Paper-based digital microfluidics

In this paper, a new fabrication method for digital microfluidics is proposed. In which, paper, graphite, and adhesive tape are used as substrate, electrodes, and dielectric layer, respectively. The graphite is sprayed over a template on the paper substrate. Two different water repellants are used as the hydrophobic layer, which replace with expensive materials such as Teflon-AF^®. The paper substrate is low cost, available, and flexible. The proposed device is disposable, and its fabrication procedure is simple, fast, and low cost which allows creation of a new device for each individual experiment. Therefore, problems such as adsorption and dielectric breakdown will not occur in this type of digital microfluidics. This device can perform two types of droplet operations, merging and moving on droplets in volumes of 15–50 μL.     

Purification of a droplet using negative dielectrophoresis traps in digital microfluidics

The development of techniques for manipulating particles and integrating them into the digital microfluidic (DMF) devices has been the subject of several studies in recent years. This paper presents a dielectrophoretic-based method that uses triangular traps to manipulate particles and purify a droplet in DMF platforms. Numerical and experimental studies are conducted to show the effectiveness of the proposed trap geometry which is also compatible with the other operators in the DMF platform. The triangular trap geometry is used to move the polystyrene particles to one side of the trap using negative dielectrophoresis (nDEP). The droplet is then split into two smaller droplets with very low and high concentrations of particles using the electrowetting on dielectric technique. The average velocity of the particles (as they move along the trap) as a function of the vertex angle of the triangular trap and the gap between the top and bottom plate is examined. It is observed that the vertex angle of the trap plays more important role on the motion of the particles than the gap. Thus, to enhance the motion of the particles and minimize the effect of splitting on the purification process, the vertex angle and the slope of the side arms of the triangular trap are modified based on the results of the numerical model simulating the dielectrophoretic force on the particle. The enhanced geometry is fabricated and tested experimentally to show the effectiveness and ease-of-use of the proposed technique in purifying (or concentrating) a droplet in DMF. The results show that using the proposed nDEP electrode geometry purification (or concentration) can be performed with the efficiency of 90 %.     

Real-time droplet caliper for digital microfluidics

We developed an optical, microfabrication-free approach for performing real-time measurements of individual droplet characteristics (frequency of production, velocity, and length) flowing in a transparent microfluidic channel. Our approach consists in an interpretation of the differential signal produced by a pair of photodiodes connected head-to-tail due to the variations of illumination at the passage of a droplet. We checked the relevance of this zero-background method by comparing results to video measurements, and observed a very good agreement at rates up to the kHz range. Moreover, since the measured values are stored in a simple text file, flow characterization over very long times (several hours) becomes accessible. We applied this facility to perform three examples of long-term studies: stationary regimes, transient regimes, and the effect of an external forcing. Several unexpected features, like long-period fluctuations, can thus be evidenced.     

Numerical modeling of electrowetting transport processes for digital microfluidics

Electrical actuation and control of liquid droplets in Hele-Shaw cells have significant importance for microfluidics and lab-on-chip devices. Numerical modeling of complex physical phenomena like contact line dynamics, dynamic contact angles or contact angle hysteresis involved in these processes do challenge in a significant manner classical numerical approaches based on macroscopic Navier–Stokes partial differential equations. In this paper, we analyze the efficiency of a numerical lattice Boltzmann model to simulate basic transport operations of sub-millimeter liquid droplets in electrowetting actuated Hele-Shaw cells. We use a two-phase three-dimensional D3Q19 lattice Boltzmann scheme driven by a Shan–Chen-type mesoscopic potential in order to simulate the gas–liquid equilibrium state of a liquid droplet confined between two solid plates. The contact angles at the liquid–solid–gas interface are simulated by taking into consideration the interaction between fluid particles and solid nodes. The electrodes are designed as regions of tunable wettability on the bottom plate and the contact angles adjusted by changing the interaction strength of the liquid with these regions. The transport velocities obtained with this approach are compared to predictions from analytical models and very good agreement is obtained.     

Wafer-level BCB bonding using a thermal press for microfluidics

Benzocyclobutene (BCB) is a thermosetting polymer that can form microfluidics and bond top and bottom layers of the microfluidics at the same time, and yields high repeatability and high bonding strength. This paper reports using photosensitive BCB to fabricate microfluidics and to bond with a thermal press for 4 in. wafers. By optimizing the parameters for pattern development and using a three-stage temperature and pressure increment BCB bonding, we realize the whole wafer glass–Si or glass–glass bonding in thermal press without any crack. The wafer-level bonding shows a bonding percentage above 70%, a tensile stress above 4.94 MPa, and a bonding repeatability over 95%. Furthermore, the bonding is compatible with thick electrode integration, that microfluidics with 380 nm thick electrodes underneath can be well-bonded. Our bonding method much reduces the cost compared with bonding BCB in a wafer bonding machine. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Droplet digital signal generation in microfluidic networks using model predictive control

The ability to actively control the spatial and temporal dynamics of droplets in microfluidic networks can be harnessed for several applications. Achieving such control is nontrivial due to the nonlinear and interactive nature of such systems, where droplets in different branches of the network can affect each other through resistive signalling. In our previous work we have demonstrated the application of a Model Predictive Control (MPC) framework for sort-synchronization in a simple microfluidic loop device, assuming that the final control elements are elastomeric valves. In this paper, we explore the ability of the MPC framework for more intricate control, where the relative drop distances at the exit of a loop are required to conform to desired profiles. We demonstrate that through appropriate MPC objective function choices, a variety of digital signals based on the relative exit distance can be generated. The importance of such control is highlighted.     

Low-cost,rapid-prototyping of digital microfluidics devices

An innovative and simple microfabrication method for digital microfluidics is presented. In this method, devices are formed from copper substrates or gold compact disks using rapid marker masking to replace photolithography. The new method is capable of forming devices with inter-electrode gaps as small as 50 μm. Saran™ wrap (polyethylene film) and commercial water repellants were used as dielectric and hydrophobic coatings, respectively, to replace commonly used and more expensive materials such as parylene-C and Teflon-AF. Devices formed by the new method enabled single- and two-plate actuation of droplets with volumes of 1–12 μL. Fabricated devices were successfully tested for droplet manipulation, merging and splitting. We anticipate that this fabrication method will bring digital microfluidics within the reach of any laboratory with minimal facilities. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Intelligent digital redesign for nonlinear systems using a guaranteed cost control method

In this paper, a novel intelligent digital redesign (IDR) technique using the guaranteed cost control method is proposed for nonlinear systems which can be represented by a Takagi-Sugeno (T-S) fuzzy model. The IDR technique, which is one of the sampled-data fuzzy controller design methods, guarantees not only the stability condition of the sampled-data closed-loop system with the sampleddata fuzzy controller and the state-matching error is presented. By using the concept of the guaranteed cost control method, sufficient conditions are obtained for both minimization of the state-matching error and stabilization of the sampled-data closed-loop system and derived in terms of linear matrix inequalities (LMIs). Finally, a numerical example is provided to verify the effectiveness of the proposed technique.     

Piezo-driven acoustic streaming in an electrowetting-on-dielectric digital microfluidics device

We report the integration of a lead zirconate titanate, \(\hbox {Pb[Zr}_{x}\hbox {Ti}_{1-x}\hbox {O}_{3}\)] (PZT), piezoelectric transducer disk into the top plate of an otherwise conventional electrowetting-on-dielectric (EWD) digital microfluidics device to demonstrate on-demand induction of circulating fluid flow within single 200 nL droplets. Microparticle image velocimetry was used to measure in-plane velocity distributions for PZT excitation voltages that ranged from 0 to 50 \(\hbox {V}_{\text {RMS}}\). Intra-droplet streaming velocities in excess of 2.0 \(\hbox {mm}\cdot \hbox {s}^{-1}\) were observed without droplet breakup or damage to the EWD device layer. Additionally, we found median intra-droplet streaming velocity to depend quadratically on PZT excitation voltage up to the stress limit of the interfacial boundary. Our approach offers an alternative device architecture for active micromixing strategies in EWD digital microfluidics laboratory-on-chip systems.     

Non-encapsulated thermo-liquid crystals for digital particle tracking thermography/velocimetry in microfluidics

The ever accelerating state of technology has powered an increasing interest in heat transfer solutions and process engineering innovations in the microfluidics domain. In order to carry out such developments, reliable heat transfer diagnostic techniques are necessary. Thermo-liquid crystal (TLC) thermography, in combination with particle image velocimetry, has been a widely accepted and commonly used technique for the simultaneous measurement and characterization of temperature and velocity fields in macroscopic fluid flows for several decades. However, low seeding density, volume illumination, and low TLC particle image quality at high magnifications present unsurpassed challenges to its application to three-dimensional flows with microscopic dimensions. In this work, a measurement technique to evaluate the color response of individual non-encapsulated TLC particles is presented. A Shirasu porous glass membrane emulsification approach was used to produce the non-encapsulated TLC particles with a narrow size distribution and a multi-variable calibration procedure, making use of all three RGB and HSI color components, as well as the proper orthogonally decomposed RGB components, was used to achieve unprecedented low uncertainty levels in the temperature estimation of individual particles, opening the door to simultaneous temperature and velocity tracking using 3D velocimetry techniques.     

Lattice Boltzmann method for microfluidics: models and applications

The lattice Boltzmann method (LBM) has experienced tremendous advances and has been well accepted as a useful method to simulate various fluid behaviors. For computational microfluidics, LBM may present some advantages, including the physical representation of microscopic interactions, the uniform algorithm for multiphase flows, and the easiness in dealing with complex boundary. In addition, LBM-like algorithms have been developed to solve microfluidics-related processes and phenomena, such as heat transfer, electric/magnetic field, and diffusion. This article provides a practical overview of these LBM models and implementation details for external force, initial condition, and boundary condition. Moreover, recent LBM applications in various microfluidic situations have been reviewed, including microscopic gaseous flows, surface wettability and solid–liquid interfacial slip, multiphase flows in microchannels, electrokinetic flows, interface deformation in electric/magnetic field, flows through porous structures, and biological microflows. These simulations show some examples of the capability and efficiency of LBM in computational microfluidics.     

Maskless writing of microfluidics: Rapid prototyping of 3D microfluidics using scratch on a polymer substrate

We present a new rapid prototyping method designed for simple fabrication of 3D microfluidics using a maskless direct writing technique on polymer substrates. The entire process is enabled by a commercial cutter plotter with 10 μm resolution precision and high speed. A CAD design of top and bottom microstructures is directly written on a polymer substrate using a cutter plotter after setting up the suitable force. The smallest channel width of 20 μm was obtained with the minimum force and 100 μm from the maximum. Also the written depth increased linearly with force from 30 to 130 μm. Several 3D microfluidic devices are demonstrated using a maskless writing technique. The entire fabrication process from CAD layout to a final 3D device can be completed in 30 min outside the clean room facilities.     

Iterative functional modification method for solving a transportation problem

We propose a new method for solving transportation problems based on decomposing the original problem into a number of two-dimensional optimization problems. Since the solution procedure is integer-valued and monotonic in the objective function, the required computation is finite. As a result, we get not only a single optimal solution of the original transportation problem but a system of constraints that can yield all optimal solutions. We give numerical examples that illustrate the constructions of our algorithm.     

Spatially controlled microfluidics using low-voltage electrokinetics

Most electrokinetic microfluidic devices currently require high voltages (>50 V) to generate sustained electric fields. However, two long-standing limitations remain, namely: (i) the resulting electrolysis of water produces bubbles, forcing electrodes to be placed in reservoirs outside the channels, and (ii) direct integration with low-voltage microelectronics cannot be achieved. A further limitation is the lack of spatial control within the microchannel. This work presents a method to achieve low-voltage (/spl les/1 V) electrokinetic transport using micropatterned Ag-AgCl electrode arrays, which allows spatial flow control within microchannels. We demonstrate bidirectional electrophoretic control of microparticles within microfluidic channels using /spl plusmn/1 V.     

Progress in computational microfluidics using TransAT

The paper reports on the progress made in predicting single- and multi-phase microfluidics flows using the computational multi-fluid flow dynamics (CMFD) code TransAT. In the multi-phase context, the code uses either the level-set approach or the phase-field variant as the “Interface Tracking Methods”. The solver incorporates phase-change capabilities, triple-line dynamics models, Marangoni effects, electro-wetting and a micro-film sub-grid scale model for lubrication. Subtle microfluidics problems like droplet generation or surface tension-driven flows are shown to be within reach of modern CMFD techniques using interface tracking, with relatively fast CPU response times. In most of the cases presented here, the comparison between CMFD and experiments is rather satisfactory.     

Hybrid method for driver accommodation using optimization-based digital human models

Many applications exist where humans are required to perform a task in a seated position, such as operating a vehicle. Seated posture inside a vehicle influences driver performance and control of the vehicle. For people of extreme stature, tall or short, and for people of extreme width, obese or pregnant populations, it can be difficult to safely operate a vehicle if there is not enough room in the cab or if some controls cannot be reached. This study proposes a hybrid method for predicting the optimum driver seat adjustment range to accommodate diverse drivers in any vehicle based on a direct optimization-based posture prediction method. The proposed hybrid method combines three approaches: a boundary manikin approach, a population sampling approach, and a special population approach. The boundary manikin approach places two boundary manikins (5% female and 95% male) inside a virtual vehicle cab to perform tests. The population sampling approach spans a multitude of test subjects ranging in stature from 158 to 185 cm, determining the range from a plot of predicted hip point distances from the point of contact of the right heel and the floor. The special population approach studies the effect that size and shape changes, such as pregnancy, have on seated posture inside a vehicle. Also given is an indication of discomfort through the output values of the multi-objective function in the optimization formulation. A combination of the three approaches is used to determine an optimal adjustment range for the driver seat, thus allowing most people to safely operate the vehicle. Results of the simulations are validated using experimental determinations of the driver seat adjustment range from the literature. The main benefit of using this method is that the human aspect of design can be included early in the design process, thereby reducing or eliminating prototypes. Another benefit of the simulation is that it can be adapted to other seating tasks such as: occupant seat check inside a vehicle; workstation design; and issues related to other special populations such as obese individuals, dwarfs, and children.     

A method using long digital straight segments for fingerprint recognition

In this paper, we proposed a new method using long digital straight segments (LDSSs) for fingerprint recognition based on such a discovery that LDSSs in fingerprints can accurately characterize the global structure of fingerprints. Different from the estimation of orientation using the slope of the straight segments, the length of LDSSs provides a measure for stability of the estimated orientation. In addition, each digital straight segment can be represented by four parameters: x-coordinate, y-coordinate, slope and length. As a result, only about 600 bytes are needed to store all the parameters of LDSSs of a fingerprint, as is much less than the storage orientation field needs. Finally, the LDSSs can well capture the structural information of local regions. Consequently, LDSSs are more feasible to apply to the matching process than orientation fields. The experiments conducted on fingerprint databases FVC2002 DB3a and DB4a show that our method is effective.     

Precise droplet volume measurement and electrode-based volume metering in digital microfluidics

In this work, for the first time, we demonstrate nanoscale droplet generation from a continuous electrowetting microchannel using a simple and precise image-based droplet volume metering technique. One of the most popular ways of droplet generation in electrowetting devices is to split a droplet from a preloaded volume as a fluid reservoir. This method is effective, but lowers volume consistency after multiple droplets are generated. Impedance- and capacitance-based methods of volume metering have been successfully used in digital microfluidics, but require complex circuitry and feedback signal processing. In this work, we demonstrate nanoliter droplet generation from a continuous electrowetting channel used as a replenishable fluid reservoir which compensates for the loss of reservoir volume as droplets are sequentially split. This improves volume consistency especially for applications requiring multi-droplet generation. Based on the area of the electrode, the volume of each droplet split from the electrowetting channel can be obtained by a simple and precise image processing technique with no need for additional hardware and measurement errors of ±0.05 %. This simple technique can be used in a wide range of applications that require precise volume metering, such as immunoassay.