Nanocomposites: The Little Things Matter a Lot

By RFBSIP / stock.adobe.com

When it comes to the size of the filler materials in your dental composite resin, downsizing is the way to go. Many of the top-performing materials today are nanocomposites. Nanocomposite filler materials create materials that not only look beautiful but wear well. Since the fillers are small enough to be measured on an atomic scale, nanocomposites prove that the little things matter a lot.

So why nanocomposites? Jason Goodchild, DMD, Vice President of Clinical Affairs for Premier Dental Products Company, says it was in pursuit of finding the perfect restorative materials that simplified the choosing process for various indications. Like all materials science, nanocomposites are the next step toward the ideal materials that work for everything, everywhere, without giving up anything in the process.

"The reason why people wanted to use nanoparticles is to eliminate compromises in composite technology. In the early days of composite materials, you either got esthetics, or you got physical properties. Nanocomposites try to marry the two to get to an ideal dental material. That was the idea of nanocomposites," Dr Goodchild says.

However, Dr Goodchild thinks that combining the esthetic performance with the strength and durability is not the only potential benefit for these materials. He sees a future where the nanoparticles can act as vehicles for biologics. Biologics are medical products taken from a biological source or manufactured in one and include human antibodies or variations of fusion proteins.¹

"You could end up with composites that have antimicrobial properties, or you have adhesives that contain nanoparticles that can become antimicrobial adhesives," Dr Goodchild says.

Nathaniel Lawson, DMD Ph.D., Associate Professor and Director, Division of Biomaterials for the UAB School of Dentistry, says understanding nanocomposites starts with the idea behind composite materials. Dental composites get their name from being a "composite" of resin polymer and ceramic filler particles.

"The function of the filler is to reinforce the resin and also reduce the polymerization shrinkage that occurs after light curing; resin shrinks and filler does not. The resin in different dental composites does not vary much, but the ceramic filler particles do," Dr Lawson says. 

The ceramic filler particles weren't always small. In the mid-twentieth century, composites were macrofills, with particle filters in the 10 to 100 μm (micrometers). Original composite from the 1950s and 1960s had large filler particles of crushed up pieces of glass, Dr Lawson explains. These glass particles could be up to 100 microns in size. With individual particles that you could see with the naked eye, these materials were strong but not that great looking. Dr Lawson says there were several problems with these big glass particles. 

"First, you can only pack them in so tight," Dr Lawson says. "Think of how densely you can pack sand into a bucket versus how densely you can pack golf balls in the same bucket. If the particles are not densely packed, then there will be a lower ratio of filler particles in the composite."

"Aside from its effect on strength and polymerization shrinkage," Dr Lawson continues, "the lower filler density also led to more resin being exposed on the surface of the composite. This exposed resin was susceptible to wear."

Dr Lawson says one of the early modifications to composite was the introduction of smaller silica fillers added to the crushed glass. These 40nm (nanometers) spherical silica particles could fill in the spaces between the large glass particles and achieve a higher filler density. Because these composites were a hybrid of large glass fillers and 40nm silica, they were called "hybrid composites." Another benefit of these tiny 40nm filler particles is that they are smaller than the wavelength of visible light (380-740nm).

"Therefore, when you polish the surface of a composite containing the silica filler particles, any divots on the surface that are created from the loss of a filler particle are not visually perceptible. That allows these composites to achieve a high gloss. So, attempts were then made to create a composite with only 40nm silica particles," Dr Lawson says.

Dr Lawson says the problem was that packing a composite with only 40nm filler particles led to agglomeration of particles and poor handling properties of the composite. In other words, he says, it was too stiff.

"To get around this issue, the silica particles were loaded into a resin at a high concentration, then polymerized, then crushed into little pieces. These little pieces of pre-polymerized composites were then used as filler particles. This type of composite is known as a 'microfill,'" Dr Lawson says. "Microfills can achieve a very high polish. But they often lack strength because they can fail at the interface between the pre-polymerized composite pieces and the resin in which they are embedded."

This opportunity led to the development of nanocomposites. Dr Lawson explains that nanocomposites contain agglomerates of the 40nm silica and other nano-scale ceramic fillers.

"By intentionally agglomerating the nano-sized particles, these particles can be loaded into resin at a higher filler concentration. Therefore, the composite gains the strength and shrinkage benefits of a high filler loading as well as the high gloss from having filler particles smaller than the wavelength of light," Dr Lawson says.

Because of their size, the nanocomposites filler materials organize the microstructure of the composite at the atomic and molecular level, which affects their performance in the physical properties.² One way to think of it is that the filler materials group together to achieve the functionality of larger particles also called a nanocluster.

Research indicates multiple benefits to using nanocomposites in composite restorations:³

• An improved interface that promotes stability and equal distribution of stress between the tooth and material
• Higher filler content that provides superior mechanical properties
• Smooth surface, which discourages plaque accumulation and gum irritation and improves esthetics with proper shade matching, high translucency, and increased polish retention
• Life-like appearance enabled by the light that "scatters" through the translucency, which improves blending with the surrounding dentition

Dr Goodchild also thinks it a considerable benefit to having a material that will give you everything you need. He says that the goal is to pick the right material for the right indication because you get the best possible results. However, he also says that sometimes practicing clinicians don't want to think about the choice; clinicians want a composite like a nanocomposite that will give them everything they need and streamline the decision.

"That's the Holy Grail," Dr Goodchild says.

However, Dr Goodchild thinks it is important to remember that there's never a perfect dental material because it's still a human-made thing. That said, taking the strength of macrofills and combining it with the polishability of microfills moves closer to the ideal.

"But if you're able to get closer to a perfect dental material without compromise, that is the whole idea of a nanocomposite," Dr Goodchild says.

References

1. Georgakopoulou, E., Scully, C. Biological agents: what they are, how they affect oral health and how they can modulate oral healthcare. Br Dent J 218, 671–677 (2015). https://doi.org/10.1038/sj.bdj.2015.439. Published 2015. Accessed May 14, 2021.

2. Sachdeva S, Kapoor P, Ak T, Noor R. NANO-COMPOSITE DENTAL RESINS: AN OVERVIEW. Semanticscholar.org. https://www.semanticscholar.org/paper/NANO-COMPOSITE-DENTAL-RESINS%3A-AN-OVERVIEW-Sachdeva-Kapoor/714a35db214c61155cb4e37fa9050e9f3f5c2129?p2df. Published 2015. Accessed May 10, 2021.