Applying physics expertise to making smiles brighter

In general, ceramics don’t have a reputation for being a rough-and-tumble material. Rather they seem prone to damage: Imagine a vase or decorative dish, for example, capable of shattering with an errant knock. 

Yet ceramics, specifically a material called zirconium dioxide, or zirconia for short, is much stronger than those decorative objects, and has become increasingly relied upon in dentistry. Used in crowns, bridges, and other applications, the resulting products can restore function and beauty to patients’ smiles. 

“Ceramics are hard but brittle,” says Yu Zhang, a professor in the School of Dental Medicine’s Department of Preventive and Restorative Sciences. “They can be beautiful, but this property of being susceptible to fracture has proved a problem in dentistry.”

Penn is a university known for embracing interdisciplinarity, and Zhang is a prime example. Having joined the Penn faculty in July, he brings a unique background in both physics and materials science. With 130 scientific publications and counting, Zhang has for the last two decades applied this expertise to the problem of how to improve dental materials, making them both a good aesthetic match for natural teeth while also enhancing their strength and durability. 

“I came into the dental field with experience in material properties and material characterization techniques,” Zhang says. “By bringing all that into dentistry, where researchers had not been exposed to the types of research my colleagues and I had been doing in physics and engineering, it has allowed us to make some interesting insights and advancements and continue improving the materials dentists use to treat patients.”

For Zhang’s new colleagues at Penn Dental Medicine, including Markus Blatz, professor and chair of the Department of Preventive and Restorative Sciences, his presence in the School opens a host of possibilities for collaboration and innovation.

“Dr. Zhang is one of the most respected if not the most respected dental ceramic researchers in the world,” says Blatz. “His research, bridging the gap between dental material sciences and clinical practice, will take our scholarly activities and global reputation to the next level. And his expertise in CAD/CAM ceramics is an especially nice fit, given our enhanced focus on digital innovation.”

An unexpected route

As an undergraduate and master’s student in Shanghai, China, and Victoria, Australia, respectively, Zhang enjoyed the intellectual pull of physics. He also spent several years working in industry as an engineer. But as he was concluding his master’s degree, it became clear that stopping his education there would limit his career options. But he was married with a child at that point, and securing an income was important. 

“I was fortunate to get an Australian post-graduate award from the government, which was very generous, and would allow me to pursue a Ph.D. in physics at the University of Melbourne,” Zhang says. 

He accepted the award and began his studies, but after “a lot of soul searching” and consideration of his family’s future, he saw more possibilities in engineering. He went on to transfer to Monash University, where he earned a Ph.D. in 2002 in materials science and engineering.

His background in physics came in handy while pursuing materials research. “There was a lot of talk about crystal structure,” he says, “so that was a nice transition.”

In that time period, Zhang recalls absorbing the insights of seminal papers from Penn faculty members, including a 1997 Nature paper by I-Wei Chen of the School of Engineering and Applied Science, focused on a lightweight yet hard and tough ceramic material. 

Reading such works about how to develop stronger, more durable materials inspired him, and he was able to put that motivation to use in a postdoctoral position, working with Brian Lawn, an Australian who worked at the U.S. government’s National Institute of Standards and Technology, based in the Maryland suburbs of Washington, D.C. 

His move to the United States also marked Zhang’s transition into the dental field. Lawn placed him on a project aimed at reducing the brittleness of ceramic materials. Zirconia was introduced as a material used in restorative dentistry around this time, beginning to replace the traditional gold crowns and porcelain-fused-to-metal restorations.

“In dentistry people look to ceramics from an aesthetic point of view—how beautiful it can be in restorations—but frequently overlook how susceptible to fracture it is,” Zhang says. 

Combining Zhang’s training in materials with Lawn’s expertise in mechanics, the two began investigating how manipulating materials could achieve more desirable properties. Their work attracted attention from the dental community. 

Beauty and might

These successes also helped land Zhang his first position as an independent investigator, at New York University’s College of Dentistry, where he would remain for 15 years. A focus over that time was on zirconia, an attractive restorative material for its strength, if not its durability. But as a white, opaque material, it was far from lifelike.

“It’s pure white,” says Zhang. “Maybe that’s good if you’re in Hollywood, but for ordinary people it’s not a good match for the various shades of natural-looking teeth.”

The conventional solution is to place a layer of porcelain over the zirconia. But when these two materials are fused one on top of the other, the porcelain becomes more susceptible to fracture. Zhang recognized this as a problem that could be addressed with physics. “When you put two different materials together, there are thermomechanical stresses that arise,” Zhang says. “I thought up a way to circumvent this problem.”

The solution was to avoid the “sharp” interface altogether by instead creating a product in which one side of the material was all zirconia, and the other was all porcelain, but the fractions of each gradually change across the interface. Working with Lawn, Zhang got a grant to develop and study such gradient materials in 2007, work that continued being funded for 10 years.

Additional composition manipulations could even give ceramic materials bioactive properties, perhaps repelling infection. “This could be of great use in dental restorations as well as craniofacial implants,” he says.

To address the somewhat unnatural opaque appearance of zirconia, Zhang had another innovative approach. “The goal is to make the material translucent with respect to visible light,” he says. “If it’s translucent in the infrared or ultraviolet wavelengths of light, it won’t matter because we don’t see those.”

Instead Zhang picked the light to which the human eye is most sensitive: green light at a wavelength of 555 nanometers. “If we shine this green light through zirconia, we’ll perceive it as translucent,” he says. Based on work in classical physics light-scattering theory, which he had studied during his undergraduate and master’s years, he proposed adjusting the microstructure to make zirconia translucent while maintaining its durability, publishing a paper on the breakthrough in 2014. 

“We use light-scattering theory to calculate what microstructures we desire,” says Zhang. “Then when we got to the lab to make the materials, we use an idea that I-Wei Chen published in Nature in 2000 that shows you can densify ceramic if you bring it to a very high temperature for a moment, then drop the temperature.” That two-step sintering process allows for the precise manipulation of material microstructure.

Next-generation materials

Other applications of this type of manipulation abound. Zhang notes that in conversation with Mark Wolff, Penn Dental Medicine’s Morton Amsterdam Dean, who overlapped with Zhang during his tenure at NYU, they have discussed how X-rays do not penetrate zirconia, making it difficult to see if there is decay underneath a crown or other restoration in the mouth. If Zhang applied a similar principal to zirconia as he did when making it translucent under green light, he could create an imaging method that would allow dentists to “see” under restorations to maintain patients’ oral health. Working with Jian Xu of Electrical & Computer Engineering, Louisiana State University, the team is currently fine-tuning wavelengths to image through zirconia and other dental ceramic materials.

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Another novel approach to dental-materials creation that Zhang is looking forward to exploring at Penn has to do with the processes by which dental products are crafted into the size and shape required for use in patients. Typically, dental technicians grind ceramic restoration materials using diamond burs to give them the correct final form. This grinding requires frequent replacement of the expensive burs, plus it exerts a lot of stresses on the zirconia, effectively chipping away small amounts of material. By potentially introducing tiny cracks into the finished product, this approach can compromise the long-term stability of products, as the cracks serve as weak points that can lead to product failure when subject to the normal stresses of chewing and grinding.

By using a different approach, known as “ductile machining,” it avoids introducing these defects, achieving an accuracy of form, maintaining the integrity of the material while preserving diamond burs. Zhang is in the process of submitting a grant to pursue this study in the upcoming weeks. “It’s the best of both worlds,” he says. “You preserve the tool and at the same time you get a better product.”

Synergistic strengths

Zhang was eager to come to Penn to collaborate with Wolff, Blatz, Chen, and others, including Michael Bergler of the Center for Virtual Treatment Planning, “a craftsman,” as Zhang describes him.

“Michael has a lot of experience in machining and grinding materials, because he’s a superb technician,” Zhang says. “I benefit a lot by talking with him, by working with him, and I’m hoping he can be part of the grant I’m working on to develop new protocols for machining ceramics.”

Already in close collaboration with Mark Wolff and Markus Blatz on improving materials for prosthodontic procedures, Zhang also hopes to expand collaborations across the University, reaching the medical and engineering schools as well.

“We have so many strong clinicians and researchers here,” he says. “I want to strengthen my own research by learning from all of them.”