Exploring Biomimetic Remineralization to Restore Carious Lesions
Franklin R. Tay, PhD, associate professor of endodontics in the department of oral biology and maxillofacial pathology at the Medical College of Georgia School of Dentistry, and his colleague, Dr. David Pashley, are conducting biomimetic mineralization research with an emphasis on dental applications. They are intrigued by the manner in which apatite, the calcium-phosphate minerals found in bone and dentin, are arranged inside the organic collagen fibrillar matrix. These nanocrystals have dimensions in the nanometer scale, approximately 40 nm to 50 nm long. What is even more amazing, Tay says, is the order in which these nanocrystals are arranged inside the collagen fibrils.
Tay explains that there are two forms of crystallites: those formed inside the collagen fibrils (ie, intrafibrillar crystallites) and those that are formed around the collagen fibrils (ie, extrafibrillar crystallites). For intrafibrillar crystallites, there is a particular order in which they are deposited.
"These nanocrystallites are deposited in the gaps between collagen molecules similar to a stack of overlapping cards. The way nature makes these crystals small enough to fit inside collagen fibrils is through protein molecules, the so-called non-collagenous proteins," Tay says. "The biomineralization process templated by proteins is a classic example of how nature uses nanotechnology to strengthen the skeletal system in vertebrates."
It is a controlled process in which the mineral supersaturation condition, and precipitate phase nucleation and growth, are regulated to lay down the mineral phase in nanocrystalline form at the intrafibrillar and extrafibrillar space associated with the collagen fibrillar assembly. Such a mineral phase plays out within the connective tissue, strengthening the structure and enabling it to resist excessive deformation when stress is applied during function, Tay says.
In many ways, regulation of biomineralization in bone and dentin is similar to how minerals are deposited in other invertebrates in the animal kingdom. The regulation of precipitate-phase morphology during biomineralization is believed to exhibit two main characteristics, Tay notes. The first is the sequestration of precipitation processes into microscopic compartmental entities known as vesicles. The second characteristic of biomineralization is the templating of mineral nucleation through noncollagenous proteins. These polyanionic protein molecules are believed to bind to the collagen substrate at specific sites, such as gap regions and pores of the fiber assembly. The bound (or immobilized) proteins present anionic charge sites for calcium binding and apatite formation. Because of immobilization of the proteins at critical sites of the collagen matrix, precipitation is facilitated at both intrafibrillar and extrafibrillar locations, he says.
"For many years, scientists have extracted noncollagenous proteins from bone, dentin, and also enamel and found that they are able to either promote or inhibit crystal nucleation or growth. While the extraction of these molecules or their synthesis via recombinant DNA technology has immensely substantiated our understanding of how these molecules contribute to biomineralization, scientists are far from being able to synthesize these molecules in a quantity that is practical or economic enough for clinical applications," Tay admits. "Thus, for the past couple of decades, other scientists have resorted to using simpler synthetic polyanionic analogs of these noncollagenous proteins in order to mimic their functions in biomineralization."
In Tay's and Pashley's laboratory, they have adopted this biomimetic approach in utilizing two biomimetic analogs in remineralizing dentin that has been acid-etched to remove a surface layer of minerals. As a result, they have been able to observe both interfibrillar and extrafibrillar remineralization with the use of these biomimetic analogs, in a way that is similar to how minerals are deposited in nature within collagen fibrils. They have further extended this technique, which they call "Guided Tissue Remineralization," to try to salvage failing bonds created by the application of contemporary dental adhesives to dentin.
"We have been successful at the proof-of-concept stage, and will be working within the next 5 years on developing delivery systems that enable clinicians to deliver these remineralizing biomimetic analogs to dentin," Tay says. "If successful, this will help, firstly, to extend the durability of resin-dentin bonds which, unfortunately, are not as durable as manufacturers claimed. Secondly, we are hoping to extend the application of our biomimetic strategies to remineralize dentinal caries. We anticipate that in the next 10 years, dentists can apply biomimetic molecules to dentin to create remineralized dentin that is similar to that created by our own cells during the developmental stage of our teeth."