“Application of plasma treatment in dental restoration procedures will effectively disinfect cavity-causing bacteria, reduce the use of the painful and destructive drilling currently practiced in dental clinics, and consequently save healthy dental tissues,” says one of the technology’s
inventors, Qingsong Yu, PhD, an assistant professor of mechanical and aerospace engineering at the University of Missouri-Columbia (UMC). “It also improves the bonding strength of restoration composites and prolongs the restoration life. Furthermore, it requires no complicated or expensive equipment.” Calling the technology “novel,” University of Detroit Mercy School of Dentistry assistant professor Timothy Kosinski, DDS, MAGD, says it could “play a prominent role in all of our dental practices.” 

The British physicist Sir William Crookes identified the fourth state of matter in 1879, but it was not called “plasma” until Irving Langmuir, an American chemist, applied the name in 1929. As the most common form of matter, making up more than 99 percent of the visible universe, plasma is a collection of stripped particles. When electrons are stripped from atoms and molecules, those partials change state and become plasma. Plasmas are naturally energetic because stripping electrons takes constant energy. If the energy dissipates, the electrons reattach and the plasma particles become a gas once again.

Unlike ordinary matter, plasmas can exist in a wide range of temperatures without changing state. The aurora borealis, or northern lights, is ice cool, for instance, while the core of a distant star is white hot. Other well-known plasmas include lightning, neon signs, and fluorescent lights. Outside of a container, plasma resembles gas—the particles don’t have a definite shape. But unlike gas, magnetic and electric fields can control plasma and shape it into useful, malleable structures.

Harnessing power

Dr. Yu, fellow UMC assistant professor of mechanical and aerospace engineering, Hao Li, PhD, and senior research scientist Mark Chen, PhD, have tapped into a major advance in plasma science and created a tiny plasma scrub brush that resembles a white-hot flame.

But this flame is cool to the touch. It glides across the surface of the tooth, killing bacteria and preparing the dentin and enamel, and it limits the acid etching, mechanical whirring, and phobias typically associated with a drill in the mouth. The promise of painless, fear-free dentistry is one of several factors motivating the plasma brush research, which the team has reported at conferences for organizations such as the International Association for Dental Research and the Society for Biomaterials.

“Many patients admit a fear of needles, vibration, and sound from a drill. Even the thought of pain keeps them away from the dentist,” says New York City-based cosmetic and reconstructive dentist Daniel Noor, DMD. “Although I don’t believe that the plasma brush will cure all dental problems, a well-trained dentist using it may be able to do many procedures without shots and pain.”

“Noiseless, painless cavity preparations would be a huge advance,” Dr. Kosinski concurs. “Dental lasers have attempted to address this concern but have proven expensive and slow.”

For Dr. Yu and his team, the notion of a plasma toothbrush is technology transfer at its finest. Plasma is “widely used in surface engineering, especially in cleaning, etching, and adhesion enhancement,” he explains. The technology proved to be so effective in the microelectronic industry—where preparing high-speed computer processors involves the highest levels of precision and durability—that the move to another equally important surface, the tooth, made sense.

Dental applications have emerged because a new version of plasma technology, so-called “non-thermal atmospheric plasmas,” permits surface preparation in open air at room temperature. The promise of plasma as a dental preparation tool is twofold, Dr. Yu explains: It will both reduce tissue damage and better prepare the dental surface for composite adhesion.

“Our preliminary data has shown that plasma treatment increases bonding strength at the dentin/composite interface by roughly 60 percent,” Dr. Li explains. “With that interface-bonding enhancement we expect to significantly improve composite performance, durability, and longevity.”

That’s good news to Dr. Kosinski. “If the plasma brush prepares the tooth surface to create a stronger attachment with our bonded composite restorations, we may indeed have a winner,” he says. 

Prepare for takeoff

According to Drs. Yu and Li, plasmas have been used extensively for fabricating semiconductor devices, modifying the surfaces of materials, sterilization, and other applications. But in this case, they are developing a handheld plasma apparatus that can be used by dentist for multiple dental clinical applications. The apparatus applies small amounts of electricity to a non-toxic gas through a “narrow slit chamber” to generate plasma with a brush-like shape. Such design and development works are primarily conducted at Nanova Inc., a high-tech company focusing on medical devices in dental, orthopedic, and cardiovascular areas.

Unlike virtually any other apparatus in the dentist’s tool palette, the plasma brush is entirely malleable. Its dimensions “are determined by a gas flow rate from 0.01 to several liters per minute, the amount of electrical power, and the composition of the gases,” the inventors note. Low power consumption, on the order of a few watts, ensures low-temperature plasma. Add power or increase the gas flow and the plasma’s temperature increases or decreases as well.

The researchers tested this technique several ways. At 15 watts, the plasma temperature was about 160 degrees Celsius. Tripling the gas flow lowered the temperature to 53 degrees Celsius. Decreasing the power by 5 watts lowered the temperature even more, to about 40 degrees Celsius. At the decreased power level, an additional gas flow increase brought the plasma to room temperature, about 20 degrees Celsius. “The low-power, low-heat feature not only makes it economical, but applicable to heat-sensitive materials,” they write. 

To test the brush on bacteria, Drs. Yu and Li sterilized a series of filter paper discs, which were contaminated with dental bacterium, with argon plasma at room temperature for less than 30 seconds. Counting live bacteria on the discs before and after treatment with a scanning electron microscope, they found 100 percent clearance. “The plasma brush requires much less sterilization time compared to traditional methods,” they report.

Treating low-density polyethylene (LDPE) yielded similar timesaving results, changing the surface from hydrophobic (repelling, tending not to combine with, or incapable of dissolving in water) to hydrophilic (water soluble) in about 30 seconds, compared with one to two minutes using conventional methods. The ability to quickly modify a surface—either polymer or tooth—may prove to be the device’s most compelling feature.

According to the National Institute of Dental and Craniofacial Research (NIDCR), an association of the National Institutes of Health (NIH), more than half of dental restoration that use current dental composites fail in 10 years. The second or third restoration not only significantly increase our nation’s dental care expenses, but also create pain and discomfort with a risk of losing the tooth. As a result, durable dental restoration has been a priority for NIDCR. With most researchers trying to develop stronger and more durable dental composites, this research, which addresses the interfacial bonding with an innovative method, is indeed unique and timely.

Failed filling replacement consumes enormous resources, accounting for nearly 75 percent of all operative dentistry, explains Yong Wang, PhD, associate professor in the Department of Oral Biology at the University of Missouri-Kansas City (UMKC) School of Dentistry. Heightened concern about mercury in dental amalgam has only exacerbated the problem, as dentists remove old fillings to make way for composite resins.

“The failure rate for large to moderate posterior composite restorations can be two to three times that of amalgam containing mercury,” states Dr. Wang, who sits on the editorial board for the Journal of Dental Research. “The higher failure rate means increased frequency of replacement, the inevitable loss of additional tooth structure, and more complex restorations with increased costs for the patient.”

For years, Dr. Wang has studied premature composite restoration failure, which he says results from breakdown of the bond at the dentin/composite interface. “After more than three decades of research, breakdown at the dentin/adhesive interface continues to plague the long-term clinical success of large composite restorations,” he explains.

That’s largely because, while much has changed in composite fabrication, little has changed in surface preparation. “Current bonding mechanisms rely on a two-step process that provides micro-mechanical retention,” Dr. Wang explains. “First, the mineral phase must be extracted from the dentin substrate without altering the collagen matrix, and second, the voids left by the mineral must be filled with an adhesive resin that undergoes complete in situ polymerization—the formation of a resin-reinforced or hybrid layer.” An ideal hybrid layer is a three-dimensional integrated polymer/collagen network, he adds, that it “provides both a continuous and stable link between the bulk adhesive and the subjacent, intact dentin substrate.” But right now, “substantial evidence” suggests that the ideal is “impossible to achieve under current clinical conditions. Instead of serving as a stable connection between the bulk adhesive and subjacent intact dentin, the hybrid layer is the weakest link,” Dr. Wang says.

Current clinical practice relies on mechanical bonding when it should rely on chemical bonding, he explains. The culprit that foils mechanical methods is a protein layer, the so-called “smear layer,” which is primarily composed of type I collagen that develops at the dentin/adhesive junction. To create a porous surface that the adhesive can infiltrate, current preparation techniques etch and demineralize dentin. Dr. Wang and his colleagues say they have found that interactions between demineralized dentin and adhesive gives rise to the smear layer, which actually inhibits adhesive diffusion throughout the prepared dentin surface. “This protein layer may be responsible, in part, for causing premature failure of the composite restoration,” Dr. Wang says. “It contributes to inadequate bonding that can leave exposed, unprotected collagen at the dentin-adhesive interface, allowing bacterial enzymes to enter and further degrade the interface and the tissue.” 

The solution: introduce bonds that depend on surface chemistry rather than surface porosity. According to Dr. Wang, using plasma etching promises such a result.

Under the scope

All these technological wonders sound good to Terry S. Shapiro, DMD, an East Setauket, N.Y.-based restorative and cosmetic dentist, but as she’s quick to remind, “many technologies that initially seemed very promising now sit in many a dental basement.” Dr. Kosinski agrees, but adds that he’s willing to take a closer look: “If it’s cost-effective and works as advertised, I would put it into my practice and let my patients know of its potential and limitations.”

With chemical bonding enhancement, Dr. Noor wants to know about a potentially unintended consequence. “Keep in mind that since the plasma brush involves chemical reactions, research needs to be done to determine safe use of the brush next to old—but intact—amalgam or alloy fillings, which contain silver, mercury, and other metals,” he advises. “You wouldn’t want any of that material to be released.” Responding to this concern, Dr. Yu states, “Dr. Noor raised a very important question. But because no toxic and etching gases will be used, we don’t expect any potential safety problems.” Nanova Inc. has planned to conduct a thorough investigation to ensure the safe use of the plasma device.

Cost-effectiveness, safety, and reliability may still not be enough, says Daniel Smith, DDS, FAGD, an Agoura Hills, Calif.-based prosthetics and implant dentist, who 10 years ago installed a device that used ozone to kill bacteria. “The price wasn’t bad, and it worked 100 percent of the time,” Dr. Smith says. But then something odd and unexpected happened—one of those unintended consequences. “With the ozone generator, we were able to kill decay-causing bacteria in such a short time that patients actually complained about the cost of the procedure.” They thought their dentist was making a buck too quickly. Ultimately, the ozone sterilization technique never caught on, even though it was extremely effective. If it delivers a little too well on its many promises, “a similar effect could occur with plasma technology,” Dr. Smith says. 

Though he too has plenty of questions, AGD Impact “Testing the Tools” columnist Howard Glazer, DDS, FAGD, is more optimistic. “I’m in favor of the plasma technology,” he says. “Patients definitely do not like the sound, feel, and vibration of a drill.”

Characterizing dentistry as “preventive, reparative, and restorative,” Dr. Glazer—past president of the Academy of General Dentistry (AGD), practitioner of general restorative and cosmetic dentistry, and a widely-recognized expert in cosmetic and forensic dentistry—says readers know him for the simple benchmarks he applies to any new tool: Is it faster? Is it easier? Is it better for me as a clinical practitioner? And, is it reasonably priced? Is it better for the ultimate end user, the patient?

Recalling the trajectories of other dental technologies, Dr. Glazer says that while dentists are eager adopters—witness the ongoing move from analog to digital X-rays—they also can be conservative. And with a down economy and benefits reductions at many companies, price matters more than ever.

“Lasers are a great example of how cost impacts adoption,” Dr. Glazer says. “Lasers have reduced a lot of procedural pain, but they’re expensive.” However, Dr. Glazer adds, “I certainly don’t mind spending more for something that substantially improves my practice.”

Dr. Kosinski agrees. “The cost of equipment and maintenance and the time saved chairside are the two most important issues to me.”

Without seeing the plasma brush in action, the improvement that makes the greatest sense to Dr. Glazer is better surface preparation for better bonding. Over the years, bonding agents have dramatically improved, increasing adhesive strength “from 17 baseline megapascals [MPa] to as high as 45 MPa,” Dr. Glazer says.

But surface preparation, he adds, hasn’t improved nearly as much. Adding even 4 to 5 MPa of bonding strength to existing composite materials “would be huge,” he says. “I’m all for it.” According to Drs. Yu and Li at UMC and Dr. Wang at UMKC, their studies have demonstrated an increase in bonding strength of approximately 60 percent. But for the team to sell the idea to “lots of skeptical dentists,” their approach will require several joint-studies with adhesive manufacturers.

“It’s easy to say that removing drills will improve the patient experience,” Dr. Glazer explains. “But it’s harder to say whether or not your technique will improve bond strengths in fifth, sixth, and seventh generation bonding agents. It’s harder to say what constitutes a ‘failed filling’ and to what extent failed fillings are a widespread problem.”

Dr. Glazer, for instance, isn’t sure nearly 75 percent of all operative dentistry involves filling replacements. And unless a filling lasts a lifetime, failure, at some point, is an unavoidable fact of life. “Most alloys last better than 15 years, composites better than 10,” he explains. “I’ve got patients with fillings that are still going strong at 16 to 17 years. Unless a technology can put in a filling for life, it cannot be said to have cured the problem of failure.”

Despite all the cautious caveats, Dr. Glazer says the plasma brush technology makes good overall sense. Toward its eventual use, he points to a generation of younger practitioners thirsty for innovative technology. “The engineers behind the plasma brush are approaching problems from the right perspectives,” Dr. Glazer says. “FDA [Food and Drug Administration] approval, which will take about three years, will also go a long way in answering my questions.”

Commercial coverage

The plasma toothbrush received its greatest vote of confidence from a $270,000 National Science Foundation (NSF) grant just as it left the drawing board for the laboratory. Funding came from the NSF Division of Chemical, Bioengineering, Environmental, and Transport Systems after reviewers agreed that acid etching, demineralization, and drilling seemed like harmful alternatives to what the researchers described as “plasma-induced elimination of bacteria.”

The grant also funds an outreach effort designed to engage underrepresented groups in engineering and promote student and public awareness of engineering principles applied to biomedical problems. Public awareness, Dr. Noor says, is critical to the technology’s eventual success; however, he thinks getting the word out will take at least 10 years. “We could see great demand for the plasma brush, but we will first have to see clinical training on its use at dental schools, residency programs, continuing education courses, and involvement from dental associations to promote the shift from drills,” he explains.

A second vote of confidence recently came from the NIDCR, which awarded Nanova Inc. a Small Business Innovation Research (SBIR) grant. Electrical engineer Meng Chen, PhD, and the rest of the Nanova Inc. team is dedicated to commercializing this plasma technology. Dr. Chen expects the plasma brush device to be widely used in dentistry in the future with huge commercialization potential. He anticipates that they will have prototypes in six months and FDA approval in about two years. And while they haven’t established a price, “Dr. Chen assures us that using the plasma brush is indeed cost-effective and will save significant time and money,” Dr. Yu says. 

That’s a good thing, says Dr. Smith, who directs the Focus Dental Institute for Implant Reconstruction. Yes, reducing discomfort is important, and yes, increasing bond strength would help, but patients and insurance companies ultimately decide how much they’re willing to pay—and the cheaper technique usually wins. “Most people don’t have problems with limited fillings using a high-speed hand piece,” Dr. Smith explains. “If we still need traditional cavity preparations and we just use plasma technology to provide a bacteria-free surface with bond strength increasing to 37 MPa, then it’s not something I would utilize. On the other hand, if plasma technology speeds up procedures without anesthesia and provides greater bond strength, then its value is much greater.”

Generation next

Although Dr. Glazer says he doesn’t see many patients with true dental phobia, fear of the dentist is widespread and well-documented enough that the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) has a name for it: odontophobia.

The problem of fear is more an issue with children and under-served communities, where education and familiarity with the dentist’s chair are, by definition, limited. For that reason, “I see the plasma brush as an extremely valuable tool among pedodontists and dentists in our less-serviced communities, where individuals do not readily obtain proper dental care,” Dr. Kosinski says.

Once he’s absolutely convinced that the plasma flame is indeed room temperature—“the researchers need to show me this”—Dr. Glazer says eliminating the heat a drill generates would mark another pain- and phobia-reducing milestone. Heat causes expansion of the fluid inside the dentinal tubules, which pressures nerve endings.

In the end, however, the biggest innovation for which Dr. Yu and his team may be responsible is simply the idea itself: introducing dentistry to a technique used to engineer hard surfaces rather than to manufacture them. The chemistry of alloys, resins, ceramics, adhesives, and all the other products now available to dentists mostly involve creation, not manipulation.

The point isn’t lost on Dr. Kosinski. “Our weakest link has always been technique,” he says.  “And though plasma technology isn’t an end-all to all the techniques we perform, it could well become a valuable tool.”

 

Mike Martin is a former senior science writer for United Press International. He is member of the National Press Club and the National Association of Science Writers. Mr. Martin specializes in news about early-stage university and laboratory research. To comment on this article, send an e-mail to impact@agd.org.