Formation of Complex Non-Equilibrium Morphologies of Calcite via Biomimetic Processing
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Formation of Complex Non-Equilibrium Morphologies of Calcite via Biomimetic Processing Yi-yeoun Kim and Laurie B. Gower Department of Materials Science and Engineering, University of Florida, Gainesville, FL, 32611, USA ABSTRACT Our biomimetic approach for fabricating organic-inorganic composites with structures similar to biominerals is based on a novel mineralization process, called the Polymer-InducedLiquid-Precursor (PILP) process. This process enables the deposition of non-equilibrium mineral morphologies of calcite under low-temperature and aqueous-based conditions [1], including patterned thin films of calcite. We have recently found that when a surplus of acidic polymer is added, the patterned mineral films act as a secondary template for directing new crystal outgrowths, which form into complex morphologies of calcite with time, such as fibrous mats and “horsetails”. Two interdependent factors, the polymer and Ca-ion concentration, which change the local solution environment over time, appear to modulate the creation of these different structures. Such observations may provide clues for unraveling the long-standing mystery of how biological systems fabricate their sophisticated and complex morphologies. INTRODUCTION Recently, the synthesis of inorganic materials with controlled morphology has attracted significant interest in the materials science field. Biomimetic synthetic strategies, which mimic the biological mechanisms used in biomineralization to form complex organic-inorganic composites, have shown promising results for attaining morphological control of inorganic materials [2-5]. In particular, the use of organic templates and polymeric crystal growth modifiers, which are inspired by biomineralization processes, have produced a wide array of interesting materials, such as structures ranging from one-dimensional fiber morphologies, twodimensional patterned films, to three-dimensional “molded” morphologies, all of which are generated under benign processing conditions [1, 6-9]. However, many questions remain as to the interplay between the various mechanisms utilized by biological systems for the regulation of their sophisticated morphologies. We have put forth the hypothesis that a polymer-induced liquid-precursor (PILP) process plays a fundamental role in biomineralization (in both vertebrates and invertebrates) [1, 3]. In the PILP process, micromolar quantities of acidic polymers are added to the crystallizing solution of an inorganic salt (such as calcite), and the charged polymer sequesters the ions and generates liquid-liquid phase separation. The phase boundaries of the minor phase ultimately define the shape of the final crystal products that form upon solidification and densification of the precursor phase, thus producing a variety of non-equilibrium crystal morphologies. In the PILP process, an important distinction in terms of morphological control is that a phase segregated precursor phase can conceivably be manipulated, molded and shaped by a compartment since the precursor is flu
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