High-current density DC magenetohydrodynamics micropump with bubble isolation and release system

One of the major challenges for integrated Lab-on-a-Chip (LOC) systems is the precise control of fluid flow in a micro-flow cell. Magnetohydrodynamics (MHD) micropumps which contain no moving parts and capable of generating a continuous flow in any ionic fluid offer an ideal solution for biological applications. MHD micropumping has been demonstrated by using both AC and direct current (DC) currents by a number of researchers with varying degrees of success. However, current MHD designs based on DC do not meet the flow rate requirements for fully automated LOC applications (>100 μl/min). In this research, we introduce a novel DC-based MHD micropump which effectively increases flow rate by limiting the effects of electrolysis generated bubbles at the electrode–electrolyte interface through isolation and a mechanism for their release. Gas bubbles, particularly, hydrogen generated by high current density at the electrodes are the main culprit in low experimental flow rate compared with theoretical values. These tiny bubbles coalesce in the flow channel thus obstructing the flow of fluid. Since hydrolysis is inevitable with DC excitation, compartmentalized electrode channels with bubble isolating and coalescence retarding mechanisms and bubble release systems are implemented to prevent the coalescence of these bubbles and minimize their effects on flow rate. In this novel design called bubble isolation and release system, flow rate of up to 325 μl/min is achieved using 1 M NaCl solution in DC mode with potentials of 5 V and current density of about 5,000 A/m² for a main channel of 800 μm × 800 μm cross-section and 6.4 mm length.     

Equivalent electrical network for performance characterization of piezoelectric peristaltic micropump

Utilizing an electronic–hydraulic analogy, this study develops an equivalent electrical network of a piezoelectric peristaltic micropump which has not been modeled the whole system operation completely by computational fluid dynamics (CFD) or equivalent electrical network so far due to its excessive complicated structure. The validity of the proposed model is verified by comparing the simulation results obtained using the SPICE (simulation program with integrated circuit emphasis) software package for flow rate spectrum and its maximum state of a typical micropump with the experimental observations for two working fluids, namely DI water and blood. The simulation results predict a maximum flow rate frequency and flow rate of 280 Hz and 43.23 μL/min, respectively, for water, and 210 Hz and 24.12 μL/min for blood. The corresponding experimental results are found to be 300 Hz and 41.58 μL/min for water and 250 Hz and 23.75 μL/min for blood. The relatively poorer agreement between the two sets of results when using blood as the working fluid is thought to be the result of the non-Newtonian nature of blood, which induces a more complex, non-linear flow behavior within the micropump. Having validated the proposed model, the equivalent network is used to perform a systematic analysis of the correlation between the principal micropump design parameters and operating conditions and the micropump performance. The results confirm the validity of the equivalent electrical network model as the first microfluidic modeling tool for optimizing the design of peristaltic micropumps and for predicting their performance.     

A micropump based on water potential difference in plants

In land plants, water vapor diffuses into the air through the stomata. The loss of water vapor creates a water potential difference between the leaf and the soil, which draws the water upward. Quantitatively, the water potential difference is 1–2 MPa which can support a water column of 100–200 m. Here we present the design and operation of a biomimetic micropump. The micropump is mainly composed of a 48-μm thick metal screen plate with a group of 102-μm diameter micropores and an agarose gel sheet with nanopores of 100 nm diameter. The micropores in the screen plate imitate the stomata to regulate the flow rate of the micropump. The agarose gel sheet is used to imitate the mesophyll cells around the stomata. The lost of water from the nanopores in the gel sheet can generate a water potential difference (more than 30 kPa) which can drive solution flow in a microfluidic chip. Results have shown that a precise flow rate of 4–8 nl/min can be obtained by using this micropump, and its ultra-high flow rate is 113–126 nl/min. The advantages of this biomimetic micropump include adjustable flow rate, simple structure and low fabrication cost. It can be used as a “plug and play” fluid-driven unit in microfluidic chips without any external power sources or equipments.     

LIGA fabrication and test of a DC type magnetohydrodynamic (MHD) micropump

 This paper reports a research effort to design, microfabricate and test a DC type magnetohydrodynamic (MHD) micropump using LIGA method (Menz et al., 1991). The micropump is driven using the Lorentz force and can be used to deliver electrically conductive fluids. In operation, a DC voltage is supplied across the electrodes to generate the distributed body force on the fluid in the pumping chamber, and therefore a constant pressure difference along the pumping chamber. The external magnetic field was supplied using permanent magnets. The major advantage of a MHD-based micropump is that it does not contain any moving parts. It may have potential applications in medicine delivery, biological and biomedical studies. The test of the DC prototype micropump shows that bubble generation mechanism affect the performance significantly and an AC driving mechanism may be used to improve the performance.     

Flow characterization of valveless micropump using driving equivalent moment: theory and experiments

A valveless micropump, actuated by a PZT disk bonded to a glass plate, can generate positive flow. In order to estimate flow characteristics of micropumps, it is necessary to theoretically analyze the radial expansion (more specifically, the equivalent moment) of the PZT disk according to the voltage input. Using the equivalent moment, deflection equations are derived for the tri-layer disk (PZT, epoxy bonder and glass plate) and are confirmed to match well with experiments. The flow rate of the valveless micropump is also theoretically and experimentally investigated in terms of input voltage and oscillation frequency. The flow increased at a rate of 0.1 μL/min/V, and the maximum flow rate was obtained at the driving frequency of around 225 Hz.     

A study of PZT valveless micropump with asymmetric obstacles

The results of a study on a new type of PZT valveless micropump with asymmetric obstacles are presented in this paper. The valveless micropump was made through a MEMS fabrication process by using simply only one photo mask. Asymmetric obstacles are used for the flow directing device instead of the diffuser/nozzle elements used in previous studies. In this study, numerical simulations were also carried out to evaluate the design and the performance of the new micropump. The main feature of the present micropump is that it has a uniform cross-section area across the micro-channel, which gives many advantages. The differential pressure head and the pumping flow rate can be adjusted easily by using obstacles of different shapes and changing the PZT operating frequency without changing the dimensions of the micro-channel. In this experiment, the performance of the micropump was evaluated by measuring the pressure head difference and the flow rate as the input voltage ranged from 20 to 40 V, a range much lower than those in previous studies. The pumping pressure can reach a maximum of 1.2 kPa, and the maximum net volume flow rate is 156 μl/min. These test data indicate that this micropump fulfills the demands for most micro-fluidic systems. Moreover, the present device can be easily applied to complex systems with combinations of several pumps and microchannels in the future.     

Particle streaming and separation using dielectrophoresis through discrete periodic microelectrode array

This study presents a particle manipulation and separation technique based on dielectrophoresis principle by employing an array of isosceles triangular microelectrodes on the bottom plate and a continuous electrode on the top plate. These electrodes generate non-uniform electric fields transversely across the microchannel. The particles within the flowing fluid experience a dielectrophoretic force perpendicular to the fluid flow direction due to the non-uniform electric fields. The isosceles triangular microelectrodes were designed to continuously exert a small dielectrophoretic force on the particles. Particles experiencing a larger dielectrophoretic force would move further in the perpendicular direction to the fluid flow as they traveled past each microelectrode. Polystyrene microspheres were used as the model particles, with particles of ∅20 μm employed for studying the basic characteristics of this technique. Particle separation was subsequently demonstrated on ∅10 and ∅15 μm microspheres. Using an applied sinusoidal voltage of 20 V_pp and frequency of 1 MHz, a mean separation distance of 0.765 mm between them was achieved at a flow rate of 3 μl/min (~1 mm/s), an important consideration for high throughput separation capability in a micro-scale technology device. This unique isosceles triangular microelectrodes design allows heterogeneous particle populations to be separated into multiple streams in a single continuous operation.     

Microfluidic pump based on the phenomenon of electroosmosis of the second kind

A micropump based on strong polarization of ion-exchange beads and corresponding actuation by electroosmosis of the second kind was designed and fabricated. Experimental results from operation with AC and DC voltage showed a close to second order relationship between flow and voltage, in good agreement with theory. The difference between experimental and theoretical flow rates and pressures is attributed to the hydrodynamic resistance of the channel network. A modified pump design which should yield higher flow rates and pressures was suggested.     

A numerical hydrodynamic and thermal characterization of an inter-strata liquid cooling solution for 3D ICs

A novel integrated thermal management solution is proposed to alleviate hot spots in a contemporary 3D IC architecture. The solution employs a series of integrated microchannels, interconnected through each stratum by through silicon fluidic vias (TSFVs), and permits the transfer of heat, via a coolant, from hot to cold zones. This microfluidic system is driven by an integrated AC electrokinetic pump embedded in the channel walls. Recent advancements in electrokinetic micropump technology have allowed greater increases in fluid velocity (mm/s) while operating within the voltage constraints of a 3D IC. This paper presents a 2D simulation of an electrokinetic micropump operating at V_pp = 1.5 V in a 40 μm channel and examines its velocity profile for six frequencies in the range 100 ≤ ω ≤ 100 MHz. An optimum frequency of 100 kHz was established within this range and this was further examined with a constant heat flux of 186 W/cm2 imposed on the wall for an inlet fluid temperature of 40°C. Temperature profiles are presented at the channel-silicon interface and compared with theory.     

AC electrokinetic pumping on symmetric electrode arrays

AC electro-osmotic (ACEO) pumping is experimentally demonstrated on a symmetric gold electrode array. Using asymmetric connection of electrodes to the applied AC voltage, spatial asymmetry along the array is created, which produces unidirectional flow of electrolyte. An aqueous solution of 100 μM KCl is selected as the pumping fluid. The liquid velocity obtained as a function of voltage and frequency is compared to that generated using travelling-wave electroosmosis (TWEO) with the same electrode array. The expected velocities from the linear electrokinetic models of ACEO and TWEO are computed numerically. The comparison shows that TWEO generates greater velocity amplitudes and the streamlines are smoother than those generated by ACEO.     

A three phase serpentine micro electrode array for AC electroosmotic flow pumping

A new three-phase electrode array with a serpentine electrode is designed and prototyped using PolyMUMPs process for micro flow pumping. Numerical model of the micropump has been developed using COMSOL Multiphysics™. Experimental testing is conducted and time-averaged flow velocities from testing and simulation agree well. Peak time-averaged flow velocity of 270 μm/s is achieved at 30 Hz using ethanol.     

A pneumatic micropump incorporated with a normally closed valve capable of generating a high pumping rate and a high back pressure

This study reports on a new pneumatic micropump integrated with a normally closed valve that is capable of generating a high pumping rate and a high back pressure. The micropump consists of a sample flow microchannel, three underlying pneumatic air chambers, resilient polydimethylsiloxane (PDMS) membrane structures and a normally closed valve. The normally closed valve of the micropump is a PDMS-based floating block structure located inside the sample flow microchannel, which is activated by hydraulic pressure created by the peristaltic motion of the PDMS membranes. The valve is used to effectively increase pumping rates and back pressures since it is utilized to prevent backflow. Experimental results indicate that a pumping rate as high as 900 μL/min at a driving frequency of 90 Hz and at an applied pressure of 20 psi (1.378 × 10⁵ Nt/m²) can be obtained. The back pressure on the micropump can be as high as 85 cm-H₂O (8,610.5 Nt/m²) at the same operation conditions. The micropump is fabricated by soft lithography processes and can be easily integrated with other microfluidic devices. To demonstrate its capability to prevent cross contamination during chemical analysis applications, two micropumps and a V-shape channel are integrated to perform a titration of two chemical solutions, specifically sodium hydroxide (NaOH) and benzoic acid (C₆H₅COOH). Experimental data show that mixing with a pH value ranging from 2.8 to 12.3 can be successfully titrated. The development of this micropump can be a promising approach for further biomedical and chemical analysis applications.     

Microfocusing using the thermal actuation of microbubbles

This paper describes the microfocusing in a microchannel using the thermal actuation of a pair of microbubbles. A microbubble was produced from de-ionized (DI) water with an integrated microheater, and the volume was controlled by applying voltage. The microfocusing was demonstrated with a polydimethylsiloxane (PDMS) device consisting of two layers. The top layer included a microchannel that was 300 μm wide and 50 μm high. It was flanked by a pair of reservoirs. The bottom layer provided a microheater underneath the reservoir. Upon heating, DI water boiled immediately over the microheater and formed a microbubble that came out of the reservoir in a perpendicular direction toward the fluid. The fluid was focused from 300 to 22 μm, as the distance between the apexes of the arch-shaped microbubbles was shortened due to expansion, which was maintained at a flow velocity up to approximately 17.8 mm s⁻¹. The temperature of the water in the reservoir was estimated to reach the boiling point within 62 or 160 ms, depending on the substrate.     

In-channel focusing of flowing microparticles utilizing hydrodynamic filtration

We present herein microfluidic systems to continuously focus the positions of flowing particles onto the center of a microchannel, which is indispensable to various applications for manipulating particles or cells such as flow cytometry and particle-based bioassay. A scheme called ‘hydrodynamic filtration’ is employed to repeatedly split fluid flows from a main stream, while remaining particles in the main stream. By re-injecting the split flows into the main channel, these flows work as sheath flows, focusing the positions of the particles onto the center of the microchannel without the help of sheath flows or complicated devices generating physical forces. In this study, we proposed two schemes, and compared the focusing efficiencies between the two schemes using particles 5.0 μm in diameter. Also, we confirmed that the flow speed did not affect the focusing efficiency, demonstrating the versatility and applicability of the presented systems. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Design of a relaying electroosmosis pump driven by low-voltage DC

Electroosmosis pumps (EOPs) have been widely used for manipulating small amounts of reagents for chemical and biological analysis. Traditionally, a high-voltage DC has to be applied in order to achieve the required flow rate. One alternative is to use low AC voltage. Here we propose another solution, which, instead of using a high-voltage DC or low AC voltage, adds a low-voltage DC to an array of electrodes. This design of EOP is called a relaying EOP or cascade EOP. In this study, we intend to push the limit of the low-voltage further down to 2 V by patterning a dense electrode array in a straight microchannel. Two patterns of interdigitated electrodes, symmetric with equal size electrodes and asymmetric with unequal size electrodes, are proposed. Simulations are performed to optimize the distribution and geometrical parameters of the electrode array in order to achieve the maximum flow rate. The proposed low-voltage DC electroosmosis pump shows an advantage in integrating EOP into portable Lab-on-a-chip devices. In addition, the low-voltage DC EOP shows a good promise for in vivo biomedical applications such as drug delivery.     

Microfluidic phase change valve with a two-level cooling/heating system

A phase change (PC) microvalve with an integrated two-level cooling/heating system is developed for microfluidic applications in this article. This PC microvalve utilizes the liquid–solid PC of a small portion of the working medium in a microchannel to switch on/off the flow in the microchannel. The size of the working medium for the PC microvalve is 5-mm long, 50-μm high, and 80-μm wide (50 μm × 80 μm is the cross-sectional area of the channel) in this study. The switch is actuated by using a two-level cooling/heating system integrated on the chip. The first-level cooling/heating unit keeps the working medium in the valve area in the temperature range of supercooling state. Based on the supercooling state, the second-level cooling/heating unit either heats up or cools down the medium in the valve area to trigger its PC between liquid and solid for valving purposes. The proposed microfluidic PC microvalve is characterized experimentally in microfluidic chips. The thermal impact of one PC microvalve in one particular microchannel on its adjacent channels is discussed by establishing a preliminary analytical model and a numerical model. In addition to no leakage and no moving element, this PC microvalve with a two-level cooling/heating system can achieve a very short cooling time (i.e., 2.72 s).     

A novel thermo-pneumatic peristaltic micropump with low temperature elevation on working fluid

This work presents a novel thermo-pneumatic peristaltic micropump with low temperature elevation on working fluid. The proposed device, which consists of two separate zones for air-heating and fluid-squeezing, is realized by using micromachining techniques. Also, the device can be operated by using a small and simple actuation circuitry with low applied voltages. Under similar operational conditions, the proposed micropump shows similar fluid-pumping performance when compared with the conventional design of thermo-pneumatic micropumps. However, for the proposed design, the temperature elevation on the fluid-pumping area is as small as about 2.0 K, which is less than 8% of that generated by the conventional design. Furthermore, by applying higher voltages, larger flow rate can be achieved with relatively small increase in temperature elevation. Due to low temperature elevation on working fluid, the proposed device is suitable for the applications such as DNA chips or protein chips. In addition, because of its small size and simple actuation scheme, potentially the proposed device can be integrated into the devices for point-of-care applications.     

Development of peristaltic antithrombogenic micropumps for in vitro and ex vivo blood transportation tests

This study involves the design, fabrication and characterization of a biocompatible silicon micropump. Three experiments were conducted to study the performance of this pump in clinical environments. They were a blood compatibility test, and in vitro and ex vivo studies. Whole blood is an intrinsically complex material and difficult to manipulate using a microsystem device. In the blood compatibility experiments, two materials N-(triethosilylpropyl)-O-polyethylene oxide urethane (PEOU) and polyethylene glycol (PEG) were employed to form a self-assembled monolayer (SAM) on a chip surface. According to the platelet remaining test and a 30-min blood transportation test, PEOU protected the micropump from thrombus. In the second experiment, the micropump handled several liquids, including DI water and whole blood. When the pump was operated at a voltage of 140 V_pp, the flow rates of the DI water and whole blood were 121.6 μl/min at 500 Hz and 50.2 μl/min at 450 Hz, respectively. The maximum back pressure of the water and the blood in the micropump were 3.2 and 1.8 kPa, respectively. Finally, the micropump injected phosphated buffered saline (PBS) and whole blood into the veins of rats. The pump was characterized ex vivo and discussed. The third experiment reveals that the micropump fulfilled the dosing condition for clinical medicine and did not affect the physiological function of the rats. This pump is highly promising for biomedical applications, such as in drug delivery for patients, or in clinical care. Moreover, the pump has potentials to control precisely medication to improve conventional clinical treatments.     

Continuous-flow particle and cell separations in a serpentine microchannel via curvature-induced dielectrophoresis

Particle and cell separations are critical to chemical and biomedical analyses. This study demonstrates a continuous-flow electrokinetic separation of particles and cells in a serpentine microchannel through curvature-induced dielectrophoresis. The separation arises from the particle size-dependent cross-stream dielectrophoretic deflection that is generated by the inherent electric field gradients within channel turns. Through the use of a sheath flow to focus the particle mixture, we implement a continuous separation of 1 and 5 μm polystyrene particles in a serpentine microchannel under a 15 kV/m DC electric field. The effects of the applied DC voltages and the serpentine length on the separation performance are examined. The same channel is also demonstrated to separate yeast cells (range in diameter between 4 and 8 μm) from 3 μm particles under an electric field as low as 10 kV/m. The observed focusing and separation processes for particles and cells in the serpentine microchannel are reasonably predicted by a numerical model.     

Modeling and analysis of a 2-DOF bidirectional electro-thermal microactuator

In this paper, a four hot-arm U-shape electro-thermal actuator that can achieve bidirectional motion in two axes is introduced. By selectively applying voltage to different pairs of its four arms, the device can provide actuation in four directions starting from its rest position. It is shown that independent in-plane and out-of-plane motions can be obtained by tailoring the geometrical parameters of the system. The lumped model of the microactuator was developed using electro-thermal and thermo-mechanical analyses and validated using finite element simulations. The device has been fabricated using PolyMUMPs and experimental results are in good agreement with the theoretical predictions. Total in-plane deflections of 4.8 μm (2.4 μm in either direction) and upward out-of-plane deflections of 8.2 μm were achieved at 8 V of input voltage. The large achievable deflections and the higher degree-of-freedom of the proposed device compared to its counterparts, foresee its use in diverse MEMS applications.