Mechanical Behaviour of Single Crystal, Pollycrystalline and Nanocrystalline Metallic Nanopillars Under Compression

Fabrication techniques, and mechanical behaviours of vertically aligned cylindrical nanopillars of various metals, including tin, bismuth, palladium, indium, rhodium and cobalt have been presented in this work. These, vertically aligned, cylindrical nanopillars of various diameters have been fabricated via an electron beam lithography and electroplating method. Microstructural properties of these pillars were studied using high resolution scanning electron and transmission electron microscopy. A non-destructive synchrotron X-ray microdiffraction (μSXRD) technique was used for the characterization of tin and indium nanopillars. The results indicated single crystal body-centered tetragonal structured tin, polycrystalline rhombohedral bismuth, single crystal tetragonal indium, and nanocrystalline palladium, cobalt and rhodium nanopillars. The mechanical properties of these structures were studied by uniaxial compression under a nanoindenter outfitted with a flat punch diamond tip. The strain rate sensitivities and flow stresses were analyzed for each material. Single crystal tin and indium nanopillars showed size-dependent flow stresses where smaller diameter pillars exhibit greater attained strengths. The observed size-dependence matches closely to that previously reported for single-crystalline face centered cubic metals at the nanoscale. Polycrystalline bismuth nanopillars showed a size effect with a change in the deformation mechanism from grain boundary mediated mechanisms to dislocation processes as the pillar diameter approached the average grain size. Nanocrystalline palladium pillars showed an inverse size effect where a decrease in strength was seen for pillars with smaller diameters. Finally, a thorough study is presented on the buckling behaviours of 130nm diameter palladium, cobalt and rhodium nanocrystalline pillars with various height-to-diameter ratios and the elastic moduli of these materials are extracted

Role of KASH domain lengths in the regulation of LINC complexes.

The linker of the nucleoskeleton and cytoskeleton (LINC) complex is formed by the conserved interactions between Sad-1 and UNC-84 (SUN) and Klarsicht, ANC-1, SYNE homology (KASH) domain proteins, providing a physical coupling between the nucleoskeleton and cytoskeleton that mediates the transfer of physical forces across the nuclear envelope. The LINC complex can perform distinct cellular functions by pairing various KASH domain proteins with the same SUN domain protein. For example, in Caenorhabditis elegans, SUN protein UNC-84 binds to two KASH proteins UNC-83 and ANC-1 to mediate nuclear migration and anchorage, respectively. In addition to distinct cytoplasmic domains, the luminal KASH domain also varies among KASH domain proteins of distinct functions. In this study, we combined in vivo C. elegans genetics and in silico molecular dynamics simulations to understand the relation between the length and amino acid composition of the luminal KASH domain, and the function of the SUN-KASH complex. We show that longer KASH domains can withstand and transfer higher forces and interact with the membrane through a conserved membrane proximal EEDY domain that is unique to longer KASH domains. In agreement with our models, our in vivo results show that swapping the KASH domains of ANC-1 and UNC-83, or shortening the KASH domain of ANC-1, both result in a nuclear anchorage defect in C. elegans

Cellular and Molecular Responses to Mechanical Cues: From the Extracellular Matrix to the Nucleus

Mechanical signals affect virtually every fundamental single- and multi-cellular process in biology. The local responses of individual molecules to mechanical stimuli at the interface of cell with its adjacent microenvironment (extracellular matrix or material) elicit global responses at the cell and tissue scales. Understanding and manipulating the cell-material interaction can be leveraged to design biomaterials with unique characteristics tailored towards a wide variety of biological applications such as platforms that direct stem cell differentiation for tissue engineering, sensors that can record accurate electrical signals in single cells for neuroscience, and implants that are susceptible to cell adhesion for biomedical applications. In this thesis I present work characterizing the response of cells to mechanical stimuli at the single cell and single molecule scales. At the single cell scale, we provide insights into how mechanical signals such as micro- and nano-topography of metallic and metallic surfaces affect cell adhesion, both in mammalian and bacterial cells. Next we characterize the mechanical response of protein complexes involved in the transmission of mechanical signals across the cytoskeleton to the nucleus. The four main contributions of the work presented in this thesis are as follows: 1) We used high resolution scanning electron microscopy to characterize the cell-nanostructure interface and provide insights into the response of individual mammalian cells to nanostructures with complex geometries. 2) We provide a first look at how individual bacterial cells adhere to metallic nanostructures, which could lead to new techniques to thwart infections. 3) We proposed a novel technique to control the growth and arrangements of bacterial cell communities. This method will allow precise small-scale mechanical manipulation of bacterial cells and could be utilized for unraveling the understudied mechanisms of bacterial mechanosensitivity. 4) We performed the first molecular dynamics study on the mechanisms of force transmission to the nucleus of eukaryotic cells through protein complexes known as linkers of the nucleoskeleton and cytoskeleton (LINC complexes). We showed that LINC complexes are highly stable under tensile forces, and that the transmission of force across the complex depends highly on the unique intermolecular covalent bonds formed between the two proteins that construct the complex. Finally, we presented a model for the molecular mechanisms of LINC complex activation and regulation at the nuclear envelope