Development of Microdroplet Generation Method for Organic Solvents Used in Chemical Synthesis

Recently, chemical operations with microfluidic devices, especially droplet-based operations, have attracted considerable attention because they can provide an isolated small-volume reaction field. However, analysis of these operations has been limited mostly to aqueous-phase reactions in water droplets due to device material restrictions. In this study, we have successfully demonstrated droplet formation of five common organic solvents frequently used in chemical synthesis by using a simple silicon/glass-based microfluidic device. When an immiscible liquid with surfactant was used as the continuous phase, the organic solvent formed droplets similar to water-in-oil droplets in the device. In contrast to conventional microfluidic devices composed of resins, which are susceptible to swelling in organic solvents, the developed microfluidic device did not undergo swelling owing to the high chemical resistance of the constituent materials. Therefore, the device has potential applications for various chemical reactions involving organic solvents. Furthermore, this droplet generation device enabled control of droplet size by adjusting the liquid flow rate. The droplet generation method proposed in this work will contribute to the study of organic reactions in microdroplets and will be useful for evaluating scaling effects in various chemical reactions.     

Designing tubular conducting polymer actuators for wireless electropumping

Rational design and shaping of soft smart materials offer potential applications that cannot be addressed with rigid systems. In particular, electroresponsive elastic materials are well-suited for developing original active devices, such as pumps and actuators. However, applying the electric stimulus requires usually a physical connection between the active part and a power supply. Here we report about the design of an electromechanical system based on conducting polymers, enabling the actuation of a wireless microfluidic pump. Using the electric field-induced asymmetric polarization of miniaturized polypyrrole tubes, it is possible to trigger simultaneously site-specific chemical reactions, leading to shrinking and swelling in aqueous solution without any physical connection to a power source. The complementary electrochemical reactions occurring at the opposite extremities of the tube result in a differential change of its diameter. In turn, this electromechanical deformation allows inducing highly controlled fluid dynamics. The performance of such a remotely triggered electrochemically active soft pump can be fine-tuned by optimizing the wall thickness, length and inner diameter of the material. The efficient and fast actuation of the polymer pump opens up new opportunities for actuators in the field of fluidic or microfluidic devices, such as controlled drug release, artificial organs and bioinspired actuators.

Tubular conducting polymer actuators are used for developing a wireless electropumping device. Bipolar electrochemistry, allowing symmetry breaking in terms of polarization and electrochemical reactions, is the key ingredient for efficient pumping.     

Multilayer soft lithography of perfluoropolyether based elastomer for microfluidic device fabrication

The compatibility of microfluidic devices with solvents and other chemicals is extremely important for many applications such as organic synthesis in microreactors and drug screening. We report the successful fabrication of microfluidic devices from a novel perfluoropolyether based polymer utilizing the Multilayer Soft Lithography? (MSL) technique with simple, straightforward processing. The perfluorinated polymer SIFEL X-71 8115 is a highly chemically resistant elastomeric material. We demonstrate fabrication of a microfluidic device using an off-ratio bonding technique to bond multiple SIFEL layers, each patterned lithographically. The mechanical properties of the SIFEL MSL valves (including actuation pressures) are similar to PDMS MSL valves of the same geometry. Chemical compatibility tests highlight SIFEL's remarkable resistance to organic solvents, acids and alkalis.     

Characterization of electrode alignment for optimal droplet charging and actuation in droplet‐based microfluidic system

The actuation method using electric force as a driving force is utilized widely in droplet‐based microfluidic systems. In this work, the effects of charging electrode alignment on direct charging of a droplet on electrified electrodes and a subsequent electrophoretic control of the droplet are investigated. The charging characteristics of a droplet according to different electrode alignments are quantitatively examined through experiments and systematic numerical simulations with varying distances and angles between the two electrodes. The droplet charge acquired from the electrified electrode is directly proportional to the distance and barely affected by the angle between the two electrodes. This implies that the primary consideration of electrode alignment in microfluidic devices is the distance between electrodes and the insignificant effect of angle provides a great degree of freedom in designing such devices. Not only the droplet charge acquired from the electrode but also the force exerted on the droplet is analyzed. Finally, the implications and design guidance for microfluidic systems are discussed with an electrophoresis of a charged droplet method‐based digital microfluidic device.     

Microfluidic operations using deformable polymer membranes fabricated by single layer soft lithography

We show that it is possible to use single layer soft lithography to create deformable polymer membranes within microfluidic chips for performing a variety of microfluidic operations. Single layer microfluidic chips were designed, fabricated, and characterized to demonstrate pumping, sorting, and mixing. Flow rates as high as 0.39 microl min(-1) were obtained by peristaltic pumping using pneumatically-actuated membrane devices. Sorting was attained via pneumatic actuation of membrane units placed alongside the branch channels. An active mixer was also demonstrated using single-layer deformable membrane units.     

Green microfluidic devices made of corn proteins

An alternative green microfluidic device made of zein, a prolamin of corn, can be utilized as a disposable environmentally friendly microchip especially in agriculture applications. Using standard soft lithography and stereo lithography techniques, we fabricated thin zein films with microfluidic chambers and channels. These were bonded to both a glass slide and another zein film. The zein film with microfluidic features bonds irreversibly with other surfaces by vapor-deposition of ethanol to create an adhesive layer resulting in very little or no trapped air and small shape distortion. Zein-zein and zein-glass microfluidic devices demonstrated sufficient strength to facilitate fluid flow in a complex microfluidic design that showed no leakage under high hydraulic pressure. Zein-glass microfluidic devices with serpentine mixing design showed successful fluid manipulation as a concentration gradient of Rhodamine B solution was generated. The ease of fabrication and bonding and the flexibility and moldability of zein offer attractive possibilities for microfluidic device design and manufacturing. These devices can include several unit operations with mixing being one of the most commonly used. The zein-based microfluidic devices, made entirely from a biopolymer from agricultural origin, offer alternative environmentally friendly material choices that are less dependent on limited petroleum based polymer resources.     

Electrical actuation of dielectric droplets by negative liquid dielectrophoresis

This paper presents an electrical actuation scheme of dielectric droplet by negative liquid dielectrophoresis. A general model of lumped parameter electromechanics for evaluating the electromechanical force acting on the droplets is established. The model reveals the influence of actuation voltage, device geometry, and dielectric parameter on the actuation force for both conductive and dielectric medium. Using this model, we compare the actuation forces for four liquid combinations in the parallel-plate geometry and predict the low voltage actuation of dielectric droplets by negative dielectrophoresis. Parallel experimental results demonstrate such electric actuation of dielectric droplets, including droplet transport, splitting, merging, and dispending. All these dielectric droplet manipulations are achieved at voltages < 100 V_rms. The frequency dependence of droplet actuation velocity in aqueous solution is discussed and the existence of surfactant molecules is believed to play an important role by realigning with the AC electric field. Finally, we present coplanar manipulation of oil and water droplets and formation of oil-in-water emulsion droplet by applying the same low voltage.     

Spontaneous generation of emulsion droplets by autonomous fluid-pumping using the gas permeability of poly(dimethylsiloxane) (PDMS)

Conventional droplet-based microfluidic systems require expensive, bulky external apparatuses, such as electric power supplies and pressure-driven pumps for fluid transportation. This study demonstrates an alternative way to produce emulsion droplets by autonomous fluid-handling based on the gas permeability of poly(dimethylsiloxane) (PDMS). Furthermore, basic concepts of fluid-handling are expanded to control the direction of the microfluid in the microfluidic device. The alternative pumping energy resulting from the high gas permeability of PDMS is used to generate water-in-oil (W/O) emulsions, which require no additional structures apart from microchannels. We can produce emulsion droplets by simple loading of the oil and aqueous solutions into the inlet reservoirs. During the operation of the microfluidic device, changes in droplet size, volumetric flow rate, and droplet generation frequency were quantitatively analyzed. As a result, we found that changes in the wetting properties of the microchannel greatly influence the volumetric flow rate and droplet generation frequency. This alternative microfluidic approach for preparing emulsion droplets in a simple and efficient manner is designed to improve the availability of emulsion droplets for point of care bioanalytical applications, in situ synthesis of materials, and on-site sample preparation tools.     

In situ fabrication of macroporous polymer networks within microfluidic devices by living radical photopolymerization and leaching

Novel fabrication techniques and polymer systems are being explored to enable mass production of low cost microfluidic devices. In this contribution we discuss a new fabrication scheme for making microfluidic devices containing porous polymer components in situ. Contact lithography, a living radical photopolymer (LRPP) system and salt leaching were used to fabricate multilayer microfluidic devices rapidly with various channel geometries and covalently attached porous polymer plugs made of various photopolymerizable substrates. LRPP systems offer the advantages of covalent attachment of microfluidic device layers and facile surface modification via grafting. Several applications of the porous plugs are also explored, including a static mixer, a high surface area-to-volume reactor and a rapidly responding hydrogel valve. Quantitative and qualitative data show an increase in mixing of a fluorescein and a water stream for channels containing porous plugs relative to channels with no porous plugs. Confocal laser scanning microscopy images demonstrate the ability to graft a functional material onto porous plug surfaces. A reaction was carried out on the grafted pore surfaces, which resulted in fluorescent labelling of the grafted material throughout the pores of the plug. Homogenous fluorescence throughout the depth of the porous plug and along pore surfaces indicated that the porous plugs were surface modified by grafting and that reactions can be carried out on the pore surfaces. Finally, porous hydrogel valves were fabricated which swelled in response to contact with various pH solutions. Results indicate that a porous hydrogel valve will swell and close more rapidly than other valve geometries made with the same polymer formulation. The LRPP-salt leaching method provides a means for rapidly incorporating porous polymer components into microfluidic devices, which can be utilized for a variety of pertinent applications upon appropriate selection of porous plug materials and surface treatments.     

Solvent-resistant photocurable liquid fluoropolymers for microfluidic device fabrication [corrected

We report the first fabrication of a solvent-compatible microfluidic device based on photocurable "Liquid Teflon" materials. The materials are highly fluorinated functionalized perfluoropolyethers (PFPEs) that have liquidlike viscosities that can be cured into tough, highly durable elastomers that exhibit the remarkable chemical resistance of fluoropolymers such as Teflon. Poly(dimethylsiloxane) (PDMS) elastomers have rapidly become the material of choice for many recent microfluidic device applications. Despite the advantages of PDMS in relation to microfluidics technology, the material suffers from a serious drawback in that it swells in most organic solvents. The swelling of PDMS-based devices in organic solvents greatly disrupts the micrometer-sized features and makes it impossible for fluids to flow inside the channels. Our approach to this problem has been to replace PDMS with photocurable perfluoropolyethers. Device fabrication and valve actuation were accomplished using established procedures for PDMS devices. The additional advantage of photocuring allows fabrication time to be decreased from several hours to a matter of minutes. The PFPE-based device exhibited mechanical properties similar to those of Sylgard 184 before and after curing as well as remarkable resistance to organic solvents. This work has the potential to expand the field of microfluidics to many novel applications.     

A surface topography assisted droplet manipulation platform for biomarker detection and pathogen identification

This paper reports a droplet microfluidic, sample-to-answer platform for the detection of disease biomarkers and infectious pathogens using crude biosamples. The platform exploited the dual functionality of silica superparamagnetic particles (SSP) for solid phase extraction of DNA and magnetic actuation. This enabled the integration of sample preparation and genetic analysis within discrete droplets, including the steps of cell lysis, DNA binding, washing, elution, amplification and detection. The microfluidic device was self contained, with all reagents stored in droplets, thereby eliminating the need for fluidic coupling to external reagent reservoirs. The device incorporated unique surface topographic features to assist droplet manipulation. Pairs of micro-elevations were created to form slits that facilitated efficient splitting of SSP from droplets. In addition, a compact sample handling stage, which integrated the magnet manipulator, the droplet microfluidic device and a Peltier thermal cycler, was built for convenient droplet manipulation and real-time detection. The feasibility of the platform was demonstrated by analysing ovarian cancer biomarker Rsf-1 and detecting Escherichia coli with real time polymerase chain reaction and real time helicase dependent amplification.     

Role,effect, and influences of micro and nano‐fillers on various properties of polymer matrix composites for microelectronics: A review

Ever since the discovery of polymer composites, its potential has been anticipated for numerous applications in various fields such as microelectronics, automobiles, and industrial applications. In this paper, we review filler reinforced polymer composites for its enormous potential in microelectronic applications. The interface and compatibility between matrix and filler have a significant role in property alteration of a polymer nanocomposites. Ceramic reinforced polymeric nanocomposites are promising candidate dielectric materials for several micro‐ and nano‐electronic devices. Because of its synergistic effect like high thermal conductivity, low thermal expansion, and dielectric constant of ceramic fillers with the polymer matrix, the resultant nanocomposites have high dielectric breakdown strength. The thermal and dielectric properties are discussed in the view of filler alignment techniques and its effect on the composites. Furthermore, the effect of various surface modified filler materials in polymer matrix, concepts of network forming using filler, and benefits of filler alignment are also discussed in this work. As a whole, this review article addresses the overall view to novice researchers on various properties such as thermal and dielectric properties of polymer matrix composites and direction for future research to be carried out.     

Monitoring spatial distribution of ethanol in microfluidic channels by using a thin layer of cholesteric liquid crystal

Monitoring spatial distribution of chemicals in microfluidic devices by using traditional sensors is a challenging task. In this paper, we report utilization of a thin layer of cholesteric liquid crystal for monitoring ethanol inside microfluidic channels. This thin layer can be either a polymer dispersed cholesteric liquid crystal (PDCLC) layer or a free cholesteric liquid crystal (CLC) layer separated from the microfluidic device by using a thin film of PDMS. They both show visible colorimetric responses to 4% of ethanol solution inside the microfluidic channels. Moreover, the spatial distribution of ethanol inside the microfluidic channel can be reflected as a color map on the CLC sensing layers. By using this device, we successfully detected ethanol produced from fermentation taking place inside the microfluidic channel. These microfluidic channels with embedded PDCLC or embedded CLC offer a new sensing solution for monitoring volatile organic compounds in microfluidic devices.     

Synthesis of polymer dielectric layers for organic thin film transistors via surface-initiated ring-opening metathesis polymerization

The use of surface-initiated ring-opening metathesis polymerization (SI-ROMP) for producing polymer dielectric layers is reported. Surface tethering of the catalyst to Au or Si/SiO2 surfaces is accomplished via self-assembled monolayers of thiols or silanes containing reactive olefins. Subsequent SI-ROMP of norbornene can be conducted under mild conditions. Pentacene semiconducting layers and gold drain/source electrodes are deposited over these polymer dielectric films. The resulting field effect transistors display promising device characteristics, demonstrating for the first time that SI-ROMP can be used in the construction of organic thin-film electronic devices.     

A digital microfluidic approach to heterogeneous immunoassays

A digital microfluidic (DMF) device was applied to a heterogeneous sandwich immunoassay. The digital approach to microfluidics manipulates samples and reagents in the form of discrete droplets, as opposed to the streams of fluid used in microchannels. Since droplets are manipulated on relatively generic 2-D arrays of electrodes, DMF devices are straightforward to use, and are reconfigurable for any desired combination of droplet operations. This flexibility makes them suitable for a wide range of applications, especially those requiring long, multistep protocols such as immunoassays. Here, we developed an immunoassay on a DMF device using Human IgG as a model analyte. To capture the analyte, an anti-IgG antibody was physisorbed on the hydrophobic surface of a DMF device, and DMF actuation was used for all washing and incubation steps. The bound analyte was detected using FITC-labeled anti-IgG, and fluorescence after the final wash was measured in a fluorescence plate reader. A non-ionic polymer surfactant, Pluronic F-127, was added to sample and detection antibody solutions to control non-specific binding and aid in movement via DMF. Sample and reagent volumes were reduced by nearly three orders of magnitude relative to conventional multiwell plate methods. Since droplets are in constant motion, the antibody–antigen binding kinetics is not limited by diffusion, and total analysis times were reduced to less than 2.5 h per assay. A multiplexed device comprising several DMF platforms wired in series further increased the throughput of the technique. A dynamic range of approximately one order of magnitude was achieved, with reproducibility similar to the assay when performed in a 96-well plate. In bovine serum samples spiked with human IgG, the target molecule was successfully detected in the presence of a 100-fold excess of bovine IgG. It was concluded that the digital microfluidic format is capable of carrying out qualitative and quantitative sandwich immunoassays with a dramatic reduction in reagent usage and analysis time compared to macroscale methods.     

Planar thin film device for capillary electrophoresis

Hollow tubular microfluidic channels were fabricated on quartz substrates using sacrificial layer, planar micromachining processes. The channels were created using a bottom-up fabrication technique, namely patterning a photoresist/aluminum sacrificial layer and depositing SiO(2) over the substrate. The photoresist/aluminum layer was removed by etching first with HCl/HNO(3), followed by etching in Nano-Strip, a more stable form of piranha (H(2)SO(4)/H(2)O(2)) stripper. Rapid separation of fluorescently labeled amino acids was performed on a device made with these channels. The fabrication process presented here provides unique control over channel composition and geometry. Future work should allow the fabrication of highly complex and precise devices with integrated analytical capabilities essential for the development of micro-total analysis systems.     

Conjugation expansion strategy enables highly stable all-polymer solar cells

The stability issue is one of the key factors hindering the commercial application of organic solar cells. All-polymer organic solar cell is one of the effective ways to solve the stability problem. In this work, we designed and synthesized two polymer donor materials PBDT and PDTBDT with different conjugation ranges, and demonstrated for the first time that extending the conjugation range of donor materials in all polymer solar cells can significantly improve device efficiency and stability. The experimental results of materials and devices show that PDTBDT with a larger conjugation range has stronger crystallinity and a more planar structure, which endows the active layer in its corresponding device with higher exciton dissociation probability, lower carrier recombination probability, more balanced charge transport properties and more favorable film morphology. As a result, the PDTBDT:PYF-T-o devices display an outstanding PCE of 13.38%, which is much higher than PBDT with smaller conjugation range based devices. Moreover, the PDTBDT:PYF-T-o device retains 0.86 of the initial PCE after over 500 h in the air atmosphere, exhibiting significantly improved stability. The improved stability is attributed to the enhanced moisture and air tolerance of active layer film thanks to the strong crystallinity of the donor material. These results demonstrate that the conjugation expansion strategy is one of the effective ways to obtain efficient and stable all-polymer organic solar cells.     

Achieving high electric energy storage in a polymer nanocomposite at low filling ratios using a highly polarizable phthalocyanine interphase

Polymer nanodielectrics have become attractive for practical applications such as electric energy storage and electromechanical actuation. However, to enhance the apparent dielectric constant of polymer nanodielectrics, a significant amount (>30 vol %) of spherical particles needs to be incorporated into the polymer matrix. As a consequence, melt-processing of polymer nanodielectrics into uniform thin films becomes difficult at such a high filler content, and electric breakdown strength will greatly decrease. In this work, we describe a three-phase composite approach towards high energy density nanodielectrics at low filling ratios. In this approach, a highly polarizable tetrameric metallophthalocyanine (TMPc) initiator is coated onto 68 nm BaTiO₃ nanoparticles, from which poly(methyl methacrylate) (PMMA) brushes are grafted using atom transfer radical polymerization for the nanoparticles to be uniformly dispersed in a poly(vinylidene fluoride-co-hexafluoropropylene) [P(VDF-HFP)] matrix. For comparison, two-phase P(VDF-HFP)/BaTiO₃ composites without the TMPc interfacial layer are also prepared. Owing to the high polarizability of the TMPc interface layer, the three-phase composite films exhibit higher dielectric constant and thus higher energy density than the two-phase composite films at volume-filling ratios below 5 vol %. Therefore, these high energy density three-phase nanodielectrics with a low filling ratio are promising for melt-processing into thin dielectric films. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1669–1680     

A linear dilution microfluidic device for cytotoxicity assays

A two-layer polymer microfluidic device is presented which creates nine linear dilutions from two input fluid streams mixed in varying volumetric proportions. The linearity of the nine dilutions is conserved when the flow rate is held constant at 1.0 microl min(-1) (R(2) = 0.9995) and when it is varied from 0.5-16 microl min(-1) (R(2) = 0.9998). An analytical expression is presented for designing microfluidic devices with arbitrary numbers of linear dilutions. To demonstrate the efficacy of this device, primary human epidermal keratinocytes (HEK) were stained with nine dilutions of calcein, resulting in a linear spread of fluorescent intensities (R(2) = 0.94). The operating principles of the device can be scaled up to incorporate any number of linear dilutions. This scalability, coupled with an intrinsic ability to create linear dilutions under a variety of operating conditions, makes the device applicable to high throughput screening applications such as combinatorial chemistry or cytotoxicity assays.     

A microfluidic device for self-synchronised production of droplets

The primary requirement for a mixing operation in droplet-based microfluidic devices is an accurate pairing of droplets of reaction fluids over an extended period of time. In this paper, a novel device for self-synchronous production of droplets has been demonstrated. The device uses a change in impedance across a pair of electrodes introduced due to the passage of a pre-formed droplet to generate a second droplet at a second pair of electrodes. The device was characterised using image analysis. Droplets with a volume of ~23.5 ± 3.1 nl (i.e.~93% of the volume of pre-formed droplets) were produced on applying a voltage of 500 V. The synchronisation efficiency of the device was 83%. As the device enables self-synchronised production of droplets, it has a potential to increase the reliability and robustness of mixing operations in droplet-based microfluidic devices.