Technical Part: Bone is a hierarchically-structured composite material which at the nanoscale comprises an interpenetrating network of hydroxyapatite (HA) nanocrystals dispersed within the interstices of self-assembled collagen fibrils. The goal of the proposed research is to use a combination of biomimetic processing techniques to synthesize collagen-hydroxyapatite composites with a hierarchical structure emulating bone, and thus reproduce its mechanical properties. The proposed work extends prior success in mimicking the interpenetrating nanostructure of bone, by integrating it to the next level of hierarchy: the lamellar microstructure of bone. To accomplish this goal, densely-packed arrays of highly organized collagen will be produced using liquid-crystalline precursor collagen solutions and then laminated to mimic the twisted plywood structure found in the lamellae of osteonal bone. The microlaminated structures will then be mineralized by the PILP process, which has been shown to lead to intrafibrillar mineralization of collagen, with oriented nanocrystals of hydroxyapatite embedded throughout the interstices of the collagen fibrils. Compositions matching bone, with 60-70 wt% mineral, can be achieved with the PILP process; however, even with this high degree of mineralization, the inherent porosity of reconstituted collagen sponges prevents the composites from supporting high loads. Thus, a highly-organized collagen, as is found in bone, is needed for reproducing the properties of bone. If the densely-packed collagen arrays can be mineralized via the PILP process, it should be possible, for the first time, to match the modulus, strength and toughness of bone. Micromechanical testing will be performed on the multilevel composites generated with different lamination strategies to assess the quality of the structures that are produced, and how the various structural parameters (such as lamellar thickness, fiber diameter/organization, degree of mineralization, etc.) correlate to mechanical properties. Finally, because bone is a mechanical structure, the PILP process will be conducted on both loaded and unloaded collagen arrays to determine if load might play a key role in bone morphogenesis.
Non-Technical Part: Bone is a remarkable composite from which its underlying structural design may help guide the design of future composite materials. There have been many studies on the mechanical properties of bone, but because of its hierarchical structure, it is difficult to isolate the effects of various strengthening and toughening mechanisms that underlie each level of structure. By developing an in vitro model system that can mimic separate levels of bone structure, such properties might be examined without this overriding complication, and benefit from the ability to tailor individual components of the composites. This work also has important biomedical implications, both at the fundamental science level with respect to understanding bone formation and properties, and the applications side of developing the next generation of orthopedic biomaterials. This biomimetic approach has the potential to lead to load-bearing bioresorbable bone substitutes which are remodeled through the cellular processes that occur during natural bone remodeling. With respect to education, bone, due to its hierarchical structure, provides an interesting forum for training students about composites in the materials science field. Outreach programs such as Research Experience for Teachers (RET) and Biomimetic-Materials Outreach Program (BMOP) will be continued, which have included training and outreach to students at the graduate, undergraduate, and K-12 levels. A new outreach program is also proposed, in which public lectures will be provided to aquarists in nearby cities to show them the interesting materials science behind the invertebrate biominerals they enjoy as a hobby (such as sea urchins and mollusk shells), and the similarities/differences between these biominerals and vertebrate biominerals, to show how biomimetic engineers are developing novel hierarchically-structured composite materials.
