Hard Materials and Composites
The diversity of mineralized structures formed in biology has inspired the development of synthetic routes to organic-inorganic composites with unusual morphologies and physical properties. Organisms are able to produce structurally sound multi-functional components from readily available and inexpensive constituents using very little energy at room temperature. For example, organisms ranging from algae to mollusks use minerals ranging from crystalline calcitic (CaCO3) to amorphous silica structures for protection and skeletal support. The mechanical properties of these skeletal materials are far superior to the individual components (i.e., brittle mineral and compliant organic polymers). Organisms are also able to control the crystal morphology, and often stabilize less thermodynamically favorable phases in their skeletons. Perhaps even more remarkably, polycrystalline, hierarchical composites such as mammalian teeth are highly mineralized, hard, and abrasion-resistant. Still other tissues, such as the bone-cartilage interface, display controlled gradients of mechanical properties that are defined by compositional gradients of mineral to organic matrix. The mechanisms by which organisms control crystal growth and the resulting properties of the final products in all of these systems are still relatively poorly understood. Within the larger field of biomaterials research, there are three specific focuses of the hard materials and composites community: 1) the structural and mechanical characterization of naturally occurring biominerals; 2) the application of strategies learned from biology to create new materials with controlled materials properties; and 3) the design of in vitro systems to interact with biology and to answer questions from biology.
The ultimate goal the advanced characterization of hard biological and synthetic materials is to understand the structure-property-function profiles of the biological organic-inorganic composites (and similarly for the synthetic materials). Structural characterization of biological composites presents several challenges since they are structured on multiple length scales and often contain organic material embedded, non-periodically, at low concentrations in a host matrix with a high electron-scattering cross-section. Given these challenges, in recent years, multiple high-resolution techniques, some of which do not depend on periodicity and can provide simultaneously structure and composition information, have been used to obtain nanometer-scale images of biological composites, such as bone, teeth, mollusk shells, and other marine animal skeletal units. These techniques include X-ray scattering techniques (e.g., XANES, EXAFS, X-PEEM, combined SAXS/WAXD), electron microscopy and tomography, atom-probe tomography, solid-state NMR, and vibrational spectromicroscopies (IR and Raman) as well as more traditional biochemical techniques for protein characterization. To complement the structural characterization, there are also efforts devoted to characterizing the materials properties of the natural and synthetic composites. Most work to date has focused on mechanical properties (e.g., nano- and microindentation), however, there has also been some work looking at the optical properties and magnetic properties of some biominerals. The current challenge now is to “close the loop” and identify the structural features that lead to a given properties profile. Applying this knowledge to synthetic systems represents the next research direction.
Based upon the attractive structure-property-function profiles of biological composites, much effort has been devoted to developing synthetic approaches to replicate certain features of these materials on the benchtop. These efforts can be divided into two different approaches: 1) bio-inspired approaches that apply strategies learned from biology to create new materials with controlled morphologies and materials properties and 2) bio-enabled approaches that harness biological organisms and/or components to make new materials. The main successes of these types of approaches include the design of organic small molecules, polymers, and surfaces that can direct crystal growth, the use of phage display and related techniques to identify mineral-binding and nucleating peptides, the manipulation of organisms such as diatoms and sea urchins to grow patterned mineral structures, and the use of naturally occurring proteins from, e.g., sponges, to catalyze the growth of a wide range of inorganic materials. Future challenges include moving beyond “pretty pictures” to design, de novo, materials with the desired properties profile. There is also a continuous need to evaluate which structures from biology are worth copying/mimicking and which are not. As the field matures, this question will have to be addressed.
The design and development of the next generation of biomaterials for the repair of hard tissues (e.g., bone and teeth) and the interfaces between hard and soft tissues (e.g., bone-cartilage, bone-tendon, bone-ligament) requires an understanding of the how cells interact with and process mineral-containing biomaterials (see Cell-Material Interactions). While interface gradients of composition and mechanical properties exist in native mineralized tissues, current generation biomaterials are usually homogenous materials, and one of the biggest challenges that remains is the integration of these synthetic, homogenous materials into the living tissue. Topics to be addressed at the workshop include identifying fundamental science questions, as opposed to biomedical application driven research.