Fabrication of microfluidic channels based on melt-electrospinning direct writing

Microfluidics is a flourishing field, enabling a wide range of applications. However, the current fabrication methods for creating the microchannel structures of microfluidic devices, such as photolithography and 3D printing, mostly have the problems of time-consuming, high cost or low resolution. In this work, we developed a simple and flexible method to fabricate PDMS microfluidic channels, based on poly(ε-caprolactone) (PCL) master mold additive manufactured by a technique termed melt-electrospinning direct writing (MEDW). It relies on the following steps: (1) direct writing of micrometric PCL 2D or 3D pattern by MEDW. (2) Casting PDMS on the printed PCL pattern. (3) Peeling off of patterned PDMS from the embedded sacrificial PCL layer. (4) Bonding the PDMS with microchannel to another PDMS layer by hot pressing. The process parameters during MEDW such as collector speed, nozzle dimension and temperature were studied and optimized for the quality and dimension of the printed micropatterns. Multilayer fiber deposition was developed and applied to achieve microscale architectures with high aspect ratio. Thus, the microchannels fabricated by the proposed approach could possess tunable width and depth. Finally, T-shape and cross-channel devices were fabricated to create either laminar flow or microdroplets to illustrate the applicability and potential of this method for microfluidic device manufacture.     

3D-printed membrane microvalves and microdecoder

A microfluidic system for multichannel switching and multiphase flow control has potential uses in pneumatic soft robotics and biological sampling systems. At present, the membrane microvalves used in microfluidic systems are mostly constructed using a multilayer bonding process so that the device cannot withstand high pressures. In this paper, we demonstrate a design method and the properties of a bondless membrane microvalve fabricated using a commercial 3D printer. We used a multijet (MJP) 3D printer to print a 100-μm-thick and 6-mm-diameter membrane from a relatively hard material (1700 MPa). The membrane’s high toughness ensures that it does not need negative pressure to reopen. The measured operation frequency was less than 2.5 Hz under a pneumatic pressure of 14.5 kPa. We also 3D-printed an integrated Quake-style microfluidic decoder network by combining 8 valves in series to demonstrate the integrability of the microvalve. The decoder chip was demonstrated to control the ON/OFF state of the four coded fluidic channels, with the droplets being generated from selected channels according to the valve action. Therefore, such 3D-printed microvalves are highly integrable, have a high manufacturing efficiency, and can be applied in pneumatic controllers, sample switchers and integrated print heads.

     

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.     

3D printing: an emerging tool for novel microfluidics and lab-on-a-chip applications

In the past few years, 3D printing technology has witnessed an explosive growth, penetrating various aspects of our lives. Current best-in-class 3D printers can fabricate micrometer scale objects, which has made fabrication of microfluidic devices possible. The highest achievable resolution is already at nanometer scale, which is continuing to drop. Since geometric complexity is not a concern for 3D printing, novel 3D microfluidics and lab-on-a-chip systems that are otherwise impossible to produce with traditional 2D microfabrication technology have started to emerge in recent years. In this review, we first introduce the basics of 3D printing technology for the microfluidic community and then summarize its emerging applications in creating novel microfluidic devices. We foresee widespread utilization of 3D printing for future developments in microfluidic engineering and lab-on-a-chip technology.     

聚二甲基硅氧烷微流体芯片的制作技术

基于MEMS技术的微流体芯片在分析化学和生物医学领域显示了巨大的应用潜力。作为构建微流体芯片的基底材料———聚二甲基硅氧烷(PDMS)已经表现出了许多的优点:良好的电绝缘性、较高的热稳定性、优良的光学特性以及简单的加工工艺等。采用浇注法制作了PDMS电泳微芯片,对PDMS微流体芯片的加工工艺、封装方法和结构特征进行了探讨,并提出了相应的解决方案。     

新型压电微泵的结构设计与理论分析

微泵在微流控化学分析芯片中有很大的应用前景,日益成为人们研究的热点。从结构设计、理论分析和工艺加工3个方面研究了微阀与微泵,设计出用压电驱动和聚二甲基硅氧烷(PDMS)作为泵膜的集成微阀与微泵,其特点是原理新颖、结构简单、易于加工、操作方便。结构主要是由PDMS泵膜、硅片和压电驱动器组成,其中,PDMS既是泵膜和缓冲单元,也是主动阀片。在直流电压的驱动下,其工作状态是微阀,阻止流体的单向流通,在方波信号的驱动下,其工作状态是微泵,实现流体的吸入与泵出。给出各种几何参数、工作原理和工艺流程。     

Enhancement of the surface free energy of PDMS for reversible and leakage-free bonding of PDMS–PS microfluidic cell-culture systems

Polydimethylsiloxane (PDMS) has become one of the most widely used materials in the fabrication of microfluidic systems bonded onto glass substrates, especially for cell biology applications. However, PDMS is often unsuitable for building microfluidic systems onto polystyrene (PS) which is the preferred substrate in most cell-culture protocols. In particular, PS is required for culturing many stem cell and primary cell types. Here, we propose a novel approach to building PDMS–PS microfluidic cell-culture systems, specifically realizing a strong and reversible bonding of PDMS on PS without using chemical agents which can have negative effects on cell viability. Our strategy to strengthen the bonding of PDMS to PS surfaces is to increase the surface free energy (SFE) by adjusting the mixing ratio of PDMS base to curing agent and by treating the surfaces of PDMS and PS with O₂ plasma and annealing. Our results show that using this method for PDMS–PS bonding, we are able to produce reliable reversible and leakage-free PDMS–PS microfluidic cell-culture systems.     

A rapid polymerisation process realizing 3D biocompatible structures in a microfluidic channel suitable for genetic analysis

Surface probe immobilisation is a complex and time consuming task undertaken prior to microfluidic integration, this requires surface functionalisation, biomolecule spotting, incubation and blocking steps. Traditional bonding techniques (anodic, thermal, etc.) or adhesives (UV cured) used to seal fluidic systems may denature biomolecules due to high temperature or vapour effects, thus bonding techniques such as thin film laminate or PDMS are used to seal systems, with substrate-fluidic alignment required prior to bonding. We propose a technique allowing probe DNA molecules to be immobilised in a sealed microfluidic system using (3D) hydrogel structures without any alignment steps. A prepolymer solution is introduced to the channels where photo-polymerisation is undertaken forming 3D structures covalently attached to the channel surface. We use a photo-initiated prepolymer material poly-ethylene-glycol (PEG) to form structures containing probe DNA. This process is fast compared to conventional biomolecule immobilisation techniques and is also biocompatible, this direct write approach removes overnight immobilisation/incubation of the probe DNA, it also facilitates immobilisation within a sealed fluidic system where conventionally DNA probe spots must be immobilised prior to channel sealing. We consider the transport of target DNA from bulk analyte to the 3D gel structure and evaluate hybridisation within the microfluidic system.     

Fabrication of layered polydimethylsiloxane/perfluoropolyether microfluidic devices with solvent compatibility and valve functionality

The development of multilayer soft lithography methodology has seen polydimethysiloxane (PDMS) as the preferred material for the fabrication of microfluidic devices. However, the functionality of these PDMS microfluidic chips is often limited by the poor chemical resistance of PDMS to certain solvents. Here, we propose the use of a photocurable perfluoropolyether (PFPE), specifically FOMBLIN^® MD40 PFPE, as a candidate material to provide a solvent-resistant buffer layer to make the device substantially impervious to chemically induced swelling. We first carried out a systematic study of the solvent resistance properties of FOMBLIN^® MD40 PFPE as compared with PDMS. The comparison presented here demonstrates the superiority of FOMBLIN^® MD40 PFPE over PDMS in this regard; moreover, the results permitted to categorize solvents in four different groups depending on their swelling ratio. We then present a step-by-step recipe for a novel fabrication process that uses multilayer lithography to construct a comprehensive solvent-resistant device with fluid and control channels integrated with a valve structure and also permitting easy establishment of outside connections.     

SU-8 microchannels for live cell dielectrophoresis improvements

In this work a novel highly precise SU-8 fabrication technology is employed to construct microfluidic devices for sensitive dielectrophoretic (DEP) manipulation of budding yeast cells. A benchmark microfluidic live cell sorting system is presented, and the effect of microchannel misalignment above electrode topologies on live cell DEP is discussed in detail. Simplified model of budding Saccharomyces cerevisiae yeast cell is presented and validated experimentally in fabricated microfluidic devices. A novel fabrication process enabling rapid prototyping of microfluidic devices with well-aligned integrated electrodes is presented and the process flow is described. Identical devices were produced with standard soft-lithography processes. In comparison to standard PDMS based soft-lithography, an SU-8 layer was used to construct the microchannel walls sealed by a flat sheet of PDMS to obtain the microfluidic channels. Direct bonding of PDMS to SU-8 surface was achieved by efficient wet chemical silanization combined with oxygen plasma treatment of the contact surface. The presented fabrication process significantly improved the alignment of the microstructures. While, according to the benchmark study, the standard PDMS procedure fell well outside the range required for reasonable cell sorting efficiency. In addition, PDMS delamination above electrode topologies was significantly decreased over standard soft-lithography devices. The fabrication time and costs of the proposed methodology were found to be roughly the same.

     

Investigation and improvement of reversible microfluidic devices based on glass–PDMS–glass sandwich configuration

Reversibly assembled microfluidic devices are dismountable and reusable, which is useful for a number of applications such as micro- and nano-device fabrication, surface functionalization, complex cell patterning, and other biological analysis by means of spatial–temporal pattern. However, reversible microfluidic devices fabricated with current standard procedures can only be used for low-pressure applications. Assembling technology based on glass–PDMS–glass sandwich configuration provides an alternative sealing method for reversible microfluidic devices, which can drastically increase the sealing strength of reversibly adhered devices. The improvement mechanism of sealing properties of microfluidic devices based on the sandwich technique has not been fully characterized, hindering further improvement and broad use of this technique. Here, we characterize, for the first time, the effect of various parameters on the sealing strength of reversible PDMS/glass hybrid microfluidic devices, including contact area, PDMS thickness, assembling mode, and external force. To further improve the reversible sealing of glass–PDMS–glass microfluidic devices, we propose a new scheme which exploits mechanical clamping elements to reinforce the sealing strength of glass–PDMS–glass sandwich structures. Using our scheme, the glass–PDMS–glass microchips can survive a pressure up to 400 kPa, which is comparable to the irreversibly bonded PDMS microdevices. We believe that this bonding method may find use in lab-on-a-chip devices, particularly in active high-pressure-driven microfluidic devices.     

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.     

A microfluidic capacitance sensor for fluid discrimination and characterization

A microfluidic device with embedded capacitive sensing is proposed. The purpose of the device is fluid discrimination and characterization in a microchannel on the basis of the dielectric permittivity. The device is fabricated in a hybrid cost-effective technology which innovatively combines PDMS (PolyDiMethylSiloxane) soft photolithography and screen printing techniques. A microchannel, realized in a PDMS layer, is placed in the field of a sensing capacitor formed by electrodes screen-printed on a glass substrate. Fluids inside the microchannel affect the capacitance, that is in the order of femtofarads, which is measured by a tailored electronic interface system. The electronic system features a sensitivity of 100 V/pF and a resolution threshold of 0.06 fF. Experimental results obtained for different fluids injected in the microchannel demonstrate the ability of the system to discriminate the fluids and to estimate their dielectric permittivity both as pure samples and as mixtures at varying solute fractions. This makes the device a promising building block for fluid mixing monitoring in microfluidic systems.     

Ink-jet printed nanoparticle microelectromechanical systems

Reports a method to additively build three-dimensional (3-D) microelectromechanical systems (MEMS) and electrical circuitry by ink-jet printing nanoparticle metal colloids. Fabricating metallic structures from nanoparticles avoids the extreme processing conditions required for standard lithographic fabrication and molten-metal-droplet deposition. Nanoparticles typically measure 1 to 100 nm in diameter and can be sintered at plastic-compatible temperatures as low as 300°C to form material nearly indistinguishable from the bulk material. Multiple ink-jet print heads mounted to a computer-controlled 3-axis gantry deposit the 10% by weight metal colloid ink layer-by-layer onto a heated substrate to make two-dimensional (2-D) and 3-D structures. We report a high-Q resonant inductive coil, linear and rotary electrostatic-drive motors, and in-plane and vertical electrothermal actuators. The devices, printed in minutes with a 100 μm feature size, were made out of silver and gold material with high conductivity,and feature as many as 400 layers, insulators, 10:1 vertical aspect ratios, and etch-released mechanical structure. These results suggest a route to a desktop or large-area MEMS fabrication system characterized by many layers, low cost, and data-driven fabrication for rapid turn-around time, and represent the first use of ink-jet printing to build active MEMS     

A novel method to construct 3D electrodes at the sidewall of microfluidic channel

We report a simple, low-cost and novel method for constructing three-dimensional (3D) microelectrodes in microfluidic system by utilizing low melting point metal alloy. Three-dimensional electrodes have unique properties in application of cell lysis, electro-osmosis, electroporation and dielectrophoresis. The fabrication process involves conventional photolithography and sputtering techniques to fabricate planar electrodes, positioning bismuth (Bi) alloy microspheres at the sidewall of PDMS channel, plasma bonding and low temperature annealing to improve electrical connection between metal microspheres and planar electrodes. Compared to other fabrication methods for 3D electrodes, the presented one does not require rigorous experimental conditions, cumbersome processes and expensive equipments. Numerical analysis on electric field distribution with different electrode configurations was presented to verify the unique field distribution of arc-shaped electrodes. The application of 3D electrode configuration with high-conductive alloy microspheres was confirmed by particle manipulation based on dielectrophoresis. The proposed technique offers alternatives to construct 3D electrodes from 2D electrodes. More importantly, the simplicity of the fabrication process provides easy ways to fabricate electrodes fast with arc-shaped geometry at the sidewall of microchannel.     

Multi-height micro structures in poly(dimethyl siloxane) lab-on-a-chip

The PDMS (poly(dimethyl siloxane)) replica molding is very useful for lab-on-a-chip development. This paper describes the several fabrication schemes of master fabrication for multi-height micro structures in PDMS microchip, and gives the overview over the technologies used for PDMS replica molding and channel closing with oxygen plasma treatment. Developed processes in the paper are good solution to fabricate the more complex structures in microfluidic lab-on-a-chip.The fabrication of the mold was done with the help of K.C. Shin and the PDMS replica molding process with the help of K.M. Yi in Digital Bio Technology Co. The authors wish to acknowledge that this paper is a result of the research accomplished with the financial support of the Intelligent Microsystem Center, Seoul, Korea, which is carrying out one of the 21st centurys New Frontier R&D Projects sponsored by the Korea Ministry of Science & Technology.     

具有3D微纳结构PDMS阵列免疫传感芯片研制

介绍了一种简便快速加工微阵列免疫传感芯片的新方法。采用化学刻蚀技术加工具有μm级山脉状起伏和nm级表面粗糙度结构(简称为3D微纳表面)的玻璃阳模,以该阳模为模板浇注法制得表面具有3D微纳表面结构的PDMS基片,再借助于物理吸附,将抗体直接固定于该PDMS表面,形成具有3D微纳结构的PDMS微阵列免疫传感器。利用光学显微镜和原子力显微镜对玻璃阳模和PDMS基片表面形貌进行表征,研究了PDMS表面微纳结构化处理对抗体吸附能力的影响。结果表明:3D微纳结构的PDMS由于具有大的比表面积,能显著增强抗体的吸附能力。将研制所得的3D微纳表面结构的PDMS芯片用于微阵列荧光免疫分析,其灵敏度是平板PDMS的5倍。     

Tunable Pneumatic Microoptics

A new class of tunable and actuated microoptical devices is presented: pneumatic microoptics. Using microelectromechanical system fabrication technology extended by the use of polydimethylsiloxane (PDMS) membranes, tunable microlenses, and lens arrays, actuated micromirrors with large tilt angles and tip-tilt piston mirrors have been designed and fabricated. Actuation is by pressure: Gas- or liquid-filled microfluidic cavities are employed to distend the microfabricated PDMS structures which then act as a lens surface or as an actuator for a micromirror. Thermopneumatic actuation is also employed for completely integrated tunable optical systems in which all actuator and optical components are fabricated on-chip. The technology is particularly promising for microsystem applications in which significant movement is required but high voltages or external fields are impractical. [2007-0301].     

A positive pressure-driven PDMS pump for fluid handling in microfluidic chips

By utilizing the high gas permeability of polydimethylsiloxane (PDMS), a simple positive pressure-driven pumping method was introduced. The pump was an aerated PDMS with a central channel in it and packing with a transfusion bottle. It could be attached to the inlet of microfluidic chip using a Teflon tube to release the air into the microfluidic system and then to create a positive pressure for driving fluid. In comparison with the degas-based PDMS pump, positive pressure-driven PDMS pump offered increased system flexibility and reduced individual device fabrication complexity due to its independence and versatility. More importantly, it offered the advantages that the PDMS pump could be wrapped in transfusion bottles to meet the readily available requirements, and it also easily assembled, which only required the user use a Teflon tube to connect a PDMS pump and a microfluidic chip. This assembly provided great freedom to meet different pumping requirements. Furthermore, this PDMS pump could offer many possible configures of pumping power by adjusting the geometries of the pump or by combining different pump modules, the adjustment of pumping capacity was investigated. To help design pumps with a suitable pumping performance, the sealing effect, pumping pressure and flow rate were also investigated. The results indicated that the performance of the positive pressure-driven PDMS pump was reliable. Finally, we demonstrated the utility of this pumping method by applying it to a PDMS-based viscometer microfluidic chip.     

ZomeFab: Cost-Effective Hybrid Fabrication with Zometools

In recent years, personalized fabrication has received considerable attention because of the widespread use of consumer-level three-dimensional (3D) printers. However, such 3D printers have drawbacks, such as long production time and limited output size, which hinder large-scale rapid-prototyping. In this paper, for the time- and cost-effective fabrication of large-scale objects, we propose a hybrid 3D fabrication method that combines 3D printing and the Zometool construction set, which is a compact, sturdy and reusable structure for infill fabrication. The proposed method significantly reduces fabrication cost and time by printing only thin 3D outer shells. In addition, we design an optimization framework to generate both a Zometol structure and printed surface partitions by optimizing several criteria, including printability, material cost and Zometool structure complexity. Moreover, we demonstrate the effectiveness of the proposed method by fabricating various large-scale 3D models.