Micropost Arrays Research Articles

Thermal atomization during droplet impingement on superhydrophobic surfaces: Influence of Weber number and micropost array configuration

An experimental study of thermal atomization intensity during droplet impingement on superheated hydrophobic and superhydrophobic surfaces of varying microstructure was performed. Thermal atomization in these scenarios is the result of droplet boiling, where vapor bubbles burst upwards through the droplet lamella, causing a fine spray of secondary droplets. A smooth hydrophobic surface and three post-patterned superhydrophobic surfaces of similar solid fraction but differing post size were investigated over a range of surface temperatures from 120 ∘C to 320 ∘C and Weber numbers from 20 to 200. Trends in atomization intensity were characterized using a high-speed image processing technique. Changes in surface temperature, Weber number, and microstructure configuration were shown to significantly influence atomization intensity, and these parameters are thought to be directly linked to three main mechanisms accounting for atomization dynamics in impingement scenarios. These mechanisms are vapor generation at the liquid-solid interface of the impinging droplet, vapor bursting through the spreading lamella, and vapor escape laterally beneath the droplet. Vapor generation increases with an increase in heat transfer to the droplet, which may be produced by increasing surface temperature or increasing liquid-solid contact through droplet wetting. Vapor bursting upwards through the lamella depends mainly on lamella thickness which decreases with increasing Weber number. Finally, vapor escape beneath the droplet may occur as vapor flows laterally through the micropost arrays. This is found to be enhanced by increasing the spacing between structures. These competing mechanisms result in thermal atomization, which generally increases with increasing Weber number and decreasing pitch. Additionally, the Leidenfrost point was also found to increase with increasing Weber number and decreasing pitch. A scale analysis was performed to explore the effect of resistance to vapor escape through micropost arrays on thermal atomization, and the resulting scaling describes the experimental findings well.

Hydrogel Micropost Arrays with Single Post Tunability to Study Cell Volume and Mechanotransduction.

The extracellular matrix varies considerably in mechanical properties at the microscale. It remains unclear how cells respond to these properties, in part, due to lack of tools to create precisely defined microenvironments in a discrete manner. Here, freeform stereolithography is leveraged to control the placement and elastic modulus of individual hydrogel microposts that serve as discrete matrix signals to interface with cells. Mesenchymal stromal cells (MSCs) located in the interstitial spaces between microposts above a base layer are analyzed. Cell volume is higher when MSCs interact with more microposts. MSCs show higher strain energy when they interact simultaneously with 4-kPa and 20-kPa microposts than with mechanically homogeneous micropost arrays. MSCs are sensitive to pharmacological inhibition of Rho-associated protein kinase in 4-kPa arrays, but resistant when presented together with 20-kPa arrays. Yes-associated protein (YAP) activity increases with higher cell volume and elastic modulus of microposts. Surprisingly, YAP activity becomes less variable with higher cell volume and decreases with higher average force and strain energy per post when MSCs interact with both 4-kPa and 20-kPa microposts simultaneously. Together, these results describe a material system for systematically investigating how the placement and intrinsic properties of discrete matrix signals impact cell volume and mechanotransduction.

A High-Performance Flexible Pressure Sensor Realized by Overhanging Cobweb-like Structure on a Micropost Array.

Recent years have seen a rapid development of electronic skin for wearable devices, autonomous robotics, and human-machine interaction. As a result, the demand for flexible pressure sensors as the critical sensing element in electronic skin is also increasing. These sensors need to feature high sensitivity, short response time, low detection limit, and so on. In this paper, inspired from the cobweb in nature, we propose a piezoresistive pressure sensor by forming a cobweb-like network made of a zinc octaethylphorphyrin (ZnOEP)/carbon nanotube (CNT) hybrid on an array of polydimethylsiloxane (PDMS) microposts. The hybrid material exhibits excellent adhesion to PDMS, benefitting from ZnOEP's low Young's modulus and the nonpolar bonding between ZnOEP and PDMS such that no delamination and resistance variation are found after thousands of cycles of bending and twisting. With the overhanging morphology of the ZnOEP/CNT network on the micropost array, we realized a pressure sensor with an ultrahigh sensitivity of 39.4 kPa-1, a super-fast response time of 3 ms, a low detection limit of 10 Pa, and a reproducible response without degradation after 5000 cycles of pressure loading/unloading. The sensor can be employed for a variety of applications, including wrist pulse measurement, sound level detection, mechanical vibration monitoring, etc., proving its great potential for use in electronic skin systems.

Visualizing Molecular Chaperone Controlled Resilient Cell Traction Force by Micropost Arrays Fabricated by Two-Photon Initiated Polymerization

Visualizing Molecular Chaperone Controlled Resilient Cell Traction Force by Micropost Arrays Fabricated by Two-Photon Initiated Polymerization

Response of cells on a dense array of micro-posts

We have analysed the response of cells on a bed of micro-posts idealized as a Winkler foundation using a homeostatic mechanics framework. The framework enables quantitative estimates of the stochastic response of cells along with the coupled analysis of cell spreading, contractility and mechano-sensitivity. In particular the model is shown to accurately predict that: (i) the extent of cell spreading, actin polymerisation as well as the traction forces that cells exert increase with increasing stiffness of the foundation; (ii) the traction forces that cells exert are primarily concentrated along the cell periphery; and (iii) while the total tractions increase with increasing cell area the average tractions are reasonably independent of cell area, i.e. for a given substrate stiffness, the average tractions that are normalized by cell area do not vary strongly with cell size. These results thus suggest that the increased foundation stiffness causes both the cell area and the average tractions that the cells exert to increase through higher levels of stress-fibre polymerization rather than the enhanced total tractions being directly linked through causation to the larger cell areas. A defining feature of the model is that its predictions are statistical in the form of probability distributions of observables such as the traction forces and cell area. In contrast, most existing models present solutions to specific boundary value problems where the cell morphology is imposed a priori. In particular, in line with observations we predict that the diversity of cell shapes, sizes and measured traction forces increase with increasing foundation stiffness. The homeostatic mechanics framework thus suggests that the diversity of observations in in vitro experiments is inherent to the homeostatic equilibrium of cells rather than being a result of experimental errors.

Self-Powered Gas Sensor Based on a Photovoltaic Cell and a Colorimetric Film with Hierarchical Micro/Nanostructures.

We report a new type of self-powered gas sensors based on the combination of a colorimetric film with hierarchical micro/nanostructures and organic photovoltaic cells. The transmittance of the colorimetric film with micro/nanostructures coated with N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) changes by reacting with NO2 gas, and it is measured as a current output of the photovoltaic cell. For this purpose, materials for the organic photovoltaic cells were carefully chosen to match the working wavelength of the TMPD. Micropost arrays and nanowires increase the surface area for the gas reaction and thus improve the transmittance changes by NO2 gas (6.7% change for the plain film vs 27.7% change for the film with hierarchical micro/nanostructures to 20 ppm of NO2). Accordingly, the colorimetric device with the hierarchical structures showed a response of ΔI/I0 = 0.27-20 ppm of NO2, which is a 71% improvement compared to that of the plain sensing film. Furthermore, it showed a high selectivity against other gases such as H2S and CO with almost negligible responses. Since the current output change of the photovoltaic cell is utilized as a sensor signal, no extra electrical power is required for the operation of gas sensors. We also integrated the sensor device with an electrical module and demonstrated a self-powered gas alarm system.

Manufacturing Process Design of a Micro-Scale Liquid-Liquid Extractor and Multi-phase Separator

Abstract Advances in process innovation within the chemical process industry are impeded by the cost of producing new innovative components. In this paper, various manufacturing process routes were evaluated for scaling up a micro-scale liquid-liquid extractor and multi-phase separator capable of separating immiscible phases that are used for the extraction of a chemical product into a lower boiling point solvent to make downstream distillation easier. A manufacturing process design methodology was used to evaluate both photochemical machining and binder jetting as a means for producing the micropost arrays within the separator. Cost curves show that for a 0.3 mm deep micropost array, photochemical machining was found to be much cheaper than binder jetting. However, because binder jetting processes enable microposts with over 13 times higher depth-to-width aspect ratios, binder jetting was found to decrease the overall costs of the extractor/separator by more than half for a 500 barrel per day plant, with breakeven as low as 15 barrels per day. These economics were confirmed by building extractor/separator plates and characterizing them dimensionally and functionally. The binder jet plates were found to perform better than photochemical plates leading to the complete separation of the organic phase from water. The use of slower sintering cycles combined with surface grinding provided acceptable dimensional tolerances for the binder jet plates. Using pressurized air during depowdering improved the economics for the binder jetting process.

Shape-Programmed Fabrication and Actuation of Magnetically Active Micropost Arrays.

Micro- and nanotextured surfaces with reconfigurable textures can enable advancements in the control of wetting and heat transfer, directed assembly of complex materials, and reconfigurable optics, among many applications. However, reliable and programmable directional shape in large scale is significant for prescribed applications. Herein, we demonstrate the self-directed fabrication and actuation of large-area elastomer micropillar arrays, using magnetic fields to both program a shape-directed actuation response and rapidly and reversibly actuate the arrays. Specifically, alignment of magnetic microparticles during casting of micropost arrays with hemicylindrical shapes imparts a deterministic anisotropy that can be exploited to achieve the prescribed, large-deformation bending or twisting of the pillars. The actuation coincides with the finite element method, and we demonstrate reversible, noncontact magnetic actuation of arrays of tens of thousands of pillars over hundreds of cycles, with the bending and twisting angles of up to 72 and 61°, respectively. Moreover, we demonstrate the use of the surfaces to control anisotropic liquid spreading and show that the capillary self-assembly of actuated micropost arrays enables highly complex architectures to be fabricated. The present technique could be scaled to indefinite areas using cost-effective materials and casting techniques, and the principle of shape-directed pillar actuation can be applied to other active material systems.

Cell Positioning Culture Chip Based on Functional Micro Topological Substrate

There are two main categories of cell positioning culture chips:chemical modification of surfaces and physical structure methods. The chemical methods have the advantage of providing a strong binding force, but the instability of chemical materials limits its application. The physical methods have the advantage of good stability for cell positioning, while the nonspecificity of physical structure weakens the ability to regulate cells. Here, we propose a functional micro topological chip. The chip is composed of micro-platform and micro-post array with different adhesion characteristics. At the same time, the chip is prepared by a polymer material with good biocompatibility and high chemical stability; therefore, it has the advantages of strong cell constraint ability and good localization stability. The result of chip performance verification shows that the cell localization rate of the functional micro topological chip is up to 90%, and the chip structure and dimension stability are good. The study provides an important supporting tool for the research of stem cells and their differentiation that need long-term cell culture.

Mitigation of Channel Clogging in a Microfluidic Device for Capturing Circulating Tumor Cells

Deterministic lateral displacement (DLD) based microfluidic devices have been developed for capturing circulating tumor cells (CTCs) from the peripheral blood. There was frequent and problematic channel clogging around the micro-post array formed on a microchannel of the device. In this study, various agents were dispersed into the blood specimen to avoid clogging. At first, platelet aggregation was considered to be the cause of the clogging, but even plasmin, which was assumed to decompose platelet aggregations, did not show obvious inhibition of the clogging. Then, enzymes used for cell detachment from tissue were examined and decomposition of the clogging residue was observed. Finally, dispersion of deoxyribonuclease into a blood specimen was found to be effective for the inhibition of clogging. The existence of DNA in the clogging residue was also confirmed by propidium iodide (PI) staining, suggesting DNA adhering to the micro-post.

Solar energy harvesting and a water droplet cleaning of micropost arrays surfaces

Solar energy harvesting in dusty environments initiates many challenges for sustainable operation of photovoltaic panels. Environmental dust reduces photovoltaic device performance and requires regular cleaning of active surfaces. Self-cleaning of surfaces by water droplets offers advantages in terms of reduced cost and sustainable operation. The present study investigates dust removal from hydrophobic and optically transparent micropost arrays surfaces in relation to solar energy applications. The micropost arrays are replicated using polydimethylsiloxane (PDMS) casting on textured silicon wafers. The replicated micropost arrays result in hydrophobic surface with contact angle of about 147.6° ± 5° and the hysteresis of 16° ± 2°. In relation to self-cleaning, water droplet behavior on the inclined replicated micropost arrays is simulated and droplet movement is examined experimentally. The optical transmittance of micropost arrays is tested in outdoor dusty environments. It is found that droplet slides on the inclined micropost surface and expanding droplet size enhances the sliding velocity. Alkaline dust compounds dissolve in droplet while increasing pH from 4.35 to 7.94 and surface tension from 0.072 to 0.120 N/m. This alters droplet pinning and lowers the sliding velocity from 0.2 to 0.15 m/s. Optical transmittance of the samples was tested in outdoor dusty environments improve considerably by sliding water droplet cleaning on daily bases (transmittance reduces only 3% or less as reference to surface being kept indoor environments). Hence, introducing micropost arrays on photovoltaic panel protective cover surface can provide self-cleaning effect, via droplet rolling, while slightly reducing optical transmittance of cover glass (approximately 2.5%-3%) in outdoor environments.

Capturing and Clinical Applications of Circulating Tumor Cells with Wave Microfluidic Chip.

As a "liquid biopsy," circulating tumor cell (CTC) enumeration with microfluidic chips has great significance in cancer prognosis. CTCs carry significant information as the original tumor. Integrated microfluidic chips are combining with affinity- and physical-based such as wave chip offers a new way to segregate CTCs. In this work, we further study capturing clinical applications of CTCs with wave chip. When cell suspension moves across the microposts array, CTCs squeeze out from narrow gaps organized by microposts. This movement renders CTCs to obtain a tilted velocity to fluid direction. This tilted velocity would direct CTCs to be captured by the smaller neighboring gaps next array. Simultaneously, interaction or friction time is longer due to barrier of modified microposts. These microposts would be effective for realizing binding of antigen and antibody. Therefore, both antibody-coated and physical-based isolations could be combined in isolating CTCs. Capture percentage concentrated on the first several arrays is shown theoretically and experimentally. Efficient capture could be obtained for artificial patient blood. Clinically, CTCs were tested positive for three metastatic human breast cancer patient samples. This wave chip is prospectively to be a valid tool for clinical enumeration of CTCs, carrying out anti-cancer drug assay.

Dissecting fat-tailed fluctuations in the cytoskeleton with active micropost arrays

The ability of animal cells to crawl, change their shape, and respond to applied force is due to their cytoskeleton: A dynamic, cross-linked network of actin protein filaments and myosin motors. How these building blocks assemble to give rise to cells' mechanics and behavior remains poorly understood. Using active micropost array detectors containing magnetic actuators, we have characterized the mechanics and fluctuations of cells' actomyosin cortex and stress fiber network in detail. Here, we find that both structures display remarkably consistent power law viscoelastic behavior along with highly intermittent fluctuations with fat-tailed distributions of amplitudes. Notably, this motion in the cortex is dominated by occasional large, step-like displacement events, with a spatial extent of several micrometers. Overall, our findings for the cortex appear contrary to the predictions of a recent active gel model, while suggesting that different actomyosin contractile units act in a highly collective and cooperative manner. We hypothesize that cells' actomyosin components robustly self-organize into marginally stable, plastic networks that give cells' their unique biomechanical properties.

Comparison of Two-Dimensional and Three-Dimensional Carbon Electrode Geometries Affecting Bidirectional Electroosmotic Pumping

This study compares fluid velocity magnitude and direction for three different glassy carbon (GC) electrode systems effecting alternating current (AC) electroosmotic pumping. The flow behavior is analyzed for electroosmotic pumping performed with asymmetric coplanar electrodes. Subsequently, effects of adding microposts array of two different heights (40 μm and 80 μm) are studied. Experimental results demonstrate that as peak-to-peak voltage is increased above 10 V peak-to-peak, the flow reversal is achieved for planar electrodes. Utilization of microposts-enhanced asymmetric electrodes blocks the flow reversal and alters the magnitude of the fluid velocity at the application of larger voltages (above 10 V peak-to-peak). Understanding of the consequences of three-dimensional geometry of asymmetric electrodes would allow designing the electrode system for AC electroosmotic pumping and mixing, as well as bidirectional fluid driving with equal forward and backward flow velocities.

On the design of deterministic dielectrophoresis for continuous separation of circulating tumor cells from peripheral blood cells.

Detection and analysis of circulating tumor cells (CTCs) have emerged as a promising way to diagnose cancer, study its cellular mechanism, and test or develop potential treatments. However, the rarity of CTCs among peripheral blood cells is a big challenge toward CTC detection. In addition, in cases where there is similar size range between certain types of CTCs (e.g. breast cancer cells) and white blood cells (WBCs), high-resolution techniques are needed. In the present work, we propose a deterministic dielectrophoresis (DEP) method that combines the concept of deterministic lateral displacement (DLD) and insulator-based dielectrophoresis (iDEP) techniques that rely on physical markers such as size and dielectric properties to differentiate different type of cells. The proposed deterministic DEP technology takes advantage of frequency-controlled AC electric field for continuous separation of CTCs from peripheral blood cells. Utilizing numerical modeling, different aspects of coupled DLD-DEP design such as the required applied voltages, velocities, and geometrical parameters of DLD arrays of microposts are investigated. Regarding the inevitable difference and uncertainty ranges for the reported crossover frequencies of cells, a comprehensive analysis is conducted on applied electric field frequency as design's determinant factor. Deterministic DEP design provides continuous sorting of CTCs from WBCs even with similar size and has the future potential for high throughput and efficiency.

Cell migration on microposts with surface coating and confinement.

Understanding cell migration in a 3D microenvironment is essential as most cells encounter complex 3D extracellular matrix (ECM) in vivo. Although interactions between cells and ECM have been studied previously on 2D surfaces, cell migration studies in 3D environment are still limited. To investigate cell migration under various degrees of confinements and coating conditions, 3D platforms with micropost arrays and controlled fibronectin (FN) protein coating were developed. MC3T3-E1 cells spread and contacted the top surface of microposts if FN was coated on top. When FN was coated all over the microposts, cells were trapped between microposts with 3 μm spacing and barely moved. As the spacing between microposts increased from 3 to 5 μm, cells became elongated with limited cell movement of 0.18 μm/min, slower than the cell migration speed of 0.40 μm/min when cells moved on top. When cells were trapped in between the microposts, cell nuclei were distorted and actin filaments formed along the sidewalls of microposts. With the addition of a top cover to introduce cell confinement, the cell migration speed was 0.23 and 0.84 μm/min when the channel height was reduced from 20 to 10 μm, respectively. Cell traction force was monitored at on the top and bottom microposts with 10 μm channel height. These results show that the MC3T3-E1 cell morphology, migration speed, and movement position were affected by surface coating and physical confinement, which will provide significant insights for in vivo cell migration within a 3D ECM.

Probing Fluctuations and Avalanches in the Cytoskeleton with Active Micropost Arrays

The cellular actomyosin network plays an important role in a wide range of cell behavior, including motility, morphology and mechanotransduction. Although the building blocks of actomyosin networks are well understood at the molecular level, the picture connecting these microscale units to cell-scale behavior remains incomplete. Here we present results using active micropost array detectors (AMPADS) to characterize the dynamical fluctuations and local rheology of cellular actomyosin networks in detail. AMPADS are poly(dimethylsiloxane) (PDMS) micropillar arrays with embedded magnetic nanowires that enable mechanical actuation of an adherent cell. We classified the cell-associated microposts based on their traction force, and identified two distinct populations, one containing posts coupled to stress fibers and the other containg posts coupled to the actomyosin cortex. Both groups show weak power law rheology, but we find that the fluctuations of stress fiber-associated posts are more active and highly anisotropic in comparison to the cortically-associated posts. Notably, the fluctuations of both populations are highly heterogenous and resemble a Lévy process. Specifically, the mean-squared displacements of both types of posts display broadly distributed amplitudes and super-diffusive behavior. We find that the amplitude distribution is fat-tailed, and is dominated by a corresponding distribution of intermittent, large step-like displacements, resembling avalanches and earthquakes in physical systems. Our regular array of detectors also allows us to determine the spatial extent and average symmetry of the largest rearrangements in the cortex, which are both spatially and temporally complex, resembling those in plastic solids. These results suggest that the dynamics of the cellular actomyosin network resembles what is seen in soft matter systems such as sandpiles and foams, where constituent components self-organize into marginally stable, plastic networks, with significant implications for future models of the cortex.

Hyaluronan-CD44 interactions mediate contractility and migration in periodontal ligament cells

ABSTRACTThe role of hyaluronan (HA) in periodontal healing has been speculated via its interaction with the CD44 receptor. While HA-CD44 interactions have previously been implicated in numerous cell types; effect and mechanism of exogenous HA on periodontal ligament (PDL) cells is less clear. Herein, we examine the effect of exogenous HA on contractility and migration in human and murine PDL cells using arrays of microposts and time-lapse microscopy. Our findings observed HA-treated human PDL cells as more contractile and less migratory than untreated cells. Moreover, the effect of HA on contractility and focal adhesion area was abrogated when PDL cells were treated with Y27632, an inhibitor of rho-dependent kinase, but not when these cells were treated with ML-7, an inhibitor of myosin light chain kinase. Our results provide insight into the mechanobiology of PDL cells, which may contribute towards the development of therapeutic strategies for periodontal healing and tissue regeneration.

Cell traction force in a confined microenvironment with double-sided micropost arrays.

Three-dimensional (3D) cell migrations are regulated by force interactions between cells and a 3D extracellular matrix (ECM). Mapping the 3D traction force generated by cells on the surrounding ECM with controlled confinement and contact area will be useful in understanding cell migration. In this study, double-sided micropost arrays were fabricated. The cell traction force was mapped by microposts on the top and bottom of opposing surfaces with a controlled separating distance to create different confinements. The density of micropost arrays was modified to investigate the effect of cell contact area on 3D traction force development. Using MC3T3-E1 osteoblastic cells, the leading traction force was found to increase with additional contact surface on the top. Summing force vectors on both surfaces, a large force imbalance was found from the leading to trailing regions for fast migrating cells. With 10 μm separation and densely arranged microposts, the traction force on the top surface was the largest at 28.6 ± 2.5 nN with the highest migration speed of 0.61 ± 0.07 μm min−1. Decreasing the density of the top micropost arrays resulted in a reduced traction force on the top and lower migration speed. With 15 μm separation, the cell traction force on the top and migration speed further decreased simultaneously. These results revealed traction force development on 3D ECM with varied degrees of confinement and contact area, which is important in regulating 3D cell migration.

Microfluidic viscometry using magnetically actuated micropost arrays.

Here we describe development of a microfluidic viscometer based on arrays of magnetically actuated micro-posts. Quantitative viscosities over a range of three orders of magnitude were determined for samples of less than 20 μL. This represents the first demonstration of quantitative viscometry using driven flexible micropost arrays. Critical to the success of our system is a comprehensive analytical model that includes the mechanical and magnetic properties of the actuating posts, the optical readout, and fluid-structure interactions. We found that alterations of the actuator beat shape as parameterized by the dimensionless “sperm number” must be taken into account to determine the fluid properties from the measured actuator dynamics. Beyond our particular system, the model described here can provide dynamics predictions for a broad class of flexible microactuator designs. We also show how the model can guide the design of new arrays that expand the accessible range of measurements.