Micropillar Arrays Research Articles

Theoretical optimization of micropillar arrays for structurally stable bioinspired dry adhesives

Inspired by the excellent adhesion performances of setae structure from organisms, micro/nano-pillar array has become one of the paradigms for adhesive surfaces. The micropillar arrays are composed of the resin pillars for adhesion and the substrate with different elastic modulus for supporting. The stress singularity at the bi-material corner between the pillars and the substrate can induce the failure of the micropillar-substrate corner and further hinder the fabrication and application of micropillar arrays, yet the design for the stability of the micropillar array lacks systematical and quantitative guides. In this work, we develop a semi-analytical method to provide the full expressions for the stress distribution within the bi-material corner combining analytical derivations and numerical calculations. The predictions for the stress within the singularity field can be obtained based on the full expressions of the stress. The good agreement between the predictions and the FEM results demonstrates the high reliability of our method. By adopting the strain energy density factor approach, the stability of the pillar-substrate corner is assessed by predicting the failure at the corner. For the elastic mismatch between the pillar and substrate given in this paper, the stability can be improved by increasing the ratio of the shear modulus of the substrate to that of the micropillar. Our study provides accurate predictions for the stress distribution at the bi-material corner and can guide the optimization of material combinations of the pillars and the substrate for more stable bioinspired dry adhesives.

Micro-feature fabrication on SS304 stainless steel using WEDM process

ABSTRACT Fabricating micro-pillars is a tough procedure as these features are highly debile. Wire Electro-Discharge Machining (WEDM) is a popular micromachining process to fabricate complex features on electrical conducting materials. SS304 stainless steel is commonly used in manufacturing of heating and cooling devices. A simple method of micro-pillars array fabrication on SS304 Stainless Steel material using WEDM was used through perpendicular crossed wire movement, keeping specified amount of step-over distance to form pillars. An analytical model for material removal rate prediction was developed using input parameters like spark duration, off time, voltage and current. Also, the influence of the process parameters was studied on depth and kerf width produced. Voltage and pulse-on time were found to be the major influencing parameters. Micro-featured surfaces fabricated for minimum kerf width and maximum feature depth were analysed using SEM and EDS along with the wettability study. The fabricated surfaces remained hydrophobic even after 100s.

CTAB-Modulated Electroplating of Copper Micropillar Arrays for Non-Enzymatic Glucose Sensing with Improved Sensitivity.

Micropillar array electrodes represent a promising avenue for enhancing detection sensitivity and response current. However, existing methods for depositing electrode materials on micropillar arrays often result in uneven distribution, with the thin sidewall layer being less conductive and prone to corrosion. In addressing this issue, this study introduces electroplating to enhance the copper layer on the sidewall of micropillar array electrodes. These electrodes, fabricated through standard microelectronics processes and electroplating, are proposed for non-enzymatic glucose detection, with the copper layer deposited via electroplating significantly enhancing sensitivity. Initially, the impact of cetyltrimethylammonium bromide (CTAB) concentration as an inhibitor on the surface morphology and sensitivity of the plated layer was investigated. It was discovered that CTAB could decrease surface roughness, hinder the development of large and coarse grains, generate small particles, and boost sensitivity. Compared to the uncoated electrode and plating without CTAB, sensitivity was elevated by a factor of 1.66 and 1.62, respectively. Subsequently, the alterations in plating morphology and detection performance within a range of 0.3 ASD to 3 ASD were examined. Sensitivity demonstrated a tendency to increase initially and then decrease. The electrode plated at 0.75 ASD achieved a maximum sensitivity of 3314 μA·mM-1·cm-2 and a detection limit of 15.9 μM. Furthermore, a potential mechanism explaining the impact of different morphology on detection performance due to CTAB and current density was discussed. It was believed that the presented effective strategy to enhance the sensitivity of micropillar array electrodes for glucose detection would promote the related biomedical detection applications.

Capacitive Pressure Sensor Combining Dual Dielectric Layers with Integrated Composite Electrode for Wearable Healthcare Monitoring.

Foot activity can reflect numerous physiological abnormalities in the human body, making gait a valuable metric in health monitoring. Research on flexible sensors for gait monitoring has focused on high sensitivity, wide working range, fast response, and low detection limit, but challenges remain in areas such as elasticity, antibacterial activity, user-friendliness, and long-term stability. In this study, we have developed a novel capacitive pressure sensor that offers an ultralow detection limit of 1 Pa, wide detection ranges from 1 Pa to 2 MPa, a high sensitivity of 0.091 kPa-1, a fast response time of 71 ms, and exceptional stability over 6000 cycles. This sensor not only has the ability of accurately discriminating mechanical stimuli but also meets the requirements of elasticity, antibacterial activity, wearable comfort, and long-term stability for gait monitoring. The fabrication method of a dual dielectric layer and integrated composite electrode is simple, cost-effective, stable, and amenable to mass production. Thereinto, the introduction of a dual dielectric layer, based on an optimized electrospinning network and micropillar array, has significantly improved the sensitivity, detection range, elasticity, and antibacterial performance of the sensor. The integrated flexible electrodes are made by template method using composite materials of carbon nanotubes (CNTs), two-dimensional titanium carbide Ti3C2Tx (MXene), and polydimethylsiloxane (PDMS), offering synergistic advantages in terms of conductivity, stability, sensitivity, and practicality. Additionally, we designed a smart insole that integrates the as-prepared sensors with a miniature instrument as a wearable platform for gait monitoring and disease warning. The developed sensor and wearable platform offer a cutting-edge solution for monitoring human activity and detecting diseases in a noninvasive manner, paving the way for future wearable devices and personalized healthcare technologies.

Droplet evaporation on two-tier hierarchical micro-pillar array surface

As surface modification becomes a promising approach to thermal management, quantifying effects of surface structure geometrical dimension is of great significance to optimize heat transfer performance. In this paper, we emphatically report droplet evaporation on three types of two-tier hierarchical micro-pillar array (THM) surfaces. Utilizing the lattice Boltzmann model, evaporation characteristics including droplet morphology, triple-phase contact line (TPCL) behavior, liquid–vapor interface evolution and heat flux distribution are elucidated. In comparison to single-tier micro-pillar array (SM) surfaces, THM surfaces enable remarkable enhancement in heat transfer when secondary pillars are configured on the side of the primary pillar or configured parallelly to the primary pillar on the substrate, which is attributed to the synergistic effect of TPCL evaporation and liquid–vapor interface evaporation. The varying trend of droplet lifetime with the primary pillar geometrical dimension on all THM surfaces are also summarized, giving a guideline for optimization of heat dissipation by tailoring structures.

Near-IR Photoinduced Electrochemiluminescence Imaging with Structured Silicon Photoanodes.

Infrared (IR) imaging devices that convert IR irradiation (invisible to the human eye) to a visible signal are based on solid-state components. Here, we introduce an alternative concept based on light-addressable electrochemistry (i.e., electrochemistry spatially confined under the action of a light stimulus) that involves the use of a liquid electrolyte. In this method, the projection of a near-IR image (λexc = 850 or 840 nm) onto a photoactive Si-based photoanode, immersed into a liquid phase, triggers locally the photoinduced electrochemiluminescence (PECL) of the efficient [Ru(bpy)3]2+-TPrA system. This leads to the local conversion of near-IR light to visible (λPECL = 632 nm) light. We demonstrate that compared to planar Si photoanodes, the use of a micropillar Si array leads to a large enhancement of local light generation and considerably improves the resolution of the PECL image by preventing photogenerated minority carriers from diffusing laterally. These results are important for the design of original light conversion devices and can lead to important applications in photothermal imaging and analytical chemistry.

Microsphere-Enabled Micropillar Array for Whispering Gallery Mode Virus Detection.

Rapid detection of pathogens and analytes at the point of care offers an opportunity for prompt patient management and public health control. This paper reports an open microfluidic platform coupled with active whispering gallery mode (WGM) microsphere resonators for the rapid detection of influenza viruses. The WGM microsphere resonators, precoated with influenza A polyclonal antibodies, are mechanically trapped in the open micropillar array, where the evaporation-driven flow continuously transports a small volume (∼μL) of sample to the resonators without auxiliaries. Selective chemical modification of the pillar array changes surface wettability and flow pattern, which enhances the detection sensitivity of the WGM resonator-based virus sensor. The optofluidic sensing platform is able to specifically detect influenza A viruses within 15 min using a few microliters of sample and displays a linear response to different virus concentrations.

Modified Graphene Micropillar Array Superhydrophobic Coating with Strong Anti-Icing Properties and Corrosion Resistance

Ice accumulation on outdoor surfaces tends to cause a series of serious accidents in extreme climates. In recent years, superhydrophobic coatings have garnered significant interest in the passive anti-icing field owing to their exceptional ability to repel water. However, these superhydrophobic coatings displayed inadequate anti-icing properties in conditions of low temperature and high humidity, as well as poor corrosion resistance after prolonged immersion. Herein, we prepared a series of modified graphene micropillar array superhydrophobic coatings that have various micropillar widths based on simple physical embossing and spraying techniques. These coatings combined the graphene nanosheets modified by perfluorodecyl triethoxysilane (AC-FAS) and the fluorocarbon varnish coating with a micropillar array structure to gain great superhydrophobicity and anti-icing properties. Among these, the average freezing delay time of the Q200G coating increased over 500 times relative to the unmodified Al surface (about 10 s), and its average deicing shear force was as low as 0.1 N. Meanwhile, the corrosion resistance increased steadily as the micropillar width decreased, based on the electrochemical impedance spectroscopy test results. More importantly, such facile fabrication methods and multi-functionality make superhydrophobic coatings hold great application prospects.

Numerical study on heat and mass transfer characteristics of surface wettability

It has been shown that wettability has an important effect on the surface’s heat transfer performance. In this article, the effects of different wettability of the surfaces of the micropillar array on the heat transfer performance under the same working condition (single-wettability and mixed-wettability surfaces) are compared by numerical simulation, the effects of the surface’s geometrical structure on the flow state of the workpiece and the behavior of the bubbles are also described. It is found that the mixed wettability surfaces combine the characteristics of hydrophilic and hydrophobic surfaces can effectively reduce the wall temperature. The microcolumn array further enhances the boiling heat transfer effect by perturbing the flow process of the workpiece while increasing the heated area. The surface structure of the hydrophobic top combined with hydrophilic sidewalls can significantly improve the heat transfer performance by 10%∼40% during the stable boiling process.

Column-Only Band Broadening in a Porous Shell Radially Elongated Pillar Array Column.

In the quest for better performing separation media for liquid chromatography, micropillar array columns have received great interest over the past years. While previous research was mainly focused around micropillar array columns (μPACs) filled with cylindrical pillars, this contribution discusses μPACs with rectangular pillars, which, for the first time, have been anodized and hence carry a mesoporous shell. We report on a series of on-chip measurements of the band broadening and flow permeability in a μPAC with very wide radially elongated pillars (3·75 μm) and with an interpillar distance (2 μm) between that of the first (2.5 μm) and second generation (1.25 μm) of cylindrical μPACs. Because of the extreme flow path tortuosity, this type of μPAC can produce very large plate numbers over a short distance. Despite the relatively large interpillar distance, we obtain Hmin = 0.26 μm for a nearly unretained component (phase retention factor, k' ≈ 0.24) and Hmin = 0.79 μm for a retained component with k' ≈ 3. The kinetic performance in terms of separation impedance (Ei = 19) is considerably improved compared to cylindrical pillar μPACs (Ei in range 40-50) and is in excellent agreement with the theoretical value for an open tubular channel with a rectangular cross-section (Ei = 18). This shows that rectangular μPACs can be represented as a parallel bundle of interconnected open-tubular channels.

Geometry-Dependent Elastic Flow Dynamics in Micropillar Arrays.

Regular device-scale DNA waves for high DNA concentrations and flow velocities have been shown to emerge in quadratic micropillar arrays with potentially strong relevance for a wide range of microfluidic applications. Hexagonal arrays constitute another geometry that is especially relevant for the microfluidic pulsed-field separation of DNA. Here, we report on the differences at the micro and macroscopic scales between the resulting wave patterns for these two regular array geometries and one disordered array geometry. In contrast to the large-scale regular waves visible in the quadratic array, in the hexagonal arrays, waves occur in a device-scale disordered zig-zag pattern with fluctuations on a much smaller scale. We connect the large-scale pattern to the microscopic flow and observe flow synchronization that switches between two directions for both the quadratic and hexagonal arrays. We show the importance of order using the disordered array, where steady-state stationary and highly fluctuating flow states persist in seemingly random locations across the array. We compare the flow dynamics of the arrays to that in a device with sparsely distributed pillars. Here, we observe similar vortex shedding, which is clearly observable in the quadratic and disordered arrays. However, the shedding of these vortices couples only in the flow direction and not laterally as in the dense, ordered arrays. We believe that our findings will contribute to the understanding of elastic flow dynamics in pillar arrays, helping us elucidate the fundamental principles of non-Newtonian fluid flow in complex environments as well as supporting applications in engineering involving e.g., transport, sorting, and mixing of complex fluids.

Micropillar arrays, wide window acquisition and AI-based data analysis improve comprehensiveness in multiple proteomic applications.

Comprehensive proteomic analysis is essential to elucidate molecular pathways and protein functions. Despite tremendous progress in proteomics, current studies still suffer from limited proteomic coverage and dynamic range. Here, we utilize micropillar array columns (µPACs) together with wide-window acquisition and the AI-based CHIMERYS search engine to achieve excellent proteomic comprehensiveness for bulk proteomics, affinity purification mass spectrometry and single cell proteomics. Our data show that µPACs identify ≤50% more peptides and ≤24% more proteins, while offering improved throughput, which is critical for large (clinical) proteomics studies. Combining wide precursor isolation widths of m/z 4-12 with the CHIMERYS search engine identified +51-74% and +59-150% more proteins and peptides, respectively, for single cell, co-immunoprecipitation, and multi-species samples over a conventional workflow at well-controlled false discovery rates. The workflow further offers excellent precision, with CVs <7% for low input bulk samples, and accuracy, with deviations <10% from expected fold changes for regular abundance two-proteome mixes. Compared to a conventional workflow, our entire optimized platform discovered 92% more potential interactors in a protein-protein interaction study on the chromatin remodeler Smarca5/Snf2h. These include previously described Smarca5 binding partners and undescribed ones including Arid1a, another chromatin remodeler with key roles in neurodevelopmental and malignant disorders.

Suppressed Droplet Splashing on Positively Skewed Surfaces for High-Efficiency Evaporation Cooling.

Two types of functional surfaces with the same roughness but completely different surface topographies are prepared, namely positively skewed surfaces filled with micropillar arrays (Sa ≈4.4µm, Ssk >0) and negatively skewed surfaces filled with microcavity arrays (Sa ≈4.4µm, Ssk <0), demonstrating promoting droplet splashing. Remarkably, the critical Weber number for generating satellite droplets on the negatively skewed surfaces is significantly lower than that on the positively skewed surfaces, indicating that the negatively skewed surface with microcavity arrays is more likely to promote droplet splashing. It is mainly attributed to the fact that air on the negatively skewed surface can make the liquid film take on a Cassie-Baxter state on the surface so that the stabilizing capillary force of the liquid film exceeds the destabilizing stress of the air film. Moreover, the surface topography promoting droplet spreading and the mechanical properties of three-phase moving contact lines are analyzed from the perspective of microscopic interface mechanics. Finally, it is demonstrated the designed positively skewed surfaces can be employed for large-area heat dissipation by means of high-efficiency evaporation.

Tailoring Interfaces for Enhanced Methanol Production from Photoelectrochemical CO2 Reduction.

Efficient and stable photoelectrochemical reduction of CO2 into highly reduced liquid fuels remains a formidable challenge, which requires an innovative semiconductor/catalyst interface to tackle. In this study, we introduce a strategy involving the fabrication of a silicon micropillar array structure coated with a superhydrophobic fluorinated carbon layer for the photoelectrochemical conversion of CO2 into methanol. The pillars increase the electrode surface area, improve catalyst loading and adhesion without compromising light absorption, and help confine gaseous intermediates near the catalyst surface. The superhydrophobic coating passivates parasitic side reactions and further enhances local accumulation of reaction intermediates. Upon one-electron reduction of the molecular catalyst, the semiconductor-catalyst interface changes from adaptive to buried junctions, providing a sufficient thermodynamic driving force for CO2 reduction. These structures together create a unique microenvironment for effective reduction of CO2 to methanol, leading to a remarkable Faradaic efficiency reaching 20% together with a partial current density of 3.4 mA cm-2, surpassing the previous record based on planar silicon photoelectrodes by a notable factor of 17. This work demonstrates a new pathway for enhancing photoelectrocatalytic CO2 reduction through meticulous interface and microenvironment tailoring and sets a benchmark for both Faradaic efficiency and current density in solar liquid fuel production.

Prediction of Plate Height Curves of Porous-Shell Pillar Array Columns Micro-Pillar Array Columns

We investigate band broadening in the most widely adopted configuration of micro-pillar array columns (μPACs)—specifically, a cylindrical pillar array where both the pillar walls and the channel bottom are coated with a thin layer of mesoporous material. The two-zone moment analysis method is adopted to investigate the dispersion properties of μPACs in a broad range of shell thicknesses, reduced fluid velocities, and retention factors. Three different models of the unit cell, of increasing complexity, have been implemented, namely a two-dimensional model and two different three-dimensional models with and without the retentive bottom layer, the presence of which seems to have a very significant effect on the plate height curves. Model predictions are compared with experimental van Deemter curves for uncoated and coated porous layers, and a robust relationship between the intra-particle (porous-zone) diffusion coefficient Dpz and the retention factor k′ is established.

Density of Micropillar Array Influences Shear Traction of Individual Pillars on Soft Substrates

Density of Micropillar Array Influences Shear Traction of Individual Pillars on Soft Substrates

Laser Powder Bed Fusion of Stainless Steel 316L for Rectangular Micropillar Array with High Geometrical Accuracy and Hardness

Laser Powder Bed Fusion of Stainless Steel 316L for Rectangular Micropillar Array with High Geometrical Accuracy and Hardness

Advances in micropillar arrays in cellular biomechanics detection and tissue engineering

Advances in micropillar arrays in cellular biomechanics detection and tissue engineering

Centrifugal microfluidic system for colorimetric sample-to-answer detection of viral pathogens.

We describe a microfluidic system for conducting thermal lysis, polymerase chain reaction (PCR) amplification, hybridization, and colorimetric detection of foodborne viral organisms in a sample-to-answer format. The on-chip protocol entails 24 steps which are conducted by a centrifugal platform that allows for actuating liquids pneumatically during rotation and so facilitates automation of the workflow. The microfluidic cartridge is fabricated from transparent thermoplastic polymers and accommodates assay components along with an embedded micropillar array for detection and read-out. A panel of oligonucleotide primers and probes has been developed to perform PCR and hybridization assays that allows for identification of five different viruses, including pathogens such as norovirus and hepatitis A virus (HAV) in a multiplexed format using digoxigenin-labelled amplicons and immunoenzymatic conversion of a chromogenic substrate. Using endpoint detection, we demonstrate that the system can accurately and repetitively (n = 3) discriminate positive and negative signals for HAV at 350 genome copies per μL. As part of the characterization and optimization process, we show that the implementation of multiple (e.g., seven) micropillar arrays in a narrow fluidic pathway can lead to variation (up to 50% or more) in the distribution of colorimetric signal deriving from the assay. Numerical modeling of flow behaviour was used to substantiate these findings. The technology-by virtue of automation-can provide a pathway toward rapid detection of viral pathogens, shortening response time in food safety surveillance, compliance, and enforcement as well as outbreak investigations.

Enhancing thermal performance in laser-driven illumination with metal–micropillars array three-dimensional substrate

Enhancing thermal performance in laser-driven illumination with metal–micropillars array three-dimensional substrate