Geometrically Confined Research Articles

Current-Induced Reversible Split of Elliptically Distorted Skyrmions in Geometrically Confined Fe3 Sn2 Nanotrack.

Skyrmions are swirling spin textures with topological characters promising for future spintronic applications. Skyrmionic devices typically rely on the electrical manipulation of skyrmions with a circular shape. However, manipulating elliptically distorted skyrmions can lead to numerous exotic magneto-electrical functions distinct from those of conventional circular skyrmions, significantly broadening the capability to design innovative spintronic devices. Despite the promising potential, its experimental realization so far remains elusive. In this study, the current-driven dynamics of the elliptically distorted skyrmions in geometrically confined magnet Fe3 Sn2 is experimentally explored. This study finds that the elliptical skyrmions can reversibly split into smaller-sized circular skyrmions at a current density of 3.8× 1010 A m-2 with the current injected along their minor axis. Combined experiments with micromagnetic simulations reveal that this dynamic behavior originates from a delicate interplay of the spin-transfer torque, geometrical confinement, and pinning effect, and strongly depends on the ratio of the major axis to the minor axis of the elliptical skyrmions. The results indicate that the morphology is a new degree of freedom for manipulating the current-driven dynamics of skyrmions, providing a compelling route for the future development of spintronic devices.

Cyclic stretch modulates the cell morphology transition under geometrical confinement by covalently immobilized gelatin.

Fibroblasts geometrically confined by photo-immobilized gelatin micropatterns were subjected to cyclic stretch on the silicone elastomer. By using covalently micropatterned surfaces, the cell morphologies such as cell area and length were quantitatively investigated under a cyclic stretch for 20 hours. The mechanical forces did not affect the cell growth but significantly altered the cellular morphology on both non-patterned and micropatterned surfaces. It was found that cells on non-patterns showed increasing cell length and decreasing cell area under the stretch. The width of the strip micropatterns provided a different extent of contact guidance for fibroblasts. The highly extended cells on the 10 μm pattern under static conditions would perform a contraction behavior once treated by cyclic stretch. In contrast, cells with a low extension on the 2 μm pattern kept elongating according to the micropattern under the cyclic stretch. The vertical stretch induced an increase in cell area and length more than the parallel stretch in both the 10 μm and 2 μm patterns. These results provided new insights into cell behaviors under geometrical confinement in a dynamic biomechanical environment and may guide biomaterial design for tissue engineering in the future.

Phase field simulations for the crossover from sharp martensitic transformation into smooth strain glass transition by fine precipitates

Recent studies have shown an intriguing phenomenon that by introducing nano-precipitates, its sharp first order martensitic transformation (MT) can be changed into smooth strain glass transition which is characterized by sluggish evolution of nano-sized martensitic domains. However, it remains unclear how the precipitates can fundamentally change the nature of the normal MT. In the present study, we reveal the origin of this phenomenon and explore the novel properties associated with the precipitate-induced strain glass using phase field simulations. Our model and phase field simulations show that the shape and size of the Ni4Ti3 precipitates influence the geometrical confinement. The geometrical confinement caused by precipitates with disk shape and high density could produce strong resistance to the growth of large-scale martensitic domains, and can lead to the freezing of nano-sized martensitic domains. When the average size of the B2 pocket between adjacent Ni4Ti3 precipitates is below a critical value (∼16 nm), the avalanche-like behavior of the normal MT is altered to a smooth nanoscale MT with nearly zero transition hysteresis, which exhibits interesting hysteresis-free superelasticity over a wide temperature range. Our results further show that the nanoscale MT exhibits characteristic signatures of strain glass, such as broken ergodicity and Vogel-Fulcher frequency dispersion of internal friction peaks and suggest the non-ergodic nature of the geometrically confined MT. Through precipitate engineering, it is possible to regulate and modify the normal MT into the nanoscale MT and consequently achieve novel properties.

Spontaneous formation and spatial self-organization of mechanically induced mesenchymal-like cells within geometrically confined cancer cell monolayers

There is spatiotemporal heterogeneity in cell phenotypes and mechanical properties in tumor tissues, which is associated with cancer invasion and metastasis. It is well-known that exogenous growth factors like transforming growth factor (TGF)-β, can induce epithelial-mesenchymal transition (EMT)-based phenotypic transformation and the formation of EMT patterning on geometrically confined monolayers with mechanics heterogeneity. In the absence of exogenous TGF-β stimulation, however, whether geometric confinement-caused mechanics heterogeneity of cancer cell monolayers alone can trigger the EMT-based phenotypic heterogeneity still remains mysterious. Here, we develop a micropattern-based cell monolayer model to investigate the regulation of mechanics heterogeneity on the cell phenotypic switch. We reveal that mechanics heterogeneity itself is enough to spontaneously induce the emergence of mesenchymal-like phenotype and asymmetrical activation of TGF-β-SMAD signaling. Spatiotemporal dynamics of patterned cell monolayers with mesenchymal-like phenotypes is essentially regulated by tissue-scale cell behaviors like proliferation, migration as well as heterogeneous cytoskeletal contraction. The inhibition of cell contraction abrogates the asymmetrical TGF-β-SMAD signaling activation level and the emergence of mesenchymal-like phenotype. Our work not only sheds light on the key regulation of mechanics heterogeneity caused by spatially geometric confinement on regional mesenchymal-like phenotype of cancer cell monolayers, but highlights the key role of biophysical/mechanical cues in triggering phenotypic switch.

Geometric Confinement Guides the Expression of Cancer Stem Cell Molecular Markers CD44 via Cell Traction Forces.

Cancer stem cells (CSCs) play a critical role in the cancer metastasis and account for tumor heterogeneity. Growing evidence indicates that the CSC phenotypes are related to the tumor microenvironment. In this study, we report that the gradient of mechanical stresses guides the spatial patterning of the expression of CD44 and Yes-associated protein (YAP) in the geometrically confined multicellular sheets. Our study shows that the cytoskeletal contraction regulates the expression of CD44 through the translocation of YAP into the nucleus. The results demonstrate that geometric confinement and mechanical stresses are the regulators in the spatial patterning of CSC. It may help to understand the relationship between the tumor microenvironment and oncogenesis.

IR Spectroscopy of Bidistilled and Deuterium Water under Conditions of Geometric Limitation in Glass Nanopores

A study of IR spectra of H2O and D2O in nanoporous glass (PG) matrices with pores of different sizes has been carried out under geometric confinement. It has been established that the fraction of water with a strong H bond decreases and that with a weak H bond increases with increasing pore size in the region of 2 nm 6 nm, there is no change in the structure of geometrically confined H2O and D2O. However, this effect is weaker for D2O (because of the D–O bond being stronger than the H–O bond). It has been shown that H2O molecules that form bonds with the pore surface are mainly formed of the molecules of a hydrogen bond framework, oscillations of which do not occur in phase. At the same time, such bonds with the surface of the pores in D2O are formed of the molecules of the hydrogen bond framework, oscillations of which occur in phase.

Active particles in geometrically confined viscoelastic fluids

We experimentally study the dynamics of active particles (APs) in a viscoelastic fluid under various geometrical constraints such as flat walls, spherical obstacles and cylindrical cavities. We observe that the main effect of the confined viscoelastic fluid is to induce an effective repulsion on the APs when moving close to a rigid surface, which depends on the incident angle, the surface curvature and the particle activity. Additionally, the geometrical confinement imposes an asymmetry to their movement, which leads to strong hydrodynamic torques, thus resulting in detention times on the wall surface orders of magnitude shorter than suggested by thermal diffusion. We show that such viscoelasticity-mediated interactions have striking consequences on the behavior of multi-AP systems strongly confined in a circular pore. In particular, these systems exhibit a transition from liquid-like behavior to a highly ordered state upon increasing their activity. A further increase in activity melts the order, thus leading to a re-entrant liquid-like behavior.

Buckling of geometrically confined shells.

We study the periodic buckling patterns that emerge when elastic shells are subjected to geometric confinement. Residual swelling provides access to range of shapes (saddles, rolled sheets, cylinders, and spherical sections) which vary in their extrinsic and intrinsic curvatures. Our experimental and numerical data show that when these moderately thick structures are radially confined, a single geometric parameter - the ratio of the total shell radius to the amount of unconstrained material - predicts the number of lobes formed. We present a model that interprets this scaling as the competition between radial and circumferential bending. Next, we show that reducing the transverse confinement of saddles causes the lobe number to decrease with a similar scaling analysis. Hence, one geometric parameter captures the wave number through a wide range of radial and transverse confinement, connecting the shell shape to the shape of the boundary that confines it. We expect these results to be relevant for an expanse of shell shapes, and thus applicable to the design of shape-shifting materials and the swelling and growth of soft structures.

Quantifying the mechanics of locomotion of the schistosome pathogen with respect to changes in its physical environment.

Schistosomiasis is a chronic and morbid disease of poverty affecting approximately 200 million people worldwide. Mature schistosome flatworms wander in the host's hepatic portal and mesenteric venous system where they encounter a range of blood flow conditions and geometrical confinement. However, the mechanisms that support schistosome locomotion and underlie the pathogen's adaptation to its physical environment are largely unknown. By combining microfabrication and traction force microscopy, we developed various in vitro assays to quantify the mechanics of locomotion of adult male Schistosoma mansoni in different physiologically relevant conditions. We show that in unconfined settings, the parasite undergoes two-anchor marching mediated by the coordinated action of its oral and ventral suckers. This mode of locomotion is maintained when the worm faces an external flow, to which it responds by adjusting the strength of its suckers. In geometrically confined conditions, S. mansoni switches to a different crawling modality by generating retrograde peristaltic waves along its body, a mechanism shared with terrestrial and marine worms. However, while the surface of most worms has backward-pointing bristles that rectify peristaltic waves and facilitate forward locomotion, S. mansoni has isotropically oriented tubercles. This requires tight coordination between muscle contraction and substrate friction but gives S. mansoni the ability to reverse its direction of locomotion without turning its body, which is likely advantageous to manoeuvre in narrow-bore vessels. We show that the parasite can also coordinate the action of its suckers with its peristaltic body contractions to increase crawling speed. Throughout this study, we report on a number of biomechanical parameters to quantify the motility of adult schistosomes (e.g. sucker grabbing strength, the rate of detachment under flow, peristaltic wave properties and traction stresses). The new series of in vitro assays make it possible to quantify key phenotypical aspects of S. mansoni motility that could guide the discovery of new drugs to treat schistosomiasis.

Nonlinear compliance of elastic layers to indentation.

Thin samples adherent to a rigid substrate are considerably less compliant to indentation when compared to specimens that are not geometrically confined. Analytical corrections to this so-called substrate effect exist for various types of indenters but are not applicable when large deformations are possible, as is the case in biological materials. To overcome this limitation, we construct a nonlinear scaling model characterized by one single exponent, which we explore employing a parametric finite element analysis. The model is based on asymptotes of two length scales in relation to the sample thickness, i.e., indentation depth and radius of the contact area. For small indentation depth, we require agreement with analytical, linear models, whereas for large indentation depth and extensive contact area, we recognize similarity to uniaxial deformation, indicating a divergent force required to indent nonlinear materials. In contrast, we find linear materials not to be influenced by the substrate effect beyond first order, implying that nonlinear effects originating from either the material or geometric confinement are clearly separated only in thin samples. Furthermore, in this regime the scaling model can be derived by following a heuristic argument extending a linear model to large indentation depths. Lastly, in a large indentation setting where the contact is small in comparison with sample thickness, we observe nonlinear effects independent of material type that we attribute to a higher-order influence of geometrical confinement. In this regime, we define a scalar as the ratio of strains along principal axes as obtained by comparison with the case of a point force on a half-space. We find this scalar to be in quantitative agreement with the scaling exponent, indicating an approach to distinguish between nonlinear effects in the scaling model. While we conjecture our findings to be applicable to other flat-ended indenters, we focus on the case of a flat-ended cylinder in normal contact with a thin layer. The analytical solution for small indentation associated with this problem has been given by Hayes et al. (J Biomech 5:541-551, 1972), for which we provide a convenient numerical implementation.

Wafer-scale single-crystal perovskite patterned thin films based on geometrically-confined lateral crystal growth

We report a facile roll-printing method, geometrically confined lateral crystal growth, for the fabrication of large-scale, single-crystal CH3NH3PbI3 perovskite thin films. Geometrically confined lateral crystal growth is based on transfer of a perovskite ink solution via a patterned rolling mould to a heated substrate, where the solution crystallizes instantly with the immediate evaporation of the solvent. The striking feature of this method is that the instant crystallization of the feeding solution under geometrical confinement leads to the unidirectional lateral growth of single-crystal perovskites. Here, we fabricated single-crystal perovskites in the form of a patterned thin film (3 × 3 inch) with a high carrier mobility of 45.64 cm2 V−1 s−1. We also used these single-crystal perovskite thin films to construct solar cells with a lateral configuration. Their active-area power conversion efficiency shows a highest value of 4.83%, which exceeds the literature efficiency values of lateral perovskite solar cells.

Geometrical confinement directed albumin-based nanoprobes as enhanced T1 contrast agents for tumor imaging.

There is an urgent demand for the development of new magnetic resonance imaging (MRI) contrast agents (CAs) with high T1 contrast ability and good biocompatibility. Herein, we report a novel albumin-based nanoprobe loaded with ibuprofen-modified gadolinium chelates, named Ibu-Gd-BSA nanoparticles (NPs). The interfacial pore structure among the albumin molecules endows the Ibu-Gd-BSA NPs with geometrical confinement, which could prolong the rotational correlation time (τR) of CAs and the diffusion correlation time (τD) of water molecules trapped within the pores. As a result, the Ibu-Gd-BSA NPs exhibited an extremely high relaxivity of 48.9 mM-1 s-1, which is about 9 times higher than that of the clinical contrast agent Gd-DOTA (Dotarem®). In addition, the Ibu-Gd-BSA NPs showed good biocompatibility in vitro and in vivo due to the intrinsically biocompatible property of each component. Moreover, the Ibu-Gd-BSA NPs showed much longer blood circulation half-life and higher accumulation in tumors due to the enhanced permeability and retention effect compared to small molecular CAs. In vivo T1-weighted MR imaging confirmed that Ibu-Gd-BSA NPs could serve as an optimal candidate for sensitive tumor imaging. This study provides a facile strategy to assemble geometrically confined albumin-based nanoparticles as T1 CAs with high biocompatibility and enhanced contrast ability, which have great potential for diverse uses in biomedical imaging and disease detection.

Stable large-scale solver for Ginzburg–Landau equations for superconductors

Understanding the interaction of vortices with inclusions in type-II superconductors is a major outstanding challenge both for fundamental science and energy applications. At application-relevant scales, the long-range interactions between a dense configuration of vortices and the dependence of their behavior on external parameters, such as temperature and an applied magnetic field, are all important to the net response of the superconductor. Capturing these features, in general, precludes analytical description of vortex dynamics and has also made numerical simulation prohibitively expensive. Here we report on a highly optimized iterative implicit solver for the time-dependent Ginzburg–Landau equations suitable for investigations of type-II superconductors on massively parallel architectures. Its main purpose is to study vortex dynamics in disordered or geometrically confined mesoscopic systems. In this work, we present the discretization and time integration scheme in detail for two types of boundary conditions. We describe the necessary conditions for a stable and physically accurate integration of the equations of motion. Using an inclusion pattern generator, we can simulate complex pinning landscapes and the effect of geometric confinement. We show that our algorithm, implemented on a GPU, can provide static and dynamic solutions of the Ginzburg–Landau equations for mesoscopically large systems over thousands of time steps in a matter of hours. Using our formulation, studying scientifically-relevant problems is a computationally reasonable task.

On the theory of thermodynamic properties of geometrically confined disordered ferroelectrics

We propose a theoretical approach to calculate the thermodynamic properties of thin films fabricated from disordered ferroelectrics. To calculate the above thermodynamic properties, we use so-called random field method, modified for the case of thin films. The essence of modification is the altering of the interaction between impurity dipoles by geometrical confinement. We show that in thin films the ferroelectric phase transition is inhibited as compared to the case of bulk samples. Our theory is generalizable to ferroelectrics of other shapes as well as magnets and multiferroics.

Enhanced MRI relaxivity of Gd3+‐based contrast agents geometrically confined within porous nanoconstructs

Gadolinium chelates, which are currently approved for clinical MRI use, provide relaxivities well below their theoretical limit, and they also lack tissue specificity. Recently, the geometrical confinement of Gd(3+) -based contrast agents (CAs) within porous structures has been proposed as a novel, alternative strategy to improve relaxivity without chemical modification of the CA. Here, we have characterized and optimized the performance of MRI nanoconstructs obtained by loading [Gd(DTPA)(H(2) O)](2-) (Magnevist®) into the pores of injectable mesoporous silicon particles. Nanoconstructs with three different pore sizes were studied, and at 60 MHz, they exhibited longitudinal relaxivities of ~24 m m(-1) s(-1) for 5-10 nm pores and ~10 m m(-1) s(-1) for 30 - 40 nm pores. No enhancement in relaxivity was observed for larger pores sizes. Using an outer-sphere compound, [GdTTHA](3-) , and mathematical modeling, it was demonstrated that the relaxivity enhancement is due to the increase in rotational correlation times (CA adsorbed on the pore walls) and diffusion correlation times (reduced mobility of the water molecules), as the pore sizes decreases. It was also observed that extensive CA adsorption on the outer surface of the silicon particles negates the advantages offered by nanoscale confinement. Upon incubation with HeLa cells, the nanoconstructs did not demonstrate significant cytotoxicity for up to 3 days post incubation, at different particle/cell ratios. In addition, the nanoconstructs showed complete degradation after 24 h of continuous agitation in phosphate-buffered saline. These data support and confirm the hypothesis that the geometrical confinement of Gd(3+) -chelate compounds into porous structures offers MRI nanoconstructs with enhanced relaxivity (up to 6 times for [Gd(DTPA)(H(2) O)](2-) , and 4 times for [GdTTHA](3-) ) and, potentially, improved stability, reduced toxicity and tissue specificity.

Quantifying the Stress Relaxation Modulus of Polymer Thin Films via Thermal Wrinkling

The viscoelastic properties of polymer thin films can have a significant impact on the performance in many small-scale devices. In this work, we use a phenomenon based on a thermally induced instability, termed thermal wrinkling, to measure viscoelastic properties of polystyrene films as a function of geometric confinement via changes in film thickness. With application of the appropriate buckling mechanics model for incompressible and geometrically confined films, we estimate the stress-relaxation modulus of polystyrene films by measuring the time-evolved wrinkle wavelength at fixed annealing temperatures. Specifically, we use time-temperature superposition to shift the stress relaxation curves and generate a modulus master curve for polystyrene films investigated here. On the basis of this master curve, we are able to identify the rubbery plateau, terminal relaxation time, and viscous flow region as a function of annealing time and temperatures that are well-above its glass transition. Our measurement technique and analysis provide an alternative means to measure viscoelastic properties and relaxation behavior of geometrically confined polymer films.

Full Quantum Simulation of Silicon-on-Insulator Single-Electron Devices

We present a method which extends the range of applicability of the domain decomposition approach to tunneling transport. Thereby we gain the ability to simulate e.g. structures with geometrically confined semiconductor quantum dots surrounded by very thin layers of dielectric or quantum dots that are defined through a combination of electrostatic forces and geometric confinement. Recently, experimental data of single electron devices on the 10 nm length-scale have become available, but due to the smallness of the devices detailed information on their geometry is hard to come by. Thus the simulations presented in this paper are intended as proof of principle rather than quantitative results for a real device. For predictive simulations more detailed knowledge of the experimental geometry is required.

Soliton dynamics of geometrically confined ferroelectric liquid crystals

We show that geometrical confinement of Sm C* type ferroelectric liquid crystals between two parallel boundaries separated by a distance d induces a soliton structure similar to the one obtained in a transverse external magnetic field. For non-polar surface anchoring (i.e., quadrupolar boundary conditions) the (T, d) phase diagram in fact closely resembles the (T, H) phase diagram and the unwinding of the smectic C* helix is preceded by a transition from the plane-wave to the soliton like helical structure. The periodic potential of the soliton lattice induces a gap in the phason dispersion relation similar to the gaps between the valence and conduction bonds of an electron in a periodic potential. For a system with polar boundary conditions the splay distortion of the director field is accompenied by a splayed polarization and a soliton like deformation of the phase profile across the cell. A transverse solitary wave excitation with a (l/d 2) relaxation rate thickness dependence and a finite frequency gap at l/d 2 = 0 has been observed at small thicknesses. A cross-over to a gapless plane-wave like dynamics has been found at large thicknesses.