Scientists reveal how natural mineralized materials work
An interdisciplinary team of scientists at the Charité – Universitätsmedizin Berlin has succeeded in unraveling an inbuilt crack-stopping mechanism acting in dentin, the bone-like material that comprises the bulk of human teeth. Contrary to normal bone tissue, cracks and damage occurring in dentin do not heal by tissue replacement or remodeling. So how do teeth survive decades of harsh mechanical wear and tear in the mouth? Scientists have now revealed for the first time: internal stress in denin act to prevent damage by stopping cracks from propagating into the tooth insides. These findings have now been published in the materials-science journal Nano Letters*.
Human teeth should serve for entire lifetimes, despite the daily exposure to damaging forces. Several theories have emerged, attempting to explain why dentin − a bone-like substance that form the major part of all human teeth − is able to withstand such deleterious stress. Now a team headed by Dr. Paul Zaslansky at the Julius Wolff Institute (JWI) of the Charité has studied the nanostructures of dentin under various stress conditions. Their findings reveal that the mineral nanoparticles of dentin tightly interact with the dense network of collagen fibers in which they are embedded. The mineral particles become increasingly stressed as a reaction to the compression of the fibers. The ensuing internal compression increases the resistance of the tissue to propagation of cracks into the tooth bulk..
Studying the tiny nanostructures was made possible through access to advanced multi-disciplinary research equipment, capable of generating high-brilliance and high quality synchrotron radiation facilities: large lab of BESSY II of the Helmholtz-Zentrum Berlin für Materialien und Energie and the ESRF – European Synchrotron Radiation Facility in Grenoble, France. Engineers use internal stresses to purposely strengthen materials for technical applications, for example in gears or turbine blades. Now it seems that evolution understood this ‘trick’ long ago, and uses this to strengthen our teeth.
The researchers altered the humidity of dentin samples in order to study the underlying stress-generation mechanism. Their measurements showed that the compression occurring in the mineral particles increased when the structural fibers shrank. “The compressed state helps to prevent cracks from developing and we found that compression takes place in such a way that cracks cannot easily reach the tooth inner parts, which might damage the sensitive pulp”, explains Dr. Paul Zaslansky of the Julius Wolff Institute of the Charité.
In further experiments, scientists demonstrated that heat significantly weakens the link between mineral particles and collagen fibers, which in turn results in a brittle behavior of dentin. “We therefore believe that the balance of stresses between the particles and the protein is important for the extended survival of teeth in the mouth”, reasons Jean-Baptiste Forien, PhD student and first author of the study. These findings explain why dental prostheses are not as resilient as healthy human tooth substance: ceramic-based materials are simply too ‘passive’ and do not counteract against the stress to which they are exposed. One reason might be that they do not possess the same internal mechanisms employed by the natural tooth substance. Dr. Zaslansky says: “Our results might inspire the development of tougher ceramic structures for tooth repair or replacement”.
*Jean-Baptiste Forien, Claudia Fleck, Peter Cloetens, Georg Duda, Peter Fratzl, Emil Zolotoyabko, Paul Zaslansky. Compressive Residual Strains in Mineral Nanoparticles as a Possible Origin of Enhanced Crack Resistance in Human Tooth Dentin. Nano Letters. 2015 May 29. doi: 10.1021/acs.nanolett.5b00143.
Working closely with the DFG funded Charité scientists, the work was made possible through collaboration with researchers from the Technische Universität Berlin, the Max Planck Institute of Colloids and Interfaces, Potsdam, and the Technion – Israel Institute of Technology, Haifa, Israel.
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