PICN Nanocomposite as Dental CAD/CAM Block Comparable to Human Tooth in Terms of Hardness and Flexural Modulus

Polymer infiltrated ceramic network (PICN) composites are an increasingly popular dental restorative material that offer mechanical biocompatibility with human enamel. This study aimed to develop a novel PICN composite as a computer-aided design and computer-aided manufacturing (CAD/CAM) block for dental applications. Several PICN composites were prepared under varying conditions via the sintering of a green body prepared from a silica-containing precursor solution, followed by resin infiltration. The flexural strength of the PICN composite block (107.8–153.7 MPa) was similar to a commercial resin-based composite, while the Vickers hardness (204.8–299.2) and flexural modulus (13.0–22.2 GPa) were similar to human enamel and dentin, respectively. The shear bond strength and surface free energy of the composite were higher than those of the commercial resin composites. Scanning electron microscopy and energy dispersive X-ray spectroscopic analysis revealed that the microstructure of the composite consisted of a nanosized silica skeleton and infiltrated resin. The PICN nanocomposite block was successfully used to fabricate a dental crown and core via the CAD/CAM milling process.

CAD/CAM blocks that offer excellent biocompatibility and mechanical properties in the oral environment have been practically implemented, but their mechanical properties differ from those of human tooth [16]. To overcome this issue, dental material development should consider biomimetics [17,18]. Biomimetic materials imitate a biological function and tissue morphology, where such dental materials have been previously investigated and reported [16,[19][20][21]. Biomimetic dental restorative materials for prostheses should

Materials and Methods
The composition of the precursor solution was optimized to obtain a monolithic block without fatal cracks, and six PICN composites were prepared under different preparation conditions (sintering time, type of infiltration resin monomer, and polymerization schedule) (see Appendix A). The mechanical properties (flexural strength, flexural modulus, and Vickers hardness) of the PICN composite blocks were evaluated, and the bonding properties to resin cement were assessed based on shear bond strength (SBS) and surface free energy (SFE). Further, the microstructure of the PICN composite was determined using scanning electron microscopy (SEM). The resultant PICN composite block was used to produce a dental crown and core via CAD/CAM milling.

Materials
The regents used to produce the PICN composite are listed in Table 1. The resulting PICN composites were compared to the commercial composites (i.e., control samples) listed in Table 2.

Preparation of PICN Composite
The PICN composites were produced using a novel process, as illustrated in Figure 1. This process included seven steps, as follows: (I) preparation of light-curable precursor solution, (II) molding of precursor, (III) light-curing of precursor to form a green body, (IV) sintering of green body to form a porous body, (V) infiltration of resin monomer into sintered porous body, (VI) heat-polymerization of the infiltrated body, and (VII) cutting the PICN composite to give CAD/CAM blocks. Six different PICN composites were produced by varying the preparation conditions, namely the sintering duration at 1150 • C, type of infiltrated resin monomer, and polymerization schedule for the infiltrated resin monomer.
The precursor solution (PS-1, see Appendix A and Table A1) were prepared with varying proportions of monomers (2-hydroxyethy methacrylate (HEMA) and triethylene glycol di-methacrylate (TEGDMA)) and solvents (2-phenoxyethanol (POE) and 1-propanol (PrOH)) with a fixed content of SiO 2 nanoparticles and light initiator (phenylbis (2, 4, 6-trimethyl-benzoyl) phosphine oxide (BAPO). The reagents were mixed using a planetary centrifugal mixer (ARE-310, THINKY Corp., Tokyo, Japan) at 2000 rpm for 6 min, and defoamed for 1 min using the defoam mode of the mixer to remove microbubbles from the solution. The precursor solution was poured into transparent silicone mold (height = 20 mm; diameter = 18 mm) and light-cured using a light-irradiator (α-LIGHT II N, J. Morita Corp., Suita, Japan) for 10 min. The samples were dried in an oven at 80 • C for 1 week to fabricate a green body. The green bodies were sintered in a furnace according to the following heating schedule: heating from room temperature to 220 • C at 50 • C/h; isothermal hold at 220 • C for 6 h; heating to 600 • C at 100 • C/h; isothermal hold for 3 h; heating to 1150 • C at 100 • C/h; isothermal hold for 1, 2, or 3 h (Table 3); and cooling to room temperature inside the furnace. The sintered body was a porous silica block, which was immersed in a silane solution of 7-MPTS (0.5 g), ethanol (8.5 g), distilled water (1.0 g), and 1M HNO 3 (100 µL) at room temperature for 3 h and dried in an oven (DY300, Yamato Scientific Co., Ltd., Tokyo, Japan) at 80 • C for 3 h. The silanized porous silica block was immersed in a resin monomer containing 0.5 wt% BPO at room temperature for 3 days. The monomer infiltrated silica block was heat-polymerized using the appropriate schedule for the monomer composition (Table 3) to give the PICN composite. The PICN composite was cut into blocks (12 × 15 × 10 mm 3 ) to obtain CAD/CAM blocks. heating to 1150 °C at 100 °C/h; isothermal hold for 1, 2, or 3 h (Table 3); and cooling to room temperature inside the furnace. The sintered body was a porous silica block, which was immersed in a silane solution of ɤ-MPTS (0.5 g), ethanol (8.5 g), distilled water (1.0 g), and 1M HNO3 (100 μL) at room temperature for 3 h and dried in an oven (DY300, Yamato Scientific Co., Ltd., Tokyo, Japan) at 80 °C for 3 h. The silanized porous silica block was immersed in a resin monomer containing 0.5 wt% BPO at room temperature for 3 days. The monomer infiltrated silica block was heat-polymerized using the appropriate schedule for the monomer composition (Table 3) to give the PICN composite. The PICN composite was cut into blocks (12 × 15 × 10 mm 3 ) to obtain CAD/CAM blocks.

Three-Point Bending Test
Each sample was cut and polished using emery papers up to #2000 to produce barshaped samples (width = 4 mm; length = 14 mm; thickness = 1.2 mm) (n = 10). The flexural strength and modulus of the samples were determined via three-point bending testing according to the standard procedure given in ISO 6872: 2008 [45]. A universal testing machine (AGS-H, Shimadzu Corp., Kyoto, Japan) with a support span of 12 mm and crosshead speed of 1 mm/min was used [10].

Vickers Hardness
After the three-point bending test, the fractured samples were used for the measurement of Vickers hardness according to the standard procedure given in ISO 6872: 2008 [45]. A hardness tester (HMV-G21ST, Shimadzu Corp., Kyoto, Japan) with a load of 200 g and dwell time of 15 s was used (n = 10) [39].

Inorganic Content
After hardness testing, the samples were weighed using an electric balance and calcined at 600 • C for 3 h in air to remove all organic matter. According to the literature [46], the organic matter in the sample, such as poly-UDMA and poly-TEGDMA, would be completely combusted at that temperature. The residue after calcination was weighed, and the inorganic content of the sample was calculated as the difference between the specimen weight before and after calcination (n = 10).

Shear Bond Strength
The SBS between the samples and a commercial resin cement was measured using a conventional procedure [47]. Disk-shaped samples (diameter = 10 mm, thickness = 1.5 mm) (n = 20) were polished using emery papers up to #1000. Silane primer (Porcelain primer, SHOFU Inc., Kyoto, Japan) was applied on the sample surface, and the resin cement (Resicem, SHOFU Inc., Kyoto, Japan) was loaded on the sample surface and cured using the light irradiator for 5 min. The cement-cured sample was held under ambient conditions for 60 min, and stored in distilled water at 37 • C for 24 h. The samples were divided into two groups to establish the properties before and after thermocycling, denoted as the 0-thermocycle and 20,000-thermocycle groups, respectively. Thermocycling was conducted by alternately immersing the samples in water baths at 5 and 55 • C for 20,000 cycles of 60 s in each bath. SBS testing of the 0-thermocycle and 20,000-thermocycle group samples was performed using the universal testing machine (n = 10). After SBS testing, the cementdebonded surface was observed using an optical microscope to classify the failure modes as one of three types, namely adhesive failure at the cement-sample interface, cohesive failure within the sample, or mixed adhesive and cohesive failure.

Surface Free Energy
The SFE of the samples (n = 10) was determined based on the contact angles between the sample surface and two liquids, namely distilled water and diiodomethane (Kanto Chemical Co., Inc. Tokyo, Japan). A contact angle meter (DMe-211, Kyowa Interface Science Co., Ltd., Saitama, Japan) was used under ambient conditions at 20 ± 3 • C (n = 10). The SFE was calculated using the Owens-Wendt theory [48] as follows: where θ denotes the contact angle for the liquids, the subscript indices L1 and L2 indicate water and diiodomethane, respectively, and γ total , γ p , and γ d are the total SFE, polar (hydrogen) SFE component, and dispersive SFE component of the sample, respectively. The SFE values for water and diiodomethane were based on previously reported values [48].

Microstructural Analysis
SEM and elemental mapping images of the samples were acquired using SEM (JCM-6000Plus NeoScope, JEOL Ltd., Tokyo, Japan) equipped with an energy dispersive X-ray spectroscopy (EDX) spectrometer.

CAD/CAM Milling of PICN Composite Block
The PICN composite block was milled to form a dental crown (maxillary right first premolar) (n = 1) and dental core (maxillary right first premolar) (n = 1) using a commercial CAD/CAM system (inLab MC X5, Dentsply Sirona Inc., Charlotte, NC, USA).

Statistical Analysis
Statistical analysis was performed using EZR software (Saitama Medical Center, Jichi Medical University, Saitama, Japan). Analysis of the flexural strength, flexural modulus, Vickers hardness, SBS and SFE was conducted using one-way analysis of variance (ANOVA) for multiple comparisons in the groups. Tukey's post hoc test was performed for the statistically significant groups. A significance level (p) of 0.05 was used for all analyses.

Mechanical Properties
The mechanical properties and inorganic contents of the PICN composites and commercial composites are given in Table 4. The flexural strength of the PICN composites was influenced by the preparation conditions, namely sintering time, infiltrated resin monomer, and polymerization schedule, where the highest flexural strength (153.7 MPa) was achieved in sample 2h-U-60. Further, the flexural modulus and Vickers hardness of the PICN composites increased with sintering time. The inorganic content of the PICN composites increased with increasing the sintering time from 71.2 wt% to 89.6 wt%. The 2h-U-60 composite was chosen as the representative PICN composite for the subsequent steps, including SBS analysis, SFE analysis, SEM-EDX analysis, and CAD/CAM milling fabrication. Table 4. Mechanical properties and inorganic content of the PICN composites and commercial composites (DC and AV) given as mean values (with standard deviation). Different letters indicate a significant difference between the groups (p < 0.05, Tukey test, n = 10).

Shear Bond Strength
The SBS test results of the PICN composite (2h-U-60) and commercial composites (DC and AV) before and after 20,000 thermocycles are given in Figure 2. Before thermocycling groups, there was difference between the PICN composite and AV. After thermocycling, the SBS of the PICN composite was significantly higher than those of DC and AV. Further, there was no significant change in the SBS value of the PICN composite between before and after thermocycling, while the SBSs of DC and AV significantly decreased.
AV exhibited the fewest cohesive failures before thermocycling, followed by the PICN composite and then DC (Figure 3). After thermocycling, AV exhibited the fewest, followed by DC and PICN composite. There was no difference in the incidence of cohesive failure of the PICN composite before and after thermocycling. before and after thermocycling, while the SBSs of DC and AV significantly decreased.
AV exhibited the fewest cohesive failures before thermocycling, followed by the PICN composite and then DC (Figure 3). After thermocycling, AV exhibited the fewest, followed by DC and PICN composite. There was no difference in the incidence of cohesive failure of the PICN composite before and after thermocycling.

Surface Free Energy
The PICN composite (2h-U-60) exhibited a higher total SFE (Figure 4a) and polar SFE component (Figure 4b) than the commercial composites (DC and AV), as well as the lowest dispersive SFE component (Figure 4c). before and after thermocycling, while the SBSs of DC and AV significantly decreased. AV exhibited the fewest cohesive failures before thermocycling, followed by the PICN composite and then DC (Figure 3). After thermocycling, AV exhibited the fewest, followed by DC and PICN composite. There was no difference in the incidence of cohesive failure of the PICN composite before and after thermocycling.

Surface Free Energy
The PICN composite (2h-U-60) exhibited a higher total SFE (Figure 4a) and polar SFE component (Figure 4b) than the commercial composites (DC and AV), as well as the lowest dispersive SFE component (Figure 4c).

Surface Free Energy
The PICN composite (2h-U-60) exhibited a higher total SFE ( Figure 4a) and polar SFE component (Figure 4b) than the commercial composites (DC and AV), as well as the lowest dispersive SFE component (Figure 4c).
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Microstructure
The EDX spectra of the PICN composite (2h-U-60) was compared to those of the commercial composites (DC and AV) ( Figure 5). The PICN composite exhibited peaks at-

Microstructure
The EDX spectra of the PICN composite (2h-U-60) was compared to those of the commercial composites (DC and AV) ( Figure 5). The PICN composite exhibited peaks attributed to silicon and oxygen, which corresponded to the silica skeleton, as well as a carbon peak due to the infiltrated resin. AV exhibited silicon and oxygen peaks related to its silica fillers, and carbon peaks due to the resin matrix, while DC exhibited peaks attributed to silicon, oxygen and carbon, as well as aluminum, barium, zirconium due to the barium glass and zirconia fillers.

Microstructure
The EDX spectra of the PICN composite (2h-U-60) was compared to those of the commercial composites (DC and AV) ( Figure 5). The PICN composite exhibited peaks attributed to silicon and oxygen, which corresponded to the silica skeleton, as well as a carbon peak due to the infiltrated resin. AV exhibited silicon and oxygen peaks related to its silica fillers, and carbon peaks due to the resin matrix, while DC exhibited peaks attributed to silicon, oxygen and carbon, as well as aluminum, barium, zirconium due to the barium glass and zirconia fillers.
SEM and EDX elemental mapping images were acquired to evaluate the silica (SiO2) inorganic component (oxygen and silicon) and the resin component (carbon) (Figure 6). The PICN composite exhibited a uniform PICN nanostructure, while DC and AV comprised nano-and in microsized dispersed-filler structures, respectively.

CAD/CAM Milling
The PICN composite was used to produce a CAD/CAM block, which was milled to give a dental crown and dental core (Figure 7). The prepared PICN composite monolith

CAD/CAM Milling
The PICN composite was used to produce a CAD/CAM block, which was milled to give a dental crown and dental core (Figure 7). The prepared PICN composite monolith block did not exhibit any cracks, while the milled crown and core exhibited no fatal damage such as edge chipping. Figure 6. SEM images and EDX elemental mapping images of silicon (Si), oxygen (O), and carbon (C) of (a) PICN composite (2h-U-60); (b) DC commercial composite; and (c) AV commercial composite. The white arrow in (Figure c) indicates the filler. The silica skeleton (Figure a) and the silica nanoparticles (Figure c) were homogeneous in nanoscale.

CAD/CAM Milling
The PICN composite was used to produce a CAD/CAM block, which was milled to give a dental crown and dental core (Figure 7). The prepared PICN composite monolith block did not exhibit any cracks, while the milled crown and core exhibited no fatal damage such as edge chipping.

Discussion
The effect of the PICN composite preparation conditions on the mechanical properties was evaluated ( Table 4). The infiltrated resin monomer affected the flexural strength, where the addition of UDMA (2h-U-60; 153.7 MPa) significantly enhanced the flexural strength compared to the composite prepared with only TEGDMA (2h-T-60; 117.7 MPa). TEGDMA has a lower strength than UDMA, and is usually used to dilute UDMA [49,50], which led to the superior flexural strength of the UDMA-infiltrated samples compared to the TEGDMA-infiltrated samples. The flexural strength was also affected by polymerization schedule, and was significantly higher in the sample polymerized at 60 °C for 5 days followed by 80 °C for 1 day (2h-U-60; 153.7 MPa) compared that polymerized at 100 °C for 1 day (2h-U-100; 119.0 MPa). Polymerization led to volume shrinkage, which typically

Discussion
The effect of the PICN composite preparation conditions on the mechanical properties was evaluated ( Table 4). The infiltrated resin monomer affected the flexural strength, where the addition of UDMA (2h-U-60; 153.7 MPa) significantly enhanced the flexural strength compared to the composite prepared with only TEGDMA (2h-T-60; 117.7 MPa). TEGDMA has a lower strength than UDMA, and is usually used to dilute UDMA [49,50], which led to the superior flexural strength of the UDMA-infiltrated samples compared to the TEGDMA-infiltrated samples. The flexural strength was also affected by polymerization schedule, and was significantly higher in the sample polymerized at 60 • C for 5 days followed by 80 • C for 1 day (2h-U-60; 153.7 MPa) compared that polymerized at 100 • C for 1 day (2h-U-100; 119.0 MPa). Polymerization led to volume shrinkage, which typically generates internal stress within the sample [51]. Slower polymerization moderated internal stress in the sample [52], thus the internal stress during polymerization of the infiltrated monomer resin in the 2h-U-60 sample was less than that of the 2h-U-100 sample. Sintering time affected both the Vickers hardness and flexural modulus of the PICN composite, which increased with increasing sintering time in 1h-U-60, 2h-U-60, and 3h-U-60. Sintering of the silica particles progressed over time, which led to a stronger silica skeleton after a longer sintering time. This phenomenon was supported by the increase in inorganic (silica) content of the sample from 73.2 wt% for 1 h sintering (1h-U-60) to 89.6 wt% for 3 h (3h-U-60).
The superior SBS of the PICN composite with the resin cement compared to the commercial composites (DC and AV) led to the PICN composite undergoing cohesive failure after thermocycling more often than the other composites. This was attributed to the preferable bond durability between the PICN composite and resin cement, which was related to its surface properties. The SFE analysis revealed that the polar SFE component and total SFE of the PICN composite were significantly higher than those of commercial composites. A previous study [54] demonstrated that the large polar SFE component of this type of composite is indicative of a large number of surface silanol groups, where the active site of the silane coupling agent allowed for higher bond strength to the resin cement. This facilitated effective bonding between the resin cement (with silane primer) and the PICN composite.
The microstructure of the PICN was too fine for observation using SEM-EDX analysis ( Figure 6). This demonstrated that the structure of the proposed PICN composite comprised a nanoscale silica skeleton with infiltrated resin. Thus, the proposed nanocomposite had a finer ceramic skeleton than previously reported microscale PICN composites [34,36,37].
To demonstrate the possible fabrication of a dental crown or core using the prepared PICN nanocomposite block, we attempted to mill the PICN nanocomposite block using the commercial CAD/CAM milling system. The PICN composite CAD/CAM block was successfully milled to form a dental crown and core without fatal damages (Figure 7).
Within the limitation of this study, the Vickers hardness and elastic modulus of the PICN nanocomposite block are comparable to those of enamel and dentin. These findings suggest the application potential of the proposed PICN nanocomposite as a biomimetic dental restorative material. The presented PICN nanocomposite clearly exhibited comparable Vickers hardness and lower elastic modulus than those of the alkali-aluminosilicate-glass skeleton (e.g., VITA ENAMIC; H V = ca. 177-190, E = ca. 29-38 GPa [10,34] or zirconia skeleton (H V = ca. 300, E = ca. 44 GPa [55]). Thus, the elastic modulus of the presented PICN nanocomposite is relatively similar to that of dentin. This can be ascribed to the microstructure of the presented PICN nanocomposite because the ceramic skeleton is consistent with the nanosized silica. The restorative material (e.g., a crown) developed using the presented PICN nanocomposite may overcome the problems caused by the difference in hardness between the opposite tooth and restorative material and by the difference in elastic modulus between the abutment tooth and restorative material. In the future, the wear and fatigue behaviors of the PICN nanocomposite are expected to be studied. In addition, in vivo studies will be conducted to compare the mechanical behaviors of such materials with those of conventional restorative materials.

Conclusions
A monolithic PICN nanocomposite block comprising a silica skeleton and infiltrated UDMA-based resin was prepared by optimizing the processing conditions. The PICN nanocomposite exhibited a similar Vickers hardness to enamel and flexural modulus to dentin, as well as excellent bond properties with resin cement. The PICN nanocomposite block was used to form a biomimetic dental crown and core via CAD/CAM milling. The proposed PICN nanocomposite shows great promise as a mechanically biocompatible restorative material.

Data Availability Statement:
The date presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.

Optimization of Precursor Solution
Six precursor solutions, referred to as PS-1 to PS-6, were prepared with varying proportions of monomers (HEMA and TEGDMA) and solvents (POE and PrOH) with a fixed content of SiO 2 nanoparticles and light initiator (BAPO) (supplemental Table). The reagents were mixed using the planetary centrifugal mixer. Monolithic porous silica blocks with a cylindrical shape (height = 20 mm; diameter = 18 mm) were formed using the precursor solutions via sintering. The green bodies were sintered in a furnace according to the following heating schedule: heating from room temperature to 220 • C at 50 • C/h; isothermal hold at 220 • C for 6 h; heating to 600 • C at 100 • C/h; isothermal hold for 3 h; heating to 1150 • C at 100 • C/h; isothermal hold for 2 h; and cooling to room temperature inside the furnace.
The monolithic porous silica blocks produced using precursor solutions PS-2, PS-3, PS-4, PS-5 and PS-6 formed fatal cracks during the sintering due to shrinkage stress. However, the monolithic porous silica block formed using PS-1 exhibited no cracks despite shrinking during the sintering process. Thus, PS-1 was used further in the present study, and the resulting monolithic porous silica blocks were successfully used to fabricate monolithic PICN composite blocks via the subsequent infiltration and polymerization processing steps.
Crack generation is a complicated phenomenon, and the mechanism through which cracking was suppressed in the PS-1 PICN composite has not yet been clarified. However, it is speculated that the appropriate ratio of resin monomers (HEMA and TEGDMA) and solvents (POE and PrOH) provided sufficient mechanical strength within the green body during light curing, which allowed for the structure to overcome the shrinkage stress generated during the subsequent sintering step. A PICN composite CAD/CAM block material must be capable of forming a monolithic block without fatal cracks. However, typical PICN composites tend to crack due to shrinkage during the sintering process. Therefore, determination of the optimal precursor solution composition was a critical step to ensure that monolithic blocks without fatal cracks were produced.