Fluctuations in local shear-fault energy produce unique and dominating strengthening in metastable complex concentrated alloys.

Local chemical short-range ordering (SRO) and spatial fluctuations of planar fault energy are important features of multi-element and metastable complex concentrated alloys (CCAs). Arising from them, dislocations in such alloys are distinctively wavy in both static and migrating conditions; yet, such effects on strength have remained unknown. In this work, molecular dynamics simulations are used to show that the wavy configurations of dislocations and their jumpy motion in a prototypic CCA of NiCoCr are due to the local fluctuations of the energy of SRO shear-faulting that accompanies dislocation motion, with the dislocation getting pinned at sites of hard atomic motifs (HAMs) associated with high local shear-fault energies. Unlike the global averaged shear-fault energy which in general will subdue on successive dislocation passes, the local fluctuations in the fault energy always remain in a CCA, thus offering a strength contribution that is unique in such alloys. Analysis of the magnitude of this form of dislocation resistance shows that this is dominating over contributions due to elastic misfit of alloying elements and is in good agreement with strengths predicted from molecular dynamics simulations and experiments. This work has unfolded the physical basis of strength in CCAs, which is important for the development of these alloys into useful structural materials.

A dual organelle-targeting mechanosensitive probe.

Cells are responsive to the mechanical environment, but the methods to detect simultaneously how different organelles react in mechanobiological processes remain largely unexplored. We herein report a dual organelle-targeting fluorescent probe, (E)-1-[3-(diethoxyphosphoryl)propyl]-4-[4-(diethylamino)styryl]pyridin-1-ium bromide (ASP-PE), for mechanical mapping in live cells. ASP-PE is aggregation-induced emission active and is sensitive to the local mechanical environment. It targets the plasma membrane (PM) and intracellular mitochondria in cells by its phosphonate moiety and pyridinium. In this work, through ASP-PE staining, changes of membrane tension in the PM and mitochondria in response to varied osmotic pressure and substrate stiffness are visualized using fluorescence lifetime imaging microscopy. The mechanobiological importance of actin filaments and microtubules in the PM and mitochondria is also investigated using this probe. Computational simulations are applied to study the sensing mechanism of the probe. This study introduces a unique tool for mapping the membrane tension in the PM and mitochondria together, providing us great opportunities to study organelle's interactions in mechanobiology.

Relating the blood-thinning effect of pentoxifylline to the reduction in the elastic modulus of human red blood cells: an in vivo study.

The blood thinning properties of pentoxifylline have been attributed to its ability to increase the deformability of red blood cells and improve their rheological properties. To interpret and substantiate these observations a novel approach is taken by measuring the stiffness of individual red blood cells from healthy humans before and after subscription to pentoxifylline for nine days. Atomic force microscopy nanoindentation experiments reveal that the elastic modulus of the red blood cells decreased by 30%-40%, after pentoxifylline subscription. This decrease in elastic modulus is related to the ability of pentoxifylline to increase the production of ATP and lower Ca2+ concentrations in red blood cells. The present in vivo experiments provide a deeper understanding of the mode of action of pentoxifylline, and pave the way to using indentation in medicine. A further unique advantage of this study is that it was performed on healthy volunteers, rather than requiring in vitro incubation.

Light-stimulated actuators based on nickel hydroxide-oxyhydroxide.

Light-induced actuators that are self-contained and compact can be used as artificial muscles for microrobotics because their actuation can be induced wirelessly, which reduces the complexity of the device or system. Here, we report a material system, nickel hydroxide-oxyhydroxide, that could actuate because of a volume change stimulated by illumination of visible light of low intensities. The actuating material here exhibited a turbostratic crystal structure capable of intercalating water, and we show that the intercalated water can be rapidly and reversibly desorbed into the environment under visible light of low intensities, resulting in fast actuation driven wirelessly by light. By electroplating the actuating material on passive substrates, we have fabricated film actuators capable of undergoing reversible bending and curling with an intrinsic actuating stress of 5 to 65 megapascals at response rates in the order of tens to hundreds of degrees per second depending on the light intensity, which are comparable to mammalian skeletal muscles. By intentionally electroplating the nickel hydroxide-oxyhydroxide on selected areas of the substrate, a hinged actuator that can lift objects ~100 times the weight of the actuating material is achieved. Other demonstrations show the potential uses in robotic devices, including sunlight-induced actuation, a biomimicked "sensitive plant" with rapid leaf movement, and a light-powered walking bot.

Multiphoton Fabrication of Fibronectin-Functionalized Protein Micropatterns: Stiffness-Induced Maturation of Cell-Matrix Adhesions in Human Mesenchymal Stem Cells.

Cell-matrix adhesions are important structures governing the interactions between cells and their microenvironment at the cell-matrix interface. The focal complex (FC) and focal adhesion (FA) have been substantially investigated in conventional planar culture systems using fibroblasts as an in vitro model. However, the formation of more mature types of cell-matrix adhesion in human mesenchymal stem cells (hMSCs), including fibrillar adhesion (FBA) and 3D matrix adhesion (3DMA), have not been fully elucidated. Here we investigate the niche factor(s) that influence(s) the maturation of FBA and 3DMA by using multiphoton fabrication-based micropatterning. First, the bovine serum albumin (BSA)-made protein micropatterns were functionalized by incorporating various concentrations of fibronectin (FN) in fabrication solution. The amount of cross-linked FN is positively correlated with the initial concentration of FN in the reaction liquid, as verified by immunofluorescence staining. On the other hand, the anisotropic FN-functionalized micropatterns were fabricated by varying the length (i.e., in-plane stiffness) and height (i.e., bending stiffness) of micropatterns, respectively. Finally, hMSCs were cultured on these micropatterns for 2 h and 1 day to determine the formation of FBA and 3DMA, respectively, using immunofluorescence staining. Results demonstrated that FN-functionalized micropatterns with high anisotropy in x-y dimension benefit FBA maturation. Furthermore, niche factors such as higher bending and in-plane stiffness and the presence of abundant fibronectin have a positive effect on the maturation of FN-based cell-matrix adhesion. These findings could provide some new perspectives on designing platforms for further cell niche study and rationalizing scaffold design for tissue engineering.

Fast and Reversible Actuation of Metallic Muscles Composed of Nickel Nanowire-Forest.

Surface-charge-induced reversible and millimeter-scale deflection is found in a bilayered Ni cantilever upon cyclic potential triggering. The nanowire-forest structure, in which unidirectional primary nanowires are evenly separated by cross-linking subnanowires, ensures fast ion transport leading to a record-high strain response time ≈0.1 s. The actuation is sustainable beyond 800 cycles; the strain energy is compatible with human skeletal muscles.

Reversible electrochemical actuation of metallic nanohoneycombs induced by pseudocapacitive redox processes.

Current metallic-based electrochemical actuators are limited to nanoporous gold/platinum with randomly distributed pores, where the charge-induced reversible strain is mainly due to the nonfaradic charging/discharging processes along the capacitive electrochemical double layer. Here, we report an electrochemical actuating property of nanohoneycomb-structured nickel, with the actuation mechanism mainly due to a pseudocapacitive behavior by means of reversible faradic redox reactions. By using a dual-template synthesis method, a bilayered cantilever, comprising a nanohoneycomb layer backed by a solid layer of the same metal, was fabricated. Reversible bending of the cantilever upon cyclic potential triggering was observed. The strain of the cantilever increases nonlinearly with both potential and charge due to redox reactions. The maximum strain that can be achieved under a certain scan rate complies with a linear relationship with the capacity. Benefiting from the stable Ni(II)/Ni(III) redox couples at the electrode surface, the reversible actuation is very stable in hydroxide solutions.

Mesenchymal stem cells reduce intervertebral disc fibrosis and facilitate repair.

Intervertebral disc degeneration is associated with back pain and radiculopathy which, being a leading cause of disability, seriously affects the quality of life and presents a hefty burden to society. There is no effective intervention for the disease and the etiology remains unclear. Here, we show that disc degeneration exhibits features of fibrosis in humans and confirmed this in a puncture-induced disc degeneration (PDD) model in rabbit. Implantation of bone marrow-derived mesenchymal stem cells (MSCs) to PDD discs can inhibit fibrosis in the nucleus pulposus with effective preservation of mechanical properties and overall spinal function. We showed that the presence of MSCs can suppress abnormal deposition of collagen I in the nucleus pulposus, modulating profibrotic mediators MMP12 and HSP47, thus reducing collagen aggregation and maintaining proper fibrillar properties and function. As collagen fibrils can regulate progenitor cell activities, our finding provides new insight to the limited self-repair capability of the intervertebral disc and importantly the mechanism by which MSCs may potentiate tissue regeneration through regulating collagen fibrillogenesis in the context of fibrotic diseases.

Thermally induced solid-solid structural transition of copper nanoparticles through direct geometrical conversion.

Molecular dynamics simulations of small Cu nanoparticles using three different interatomic potentials at rising temperature indicate that small nanoparticles can undergo solid-solid structural transitions through a direct geometrical conversion route. The direct geometrical conversion can happen for cuboctahedral nanoparticles, which turn into an icosahedra shape: one diagonal of the square faces contracts, and the faces are folded along the diagonal to give rise to two equilateral triangles. The transition is a kinetic process that cannot be fully explained through an energetic point of view. It has low activation energy and fast reaction time in the simulations. The transition mechanism is via the transmission of shear waves initiated from the particle surface and does not involve dislocation activity.

Fast fabrication of self-ordered anodic porous alumina on oriented aluminum grains by high acid concentration and high temperature anodization.

Anodic porous alumina, which exhibits a characteristic nanohoneycomb structure, has been used in a wide range of nanotechnology applications. The conventional fabrication method of mild anodization (MA) requires a prolonged anodization time which is impractical for batch processing, and self-ordered porous structures can only be formed within narrow processing windows so that the dimensions of the resultant structures are extremely limited. The alternative hard anodization (HA) may easily result in macroscopic defects on the alumina surface. In this work, by systematically varying the anodization conditions including the substrate grain orientation, electrolyte concentration, temperature, voltage, and time, a new oxalic acid based anodization method, called high acid concentration and high temperature anodization (HHA), is found, which can result in far better self-ordering of the porous structures at rates 7-26 times faster than MA, under a continuous voltage range of 30-60 V on (001) oriented Al grains. Unlike HA, no macroscopic defects appear under the optimum self-ordered conditions of HHA at 40 V, even for pore channels grown up to high aspect ratios of more than 3000. Compared to MA and HA, HHA provides more choices of self-ordered nano-porous structures with fast and mechanically stable formation features for practical applications.

Correlation between compositional and mechanical properties of human mesenchymal stem cell-collagen microspheres during chondrogenic differentiation.

Mesenchymal stem cell (MSC)-based engineering is promising for cartilage repair. However, the compositional mechanical relationship of the engineered structures has not been extensively studied, given the importance of such relationship in native cartilage tissues. In this study, a novel human MSC-collagen microsphere system was used to study the compositional mechanical relationship during in vitro chondrogenic differentiation using histological and biochemical methods and a microplate compression assay. The mechanical property was found positively correlating with newly deposited cartilage-relevant matrices, glycosaminoglycan, and type II collagen, and with the collagen crosslinker density, in agreement with the presence of thick collagen bundles upon structural characterization. On the other hand, the mechanical property negatively correlates with type I collagen and total collagen, suggesting that the initial collagen matrix scaffold of the microsphere system was being remodeled by the differentiating human MSCs. This study also demonstrated the application of a simple, sensitive, and nondestructive tool for monitoring the progression of chondrogenic differentiation of MSCs in tissue-engineered constructs and therefore contributes to future development of novel cartilage repair strategies.

Nanostructure of collagen fibrils in human nucleus pulposus and its correlation with macroscale tissue mechanics.

Collagen fibrils are the main structural components of the nucleus pulposus tissue in the intervertebral discs. The structure-property relationship of the nucleus pulposus (NP) tissues is still unclear. We investigated the structure of individual collagen fibrils of the NP and evaluated its correlation with the bulk mechanical properties of the tissue. Collagen fibrils were extracted from the NP of discs retrieved from adolescents during scoliosis correction surgery, and the extracts were confirmed by SDS-PAGE. The diameters of the individual collagen fibrils were measured through atomic force microscopy, and the compressive mechanical properties of the tissues were evaluated by confined compression. The correlations between the nanoscale morphology of the collagen fibrils and the macroscale mechanical properties of the tissues were evaluated by linear regression. The SDS-PAGE results showed that the fibril extracts were largely composed of type II collagen. The mean diameter of the collagen fibrils was 92.1 +/- 26.54 nm; the mean swelling pressure and compressive modulus of the tissues were 6.15 +/- 4.3 kPa and 1.23 +/- 0.7 MPa, respectively. The mean fibril diameter had no linear correlation (R(2) = 0.30) with the swelling pressure of the tissues. However, it had a mild linear correlation with the compressive modulus (p = 0.023, R(2) = 0.68). This is the first study, to our knowledge, to evaluate the nanostructure of the individual collagen fibrils of the nucleus pulposus and its relationship with macroscale mechanical properties of the NP tissues.