On-chip fabrication of magnetic alginate hydrogel microfibers by multilayered pneumatic microvalves

Alginate hydrogel has widespread applications in tissue engineering, cancer therapy, wound management and drug/cell/growth factor delivery due to its biocompatibility, hydrated environment and desirable viscoelastic properties. However, the lack of controllability is still an obstacle for utilizing it in the fabrication of 3D tissue constructs and accurate targeting in mass delivery. Here, we proposed a new method for achieving magnetic alginate hydrogel microfibers by dispersing magnetic nanoparticles in alginate solution and solidifying the magnetic alginate into hydrogel fiber inside microfluidic devices. The microfluidic devices have multilayered pneumatic microvalves with hemicylindrical channels to fully stop the fluids. In the experiments, the magnetic nanoparticles and the alginate solution were mixed and formed a uniform suspension. No aggregation of magnetic nanoparticles was found, which is crucial for flow control inside microfluidic devices. By regulating the flow rates of different solutions with the microvalves inside the microfluidic device, magnetic hydrogel fibers and nonmagnetic hydrogel fibers were fabricated with controlled sizes. The proposed method for fabricating magnetic hydrogel fiber holds great potential for engineering 3D tissue constructs with complex architectures and active drug release.     

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.     

Integrated CMOS optical sensor for cell detection and analysis

To circumvent the complexity of the detection systems of microfluidic devices, Hartley et al. recently reported on a CMOS optical active pixel sensor (APS) for near-field detection and counting of microscopic particles. To further enhance the digital cytometric capabilities of the original sensor, we modified and utilized a dual APS-array scheme to facilitate the determination of the velocity and size of particles flowing in microfluidic channels. Our findings indicate that the prototype dual-APS sensor is capable of detecting particle velocities up to ～500 μm/s and particles with diameter in the range of 5–15 μm. The dual APS CMOS sensor, as a result of the hybrid integration with a microfluidic, provides a low cost and practical means of noninvasively monitoring the contents of microfluidic and lab-on-a-chip devices.     

On-Line Testing of Lab-on-Chip Using Reconfigurable Digital-Microfluidic Compactors

Dependability is an important system attribute for microfluidic lab-on-chip devices. On-line testing offers a promising method for detecting defects, fluidic abnormalities, and bioassay malfunctions during chip operation. However, previous techniques for reading test outcomes and analyzing pulse sequences are cumbersome, sensitive to the calibration of capacitive sensors, and error-prone. We present a built-in self-test (BIST) method for on-line testing of digital microfluidic lab-on-chip. This method utilizes microfluidic compactors based on droplet-based AND gates, which are implemented using digital microfluidics. An optimization method is proposed to schedule logic AND operations in the compactor to minimize the end time for the compaction procedure. Dynamic reconfiguration of these compactors ensures low area overhead and it allows BIST to be interleaved with bioassays in functional mode. We evaluate the on-line testing method using a multiplexed in vitro diagnostics bioassay.     

A serial sample loading system: interfacing multiwell plates with microfluidic devices

There is an increasing demand for novel high-throughput screening (HTS) technologies in the pharmaceutical and biotechnological industries. The robotic sample-handling techniques currently used in these industries, although fast, are still limited to operating in multiwell plates with the sample volumes per reaction in the microliter regime. Digital microfluidics offers an alternative for reduction in sample volume consumption for HTS but lacks a reliable technique for transporting a large number of samples to the microfluidic device. In this report, we develop a technique for serial delivery of sample arrays to a microfluidic device from multiwell plates, through a single sample inlet. Under this approach, a serial array of sample plugs, separated by an immiscible carrier fluid, is loaded into a capillary and delivered to a microfluidic device. Similar approaches have been attempted in the past, however, either with a slower sample loading device such as a syringe pump or vacuum-based sample loading with limited driving pressure. We demonstrated the application of our positive-pressure-based serial sample loading (SSL) system to load a series of sample plugs into a capillary. The adaptability of the SSL system to generate sample plugs with a variety of volumes in a predictable manner was also demonstrated.     

Characterization of electrowetting,contact angle hysteresis,and adhesion on digital microfluidic devices with inkjet-printed electrodes

This investigation characterizes electrowetting performance, contact angle hysteresis, contact line pinning force, and adhesion work on digital microfluidic devices with inkjet-printed electrodes. It also demonstrates electrowetting-induced droplet detachment on these devices. Average performance was similar to cleanroom-fabricated devices in all experimental measurements, but variability was persistently higher on inkjet-printed devices. This appears to be consistent with increased defect density and variation in local electrowetting number caused by increased roughness of printed electrodes. This work suggests that inkjet-printed devices are suitable for the study of colloidal transport and deposition under electric fields and electrowetting-induced droplet detachment when accompanied by rigorous uncertainty analysis.     

A plug-based microfluidic system for dispensing lipidic cubic phase (LCP) material validated by crystallizing membrane proteins in lipidic mesophases

This article presents a plug-based microfluidic system to dispense nanoliter-volume plugs of lipidic cubic phase (LCP) material and subsequently merge the LCP plugs with aqueous plugs. This system was validated by crystallizing membrane proteins in lipidic mesophases, including LCP. This system allows for accurate dispensing of LCP material in nanoliter volumes, prevents inadvertent phase transitions that may occur due to dehydration by enclosing LCP in plugs, and is compatible with the traditional method of forming LCP material using a membrane protein sample, as shown by the successful crystallization of bacteriorhodopsin from Halobacterium salinarum. Conditions for the formation of LCP plugs were characterized and presented in a phase diagram. This system was also implemented using two different methods of introducing the membrane protein: (1) the traditional method of generating the LCP material using a membrane protein sample and (2) post LCP-formation incorporation (PLI), which involves making LCP material without protein, adding the membrane protein sample externally to the LCP material, and allowing the protein to diffuse into the LCP material or into other lipidic mesophases that may result from phase transitions. Crystals of bacterial photosynthetic reaction centers from Rhodobacter sphaeroides and Blastochloris viridis were obtained using PLI. The plug-based, LCP-assisted microfluidic system, combined with the PLI method for introducing membrane protein into LCP, should be useful for minimizing consumption of samples and broadening the screening of parameter space in membrane protein crystallization.     

Multi-scale,multi-depth lithography using optical fibers for microfluidic applications

This paper proposes and demonstrates a method for multi-scale, multi-depth three-dimensional (3D) lithography. In this method, 3D molds for replicating microchannels are fabricated by passing a non-focused laser beam through an optical fiber, whose tip is immersed in a droplet of photopolymer. Line width is adjustable from 1 to 980 µm using eight kinds of optical fibers with different core diameters. The height of line drawing can be controlled by adjusting the distance between the tip of the optical fiber and a substrate. The surface roughness (R_a, R_z) of a single line and plane was evaluated. The method was employed to fabricate a 3D mold of a microchannel containing tandem chambers, which was then successfully replicated in PDMS. Multi-scale, multi-depth 3D lithography can provide a simple, flexible tool for producing PDMS microfluidic devices.     

Spiral-shaped inertial stem cell device for high-throughput enrichment of iPSC-derived neural stem cells

Stem cell enrichment plays a critical role in both research and clinical applications. The typical method for stem cell enrichment may use invasive processes and takes a long period of time. Spiral-shaped microfluidic devices, which combine lift and Dean drag forces to direct cells of different sizes into separate trajectories, can be used to noninvasively process samples at a rate of milliliters per minute. This paper presents a simple 2-loop spiral-shaped inertial microfluidic devices with the aid of sheath flow to enrich neural stem cells (NSCs), derived from induced pluripotent stem cells. NSCs and spontaneously differentiated non-neural cells were mixed and flowed through the spiral-shaped devices. Samples collected at the outlets were analyzed for purity and recovery. It was found that the device focused the NSCs into a narrow trajectory, which could then be collected in two out of the eight outlets. The device was tested at different flow rates and found that the most highly enriched fractions (2.1×) with NSCs recovery 93% were achieved at the flow rate (3 ml/min). Next, we extended our investigation from 2-loop design to 10-loop design to eliminate the use of sheath flow. NSCs were enriched to 2.5×, but only 38% of the NSCs were recovered from the most enriched fractions. Spiral-shaped microfluidic devices are capable of rapid, label-free enrichment of target stem cells, and have great potential in point-of-care tissue preparation.     

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.     

Non-woven fabric-based microfluidic devices with hydrophobic wax barrier

This study proposed a novel method for the fabrication of non-woven based microfluidic devices with a wax hydrophobic barrier. Current microfluidic devices were fabricated with glass or polymer material, and paper is also widely used for the fabrication of low-cost microfluidic devices. The application of non-woven fabric based microfluidic devices provides a new option of bulk materials for microfluidics. Compared with the glass or polymer material used in microfluidics, non-woven fabric is low-cost, easy to process and disposable. Fluid can penetrate through the non-woven fabric material with capillary force without the requirement of external pumps. As fiber-based material, comparing with paper, non-woven fabric material is more durable with higher mechanical strength, and various types of non-woven fabric material also provide a board choice of surface chemical/physical properties for microfluidic applications. In this study, the hydrophilic non-woven fabric is chosen as the bulk material for microfluidic devices, a wax pattern transfer protocol is also proposed in this study for the deposition of hydrophobic barriers. For a demonstration of the proposed fabrication technique, a microfluidic mixer was also fabricated in this study.

     

Integration of impedance spectroscopy sensors in a digital microfluidic platform

Digital microfluidics combines the advantages of a low consumption of reagents with a high flexibility of processing fluid samples. For applications in life sciences not only the processing but also the characterization of fluids is crucial. In this contribution, a microfluidic platform, combining the actuation principle of electrowetting on dielectrics for droplet manipulations and the sensor principle of impedance spectroscopy for the characterization of the fluid composition and condition, is presented. The fabrication process of the microfluidic platform comprises physical vapor deposition and structuring of the metal electrodes onto a substrate, the deposition of a dielectric isolator and a hydrophobic top coating. The key advantage of this microfluidic chip is the common electric nature of the sensor and the actuation principle. This allows for fabricating digital microfluidic devices with a minimal number of process steps. Multiple measurements on fluids of different composition (including rigid particles) and of different conditions (temperature, sedimentation) were performed and process parameters were monitored online. These sample applications demonstrate the versatile applications of this combined technology.     

Microfluidic inverse phase ELISA via manipulation of magnetic beads

We report a new technique for conducting immuno-diagnostics on a microfluidic platform. Rather than handling fluid reagents against a stationary solid phase, the platform manipulates analyte-coated magnetic beads through stationary plugs of fluid reagents to detect an antigenic analyte. These isolated but accessible plugs are pre-encapsulated in a microchannel by capillary force. We call this platform microfluidic inverse phase enzyme-linked immunosorbent assay (μIPELISA). μIPELISA has distinctive advantages in the family of microfluidic immunoassay. In particular, it avoids pumping and valving fluid reagents during assaying, thus leading to a lab-on-a-chip format that is free of instrumentation for fluid actuation and control. We use μIPELISA to detect digoxigenin-labeled DNA segments amplified from E. coli O157:H7 by polymerase chain reaction (PCR), and compare its detection capability with that of microplate ELISA. For 0.259 ng μl⁻¹ of digoxigenin-labeled amplicon, μIPELISA is as responsive as the microplate ELISA. Also, we simultaneously conduct μIPELISA in two parallel microchannels.     

Droplet transport by electrowetting: lets get rough!

Since the pioneering works of Wenzel and Cassie Baxter in the 1930s, and now with the trivialization of the micro- and nanotechnology facilities, superhydrophobic surfaces have been announced as potentially amazing components for applications such as fluidic, optical, electronic, or thermal devices. In this paper, we show that using superhydrophobic surfaces in digital microfluidic devices could solve some usual limitations or enhance their performances. Thus, we investigate a specific monophasic (air environment) microfluidic device based on electrowetting integrating either a hydrophobic or a superhydrophobic surface as a counter-electrode. The droplet transport using a superhydrophobic surface compared with a classical hydrophobic system led to some original results. Characterization of the dynamic contact angle and the droplet shape allows us to get new insight of the fluid dynamics. Among the remarkable properties reported, a 30 % lower applied voltage, a 30 % higher average speed with a maximum instantaneous speed of 460 mm/s have been measured. Furthermore, we have noticed a huge droplet deformation leading to an increase by a factor 5 of the Weber number (from 1.4 to 7.0) on SH compared to hydrophobic surfaces. Finally, we discuss some of the repercussions of this behaviour especially for microfluidic device.     

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.     

Micro-injection moulding of polymer microfluidic devices

Microfluidic devices have several applications in different fields, such as chemistry, medicine and biotechnology. Many research activities are currently investigating the manufacturing of integrated microfluidic devices on a mass-production scale with relatively low costs. This is especially important for applications where disposable devices are used for medical analysis. Micromoulding of thermoplastic polymers is a developing process with great potential for producing low-cost microfluidic devices. Among different micromoulding techniques, micro-injection moulding is one of the most promising processes suitable for manufacturing polymeric disposable microfluidic devices. This review paper aims at presenting the main significant developments that have been achieved in different aspects of micro-injection moulding of microfluidic devices. Aspects covered include device design, machine capabilities, mould manufacturing, material selection and process parameters. Problems, challenges and potential areas for research are highlighted.     

PIV measurements of flow within plugs in a microchannel

The use of two-phase flow in lab-on-chip devices, where chemical and biological reagents are enclosed within plugs separated from each other by an immiscible fluid, offers significant advantages for the development of devices with high throughput of individual heterogeneous samples. Lab-on-chip devices designed to perform the polymerase chain reaction (PCR) are a prime example of such developments. The internal circulation within the plugs used to transport the reagents affects the efficiency of the chemical reaction within the plug, due to the degree of mixing induced on the reagents by the flow regime. It has been hypothesised in the literature that all plug flows produce internal circulation. This work demonstrates experimentally that this is false. The particle image velocimetry (PIV) technique offers a powerful non-intrusive tool to study such flow fields. This paper presents micro-PIV experiments carried out to study the internal circulation of aqueous plugs in two phase flow within 762 μm internal diameter FEP Teflon tubing with FC-40 as the segmenting fluid. Experiments have been performed and the results are presented for plugs ranging in length from 1 to 13 mm with a bulk mean flow velocity ranging from 0.3 to 50 mm/s. The results demonstrate for the first time that circulation within the plugs is not always present and requires fluidic design considerations to ensure their generation.     

Modular component design for portable microfluidic devices

A new modular design concept for microfluidic devices is proposed and demonstrated in this study. We designed three key modular microfluidic components: pumps, valves, and reservoirs, and demonstrated that a microfluidic device with specific functions can be easily assembled with those key modular components. Our pumps are man-powerable so that the assembled microfluidic devices require no any other power sources like expensive syringe pumps or air compressors. This feature makes the assembled microfluidic devices completely portable. We also combined our assembled device with other existing mixing microchannels to serve as the mixing and loading system in polymerase chain reaction experiment to amplify DNA successfully. This result shows that those modular components can be integrated into other microchannels, implying great potential applications of the modular design.     

Numerical study of the microdroplet actuation switching frequency in digital microfluidic biochips

In this article, an electrohydrodynamic approach is used to study the microdroplet actuation in contemporary digital microfluidic biochips. The model is employed to analyze the microdroplet motion, and investigate the effects of the key parameters on the devices performance. The modeling results are compared to the experimental observations, and it is shown that the model provides an accurate representation of digital microfluidic transport. An extensive parametric variation is used to derive the maximum actuation switching frequency for ranges of the microdroplet size, gap spacing between the top and bottom plates and electrode pitch size. As a result, scalability of the devices is investigated, and it is shown that the microdroplet transfer rates change inversely with the system size, and microdroplet average velocity is nearly the same for different system scales. As a result of this study, an adjustable force-based actuation switching frequency implementation is proposed, and it is shown that faster microdroplet motion is obtained by in situ adjusting of the switching frequency. Finally, it has been observed that fastest microdroplet motion, despite similar studies conducted in the literature, is not achieved via actuating the next electrode as soon as the microdroplet touches it. Indeed, the switching frequency spectrum is dependent on the physical and geometrical properties of the system.     

Electrohydrodynamic modeling of microdroplet transient dynamics in electrocapillary-based digital microfluidic devices

In this article, a multiphysics approach is used to develop a model for microdroplet motion and dynamics in contemporary electrocapillary-based digital microfluidic systems. Electrostatic and hydrodynamic pressure effects are combined to calculate the driving and opposing forces as well as the moving boundary of the microdroplet. The proposed methodology accurately predicts the microdroplet electrohydrodynamics which is crucial for the design, control and fabrication of such devices. The results obtained from the model are in excellent agreement with expected trends and experimental results.