45 research outputs found
Plastic deformation of metallic glasses: Size of shear transformation zones from molecular dynamics simulations
Plastic deformation in metallic glasses well below their glass transition temperatures Tg occurs spatially heterogeneously within highly localized regions, termed shear transformation zones (STZs). Yet, their size and the number of atoms involved in a local shear event, remains greatly unclear. With the help of classical molecular dynamics (MD) computer simulations on plastic deformation of the model glass CuTi during pure shearing, we address this issue by evaluating correlations in atomic-scale plastic displacements, viz. the displacement correlation function. From the correlation length, a universal diameter of about 15 Ă
, or, equivalently, approximately 120 atoms is derived for a variety of conditions, such as variable strains, strain rates, temperatures, and boundary conditions. Our findings are consistent with a recent model proposed by Johnson and Samwer [Phys. Rev. Lett. 95, 195501 (2005)]
Adhesion of Neurons and Glial Cells with Nanocolumnar TiN Films for Brain-Machine Interfaces
Coupling of cells to biomaterials is a prerequisite for most biomedical applications;
e.g., neuroelectrodes can only stimulate brain tissue in vivo if the electric signal is transferred
to neurons attached to the electrodesâ surface. Besides, cell survival in vitro also depends on the
interaction of cells with the underlying substrate materials; in vitro assays such as multielectrode
arrays determine cellular behavior by electrical coupling to the adherent cells. In our study, we
investigated the interaction of neurons and glial cells with different electrode materials such as TiN
and nanocolumnar TiN surfaces in contrast to gold and ITO substrates. Employing single-cell force
spectroscopy, we quantified short-term interaction forces between neuron-like cells (SH-SY5Y cells)
and glial cells (U-87 MG cells) for the different materials and contact times. Additionally, results
were compared to the spreading dynamics of cells for different culture times as a function of the
underlying substrate. The adhesion behavior of glial cells was almost independent of the biomaterial
and the maximum growth areas were already seen after one day; however, adhesion dynamics of
neurons relied on culture material and time. Neurons spread much better on TiN and nanocolumnar
TiN and also formed more neurites after three days in culture. Our designed nanocolumnar TiN
offers the possibility for building miniaturized microelectrode arrays for impedance spectroscopy
without losing detection sensitivity due to a lowered self-impedance of the electrode. Hence, our
results show that this biomaterial promotes adhesion and spreading of neurons and glial cells, which
are important for many biomedical applications in vitro and in vivo
The impact of jamming on boundaries of collectively moving weak-interacting cells
Collective cell migration is an important feature of wound healing,
as well as embryonic and tumor development. The origin of collective cell
migration is mainly intercellular interactions through effects such as a line
tension preventing cells from detaching from the boundary. In contrast, in this
study, we show for the first time that the formation of a constant cell front of a
monolayer can also be maintained by the dynamics of the underlying migrating
single cells. Ballistic motion enables the maintenance of the integrity of the
sheet, while a slowed down dynamics and glass-like behavior cause jamming of
cells at the front when two monolayersâeven of the same cell typeâmeet. By
employing a velocity autocorrelation function to investigate the cell dynamics
in detail, we found a compressed exponential decay as described by the
KohlrauschâWilliamâWatts function of the form C(δx)t âź exp (â(x/x0(t))β(t)),
with 1.5 6 β(t) 6 1.8. This clearly shows that although migrating cells are an
active, non-equilibrium system, the cell monolayer behaves in a glass-like way,
which requires jamming as a part of intercellular interactions. Since it is the
dynamics which determine the integrity of the cell sheet and its front for weakly
interacting cells, it becomes evident why changes of the migratory behavior
during epithelial to mesenchymal transition can result in the escape of single
cells and metastasis
Laminin Adsorption and Adhesion of Neurons and Glial Cells on Carbon Implanted Titania Nanotube Scaffolds for Neural Implant Applications
Interfacing neurons persistently to conductive matter constitutes one of the key challenges when designing brain-machine interfaces such as neuroelectrodes or retinal implants. Novel materials approaches that prevent occurrence of loss of long-term adhesion, rejection reactions, and glial scarring are highly desirable. Ion doped titania nanotube scaffolds are a promising material to fulfill all these requirements while revealing sufficient electrical conductivity, and are scrutinized in the present study regarding their neuronâmaterial interface. Adsorption of laminin, an essential extracellular matrix protein of the brain, is comprehensively analyzed. The implantation-dependent decline in laminin adsorption is revealed by employing surface characteristics such as nanotube diameter, (Formula presented.) -potential, and surface free energy. Moreover, the viability of U87-MG glial cells and SH-SY5Y neurons after one and four days are investigated, as well as the materialâs cytotoxicity. The higher conductivity related to carbon implantation does not affect the viability of neurons, although it impedes glial cell proliferation. This gives rise to novel titania nanotube based implant materials with long-term stability, and could reduce undesirable glial scarring
Recommended from our members
Mechanical spectroscopy of retina explants at the protein level employing nanostructured scaffolds
Development of neuronal tissue, such as folding of the brain, and formation of the fovea centralis in the human retina are intimately connected with the mechanical properties of the underlying cells and the extracellular matrix. In particular for neuronal tissue as complex as the vertebrate retina, mechanical properties are still a matter of debate due to their relation to numerous diseases as well as surgery, where the tension of the retina can result in tissue detachment during cutting. However, measuring the elasticity of adult retina wholemounts is difficult and until now only the mechanical properties at the surface have been characterized with micrometer resolution. Many processes, however, such as pathological changes prone to cause tissue rupture and detachment, respectively, are reflected in variations of retina elasticity at smaller length scales at the protein level. In the present work we demonstrate that freely oscillating cantilevers composed of nanostructured TiO2 scaffolds can be employed to study the frequency-dependent mechanical response of adult mammalian retina explants at the nanoscale. Constituting highly versatile scaffolds with strong tissue attachment for long-term organotypic culture atop, these scaffolds perform damped vibrations as fingerprints of the mechanical tissue properties that are derived using finite element calculations. Since the tissue adheres to the nanostructures via constitutive proteins on the photoreceptor side of the retina, the latter are stretched and compressed during vibration of the underlying scaffold. Probing mechanical response of individual proteins within the tissue, the proposed mechanical spectroscopy approach opens the way for studying tissue mechanics, diseases and the effect of drugs at the protein level
Recommended from our members
Nanoporous Morphogenesis in Amorphous Carbon Layers: Experiments and Modeling on Energetic Ion Induced SelfâOrganization
Nanoporous amorphous carbon constitutes a highly relevant material for a multitude of applications ranging from energy to environmental and biomedical systems. In the present work, it is demonstrated experimentally how energetic ions can be utilized to tailor porosity of thin sputter deposited amorphous carbon films. The physical mechanisms underlying self-organized nanoporous morphogenesis are unraveled by employing extensive molecular dynamics and phase field models across different length scales. It is demonstrated that pore formation is a defect induced phenomenon, in which vacancies cluster in a spinodal decomposition type of self-organization process, while interstitials are absorbed by the amorphous matrix, leading to additional volume increase and radiation induced viscous flow. The proposed modeling framework is capable to reproduce and predict the experimental observations from first principles and thus opens the venue for computer assisted design of nanoporous frameworks
Thermal instability of cell nuclei
DNA is known to be a mechanically and thermally stable structure. In its double
stranded form it is densely packed within the cell nucleus and is thermo-resistant
up to 70 °C. In contrast, we found a sudden loss of cell nuclei integrity at
relatively moderate temperatures ranging from 45 to 55 °C. In our study, suspended
cells held in an optical double beam trap were heated under controlled
conditions while monitoring the nuclear shape. At specific critical temperatures,
an irreversible sudden shape transition of the nuclei was observed. These temperature
induced transitions differ in abundance and intensity for various normal
and cancerous epithelial breast cells, which clearly characterizes different cell
types. Our results show that temperatures slightly higher than physiological
conditions are able to induce instabilities of nuclear structures, eventually
leading to cell death. This is a surprising finding since recent thermorheological
cell studies have shown that cells have a lower viscosity and are thus more
deformable upon temperature increase. Since the nucleus is tightly coupled to
the outer cell shape via the cytoskeleton, the force propagation of nuclear
reshaping to the cell membrane was investigated in combination with the
application of cytoskeletal drugs
Validity of temperature and time equivalence in metallic glasses during shear deformation
Competing internal and external time scales, which are determined by temperature and experimental sampling timeâviz., reciprocal frequencyârespectively, are essentials for understanding the physics of glasses and the glass transition. A temperature increase should ideally affect thermally activated phenomena in a similar manner as an increase of sampling time at constant temperature. We investigate the validity of this empirical principle by its manifestations in mechanical propertiesâviz., the temperature and strain rate dependence of the shear modulus and yield stress of a CuTi model glass in molecular dynamics computer simulations. In equivalence to the temperature-dependent glass transition, we identify a shear-rate-dependent glass transition below a certain threshold. Beyond that, deviations occur in a highly non-Newtonian regime
Recommended from our members
Employing Nanostructured Scaffolds to Investigate the Mechanical Properties of Adult Mammalian Retinae Under Tension
Numerous eye diseases are linked to biomechanical dysfunction of the retina. However, the underlying forces are almost impossible to quantify experimentally. Here, we show how biomechanical properties of adult neuronal tissues such as porcine retinae can be investigated under tension in a home-built tissue stretcher composed of nanostructured TiO2 scaffolds coupled to a self-designed force sensor. The employed TiO2 nanotube scaffolds allow for organotypic long-term preservation of adult tissues ex vivo and support strong tissue adhesion without the application of glues, a prerequisite for tissue investigations under tension. In combination with finite element calculations we found that the deformation behavior is highly dependent on the displacement rate which results in Youngâs moduli of (760â1270) Pa. Image analysis revealed that the elastic regime is characterized by a reversible shear deformation of retinal layers. For larger deformations, tissue destruction and sliding of retinal layers occurred with an equilibration between slip and stick at the interface of ruptured layers, resulting in a constant force during stretching. Since our study demonstrates how porcine eyes collected from slaughterhouses can be employed for ex vivo experiments, our study also offers new perspectives to investigate tissue biomechanics without excessive animal experiments. Š 2020 by the authors. Licensee MDPI, Basel, Switzerland
Recommended from our members
Energetic electron assisted synthesis of highly tunable temperature-responsive collagen/elastin gels for cyclic actuation: macroscopic switching and molecular origins
Thermoresponsive bio-only gels that yield sufficiently large strokes reversibly and without large hysteresis at a well-defined temperature in the physiological range, promise to be of value in biomedical application. Within the present work we demonstrate that electron beam modification of a blend of natural collagen and elastin gels is a route to achieve this goal, viz. to synthesize a bioresorbable gel with largely reversible volume contractions as large as 90% upon traversing a transition temperature that can be preadjusted between 36 °C and 43 °C by the applied electron dose. Employing circular dichroism and temperature depending confocal laser scanning microscopy measurements, we furthermore unravel the mechanisms underlying this macroscopic behavior on a molecular and network level, respectively and suggest a stringent picture to account for the experimental observations. Š 2019, The Author(s)