Nanodentistry: Big Impacts from Small Science

Imagine your toothpaste contains an army of tiny cleaning robots. You brush, rinse, and spit, but the robots keep on working. They travel around, metabolizing the leftover pizza and soda scum on the surface of your teeth into harmless, odorless vapors. They fight off all of the decay-causing bacteria and then, mission complete, deactivate and pass on to the next life.

These “nanorobots” are still hypothetical, but they are one of many possibilities that emerge when you design tools, materials, and even robots on the scale of atoms—possibilities that include reducing the anxiety, fear, and pain that millions of people associate with the dentist. And, strangely enough, doing so by applying physics—a subject that also causes anxiety and fear for many people.

A dentist’s office may seem primitive, with the drilling and tooth pulling, but most dentists rely on pretty sophisticated scientific tools. For example, lasers are now commonly used to treat gum disease and ulcers, prepare teeth for fillings, and perform oral surgeries. Many of these procedures are more accurate and less painful when done with a laser than with traditional methods. Dentistry has come a long way since the pre-Enlightenment days, when people in many cultures believed that cavities were holes bored by tooth worms.

Nanodentistry is a new and growing field that merges nanotechnology with dentistry. Objects on the nanoscale are measured in nanometers and are extremely tiny, around the width of DNA. Just one inch is 25.4 million nanometers.

When you zoom in to this level, things get weird. The properties of a material – like color and melting point – depend on the size of the particle, they aren’t intrinsic to the material like they are on the visible scale. For example, gold nanoparticles of different sizes interact with light differently. Solutions made with gold nanoparticles of different sizes are different colors. At the nanoscale you can engineer a material to have a specific property by changing its size.

Solutions of gold nanoparticles of various sizes. The size difference causes the difference in colors.

Teeth are way off the nanoscale charts, but many biological process take place on that scale. This is the world of atoms (0.1-0.3 nm), proteins (3-6 nm), viruses (30-50 nm), and many cellular processes. Understanding physics at the nanoscale enables us to design devices and structures that interact with these biological processes in new and targeted ways.

Good dental care is a combination of preventing problems, treating problems, and making your smile look good. Nanodentistry is leading to improvements all of these areas through new products and techniques. Some are already available while others are in various stages of development and testing.

On the Market

Nanodentistry made its debut in a less-than-glamorous fashion: filling cavities.

Archeological evidence suggests that ancient Greeks may have treated cavities with linen soaked in medicine, and a cavity in a tooth 65-centuries old is still packed with beeswax. For the last 150+ years, most cavities have been filled with amalgam, a mix of silver, tin, copper, mercury, and small amounts of other metals.

Image Credit: David Shankbone, CC BY

Fillings are a materials challenge. Dentists need a material that is pliable and conforms easily to a hole in the tooth, but that hardens quickly and is strong enough to withstand daily chomping. Amalgam does this relatively easily and cheaply and has been used to fill billions of cavities. However, it has been plagued by concerns about possible health risks associated with its mercury content despite the approval of major health organizations including the US Food and Drug Administration. The mercury is essential—it is only by adding the mercury that the amalgam becomes pliable. The mixture starts to harden almost immediately after the mercury is added. Aside from the mercury concerns, the main downside to amalgam is its color. Many people prefer fillings that match the color of their teeth over the silver-colored amalgam.

The most common alterative to amalgam is composite resin. Composite resins are essentially a mixture of a plastic resin, filler particles, and a “glue” that holds them together. The plastic resin controls the consistency of the mixture. It starts off soft and doughy so that the filling can conform to the cavity. After the composite resin is placed, the dentist shines a bright, blue light on the tooth to harden the filling. The energy from this light initiates a process called polymerization within the resin. During this process the single molecules that compose the resin bond together into polymer chains, making the filling more rigid.  The filler particles, often quartz, glass, or silica, affect a filling’s appearance, strength, and durability. Unlike amalgam, composite resins can be color-matched to a tooth. They tend to cost more though, even for patients with insurance, and they wear out sooner in large cavities.

Dentists can choose from many kinds of composite resins, which are often categorized by the size of the filler particles. Nano-composite resins use nano-sized filler particles and have several advantages. Composite resins with a higher volume of filler particles are stronger and more resistant to wear. Like packing balls in a box, you can fit a greater volume of small particles in a composite resin than large particles.

In addition, the size of the filler particles impacts the smoothness of the surface. Larger particles lead to a rougher surface. Because nanoparticles are so small, dentists can polish nano-composites until they are extremely smooth. Smoother surfaces offer fewer places for plaque to stick and hide. From an aesthetic perspective, nano-composite fillings interact with light more like real teeth and defects in the surface are practically invisible because the nanoparticles are smaller than the wavelength of visible light.