Synthesis, Characterization, and Application of Gel-Grown, Polymer-Reinforced Single Crystals (Funded by DMR/NSF)
Recent observations, both in Nature and in the laboratory, of macroscopic, single crystals with incorporated polymer fibers and other macromolecules have generated immense interest amongst materials scientists, chemists, and biologists. The mechanisms by which they form, however, are poorly understood. The research goals of this project are: to understand the fundamental principles that govern the incorporation of polymer networks and macromolecules into single crystals, to develop synthetic routes to a wide range of polymer-reinforced crystals with detailed control over structure and porosity, to characterize the internal structure of these crystals, and to establish structure-mechanical property correlations for model systems. To accomplish these goals, we will develop the use of crystal growth in gels to produce polymer-reinforced single crystals of ionic, molecular, and covalent crystals. When successful, this work has the potential to transform our current understanding of the mechanisms that lead to the incorporation of large amounts of polymeric impurities within crystals, without disrupting their single crystal nature. The particular strength of this program lies in the interdisciplinary approach that combines our demonstrated expertise in crystal growth, organic synthesis, and materials characterization methods. By combining all of these efforts, the project aims to generate a fundamental understanding of the underlying mechanisms governing polymer fiber incorporation into single crystals. This understanding, in turn, will allow us to generalize the effects to materials of interest (e.g., bioactive minerals, inorganic semiconductors, pharmaceutical compounds). The ultimate goals of the project are to be able to design a gel system for a given crystalline material and to predict the growth conditions required to obtain single crystals of that material with incorporated networks.
The agarose gel network visualized inside of a single crystal of calcite using annular dark field scanning transmission electron microscopy (ADF-STEM) tomography.
Nanostructured Block-copolymer Reinforced Calcium Phosphate Composites
(Funded by NIDCR/NIH)
Collaboration with U. Wiesner
The broad objective of this project is to develop synthetic methodologies towards next generation dental restorative composites that have a nanostructured arrangement of the inorganic and organic components, similar to the structures found in dentin and enamel. Our approach is to combine calcium phosphate minerals with the well-established ability of amphiphilic (hydrophobic/hydrophilic) block copolymers to direct the assembly of inorganic materials into mesostructured hybrids (with e.g., cylindrical, lamellar, or bicontinuous morphologies). The synthesis of the materials will be completed with four specific aims: 1) Synthesis and characterization of stable amorphous calcium phosphate (ACP) particles (< 100 nm) capable of forming stable suspensions in both aqueous and organic solvents. 2) Use of amphiphilic diblock copolymers to direct the assembly of ACP particles into materials with defined 1-D, 2-D, and 3-D nanostructures. 3) Controlled crystallization of the amorphous, inorganic phase of the nanostructured composites. 4) Development of self-hardening composites by mixing powders of the nanostructured composites developed in Aims 2 and 3 with an aqueous liquid phase to induce secondary crystallization. The desgin of these new composites for dental restoration represents a paradigm shift away from traditional materials with isolated inorganic particles embedded within a continous polymer matrix to composites with a continuous inorganic phase penetrated by polymer. The benefits of a continuous inorganic phase include a reduction (or even elimination) of shrinkage of the composites as they harden, improved abrasion (wear) resistance, and stronger materials. Mechanically, the amphiphilic block-copolymer as the organic component will provide toughness and fracture resistance to the composite. The adhesion of these self-hardening composites to the natural tooth should be better than the traditional resins since we are using a water-based system that will be able to infiltrate the hydrated dentin. The flexibility of the block copolymer system will allow us in the future to tailor the mesostructure of the composites to form materials whose mechanical properties best match the part of the tooth (e.g., dentin or enamel) being repaired.
Crystalline and Amorphous Nanomaterials in Breast Cancer Bone Metastasis (Funded by CCMR Seed Grant)
Collaboration with Claudia Fischbach-Teschl (BME)
Metastatic bone disease is a frequent cause of morbidity in patients with advanced breast cancer, and the physicochemical characteristics of bone may be critical to this condition. At bone metastatic sites, breast cancer cells interact closely with the biomineralized bone matrix, a composite structure of collagen fibers reinforced with nano-scale crystals of hydroxyapatite (Ca10(PO4)6(OH)2). The nanostructure of the biogenic hydroxyapatite (HA) crystals (i.e., crystallinity, chemical composition, size, and aspect ratio) play an important role in determining the biological and physicochemical characteristics of the particles, and may vary as a function of bone age, location, and disease state. It may be possible that these intrinsic differences impact malignant progression and bone degradation (osteolysis) in breast cancer patients, however, it remains unclear which factors are most important. This lack of understanding is partly due to a paucity of pathologically relevant culture systems to study changes in tumor cell behavior as a function of varying HA nanoscale properties. Therefore, we sought to utilize synthetically prepared HA nanoparticles to determine the impact of particle size and crystallinity on mammary cancer cell activity. The long-term goal of this SEED project is to elucidate the functional relationships between the nano-scale characteristics of HA, mammary tumor cell behavior, and osteolytic bone metastasis, potentially establishing a new paradigm for the induction and progression of tumor-bone interactions during metastatic breast cancer.
Patterning Mineralization in Gels: Surfaces, Gradients, and Cells (Funded by NIAMS/NIH: pending)
Collaboration with Adele Boskey (HSS)
The long-range goal of these studies is to develop a multi-disciplinary approach to the design and synthesis of composite structures with controlled gradients of mechanical properties dictated by the distribution of apatite crystals and cells within a gel matrix. Ultimately, this research will lead to the development of strategies for the design of new biomaterials for the repair of damaged hard tissues. Specifically, the proposed research focuses on the use of a hydrogel-based double-diffusion system (DDS), developed by the investigators, that contains nucleating surfaces and chondrocytes to study, in vitro, the orchestrated events that lead to biomineralization in the growth plate. While interface gradients exist in native mineralized tissues, current generation biomaterials are usually homogenous materials and one of the biggest challenges for bioengineering remains the integration of these synthetic, homogenous materials into the living tissue. Towards this objective, three hypotheses will be tested: 1) Inclusion of nucleating surfaces into the DDS will lead to more control over the physicochemical properties of the hydroxyapaptite crystals than systems without such surfaces. 2) Gradients of mineralization inhibitors and/or promoters will yield a physiologic mineralized matrix with spatial gradients of mechanical properties similar to the native tissue. 3) Exposure of mesenchymal cells, which differentiate into growth plate chondrocytes, to chemical and physical gradients will alter their differentiation, maturation, and mineralization and will result in a construct that resembles the in situ tissue. There are three specific aims designed to test these hypotheses: Aim 1) Introduction of functionalized, porous silicon membranes into the DDS to serve as “nucleating surfaces”. The effect of surface functionality and gel/surface interactions on the size, shape (aspect ratio), and crystallographic orientation of the resulting hydroxyapatite crystals will be determined. A combination of IR spectroscopy, x-ray diffraction, and electron microscopy will be used to analyze the mineral. Aim 2) Development and evaluation of methods to introduce physiologically-relevant gradients of mineralization inhibitors and/or promoters into the DDS using enzymes trapped within the gel. These chemical gradients will be translated into gradients of mineral density in the gels. The gradients in mechanical properties that result from the mineralization gradients will be determined and compared to the native tissue. Aim 3) Introduction of chick-limb bud mesenchymal cells into persistent, 3-D gradients created in the DDS. The effect of these gradients on the differentiation, maturation, and activity of growth plate chondrocytes in vitro, will be assessed and compared to the native tissue. This work combines approaches from bioengineering, chemistry (crystal growth and nucleation) and biology (cell culture in gradients) to address fundamental questions regarding the creation of complex physical and chemical gradients in mineralized tissues.