Microfluidic manipulation by AC Electrothermal effect

AC Electrokinetics (ACEK) has attracted much research interest for microfluidic manipulation for the last few years. It shows great potential for functions such as micropumping, mixing and concentrating particles. Most of current ACEK research focuses on AC electroosmosis (ACEO), which is limited to solutions with conductivity less than 0.02 S/m, excluding most biofluidic applications. To solve for this problem, this dissertation seeks to apply AC electrothermal (ACET) effect to manipulate conductive fluids and particles within, and it is among the first demonstration of ACET devices, a particle trap and an ACET micropump. The experiments used fluids at a conductivity of 0.224 S/m that is common in bio-applications. Pumping and trapping were demonstrated at low voltages, reaching ~100 um/s for no more than 8 Vrms at 200 kHz. The flow velocity was measured to follow a quadratic relationship with applied voltage which is in accordance with theory. This research also studies ACET effect on low ionic strength microfluidics, since Joule heating is ubiquitous in electrokinetic devices. One contribution is that our study suggested ACET as one possible reason of flow reversal, which has intrigued the researchers in ACEK field. Electrically, a microfluidic cell can be viewed as an impedance network of capacitances and resistors. Heat dissipation in those elements varies with AC frequency and fluid properties, so changes the relative importance of heat generation at the electrode/electrolyte interface and in the resistive fluid bulk, which could change the temperature gradient in the device, hence changing the flow direction. Another contribution of this dissertation is the reaction enhanced ACET micropumping. A dramatic improvement in flow rate over conventional ac micropumps is achieved by introducing a thin fluid layer of high ionic density near the electrodes. Such an ionic layer is produced by superimposing a DC offset on AC signal that induces Faradaic reaction. The velocity improvement, in some cases, is over an order of magnitude, reaching a linear velocity of up to 2.5 mm/s with only 5.4Vrms. This discovery presents an exciting opportunity of utilizing ACET effect in microfluidic applications

Development of Gate-Controlled DC Electrokinetic Micropumps

Lab-on-chip (LOC) devices have received considerable attention in research and development for automated, high-throughput biological and chemical analysis. While much progress has been accomplished; however, fluid flow control still needs improvement and reminds one of the significant challenges for the future practical LOC devices. This thesis explores the application of electroosmosis (EO) technique and field effect flow control (FEFC) technology for micropumps, an important microfluidic component of LOC systems. In this work, electroosmosis method was employed to electro-kinetically move the working fluid under a longitudinal electric field, and the FEFC technique was also utilized to manipulate the Electroosmotic Flow (EOF) through applying a normal electric field to influence the surface charge at the fluid-microchannel wall interface for an independent control over the EOF. Major accomplishments in this thesis are, study on channel geometry effect with no gate control component, and a single microchannel with gate control component. A number of micropumps with different channel geometries were fabricated using soft lithography technique. PDMS prepolymer served as a top wall and both side walls of the microchannel, with a glass slide as the bottom (in the case of gate control, Indium Tin Oxide glass slides were used). On the gate control region, through adjusting the secondary electric field over the gate, FEFC can locally manipulate EOF. It helps produce a range of flow rates, enhance flow rates, and control flow direction. Moreover, micropumps were interfaced with another microchannel section for sample delivery. To improve the microfluidic device, electro-fluid flow models were developed to describe and predict electric field distribution, velocity field distribution, flow direction, and FEFC phenomena using Finite Element Analysis tool (FEMLAB). The simulation results agreed well with experimental results

Fabrication and Characterization of a Magnetohydrodynamic Micropump from Polydimethylsiloxane (PDMS)

Microfluidics is a growing area of study in recent years, particularly for lab-on-a-chip applications. Fluids must oftentimes be transported from one location on the chip to another. This study focuses on the fabrication and characterization of a magnetohydrodynamic micropump. The device was fabricated in the Rose-Hulman Institute of Technology MiNDS facility, and consisted of a PDMS channel and titanium electrodes supported on a glass slide with a permanent magnet. An Arduino microcontroller capable of pulse width modulation (PWM) was used to control the electrical potential. An electrode design that spanned the full length of the channel was successful in driving fluid motion. Erioglaucine disodium salt blue dye was used to track the fluid motion through the channel, and appeared to have a greater impact on the apparent fluid velocity than sodium chloride. Of the PWM duty cycles tested, 75% had the highest apparent fluid velocity. The titanium that contacted the fluid anodized, indicating it is best suited for disposable applications

Micro Electromechanical Systems (MEMS) Based Microfluidic Devices for Biomedical Applications

Micro Electromechanical Systems (MEMS) based microfluidic devices have gained popularity in biomedicine field over the last few years. In this paper, a comprehensive overview of microfluidic devices such as micropumps and microneedles has been presented for biomedical applications. The aim of this paper is to present the major features and issues related to micropumps and microneedles, e.g., working principles, actuation methods, fabrication techniques, construction, performance parameters, failure analysis, testing, safety issues, applications, commercialization issues and future prospects. Based on the actuation mechanisms, the micropumps are classified into two main types, i.e., mechanical and non-mechanical micropumps. Microneedles can be categorized according to their structure, fabrication process, material, overall shape, tip shape, size, array density and application. The presented literature review on micropumps and microneedles will provide comprehensive information for researchers working on design and development of microfluidic devices for biomedical applications

Review: Electric field driven pumping in microfluidic device

Pumping of fluids with precise control is one of the key components in a microfluidic device. The electric field has been used as one of the most popular and efficient nonmechanical pumping mechanism to transport fluids in microchannels from the very early stage of microfluidic technology development. This review presents fundamental physics and theories of the different microscale phenomena that arise due to the application of an electric field in fluids, which can be applied for pumping of fluids in microdevices. Specific mechanisms considered in this report are electroosmosis, AC electroosmosis, AC electrothermal, induced charge electroosmosis, traveling wave dielectrophoresis, and liquid dielectrophoresis. Each phenomenon is discussed systematically with theoretical rigor and role of relevant key parameters are identified for pumping in microdevices. We specifically discussed the electric field driven body force term for each phenomenon using generalized Maxwell stress tensor as well as simplified effective dipole moment based method. Both experimental and theoretical works by several researchers are highlighted in this article for each electric field driven pumping mechanism. The detailed understanding of these phenomena and relevant key parameters are critical for better utilization, modulation, and selection of appropriate phenomenon for efficient pumping in a specific microfluidic application

Microfabricated All-Around-Electrode AC Electro-osmotic Micropump

This thesis presents the fabrication and characterisation of AC electro-osmotic micropumps with a simple design and velocity generation enhanced by about four times with respect to devices with simpler designs. Electro-osmosis is a convenient and effective method to pump liquids without the need for moving components. The implementation of valveless micropumps is important for the realisation of safe and robust biomedical devices, which require long-term reliability. AC electro-osmosis has the advantage, over other kinds of pumping strategies, of being implementable with relatively simple geometries and fabrication processes. Moreover, it uses low voltages and avoids undesired phenomena such as electrolysis, thus being suitable for the implementation in implantable devices that should operate in a closed environment. Whereas AC electro-osmotic pumps presented in the literature exploit planar electrode designs and fail to generate good values of velocity and pressure, the prototypes presented in this work have electrodes patterned all around the pumping channel and can generate much larger values. Moreover, with respect to other improved prototypes based on 3D electrode geometries, our devices are simpler to fabricate and give comparable enhancements of the performances. In this work, we present the development of the all-around-electrode devices and give a theoretical explanation for the measured improvements in velocity generation. The fabrication process is carried out in the cleanroom by depositing Ti/Pt electrodes on pre-structured Pyrex substrates and requires only three lithographic steps. The performances of the fabricated devices are characterised as a function of the applied voltage and frequency, and the dynamic behaviour of the prototypes is studied using the Fourier transform. In order to evaluate the suitability of the pumps for biomedical fluids, the dependence of velocity generation on the concentration of the pumped solution is also addressed. Finally, we show that the fabrication process can be adapted to an industrial batch manufacture requiring lower costs by substituting the Pyrex substrates with thin plastic foils. All-around-electrode micropumps can be successfully fabricated by patterning metal electrodes onto 12-µm-thick plastic foils and the costs might be further reduced by substituting the metal structures with inkjet-printed conductive-polymer electrodes

Fabrication and Flow Dynamics Analysis of Micromixer for Lab-on-a-Chip Devices

The miniaturized systems designed for lab-on-a-chip (LOC) technologies are generally implemented with a micro-scale mixer to provide intimate contact between the reagent molecules for interactions and chemical reactions. The exponential increase of research in microfabrication and microfluidic applications highlights the importance of understanding the theory and mechanism that governs mixing at the microscale level. In this study, the fabrication of an active and passive micromixer was discussed. The optimized state of art soft lithography and 3D printing was used as a microfabrication technique. The challenges at different fabrication steps were presented along with the modifications. Microelectrodes were integrated with the active microfluidic mixer to create an electrokinetic effect. The fluid flow field inside the micromixerwas characterized by the Micro Particulate Image Velocimetry (Micro-PIV) system. Besides, numerical simulations were performed on 2D and 3D micromixers. Finally, results obtained in experiment and numerical simulations were analyzed to get a better understanding of the micromixer design

Multi-Functional System for Biomedical Application Using AC Electrokinetics

Manipulation of fluids in a small volume is often a challenge in the field of Microfluidics. While many research groups have addressed this issue with robust methodologies, manipulating fluids remains a scope of study due to the ever-changing technology (Processing Tools) and increase in the demand for “Lab-On-a-Chip” devices in biological applications. This thesis peruses the flow pattern of the orthogonal electrode pattern and circular electrode providing, examples of the flow patterns and the process micromixing. Characteristics of a multifunctional system were demonstrated using orthogonal electrode and circular electrode patterned device. Conductivity of the fluids were chosen such they reflect perfect biological conditions to determine the working conditions of the proposed devices under different AC voltage and frequency levels. Experimental results were then compared with simulated results which were obtained using COMSOL simulation software

System Integration - A Major Step toward Lab on a Chip

Microfluidics holds great promise to revolutionize various areas of biological engineering, such as single cell analysis, environmental monitoring, regenerative medicine, and point-of-care diagnostics. Despite the fact that intensive efforts have been devoted into the field in the past decades, microfluidics has not yet been adopted widely. It is increasingly realized that an effective system integration strategy that is low cost and broadly applicable to various biological engineering situations is required to fully realize the potential of microfluidics. In this article, we review several promising system integration approaches for microfluidics and discuss their advantages, limitations, and applications. Future advancements of these microfluidic strategies will lead toward translational lab-on-a-chip systems for a wide spectrum of biological engineering applications

HIGH PERFORMANCE PIEZOELECTRIC MATERIALS AND DEVICES FOR MULTILAYER LOW TEMPERATURE CO-FIRED CERAMIC BASED MICROFLUIDIC SYSTEMS

The incorporation of active piezoelectric elements and fluidic components into micro-electromechanical systems (MEMS) is of great interest for the development of sensors, actuators, and integrated systems used in microfluidics. Low temperature cofired ceramics (LTCC), widely used as electronic packaging materials, offer the possibility of manufacturing highly integrated microfluidic systems with complex 3-D features and various co-firable functional materials in a multilayer module. It would be desirable to integrate high performance lead zirconate titanate (PZT) based ceramics into LTCC-based MEMS using modern thick film and 3-D packaging technologies. The challenges for fabricating functional LTCC/PZT devices are: 1) formulating piezoelectric compositions which have similar sintering conditions to LTCC materials; 2) reducing elemental inter-diffusion between the LTCC package and PZT materials in co-firing process; and 3) developing active piezoelectric layers with desirable electric properties. The goal of present work was to develop low temperature fired PZT-based materials and compatible processing methods which enable integration of piezoelectric elements with LTCC materials and production of high performance integrated multilayer devices for microfluidics. First, the low temperature sintering behavior of piezoelectric ceramics in the solid solution of Pb(Zr0.53,Ti0.47)O3-Sr(K0.25, Nb0.75)O3 (PZT-SKN) with sintering aids has been investigated. 1 wt% LiBiO2 + 1 wt% CuO fluxed PZT-SKN ceramics sintered at 900oC for 1 h exhibited desirable piezoelectric and dielectric properties with a reduction of sintering temperature by 350oC. Next, the fluxed PZT-SKN tapes were successfully laminated and co-fired with LTCC materials to build the hybrid multilayer structures. HL2000/PZT-SKN multilayer ceramics co-fired at 900oC for 0.5 h exhibited the optimal properties with high field d33 piezoelectric coefficient of 356 pm/V. A potential application of the developed LTCC/PZT-SKN multilayer ceramics as a microbalance was demonstrated. The final research focus was the fabrication of an HL2000/PZT-SKN multilayer piezoelectric micropump and the characterization of pumping performance. The measured maximum flow rate and backpressure were 450 μl/min and 1.4 kPa respectively. Use of different microchannel geometries has been studied to improve the pumping performance. It is believed that the high performance multilayer piezoelectric devices implemented in this work will enable the development of highly integrated LTCC-based microfluidic systems for many future applications