Microfluidic devices easily created using an office inkjet printer

Although various processes have been used for producing microfluidic devices, many of them are not so simple that ordinary end-users can produce the devices by themselves. However, in this study, microfluidic devices were easily produced using an office inkjet printer. As the components of the device, channels, manifolds, and mixers were created by printing their shapes on glass slides using the printer. A syringe pump could control the flow of fluid through the manifolds and mixers. In addition, resistivity of the device to acidic and basic solutions was tested.     

Drop-on-demand printed microfluidics device with sensing electrodes using silver and PDMS reactive inks

For this work, a cure-in-place polydimethylsiloxane (PDMS) reactive ink was developed and its utility demonstrated by printing a complete microfluidic mixer with integrated electrodes to measure fluid conductivity, concentration, and mixing completeness. First, a parameter-space investigation was conducted to generate a set of PDMS inks and printing parameters compatible with drop-on-demand (DOD) printing constraints. Next, a microfluidic mixer was fabricated using DOD-printed silver reactive inks, PDMS reactive inks, and a low-temperature polyethylene glycol fugitive ink. Lastly, the device was calibrated and tested using NaCl solutions with concentrations ranging from 0.01 to 1.0 M to show that electrolyte concentration and mixing completeness can be accurately measured. Overall, this work demonstrates a set of reactive inks and processes to fabricate sophisticated microfluidic devices using low-cost inks and DOD printing techniques.     

Quality evaluation of ink-jet paper with principal components analysis

Non-impact printing techniques have in many fields of application replaced traditional printing methods. Apart from factors such as the type and conditions of ink/paper transfer and ink properties, the quality of the image produced with an office ink-jet printer depends decisively on paper characteristics. In this study, various medium- and photo-quality ink-jet paper sorts were examined for their general, optical, surface and printing properties. Based on these results, the ranking of tested paper properties according to their significance for ink-jet paper quality on one hand and the suitability of individual paper sorts for this printing technique on the other was determined using multivariate tool principal components analysis.     

基于机器学习的彩色匹配技术

在不同的设备间保持色彩的一致性是当前彩色印刷的一项重要的世界性技术难题.从机器学习的角度,提出了基于学习的色彩空间变换方法,并利用科学发现的基本思想,较为成功地解决了彩色喷墨打印机中的彩色匹配的自动化和通用性问题,取得了理想的匹配效果.该技术目前已与日本佳能泰克公司进一步合作、开发,实现商品化.     

Fabrication of three-dimensional microfluidic channels in a single layer of cellulose paper

Three-dimensional microfluidic paper-based analytical devices (3D-μPADs) represent a promising platform technology that permits complex fluid manipulation, parallel sample distribution, high throughput, and multiplexed analytical tests. Conventional fabrication techniques of 3D-μPADs always involve stacking and assembling layers of patterned paper using adhesives, which are tedious and time-consuming. This paper reports a novel technique for fabricating 3D microfluidic channels in a single layer of cellulose paper, which greatly simplifies the fabrication process of 3D-μPADs. This technique, evolved from the popular wax-printing technique for paper channel patterning, is capable of controlling the penetration depth of melted wax, printed on both sides of a paper substrate, and thus forming multilayers of patterned channels in the substrate. We control two fabrication parameters, the density of printed wax (i.e., grayscale level of printing) and the heating time, to adjust the penetration depth of wax upon heating. Through double-sided printing of patterns at different grayscale levels and proper selection of the heating time, we construct up to four layers of channels in a 315.4-μm-thick sheet of paper. As a proof-of-concept demonstration, we fabricate a 3D-μPAD with three layers of channels from a paper substrate and demonstrate multiplexed enzymatic detection of three biomarkers (glucose, lactate, and uric acid). This technique is also compatible with the conventional fabrication techniques of 3D-μPADs, and can decrease the number of paper layers required for forming a 3D-μPAD and therefore make the device quality control easier. This technique holds a great potential to further popularize the use of 3D-μPADs and enhance the mass-production quality of these devices.     

Behavior of capillary valves in centrifugal microfluidic devices prepared by three-dimensional printing

This paper details the behavior of capillary valves in centrifugal microfluidic devices prepared by three-dimensional (3D), or solid-object, printing. Microfluidic structures containing valve channels with different widths, heights, and radial distances from the center of rotation were studied and compared with extant capillary valve theories. Due to the printing process, the produced valve channels possessed a ridged or “scalloped” pattern. Hence, actual channel widths at the widest and narrowest points of the ridged pattern were determined, and used in comparisons between theoretical and empirical values. In addition, variations in contact angle resulting from the ridged pattern were measured and employed in theoretical calculations. For 1-mm high valve channels, the critical angular frequency (rpm) required to overcome capillary valve pressure was found to be independent of width. However, as the height of the valve channel was reduced, the critical rpm was found to become progressively more width-dependent increasing more rapidly for narrower channels. Both of these observations point to a role for feature sharpness, as well as the geometry of the valve channel opening, in valve behavior. Otherwise, valves followed a predictable trend of increasing critical rpm with decreased valve height and decreased radial distance from the rotation center. Using these results as a guide, then, it is possible to prepare centrifugal microfluidic devices by 3D printing with operability comparable to devices prepared by other microfabrication techniques.     

Insight into the micro scale dynamics of a micro fluidic wetting-based conveying system by particle based simulation

We simulate a microfluidic conveying system using the many-body dissipative particle dynamics method (MDPD). The conveying system can transport micro parts to a specified spot on a surface by letting them float inside or on top of a droplet, which is pumped by changing the wetting behaviour of the substrate, e.g., with electrowetting on dielectrics. Subsequent evaporation removes the fluid; the micro part remains on its final position, where a second substrate can pick it up. In this way, the wetting control can be separate from the final device substrate. The MDPD method represents a fluid by particles, which are interpreted as a coarse graining of the fluid’s molecules. The choice of interaction forces allows for free surfaces. To introduce a contact angle model, non-moving particles beyond the substrate interact with the fluid particles by MDPD forces such that the required contact angle emerges. The micro part is simulated by particles with spring-type interaction forces.     

Experimental-based feedforward control for a DoD inkjet printhead

Markets demand continuously for higher quality, higher speed, and more energy-efficient professional printers. Drop-on-Demand (DoD) inkjet printing is considered as one of the most promising printing technologies. It offers many advantages including high speed, quiet operation, and compatibility with a variety of printing media. Nowadays, it has been used as low-cost and efficient manufacturing technology in a wide variety of markets. Although the performance requirements, which are imposed by the current applications, are tight, the future performance requirements are expected to be even more challenging. These print requirements are related to the jetted drop properties, namely, drop velocity, drop volume, drop velocity consistency, productivity, and reliability. Meeting these performance requirements is restricted by several operational issues that are associated with the design and the operation of inkjet printheads. Major issues that are usually encountered are residual vibrations and crosstalk among ink channels. These result in a poor printing quality for high-speed printing. The main objective is to design a feedforward control strategy such that variations in the velocity and volume of the jetted drops are minimized. In this article, an experimental-based feedforward control scheme is proposed to improve the performance of a professional inkjet printer.     

Printing 3D microfluidic chips with a 3D sugar printer

This study demonstrated how to quickly and effectively print two-dimensional (2D) and three-dimensional (3D) microfluidic chips with a low-cost 3D sugar printer. The sugar printer was modified from a desktop 3D printer by redesigning the extruder, so the melting sugar could be extruded with pneumatic driving. Sacrificial sugar lines were first printed on a base layer followed by casting polydimethylsiloxane (PDMS) onto the layer and repeating. Microchannels were then printed in the PDMS solvent, microfluidic chips dropped into hot water to dissolve the sugar lines after the PDMS was solidified, and the microfluidic chips did not need further sealing. Different types of sugar utilized for printing material were studied with results indicating that maltitol exhibited a stable flow property compared with other sugars such as caramel or sucrose. Low cost is a significant advantage of this type of sugar printer as the machine may be purchased for only approximately $800. Additionally, as demonstrated in this study, the printed 3D microfluidic chip is a useful tool utilized for cell culture, thus proving the 3D printer is a powerful tool for medical/biological research.     

用于微流控制备的3D打印机设计

针对微流控芯片传统加工工艺成本较高,工时较长等问题提出了一种低成本、适用于微流控芯片制备的3D打印机设计方案,该设计方案由3D打印机本体和上位机控制软件组成,其中上位机控制软件负责将事先建好的三维模型进行分析、切片,并生成G-code格式文件;3D打印控制系统负责接收、解析G-code文件及转化为打印机可识别的控制指令以完成物体的快速成型。详细阐述了3D打印机各功能模块的具体实现,给出了测试打印结果,证明该打印机具有成本低、精度高的优点。     

LED packaging by ink‐jet microdeposition of high‐viscosity resin and phosphor dispersion

Abstract— An ink‐jet‐printing method applied to the microdeposition of high‐viscosity resin, including optimization of phosphor dispersion for light‐emitting‐diode (LED) packaging was examined for the first time. An ultrasonic ink‐jet‐printing method was used, in which ink droplets are ejected by a focused ultrasonic beam from a nozzle‐less printhead. To fabricate white LEDs, high‐viscosity phosphor‐dispersed resin was deposited to form an encapsulant dome. Two types of methods to control phosphor sedimentation for color uniformity were examined; one is heating the lead frame during the resin deposition, and the other is hydrophobic surface treatment of the lead frame base enabling the fabrication of a small encapsulant dome. For light direction control, a silicone micro lens was deposited on an encapsulant dome using the ink‐jet method. The results show that ultrasonic ink‐jet printing is an applicable technique to optimize and modify on‐demand optical characteristics of LED devices.     

Self-priming of liquids in capillary autonomous microfluidic systems

We present a numerical approach to the capillary rise dynamics in microfluidic channels of complex 3D geometries. In order to optimize the delivery of specific biological fluids to target regions in microfluidic capillary autonomous systems (CAS), we analyze self-priming of liquid water into a microfluidic device consisting of a microfluidic channel that feeds a rectangular microfluidic cavity trough an appropriately designed micro-chamber. The target performance criteria in our optimization are (1) fast and complete wetting of the cavity bottom while (2) minimizing the probability of trapping air bubble in the device. The numerical model is based on the lattice Boltzmann method (LBM) and a three-dimensional single-component multiple-phase (SCMP) scheme. By using a parallel implementation of this algorithm, we investigate the physical processes related to the invasion of the liquid–gas interfaces in rectangular cavities at different liquid–solid contact angle and shapes of the transition micro-chamber. The numerical results has successfully captured important qualitative and some key quantitative effects of the liquid–solid contact angle, the roughness of the cavity edges, the depth of the holes and shape of the micro-chambers. Moreover, we present and validate experimentally simple geometrical optimizations of the microfluidic device that ensure the complete filling the microfluidic cavity with liquid. Critical parameters related to the overall priming time of the device are presented as well.     

Norland optical adhesive (NOA81) microchannels with adjustable wetting behavior and high chemical resistance against a range of mid-infrared-transparent organic solvents

Different methods to adjust the wetting behavior of surfaces of the UV-curable adhesive NOA81 were investigated and quantitatively characterized by dynamic contact angle measurements with an optical goniometer. A new method to make NOA81 surfaces hydrophobic by mixing an additive in the uncured polymer was presented. The effect was confirmed by surface roughness studies using atomic force microscopy and X-ray photoelectron spectroscopy measurements. The chemical resistance of NOA81 microfluidic channels was evaluated by flowing organic solvents therein. Emphasis was placed on IR-transparent organic solvents. A simple, low-cost method to fabricate chemically resistant, hydrophilic, hydrophobic and hybrid (hydrophilic and hydrophobic), all-polymer microfluidic channels made of NOA81 was reported. Applications like oil-in-water and water-in-oil droplet generation or handling of a multi-phase flow were presented to demonstrate the chemical resistance and the control over the wetting behavior of NOA81 microfluidic chips.     

Hydrophobic patterning of functional porous pigment coatings by inkjet printing

Paper-based analytical devices provide novel platforms for functional sensing applications, such as medical diagnostics and environmental monitoring. They are based on porous hydrophilic material, which transports the sample liquid by capillary action. The directional flow of aqueous liquids can be controlled by selective hydrophobising of pores. Earlier research in this field has concentrated on highly porous cellulose papers as base substrates, with no significant interest shown for pigment coatings. Such coatings can produce significantly thinner porous layers, thus requiring smaller sample volumes. This study investigates the hydrophobic patterning of custom-designed porous pigment coatings by functional inkjet printing. Tested coatings consisted of reference ground calcium carbonate and porous functionalised calcium carbonate (FCC) pigments combined with various binders, including microfibrillated cellulose. The hydrophobising custom-made inks contain polystyrene or alkyl ketene dimer (AKD) in p-xylene. The patterning is demonstrated by reaction arrays and simple channels. With polystyrene ink, successful hydrophobic barriers could be generated on all tested pigment/binder coatings, although generally requiring printing of multiple layers of barrier material. With AKD ink, hydrophobic patterns could be created successfully on coatings containing an organic binder, but not on coatings with inorganic sodium silicate as binder. The AKD ink generated hydrophobic barriers using fewer ink layers compared with polystyrene ink. Interestingly, AKD ink could hydrophobise the FCC pigment alone without binder, presumably due to hydroxyl groups on the pigment surface. Hydrophobic patterning of the pigment coatings is seen to require large amounts of hydrophobising agent, likely related to the high specific surface area.     

Thin-film electrode based droplet detection for microfluidic systems

We report on a droplet-producing microfluidic system with electrical impedance-based detection. The microfluidic devices are made of polydimethylsiloxane (PDMS) and glass with thin film electrodes connected to an impedance-monitoring circuit. Immiscible fluids containing the hydrophobic and hydrophilic phases are injected with syringe pumps and spontaneously break into water-in-oil droplet trains. When a droplet passes between a pair of electrodes in a medium having different electrical conductivity, the resulting impedance change signals the presence of the particle for closed-loop feedback during processing. The circuit produces a digital pulse for input into a computer control system. The droplet detector allows estimation of a droplet's arrival time at the microfluidic chip outlet for dispensing applications. Droplet detection is required in applications that count, sort, and direct microfluidic droplets. Because of their low cost and simplicity, microelectrode-based droplet detection techniques should find applications in digital microfluidics and in three-dimensional printing technology for rapid prototyping and biotechnology.     

Optimization and application of dry film photoresist for rapid fabrication of high-aspect-ratio microfluidic devices

Fabrication of high-aspect-ratio PDMS microfluidic devices with conventional SU-8 based soft photolithography is challenging, and often, the thickness of the master from which PDMS replicas are molded is non-uniform. Here, we present an optimized, low cost, fast prototyping microfabrication technique to make deep (up to 500 μm) and high-aspect-ratio (up to 10) microfluidic channels by producing masters by laminating a single or multiple layers of a thin dry film photoresist onto metal wafers. In particular, we explore the required exposure energy for different film thicknesses as well as the highest achievable channel depths and aspect ratios. The homogeneity of the depth of PDMS channels formed using these masters is quantified and found to be remarkably uniform over distances of 20 mm or more. The importance of the processing parameters, such as the exposure energy and development time on final feature size, wall angle, and channel aspect ratio, is investigated. In addition, we report some failure cases, the potential reasons, and strategies for making optimized devices. Potentially, deep microfluidic channels with a wide range of aspect ratios can be used to make long, homogenous separation devices that can be used in cell sorting, filtration, and flow cytometry. We believe the protocols we outline here will be of great utility to the microfluidics community.     

Rapid prototyping of PMMA microfluidic chips utilizing a CO<Subscript>2</Subscript> laser

A commercially available CO₂ laser scriber is used to perform the direct-writing ablation of polymethyl-methacrylate (PMMA) substrates for microfluidic applications. The microfluidic designs are created using commercial layout software and are converted into the command signals required to drive the laser scriber in such a way as to reproduce the desired microchannel configuration on the surface of a PMMA substrate. The aspect ratio and surface quality of the ablated microchannels are examined using scanning electron microscopy and atomic force microscopy surface measurement techniques. The results show that a smooth channel wall can be obtained without the need for a post-machining annealing operation by performing the scribing process with the CO₂ laser beam in an unfocused condition. The practicality of the proposed approach is demonstrated by fabricating two microfluidic chips, namely a cytometer, and an integrating microfluidic chip for methanol detection, respectively. The results confirm that the proposed unfocused ablation technique represents a viable solution for the rapid and economic fabrication of a wide variety of PMMA-based microfluidic chips.     

Plastic LC/MS microchip with an embedded microstructured fibre having the dual role of a frit and a nanoelectrospray emitter

Integrating functionality on a microfluidic platform, such as a frit or electrospray emitter for coupling with a mass spectrometer, can be complicated, costly, and introduce dead volume deleterious to liquid chromatography (LC). Here, we demonstrate the fabrication of a LC/MS microchip device with an integrated nanoelectrospray emitter that also functions as a retaining frit. The integrated emitter effectively minimizes dead volume associated with coupling the device externally to MS with electrospray ionization and complications associated with frit microfabrication. The emitter is made of microstructured optical fibre, which has an array of equivalent parallel channels. The size and spacing of these channels make the fibre amenable to retaining chromatographic packing material while simultaneously performing as a nanoelectrospray emitter. The chromatographic chip is fashioned from cyclic olefin copolymer (Zeonor) using hot embossing and thermal/solvent bonding techniques. The device is fabricated in less than 3 h with simple technique and low-cost materials while maintaining sufficient solvent resistance and mechanical strength, withstanding pressure drops of up to 100 bar. This microchip was used to effectively separate small drug molecules by isocratic elution and larger peptides using gradient elution. Robustness of the plastic LC/MS devices was demonstrated by frequent use over several months with consistent results. Such devices represent a practical approach to the facile integration of a frit and/or an ESI emitter in a microfluidic device at low cost while minimizing dead volume associated with coupling the chip to MS.     

Reversible anemometric sticker sensor applied on finalized polymeric LoC devices

In this paper, an anemometric sticker sensor is presented, which can be used as a flag to verify the completion of a particular step inside microfluidic devices, the presence of leakages or trapped air bubbles. The similarity of the fabrication method to the printing and roll-to-roll techniques offers a simple and realistic mass-producible solution for direct monitoring of microfluidic protocols inside lab-on-a-chip devices. The detector has been fabricated on a sticker, which presents low space needs and can be easily applied on the test chip with no strict alignment requirements. The introduced idea fulfills the sensing demand that currently microfluidic devices present, being reusable or disposable due to its low cost requirements.     

Clean color: Improving multi‐filament 3D prints

Fused Filament Fabrication is an additive manufacturing process by which a 3D object is created from plastic filament. The filament is pushed through a hot nozzle where it melts. The nozzle deposits plastic layer after layer to create the final object. This process has been popularized by the RepRap community. Several printers feature multiple extruders, allowing objects to be formed from multiple materials or colors. The extruders are mounted side by side on the printer carriage. However, the print quality suffers when objects with color patterns are printed – a disappointment for designers interested in 3D printing their colored digital models. The most severe issue is the oozing of plastic from the idle extruders: Plastics of different colors bleed onto each other giving the surface a smudged aspect, excess strings oozing from the extruder deposit on the surface, and holes appear due to this missing plastic. Fixing this issue is difficult: increasing the printing speed reduces oozing but also degrades surface quality – on large prints the required speed level become impractical. Adding a physical mechanism increases cost and print time as extruders travel to a cleaning station. Instead, we rely on software and exploit degrees of freedom of the printing process. We introduce three techniques that complement each other in improving the print quality significantly. We first reduce the impact of oozing plastic by choosing a better azimuth angle for the printed part. We build a disposable rampart in close proximity of the part, giving the extruders the opportunity to wipe oozing strings and refill with hot plastic. We finally introduce a toolpath planner avoiding and hiding most of the defects due to oozing, and seamlessly integrating the rampart. We demonstrate our technique on several challenging multiple color prints, and show that our tool path planner improves the surface finish of single color prints as well.