Search

Quasi‐2D Ruddlesden–Popper perovskite (RPP) have surfaced as a promising candidate for light emitting diodes (LEDs) due to its outstanding optoelectronic properties. However, a reliable approach for patterning RPPs remains elusive due to the use of polar solvents in lithographic processes, which can damage the RPP. Here, a reliable and damage‐free dry micropatterning method of RPPs is reported, which also offers a cost/time advantage compared to conventional patterning methods. The sharp edges of high aspect ratio silicon micropillars are used to cut RPPs to a pre‐defined shape and then the cut RPPs are delaminated to obtain a patterned array of RPPs. The resultant patterned array exhibits no sign of degradation or discernable difference between adjacent pixels, achieving a ≈100% yield. The obtained array is utilized to fabricate a pixelated LED where a sharp electroluminescence (EL) spectrum peaking at 410 nm with full‐width‐at‐half‐maximum (FWHM) of 10 nm is observed. The pixelated devices demonstrate the potential to suppress EQE drops as the pixel size decreases, attributed to both the damage‐free micropatterning process and the defect tolerance of RPPs. Moreover, further improvements of the patterning method are demonstrated to avoid parasitic emission and suggest a promising strategy to fabricate pixel‐accessible micro‐LEDs. [ABSTRACT FROM AUTHOR]

Bioinspired micropillar adhesives have shown broad application prospects in space capture and docking, due to their strong adhesion, good environmental adaptability, and reusability. However, when performing space missions, unavoidable contact collision with target objects may cause large deformation of the micropillars, resulting in the loss of adhesion ability. This study reports a novel micropillar adhesive through the in‐plane combination of micropillars (IPCM) with different aspect ratios, consisting of small pillars for retaining strong adhesion and large ones for resisting overload‐induced adhesion failure. It is demonstrated that the IPCM array can still maintain 85% of the adhesion peak after static large deformation compared to a general micropillar array composed of the same pillars. The impact of element size and layout of the IPCM, as well as detachment velocity on adhesion performance under high preload is discussed. Furthermore, finite element contact analysis qualitatively reproduces the experimentally observed micropillar deformations and attributes the overload‐induced adhesion failure to the redistribution of surface normal stress. Finally, the potential application of the IPCM in dynamic capture is demonstrated on different objects. The proposed IPCM opens up new design concepts for practical applications of bioinspired adhesives in space capture and docking. [ABSTRACT FROM AUTHOR]

It is now well established that, upon decreasing system sizes down to a few μ m or below, the nature of plasticity of metallic materials is changing. Two important features of this small-sizes plasticity are two size effects, which can be summed up as "smaller is stronger" and "smaller is wilder", this last observation meaning that the jerkiness of plastic deformation becomes prominent at small enough system sizes. In FCC and HCP materials, this is now rather well understood within the framework of obstacle-controlled plasticity, from the key role of a scaling ratio between the system size L and an internal scale l mainly dictated by dislocation patterning in pure materials, or by the nature of extrinsic disorder in alloys. The situation is more complex in BCC materials, for which screw dislocation motion becomes lattice-controlled, i.e. is thermally activated, below a transition temperature T a . Therefore, in small-sized BCC systems, temperature, size and strain-rate effects combine to give rise to a complex landscape. We show, from an analysis of the literature as well as micropillar compression tests on Molybdenum performed with different sample sizes, under different temperatures and different applied strain-rates, that (i) near or above T a , the plasticity of pure BCC metals is athermal and obstacle-controlled, much like at bulk scales, therefore mimicking that of pure FCC metals; (ii) below T a and for sample sizes larger than ∼ 1 μ m, BCC plasticity becomes lattice-controlled, this damping dislocation avalanches and thus reducing wildness; but (iii) for very small systems, still below T a , the role of screw dislocations on plasticity vanishes, i.e. is no more lattice-controlled, opening again the door for wild plastic fluctuations and jerkiness. [ABSTRACT FROM AUTHOR]

Abstract Bioinspired micropillar adhesives have shown broad application prospects in space capture and docking, due to their strong adhesion, good environmental adaptability, and reusability. However, when performing space missions, unavoidable contact collision with target objects may cause large deformation of the micropillars, resulting in the loss of adhesion ability. This study reports a novel micropillar adhesive through the in‐plane combination of micropillars (IPCM) with different aspect ratios, consisting of small pillars for retaining strong adhesion and large ones for resisting overload‐induced adhesion failure. It is demonstrated that the IPCM array can still maintain 85% of the adhesion peak after static large deformation compared to a general micropillar array composed of the same pillars. The impact of element size and layout of the IPCM, as well as detachment velocity on adhesion performance under high preload is discussed. Furthermore, finite element contact analysis qualitatively reproduces the experimentally observed micropillar deformations and attributes the overload‐induced adhesion failure to the redistribution of surface normal stress. Finally, the potential application of the IPCM in dynamic capture is demonstrated on different objects. The proposed IPCM opens up new design concepts for practical applications of bioinspired adhesives in space capture and docking.

Efficient bone repair relies on biomaterials that exhibit superior biocompatibility and enhanced osteogenic capabilities. In recent, micropatterning technology has emerged as a robust strategy for directing stem cell differentiation, offering a notable shift from traditional chemical induction or genetic modification methods by providing environmental cues to regulate cellular behavior. Among the various types of microstructures, micropillars have garnered significant attention due to their adjustable properties and high‐throughput experimental capabilities. These structures play a pivotal role in governing nuclear deformation and deciphering cellular responses to biomechanical cues. In this comprehensive review, the influence of micropillars on mesenchymal stem cells (MSCs), drawing upon two decades of research findings is critically assessed. The study comprehensively evaluates how micropillar substrates impact crucial cellular processes such as deformation, migration, adhesion, and ultimately, MSC fate determination toward an osteogenic lineage. By synthesizing and analyzing past studies, the potential regulatory mechanisms through which micropillars modulate MSC behavior are aimed to be elucidated. Furthermore, the utility of micropillars as a cutting‐edge biomechanical detection tool and platform for investigating cellular behaviors is explored. By projecting future applications, the review highlights the growing significance of micropillars in the dynamic field of biomechanics, underscoring their transformative potential in tissue engineering and regenerative medicine. [ABSTRACT FROM AUTHOR]

The proportion of hospital‐acquired medical device infections caused by pathogenic, multi‐drug resistant Candida species occurs in up to 10% of implantations. In this study, a unique antifungal micro‐pillared titanium surface pattern is developed, which demonstrates both fungicidal and fungistatic activity, persistently deterring biofilm formation by Candida albicans and multi‐drug resistant Candida auris fungi for up to 7 days. The Ti micropillars of 3.5 µm height are fabricated using maskless inductively coupled plasma reactive ion etching. The micro‐textured surface consistently kills ≈50% of Candida spp. irreversibly attached cells and prevent the proliferation of the remaining cells by inducing programmed cell death. Proteomic analysis reveals that Candida cells undergo extensive metabolic stress, preventing the transformation from yeast to the filamentous/hyphal cell phenotype that is essential for establishing a typical in vitro biofilm. The mechanical stress imparted following interaction with the micropillars injures attaching cells and induces apoptosis whereby the Candida cells are unable to be revived in a non‐stress environment. These findings shed new insight toward the design of durable antifungal surfaces that prevent biofilm formation by pathogenic, multi‐drug resistant yeasts. [ABSTRACT FROM AUTHOR]

Vibration-induced flow (VIF), in which a mean flow is induced around a microstructure by applying periodic vibrations, is increasingly used as an active flow-control technique at the microscale. In this study, we have developed a microdevice that actively controls the VIF patterns using elastic membrane protrusions (microballoons) actuated by pneumatic pressure. This device enables on-demand spatial and temporal fluid manipulation using a single device that cannot be achieved using a conventional fixed-structure arrangement. We successfully demonstrated that the device achieved displacements of up to 38 µm using the device within a pressure range of 0 to 30 kPa, indicating the suitability of the device for microfluidic applications. Using this active microballoon array, we demonstrated that the device can actively manipulate the flow field and induce swirling flows. Furthermore, we achieved selective actuation of the microballoon using this system. By applying air pressure from a multi-input channel system through a connection tube, the microballoons corresponding to each air channel can be selectively actuated. This enabled precise control of the flow field and periodic switching of the flow patterns using a single chip. In summary, the proposed microdevice provides active control of VIF patterns and has potential applications in advanced microfluidics, such as fluid mixing and particle manipulation. [ABSTRACT FROM AUTHOR]

Abstract Efficient bone repair relies on biomaterials that exhibit superior biocompatibility and enhanced osteogenic capabilities. In recent, micropatterning technology has emerged as a robust strategy for directing stem cell differentiation, offering a notable shift from traditional chemical induction or genetic modification methods by providing environmental cues to regulate cellular behavior. Among the various types of microstructures, micropillars have garnered significant attention due to their adjustable properties and high‐throughput experimental capabilities. These structures play a pivotal role in governing nuclear deformation and deciphering cellular responses to biomechanical cues. In this comprehensive review, the influence of micropillars on mesenchymal stem cells (MSCs), drawing upon two decades of research findings is critically assessed. The study comprehensively evaluates how micropillar substrates impact crucial cellular processes such as deformation, migration, adhesion, and ultimately, MSC fate determination toward an osteogenic lineage. By synthesizing and analyzing past studies, the potential regulatory mechanisms through which micropillars modulate MSC behavior are aimed to be elucidated. Furthermore, the utility of micropillars as a cutting‐edge biomechanical detection tool and platform for investigating cellular behaviors is explored. By projecting future applications, the review highlights the growing significance of micropillars in the dynamic field of biomechanics, underscoring their transformative potential in tissue engineering and regenerative medicine.

Abstract The proportion of hospital‐acquired medical device infections caused by pathogenic, multi‐drug resistant Candida species occurs in up to 10% of implantations. In this study, a unique antifungal micro‐pillared titanium surface pattern is developed, which demonstrates both fungicidal and fungistatic activity, persistently deterring biofilm formation by Candida albicans and multi‐drug resistant Candida auris fungi for up to 7 days. The Ti micropillars of 3.5 µm height are fabricated using maskless inductively coupled plasma reactive ion etching. The micro‐textured surface consistently kills ≈50% of Candida spp. irreversibly attached cells and prevent the proliferation of the remaining cells by inducing programmed cell death. Proteomic analysis reveals that Candida cells undergo extensive metabolic stress, preventing the transformation from yeast to the filamentous/hyphal cell phenotype that is essential for establishing a typical in vitro biofilm. The mechanical stress imparted following interaction with the micropillars injures attaching cells and induces apoptosis whereby the Candida cells are unable to be revived in a non‐stress environment. These findings shed new insight toward the design of durable antifungal surfaces that prevent biofilm formation by pathogenic, multi‐drug resistant yeasts.

Bioinspired structured adhesives exhibit promising applications in the fields of treatment, robotics, precision manufacturing, etc. The robust and strong adhesion and friction of the adhesive are highly required due to the easily damaged submicron structures. Here, we develop a simple method to prepare micropillar arrays with core–shell structure (PCS), which realizes a 2.03-fold adhesion and a 1.78-fold friction as compared to the pure PDMS micropillars array (PP). Moreover, the adhesion of PCS shows excellent durability over 100 repeating cycles of attachment/detachment. The enhancement can be attributed to the transfer of the maximum stress from the edge of the contact interface to the central region due to the existence of the core–shell structure. The study presents a novel approach for designing robust structured adhesives with strong adhesion and friction, which can be extended to other material combinations. [ABSTRACT FROM AUTHOR]

The present study investigates different design strategies to produce non-wettable micropatterned surfaces. In addition to the classical method of measuring the contact angle, the non-wettability is also discussed by means of the immersion test. Inspired by non-wettable structures found in nature, the effects of features such as reentrant cavities, micropillars, and overhanging layers are studied. We show that a densely populated array of small diameter cavities exhibits superior non-wettability, with 65% of the cavities remaining intact after 24 h of full immersion in water. In addition, it is suggested that the wetting transition time is influenced by the length of the overhanging layer as well as by the number of columns within the cavity. Our findings indicate a non-wetting performance that is three times longer than previously reported in the literature for a small, densely populated design with cavities as small as 10 μm in diameter. Such properties are particularly beneficial for neural implants as they may reduce the interface between the body fluid and the solid state, thereby minimiing the inflammatory response following implantation injury. In order to assess the effectiveness of this approach in reducing the immune response induced by neural implants, further in vitro and in vivo studies will be essential. [ABSTRACT FROM AUTHOR]

The role of a ∑3{112} incoherent twin boundary (ITB) on the shear stress of Cu at the micron scale has been investigated through microcompression of bi-crystalline pillars containing ITB, as well as single-crystalline pillars, in two different compression directions. The Cu sample containing ITBs was synthesized using magnetron sputtering on a sapphire substrate. Firstly, pillars along [111] compression direction were milled on the film surface. As multiple slip systems were activated upon loading, the dislocation-ITB interaction in this direction was dominated by the dislocation–dislocation interactions. Another set of pillars was milled from the side of the film (in the thickness of the film) in a nominally [134¯] compression direction. Compression in this direction activated a single slip in each grain, which facilitated the investigation of the interaction between dislocations and ITBs. Post-mortem images showed that slip traces were not distinctly connected at the boundary unlike ideal slip transmission in pillars containing a coherent twin boundary. Moreover, bi-crystalline pillars in the single slip direction are stronger than single-crystalline pillars. The observations indicate that ITBs are not impenetrable for dislocations, but the boundary demonstrates some resistance to transmission.

Structural colors of 2D gratings are iridescent, color-tunable, and never fade, which renders them appealing for anti-counterfeiting applications. However, for advanced security, it still remains a challenge to completely hide the encrypted color patterns and reveal them on demand. In this work, a water-responsive photonic grating consisting of a micropillar array and a hydrogel overcoat with a similar refractive index, termed “hydrocipher”, is presented. The joint effect of stimuli-reversible refractive-index (mis)match and reconfigurable grating-based diffraction coloration enables a complete encryption of the structural color and rapid decryption. The photonic structure shows a strong iridescence due to the angle-dependent diffraction when the hydrogel overcoat is swollen from water. Upon drying, the micropillars bend and the refractive index contrast disappears, which dramatically lessens the diffraction intensity and renders the surface highly transparent. The dehydrated-to-hydrated state transition can occur within 1 s, enabling fast decryption. The color switching is highly reversible over a prolonged hydration/dehydration cycle, and the dehydrated hydrogel layer protects the delicate micropillar array from external mechanical stress. The creative combination of hydrogel material and the 2D grating structure offers a new and simple strategy for realizing reversible, durable, and fast-response cryptography with potentially broad impact on the anti-counterfeiting technology market.

Micropillar compression is a method of choice to understand mechanics at small scale. It is mainly studied with electron microscopy or white‐beam micro‐Laue X‐ray diffraction. The aim of the present article is to show the possibilities of the use of diffraction with a coherent X‐ray beam. InSb micropillars in epitaxy with their pedestals (i.e. their support) are studied in situ during compression. Firstly, an experiment using a collimated beam matching the pillar size allows determination of when the sample enters the plastic regime, independently of small defects induced by experimental artefacts. A second experiment deals with scanning X‐ray diffraction maps with a nano‐focused beam; despite the coherence of the beam, the contributions from the pedestal and from the micropillar in the diffraction patterns can be separated, making possible a spatially resolved study of the plastic strain fields. A quantitative measurement of the elastic strain field is nevertheless hampered by the fact that the pillar diffracts at the same angles as the pedestal. Finally, no image reconstructions were possible in these experiments, either in situ due to a blurring of the fringes during loading or post‐mortem because the defect density after yielding was too high. However, it is shown how to determine the elastic bending of the pillar in the elastic regime. Bending angles of around 0.3° are found, and a method to estimate the sample's radius of curvature is suggested. [ABSTRACT FROM AUTHOR]

In this work, we report investigations of the swimming behaviour of Paramecium tetraurelia, a unicellular microorganism, in micro-engineered pools that are decorated with thousands of cylindrical pillars. Two types of contact interactions are measured, either passive scattering of Paramecium along the obstacle or avoiding reactions (ARs), characterized by an initial backward swimming upon contact, followed by a reorientation before resuming forward motion. We find that ARs are only mechanically triggered approximately 10% of the time. In addition, we observe that only a third of all ARs triggered by contact are instantaneous while two-thirds are delayed by approximately 150 ms. These measurements are consistent with a simple electrophysiological model of mechanotransduction composed of a strong transient current followed by a persistent one upon prolonged contact. This is in apparent contrast with previous electrophysiological measurements where immobilized cells were stimulated with thin probes, which showed instantaneous behavioural responses and no persistent current. Our findings highlight the importance of ecologically relevant approaches to unravel the motility of mechanosensitive microorganisms in complex environments.

Silicon carbide (SiC) has a variety of applications because of its favorable chemical stability and outstanding physical characteristics, such as high hardness and high rigidity. In this study, a femtosecond laser with a spiral scanning radial offset of 5 μm and a spot radius of 6 μm is utilized to process micropillars on a SiC plate. The influence of pulsed laser beam energies and laser translation velocities on the micropillar profiles, dimensions, surface roughness Ra, and material removal capability (MRC) of micropillars was investigated. The processing results indicate that the micropillar has the best perpendicularity, with a micropillar bottom angle of 75.59° under a pulsed beam energy of 50 μJ in the range of 10–70 μJ, with a pulsed repetition rate of 600 kHz and a translation velocity of 0.1 m/s. As the laser translation velocity increases between 0.2 m/s and 1.0 m/s under a fixed pulsed beam energy of 50 μJ and a constant pulsed repetition rate of 600 kHz, the micropillar height decreases from 119.88 μm to 81.79 μm, with the MRC value increasing from 1.998 μm3/μJ to 6.816 μm3/μJ, while the micropillar bottom angle increases from 68.87° to 75.59°, and the Ra value diminishes from 0.836 μm to 0.341 μm. It is suggested that a combination of a higher pulsed laser beam energy with a faster laser translation speed is recommended to achieve micropillars with the same height, as well as an improved processing efficiency and surface finish. [ABSTRACT FROM AUTHOR]

Vibration-induced flow (VIF), in which a mean flow is induced around a microstructure by applying periodic vibrations, is increasingly used as an active flow-control technique at the microscale. In this study, we have developed a microdevice that actively controls the VIF patterns using elastic membrane protrusions (microballoons) actuated by pneumatic pressure. This device enables on-demand spatial and temporal fluid manipulation using a single device that cannot be achieved using a conventional fixed-structure arrangement. We successfully demonstrated that the device achieved displacements of up to 38 µm using the device within a pressure range of 0 to 30 kPa, indicating the suitability of the device for microfluidic applications. Using this active microballoon array, we demonstrated that the device can actively manipulate the flow field and induce swirling flows. Furthermore, we achieved selective actuation of the microballoon using this system. By applying air pressure from a multi-input channel system through a connection tube, the microballoons corresponding to each air channel can be selectively actuated. This enabled precise control of the flow field and periodic switching of the flow patterns using a single chip. In summary, the proposed microdevice provides active control of VIF patterns and has potential applications in advanced microfluidics, such as fluid mixing and particle manipulation.

The present study investigates different design strategies to produce non-wettable micropatterned surfaces. In addition to the classical method of measuring the contact angle, the non-wettability is also discussed by means of the immersion test. Inspired by non-wettable structures found in nature, the effects of features such as reentrant cavities, micropillars, and overhanging layers are studied. We show that a densely populated array of small diameter cavities exhibits superior non-wettability, with 65% of the cavities remaining intact after 24 h of full immersion in water. In addition, it is suggested that the wetting transition time is influenced by the length of the overhanging layer as well as by the number of columns within the cavity. Our findings indicate a non-wetting performance that is three times longer than previously reported in the literature for a small, densely populated design with cavities as small as 10 μm in diameter. Such properties are particularly beneficial for neural implants as they may reduce the interface between the body fluid and the solid state, thereby minimiing the inflammatory response following implantation injury. In order to assess the effectiveness of this approach in reducing the immune response induced by neural implants, further in vitro and in vivo studies will be essential.

The vibration-induced flow (VIF), in which a mean flow is induced by the interaction between the system vibration and micro-structures, has been studied as a fluid/particle micro-manipulation method that does not require an external pump. While the use of VIF with a wide variety of vibrations is expected to realize sophisticated fluid manipulation, numerical tools to predict these unsteady flows remain difficult. In this study, we have performed a numerical simulation of VIF with different vibrations and micropillar cross-sections. A proposed numerical model, which directly solves the continuity and Navier-Stokes equations in the coordinate system moving with the vibrating micropillar, enables us to avoid the introduction of a moving boundary, and therefore has a significant advantage in numerical stability and accuracy. The immersed boundary technique allows us to embed arbitrary complex micro-structures in the Cartesian computational domain without requiring boundary-fitted meshes for each geometry. The dependencies of characteristics of flow on vibration parameters, such as vibration frequency, amplitude, direction, and the shape of micro-structures, were investigated and compared with the experimental results obtained by the particle image velocimetry (PIV) measurement. Excellent agreement between the numerical and experimental results validates that the present numerical approach can be a powerful tool to design functional VIF systems, such as mixing, particle/cell transport, trapping, and separation.

Soft-lithography is widely used to fabricate microstructured surfaces on plastics and elastomers for designable physical properties such as wetting and adhesions. However, it remains a big challenge to construct high-aspect-ratio microstructures on the surface of hydrogels due to the difficulty in demolding from the gel with low strength and stiffness. Demonstrated here is the engineering of tough hydrogels by soft-lithography to form well-defined micropillars. The mechanical properties of poly(acrylamide-co-methacrylic acid) hydrogels with dense hydrogen-bond associations severely depend on temperature, with Young's modulus increasing from 8.1 MPa at 15 °C to 821.8 MPa at -30 °C, enabling easy demolding at low temperatures. Arrays of micropillars are maintained on the surface of the gel, and can be used at room temperature when the gel restores soft and stretchable. The hydrogel also exhibits good shape-memory property, favoring tailoring the morphology with a switchable tilt angle of micropillars. Consequently, the hydrogel shows tunable wetting and adhesion properties, as manifested by varying contact angles and adhesion strengths. These surface properties can also be tuned by geometry and arrangement of micropillars. This facile strategy by harnessing tunable viscoelasticity of supramolecular hydrogels should be applicable to other soft materials, and broaden their applications in biomedical and engineering fields., (© 2024 Wiley‐VCH GmbH.)

Stainless steel 304 (SS304) experiences corrosion when it is exposed to a saline atmosphere, which attains severity due to its high surface wettability. Topographical modification of metallic surfaces is an effective route to reduce wettability and thereby mitigate liquid-mediated corrosion. In this work, topographical modification of stainless steel 304 flat surface in the form of micropillars was done (pillar width: 100 μm, inter-pillar distance: 100 μm and height: 80 μm). Micropillars were fabricated by a chemical etching process. Wetting and corrosion of the micropillars was studied over long-time duration in comparison with flat surface, before and after intermittent and continuous exposures to saline water for 168 h. Wetting was characterized by measuring the static water contact angle on the test surfaces and their corrosion by electrochemical polarization tests (electrolyte: 3.5 wt.% sodium chloride solution). The relationship between the nature of wetting of the test surfaces and their corrosion was examined. Micropillars showed predominantly composite wetting over a long time, which imparted an effective resistance against corrosion over a long time to the SS304 surface. When compared to the flat surface, the corrosion rates of the micropillars were lower by two orders of magnitude, prior to and also upon long-time contact with the NaCl solution. Micropillars lowered corrosion due to composite wetting, i.e., solid-liquid-air interface that reduced the area that was in contact with the NaCl solution. The efficiency of corrosion inhibition (η) of micropillars was 88% before long-time contact, 84% after intermittent contact, and 77% after continuous contact with NaCl solution. Topographical modification in the form of micropillars that can impart composite wetting is an effective route to induce long-term anticorrosion ability to the SS304 surface. [ABSTRACT FROM AUTHOR]

We demonstrate the use of an optical knife-edge technique in micropillar-based microfluidic chips, which are intended for viscosity measurements. Our demonstrations rely on the use of a single photodiode, offering a significant cost, form factor and computational efficiency advantage over CMOS camera detection. Our readout methodology involves the utilization of pillars as waveguides, such that a laser is coupled from the backside of the fluidic chip, while the output light is monitored with a photodiode using knife-edge detection. The bending of the pillars due to applied flow alters the observed light intensity, from which the bending angle could be deduced. We present ray-tracing simulations and conduct proof-of-concept experiments on a micropillar-based microfluidic chip. The bending of the pillar observed with the knife-edge technique matches well with that observed with a camera. With further development, the proposed methodology can be employed as a compact and low-cost point-of-care instrument for real-time viscosity measurements. [Display omitted] • Optical knife-edge technique is demonstrated on micropillar-based viscometer chips. • Photodiode is used to monitor the light output from micropillars acting as waveguide. • Bending of micropillars can be deduced from the the observed light intensity change. • Calculated pillar bending with knife-edge matches well with that observed with camera. • Photodiode provides material, computational cost and size advantages over camera. [ABSTRACT FROM AUTHOR]

Shear stress, pressure, and flow rate are fluid dynamic parameters that can lead to changes in the morphology, proliferation, function, and survival of many cell types and have a determinant impact on tissue function and viability. Microfluidic devices are promising tools to investigate these parameters and fluid behaviour within different microchannel geometries. This study discusses and analyses different designed microfluidic channel geometries regarding the influence of fluid dynamic parameters on their microenvironment at specified fluidic parameters. The results demonstrate that in the circular microchamber, the velocity and shear stress profiles assume a parabolic shape with a maximum velocity occurring in the centre of the chamber and a minimum velocity at the walls. The longitudinal microchannel shows a uniform velocity and shear stress profile throughout the microchannel. Simulation studies for the two geometries with three parallel microchannels showed that in proximity to the micropillars, the velocity and shear stress profiles decreased. Moreover, the pressure is inversely proportional to the width and directly proportional to the flow rate within the microfluidic channels. The simulations showed that the velocity and wall shear stress indicated different values at different flow rates. It was also found that the width and height of the microfluidic channels could affect both velocity and shear stress profiles, contributing to the control of shear stress. The study has demonstrated strategies to predict and control the effects of these forces and the potential as an alternative to conventional cell culture as well as to recapitulate the cell- and organ-specific microenvironment. [ABSTRACT FROM AUTHOR]

We report the enhancement of the optical emission between 850 and 1400 nm of an ensemble of silicon mono-vacancies (VSi), silicon and carbon divacancies (VCVSi), and nitrogen vacancies (NCVSi) in an n-type 4H-SiC array of micropillars. The micropillars have a length of ca. 4.5 μm and a diameter of ca. 740 nm, and were implanted with H+ ions to produce an ensemble of color centers at a depth of approximately 2 μm. The samples were in part annealed at different temperatures (750 and 900 °C) to selectively produce distinct color centers. For all these color centers we saw an enhancement of the photostable fluorescence emission of at least a factor of 6 using micro-photoluminescence systems. Using custom confocal microscopy setups, we characterized the emission of VSi measuring an enhancement by up to a factor of 20, and of NCVSi with an enhancement up to a factor of 7. The experimental results are supported by finite element method simulations. Our study provides the pathway for device design and fabrication with an integrated ultra-bright ensemble of VSi and NCVSi for in vivo imaging and sensing in the infrared.

Parylene-based implants or coatings introduce surfaces suffering from bacteria colonization. Here, we synthesized polyvinylpyrrolidone-stabilized selenium nanoparticles (SeNPs) as the antibacterial agent, and various approaches are studied for their reproducible adsorption, and thus the modification of parylene-C–coated glass substrate. The nanoparticle deposition process is optimized in the nanoparticle concentration to obtain evenly distributed NPs on the flat parylene-C surface. Moreover, the array of parylene-C micropillars is fabricated by the plasma etching of parylene-C on a silicon wafer, and the surface is modified with SeNPs. All designed surfaces are tested against two bacterial pathogens, Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive). The results show no antibacterial effect toward S. aureus, while some bacteriostatic effect is observed for E. coli on the flat and microstructured parylene. However, SeNPs did not enhance the antibacterial effect against both bacteria. Additionally, all designed surfaces show cytotoxic effects toward mesenchymal stem cells at high SeNP deposition. These results provide valuable information about the potential antibacterial treatment of widely used parylene-C in biomedicine.

Acoustic wave sensors are being developed for many emerging applications such as in semiconductor fabrication, biological diagnostics and polymer characterization. Traditional acoustic wave sensing devices such as quartz crystal microbalance (QCM) rely on polymer thin films coated on quartz plates to detect chemical and biological agents. It has been found that significant sensitivity enhancement of QCM devices can be achieved by simply attaching a polymer micropillar film onto the QCM substrate (QCM-P) to enable a unique coupled resonance between the micropillars and quartz substrate. In the present work, an equivalent circuit model integrating mechanical vibration of micropillars and electrical load impedance of piezoelectric substrate was developed to predict the frequency shift and Q-factor of the QCM-P devices when operating in air and liquid environments. In the model, the vibration of micropillars was solved simultaneously with the liquid loading on the pillar surface. The resultant hydraulic force was integrated into the circuit model to predict the load impedance on the sensor surface. The developed model was validated by experimental results for QCM-P devices operating in air and water with different micropillar heights. It will serve as a powerful tool to predict the performance of the QCM-P devices for different applications.

Silicon carbide (SiC) has a variety of applications because of its favorable chemical stability and outstanding physical characteristics, such as high hardness and high rigidity. In this study, a femtosecond laser with a spiral scanning radial offset of 5 μm and a spot radius of 6 μm is utilized to process micropillars on a SiC plate. The influence of pulsed laser beam energies and laser translation velocities on the micropillar profiles, dimensions, surface roughness Ra, and material removal capability (MRC) of micropillars was investigated. The processing results indicate that the micropillar has the best perpendicularity, with a micropillar bottom angle of 75.59° under a pulsed beam energy of 50 μJ in the range of 10–70 μJ, with a pulsed repetition rate of 600 kHz and a translation velocity of 0.1 m/s. As the laser translation velocity increases between 0.2 m/s and 1.0 m/s under a fixed pulsed beam energy of 50 μJ and a constant pulsed repetition rate of 600 kHz, the micropillar height decreases from 119.88 μm to 81.79 μm, with the MRC value increasing from 1.998 μm3/μJ to 6.816 μm3/μJ, while the micropillar bottom angle increases from 68.87° to 75.59°, and the Ra value diminishes from 0.836 μm to 0.341 μm. It is suggested that a combination of a higher pulsed laser beam energy with a faster laser translation speed is recommended to achieve micropillars with the same height, as well as an improved processing efficiency and surface finish.

Superomniphobic surfaces inspired by nature have been studied for decades. Recently, the development of liquid-repelling surfaces has moved from the fabrication of artificial structures to real applications that address friction associated with clothes, paper, and skin. To have superoleophobicity, re-entrant structures such as mushrooms or inverse trapezoids have been suggested. However they can be mechanically fragile, especially under shear stress, because the bottom region is narrow. Here, a facile method to obtain new re-entrant structures is proposed, namely, micro-hyperbola structures, by wetting a photocurable viscous liquid around micropillars by capillary force. It is demonstrated that the formation of the hyperbola structures depends on the spacing ratio between micropillars, and the formation mechanism is explained with a simple model. The micro-hyperbola structure demonstrates robust omniphobicity even after rubbing and abrasion tests. The advantage of the wide fabrication range and the robust superoleophobicity of micro-hyperbola structures enable the uses in practical superomniphobic applications that undergo shear forces. [ABSTRACT FROM AUTHOR]

In this study, we propose using a polymethylmethacrylate (PMMA) micropillar-based quartz crystal microbalance (micropillar-QCM) sensor for simultaneous measurement of exhaled breath temperature (EBT) and relative humidity (RH). The micropillar-QCM device, fabricated by nanoimprinting lithography, exhibits enhanced sensitivity due to the unique resonance phenomena formed between the vibrating micropillars and quartz substrate. The resonance frequency of the micropillar-QCM shifts in response to changes in the mechanical property – Young's modulus of PMMA due to the temperature and humidity variations. Experimental results demonstrate that the frequency shift is linearly dependent on the temperature change within the range of 25–35 °C and follows a power law relationship with relative humidity (RH) within the range of 50%− 90%. The sensor exhibits excellent sensitivity in temperature and humidity measurements with limit of detections (LODs) of 0.09 °C and 0.35% respectively. In addition, the temperature change has little effect on the bandwidth of the signal of micropillar-QCM while its bandwidth decreases linearly with the increasing relative humidity. As a result, the combination of the resonance frequency shift and bandwidth of the micropillar-QCM sensor can be utilized to predict the temperature and relative humidity simultaneously. The finding highlights the potential of the micropillar-QCM sensor for applications requiring accurate and real-time temperature and humidity measurements, such as breath analysis, environmental monitoring, and healthcare. [Display omitted] • A novel PMMA micropillars on QCM sensing device is fabricated, achieving exceptional sensitivity for measurement of temperature and relative humidity. • The combination of the resonance frequency shift and bandwidth of the micropillar-QCM sensor can simultaneously measure temperature and relative humidity. [ABSTRACT FROM AUTHOR]

Stainless steel 304 (SS304) experiences corrosion when it is exposed to a saline atmosphere, which attains severity due to its high surface wettability. Topographical modification of metallic surfaces is an effective route to reduce wettability and thereby mitigate liquid-mediated corrosion. In this work, topographical modification of stainless steel 304 flat surface in the form of micropillars was done (pillar width: 100 μm, inter-pillar distance: 100 μm and height: 80 μm). Micropillars were fabricated by a chemical etching process. Wetting and corrosion of the micropillars was studied over long-time duration in comparison with flat surface, before and after intermittent and continuous exposures to saline water for 168 h. Wetting was characterized by measuring the static water contact angle on the test surfaces and their corrosion by electrochemical polarization tests (electrolyte: 3.5 wt.% sodium chloride solution). The relationship between the nature of wetting of the test surfaces and their corrosion was examined. Micropillars showed predominantly composite wetting over a long time, which imparted an effective resistance against corrosion over a long time to the SS304 surface. When compared to the flat surface, the corrosion rates of the micropillars were lower by two orders of magnitude, prior to and also upon long-time contact with the NaCl solution. Micropillars lowered corrosion due to composite wetting, i.e., solid-liquid-air interface that reduced the area that was in contact with the NaCl solution. The efficiency of corrosion inhibition (η) of micropillars was 88% before long-time contact, 84% after intermittent contact, and 77% after continuous contact with NaCl solution. Topographical modification in the form of micropillars that can impart composite wetting is an effective route to induce long-term anticorrosion ability to the SS304 surface.

When a water droplet on a micropillar-structured hydrophobic surface is submitted to gradually increased pressure, the Cassie-Baxter wetting state transforms into the Wenzel wetting state once the pressure exceeds a critical value. It has been assumed that the reverse transition (Wenzel-to-Cassie-Baxter wetting state) cannot happen spontaneously after the pressure has been removed. In this paper, we report a new wetting-state transition. When external pressure is exerted on a droplet in the Cassie-Baxter wetting state on textured surfaces with high micropillars to trigger the breakdown of this wetting state, the droplet penetrates the micropillars but does not touch the base of the surface to trigger the occurrence of the Wenzel wetting state. We have named this state the suspended penetration wetting state. Spontaneous recovery from the suspended penetration wetting state to the initial Cassie-Baxter wetting state is achieved when the pressure is removed. Based on the experimental results, we built models to establish the penetration depth that the suspended penetration wetting state could achieve and to understand the energy barrier that influences the equilibrium position of the liquid surface. These results deepen our understanding of wetting states on rough surfaces subjected to external disturbances and shed new light on the design of superhydrophobic materials with a robust wetting stability. [ABSTRACT FROM AUTHOR]

Living cells sense and respond to mechanical signals through specific mechanisms generating traction force. The quantification of cell forces using micropillars can be limited by micropillar stiffness, technological aspects of fabrications, and microcontact printing of proteins. This paper develops the new design of SiO2/Parylene C micropillars with an aspect ratio of 6 and 3.5 and spring constant of 4.7 and 28 µN µm−1, respectively. The upper part of micropillars is coated with a 250 nm layer of SiO2, and results confirm protein deposition on individual micropillars via SiO2 interface and non‐adhesiveness on the micropillars' sidewalls. Results show an absence of cytotoxicity for micropillar‐based substrates and a dependence on its stiffness. Stiffer micropillars enhance cell adhesion and proliferation rate, and a stronger cellular force of ≈25 μN is obtained. The main contribution of SiO2/parylene C micropillars is the elimination of the step involving the fabrication of polydimethylsiloxane stamp because the array enables covalent binding of proteins via SiO2 chemistry. These micropillars stand on Si wafer and thus, any warping of underlying polymer membrane does not have to be considered. Additionally, SiO2/parylene C micropillars can broaden the range of stiffer substrates to be probed by cells. [ABSTRACT FROM AUTHOR]

Shear stress, pressure, and flow rate are fluid dynamic parameters that can lead to changes in the morphology, proliferation, function, and survival of many cell types and have a determinant impact on tissue function and viability. Microfluidic devices are promising tools to investigate these parameters and fluid behaviour within different microchannel geometries. This study discusses and analyses different designed microfluidic channel geometries regarding the influence of fluid dynamic parameters on their microenvironment at specified fluidic parameters. The results demonstrate that in the circular microchamber, the velocity and shear stress profiles assume a parabolic shape with a maximum velocity occurring in the centre of the chamber and a minimum velocity at the walls. The longitudinal microchannel shows a uniform velocity and shear stress profile throughout the microchannel. Simulation studies for the two geometries with three parallel microchannels showed that in proximity to the micropillars, the velocity and shear stress profiles decreased. Moreover, the pressure is inversely proportional to the width and directly proportional to the flow rate within the microfluidic channels. The simulations showed that the velocity and wall shear stress indicated different values at different flow rates. It was also found that the width and height of the microfluidic channels could affect both velocity and shear stress profiles, contributing to the control of shear stress. The study has demonstrated strategies to predict and control the effects of these forces and the potential as an alternative to conventional cell culture as well as to recapitulate the cell- and organ-specific microenvironment.

In this work, we report a microfluidics-based approach to the sorting of two-particle and three-particle mixtures using a micropillar arrangement. Using a simple retain-and-release strategy, we demonstrate the capability of the device to completely sort two-particle mixtures containing particles at various ratios. Additionally, for three-particle mixture, we demonstrate enrichment and recovery of largest size particles and generation of streams completely free of the largest particles. The efficiency of the sorting process is characterized in terms of purity and recovery using Fluorescence Activated Cell Sorting (FACS). We report high purity and recovery of the target particles for all the particle ratios investigated and the particle release strategy employed here is observed to be crucial to this. Through a systematic investigation on the effect of particle ratios on the sorting process, the present work demonstrates the ability of pillar-based microfluidic devices to sort microparticles in diverse applications, beyond the usual blood cell sorting application. [ABSTRACT FROM AUTHOR]

The combination of microstructures and nanostructures has broad application prospects. However, most existing methods are oriented to fabricating microstructures and nanostructures separately, and the fabrication of nanostructures on a microstructured surface at the wafer level is rarely studied. In this work, a new method of fabricating large‐area high aspect ratio nano‐cylinders on micro‐pillars based on a colloidal crystal mask is proposed. An experimental device for stripping and draining was designed to create a polystyrene colloidal crystal mask. Together with reactive‐ion etching and metal‐assisted chemical etching, high aspect ratio nano‐cylinders on micro‐pillars with controllable size were obtained on a 4‐inch silicon wafer. Wetting tests were performed on four groups of fabricated structures of different sizes, and the results showed that all samples were superhydrophobic. Thus, a method for fabricating uniform, size‐controllable, large‐area high aspect ratio nano‐cylinders on micro‐pillars with great potential as a superhydrophobic engineering material is proposed. [ABSTRACT FROM AUTHOR]

In this study, a novel viscosity measurement technique based on measuring the deflection of flexible (poly) dimethylsiloxane (PDMS) micropillars is presented. The experimental results show a nonlinear relationship between fluid viscosity and the deflection of micropillars due to viscoelastic properties of PDMS. A calibration curve, demonstrating this nonlinear relationship, is generated, and used to determine the viscosity of an unknown fluid. Using our method, viscosity measurements for Newtonian fluids (glycerol/water solutions) can be performed within 2–100 cP at shear rates γ = 60.5–398.4 s−1. We also measured viscosity of human whole blood samples (non-Newtonian fluid) yielding 2.7–5.1 cP at shear rates γ = 120–345.1 s−1, which compares well with measurements using conventional rotational viscometers (3.6–5.7 cP). With a sensitivity better than 0.5 cP, this method has the potential to be used as a portable microfluidic viscometer for real-time rheological studies. Image 1 • Novel viscosity measurement method: flexible micropillar displacement tracking. • Newtonian and non-Newtonian fluids viscosity measurements with microliter sample volumes. • Sensitivity of device depends on micropillar arrangement. • Device sensitivity increases with high aspect ratio of micropillars. [ABSTRACT FROM AUTHOR]

The ongoing revolution of human–robot interactions and electronic skins has created new requirements for tactile sensors, including good mechanical flexibility, high sensitivity, and the availability of distributed pixels for detecting force distribution. Here, a highly sensitive flexible sensor array based on piezoelectricity‐enhanced vertically aligned P(VDF‐TrFE) micropillars is developed for dynamic tactile sensing. The core piezoelectric sensing micropillars are fabricated using a straightforward and scalable nanoimprinting technology and then sandwiched between a pair of cross electrode arrays to construct multiplexed sensor arrays. Due to the proposed structural design and nanoimprinting methodology, the sensor pixels exhibit uniform output generation, robust output stability, and scalable fabrication ability. In addition, taking advantage of the high compressibility and enhanced strain of the piezoelectric micropillars compared to planar films, the microstructured sensors show an enhanced sensitivity of 228.2 mV N−1 and a highly linear response to loads. By integrating the flexible sensor with a portable signal processing circuit, a complete tactile sensing system is successfully developed to provide clear intuitive user interfaces. The good flexibility and robust stability of the sensor arrays enable them to be attached onto curved surface for real‐time tracking of dynamic force and imaging the force distribution. [ABSTRACT FROM AUTHOR]

Featured Application: The proposed method can be utilized to fabricate micropillar solar cells. Micropillar arrays with radial p–n junctions are attractive for photovoltaic applications, because the light absorption and carrier collection become decoupled. The main challenge in manufacturing radial p–n junctions is achieving shallow (dopant depth <200 nm) and heavy doping (>1020 cm−3) that will allow the formation of a quasi-neutral region (QNR) and space charge region (SCR) in its tiny geometry. This experimental study investigates an approach that allows shallow and heavy doping in silicon micropillars. It aims to demonstrate that silicon dioxide (SiO2) can be used to control the dopant penetration depth in silicon micropillars. [ABSTRACT FROM AUTHOR]

A new approach to generate magneto‐activatable microstructures for discrete cell adhesion to investigate cell mechanoresponsiveness is reported here. The system is based on poly(n‐butylacrylate) micropillar arrays, generated from prepolymers via C,H insertion chemistry and filled with magnetic iron oxide nanoparticles. The pillars' surfaces are modified such that only top faces of the pillars are cell attractive, while the pillar side walls and the area between the pillars are covered by a cell repelling hydrogel layer based on poly(N,N‐dimethyl acrylamide). Magnet stimulation leads to pillar deflection, which induces traction forces to adherent cells. Analysis of early mechanoresponse in human mesenchymal stem cells, specifically the localization of autophosphorylated focal adhesion kinase (pFAK Y397) and the co‐transcriptional activator yes‐associated protein (YAP), shows a clear redistribution of pFAK Y397 to the cell margins and an enhanced nuclear localization of YAP, following the magnetically induced mechanical actuation. The successful use of this technique shows that the generated magnetoactive posts are suitable tools to detect the onset (FAK) and maintenance (YAP) involved in stem cell mechanosensing and mechanotransduction. The platform combines selective cell adhesion with the possibility of magnetic actuation, thereby representing a promising technology to broaden the molecular understanding of cellular mechanobiology. [ABSTRACT FROM AUTHOR]

Herein, selective‐area growth (SAG) of lightly n‐doped GaN micropillars on masked GaN‐on‐sapphire templates is investigated. Using the micropillar SAG approach, the maximum GaN drift layer thickness in Schottky diodes on foreign substrates is increased. Thus, cost‐efficient vertical power devices with large breakdown voltages (VBD) based on heteroepitaxy are enabled. The influence of different hard‐mask materials and SAG temperatures (TSAG) on growth selectivity, morphology, and net doping concentration (ND–NA) is investigated. By using an AlOx hard mask and TSAG = 1045 °C, 3.7 μm high GaN micropillars are grown in circular mask openings. Quasi‐vertical Schottky diodes on these pillars exhibit low ND–NA = 5.2 × 1016 cm−3, VBD = 393 V, and a critical electric field EC = 2.63 MV cm−1. [ABSTRACT FROM AUTHOR]

In this work, the mechanism of focused ion beam (FIB) milling as an advanced manufacturing technique is studied to pattern micropillars at different diameter sizes of 5, 10 and 20 μm. Under the same type of ion source which is Ga ion, the main ion beam parameters that influenced the FIB machining to produce micropillars are the ion beam scanning, dose, profile, and incident angle. The mathematical modeling is presented to describe the micropillars' production by crater sputtering of the FIB milling. The effect of sputtering and redeposition on the final geometrical micropillars suggest the arrangement of the multilayer sample (Sn/Ni/Cu) that close to the ion irradiation gets highly sputtered and redeposited despite the surface binding energy and sputtering yield, ϒ of each element is known. Results show that the highest aspect ratio values are derived from the lowest diameter size of the micropillars produced from the FIB machining. [ABSTRACT FROM AUTHOR]

Neuromorphic computing has received considerable attention as promising alternatives to classical von Neumann computing architectures. An attractive concept in this field is reservoir computing which is based on coupled non-linear elements to enable for instance ultra-fast pattern recognition. We focus on the development of microlasers in a dense regular array for the implementation of photonic reservoir computing based on the diffractive coupling. The coupling relies on injection locking of microlasers and sets stringent requirements on the spectral homogeneity of the array, which needs to be on the order of the achievable locking range. We realize GaAs/AlGaAs micropillar arrays with InGaAs quantum dots as active medium. To achieve the high spectral homogeneity on the order of 100 μeV, as determined by injection locking experiments, the emission energy of each individual micropillar is adjusted to compensate for local inhomogeneities of order ∼1.3 meV in the underlying microcavity structure. The realized micropillar arrays have a spectral inhomogeneity as low as 190 μeV for an 8 × 8 array and down to 118 μeV for a 5 × 5 sub-array. The arrays have high potential to enable the implementation of powerful photonic reservoir computing, which can be extended to a reservoir of hundreds of microlasers in the future. [ABSTRACT FROM AUTHOR]

An optimization process of electroless plating of nickel was carried out with NiCl2 as the nickel ion source, NaH2PO2 as the reduction agent, CH3COONa and Na3C6H5O7 as complexing agents. Electroless plated nickel layers on sputtered aluminum corning glass substrates with a resistivity of about 75.9 μΩ cm and a nickel concentration higher than 93% were obtained. This optimum process was successfully applied in growing nickel micropillars at selected areas with a well-controlled height. The microstructure of the masking layers was fabricated by means of optical photolithography for subsequent growth of nickel micropillars on selected areas. Micropillars size was defined by the opening size and the height was controlled by adjusting the plating time at a growth rate of 0.41 μm/min. This result shows that electroless nickel plating could be a good candidate for growing micropillars for applications in microbolometers.

CO2 laser surface treatment of cemented carbide cutting tool is considered and the resulting surface wetting state and texture characteristics are examined using analytical tools. The morphological and metallurgical changes in the CO2 laser treated layer are assessed, and microhardness and fracture toughness of the resulting surface are analysed. The influence of mud formed from the environmental dust particles in humid ambient air on the CO2 laser treated surface is investigated and the tangential force required for removing the dried mud from the cemented carbide cutting tool surface is measured. It is found that the CO2 laser treatment slightly increases the microhardness and lowers the fracture toughness of the cemented carbide cutting tool surface. The CO2 laser treated surface has texture characteristic consisting of hierarchically distributed micro/nanopillars. The CO2 laser treatment reduces the surface free energy, in which case the wetting state of the cemented carbide cutting tool surface changes from hydrophilic to hydrophobic after the laser treatment process. The water condensation on to the dust particles gives rise to mud formation on the substrate surface in humid air ambient, which enhances the dust particles adhesion onto the surface upon drying. The tangential force required to remove the dried mud from the untreated cemented carbide cutting tool surface is almost twice of the CO2 laser treated surface; consequently, the laser treatment significantly lowers the dried mud adhesion on the surface. [ABSTRACT FROM AUTHOR]

• Micropillars increased cell attachment but decreased proliferation rate at early time points. • Oxygen plasma treatment enhanced both cell attachment and proliferation rate on all substrates. • Untreated hydrophobic arrays induced osteogenic differentiation of DPSCs without chemical induction over a 28 day culture period. • The expression levels of osteogenic genes were positively correlated with the degree of nuclear deformations. Use of soluble factors is the most common strategy to induce osteogenic differentiation of mesenchymal stem cells (MSCs) in vitro, but it may raise potential side effects in vivo. The topographies of the substrate surfaces affect cell behavior, and this could be a promising approach to guide stem cell differentiation. Micropillars have been reported to modulate cellular and subcellular shape, and it is particularly interesting to investigate whether these changes in cell morphology can modulate gene expression and lineage commitment without chemical induction. In this study, poly(methyl methacrylate) (PMMA) films were decorated with square prism micropillars with different lateral dimensions (4, 8 and 16 μm), and the surface wettability of the substrates was altered by oxygen plasma treatment. Both, pattern dimensions and hydrophilicity, were found to affect the attachment, proliferation, and most importantly, gene expression of human dental pulp mesenchymal stem cells (DPSCs). Decreasing the pillar width and interpillar spacing of the square prism pillars enhanced cell attachment, cell elongation, and deformation of nuclei, but reduced early proliferation rate. Surfaces with 4 or 8 μm wide pillars/gaps upregulated the expression of early bone-marker genes and mineralization over 28 days of culture. Exposure to oxygen plasma increased wettability and promoted cell attachment and proliferation but delayed osteogenesis. Our findings showed that surface topography and chemistry are very useful tools in controlling cell behavior on substrates and they can also help create better implants. The most important finding is that hydrophobic micropillars on polymeric substrate surfaces can be exploited in inducing osteogenic differentiation of MSCs without any differentiation supplements. [ABSTRACT FROM AUTHOR]

Silicon micropillars have been suggested as one of the techniques for improving the efficiency of devices. Fabrication of micropillars has been done in several ways—Metal Assisted Chemical Etching (MACE) and Reactive Ion Etching (RIE) being the most popular techniques. These techniques include etching through the surface which results in surface damage that affects the carrier lifetime. This paper presents a study that compares the carrier lifetime of micropillars fabricated using RIE and MACE methods. It also looks at increasing carrier lifetime by surface treatment using three main approaches: surface passivation by depositing Al2O3, surface passivation by depositing SiO2/SiN, and surface passivation by etching using KOH and Hydrofluoric Nitric Acetic (HNA) solution. It was concluded that passivating with SiO2 and SiN results in the highest carrier lifetime on the MACE and RIE pillars. [ABSTRACT FROM AUTHOR]