No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Rapid vacuum-driven monolayer assembly of microparticles on the surface of perforated microfluidic devices
Abstract
On the cusp of a miniaturized device era, a number of promising methods have been developed to attain large-scale assemblies of micro- and nanoparticles. In this study, a novel method is proposed to firmly capture dispersed microparticles of nominal sizes of 10 μm on a two-dimensional array (1.0 × 1.0 mm²) of through-pores on a surface. This is obtained by dispensing a droplet of the particle dispersion on the pores, which drains by applying a vacuum-driven force at the backside of the pores. The assembled particles are captured on the surface in a reversible way, making them available for direct manipulation and inspection, or subsequent transfer of the particles to a second surface. The relevant process parameters dispersant concentration, dispersant type, particle properties, and pitch distance d, are optimized to obtain (near-)perfect ordered particle arrays. Furthermore, to significantly improve the quality of the particle assembly, washing steps are added to remove excess particles from the surface. Silica or polystyrene (PS) particle assemblies with an error ratio (ER) as low as 0.2% are obtained, demonstrating the universality of the proposed method. For the smallest pitch, d = 1.25 μm, even with optimal process parameters, higher ER-values (=1.1%) are obtained.
... Figure 1a schematically depicts the proposed method encompassing various steps, using a perforated device positioned horizontally. It is noteworthy that the proposed technique is not limited to this position [2]. A specified volume of the particle dispersion is manually dispensed on the chip using a micropipette, and subsequently, the vacuum channel connected to the pores was opened. ...
... We already demonstrated the ability of the proposed method to assemble 10 μm dispersed silica and polystyrene particles on non-profiled surfaces using optimized process parameters [2]. In this respect, a supply of excess particles and the addition of at least four consecutive washing steps were imperative to obtain perfect assembled arrays (cf. ...
... However, in the smallest pitch = 1.25 μm case, the particle assembly could significantly deteriorate, as particles could inevitably get trapped between adjacent pores (cf. blue circle in Fig. 1b-right image) [2]. To overcome and improve this limitation and previous efforts, profiled perforated devices, as shown in Fig. 1c, resembling a funnel-like structure, were micromachined [3]. ...
Here, we propose a universal technique to firmly capture dispersed microparticles on any desired two-dimensional array of through-pores on a surface. The 10 μm silica or polystyrene particles are reversibly captured, making them accessible for direct manipulation and inspection or subsequent transfer to other surfaces. To obtain perfect arrays with a pitch of 1.25 μm, perforated devices with profiled surfaces were required. Additionally, the method has proven successful for both types of particles, either dispersed in water or ethanol. The assembly technique may serve as a platform for manufacturing hierarchical materials, e.g., ordered chromatography packings, or performing cellular assays.
... Another way to achieve the deposition of particles in specific locations is to use a substrate with an array of micropores. 19 If a reduced atmospheric pressure, close to a vacuum, is created under the substrate, the liquid quickly evaporates from the sessile drop through the substrate micropores. A flow of liquid occurs carrying particles toward the pores. ...
... To prevent the particles from being sucked into the vacuum region along with the liquid molecules, they must have a size exceeding the pore diameter. 19 Evaporative lithography allows the deposition of functional coatings. For example, electrically conductive ink can be deposited into microchannels of a topographically structured surface through nanoparticle transfer by the capillary flow induced by evaporation. ...
The continuing development of evaporative lithography is important for many areas such as the creation of photonic crystals for optronics and microelectronics, the development of biosensors for medical applications and biotechnology, and for the formation of functional coatings for nanotechnology, including the application of thin, protective polymer coatings. The article proposes a mathematical model that allows us to explain the basic mechanisms of the formation of thin polymer films (less than 50 μm thick) during their deposition onto a composite substrate by methanol evaporation from a solution. If the thermal conductivity of the substrate is spatially non-uniform, this results in inhomogeneous evaporation along the free film surface. Therefore, as the film dries, a patterned polymer coating is left behind on the substrate. The mathematical model described here is based on the lubrication approximation and takes into account the dependence of the solution density on the concentration. The numerical computation results are in qualitative agreement with the experimental data of other authors. The article shows that solutal Marangoni flow plays a primary role in the process under consideration. This study allows us to better understand the mechanisms that can be used in evaporative lithography.
... Many of these wet assembly techniques, e.g., convective assembly, capillary assembly, or spin-coating, apply delicate self-assembly processes on (non-) patterned substrates, where a slight deviation in one of the required parameters, such as pH, relative humidity and temperature, can yield defects in the assembled monolayers comprising microspheres or nanospheres [9,10]. Other wet assembly techniques use electric and magnetic fields or a vacuumdriven force to attain an ordered monolayer from a dispersion of nanoand microparticles [6,[11][12][13] Recently, it has been shown that these close-packed assembled arrays can be transformed into non-closely packed arrays using reactive ion etching (RIE) to tune the spacing between neighbouring particles [14,15]. However, a drawback of this method is that the particle's morphology and surface roughness can be altered by dry etching. ...
... Two types of membranes have been used: membranes with nonprofiled through-pores (straight cylindrical pores) and profiled throughpores (entrance of pore is sloped). The membranes were manufactured using state-of-the-art micromachining technology described earlier [13,21]. Experiments were performed using dry monodisperse hydrophilic silica microspheres ( = 1.85 g/cm 3 ) with a diameter of (10.02±0.32) ...
A myriad of wet assembly techniques exists to attain ordered arrays of micro- and nanoparticles. The present contribution proposes a universal and rapid (in order of a few seconds) dry assembly strategy that can be employed to assemble ordered arrays of particles with a designed spacing. This method involves shooting agglomerated monodisperse silica, polystyrene or PMMA powder microspheres with diameters ranging between 5–10 μm against an impact plating using pressure exceeding 2.5 bar. Consequently, the fluidized microspheres are attracted towards the pores of a silicon membrane device by applying a vacuum force. Furthermore, a brushing step is added to remove excess particles in undesired positions on top of the ordered arrays. An asset of the proposed method is that for the investigated particle properties, the same optimized conditions could be used to attain any desired 2-D particle arrangement that can be transferred on soft surfaces, e.g., PDMS.
... In the broader context of fixed beds with defined packing structures and applications in catalysis and liquid chromatography, two main approaches are currently investigated: selfassembly of colloidal particles into (dense) structures, e.g., pillar arrays [3], monoliths [4], or vacuum-assisted layer-wise methods [5,6]. The practicality of these approaches are currently limited by either producing too dense packings or in challenges going from single-to multi-layer specimens. ...
In this work, we present and characterise an experimental setup that allows the generation of porous packings from nanosuspensions. By defined positioning and drying of solid-containing droplets, large-scale porous structures can be generated. Examples of such structures are shown and characterised. Operational challenges are presented, and it is discussed how they can be overcome to allow the maximum degree of freedom in packing generation.
... Nevertheless, wet assembly methods are in studies still generally preferred over dry methods as the interaction forces between particles in suspensions are noticeably weaker than in a dry state. 22,23 Whereas the aforementioned studies mainly focused on the production of particle arrays and layers on flat or weakly structured surfaces, the present study focuses on the possibility to assemble microspheres (5 and 10 μm diameter) in arrays of micromachined pockets wherein the particles can be completely sunk into the substrate (depth of the pocket ≥ diameter of the particle). Particle assembly is achieved using PDMS rubbing. ...
The present contribution reports on a study aiming to find the most suitable rubbing method for filling arrays of separated and interconnected micromachined pockets with individual microspheres on rigid, uncoated silicon substrates without breaking the particles or damaging the substrate. The explored dry rubbing methods generally yielded unsatisfactory results, marked by very large percentages of empty pockets and misplaced particles. On the other hand, the combination of wet rubbing with a patterned rubbing tool provided excellent results (typically <1% of empty pockets and <5% of misplaced particles). The wet method also did not leave any damage marks on the silicon substrate or the particles. When the pockets were aligned in linear grooves, markedly the best results were obtained when the ridge pattern of the rubbing tool was moved under a 45° angle with respect to the direction of the grooves. The method was tested for both silica and polystyrene particles. The proposed assembly method can be used in the production of medical devices, antireflective coatings, and microfluidic devices with applications in chemical analysis and/or catalysis.
... To circumvent the strong interaction forces, micro-or nanoparticles are often suspended in liquids to study the selfor directed organization of these particles on substrates using a variety of techniques, such as manipulation by means of electric and magnetic fields, or solvent evaporation. [17][18][19][20][21][22][23][24][25][26][27] In this respect, a profound understanding of the competing interparticle forces and the adhesion force between the particle and substrate is key to attain well-controlled particle patterns 20,28,29 which are relevant in various applications, e.g., bio-inspired approaches to materials engineering, paints, photonic crystals, self-cleaning anti-reflective coatings, optical and biological sensors, chemical catalysis, biomimicry. [30][31][32][33][34] However, as the wet assembly techniques rely on optimized conditions, such as solvent evaporation rate, surface wettability, pH, it tends to be challenging to attain perfect ordered crystals, without any cracks, on a large scale. ...
The vibration dynamics of relatively large granular grains is extensively treated in literature, but comparable studies on the self-assembly of smaller agitated beads are lacking. In this work, we investigate...
... Most studies aiming at the production of (self-) assembled particle arrays make use of wet assembly methods, hinging predominantly on a delicate balance of the surface interaction forces and requiring an optimized evaporation rate, solvent properties, pH, temperature [2,[17][18][19][20][21][22][23][24][25][26][27]. Consequently, the number of possible particle configurations, e.g, hexagonal and cubic centred arrays, is limited. ...
At the onset of a miniaturized device era, several promising methods, primarily wet methods, have been developed to attain large-scale assemblies of microparticles. To improve the speed, versatility and robustness of the current methods for the structured assembly of microparticles, an automatable method capable of forming 2D arrays of microspheres on large silicon surfaces is devised. The method uses surfaces perforated with vacuum-suction holes, capable of aspiring and holding individual particles from a particle cloud generated by subjecting a lump of chargeable particles, e.g., silica, polystyrene, and polymethyl methacrylate (PMMA), to a strong electrical field under ambient air conditions. The microsphere levitation depends on the electrical conductivity and permittivity of the particles. A single or double brush stroke can remove excess particles covering the formed arrays. We find that silica or polystyrene microspheres with a diameter of 5 μm or 10 μm can be assembled on the order of a few seconds, independently of the array size. Owing to the reversible nature of the arresting vacuum force, the assembled layers can be transferred to another surface, such as polydimethylsiloxane (PDMS) sheets, thus providing a key step for future particle printing processes for the fabrication of hierarchical materials, e.g., photonic crystals.
... In particular, ellipsoidal particles constitute a notably appealing building block. Although spherical particles allow the attainment of a rich variety of patterns investigated in both fundamental studies and applications [7][8][9][10][11][12][13][14][15][16][17][18], ellipsoidal particles originate assemblies with distinctive features in comparison with spherical particles in both crystalline and amorphous phases [19][20][21][22][23][24]. Assemblies of ellipsoidal particles display specific angle and polarization dependent spectral properties [2,[25][26][27][28][29]. Ellipsoidal particles exhibit transport properties different from spherical particles which can be exploited for applications such as detoxification, drug delivery and biosensing (if put in circulation in the human body), as well as in bioremediation and removal of contaminants from groundwater [30][31][32]. ...
Ion beam irradiation of spherical colloidal particles is a viable route to induce particle deformation, especially to get anisotropic shapes. Even though less common in comparison with dry etching techniques, different types of morphological changes can be attained depending on the process parameters (angle of incidence, energy, fluence of the ion beam, type of ion, temperature) and on particle material and initial particle arrangement (crystalline or disordered, made up of isolated or closely-packed particles). The technique can be harnessed to get anisotropic deformation of spherical colloidal particles into an ellipsoidal shape, but also to tailor the interstices between closely-packed colloidal particles, to get particle necking and coalescence as well as particle rearrangement. As such, particle deformation based on ion irradiation can find diverse applications from synthesis of ellipsoidal particles to modified templates for colloidal lithography. In this review, we examine in detail the principles and models of colloidal particle shaping via ion beam irradiation, the influence of process parameters on particle morphology and the applications of irradiated particles.
In this work, we report on an in-depth study of how 10 μm silica and polystyrene particles interact with a target electrode after they were levitated by applying a strong electric field. The results show that, under these conditions, silica particles unexpectedly have a higher tendency to adhere on a fluorocarbon coated electrode compared to a bare, non-coated silicon electrode. Relative adherence ratios Γ up to Γ = 4.7 were observed. Using the colloidal probe technique, atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM), the observations can be explained by a mechanism where particles dissipate their energy through adhesive forces combined with permanent surface deformations during impact and charge transfer through the contact electrification phenomenon. All these processes attribute to increasing the probability that levitated particles attain velocities that are lower than a sticking velocity.
Characterization of the morphology, identification of patterns and quantification of order encountered in colloidal assemblies is essential for several reasons. First of all, it is useful to compare different self-assembly methods and assess the influence of different process parameters on the final colloidal pattern. In addition, casting light on the structures formed by colloidal particles can help to get better insight into colloidal interactions and understand phase transitions. Finally, the growing interest in colloidal assemblies in materials science for practical applications going from optoelectronics to biosensing imposes a thorough characterization of the morphology of colloidal assemblies because of the intimate relationship between morphology and physical properties (e.g. optical and mechanical) of a material. Several image analysis techniques developed to investigate images (acquired via scanning electron microscopy, digital video microscopy and other imaging methods) provide variegated and complementary information on the colloidal structures under scrutiny. However, understanding how to use such image analysis tools to get information on the characteristics of the colloidal assemblies may represent a non-trivial task, because it requires the combination of approaches drawn from diverse disciplines such as image processing, computational geometry and computational topology and their application to a primarily physico-chemical process. Moreover, the lack of a systematic description of such analysis tools makes it difficult to select the ones more suitable for the features of the colloidal assembly under examination. In this review we provide a methodical and extensive description of real-space image analysis tools by explaining their principles and their application to the investigation of two-dimensional colloidal assemblies with different morphological characteristics.
Optical tweezers have emerged as a powerful tool for the non-invasive trapping and manipulation of colloidal particles and biological cells1,2. However, the diffraction limit precludes the low-power trapping of nanometre-scale objects. Substantially increasing the laser power can provide enough trapping potential depth to trap nanoscale objects. Unfortunately, the substantial optical intensity required causes photo-toxicity and thermal stress in the trapped biological specimens³. Low-power near-field nano-optical tweezers comprising plasmonic nanoantennas and photonic crystal cavities have been explored for stable nanoscale object trapping4–13. However, the demonstrated approaches still require that the object is trapped at the high-light-intensity region. We report a new kind of optically controlled nanotweezers, called opto-thermo-electrohydrodynamic tweezers, that enable the trapping and dynamic manipulation of nanometre-scale objects at locations that are several micrometres away from the high-intensity laser focus. At the trapping locations, the nanoscale objects experience both negligible photothermal heating and light intensity. Opto-thermo-electrohydrodynamic tweezers employ a finite array of plasmonic nanoholes illuminated with light and an applied a.c. electric field to create the spatially varying electrohydrodynamic potential that can rapidly trap sub-10 nm biomolecules at femtomolar concentrations on demand. This non-invasive optical nanotweezing approach is expected to open new opportunities in nanoscience and life science by offering an unprecedented level of control of nano-sized objects, including photo-sensitive biological molecules.
The patterning of structural colored materials has a significant impact on various applications such as flexible displays, anti‐counterfeiting patches, colorimetric sensors, etc. Herein, a sessile microdroplet‐based writing board is presented to pattern magnetochromatic hydrogels with abundant structural colors and improved optical performance. It is demonstrated that predesigned hydrophilic patterns on a hydrophobic writing board can capture a mixture of polymer and Fe3O4@SiO2 magnetic nanoparticles inks with a spatial resolution of ≈100 pin per 1 cm2 while retaining magnetic field responsibility to the lower limit of 84 Gs. The inks are self‐partitioned into microdroplet arrays, which would in situ transform into structural colored hydrogels within a short time via thiol‐Michael addition. In contrast to conventional evaporation induced assembly of colloidal photonic crystals in sessile droplets, the resulting structural colored hydrogel microarrays show not only good stability and optical adjustability but tunable morphologies. In addition, the introduction of the microfluidic mixing and ink dispensing system greatly shortens the time interval from the polymer mixing to sessile droplet generation, circumvents the challenge of short operation time for the self‐crosslinking ink components, and enables the direct handwriting of high quality structural colored patterns.
The rapid progress in flexible electronic devices has attracted immense interest in many applications, such as health monitoring devices, sensory skins, and implantable apparatus. Here, inspired by the adhesion features of mussels and the color shift mechanism of chameleons, a novel stretchable, adhesive, and conductive structural color film is presented for visually flexible electronics. The film is generated by adding a conductive carbon nanotubes polydopamine (PDA) filler into an elastic polyurethane (PU) inverse opal scaffold. Owing to the brilliant flexibility and inverse opal structure of the PU layer, the film shows stable stretchability and brilliant structural color. Besides, the catechol groups on PDA impart the film with high tissue adhesiveness and self‐healing capability. Notably, because of its responsiveness, the resultant film is endowed with color‐changing ability that responds to motions, which can function as dual‐signal soft human‐motion sensors for real‐time color‐sensing and electrical signal monitoring. These features make the bio‐inspired hydrogel‐based electronics highly potential in the flexible electronics field.
Evaporation‐induced self‐assembly of colloidal particles is one of the most versatile fabrication routes to obtain large‐area colloidal crystals; however, the formation of uncontrolled “drying cracks” due to gradual solvent evaporation represents a significant challenge of this process. While several methods are reported to minimize crack formation during evaporation‐induced colloidal assembly, here an approach is reported to take advantage of the crack formation as a patterning tool to fabricate microscopic photonic structures with controlled sizes and geometries. This is achieved through a mechanistic understanding of the fracture behavior of three different types of opal structures, namely, direct opals (colloidal crystals with no matrix material), compound opals (colloidal crystals with matrix material), and inverse opals (matrix material templated by a sacrificial colloidal crystal). This work explains why, while direct and inverse opals tend to fracture along the expected {111} planes, the compound opals exhibit a different cracking behavior along the nonclose‐packed {110} planes, which is facilitated by the formation of cleavage‐like fracture surfaces. The discovered principles are utilized to fabricate photonic microbricks by programming the crack initiation at specific locations and by guiding propagation along predefined orientations during the self‐assembly process, resulting in photonic microbricks with controlled sizes and geometries. Rational engineering of crack formations in colloidal crystals is developed as a versatile route to fabricate photonic microstructures with controlled geometries and sizes. This strategy is based on a mechanistic understanding of the orientation‐dependent fracture behavior of colloidal crystals, which guides the design of patterned substrates with stress concentrators for controlled crack formation and propagation.
While microparticle (MP) assemblies have long attracted academic interest, few practical applications of assembled MPs have been achieved because of technological difficulties related to MP synthesis, MP position registration, and the absence of device concepts. The precise positioning of functional MPs in a proper stencil can produce flexible/stretchable electronic devices, even when the MPs themselves are rigid. In recent years, remarkable progress has been made in the programmable position registration of MPs, production of functional MPs, and concepts for MP‐based, pixel‐type electronic devices. This progress report reviews the recent technological advances in MP assembly and discusses the technological challenges preventing the realization of the one‐particle/one‐pixel concept.
Here the fabrication and characterization of a novel microelectrode array for electrophysiology applications is described, termed a micro sieve electrode array (µSEA). This silicon based µSEA device allows for hydrodynamic parallel positioning of single cells on 3D electrodes realized on the walls of inverted pyramidal shaped pores. To realize the µSEA, a previously realized silicon sieving structure is provided with a patterned boron doped poly-silicon, connecting the contact electrodes with the 3D sensing electrodes in the pores. A LPCVD silicon-rich silicon nitride layer was used as insulation. The selective opening of this insulation layer at the ends of the wiring lines allows to generate well-defined contact and sensing electrodes according to the layout used in commercial microelectrode array readers. The main challenge lays in the simultaneously selective etching of material at both the planar surface (contact electrode) as well as in the sieving structure containing the (3D) pores (sensing electrodes). For the generation of 3D electrodes in the pores a self-aligning technique was developed using the pore geometry to our advantage. This technique, based on sacrificial layer etching, allows for the fine tuning of the sensing electrode surface area and thus supports the positioning and coupling of single cells on the electrode surface in relation to the cell size. Furthermore, a self-aligning silicide is formed on the sensing electrodes to favour the electrical properties.
Experiments were performed to demonstrate the working principle of the µSEA using different types of neuronal cells. Capture efficiency in the pores was >70% with a 70% survival rate of the cell maintained for up to 14 DIV.
The TiSi2–boron-doped-poly-silicon sensing electrodes of the µSEA were characterized, which indicated noise levels of <15 µV and impedance values of 360 kΩ. These findings potentially allow for future electrophysiological measurements using the µSEA.
Herein, we present a large-area 3D hemispherical perforated microwell structure for a bead based bioassay. Such a unique microstructure enables us to perform the rapid and stable localization of the beads at the single bead level and the facile manipulation of the bead capture and retrieval with high speed and efficiency. The fabrication process mainly consisted of three steps: the convex micropatterned nickel
(Ni) mold production from the concave micropatterned silicon (Si) wafer, hot embossing on the polymer matrix to generate the concave micropattened acrylate sheet, and reactive ion etching to make the bottom holes. The large-area hemispherical perforated micropatterned acrylate sheet was sandwiched between two polydimethylsiloxane
(PDMS) microchannel layers. The bead solution was injected and recovered in the top PDMS microchannel, while the bottom PDMS microchannel was connected with control lines to exert the hydrodynamic force in order to alter the flow direction of the bead solution for the bead capture and release operation. The streptavidin-coated microbead capture was achieved with almost 100% yield within 1 min, and all the beads were retrieved in 10 s. Lysozyme or thrombin binding aptamer labelled microbeads were trapped on the proposed bead microarray, and the in situ
fluorescence signal of the bead array was monitored after aptamer-target protein interaction. The protein-aptamer conjugated microbeads were recovered, and the aptamer was isolated for matrix assisted laser desorption/ionization time-of-flight mass spectrometry analysis to confirm the identity of the aptamer.
When a spilled drop of coffee dries on a solid surface, it leaves a dense, ring-like deposit along the perimeter (Fig. 1a). The coffeeâinitially dispersed over the entire dropâbecomes concentrated into a tiny fraction of it. Such ring deposits are common wherever drops containing dispersed solids evaporate on a surface, and they influence processes such as printing, washing and coating1, 2, 3, 4, 5. Ring deposits also provide a potential means to write or deposit a fine pattern onto a surface. Here we ascribe the characteristic pattern of the deposition to a form of capillary flow in which pinning of the contact line of the drying drop ensures that liquid evaporating from the edge is replenished by liquid from the interior. The resulting outward flow can carry virtually all the dispersed material to the edge. This mechanism predicts a distinctive power-law growth of the ring mass with timeâa law independent of the particular substrate, carrier fluid or deposited solids. We have verified this law
Photonic crystals have proven their potential and are nowadays a familiar concept. They have been approached from many scientific and technological flanks. Among the many techniques devised to implement this technology self-assembly has always been one of great popularity surely due to its ease of access and the richness of results offered. Self-assembly is also probably the approach entailing more materials aspects owing to the fact that they lend themselves to be fabricated by a great many, very different methods on a vast variety of materials and to multiple purposes. To these well-known material systems a new sibling has been born (photonic glass) expanding the paradigm of optical materials inspired by solid state physics crystal concept. It is expected that they may become an important player in the near future not only because they complement the properties of photonic crystals but because they entice the researchers' curiosity. In this review a panorama is presented of the state of the art in this field with the view to serve a broad community concerned with materials aspects of photonic structures and more so those interested in self-assembly.
All excitable cell functions rely upon ion channels that are embedded in their plasma membrane. Perturbations of ion channel structure or function result in pathologies ranging from cardiac dysfunction to neurodegenerative disorders. Consequently, to understand the functions of excitable cells and to remedy their pathophysiology, it is important to understand the ion channel functions under various experimental conditions – including exposure to novel drug targets. Glass pipette patch-clamp is the state of the art technique to monitor the intrinsic and synaptic properties of neurons. However, this technique is labor intensive and has low data throughput. Planar patch-clamp chips, integrated into automated systems, offer high throughputs but are limited to isolated cells from suspensions, thus limiting their use in modeling physiological function. These chips are therefore not most suitable for studies involving neuronal communication. Multielectrode arrays (MEAs), in contrast, have the ability to monitor network activity by measuring local field potentials from multiple extracellular sites, but specific ion channel activity is challenging to extract from these multiplexed signals. Here we describe a novel planar patch-clamp chip technology that enables the simultaneous high-resolution electrophysiological interrogation of individual neurons at multiple sites in synaptically connected neuronal networks, thereby combining the advantages of MEA and patch-clamp techniques. Each neuron can be probed through an aperture that connects to a dedicated subterranean microfluidic channel. Neurons growing in networks are aligned to the apertures by physisorbed or chemisorbed chemical cues. In this review, we describe the design and fabrication process of these chips, approaches to chemical patterning for cell placement, and present physiological data from cultured neuronal cells.
A colloidal dispersion droplet evaporating from a surface, such as a drying coffee drop, leaves a distinct ring-shaped stain. Although this mechanism is frequently used for particle self-assembly, the conditions for crystallization have remained unclear. Our experiments with monodisperse colloidal particles reveal a structural transition in the stain, from ordered crystals to disordered packings. We show that this sharp transition originates from a temporal singularity of the flow velocity inside the evaporating droplet at the end of its life. When the deposition speed is low, particles have time to arrange by Brownian motion, while at the end, high-speed particles are jammed into a disordered phase.
We review the combinations of optical micro-manipulation with other techniques and their classical and emerging applications to non-contact optical separation and sorting of micro- and nanoparticle suspensions, compositional and structural analysis of specimens, and quantification of force interactions at the microscopic scale. The review aims at inspiring researchers, especially those working outside the optical micro-manipulation field, to find new and interesting applications of these methods.
Microlens array (MLA) is applied widely in imaging, lighting, and sensing fields. Among the many techniques for MLA fabrication, molding is considered to be a mass-production method with low-cost and good accuracy. Silicon carbide (SiC), which has a high mechanical hardness, high thermal conductivity, and low thermal expansion coefficient, is an ideal mold material. But to fabricate microlens array on SiC by the mechanical method is a challenging issue because of its high brittleness and hardness. In this study, a novel method is proposed and developed, in which we use self-assembled silica microspheres monolayer as a template for SU8 resist patterning, subsequently producing concave MLAs in the SiC substrates by Inductively Coupled Plasma (ICP) etching. Two sizes of hexagonal MLAs (diameter Φ11 μm and height 0.8 μm, diameter Φ33 μm and height 2.9 μm, respectively) have been created and tested to possess good imaging performance.
Particle (monolayer) assembly is essential to various scientific and industrial applications, such as the fabrication of photonic crystals, optical sensors, and surface coatings. Several methods, including rubbing, have been developed for this purpose. Here, we report on the serendipitous observation that microparticles preferentially partition onto the fluorocarbon-coated parts of patterned silicon and borosilicate glass wafers when rubbed with polydimethylsiloxane slabs. To explore the extent of this effect, we varied the geometry of the pattern, the substrate material, the ambient humidity and the material and size of the particles. Partitioning coefficients amounted up to a factor 12 on silicon wafers, and even ran in the 100’s on borosilicate glass wafers at zero humidity. Using Kelvin Probe Force Microscopy, the observations can be explained by tribo-electrification, inducing a strong electrostatic attraction between the particles and the fluorocarbon zones, while the interaction with the non-coated zones is insignificant or even weakly repulsive.
This article presents a comprehensive review about the current research activities on colloidal lithography, a highly efficient technology for fabricating large-area patterned functional nanostructures. Three aspects are elaborated: i) self-assembly of monolayer of colloidal crystals (MCCs) and their modifications; ii) lithographic patterning methods, including deposition and patterning of functional materials and pattern transfer onto the underlying substrates; iii) promising applications, especially in the optical and photonic-related fields in the past several years. Finally, perspectives on the current challenges and future trends in this area are given. The present review intends to inspire more ingenious designs and exciting research in colloidal lithography for advanced nanofabrication.
During the past decade, capillary assembly in topographical templates has evolved into an efficient method for the heterogeneous integration of micro- and nano-scale objects on a variety of surfaces. This assembly route has been applied to a large spectrum of materials of micrometer to nanometer dimensions, supplied in the form of aqueous colloidal suspensions. Using systems produced via bulk synthesis affords a huge flexibility in the choice of materials, holding promise for the realization of novel superior devices in the fields of optics, electronics and health, if they can be integrated into surface structures in a fast, simple, and reliable way. In this review, the working principles of capillary assembly and its fundamental process parameters are first presented and discussed. We then examine the latest developments in template design and tool optimization to perform capillary assembly in more robust and efficient ways. This is followed by a focus on the broad range of functional materials that have been realized using capillary assembly, from single components to large-scale heterogeneous multi-component assemblies. We then review current applications of capillary assembly, especially in optics, electronics, and in biomaterials. We conclude with a short summary and an outlook for future developments.
Evaporation of sessile droplets containing non-volatile solutes dispersed in a volatile solvent leaves behind ring-like solid stains. As the volatile species evaporates, pinning of the contact line gives rise to capillary flows that transport non-volatile solutes to the contact line. This phenomenon, called the coffee-ring effect, compromises the overall performance of industrially relevant manufacturing processes involving evaporation such as printing, biochemical analysis, manufacturing of nano-structured materials through colloidal and macromolecular patterning. Various approaches have been developed to suppress this phenomenon, which is otherwise difficult to avoid. The coffee-ring effect has also been leveraged to prepare new materials through convection induced assembly. This review underlines not only the strategies developed to suppress the coffee-ring effect but also sheds light on approaches to arrive at novel processes and materials. Working principles and applicability of these strategies are discussed together with a critical comparison.
Arrays of probe molecules integrated into a microfluidic cell are utilized as analytical tools to screen the binding interactions of the displayed probes against a target molecule. These assay platforms are useful in enzyme or antibody discovery, clinical diagnostics, and biosensing, as their ultraminiaturized design allows for high sensitivity and reduced consumption of reagents and target. We study here a platform in which the probes are first grafted to microbeads which are then arrayed in the microfluidic cell by capture in a trapping course. We examine a course which consists of V-shaped, half-open enclosures, and study theoretically and experimentally target mass transfer to the surface probes. Target binding is a two step process of diffusion across streamlines which convect the target over the microbead surface, and kinetic conjugation to the surface probes. Finite element simulations are obtained to calculate the target surface concentration as a function of time. For slow convection, large diffusive gradients build around the microbead and the trap, decreasing the overall binding rate. For rapid convection, thin diffusion boundary layers develop along the microbead surface and within the trap, increasing the binding rate to the idealized limit of untrapped microbeads in a channel. Experiments are undertaken using the binding of a target, fluorescently labeled NeutrAvidin, to its binding partner biotin, on the microbead surface. With the simulations as a guide, we identify convective flow rates which minimize diffusion barriers so that the transport rate is only kinetically determined and measure the rate constant.
This work introduces a robust means for excellent position registry of microparticles via a forced assembly technique on flexible or stretchable substrates. It is based on the dry powder rubbing process which allows assembly of a microparticle monolayer in a short time without requiring any solvent or thermal treatment. Elastic physical templates are used as substrates for the forced assembly in this study. Since the elastic templates can reduce the stress accumulation between the closely-packed particles, they can minimize the defect formation in the particle assembly in large areas. The method can be used with powders comprising irregularly shaped particles with a relatively large size distribution that cannot be periodically ordered by conventional self-assembly. Furthermore, non-closely packed particle array can be fabricated readily in large area, which is highly desirable for practical uses of the particle monolayers. The particle monolayers formed on the elastomer templates can be transferred to surfaces coated with thermoplastic block copolymers. Once transferred, the particle monolayers are flexible and stretchable over their entire surface. This work uses the particle monolayers on a large-area flexible substrate as photomasks to produce various photoresist patterns.
Dipolar interactions between nano- and micron sized colloids lead to their assembly into domains with well-defined local order. The particles with a single dipole induced by an external field assemble into linear chains and clusters. However, to achieve the formation of multidirectionally organized nano- or microassemblies with tunable physical characteristics, more sophisticated interaction tools are needed. Here we demonstrate that such complex interactions can be introduced in the form of two independent, non-interacting dipoles (double-dipoles) within a microparticle. We show how this can be achieved by the simultaneous application of alternating current (AC)-electric field and uniform magnetic field to dispersions of superparamagnetic microspheres. Depending on their timing and intensity, concurrent electric and magnetic fields lead to the formation of bidirectional particle chains, colloidal networks, and discrete crystals. We investigate the mechanistic details of the assembly process, and identify and classify the non-equilibrium states formed. The morphologies of different experimental states are in excellent correlation with our theoretical predictions based on Brownian dynamics simulations combined with a structural analysis based on local energy parameters. This novel methodology of introducing and interpreting double-dipolar particle interactions may assist in the assembly of colloidal coatings, dynamically reconfigurable particle networks, and bidirectional active structures.
This Review highlights the large number of methods to exploit colloidal assembly of comparably simple particles with nano- to micrometer dimensions in order to access complex structural hierarchies from nanoscopic over microscopic to macroscopic dimensions
A layer-by-layer (LbL) nanocoat (< 25 nm thick) of two polyelectrolytes, chitosan and chondroitin sulfate was self-assembled step-wise onto drug nanoparticles that were prepared by a solvent-evaporation emulsification method using eucalyptol as the oil phase. Four poorly water-soluble model drugs, furosemide, isoxyl, rifampin and paclitaxel were chosen to prepare these particles. Zeta potential, particle size measurements, and microscopic inspection of the coated particles were used to confirm the successful addition of each polyelectrolyte layer and the stability of the nanoparticles. This manufacturing process produced stable drug nanoparticles with volume mean diameters below 250 nm. Dissolution tests confirmed that although the nanocoat reduced the dissolution of nanoparticles proportional to the coat thickness; they still dissolved much faster than commercially available micronized powders of the drugs. In addition, increasing the layer thickness (still less than 50 nm thick) by adding more LbL bilayers produced sustained release nanoparticles. Ultimately, the LbL nanocoating stabilized these small particles against crystal growth and aggregation in suspension and resulted in nearly perfect controlled drug release.
A simple and novel method for avoiding the coffee ring structure has been demonstrated based on hydrophobic silicon pillar arrays during single-drop evaporation. When a drop of a colloidal suspension of latex spheres is dropped onto hydrophobic silicon pillar arrays with high contact angle hysteresis, the latex spheres are deposited at the periphery to form a porous gel foot due to the Wenzel wetting state of the drop on the substrate, which results in an effective elimination of the coffee ring structure. The coffee ring effect is avoided by relying on radially inward mass transport: a circulatory fluid flow triggered by means of the gel foot growth. Thus, uniform and ordered macroscale colloidal photonic crystals are fabricated. In the meantime, the influences of some factors, such as concentration of latex spheres, evaporation temperature, periodicity of hydrophobic silicon pillar arrays and the distance between the top rims of adjacent silicon pillars on drop deposition are investigated. This facile approach to eliminating the coffee ring structure will be of great significance for extensive applications of drop deposition in biochemical assays and material deposition.
As a strategy of autonomously organising nanoparticles into patterns or structures, colloidal self-assembly has attracted significant interests in both fundamental research and applied science. Discrete element method (DEM) coupled with a simplified fluid flow model is applied to investigate convective colloidal self-assembly. The model developed takes into account the interparticle interactions, i.e. the electrostatic repulsion, van der Waals attraction, Brownian motions, and the hydrodynamic effect. Therefore, a detailed insight of the combined influences of fluid flow field, geometrical confinement, and the interparticle interactions on the self-assembly process can be obtained. In this study, we simulated different self-assembled structures and various transition areas where a growing crystal transits from n to n+1 layer as a function of varied 3 phase contact angle, which is represented by a wedge geometry, and the velocity and direction of fluid flow. The crystal defects and the formation mechanism of different defects are theoretically studied through numerical simulation.
We present a novel technique coined directed evaporation-induced self-assembly (DEISA) that enables the formation of planarized opal-based microphotonic crystal chips in which opal crystal shape, size, and orientation are under synthetic control. We provide detailed synthetic protocols that underpin the DEISA process and formulate directed self-assembly strategies that are suited for the fabrication of opal architectures with complex form and designed optical functionality. These developments bode well for the utilization of opal-based photonic crystals in microphotonic crystal devices and chips.
The atomic force microscope (AFM) is not only a tool to image the topography of solid surfaces at high resolution. It can also be used to measure force-versus-distance curves. Such curves, briefly called force curves, provide valuable information on local material properties such as elasticity, hardness, Hamaker constant, adhesion and surface charge densities. For this reason the measurement of force curves has become essential in different fields of research such as surface science, materials engineering, and biology.
Several types of silicon-based inverse-opal films are synthesized, characterized by a range of experimental techniques, and studied in terms of electrochemical performance. Amorphous silicon inverse opals are fabricated via chemical vapor deposition. Galvanostatic cycling demonstrates that these materials possess high capacities and reasonable capacity retentions. Amorphous silicon inverse opals perform unsatisfactorily at high rates due to the low conductivity of silicon. The conductivity of silicon inverse opals can be improved by their crystallization. Nanocrystalline silicon inverse opals demonstrate much better rate capabilities but the capacities fade to zero after several cycles. Silicon–carbon composite inverse-opal materials are synthesized by depositing a thin layer of carbon via pyrolysis of a sucrose-based precursor onto the silicon inverse opals. The amount of carbon deposited proves to be insufficient to stabilize the structures and silicon–carbon composites demonstrate unsatisfactory electrochemical behavior. Carbon inverse opals are coated with amorphous silicon producing another type of macroporous composite. These electrodes demonstrate significant improvement both in capacity retentions and in rate capabilities. The inner carbon matrix not only increases the material conductivity but also results in lower silicon pulverization during cycling.
In this study, a new surfactant–solvent system was described for the preparation of periodic stripe patterns of zeolite A on solid substrates. The evaporation induced self-assembly of zeolite A particles was due to the stick–slip dynamics of the three-phase contact line of the colloid solutions in acetone containing 10% (v/v) poly(dimethylsiloxane) (PDMS) fluid (2 cst.). In order to investigate the possible effects of particle size and the particle concentration on the stick–slip dynamics, three types of zeolite A samples with different particle sizes (zeolite A-I: 250–500 nm, zeolite A-II: 100–250 nm and zeolite A-III: 0–100 nm) were utilized to prepare 0.007–0.06% (w/v) colloidal dispersions. Zeolite A micropatterns were self-assembled on the surface of glass, high density polyethylene (HDPE) and poly(tetrafluoroethylene) (PTFE) substrates, which were placed vertically inside the colloid solutions and held against the wall of the cylindrical vial during the evaporation of acetone. The stripe patterns of zeolite A particles were analyzed with field emission scanning electron microscope (FE-SEM) and optical microscope. The widths of microstripes and the distance between the stripes were found as 2–20 μm and 40–60 μm respectively depending on the particle concentration. By using the stick–slip dynamics of colloids, the linear micropatterns of zeolite A nanocrystals were prepared with low cost and low energy.Graphical abstractIn this study, a new surfactant–solvent system was described for the preparation of periodic stripe patterns of zeolite A. The evaporation induced self-assembly of particles was due to the stick–slip dynamics of the three-phase contact line of the colloids in acetone containing 10% poly(dimethylsiloxane). The effects of particle size and the particle concentration were investigated. The width of microstripes was found as 2–20 μm.
The synthesis and characterization of mechanically robust, optically tunable inverse opal hydrogel through colloidal crystal templating is presented. Mesoporous hydrogels based on HEMA-AA copolymers exhibit pH-dependent shifts in optical diffraction, the magnitude of which can be tailored by varying the AA concentration. In addition, such hydrogel also demonstrate ionic-strength-sensitive diffraction corresponding to both Donnan potential-induced welling and nonspecific polymer-water interactions.
The symmetry control of polymer colloidal crystals and monolayers using electrophoretic deposition onto patterned electrode surfaces was investigated. The method consisted of the electrophoretic deposition and laser-interference lithography (LIL) was used for the growth of colloidal monolayers or of colloidal crystals. In the absence of electric potential, the capillary forces between the colloidal particles were found to be higher than the forces acting between the colloids and the patterned substrate. The results show that method controls and changes the colloidal-crystal structure by introducing different patterns in a dielectric layer on top of of the electrodes.
Several thin-film deposition and etching techniques of the polymer fluorocarbon are investigated and the resulting thin-film properties will be compared with those of commercially available bulk polytetrafluoroethylene. The most promising deposition technique is performed in a conventional reactive ion etcher using a carbonhydrotrifluoride (CHF3) plasma. By changing the deposition parameters, control of the properties and step coverage of the deposited thin films within a certain range is possible, eg., uni-directional and conformal step coverage of deposited thin films can be obtained. Etching is performed with the help of an evaporated aluminium oxide mask using an oxygen, nitrogen, or sulfurhexafluoride plasma for isotropic etching, or a CHF3 plasma giving a directional etch profile. The combination of the unique properties, deposition and etching techniques make fluorocarbon thin films a promising tool for micromachining; a number of applications will be discussed and demonstrated.
We report the development of a microfabricated electrophoretic device for assembling high-density arrays of antibody-conjugated microbeads for chip-based protein detection. The device consists of a flow cell formed between a gold-coated silicon chip with an array of microwells etched in a silicon dioxide film and a glass coverslip with a series of thin gold counter electrode lines. We have demonstrated that 0.4 and 1 μm beads conjugated with antibodies can be rapidly assembled into the microwells by applying a pulsed electric field across the chamber. By assembling step-wise a mixture of fluorescently labeled antibody-conjugated microbeads, we incorporated both spatial and fluorescence encoding strategies to demonstrate significant multiplexing capabilities. We have shown that these antibody-conjugated microbead arrays can be used to perform on-chip sandwich immunoassays to detect test antigens at concentrations as low as 40 pM (6 ng/mL). A finite element model was also developed to examine the electric field distribution within the device for different counter electrode configurations over a range of line pitches and chamber heights. This device will be useful for assembling high-density, encoded antibody arrays for multiplexed detection of proteins and other types of protein-conjugated microbeads for applications such as the analysis of protein-protein interactions.
We have extended the widely used technique of nanosphere lithography to produce nanosphere templates with significantly improved long-range order. Single, ordered domains stretching over areas greater than 1 cm2 have been achieved by assembling spheres with the correct surface chemistry on a water/air interface. Self-assembly over macroscopic areas is facilitated by a combination of electrostatic and capillary forces. The presented technique is easily implemented, and the assembled monolayers can be transferred onto almost any surface, thus making the procedure applicable to a broad range of nanoscale research. We demonstrate this through the fabrication of hexagonally ordered, macroscopic arrays of magnetic nanostructures with modified magnetic properties.