INTELLECTUAL MERIT: There is a need for new light-weight structural materials with high strength and durability that are low-cost and recyclable. Nature has evolved efficient strategies, exemplified in the mineralized tissues of numerous species, to synthesize materials that often exhibit exceptional mechanical properties. These biological systems demonstrate the ability to control nano- and microstructural features that significantly improve the mechanical performance of otherwise brittle materials. One such example is found in the hyper-mineralized combative dactyl club of the stomatopods, a group of highly aggressive marine crustaceans. This ultrahard organic-inorganic composite structure is capable of inflicting significant damage following impact with a wide variety of biomineralized structures (e.g., mollusk shells, crab exoskeletons, the skulls of small fish). This project combines expertise in a number of areas including biologically inspired materials synthesis, structural characterization, mechanical testing, solid-state nuclear magnetic resonance (NMR) spectroscopy, protein-mineral interaction modeling, and a thorough knowledge of the biological system to be studied. Using these approaches, the project will investigate the structure-property relationships of the hyper-mineralized stomatopod dactyl clubs and elucidate the primary toughening mechanisms of these unique composite materials through the following potentially transformational investigations: (1) A detailed, three-dimensional map of the nano- and microstructural features, with specific mineral composition and phase information. (2) A complementary mechanical investigation of the regional structures. (3) In-vitro mineralization experiments to understand organic-inorganic interactions that control mineral composition and phase.
BROADER IMPACTS: This strongly interdisciplinary research has the potential to enable significant progress in the emerging fields of nanotechnology and nanomanufacturing by exploiting control mechanisms established by nature to make novel materials and devices exhibiting paradigm-shifting properties. By investigating the structure-property relationships of these unique impact-tolerant mineralized structures using modern chemical, morphological, and mechanical characterization techniques, the long-term aim will be to develop the necessary tools for the design and fabrication of cost-effective and environmentally friendly engineering materials that mimic the various design elements and performance properties present in biological systems. The PI, a new faculty member at UC Riverside, has demonstrated an ability to attract a diverse group of students to his lab. Of the six graduate students, three are female. He has 12 undergraduates working in his lab, including four females, four Hispanics, and two African Americans. The co-PI has an equally strong record in attracting a diverse group of students. The UC Riverside campus is noted for its success in attracting underrepresented minorities to a Research I environment and seeing them complete their studies successfully. Encouragement of undergraduate research is an important component of this success, and both PIs are making strong contributions. Although not required of proposals submitted prior to January 5, 2009, the proposal describes a clear and effective plan for training of the postdoctoral who will be trained on the project. The PI has also described plans for involving high school students and teachers from the diverse community surrounding the university in his research. A web camera set up in the Kisailus lab will enable students in local and more distant high schools to observe the stomatopod rearing facilities and the experimental research as it progresses.
¬(0906770) PI: David Kisailus, UC Riverside The stomatopods are an ancient group of marine tropical and subtropical crustaceans with a fossil record that dates back more than 300 million years. Modern representatives can reach lengths of nearly 40 cm, although most species are significantly smaller (e.g., 15 cm). To the casual observer, they superficially resemble heavily armored caterpillars and the group is best known for their complex visual systems, their solitary nature, and aggressive hunting strategies. In stomatopods, the second pair of thoracic appendages is highly modified and specifically adapted for powerful close-range combat. The dactyl modifications (the terminal segment) of these appendages divide the group into those that either hunt by impaling their prey with spear-like structures or those that smash them with a powerful blow from a heavily mineralized club. This robust hammer-like composite structure is capable of inflicting significant damage following impact with a wide variety of heavily mineralized biological structures (e.g., mollusk shells, crab exoskeletons, the skulls of small fish, and the occasional weary fisherman). Significantly, many of these prey items represent model systems for the study of tough and damage tolerant biological materials and investigation into their micro- and nano-architectural features have provided critical insight into the design of robust synthetic analogs. This observation highlights the unique structure and impressive performance of the stomatopod dactyl club, and the important lessons that can be learned from its investigation. We report a comprehensive study of the structural complexity and mechanical properties of the dactyl clubs of Odontodactylus scyllarus, a common reef-associated stomatopod from the tropical Indo-Pacific. As described by Patek et al., these formidable structures are capable of accelerations to 10,400 g and speeds of 23 m/s from a stationary position. Their rapid strike can generate cavitation bubbles between the appendage and their prey, with the collapse of these bubbles producing significant stresses at the contact point, in addition to the instantaneous forces upwards of 700 N resulting from the direct impact. Despite these significant loads, the dactyl clubs are extremely damage tolerant and are able to withstand thousands of highly energetic blows. Our studies show that the stomatopod dactyl club represents a significant departure from previously studied damage tolerant biological composites, in that it is specifically employed for high velocity offensive strikes. Our structural investigations coupled with nano-mechanical characterization and finite element simulations have shown that the club consists of several microstructural features (Figure 1) that permit the infliction of crippling impacts while simultaneously minimizing internal damage within the club. Caption for Figure 1. Morphological features of the stomatopod dactyl club. (A) A generalized stomatopod body plan and (B) a close up of the anterior end of Odontodactylus scylarus. The arrows denote the location of the dactyl club’s impact surface. (C) Backscattered scanning electron micrograph of the club’s external morphology and (D) a microCT longitudinal section though the anterior half of a complete specimen showing the constituent dactyl, D and propodus, P segments, revealing their differences in electron density (the second thoracic appendage with its terminal dactyl club modification is highlighted in red). (E) Cross-sectional analysis of the club illustrates the three distinct structural domains: (i) The impact region (blue), (ii) the periodic region [further subdivided into two discrete zones: medial (red) and lateral (yellow)], and (iii) the striated region (green). The periodic region of the propodus is shown in orange. (F) Optical micrographs, revealing the buckled rotated plywood structural motif of the impact region, the pseudo-laminations of the periodic region, and the thickened circumferential band with parallel chitin fibers in the striated region (A adapted from (36), B courtesy of Silke Baron, and D courtesy of DigiMorph.org). These characteristics include a pitch-graded helicoidal architecture constructed from mineralized chitin fibers that can dissipate the energy released by propagating microcracks, an oscillating elastic modulus that provides further shielding against catastrophic crack propagation, a modulus mismatch in the impact region that acts as a crack deflector near the impact surface, and an ultra-hard outer layer correlated with a high level of mineralization and a radial organization of apatitic crystallites. The structural lessons gained from the study of this multi-phase biological composite could thus provide important design insights into the fabrication of tough ceramic/organic hybrid materials in structural applications where components are subjected to intense repetitive loading. We have initiated synthesis of biomimetic composites, which have demonstrated incredible impact resistance to ballistics. However, our ultimate goal will be to develop a new generation of high-performance multifunctional materials for a wide range of technologically relevant applications, ranging from energy storage and conversion to high-strength/low-weight structural materials. This will be included in future research.
