Paper-Based Microfluidics Perform Mixing Effects by Utilizing Planar Constricted-Expanded Structures to Enhance Chaotic Advection.

This study aimed to design and fabricate planar constricted-expanded structures that are integrated into paper-based channels in order to enhance their chaotic advection and improve their mixing performance. Chromatography papers were used to print paper-based microfluidics using a solid-wax printer. Three different constricted-expanded structures-i.e., zigzag, crossed, and curved channels-were designed in order to evaluate their mixing performance in comparison with that of straight channels. A numerical simulation was performed in order to investigate the mixing mechanism, and to examine the ways in which the planar constricted-expanded structures affected the flow patterns. The experimental and numerical results indicated that the proposed structures can successfully mix confluents. The experimental results revealed that the mixing indices (σ) rose from the initial 20.1% (unmixed) to 34.5%, 84.3%, 87.3%, and 92.4% for the straight, zigzag, curved, and cross-shaped channels, respectively. In addition, the numerical calculations showed a reasonable agreement with the experimental results, with a variation in the range of 1.0-11.0%. In future, we hope that the proposed passive paper-based mixers will be a crucial component in the application of paper-based microfluidic devices.

Characterization of microparticle separation utilizing electrokinesis within an electrodeless dielectrophoresis chip.

This study demonstrated the feasibility of utilizing electrokinesis in an electrodeless dielectrophoresis chip to separate and concentrate microparticles such as biosamples. Numerical simulations and experimental observations were facilitated to investigate the phenomena of electrokinetics, i.e., electroosmosis, dielectrophoresis, and electrothermosis. Moreover, the proposed operating mode can be used to simultaneously convey microparticles through a microfluidic device by using electroosmotic flow, eliminating the need for an additional micropump. These results not only revealed that the directions of fluids could be controlled with a forward/backward electroosmotic flow but also categorized the optimum separating parameters for various microparticle sizes (0.5, 1.0 and 2.0 µm). Separation of microparticles can be achieved by tuning driving frequencies at a specific electric potential (90 Vpp·cm(-1)). Certainly, the device can be designed as a single automated device that carries out multiple functions such as transportation, separation, and detection for the realization of the envisioned Lab-on-a-Chip idea.

Magnetic Beads inside Droplets for Agitation and Splitting Manipulation by Utilizing a Magnetically Actuated Platform.

We successfully developed a platform for the magnetic manipulation of droplets containing magnetic beads and examined the washing behaviors of the droplets, including droplet transportation, magnetic bead agitation inside droplets, and separation from parent droplets. Magnetic field gradients were produced with two layers of 6 × 1 planar coils fabricated by using printed circuit board technology. We performed theoretical analyses to understand the characteristics of the coils and successfully predicted the magnetic field and thermal temperature of a single coil. We then investigated experimentally the agitation and splitting kinetics of the magnetic beads inside droplets and experimentally observed the washing performance in different neck-shaped gaps. The performance of the washing process was evaluated by measuring both the particle loss ratio and the optical density. The findings of this work will be used to design a magnetic-actuated droplet platform, which will separate magnetic beads from their parent droplets and enhance washing performance. We hope that this study will provide digital microfluidics for application in point-of-care testing. The developed microchip will be of great benefit for genetic analysis and infectious disease detection in the future.

Characterization of a Droplet Containing the Clustered Magnetic Beads Manipulation by Magnetically Actuated Chips.

A magnetically actuated chip was successfully developed in this study to perform the purpose of transportation for a droplet containing clustered magnetic beads. The magnetic field gradient is generated by the chip of the two-layer 4 × 4 array micro-coils, which was commercially fabricated by printing circuit board (PCB) technology. A numerical model was first established to investigate the magnetic field and thermal field for such a micro-coil. Consequently, the numerical simulations were in reasonable agreement with the experimental results. Moreover, a theoretical analysis was derived to predict the dynamic behaviors of the droplets. This analysis will offer the optimal operation for such a magnetically actuated chip. This study aims to successfully implement the concept of "digital microfluidics" in "point-of-care testing" (POCT). In the future, the micro-coil chip will be of substantial benefit to genetic analysis and infectious disease detection.

Design and Fabrication of Pneumatically Actuated Valveless Pumps.

In this study, a valveless pump was successfully designed and fabricated for the purpose of medium transportation. Different from traditional pumps, the newly designed pump utilizes an actuated or a deflected membrane, and it serves as the function of a check valve at the same time. For achieving the valveless property, an inlet or outlet port positioned in an upper- or lower-layer thin membrane was designed to be connected to an entrance or exit channel. Theoretical analysis and numerical simulation were conducted simultaneously to investigate the large deformation characteristics of the membranes and to determine the proper location of the inlet or outlet port on the proposed pump. Then, the valveless pump was fabricated on the basis of the proposed design. In the experiment, the maximum flow rate of the proposed pump exceeded 12.47 mL/min at a driving frequency of 5.0 Hz and driving pressure of 68.95 kPa.

Application of indium tin oxide (ITO)-based microheater chip with uniform thermal distribution for perfusion cell culture outside a cell incubator.

This study reports a transparent indium tin oxide (ITO)-based microheater chip and its applicability for perfusion cell culture outside a cell incubator. The attempt of the proposed ITO microheater is to take the role of conventional bulky incubator for cell culture in order to improve integratability with the experimental setup for continuous/perfusion cell culture, to facilitate microscopic observation or other online monitoring activities during cell culture, or even to provide portability of cell culture operation. In this work, numerical simulation and experimental evaluation have been conducted to justify that the presented device is capable of providing a spatially uniform thermal environment and precise temperature control with a mild deviation of +/-0.2 degrees C, which is suitable for a general cell culture practice. Besides, to testify that the thermal environment generated by the presented device is well compatible with conventional cell incubator, chondrocyte perfusion culture was carried out. Results demonstrated that the physiology of the cultured chondrocytes on the developed ITO microheater chip was consistent with that of an incubator. All these not only demonstrate the feasibility of using the presented ITO microheater as a thermal control system for cell culture outside a cell incubator but also reveal its potential for other applications in which excellent thermal control is required.

Development of high throughput optical sensor array for on-line pH monitoring in micro-scale cell culture environment.

On-line pH detection of cell culture environment is necessary in a bioprocess or tissue engineering. Devices by means of electrochemical mechanisms for this purpose have been reported to be less suitable compared with optical-based sensing principles. More recently, some non-invasive optical sensing systems have been proposed for online pH monitoring of cell culture environment. However, these devices are not for multi-target pH monitoring purpose, and are large in scale and thus not appropriate for the pH monitoring at a micro scale such as in microbioreactor or microfluidic-based cell culture platform. To tackle these issues, an optical fiber sensor array for on-line pH monitoring was proposed using microfluidic technology. The working principle is based on the optical absorption of phenol red normally contained in culture medium. Different from other device of the similar working principle, the proposed device requires less liquid volume (less than 0.8 microl), is non-invasive, and particularly can be configured as an array for high throughput pH monitoring. The present device has been optimized for the shape of detection chamber in a microfluidic chip with the aid of computational fluid dynamics (CFD) simulation, to avoid flow dead zone and thus to reduce the response time of detection. Both simulation and experimental results revealed that the design of oval detection chamber (axis, 1.5 and 2.0 mm) can considerably reduce the response time. Preliminary test has proved that the optical pH detection device is able to detect pH with average detection sensitivity of 0.83 V/pH in the pH range of 6.8-7.8, which is normally experienced in mammalian cell culture.

Purification and enrichment of virus samples utilizing magnetic beads on a microfluidic system.

This study reports a new microfluidic system with three integrated functional devices for pumping, mixing and separation of bio-samples by utilizing micro-electro-mechanical-systems technology. By using antibody-conjugated magnetic beads, the developed system can be used to purify and enrich virus samples such that the subsequent detection of viruses can be performed with a higher sensitivity. The target viruses were first captured by the antibody coated onto the magnetic beads by using a rotary micromixer which performed the incubation process. The viruses were then purified and enriched by a magnetic field generated by planar microcoils. The integrated microfluidic system can perform the whole purification and enrichment process automatically using a rotary micropump and appropriate microvalves. In addition, a numerical simulation was also employed to optimize the design of the microcoils and to investigate the magnetic field strength and distribution. The simulation results were consistent with experimental observations. Finally, the developed system was used to successfully perform the purification and enrichment of Dengue viruses. The detectable limit of Dengue viruses was found to be as low as 10(2) pfu ml(-1) by using this approach. Therefore, the integrated microsystem can perform incubation, transportation, mixing and purification of virus samples, possibly making it a promising platform for future biological and medical applications.

Optimization of Micropump Performance Utilizing a Single Membrane with an Active Check Valve.

In this study, we successfully designed and tested a new micropump that utilizes an active check valve and bottom-protruding structure to achieve sample transportation. We performed theoretical analyses and numerical simulations to determine the optimal location of the active check valve. We also experimentally analyzed variations in the generated flow rate with respect to the pneumatic frequencies, actuated air pressures, and locations of the active check valve. The experimental results indicate the optimum air pressure, driving frequency, and location of the active check valve to be 68.9 kPa, 26.0 Hz, and 2.0 mm, respectively. We obtained a maximum pumping rate of 488 µL/min and a maximum pumping efficiency of 35.4%. The proposed micropump could perform a crucial function in the transportation of microfluids and could be incorporated into micro total analysis systems.

Rapid whole-cell sensing chip for low-level arsenite detection.

A novel whole-cell sensing chip system consisted of a micro-concentrator, a set of electrochemical detection electrodes, and a microfluidic channel was developed for rapid detection of arsenite in water. Firstly, the E. coli cells transformed with arsenited-regulated reporter plasmids were incubated with solution contained arsenite. Under this condition, the level of reporter protein, ß-galactosidase, expressed by E. coli cells is dependent on the concentration of arsenite. Using the dielectrophoretic force, the micro-concentrator continuously enriched the E. coli cells into a small area above the embedded detection electrodes. And then the relative expression levels of ß-galactosidase were obtained using the electrochemical method to measure the amount of p-aminophenol (PAP) which converted from the p-aminophenyl-ß-D-galactopyranoside (PAPG) by ß-galactosidase. From the result, it indicates this device can detect as low as 0.1 ppm of arsenite within 30 min. Compared with other traditional detection methods, our new device provides better performance like higher sensitivity, shorter analysis time, and lower cost in detecting the arsenite.

Integrated polymerase chain reaction chips utilizing digital microfluidics.

This study reports an integrated microfluidic chip for polymerase chain reaction (PCR) applications utilizing digital microfluidic chip (DMC) technology. Several crucial procedures including sample transportation, mixing, and DNA amplification were performed on the integrated chip using electro-wetting-on-dielectric (EWOD) effect. An innovative concept of hydrophobic/hydrophilic structure has been successfully demonstrated to integrate the DMC chip with the on-chip PCR device. Sample droplets were generated, transported and mixed by the EWOD-actuation. Then the mixture droplets were transported to a PCR chamber by utilizing the hydrophilic/hydrophobic interface to generate required surface tension gradient. A micro temperature sensor and two micro heaters inside the PCR chamber along with a controller were used to form a micro temperature control module, which could perform precise PCR thermal cycling for DNA amplification. In order to demonstrate the performance of the integrated DMC/PCR chips, a detection gene for Dengue II virus was successfully amplified and detected. The new integrated DMC/PCR chips only required an operation voltage of 12V(RMS) at a frequency of 3 KHz for digital microfluidic actuation and 9V(DC) for thermal cycling. When compared to its large-scale counterparts for DNA amplification, the developed system consumed less sample and reagent and could reduce the detection time. The developed chips successfully demonstrated the feasibility of Lab-On-a-Chip (LOC) by utilizing EWOD-based digital microfluidics.

Model description of contact angles in electrowetting on dielectric layers.

The electrowetting on dielectric (EWOD) technique has considerable potential for microfluidic and biomedical applications. The Lippmann-Young model based on the force balance concept has long been used to predict the contact angles of droplets under electrowetting. However, recent experimental evidence has indicated that this model fails to provide accurate predictions of the lower contact angles associated with saturation conditions at higher electric potentials. Hence, the study simulates the internal flow in an actuated droplet and treats it as stagnation-point flow. This kinetic energy is then taken into consideration while calculating the contact angles using an energy balance model. The energy of an actuated droplet is contributed by the combination of the side surface tension energy, the base tension energy, the dielectric energy, and the kinetic energy when deriving the energy balance model. Consequently, the new energy balance model modifies the Lippmann-Young equation, thereby providing enhanced reasonable predictions of the droplet contact angle across the higher electric potential where the contact angles are close to the saturated condition.

Active mixing inside microchannels utilizing dynamic variation of gradient zeta potentials.

This study presents a new active micromixer with high mixing efficiency achieved by means of a gradient distribution of the surface zeta potential controlled by changing the frequency of voltage applied on shielding electrodes. Gradient surface zeta potential is generated by applying a high voltage to inclined buried shielding electrodes. While alternating the frequency of driving voltage, the zeta potential could be changed accordingly, thus providing a significant mixing effect inside microchannels. A theoretical model is proposed to predict the distribution of zeta potential. The results from this model are critically compared with the well-developed three-capacitor model. Additionally, two time-factor scales, the charge time of capacitor and mixing length flow time, are used to predict the optimum frequency. The prediction of optimum frequency, 0.5 Hz, is consistent with experimental results. Moreover, a five-pair inclined shielding electrode with a frequency of 0.5 Hz leads to a significant improvement in the mixing performance of the active micromixer. Numerical results indicate that a localized flow circulation is generated when the control voltage is applied to the inclined shielding electrodes. Furthermore, the streamlines are experimentally observed by using fluorescent beads. The shape of this circulation is dependent on the distribution of gradient zeta potential, which is determined by the arrangement of electrodes. The effects of the number of electrode pairs and the layout of shielding electrodes on the mixing performance of micromixer are also explored both numerically and experimentally. It is revealed that five-pair inclined electrodes at 0.5 Hz provide the highest mixing efficiency. Finally, a reaction between N-benzoyl-L-arginine-p-nitroanilide and trypsin enzyme is performed to verify the capability of micromixers. The experimental results reveal that the reaction can achieve a higher performance indicating a higher mixing efficiency. The active micromixers could be used in microfluidic systems for improving the mixing efficiency and thus enhancing the bioreaction.