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Nanoscale fiber strengthens composites

NovaPro Flow, designed and created by Nanova Biomaterials, is the first commercial dental composite reinforced by nanofibers. The in-house made hydroxyapatite nanofibers, composed of calcium phosphate minerals, are the secret to NovaPro Flow’s high strength, great handling properties and esthetic effects, according to the company. Teeth, bones and nanofibers have the same structure, all composed of a hard inorganic mineral in a soft organic matrix. For example, the enamel of a tooth is made up of high-volume, needle-like mineral crystals (approximately 15–20 nanometers thick and 1,000 nanometers long) in a small-volume, soft protein matrix. Bone and dentin are made up of plate-like crystals (approximately 2–4 nanometers thick and up to 100 nanometers long) embedded in a collagenrich protein matrix. By comparison, the nanofibers have a thickness under 100 nanometers in diameter, or 1,000 times smaller than human hair. Most dental composites comprise only nanoparticles, while NovaPro Flow reinforces the composites with nanofibers, adding several advantages. The strength found in the nanofibers is due to its one-dimensional nature and large surface area. If you apply a shear force to a bunch of nanoparticles, it is easier for the particles to slide by each other. A fiber, on the other hand, has a solid connection that is stronger to resist bending, shear and tensile forces. In comparison, synthetic bulk h y d r o x y a p a t i t e typically has a tensile strength of approximately 50 MPa, which would not be able to reinforce a dental composite. The large size would also lead to polishing problems. The larger surface area of nanofibers provide better transfer of strength for the fiber and the dental composite alike. During World War I, A. A. Griffith discovered the correlation between strength and the small size of ceramic materials, such as enamel, by testing different thicknesses of glass. According to the Griffith theory, these needle-like mineral crystals can reach their theoretical strength, or maximum strength, of a material (several or tens of GPa) when their diameters are in the nanoscale. Microscopic flaws cause a material to fail, so by creating a nanoscale fiber, statistically, it limits the amount of flaws and provides the ability to reach the material’s maximum strength. In addition, such mineral crystals at nanoscale are not sensitive to cracks or stress concentration, which typically degrades when produced on a micro or macro scale. The fibers are able to bend without breaking. The high strength and low sensitivity to cracks found in natural nanocomposites are the same advantages provided by the nanofiber reinforced NovaPro Flow.