Ceramics in Biomedicine

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ceramics in biomedicine Strong, tough, and uncrushable—How Mother Nature designs structural biological materials Nature has designed some ingenious materials. Spider thread, for example, has amazing tensile and stretching properties. Abalone shells resist the erosion of ocean floor environments that polish materials with similar compositions into shiny, pretty baubles prized by artists. Why do bird beaks not break? How is the structure of seahorse spines an advantage? University of California, San Diego, researchers Marc Meyers and Joanna McKittrick, and Po-Yu Chen, now at National Tsing Hua University, Taiwan, recently Science wrote a review article in on the mechanics of structural biological materials. The paper, “Structural biological materials: Critical mechanics-materials connections,” reports on the search Nature is replete with ingenious structures to make life not just possible, but better. The bony plates of seahorse skeletons, for example, slide past each other, giving the creature incredible flexibility. Materials scientists at the University of California, San Diego, work to unlock the secrets. for connections between the structure and properties of biological materials, with an eye toward understanding how to engineer similar structures and properties in synthetic materials. Their review focused on three properties: strength under tension, toughness, and resistance to buckling or torsion. They note that there are seven distinguishing characteristics of structural biological materials: self assembly, multifunctionality, hierarchy (different structures at different scales for different purposes), hydration, mild synthesis conditions (low temperature and pressure in aqueous environments), constraints imposed by evolution and environment, and self-healing ability. Biological materials fall into two broad structural categories: “soft” structures, which are nonmineralized; and “hard” structures, which are composites of minerals and fibrous organic biopolymers. (See the next story for more about this.) Examples of soft structures include collagen, keratin, elastin, chitin, lignin, and others. Mineralized composites consist of a mineral reinforcement phase, such as hydroxyapatite, calcium carbonate, or siica, embedded in a biopolymer matrix, such as collagen or chitin. Examples from nature provide insights into the mechanics of structural biological materials. “Mother (Credit: Joanna McKittrick, UCSD.) Nature give us templates. We are trying to understand them better so we can implement them in new materials,” McKittrick says in a news release. Biological materials also have secrets to reveal regarding processing. Exoskeletal animals, like abalone, grow their shells one layer at a time. McKittrick observes in the press release that 3D printing is basically the same concept. “You could build a material similar to the abalone shell using principles we learned from nature by printing layer upon layer of mineral deposits—and do it much faster than nature would.” Besides structural biological materials, there are other familiar applications of bioinspired materials. For example, most are familiar with the invention of Velcro being inspired by the way plant burrs stuck to animal fur. Olympic sports fans may recall hearing about high-performance swimsuits—eventually banned from competition—that mimic the structure of shark skin and reduce drag in the water. New superadhesive surgical tapes are designed after gecko foot structure. Who better than Mother Nature would know about genomes and design of materials? n US ‘Repair & Protect’ toothpaste has no Bioglass This past spring, GlaxoSmithKline finally (and relatively quietly) began selling its renowned Sensodyne Repair & Protect toothpaste in the United States. The product was reported to be a godsend to people whose teeth have become sensitive to heat and cold. Typically, sensitivity increases with age— some of the tooth enamel wears off over the years, exposing the dentinal tubules that connect with the tooth nerves. The solution, in Repair & Protect toothpaste that GSK has sold outside the US for years, was a form of the 45S5 glass invented by ACerS Distinguished Life Member Larry Hench. The 45S5 glass particles in Repair & Protect trigger an ionic reaction. When the glass particles contact saliva and water, the glass releases calcium and phosphate ions to form a calcium phosphate layer. The body then converts this to hydroxyapatite, which creates a physical barrier over the tubules much like the original enamel. Unfortunately, the US formulation of the product does not contain Bioglass (sold under the trade name NovaMin). The omission of this key ingredient typically is blamed on the need to obtain approval from the US Food and Drug Administration. GSK is not talking—the company American Ceramic Society Bulletin, Vol. 92, No. 6 | www.ceramics.org 15


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