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Background: Compression experiments are widely used to study the mechanical properties of materials at micro- and nanoscale. However, the conventional engineering stress measurement method used in these experiments neglects to account for the alterations in the material's shape during loading. This can lead to inaccurate stress values and potentially misleading conclusions about the material's mechanical behavior, especially in the case of localized deformation. Objective: Our goal is to calculate true stress in cases of localized plastic deformation from standard experimental data (displacement-force curve, aspect ratio, shear band angle and elastic strain limit). Methods: We use a simple mechanical-geometrical approach based on reasonable physical assumptions to get analytic formulas of true stress and eliminating the need for finite element computations. Furthermore, in numerical simulations of pillar compression, the formula-based true stress demonstrates strong alignment with the theoretical true stress. Results: We propose analytic formulas for calculating true stress in cases of localized plastic deformation commonly encountered in experimental settings for a single band oriented in arbitrary directions with respect to the vertical axis of the pillar. Conclusions: The true stress computed with the proposed formulas provides a more precise interpretation of experimental results and can serve as a valuable and simple tool in material design and characterization. [ABSTRACT FROM AUTHOR]

In the contemporary era, the enhancement of wearable capacitive sensors is achieved through the utilization of polymeric micropillars as filler materials between electrode plates. To gain a deeper understanding of the dynamic response of the system, nonlinear coupled governing equations of a circular microplate motion resting on an array of polymeric micropillars have been derived. These equations are used to model the system's behavior. In addition, the squeezing motion of the micro-pillars is characterized using the incompressible Neo-Hookean model. Both static and dynamic responses, including transient and steady-state solutions, are investigated in detail by discretizing over spatial coordinates using a weak formulation approach. A frequency response analysis is conducted using a continuation-based method. This entails expanding the steady-state solution using a Fourier transform and employing the energy balance principle. The unknown coefficients of the expansion series are calculated using a gradient descent-based learning approach that is physically motivated. Furthermore, a dynamic step size strategy for frequency increments is employed to effectively follow the solution path. This strategy is implemented via the ARC length method. In this study, we examine the impact of varying PDMS (polydimethylsiloxane) hydrogel mechanical and geometrical configurations. It can be reasonably concluded that the mechanical properties of the pillars and the geometrical configuration of the circular plate and micropillars have a significant impact on the maximum tolerable pressure, fast transient response, and frequency response analysis. [ABSTRACT FROM AUTHOR]

Abstract In the contemporary era, the enhancement of wearable capacitive sensors is achieved through the utilization of polymeric micropillars as filler materials between electrode plates. To gain a deeper understanding of the dynamic response of the system, nonlinear coupled governing equations of a circular microplate motion resting on an array of polymeric micropillars have been derived. These equations are used to model the system’s behavior. In addition, the squeezing motion of the micro-pillars is characterized using the incompressible Neo-Hookean model. Both static and dynamic responses, including transient and steady-state solutions, are investigated in detail by discretizing over spatial coordinates using a weak formulation approach. A frequency response analysis is conducted using a continuation-based method. This entails expanding the steady-state solution using a Fourier transform and employing the energy balance principle. The unknown coefficients of the expansion series are calculated using a gradient descent-based learning approach that is physically motivated. Furthermore, a dynamic step size strategy for frequency increments is employed to effectively follow the solution path. This strategy is implemented via the ARC length method. In this study, we examine the impact of varying PDMS (polydimethylsiloxane) hydrogel mechanical and geometrical configurations. It can be reasonably concluded that the mechanical properties of the pillars and the geometrical configuration of the circular plate and micropillars have a significant impact on the maximum tolerable pressure, fast transient response, and frequency response analysis.

Graphical Abstract

Laser surface texturing and through-mask electrochemical micromachining (TMECMM) are some of the commonly used methods which include multiple steps to achieve micro-textures. However, for large-area applications, it is desirable to have an economical single-step process. In this regard, ECMM is expected to be a promising and economically viable micro-texturing process for micro-manufacturing industries. This paper proposes a novel maskless EC micro-texturing process using tool sinking technique with low voltage and short pulses. Methodology of using edges (∼30 μm each) of printed circuit boards (PCBs) for ECMM of micro-channels and micro-pillars is a unique one. Micro-pillars, micro-dimples and micro-channels are produced through a direct electrochemical cathode sinking process. Analysis of current density and prediction of width and depth of micro-dimples on a flat stainless steel surface are studied through 2D numerical simulation carried out on COMSOL 4.3a. The proposed method implies the use of less toxic electrolyte, low voltage (1-6 V), short pulses (5 to 50 μs) and selective polymer coating on the tool (cathode). A series of experiments of EC sinking for creating various micro-patterns and micro-structures has been carried out on the flat as well as curved metallic surfaces. This paper reports machining of micro-dimples of 200-300 μm diameter, micro-channels of 150-250 μm and square micro-pillars of 300-350 μm. Comparison of the predicted geometrical dimensions of the micro-dimples through the simulation shows reasonable agreement with the experimental results for the given process parameters of ECMM. The paper also reports a brief comparison of laser surface texturing (LSTex) and electrochemical surface texturing (ECSTex). [ABSTRACT FROM AUTHOR]

The physico-chemical surface design of implants influences the surrounding cells. Osteoblasts on sharp-edged micro-topographies revealed an impaired cell phenotype, function and Ca2+ mobilization. The influence of edges and ridges on the Wnt/β-catenin pathway in combination with the cells’ stress response has not been clear. Therefore, MG-63 osteoblasts were studied on defined titanium-coated micro-pillars (5 × 5 × 5 µm) in vitro and in silico. MG-63s on micro-pillars indicated an activated state of the Wnt/β-catenin pathway. The β-catenin protein accumulated in the cytosol and translocated into the nucleus. Gene profiling indicated an antagonism mechanism of the transcriptional activity of β-catenin due to an increased expression of inhibitors like ICAT (inhibitor of β-catenin and transcription factor-4). Cells on pillars produced a significant reactive oxygen species (ROS) amount after 1 and 24 h. In silico analyses provided a detailed view on how transcriptional activity of Wnt signaling is coordinated in response to the oxidative stress induced by the micro-topography. Based on a coordinated expression of regulatory elements of the Wnt/β-catenin pathway, MG-63s are able to cope with an increased accumulation of β-catenin on micro-pillars and suppress an unintended target gene expression. Further, β-catenin may be diverted into other signaling pathways to support defense mechanisms against ROS.

A micro-structured nickel insert was investigated for polyether ether ketone (PEEK) injection moulding. The micro-features were circular holes 4 μm in diameter and 2 μm deep, with a 2-μm edge-to-edge distance. Six thousand moulding cycles were operated. Half of the insert was coated by approximately 200 nm CrN. PEEK parts produced by the coated side and uncoated side were compared. Coating thickness was measured at intervals of production and employed to characterize the coating wear. Pillar geometry at fixed locations on PEEK parts was studied by scanning electron microscope (SEM). Energy-dispersive X-ray spectroscopy (EDS) was conducted on the PEEK parts in order to study the possible nickel and silver contamination. The results show that the studied coating had a very low wear, and no nickel or silver contamination on PEEK was detected for both parts produced by coated and uncoated sides. Coating improved demoulding by reducing small indentations on pillars. [ABSTRACT FROM AUTHOR]

Three dimensional dislocation dynamics ( DD ) simulations are performed to investigate the governing mechanism of size dependent plastic deformation in submicron face-centered cubic ( fcc ) micropillars under uniaxial loading. Based on previous atomistic simulations, we introduce an algorithm for dislocation nucleation at the free surface as a function of stress and temperature in the DD simulation. The simulation results show stochastic behaviors in agreement with experimental observations, and reveal that dislocation nucleation at the free surface is the dominant mechanism of plastic flow in small pillars with diameters less than 200 nm, while the operation of truncated dislocation sources is the governing mechanism in large pillars with diameters exceeding 1 μm. In between, both mechanisms come into play in a stochastic way. [ABSTRACT FROM AUTHOR]

Microglia are the resident macrophages of the central nervous system and contribute to maintaining brain's homeostasis. Current 2D "petri-dish" in vitro cell culturing platforms employed for microglia, are unrepresentative of the softness or topography of native brain tissue. This often contributes to changes in microglial morphology, exhibiting an amoeboid phenotype that considerably differs from the homeostatic ramified phenotype in healthy brain tissue. To overcome this problem, multi-scale engineered polymeric microenvironments are developed and tested for the first time with primary microglia derived from adult rhesus macaques. In particular, biomimetic 2.5D micro- and nano-pillar arrays (diameters = 0.29-1.06 µm), featuring low effective shear moduli (0.25-14.63 MPa), and 3D micro-cages (volume = 24 × 24 × 24 to 49 × 49 × 49 μm 3 ) with and without micro- and nano-pillar decorations (pillar diameters = 0.24-1 µm) were fabricated using two-photon polymerization (2PP). Compared to microglia cultured on flat substrates, cells growing on the pillar arrays exhibit an increased expression of the ramified phenotype and a higher number of primary branches per ramified cell. The interaction between the cells and the micro-pillar-decorated cages enables a more homogenous 3D cell colonization compared to the undecorated ones. The results pave the way for the development of improved primary microglia in vitro models to study these cells in both healthy and diseased conditions., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Sharaf, Roos, Timmerman, Kremers, Bajramovic and Accardo.)

Abstract: The thermal and hydrodynamic performance of passive two-phase cooling devices such as heat pipes and vapor chambers is limited by the capabilities of the capillary wick structures employed. The desired characteristics of wick microstructures are high permeability, high wicking capability and large extended meniscus area that sustains thin-film evaporation. Choices of scale and porosity of wick structures lead to trade-offs between the desired characteristics. In the present work, models are developed to predict the capillary pressure, permeability and thin-film evaporation rates of various micropillared geometries. Novel wicking geometries such as conical and pyramidal pillars on a surface are proposed which provide high permeability, good thermal contact with the substrate and large thin-film evaporation rates. A comparison between three different micropillared geometries – cylindrical, conical and pyramidal – is presented and compared to the performance of conventional sintered particle wicks. The employment of micropillared wick structure leads to a 10-fold enhancement in the maximum heat transport capability of the device. The present work also demonstrates a basis for reverse-engineering wick microstructures that can provide superior performance in phase-change cooling devices. [Copyright &y& Elsevier]

Micro-hairs are profound elements widely used in nature. A great variety of animals use them as sensors to gather information about their surroundings. To artificially fabricate these kinds of sensors, we present a method to produce micro-hairs made of polydimethylsiloxane (PDMS). We investigated the laser micro-drilling of wax and polycarbonate (PC) substrates with a 193-nm ArF laser to produce molds suitable for casting micro-hairs. Especially PC molds lead to high-quality micro-hairs. Thus, laser ablation of PC was intensively studied. In order to obtain micro-hair arrays, experiments where performed to optimize the casting techniques of PDMS in PC molds. The possibilities and limitations of the investigated method of micro-hair production are presented. [ABSTRACT FROM AUTHOR]

A microfluidic device integrated with a nanoliter volume enzyme pre-reactor and an enzyme-modified electrode was developed for the highly selective continuous measurement of glutamate (Glu). The device consists mainly of two glass plates. One plate incorporates an electrochemical cell that consists of working electrode (WE), reference electrode (RE) and counter electrode (CE). The WE is modified with a bilayer film of Os-polyvinylpyrridine-based mediator containing horseradish peroxidase (Os-gel-HRP). The WE was operated at −50 mV versus Ag. The other plate has a thin layer flow channel integrated with a pre-reactor. The reactor has a number of micropillars (20 μm in diameter, 20 μm high and separated from each other by a 20 μm gap) modified with ascorbate oxidase (AAOx) to eliminate l-ascorbic acid (AA). The enzymatic oxidation of AA is superior to that obtained with our previously reported pre-electrolysis type micro-reactor since electrochemically reversible transmitters such as catecholamines do not provide a cathodic current at the WE. In addition, the high operation potential of the pre-reactor causes unknown electroactive species, which also cause interference at the detection electrode. As a result, we were able to detect 1 μM Glu continuously at a low flow rate even when AA concentration was 100 μM. [Copyright &y& Elsevier]

The interface debonding caused by mismatched thermal expansion is a key issue in graphene nanoplatelets/Ti matrix composites (GNPs/TiMCs), which significantly decreases the composites' mechanical properties. In this study, GNPs/Ti composites were fabricated by spark plasma sintering (SPS) followed by heat treatment (HT) routes. A novel strategy that strengthening the GNPs-Ti interface bonding via tailored three-dimensional (3D) interface configuration was proposed for the first time. Characters of interface evolution (before and after HT processes) were discussed detailedly in conjunction with the in-situ formed TiB whiskers (TiBw). The interfacial microstructure displayed that GNPs was used as template for the heteroepitaxial growth of TiBw, then TiBw connected the TiC layer and the adjacent Ti matrix. Tensile tests showed that GNPs-(TiBw)/Ti composites exhibited excellent strength-plasticity compatibility as compared to GNPs/Ti. Ex-situ compression tests of micro-pillars fabricated from the bulk composites revealed that the 3D interface configuration in GNPs-(TiBw)/Ti possessed significantly higher strain accommodation and strain-hardening ability during deformation. This study highlights the importance of interface optimization, which is helpful for developing the interfacial engineering and achieving good properties in GNPs/TiMCs. [ABSTRACT FROM AUTHOR]

The physico-chemical surface design of implants influences the surrounding cells. Osteoblasts on sharp-edged micro-topographies revealed an impaired cell phenotype, function and Ca2+ mobilization. The influence of edges and ridges on the Wnt/β-catenin pathway in combination with the cells' stress response has not been clear. Therefore, MG-63 osteoblasts were studied on defined titanium-coated micro-pillars (5 × 5 × 5 µm) in vitro and in silico. MG-63s on micro-pillars indicated an activated state of the Wnt/β-catenin pathway. The β-catenin protein accumulated in the cytosol and translocated into the nucleus. Gene profiling indicated an antagonism mechanism of the transcriptional activity of β-catenin due to an increased expression of inhibitors like ICAT (inhibitor of β-catenin and transcription factor-4). Cells on pillars produced a significant reactive oxygen species (ROS) amount after 1 and 24 h. In silico analyses provided a detailed view on how transcriptional activity of Wnt signaling is coordinated in response to the oxidative stress induced by the micro-topography. Based on a coordinated expression of regulatory elements of the Wnt/β-catenin pathway, MG-63s are able to cope with an increased accumulation of β-catenin on micro-pillars and suppress an unintended target gene expression. Further, β-catenin may be diverted into other signaling pathways to support defense mechanisms against ROS. [ABSTRACT FROM AUTHOR]

This research was devoted to studying the influence of the taper angle on the micro-compression of micro-pillars fabricated from near-amorphous and nanocrystalline Mo-B-C coatings. A series of micro-pillars with a taper angle between 4–14° was fabricated by focused ion beam technique. The deformation mechanism was found to be dependent on the taper and, also, on the crystallinity of the coating. In order to obtain correct values of yield strength and Young's modulus, three empirical models of stress correction were experimentally tested, and the results were compared with nanoindentation measurements. It was shown that the average stress correction model provided comparable results with nanoindentation for the yield strength for taper angles up to ~10°. On the other hand, the average radius or area model gave the most precise results for Young's modulus if the taper angle was <10°. [ABSTRACT FROM AUTHOR]

By stabilising metastable austenite with a suitable morphology in a martensitic structure, it is possible to impart to multi-phase steels high ductility combined with tensile strengths exceeding 2000 MPa. One way to achieve such mixed structures consisting of martensite and retained austenite (RA) is the Q&P (quenching and partitioning) process. The resulting structure contains metastable austenite in the form of thin foils located between martensite laths or plates. The stability of austenite under mechanical loading is the essential factor contributing to the extraordinary plasticity of such materials during cold deformation. A steel with 0.43% of carbon, alloyed with manganese, silicon and chromium was chosen for the experiment described in the present paper. Using the Q&P process, a martensitic structure with 20% of retained austenite was obtained. As cold plastic deformation causes the austenite to transform, 10% cold deformation was applied after the Q&P process. This deformation reduced the RA fraction to 11%. Materials prepared by this method were examined using micro-pillar compression experiments. Using the focused ion beam (FIB) method, pillars of 3 × 3 μm cross-section and 8 μm length were fabricated. These were afterwards mechanically tested in situ in an electron microscope in quasi-static compression at a true strain rate of 3 × 10 −4 s −1 to different amounts of plastic strain. The experiment showed that mechanical properties of the two conditions of material differ in terms of yield strength and the strain hardening exponent. An additional metallographic analysis of structures, including the exploration of the influence of decomposition of retained austenite, was performed. [ABSTRACT FROM AUTHOR]