]. The surface roughness of the polished BioHPP specimens (0.014 µm) was comparable to the results of the manufacturer (0.0239 µm), and the results of both BioHPP and BioHPP plus were lower than the results of pressed BioHPP reported in the literature (0.034 µm), which could be a consequence of different test parameters being used (the profile length was 1.75 mm and resolution 0.01 µm) [58].
The surface roughness of all the BioHPP plus groups (untreated, sandblasted and polished specimens) was higher than that of BioHPP. The surface roughness of the BioHPP plus sandblasted with 110 µm Al 2 O 3 was lower than that of the untreated specimens, which was the opposite to BioHPP. The lowest surface roughness was characteristic of polished surfaces, the same as for BioHPP. The slightly higher surface roughness of the polished BioHPP plus specimens compared to BioHPP could be a consequence of the different methods of manufacturing. During the compression of BioHPP plus specimens, some minor porosities can evolve, which is not the case for CAD/CAM BioHPP blanks [23,26,28,30,34,43].
The SBS of the untreated BioHPP specimens with a Visio.lign composite system in our study (7.27 MPa, Table 9 and Figure 2) was higher than that reported in the literature [23,28,59], where they did not use Combo.lign preopaquer (6.35 MPa; 5.09 MPa; 5.68 MPa). These results imply that the use of Combo.lign preopaquer is necessary to achieve higher SBS. Higher results with the Visio.lign veneering composite system, as in our study, were also achieved on untreated BioHPP with the use of Visio.link adhesive, Crea.lign opaquer and Crea.lign veneering composite after the specimens were aged in distilled water at 37 • C for 24 h (7.7 MPa) [27].
The results of the SBS between the BioHPP and Visio.lign composite system, sandblasted with 110 µm Al 2 O 3 and with applied Visio.link adhesive (26.31 MPa) (Table 9 and Figure 2), were similar to, and higher than, the ones in the literature [23,28,30,34,59]. In one study [59], BioHPP specimens were milled and sandblasted with 50 µm Al 2 O 3 (24.71 MPa), whereas, in another study [30], the specimens were sandblasted with 110 µm Al 2 O 3 (10.81 MPa). Visio.link adhesive and Combo.lign cement were applied in both studies. In the study [23], the specimens were sandblasted with 110 µm Al 2 O 3, and Visio.link adhesive and a Crea.lign veneering composite were applied (12.85 MPa). In the study [34], BioHPP specimens were milled and sandblasted with 110 µm Al 2 O 3 (6.14 MPa), whereas in another study [28], the specimens were sandblasted with 50 µm Al 2 O 3 , and then the Visio.link adhesive, Crea.lign opaquer and Crea.lign veneering composite were applied (10. Table 9). In comparison to the literature, our results were lower than those of milled BioHPP specimens after the same surface treatment, which were aged for 24 h (31.1 MPa) [40] or higher (13.60 MPa) [34]. The difference in results may be due to the fact that our specimens were not subjected to ageing.
In general, the SBSs of the BioHPP plus specimens were lower than those of BioHPP; this could be the result of the thermal pretreatment during pressing, which did not occur during the manufacturing of the BioHPP discs. In the study [60], they tested the influence of thermal pretreatment during the fused deposition modelling (FDM) of carbon-reinforced PEEK (CRF-PEEK) on the degree of crystallinity. They found that the degree of crystallinity increased with higher post-processing temperatures. Thus, we can assume that the degree of crystallinity of BioHPP plus is affected by lower post-processing thermal treatment and can lead to a reduction in the tensile strength and other mechanical properties. It is known that the higher the degree of crystallinity, the higher the tensile strength and mechanical properties of the material [60]. The highest results of SBS in our study were achieved for BioHPP sandblasted with 110 µm Al 2 O 3 , with the veneering composite Visio.lign and adhesive Visio.link (26.31 MPa), as well as for BioHPP sandblasted with 110 µm Al 2 O 3 , cleaned ultrasonically with 80% ethanol, with the veneering composite VITA VM LC and adhesive Visio.link (26.28 MPa).
Other surface treatments studied in the literature are: acid etching with sulfuric acid or piranha solution and plasma treatment or laser treatment [27,28,30,34]. In the studies [30,34], BioHPP specimens were treated with a sulfuric acid solution, and they achieved the highest SBS after artificial ageing (15.82 MPa; 13.80 MPa). Other pretreatments (piranha solution, plasma pretreatment and laser treatment) led to lower SBS than pretreatment with sulfuric acid. The SBS values of BioHPP in our study with the Visio.lign composite system were higher after almost all kinds of pretreatments, except for group AV, whereas, for BioHPP plus, the SBS results were higher only for groups FV+ and GV+. By using the veneering composite SR Nexco, higher SBSs were achieved in groups EN, FN, GN, FN+ and GN+, whereas the VITA VM LC veneering composite showed higher results only in groups EVI, FVI and GVI. The SBSs for groups F and G in our study were higher than the SBSs after pretreatment with sulfuric acid (15.82 MPa; 13.80 MPa, [30,34]), only FVI+ and GVI+ showed lower SBS (15.30;14.13 MPa). The results in our study confirm the improvement of the SBS between PEEK composites and veneering composites by using Visio.link adhesive on sandblasted surfaces.
The fracture of specimens ( Figure 4) was either adhesive or mixed (Table 10). When comparing the stereomicroscope images of groups FV and FV+, we can see that a specimen of group FV had a more cohesive failure than group FV+, which means that the bond between the BioHPP and Visio.lign veneering composite with the use of Visio.link adhesive after sandblasting was established better than for BioHPP plus (group FV+). This could be a consequence of the higher hydrophilicity of sandblasted BioHPP, which ensures good wettability and a better bond with the adhesive Visio.link. Group GN+ achieved the highest SBS for the BioHPP plus specimens, but lower than group GN with BioHPP. As seen on the stereomicroscope image, the fractures of both the GN+ and GN groups were mixed, but with a higher proportion of cohesive failure for the GN specimens. The specimens of groups AN and DV+ showed an adhesive failure, which was expected at low SBSs.

Conclusions
We studied two different PEEK composites: milled BioHPP (20 wt% filler TiO 2 ) and pressed BioHPP plus (24 wt% inorganic fillers and 1 wt% pigment), to determine their viscoelastic properties, surface roughness, wettability and shear bond strength (SBS) after different surface pretreatments with three veneering composites: Visio.lign, SR Nexco and VITA VM LC. Although it was expected that the composite BioHPP plus, with a higher proportion of fillers, would achieve a greater SBS than composite BioHPP, our results did not confirm this hypothesis. We infer that the composition and proportion of the fillers in BioHPP plus may affect the reduction in its degree of crystallinity, and, consequently, also the reduction in SBS with veneering composites.
The highest surface roughness among the BioHPP and BioHPP plus specimens was measured on the untreated BioHPP plus specimens, and the lowest surface roughness on the polished BioHPP specimens. The contact angle with distilled water of BioHPP plus decreased on polished surfaces, leading to increased hydrophilicity. The surface hydrophobicity of BioHPP increased in the following order: untreated surface, sandblasted surface and polished surface. BioHPP plus also showed a higher contact angle with distilled water after surface sandblasting, whereas the contact angle on polished surfaces was slightly lower than that of BioHPP.
The highest SBS of the BioHPP specimens was achieved for specimens whose surface was sandblasted with 110 µm Al 2 O 3 , coated with Visio.link adhesive and then a Visio.lign composite system was applied (group FV, 26.31 MPa). High SBSs were determined for the sandblasted specimens with the composite system Visio.lign, where the MKZ primer was applied (group DV, 25.59 MPa); sandblasted specimens veneered with VITA VM LC with applied MKZ primer and adhesive Visio.link (group EVI, 25.51 MPa); and sandblasted specimens cleaned ultrasonically in 80% ethanol, veneered with VITA VM LC and with applied Visio.link (group GVI, 26.28 MPa). A high SBS was also achieved for BioHPP specimens sandblasted with the SR Nexco veneering composite, where adhesive Visio.link was applied (group GN, 25.00 MPa).
For BioHPP plus, the highest SBSs were determined for the sandblasted surface, cleaned ultrasonically in 80% ethanol: with the veneering composite SR Nexco and Visio.link adhesive (group GN+, 23.39 MPa); and with the veneering composite Visio.lign and Visio.link adhesive (group GV+, 21.53 MPa).
In general, higher SBSs were observed for BioHPP and not for BioHPP plus composites. We can conclude that the higher roughness of the BioHPP plus surface decreases the SBS with veneering composites.
A limitation of this study was not performing long-term water (artificial saliva) storage or thermocycling. Our study had an in vitro design, which is why we did not include the aforementioned tests, and the obtained results should be interpreted by taking into account the limitations of in vitro studies.