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Secrets of the nanoscale materials

FEATURED RESEARCH - POSTED ON MAR 13, 2014

NDSU professors, Drs. Kalpana and Dinesh Katti, use CCAST computers in multiscale modeling of nanoscale systems. Over the past 15 years, their group has made major discoveries and contributions to nanosciences that span nanomaterials of biological and geological origin. In several instances, their research led to the development of fundamental new ways for the design of synthetic nanocomposite materials. Kalpana's and Dinesh's research has induced a paradigm shift within the mechanics community. This paradigm shift changed the locus of the fundamental mechanisms that control the mechanics of nanoscale systems. They found that irrespective of the origin of the nanoscale material; it is the molecular scale interactions that are significant to the mechanical behavior of the material, and not just the mesoscale and continuum regimes that are used in modeling today. Researchers in various fields are now adopting their approach and techniques.

In addition, the discovery by their group of the role of nanoclays on influencing the biology of stem cells will have a profound impact on medicine in general and regenerative medicine in particular.

The following is a snapshot of Kalpana's and Dinesh's research activities.

Simulation-based design of biomimetic nanocomposites

Nacre, the inner layer of seashells.

Seashells are nature's armor materials that protect living organisms that reside within them. Of particular interest to materials scientists is nacre, the inner layer of seashells that exhibits extraordinary properties such as high tensile strength and remarkable fracture toughness which potentially make nacre a next-generation high-performance biomimetic material. Nacre is a biological nanocomposite as a result of nanosized organic phase sandwiched between submicron thick hexagonal aragonite platelets. Kalpana's and Dinesh's novel multiscale simulations paralleled by experiments revealed the key mechanisms that lead to these extraordinary and unique properties. Their work provides a roadmap for design of high performance biomimetic nanocomposites based on nacre. In particular, Kalpana and Dinesh found that:

  1. At the nanoscale level, the properties of these materials are dramatically different from their bulk properties.
  2. Mineral proximity strongly influences the mechanics and unfolding of neighboring proteins and thus profoundly influences the mechanics of nacre.
  3. Platelets interlock in nacre and these interlocks are the key to fracture toughness and strength in this material. The PIs also discovered platelet interlocks in nacre.

Steered molecular dynamics simulations show tremendous influence of mineral proximity on the mechanics of nanoscale organic phase

 

Discovery of platelet interlocks at NDSU by the PIs. (a) Scanning electron microscopy image showing platelet interlocking., b) mechanism of interlock formation, c) finite element model simulation of nacre with interlocks showing role of interlocks on the nacre mechanics, d) high fracture toughness (resistance to fracture) as a result of progressive failure of interlocks.

 

Multiscale investigation of human bone

Hierarchy in human bone

Bones are vital to the human body and provide structural integrity, mobility and protection to organs and constitute repositories of critical chemical and biological materials essential for human health and vitality. They are biological hierarchical nanocomposites consisting of nanosized mineral phase, collagen, water and other organic molecules and ions. This hierarchy ranges from the molecular scale to the macroscale. Kalpana and Dinesh have extensively investigated the role of molecular scale interactions between the nanosized mineral phase and the collagen molecule and the profound role of their interactions on the mechanics of bone. Their results suggest that current bone models used extensively in biomedical research and practice are inaccurate and inadequate. Some of Kalpana's and Dinesh's important findings and accomplishments under this research activity are:

 

Finite element model of fibril incorporating molecular scale interactions. Results show the important role of protein-mineral interactions on the mechanics of bone.

  1. Proximity of hydroxyapatite mineral strongly influences the mechanics of the collagen molecule. Key mechanisms that cause changes to the collagen mechanics have been identified.
  2. The first multiscale model of mineralized fibril in bone that relates molecular scale interactions to macroscale properties has been constructed.
  3. Molecular scale interactions between mineral and collagen contribute very significantly to the mechanics of fibril and consequently to the properties of the bone. Thus, models ignoring these interactions are inadequate to predict bone mechanics.
  4. The stoichiometry of nanoscale hydroxyapatite mineral is influenced by the orientation of the collagen molecule in its proximity, thus influencing biomineralization and potentially the mechanics of the bone.

 

 

Steered molecular dynamics simulations of collagen molecule in the proximity of hydroxyapatite mineral showing significant influence of mineral proximity on the mechanics of collagen

 

 

Fourier Transform Infrared Spectroscopy experiments on human cadaver bone reveal that differences in molecular interactions between collagen and mineral (due to differences in collagen orientation) during biomineralization influence the stoichiometry of the mineral phase.

 

 

Investigation of Collagen Molecule Mechanics

 

 

Discovery of third level helicity in full length collagen molecule. In the elastic stress regime, almost half the deformation in the molecule is attributed to the level-3 helicity.

Collagen, a triple chain molecule, is the most abundant protein in the human body and is responsible for vital mechanical and biological functions. Kalpana's and Dinesh's group has conducted detailed molecular studies of collagen, including generation of quantitative structure property relationships, in order to understand mechanisms contributing to mechanics of collagen and to enable accurate prediction of its behavior in normal and diseased states. Kalpana's and Dinesh's discovery of the third level helicity in collagen molecules and its critical role in collagen mechanics affords accurate prediction of the properties of this important molecule. The following are important discoveries and findings about collagen mechanics

 

  1. Discovery of the third level helicity in full length collagen molecule. Only two levels of structural helicity were known prior to this discovery.
  2. In the normal physiological deformation regime (elastic range), the third level helicity contributes as much to the deformation of collagen as the two previously known levels, thus making the newly discovered third level helix a critical structural feature that must be considered in modeling of collagen.
  3. The key molecular mechanisms that provide stability and prevent the unraveling of the triple helix were elucidated, and the important role of nonbonded interchain interactions on the mechanics of collagen was established.
  4. The wide use of short collagen molecules as representative molecules is inadequate because of the absence of the third level helicity.

 

 

Regenerative medicine -- Tissue Engineering

Nanoclay based scaffolds for tissue engineering. These scaffolds create an environment for stem cells to make bone

Design and fabrication of organs is the holy grail of bioengineering and medicine. The particularly challenging organ in tissue engineering is the human bone because of challenges in replicating both mechanics and a biological environment needed for bone regeneration in scaffolds. Kalpana and Dinesh are designing biobased as well as synthetic nanocomposite materials for bone regeneration. For the first time, nanoclays are proposed and used for synthesis of nanocomposites for biomedical applications. Stem cells need a stimulus or environment to differentiate into various cell types. These nanoclays not only significantly improve the mechanics of scaffolds but also provide a biological environment for the growth, proliferation, differentiation and tissue generation without growth factors. Therefore, Kalpana and Dinesh are paving the way for the use of innovative nanoclay based materials for biomedical applications. Their research in this area has led to these important discoveries and findings:

 

Molecular models of nanoclay-amino acid-hydroxyapatite system to optimize mechanical properties of materials for simulation based design of scaffolds

  1. Nanoclays can be used for biomedical materials.
  2. Unnatural amino acids can be used for nanoclay preparation for application in biomedical applications.
  3. Nanoclays improve mechanics as well as create a biological environment for stem cell proliferation, growth, differentiation and tissue regeneration.
  4. Scaffold material cell interactions influence cell mechanics.

 

 

Polymer clay nanocomposites

Dispersion of small amounts of nanoclays results in big changes in mechanical, thermal and diffusion properties of polymer clay nanocomposites. The reasons for such property enhancement were not known. Kalpana and Dinesh used extensive multiscale modeling and experimentation techniques to develop a new theory that explains the mechanisms that lead to the drastic enhancement of mechanical properties in polymer clay nanocomposites. This new theory, named the Altered Phase Theory, demonstrates how molecular scale interactions between the clay-modifier and polymer can alter large volumes of the polymer, resulting in dramatic changes to its crystallinity and mechanical properties in the altered zones. Kalpana's and Dinesh's research in this area have led to these important developments and findings:

 

  1. Development of the molecular Altered Phase Theory for polymer clay nanocomposites.
  2. Molecular interactions alter the crystallinity and mechanical properties of the polymer. The altered zone encompasses a significant volume of the polymer. Because of this, small amounts of nanoclay can drastically impact the macroscale properties of the composite.
  
 Altered phase model for polymer clay nanocomposites. Molecular interactions alter a significant volume of the polymer phase, thus a small amount of nanoclay dispersed in a polymer can significantly enhance the mechanical properties of the nanocomposites. Multiscale computational models bridging molecular interaction and nanostructure were built.

 

Swelling clays

Swelling clays are found in various parts of the United States (including eastern North Dakota) and many parts of the world. Swelling clays exert enormous pressure when wetted, causing tremendous damage to the nation's infrastructure and light structures. The damage caused by these clays is estimated to be of the order of $7 to $8 billion per year in the United States alone. These clays are also used as drilling muds and sealants, in oil exploration as liners and barriers to prevent contaminants from entering the groundwater, as drug delivery systems, etc. Kalpana and Dinesh are pioneering the application of steered molecular dynamics in conjunction with discrete element techniques and experimental techniques to delineate the relationship between molecular scale interactions across clays and fluids and resulting changes to the microstructure and macroscale properties of these complexes. They are bringing steered molecular dynamics to the field of geomechanics. Kalpana's and Dinesh's research in this area have led to these important developments and findings:

 

  1. First steered molecular dynamics simulation of smectite swelling clays.
  2. The breakdown of clay particles during swelling has been observed by experimental evolution of microstructure.
  3. For the first time, the role of particle breakdown on swelling and swelling pressure has been elucidated by discrete element modeling.
  4. A novel spectroscopic method has been developed to evaluate flow rate of water molecules in the sub-nanometer and nanometer interlayer space between clay sheets.
  5. Molecular interactions between clay and fluids alter the soil microstructure and together dramatically affect the macroscale properties of the swelling clays.
   
Design and fabrication of a new device to accurately measure fluid flow properties through swelling clays. Steered molecular dynamics simulatins are conducted to evaluate mechanical properties of clay interlayer. A new discrete element method incoporating particle subdivision was devloped to accurately model clay swelling.

 

Plot showing dramatic changes to fluid flow through swelling clay due to differences in the fluid dielectric constant. Fluids with higher dielectric constant have stronger molecular interactions with clay. Lower values of coefficient of permeability indicate higher fluid flow through the clay. Each major division on the y-axis is a 10 fold increase or decrease. For example, flow of toxic sol-vent such as TCE will flow about half a million times faster than water through this clay, thus making this clay an ineffective long term barrier material to contain TCE.

 

Oil Shale

Oil shale is a sedimentary rock that contains a significant amount of pre-cursor to crude oil known as Kerogen. It is estimated that, in the United States, oil shale deposits contain approximately the equivalent of 2 trillion barrels of crude oil locked in the rock matrix in the form of Kerogen. Kalpana and Dinesh are conducting multiscale modeling and experimental investigation in order to understand how the organic macromolecules are "locked" in the mineral matrix. Such understanding will allow for the development of effective methods for extraction of this valuable energy resource. Some of the key findings from Kalpana's and Dinesh's work in this area are:

  1. Kerogen "pockets" in the Green River oil shale have sizes of the order of 10s of nanometers, making oil shale a geologic nanocomposite.
  2. Kerogen macromolecules in oil shale are strongly influenced by nonbonded interactions with surrounding minerals.
  3. Molecular interactions of Kerogen moieties with clay provide detailed insight into the nature of interactions between clay and Kerogen moieties.
  4. Development of the full representative Kerogen structure model to conduct steered molecular dynamics simulations is almost complete and will be used to study detailed mechanism of Kerogen binding with minerals.
  5. Simulation test beds are being developed for testing effective Kerogen extraction techniques.
  
Scanning electron microscopy images of oil shale obtained from Colorado showing that kerogen is dispersed in small nano-sized pockets in the rock's mineral matrix. Molecu-lar modeling studies on kerogen with rock minerals are being conducted.

 


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