Inspired by the mechanisms that mussels use to adhere to inhospitable surfaces, researchers at the University of California at Santa Barbara have developed a dental composite that provides an extra layer or durability for teeth treated with fillings, crowns, and other work. It’s as hard as a typical dental restoration, the researchers note, but it is less likely to crack.
“All dental composites have microparticles to increase their rigidity and prevent their shrinkage during their curing process,” said Kollbe Ahn, PhD, a materials scientist at the school’s Marine Science Institute and corresponding author of the study. “But there’s a tradeoff. When the composite gets harder, it gets more brittle.”
Mussels need to maintain their strength and hardness as well as durability as they adhere to irregular surfaces in the variable conditions of the intertidal zone, which includes pounding waves, hot sun, and cycles of immersion in salt water and windy dryness. The byssal threads they use to affix to surfaces enable them to resist the forces that would tear them away.
“In nature, the soft, collagenous core of the mussel’s byssal threads is protected by a 5- to 10-µm thick, hard coating, which is also extensible and, thus, tough,” said Ahn.
This durability and flexibility allow mussels to stick to wet mineral surfaces in harsh environments that involve repeated push-and-pull stress. The researchers attribute this ability to dynamic or sacrificial bonding, which involves multiple reversible and weak bonds on the sub-nanoscopic molecular level that can dissipate energy without compromising the overall adhesion and mechanical properties of the load-bearing material.
“Say you have one strong bond,” Ahn said. “It may be strong, but once it breaks, it breaks. If you have several weaker bonds, you would have to break them one by one.”
Breaking each weak bond dissipates energy, so the overall energy required to break the material would be greater than with a single strong bond. This type of bonding occurs in many biological systems, including animal bone and teeth.
The mussel’s byssus include a high number of unique chemical functional groups called catechols, which are used to prime and promote adhesion to wet mineral surfaces. Ahn and his team found that using a catecholic coupling agent instead of the conventional silane coupling agent provides 10 times more adhesion and a 50% increase in toughness compared to current dental restorative resin composites. It also demonstrates a lack of cytotoxicity.
Research has proven this toughening mechanism in soft materials, but this study is one of the first to prove it with rigid and load-bearing materials. According to the researchers, the material could lead to rougher and more durable dental fillings that, in the long run, mean fewer dental visits. And since replacement fillings mean more tooth destruction, more durable restorations would extend the lifespan of the tooth, prevent tooth loss, and help preserve overall health. Next, the researchers hope to increase the material’s durability even further.
“By changing the molecular design, you could have even denser coupling agents that exist on the surface, and then we can have a stronger and more durable dental composite,” Ahn said, estimating a commercial product within a couple of years.
Ahn credits the interdisciplinary research environment at the school for the development of such load-bearing polymer composites. It involved collaboration between molecular biologists, dentists, surface physicists, physical chemists, and mechanical engineers. The study, “Significant Performance Enhancement of Polymer Resins by Bioinspired Dynamic Bonding,” was published by Advanced Materials.