A fundamental study on electrowetting by traditional and multifunctional ionic liquids: possible use in electrowetting on dielectric-based microfluidic applications

Water or aqueous electrolytes are the dominant components in electrowetting on dielectric (EWOD)-based microfluidic devices. Low thermal stability, evaporation, and a propensity to facilitate corrosion of the metal parts of integrated circuits or electronics are drawbacks of aqueous solutions. The alternative use of ionic liquids (ILs) as electrowetting agents in EWOD-based applications or devices could overcome these limitations. Efficient EWOD devices could be developed using task-specific ILs. In this regard, a fundamental study on the electrowetting properties of ILs is essential. Therefore electrowetting properties of 19 different ionic liquids, including mono-, di-, and tricationic, plus mono- and dianionic ILs were examined. All tested ILs showed electrowetting of various magnitudes on an amorphous flouropolymer layer. The effects of IL structure, functionality, and charge density on the electrowetting properties were studied. The enhanced stability of ILs in electrowetting on dielectric at higher voltages was studied in comparison with water. Deviations from classical electrowetting theory were confirmed. The physical properties of ILs and their electrowetting properties were tabulated. These data can be used as references to engineer task-specific electrowetting agents (ILs) for future electrowetting-based applications.     

Electrically Controlled Localized Charge Trapping at Amorphous Fluoropolymer–Electrolyte Interfaces

Charge trapping is a long‐standing problem in electrowetting on dielectric, causing reliability reduction and restricting its practical applications. Although this phenomenon is investigated macroscopically, the microscopic investigations are still lacking. In this work, the trapped charges are proven to be localized at the three‐phase contact line (TPCL) region by using three detecting methods—local contact angle measurements, electrowetting (EW) probe, and Kelvin probe force microscopy. Moreover, it is demonstrated that this EW‐assisted charge injection (EWCI) process can be utilized as a simple and low‐cost method to deposit charges on fluoropolymer surfaces. Charge densities near the TPCL up to 0.46 mC m^?2 and line widths of the deposited charge ranging from 20 to 300 µm are achieved by the proposed EWCI method. Particularly, negative charge densities do not degrade even after a “harsh” testing with a water droplet on top of the sample surfaces for 12 h, as well as after being treated by water vapor for 3 h. These findings provide an approach for applications which desire stable and controllable surface charges.     

On-Chip Drop-to-Drop Liquid Microextraction Coupled with Real-Time Concentration Monitoring Technique

This paper demonstrates a novel drop-to-drop liquid-liquid micro-extraction (DTD-LLME) device, which is based on an electrowetting on dielectric (EWOD) digital microfluidic chip. Droplets of two immiscible liquids, one of which is an ionic liquid, are formed in nanoliter volumes, driven along electrodes, merged and mixed for extraction, and finally separated upon the completion of the extraction process. All the steps are carried out on a microfluidic chip using combined electrowetting and dielectrophoretic forces, which act on the droplet upon the application of electric potential. Specially, the phase separation of two immiscible nanoliter-scale liquid drops was achieved for the first time on an EWOD digital microfluidic chip. To study the on-chip extraction kinetics, an image-based concentration measurement technique with suitable color parameters was studied and compared with the typical UV absorption based technique. Finally, the effect of applied ac voltage frequency on the extraction kinetics was studied. The observations on DTD-LLME, particularly phase separation, are discussed. The image-based method was found to be applicable for precise concentration measurements with the right choice of the color parameter. Results from experiments on finding the frequency dependence on extraction kinetics demonstrate that the application of higher frequencies can be a factor in accelerating the extraction on the proposed microextraction device.     

Addressable Microfluidic Polymer Chip for DNA‐Directed Immobilization of Oligonucleotide‐Tagged Compounds

A microfluidic polymer chip for the self‐assembly of DNA conjugates through DNA‐directed immobilization is developed. The chip is fabricated from two parts, one of which contains a microfluidic channel produced from poly(dimethylsiloxane) (PDMS) by replica‐casting technique using a mold prepared by photolithographic techniques. The microfluidic part is sealed by covalent bonding with a chemically activated glass slide containing a DNA oligonucleotide microarray. The dimension of the PDMS–glass microfluidic chip is equivalent to standard microscope slides (76 × 26 mm²). The DNA microarray surface inside the microfluidic channels is configured through conventional spotting, and the resulting DNA patches can be conveniently addressed with compounds containing complementary DNA tags. To demonstrate the utility of the addressable surface within the microfluidic channel, DNA‐directed immobilization (DDI) of DNA‐modified gold nanoparticles (AuNPs) and DNA‐conjugates of the enzymes glucose oxidase (GOx) and horseradish peroxidase (HRP) are carried out. DDI of AuNPs is used to demonstrate site selectivity and reversibility of the surface‐modification process. In the case of the DNA–enzyme conjugates, the patterned assembly of the two enzymes allows the establishment and investigation of the coupled reaction of GOx and HRP, with particular emphasis on surface coverage and lateral flow rates. The results demonstrate that this addressable chip is well suited for the generation of fluidically coupled multi‐enzyme microreactors.     

In Situ Two‐Step Photoreduced SERS Materials for On‐Chip Single‐Molecule Spectroscopy with High Reproducibility

A method is developed to synthesize surface‐enhanced Raman scattering (SERS) materials capable of single‐molecule detection, integrated with a microfluidic system. Using a focused laser, silver nanoparticle aggregates as SERS monitors are fabricated in a microfluidic channel through photochemical reduction. After washing out the monitor, the aggregates are irradiated again by the same laser. This key step leads to full reduction of the residual reactants, which generates numerous small silver nanoparticles on the former nanoaggregates. Consequently, the enhancement ability of the SERS monitor is greatly boosted due to the emergence of new “hot spots.” At the same time, the influence of the notorious “memory effect” in microfluidics is substantially suppressed due to the depletion of surface residues. Taking these advantages, two‐step photoreduced SERS materials are able to detect different types of molecules with the concentration down to 10^?13m . Based on a well‐accepted bianalyte approach, it is proved that the detection limit reaches the single‐molecule level. From a practical point of view, the detection reproducibility at different probing concentrations is also investigated. It is found that the effective single‐molecule SERS measurements can be raised up to ≈50%. This microfluidic SERS with high reproducibility and ultrasensitivity will find promising applications in on‐chip single‐molecule spectroscopy.     

Dynamic electrowetting on nanofilament silicon for matrix-free laser desorption/ionization mass spectrometry

Dynamic electrowetting on nanostructured silicon surfaces is demonstrated as an effective method for improving detection sensitivity in matrix-free laser desorption/ionization mass spectrometry. Without electrowetting, silicon surfaces comprising dense fields of oriented nanofilaments are shown to provide efficient ion generation and high spectral peak intensities for deposited peptides bound to the nanofilaments through hydrophobic interactions. By applying an electrical bias to the silicon substrate, the surface energy of the oxidized nanofilaments can be dynamically controlled by electrowetting, thereby allowing aqueous buffer to penetrate deep into the nanofilament matrix. The use of electrowetting is shown to result in enhanced interactions between deposited peptides and the nanofilament silicon surface, with improved signal-to-noise ratio for detected spectral peaks. An essential feature contributing to the observed performance enhancement is the open-cell nature of the nanofilament surfaces, which prevents air from becoming trapped within the pores and limiting solvent penetration during electrowetting. The combination of nanofilament silicon and dynamic electrowetting is shown to provide routine detection limits on the order of several attomoles for a panel of model peptides.     

Hybrid Nanomedicine Fabricated from Photosensitizer‐Terminated Metal–Organic Framework Nanoparticles for Photodynamic Therapy and Hypoxia‐Activated Cascade Chemotherapy

During photodynamic therapy (PDT), severe hypoxia often occurs as an undesirable limitation of PDT owing to the O₂‐consuming photodynamic process, compromising the effectiveness of PDT. To overcome this problem, several strategies aiming to improve tumor oxygenation are developed. Unlike these traditional approaches, an opposite method combining hypoxia‐activated prodrug and PDT may provide a promising strategy for cancer synergistic therapy. In light of this, azido‐/photosensitizer‐terminated UiO‐66 nanoscale metal–organic frameworks (UiO‐66‐H/N₃ NMOFs) which serve as nanocarriers for the bioreductive prodrug banoxantrone (AQ4N) are engineered. Owing to the effective shielding of the nanoparticles, the stability of AQ4N is well preserved, highlighting the vital function of the nanocarriers. By virtue of strain‐promoted azide–alkyne cycloaddition, the nanocarriers are further decorated with a dense PEG layer to enhance their dispersion in the physiological environment and improve their therapeutic performance. Both in vitro and in vivo studies reveal that the O₂‐depleting PDT process indeed aggravates intracellular/tumor hypoxia that activates the cytotoxicity of AQ4N through a cascade process, consequently achieving PDT‐induced and hypoxia‐activated synergistic therapy. Benefiting from the localized therapeutic effect of PDT and hypoxia‐activated cytotoxicity of AQ4N, this hybrid nanomedicine exhibits enhanced therapeutic efficacy with negligible systemic toxicity, making it a promising candidate for cancer therapy.     

A Lithium‐Organic Primary Battery

Lithium primary batteries are still widely used in military, aerospace, medical, and civilian applications despite the omnipresence of rechargeable Li‐ion batteries. However, these current primary chemistries are exclusively based on inorganic materials with high cost, low energy density or severe safety concerns. Here, a novel lithium‐organic primary battery chemistry that operates through a synergetic reduction of 9,10‐anthraquinone (AQ) and fluoroethylene carbonate (FEC) is reported. In FEC‐presence, the equilibrium between the carbonyl and enol structures is disabled, and replaced by an irreversible process that corresponds to a large capacity along with methylene and inorganic salts (such as LiF, Li₂CO₃) generated as products. This irreversible chemistry of AQ yields a high energy density of 1300 Wh/(kg of AQ) at a stable discharge voltage platform of 2.4 V as well as high rate capability (up to 313 mAh g^?1 at a current density of 1000 mA g^?1), wide temperature range of operation (?40 to 40 °C) and low self‐discharge rate. Combined with the advantages of low toxicity, facile and diverse synthesis methods, and easy accessibility of AQ, Li‐organic primary battery chemistry promises a new battery candidate for applications that requires low cost, high environmental friendliness, and high energy density.     

Art‐on‐a‐Chip: Preserving Microfluidic Chips for Visualization and Permanent Display

“After a certain high level of technical skill is achieved, science and art tend to coalesce in aesthetics, plasticity, and form. The greatest scientists are always artists as well.” said Albert Einstein. Currently, photographic images bridge the gap between microfluidic/lab‐on‐a‐chip devices and art. However, the microfluidic chip itself should be a form of art. Here, novel vibrant epoxy dyes are presented in combination with a simple process to fill and preserve microfluidic chips, to produce microfluidic art or art‐on‐a‐chip. In addition, this process can be used to produce epoxy dye patterned substrates that preserve the geometry of the microfluidic channels—height within 10% of the mold master. This simple approach for preserving microfluidic chips with vibrant, colorful, and long‐lasting epoxy dyes creates microfluidic chips that can easily be visualized and photographed repeatedly, for at least 11 years, and hence enabling researchers to showcase their microfluidic chips to potential graduate students, investors, and collaborators.     

Electrical Textile Valves for Paper Microfluidics

This paper describes electrically‐activated fluidic valves that operate based on electrowetting through textiles. The valves are fabricated from electrically conductive, insulated, hydrophobic textiles, but the concept can be extended to other porous materials. When the valve is closed, the liquid cannot pass through the hydrophobic textile. Upon application of a potential (in the range of 100–1000 V) between the textile and the liquid, the valve opens and the liquid penetrates the textile. These valves actuate in less than 1 s, require low energy (≈27 µJ per actuation), and work with a variety of aqueous solutions, including those with low surface tension and those containing bioanalytes. They are bistable in function, and are, in a sense, the electrofluidic analog of thyristors. They can be integrated into paper microfluidic devices to make circuits that are capable of controlling liquid, including autonomous fluidic timers and fluidic logic.     

Organic transistor based circuit as drive for planar microfluidic devices

Organic transistor based circuits are a promising candidate for acting as drivers for microfluidic devices handling discrete droplets. The ease of fabrication along with the ability to generate desired voltage levels for performing electrowetting based actuation of liquids make them an ideal match for discrete droplet based microfluidic systems. In this article, we report the implementation of an organic transistor based complementary metal-oxide semiconductor (CMOS) inverter used to actuate microliter quantities of droplets on a simple planar microfluidic device. We also present two approaches for fabricating an open-structured device for different applications. The inverter is fabricated using Pentacene and N, N′- bis (n-octyl) dicyanoperylene-3, 4:9, 10-bis (dicarboxyimide) (PDI-8CN₂) (Northwestern University). The inverter output is stable and repeatable and is used to actuate droplets over adjacent electrodes as well as in merging of discrete droplets.     

Bio‐microfluidics: Biomaterials and Biomimetic Designs

Bio‐microfluidics applies biomaterials and biologically inspired structural designs (biomimetics) to microfluidic devices. Microfluidics, the techniques for constraining fluids on the micrometer and sub‐micrometer scale, offer applications ranging from lab‐on‐a‐chip to optofluidics. Despite this wealth of applications, the design of typical microfluidic devices imparts relatively simple, laminar behavior on fluids and is realized using materials and techniques from silicon planar fabrication. On the other hand, highly complex microfluidic behavior is commonplace in nature, where fluids with nonlinear rheology flow through chaotic vasculature composed from a range of biopolymers. In this Review, the current state of bio‐microfluidic materials, designs and applications are examined. Biopolymers enable bio‐microfluidic devices with versatile functionalization chemistries, flexibility in fabrication, and biocompatibility in vitro and in vivo. Polymeric materials such as alginate, collagen, chitosan, and silk are being explored as bulk and film materials for bio‐microfluidics. Hydrogels offer options for mechanically functional devices for microfluidic systems such as self‐regulating valves, microlens arrays and drug release systems, vital for integrated bio‐microfluidic devices. These devices including growth factor gradients to study cell responses, blood analysis, biomimetic capillary designs, and blood vessel tissue culture systems, as some recent examples of inroads in the field that should lead the way in a new generation of microfluidic devices for bio‐related needs and applications. Perhaps one of the most intriguing directions for the future will be fully implantable microfluidic devices that will also integrate with existing vasculature and slowly degrade to fully recapitulate native tissue structure and function, yet serve critical interim functions, such as tissue maintenance, drug release, mechanical support, and cell delivery.     

Reliable and low-voltage electrowetting on thin parylene films

The stability of an electrowetting system is dependent upon the choice of liquids, the dielectric material and the operating voltage. Substantial progress is reported herein on use of 300 nm thick poly(α, α, α′, α′-tetrafluoro-para-xylylene) (Parylene HT) films for almost 100° of reliable electrowetting modulation at only 15 V. Not only does Parylene HT exhibit improved resistance to dielectric failure as compared to poly(2-chloro-para-xylylene) (Parylene C), but Parylene HT is shown to sustain continuous DC electrowetting to < 70° for > 6 h. Furthermore, Parylene HT has a surface energy such that when electrowetting in an alkane oil ambient, a Young's angle of about 170° can be achieved without the traditional fluoropolymer top-coat. Also presented is a discussion of electric field enhancement when electrowetting in an oil bath. It is shown that Parylene HT is a promising candidate for low-voltage and large-area electrowetting devices such as displays and lab-on-chip.     

Discontinuous Nanoporous Membranes Reduce Non‐Specific Fouling for Immunoaffinity Cell Capture

The microfluidic isolation of target cells using adhesion‐based surface capture has been widely explored for biology and medicine. However, high‐throughput processing can be challenging due to interfacial limitations such as transport, reaction, and non‐specific fouling. Here, it is shown that antibody‐functionalized capture surfaces with discontinuous permeability enable efficient target cell capture at high flow rates by decreasing fouling. Experimental characterization and theoretical modeling reveal that “wall effects” affect cell–surface interactions and promote excess surface accumulation. These issues are partially circumvented by reducing the transport and deposition of cells near the channel walls. Optimized microfluidic devices can be operated at higher cell concentrations with significant improvements in throughput.     

High Throughput‐Per‐Footprint Inertial Focusing

Matching the scale of microfluidic flow systems with that of microelectronic chips for realizing monolithically integrated systems still needs to be accomplished. However, this is appealing only if such re‐scaling does not compromise the fluidic throughput. This is related to the fact that the cost of microelectronic circuits primarily depends on the layout footprint, while the performance of many microfluidic systems, like flow cytometers, is measured by the throughput. The simple operation of inertial particle focusing makes it a promising technique for use in such integrated flow cytometer applications, however, microfluidic footprints demonstrated so far preclude monolithic integration. Here, the scaling limits of throughput‐per‐footprint (TPFP) in using inertial focusing are explored by studying the interplay between theory, the effect of channel Reynolds numbers up to 1500 on focusing, the entry length for the laminar flow to develop, and pressure resistance of the microchannels. Inertial particle focusing is demonstrated with a TPFP up to 0.3 L/(min cm²) in high aspect‐ratio rectangular microfluidic channels that are readily fabricated with a post‐CMOS integratable process, suggesting at least a 100‐fold improvement compared to previously demonstrated techniques. Not only can this be an enabling technology for realizing cost‐effective monolithically integrated flow cytometry devices, but the methodology represented here can also open perspectives for miniaturization of many biomedical microfluidic applications requiring monolithic integration with microelectronics without compromising the throughput.     

A SERS‐Assisted 3D Barcode Chip for High‐Throughput Biosensing

A surface enhanced Raman scattering (SERS)‐assisted 3D barcode chip has been developed for high‐throughput biosensing. The 3D barcode is realized through joint 2D spatial encoding with the Raman spectroscopic encoding, which stores the SERS fingerprint information in the format of a 2D array. Here, the concept of SERS‐assisted 3D barcode is demonstrated through multiplex immunoassay, where simultaneous detection of multiple targets in different samples has been achieved using a microfluidic platform. First, multiple proteins in different samples are spatially separated using a microfluidic patterned antibody barcode substrate, forming a 2D hybridization array. Then the SERS probes are used to identify and quantify the proteins. As different SERS probes are labeled with different Raman reporters, they could be employed as “SERS tags” to incorporate spectroscopic information into the 3D barcode. In this 3D barcode, the 2D spatial information helps to differentiate the samples and targets while the SERS information allows quantitative multiplex detection. It is found that the SERS‐assisted 3D barcode chip can not only accomplish one‐step multiplex detection within 30 min but also achieve an ultrasensitivity down to 10 fg mL^?1 (≈70 aM), which is expected to provide a promising tool for high‐throughput biomedical applications.     

Microfluidics: Nanostructuring of Titania Thin Films by a Combination of Microfluidics and Block‐Copolymer‐Based Sol–Gel Templating (Small 7/2011)

Sol–gel templating of titania thin films with the amphiphilic diblock copolymer poly(dimethyl siloxane)‐block‐methyl methacrylate poly(ethylene oxide) is combined with microfluidic technology to control the structure formation. Due to the laminar flow conditions in the microfluidic cell, a better control of the local composition of the reactive fluid is achieved. The resulting titania films exhibit mesopores and macropores, as determined with scanning electron microscopy, X‐ray reflectivity, and grazing incidence small angle X‐ray scattering. The titania morphology has three features that are beneficial for application in photovoltaics: 1) a large surface‐to‐volume ratio important for charge generation with disordered hexagonally arranged mesopores of 25 nm size and a film porosity of up to 0.79, 2) enhanced light scattering that enables the absorption of more light, and 3) a dense titania layer with a thickness of about 6 nm at the substrate (bottom electrode) to prevent short circuits. An optical characterization complements the structural investigation.     

Isolation and Profiling of Circulating Tumor‐Associated Exosomes Using Extracellular Vesicular Lipid–Protein Binding Affinity Based Microfluidic Device

Extracellular vesicles (EVs) are emerging as a potential diagnostic test for cancer. Owing to the recent advances in microfluidics, on‐chip EV isolation is showing promise with respect to improved recovery rates, smaller necessary sample volumes, and shorter processing times than ultracentrifugation. Immunoaffinity‐based microfluidic EV isolation using anti‐CD63 is widely used; however, anti‐CD63 is not specific to cancer‐EVs, and some cancers secrete EVs with low expression of CD63. Alternatively, phosphatidylserine (PS), usually expressed in the inner leaflet of the lipid bilayer of the cells, is shown to be expressed on the outer surface of cancer‐associated EVs. A new exosome isolation microfluidic device (^newExoChip), conjugated with a PS‐specific protein, to isolate cancer‐associated exosomes from plasma, is presented. The device achieves 90% capture efficiency for cancer cell exosomes compared to 38% for healthy exosomes and isolates 35% more A549‐derived exosomes than an anti‐CD63‐conjugated device. Immobilized exosomes are then easily released using Ca²⁺ chelation. The recovered exosomes from clinical samples are characterized by electron microscopy and western‐blot analysis, revealing exosomal shapes and exosomal protein expressions. The ^newExoChip facilitates the isolation of a specific subset of exosomes, allowing the exploration of the undiscovered roles of exosomes in cancer progression and metastasis.     

Microfluidics: Accelerated Biofluid Filling in Complex Microfluidic Networks by Vacuum‐Pressure Accelerated Movement (V‐PAM) (Small 33/2016)

Rapid fluid transport and exchange are critical operations involved in many microfluidic applications. However, conventional mechanisms used for driving fluid transport in microfluidics, such as micropumping and high pressure, can be inaccurate and difficult for implementation for integrated microfluidics containing control components and closed compartments. Here, a technology has been developed termed Vacuum–Pressure Accelerated Movement (V‐PAM) capable of significantly enhancing biofluid transport in complex microfluidic environments containing dead‐end channels and closed chambers. Operation of the V‐PAM entails a pressurized fluid loading into microfluidic channels where gas confined inside can rapidly be dissipated through permeation through a thin, gas‐permeable membrane sandwiched between microfluidic channels and a network of vacuum channels. Effects of different structural and operational parameters of the V‐PAM for promoting fluid filling in microfluidic environments have been studied systematically. This work further demonstrates the applicability of V‐PAM for rapid filling of temperature‐sensitive hydrogels and unprocessed whole blood into complex irregular microfluidic networks such as microfluidic leaf venation patterns and blood circulatory systems. Together, the V‐PAM technology provides a promising generic microfluidic tool for advanced fluid control and transport in integrated microfluidics for different microfluidic diagnosis, organs‐on‐chips, and biomimetic studies.     

A digital microfluidic system for the investigation of pre-steady-state enzyme kinetics using rapid quenching with MALDI-TOF mass spectrometry

A digital microfluidic system based on electrowetting has been developed to facilitate the investigation of pre-steady-state reaction kinetics using rapid quenching and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The device consists of individually addressable electrodes arranged to allow the combination of liquid droplets at well-defined time intervals and an integrated, electrohydrodynamically driven mixer. The device combines two droplets to initiate a reaction, then, with precise timing, combines a third droplet to quench the reaction, and finally combines a fourth droplet to form a matrix. Improvements to throughput when compared to traditional laboratory-scale methods, and previous MALDI-TOF MS digital microfluidic systems, were made. The device was tested against a model protein tyrosine phosphatase system, and results agreed well with published data. The system therefore allows for the analysis of reaction kinetics that were previously too rapid to analyze using MALDI-TOF MS.