Methods. The authors developed two types of nanofillers: nanomeric particles and nanoclusters. They used optimal combinations of these nanofillers in a proprietary resin matrix to prepare the nanocomposite system with a wide range of shades and opacities. The properties they studied were compressive, diametral tensile and flexural strengths; in vitro three-body wear; fracture resistance; polish retention; and surface morphology after toothbrush abrasion. They performed statistical analysis using analysis of variance/Tukey-Kramer paired analysis at a 95 percent confidence interval.

Results. The compressive and diametral strengths and the fracture resistance of the nanocomposite were equivalent to or higher than those of the other commercial composites tested. The three-body wear results of the nanocomposite system were statistically better than those of all other composites tested. The nanocomposite showed better polish retention than the hybrids and microhybrids tested at the extended brushing periods. After extended toothbrush abrasion, the dentin, body and enamel shades showed polish retention equivalent to that of the microfill tested, while translucent shades showed better polish retention than the microfill.

Conclusions. The dental nanocomposite system studied showed high translucency, high polish and polish retention similar to those of microfills while maintaining physical properties and wear resistance equivalent to those of several hybrid composites.

Clinical Implications. The strength and esthetic properties of the resin-based nanocomposite tested should allow the clinician to use it for both anterior and posterior restorations.

The intense interest in using nanomaterials stems from the idea that they may be used to manipulate the structure of materials to provide dramatic improvements in electrical, chemical, mechanical and optical properties.³ At a billionth of a meter, a nanometer is the essence of small. For perspective, the size of one hydrogen atom is 0.1 to 0.2 nm and of a small bacterium about 1,000 nm or 1 micrometer. A large amount of research is being devoted to the development of nanocomposites of different types for various applications, including structural materials, high performance coatings, catalysts, electronics, photonics and biomedical systems.⁴ Every property has a critical length scale, and by using building blocks smaller than the critical length scale—such as nanoparticles—one can capitalize on the manifestations of physics at small sizes. An example of this is in light scattering. When a particle shrinks to a fraction of the wavelength of visible light (0.4–0.8 µm), then it would not scatter that particular light, resulting in the human eye’s inability to detect the particles. This has tremendous implications for the optical properties of materials, as this article will demonstrate.

One of the most significant contributions to dentistry has been the development of resin-based composite technology. Adhesively bonded composites have the advantage of conserving sound tooth structure with the potential for tooth reinforcement, while at the same time providing a cosmetically acceptable restoration.⁵ However, no one composite material has been able to meet both the functional needs of a posterior Class I or II restoration and the superior esthetics required for anterior restorations.⁶ Our objective was to develop a composite dental filling material that could be used in all areas of the mouth with high initial polish and superior polish retention (typical of microfills), as well as excellent mechanical properties suitable for high stress–bearing restorations (typical of hybrid composites).⁷ To this end, we developed novel nanofillers and then nanocomposites using advanced methacrylate resins and curing technologies.

Nanofillers are very different from traditional fillers and require a shift from a top-down to a bottom-up manufacturing approach.

Nanofillers are very different from traditional fillers and require a shift from a top-down to a bottom-up manufacturing approach. To make filler particles of the mechanically strong composites of today (such as macrofills, hybrids and microhybrids) one starts from dense, large particles (mined quartz, melt glasses, ceramics) and comminutes them to small particle size. However, these milling procedures usually cannot reduce the filler particle size below 100 nm (1 nm = 1/1,000 µm). To circumvent this roadblock, our team used synthetic chemical processes to produce building blocks on a molecular scale. We then assembled these materials into progressively larger structures and transformed them into nanosized fillers suitable for a dental composite. This article describes our research toward the development of a new dental nanocomposite, Filtek Supreme Universal Restorative (3M ESPE Dental Products, St. Paul, Minn.), that has the esthetic properties required for cosmetic restorations and the mechanical properties necessary for posterior restorations. In our research, we compared the properties of these materials with those of several commercial composites.

	MATERIALS

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Nanofiller particles. We synthesized two new types of nanofiller particles for this investigation: nanomeric, or NM, particles and nanoclusters, or NCs. The NM particles are monodisperse nonaggregated and nonagglomerated silica nanoparticles. In this investigation, we used aqueous colloidal silica sols to synthesize dry powders of nanosized silica particles 20 and 75 nm in diameter. We treated the silica particles with 3-methacryloxypropyltrimethoxysilane, or MPTS, using a proprietary method. MPTS, a bifunctional material also known as a coupling agent, contains a silica ester function on one end for bonding to the inorganic surface and a methacrylate group on the other end to make the filler compatible with the resin before curing to prevent any agglomeration or aggregation. MPTS also allows chemical bonding of the NM filler to the resin matrix during curing.

We synthesized two types of NC fillers using proprietary processes. The first type consists of zirconia-silica particles synthesized from a colloidal solution of silica and a zirconyl salt. The primary particle size of this NC filler ranges from 2 to 20 nm, while the spheroidal agglomerated particles have a broad size distribution, with an average particle size of 0.6 µm. The second type of NC filler, which we synthesized from 75-nm primary particles of silica, has a broad secondary particle size distribution with a 0.6-µm average. We treated the surfaces of both types of nanocluster filler particles with an MPTS coupling agent to provide compatibility and chemical bonding with the organic resin.

Resin system. The resin system used in the nanocomposites in this investigation is the same proprietary mixture used in Filtek Z250 Universal Restorative composite (3M ESPE Dental Products): bisphenol A glycidyl dimethacrylate, ethoxylated bisphenol A dimethacrylate, triethylene glycol dimethacrylate, 1,6-bis(2'-methacryloyloxyethoxycarbonylamino) -2,4,4-trimethylhexane, photoinitiators and stabilizers.

Nanocomposite preparation. Using statistically designed experimentation methodology, we studied many combinations of NC and NM fillers to determine an optimal formulation for the Filtek Supreme Universal Restorative dental nanocomposite system. The formulations for the dentin, body and enamel shades of Filtek Supreme Standard, or FSS, pastes contain zirconia-silica NCs and silica NPs. The effective primary particle size is 20 nm. The formulations of Filtek Supreme Translucent, or FST, shades contain a filler predominantly composed of individual NM particles 75 nm in diameter and a minor amount of silica NCs.

Commercial materials. We also tested several commercial composites currently on the market for comparative purposes. Table 1 lists the name of the product, the type of product, the name of the manufacturer and the lot number of each of the composites we tested.

	METHODS

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Wear. We determined the wear rate by an in vitro three-body wear test according to a modified Academisch Centrum Trandheelkunde Amsterdam, or ACTA, method.^10,11 In this test, we loaded composite (the first body) onto a wheel and cured it, then brought the wheel into contact with another wheel acting as an "antagonistic cusp" (the second body). The two wheels counter-rotated against each other, dragging an abrasive slurry (the third body) between them. We determined dimensional loss during 156,000 cycles by profilometry at regular intervals (that is, after every 39,000 cycles). As the wear in this method typically followed a linear pattern, we plotted the data using linear regression and determined the wear rates from the slope.¹² We tested three samples of each material.

Fracture toughness. We determined the fracture resistance of materials using a short rod fracture toughness method.¹² We cut a chevron-shaped notch into the end of a cured rod of material (8 millimeters in length x 4 mm in diameter) that had been stored in water at 37 C for seven days, and we propagated a crack through the chevron by pulling the two halves of the specimen apart at a controlled rate. The values reported for fracture resistance are related to the energy required to propagate a crack.^12,13 We tested five samples of each material studied.

Flexural strength. Flexural strength was measured (n = 3) using the three-point bending test method of the International Organization for Standardization.¹⁴

Polish retention. Samples of composite paste were cured in a rectangular 20 x 9 x 3–mm mold between two pieces of polyester film. We cured each specimen with a curing unit (Visilux 2, 3M ESPE Dental Products) for 80 seconds followed by curing for 90 seconds in a light box (UniXS, Heraeus Kulzer, Armonk, N.Y.). Specimens were mounted with double-sided adhesive tape (Scotch Brand Tape, Core series 2-1300, 3M, St. Paul, Minn.) to a sample holder (Ecomet 4 Grinder/Polisher, Buehler, Lake Bluff, Ill.). The mounted samples were polished using a polisher (Buehler Ecomet 4 Grinder/Polisher) with a special polishing head (Automet 2 Polishing Head, Buehler). The following sequence of abrasives was used for each sample: 320-grit and 600-grit silicon carbide abrasive, 9-mm diamond polishing paste, 3-mm diamond polishing paste and finally a polishing suspension (Masterpolish Polishing Suspension, Buehler). A micro–tri-gloss instrument (BYK-Gardner, Columbia, Md.) was used to collect photoelectric measurements of specularly reflected light from the sample surface after polishing and after tooth brushing. We conducted the procedure according to American Society for Testing and Materials’ standard for measuring specular gloss¹⁵ for measurements made at 60 degrees geometry with the following analysis. Initial gloss after polishing (G_I) was measured for sample immediately after preparation. Final gloss was measured after 500 toothbrushing cycles (G_F). The G value was calculated with the following formula: G = (G_F) – (G_I). gloss retention (GR) = 100 x G/G_I. Each sample was brushed for a total of 500 cycles with a straight toothbrush (Oral B 40 medium, Oral B Laboratories, Belmont, Calif.) using (Crest Regular Flavor, Procter & Gamble, Cincinnati). We ran three replicates for each formulation tested.

Surface morphology after toothbrush abrasion. We mounted samples for scanning electron microscopy, or SEM, analysis on an aluminum stub using double-sided tape with the toothbrush-abraded surface facing upward. We ground the samples on the edges with colloidal graphite and gold-palladium sputter-coated for 30 seconds in a sputter coater (Desk II Cold Sputter-coater/Etch Unit, Denton Vacuum, Moorestown, N.J.). We performed SEM analysis using two models of scanning electron microscopes (models 820 and 840, JEOL USA, Peabody, Mass.). We performed SEM analysis on all dental composite samples at magnifications of x100, x500, x2,500 and x10,000 and at a stage tilt of 45 degrees.

Transmission electron microscopy. We prepared samples for transmission electron microscopy, or TEM, analysis by microtomy on a microtome (Leica UCT, Reichert Analytical Instruments, Depew, N.Y.) at room temperature conditions, using a diamond knife (at a 45-degree angle) (UCT, Diatome U.S., Fort Washington, Pa.). We did the sectioning at 0.2 mm/second to provide a section thickness of 80 nm. Sections were floated on water and collected on 200 mesh copper grids with carbon stabilized embedding material substrates. We performed TEM on a JEOL 200CX (JEOL USA) at 200 kilovolts.

Statistical analysis. We performed an analysis of variance, or ANOVA/Tukey-Kramer paired analysis using a software program (JMP 4.0 Statistical Discovery Software, SAS Institute, Cary, N.C.) at the 95 percent confidence interval, or CI.

	RESULTS

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Figure 1 shows three transmission electron micrographs:

View larger version (100K): [in this window] [in a new window]   Figure 1. Schematics and transmission electron microscopic images of composites studied. A. Composite with nanometric particles (x60,000 magnification). B. Composite with nanocluster particles (x300,000 magnification). C. Composite with hybrid fillers (x300,000 magnification). nm: Nanometers. APS: Average particle size. µm: Micrometer.

– one of a nanocomposite filled with 75-nm diameter NM particles only;

– one of an experimental nanocomposite filled with NCs alone;

– one of a commercial composite made with large-particle-size dense hybrid filler (particle size, approximately 1 µm).

Table 2 lists the mechanical properties of FSS and FST along with those of the materials with which we compared them. Figure 2 shows a comparative summary of the results of diametral tensile strength, compressive strength, flexural strength and fracture resistance. Statistical analysis by the ANOVA/Tukey-Kramer paired test showed that the compressive and diametral tensile strengths of the FSS and FST were equivalent to or higher than those of the other commercial composites tested. The flexural strength of FSS and FST was higher than that of three of the other composites and equivalent to that of the other commercial composites tested at the 95 percent CI (Table 2). Statistical analysis showed that the fracture resistance of FSS and FST was higher than that of one of the composites tested and equivalent to that of the other commercial composites tested at the 95 percent CI.

View larger version (37K): [in this window] [in a new window]   Figure 2. Comparison of mechanical properties of composites normalized to Filtek Z250 Universal Restorative composite (hybrid) (3M ESPE Dental Products, St. Paul, Minn.). Z250: Filtek Z250. TPH: TPH Spectrum (Dentsply Caulk, York, Pa.). Point 4: Point 4 (Kerr, Orange, Calif.). EsthetX: EsthetX (Dentsply Caulk). A110: Filtek A110 (3M ESPE Dental Products). FSS: Filtek Supreme Standard formulation of dentin, body and enamel shades (3M ESPE Dental Products). FST: Filtek Supreme Translucent formulation (3M ESPE Dental Products).

View larger version (33K): [in this window] [in a new window]   Figure 3. Wear resistance of Filtek Supreme Universal Restorative (3M ESPE Dental Products, St. Paul, Minn.) nanocomposites as compared with that of the other composite materials in the study. Z250: Filtek Z250 Universal Restorative composite (3M ESPE Dental Products). TPH: TPH Spectrum (Dentsply Caulk, York, Pa.) Point 4: Point 4 (Kerr, Orange, Calif.). EsthetX: EsthetX (Dentsply Caulk). A110: Filtek A110 (3M ESPE Dental Products). FSS: Filtek Supreme Standard formulation of dentin, body and enamel shades (3M ESPE Dental Products). FST: Filtek Supreme Translucent formulation (3M ESPE Dental Products).

View larger version (30K): [in this window] [in a new window]   Figure 4. Gloss retention of Filtek Supreme Universal Restorative (3M ESPE Dental Products, St. Paul, Minn.) nanocomposites as compared with that of other composites in the study. Z250: Filtek Z250 Universal Restorative composite (3M ESPE Dental Products). TPH: TPH Spectrum (Dentsply Caulk, York, Pa.). Point 4: Point 4 (Kerr, Orange, Calif.). EsthetX: EsthetX (Dentsply Caulk). A110: Filtek A110 (3M ESPE Dental Products). FSS: Filtek Supreme Standard formulation of dentin, body and enamel shades (3M ESPE Dental Products). FST: Filtek Supreme Translucent formulation (3M ESPE Dental Products).

View larger version (80K): [in this window] [in a new window]   Figure 5. Scanning electron microscopic images of toothbrush-abraded surfaces of restorative dental composites. A. Hybrid. B. Microfill. C. FSS: Filtek Supreme Standard formulation of dentin, body and enamel shades (3M ESPE Dental Products, St. Paul, Minn.). D. FST: Filtek Supreme Translucent formulation (3M ESPE Dental Products).

	DISCUSSION

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Although the average cluster size of the NCs developed for our work is similar to that in conventional hybrid fillers, NC particles are fundamentally different from hybrid filler particles. Hybrid fillers, typically, are large, dense particles of an average size of about 1 µm, as shown by schematic drawing and TEM in Figure 1C (page 1385). These particles cannot be further subdivided under normal abrasive forces in the mouth. Similar remarks apply to microhybrids, which are only slightly smaller than hybrids in average particle size. By contrast, we propose that the nano-sized primary particles in the NCs, clearly seen in the cluster domain of Figure 1B (page 1385), wear by breaking off individual primary particles (rather than plucking out the larger secondary particle from the resin). Thus, the resulting wear surfaces have smaller defects and better gloss retention. The SEMs of wear facets of toothbrush-abraded surface of the NC composite (Figure 5B) contrasts with those of a hybrid material (Figure 5A), which clearly show large particles protruding from the surface, as well as pits where particles have been plucked from the surface. Microhybrid composites contain particles somewhat smaller in size than do hybrids. However, the two microhybrid materials we tested showed significant surface roughness after 500 cycles of toothbrush abrasion. This is because the mechanism of abrasion in a microhybrid is similar to that of a hybrid. Individual filler particles were plucked out, leaving voids that are only slightly smaller than those in traditional hybrids. The resultant surface is not as rough as that of a hybrid material but certainly not as glossy as that of a microfill.

NM particles and NCs also are fundamentally different from particles in microfill fillers. Typical microfill fillers are made using pyrogenic processes, which produce materials with an average primary particle size of about 40 nm, but in which the primary particles typically aggregate in fibrous, low-density, chain-like secondary structures. The fibrous structures of microfill fillers limits paste filler loadings and results in poor handling and lower mechanical properties than are demonstrated by hybrids and microhybrids. Commercial microfills generally contain prepolymerized resin particles previously filled with fumed silica (commonly known as "organic filler") to improve the handling characteristics. Because of the small primary particle size, microfills display high gloss retention but poor bonding between the organic filler particles, and the resin matrix lowers the mechanical properties. Thus, indications for microfills usually are limited to low stress–bearing anterior restorations.

The use of spheroidal NC fillers with their broad particle distribution enabled us to obtain high filler loading, desirable handling characteristics and physical properties comparable with those of commercial hybrid composites. The diametral tensile strength, compressive strength, flexural strength and fracture resistance of the FSS and FST formulations of Filtek Supreme Universal Restorative are statistically equivalent to or higher than those of the hybrid or microhybrid composites tested and significantly higher than those of the microfill material tested. These results, combined with the wear results (and other data not reported here), support the use of these materials for the same indications as those for other universal restoratives.

Although microhybrid composites contain particles somewhat smaller in size than hybrids, the two microhybrid materials we tested showed significant surface roughness after 500 cycles of toothbrush abrasion. This is because the microhybrid’s mechanism of abrasion is similar to that of a hybrid. Individual filler particles are plucked out, leaving voids that are only slightly smaller than those in traditional hybrids. The resultant surface is not as rough as that of a hybrid but certainly not as glossy as that of a microfill.

The FST and FSS nanocomposites use combinations of NM-particle and NC fillers in optimized ratios for desirable performance. The NM particles in these formulations fill the interstitial spaces between the clusters. The resultant surface, thus, is densely packed with fillers. The FST and FSS materials consequently display high polish retention after toothbrush abrasion. When these materials undergo toothbrush abrasion, only nanosized particles are plucked away, leaving the surfaces with defects smaller than the wavelength of light. The visual appearance retains a high gloss and is consistent with the smooth surfaces displayed in the SEM of Figure 5D. In this study, the maintenance of surface smoothness of the abraded Filtek Supreme translucent material, which consists of predominantly NM particles and is qualitatively equivalent to the microfill we tested. The major portion of the FSS filler consists of NCs (that is, the NM particle concentration here is lower than that of the FST). Not surprisingly, the FSS showed a slightly rougher surface than did the FST. However, the abraded surface of FSS still was found to be very smooth compared with those of the hybrids and microhybrids in this study. It is evident from the SEMs that individual nanoparticles of the zirconia-silica NC sheared off. This is in contrast to the situation in hybrids of microhybrids, where large particles sheared off in totality, leaving much larger craters on abrasion.

Nanofillers also offer advantages in optical properties. In general, it is desirable to provide low visual opacity in unpigmented dental composites. This allows the clinician to construct a wide range of shades and opacities and, thus, provide highly esthetic restorations. In hybrid materials, fillers consist of particles averaging 1 mm in size. When particles and resin are mismatched in the refractive index, which measures the ability of the material to transmit light, the particles will scatter light and produce opaque materials. In NM-particle materials, the size of the particles is far below the wavelength of light, making them unmeasurable by the refractive index. When light comes in, long-wavelength light passes directly through and materials show high translucency. As shown in Figure 6, the disks made with hybrid and microfill fillers are rather opaque. The FST sample made predominantly with the NM particle filler is very clear, as the background can be seen through the composite. In addition, when placed on a black background, the nanoparticles preferentially scatter blue light, giving the composite an opalescent effect. The ability to create a nanocomposite with a very low opacity provides the ability to formulate a vast range of shade and opacity options from the very translucent shades needed for the incisal edge and for the final layer in multilayered restorations to the more opaque shades desired in the enamel, body and dentin shades. The commercial material is available in three translucent shades, seven enamel shades, 13 body shades and seven dentin shades. This allows the clinician the flexibility to make a choice of using a single shade or a multishade layering technique, depending on the clinical case in question.

View larger version (56K): [in this window] [in a new window]   Figure 6. Optical effect of nanocomposite material versus that of the other types of composite materials studied. FST: Filtek Supreme Translucent formulation (3M ESPE Dental Products, St. Paul, Minn.).

	CONCLUSIONS

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This article describes the use of nanotechnology to make a dental restorative composite system that offers high translucency, high polish and polish retention similar to those of microfills while maintaining physical properties and wear resistance equivalent to several commercial hybrid composites. Combinations of two types of nanofillers result in the best combination of physical properties. With the combination of superior esthetics, long-term polish retention and other optimized physical properties, it is expected that this novel nanocomposite system would be useful for all posterior and anterior restorative applications. Clinical studies are needed to confirm the laboratory findings.

View larger version (120K): [in this window] [in a new window]   Dr. Mitra is a corporate scientist, 3M ESPE Dental Products, 3M, Building 260-2B-13, 3M Center, St. Paul, Minn. 55144, e-mail "sbmitra@mmm. com". Address reprint requests to Dr. Mitra.

View larger version (137K): [in this window] [in a new window]   Dr. Wu is a research specialist, 3M ESPE Dental Products, 3M, St. Paul, Minn.

View larger version (157K): [in this window] [in a new window]   Dr. Holmes is a senior research specialist, 3M ESPE Dental Products, 3M, St. Paul, Minn.