JOULE HEATING EFFECTS ON ELECTROKINETIC TRANSPORT IN CONSTRICTION MICROCHANNELS

Microfluidic technology involving multidisciplinary studies including MEMS, chemistry, physics, fluids and heat transfer has been developed into a promising research field in the recent decade. If offers many advantages over conventional laboratory techniques like reduced reagent consumption, faster analysis, easy fabrication and low chemical waste. Microfluidic lab-on-a-chip devices have been used to manipulate cells and particles like sorting, separating, trapping, mixing and lysing. Microfluidic manipulation can be achieved through many methods and insulator based dielectrophoresis (iDEP) is one of the highly used method in the recent years. In iDEP, both DC and AC voltages can be applied to the remote electrodes positioned in end-channel reservoirs for transporting and manipulating particles. The electric field gradients are caused by the blockage of electric current due to in-channel hurdles, posts, and ridges. However, iDEP devices suffer from the issue of Joule heating due to locally amplified electric field around the insulators. A parametric study of Joule heating effects on electroosmotic fluid flow in iDEP is studied under various electric fields. It was determined that depending upon the magnitude of DC voltage, a pair of counter rotating vortices fluid circulations can occur at either downstream end or each end of the channel constriction. Moreover, pair at the downstream end appears larger in size than the upstream end due to DC electroosmotic flow. A numerical model is developed to simulate the fluid circulations occurred due to the action of electric field on Joule heating induced fluid inhomogeneities in the constriction region. Focusing particles or cells into a single stream is usually a necessary step prior to counting and separating them in microfluidic devices such as flow cytometers and cell sorters. A systematic study of Joule heating effects on electrokinetic particle transport in constriction microchannels under DC and DC biased AC electric fields is presented in this work. A numerical model is developed to capture the particle trace observed in the experiments. It was determined that particle transport is greatly affected by electrothermal effects where Joule heating is high. At very low DC magnitude where the electrothermal effects dominate the electrokinetic flow, particles in the shallow depth channel are being trapped and particles in deep channels are transported to the downstream reservoir from the constriction in a single streamline. Electrothermal flow circulations should be taken into account in the design and operation of iDEP devices, especially when highly conductive solutions and large electric fields must be employed. They may potentially be harnessed to enhance microfluidic mixing and immunoassay for lab-on-a-chip applications. A numerical study of Joule heating effects on the sample mixing performance in constriction microchannels is presented in this work. It was determined that Joule heating induced electrothermal force enhanced the sample mixing by generating circulations at the ends of the constriction under DC biased AC electric fields. Furthermore, mixing performance was also studied for various parameters like applied electric field, channel structure, channel depth and number of constrictions

Microfluidics based DNA hybridization: mathematical modeling issues and future challenges

In this paper, various mathematical modeling strategies associated with the analysis of the kinetics and the transport processes pertinent to microfluidics-based DNA hybridization methodologies are critically reviewed. In particular, the coupling of specific/non- specific hybridization kinetics with the fluid flow, heat transfer and mass transfer equations is described in detail. Methodologies for obtaining faster DNA hybridization rates are also discussed and the corresponding mathematical modeling issues are identified to define the scope of ongoing and future research endeavours

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

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

BioMEMS

As technological advancements widen the scope of applications for biomicroelectromechanical systems (BioMEMS or biomicrosystems), the field continues to have an impact on many aspects of life science operations and functionalities. Because BioMEMS research and development require the input of experts who use different technical languages and come from varying disciplines and backgrounds, scientists and students can avoid potential difficulties in communication and understanding only if they possess a skill set and understanding that enables them to work at the interface of engineering and biosciences. Keeping this duality in mind throughout, BioMEMS: Science and Engineering Perspectives supports and expedites the multidisciplinary learning involved in the development of biomicrosystems. Divided into nine chapters, it starts with a balanced introduction of biological, engineering, application, and commercialization aspects of the field. With a focus on molecules of biological interest, the book explores the building blocks of cells and viruses, as well as molecules that form the self-assembled monolayers (SAMs), linkers, and hydrogels used for making different surfaces biocompatible through functionalization. The book also discusses: Different materials and platforms used to develop biomicrosystems Various biological entities and pathogens (in ascending order of complexity) The multidisciplinary aspects of engineering bioactive surfaces Engineering perspectives, including methods of manufacturing bioactive surfaces and devices Microfluidics modeling and experimentation Device level implementation of BioMEMS concepts for different applications. Because BioMEMS is an application-driven field, the book also highlights the concepts of lab-on-a-chip (LOC) and micro total analysis system (μTAS), along with their pertinence to the emerging point-of-care (POC) and point-of-need (PON) applications

BioMEMS

As technological advancements widen the scope of applications for biomicroelectromechanical systems (BioMEMS or biomicrosystems), the field continues to have an impact on many aspects of life science operations and functionalities. Because BioMEMS research and development require the input of experts who use different technical languages and come from varying disciplines and backgrounds, scientists and students can avoid potential difficulties in communication and understanding only if they possess a skill set and understanding that enables them to work at the interface of engineering and biosciences. Keeping this duality in mind throughout, BioMEMS: Science and Engineering Perspectives supports and expedites the multidisciplinary learning involved in the development of biomicrosystems. Divided into nine chapters, it starts with a balanced introduction of biological, engineering, application, and commercialization aspects of the field. With a focus on molecules of biological interest, the book explores the building blocks of cells and viruses, as well as molecules that form the self-assembled monolayers (SAMs), linkers, and hydrogels used for making different surfaces biocompatible through functionalization. The book also discusses: Different materials and platforms used to develop biomicrosystems Various biological entities and pathogens (in ascending order of complexity) The multidisciplinary aspects of engineering bioactive surfaces Engineering perspectives, including methods of manufacturing bioactive surfaces and devices Microfluidics modeling and experimentation Device level implementation of BioMEMS concepts for different applications. Because BioMEMS is an application-driven field, the book also highlights the concepts of lab-on-a-chip (LOC) and micro total analysis system (μTAS), along with their pertinence to the emerging point-of-care (POC) and point-of-need (PON) applications

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

Test analysis & fault simulation of microfluidic systems

This work presents a design, simulation and test methodology for microfluidic systems, with particular focus on simulation for test. A Microfluidic Fault Simulator (MFS) has been created based around COMSOL which allows a fault-free system model to undergo fault injection and provide test measurements. A post MFS test analysis procedure is also described.A range of fault-free system simulations have been cross-validated to experimental work to gauge the accuracy of the fundamental simulation approach prior to further investigation and development of the simulation and test procedure.A generic mechanism, termed a fault block, has been developed to provide fault injection and a method of describing a low abstraction behavioural fault model within the system. This technique has allowed the creation of a fault library containing a range of different microfluidic fault conditions. Each of the fault models has been cross-validated to experimental conditions or published results to determine their accuracy.Two test methods, namely, impedance spectroscopy and Levich electro-chemical sensors have been investigated as general methods of microfluidic test, each of which has been shown to be sensitive to a multitude of fault. Each method has successfully been implemented within the simulation environment and each cross-validated by first-hand experimentation or published work.A test analysis procedure based around the Neyman-Pearson criterion has been developed to allow a probabilistic metric for each test applied for a given fault condition, providing a quantitive assessment of each test. These metrics are used to analyse the sensitivity of each test method, useful when determining which tests to employ in the final system. Furthermore, these probabilistic metrics may be combined to provide a fault coverage metric for the complete system.The complete MFS method has been applied to two system cases studies; a hydrodynamic “Y” channel and a flow cytometry system for prognosing head and neck cancer.Decision trees are trained based on the test measurement data and fault conditions as a means of classifying the systems fault condition state. The classification rules created by the decision trees may be displayed graphically or as a set of rules which can be loaded into test instrumentation. During the course of this research a high voltage power supply instrument has been developed to aid electro-osmotic experimentation and an impedance spectrometer to provide embedded test

Microelectromechanical Systems and Devices

The advances of microelectromechanical systems (MEMS) and devices have been instrumental in the demonstration of new devices and applications, and even in the creation of new fields of research and development: bioMEMS, actuators, microfluidic devices, RF and optical MEMS. Experience indicates a need for MEMS book covering these materials as well as the most important process steps in bulk micro-machining and modeling. We are very pleased to present this book that contains 18 chapters, written by the experts in the field of MEMS. These chapters are groups into four broad sections of BioMEMS Devices, MEMS characterization and micromachining, RF and Optical MEMS, and MEMS based Actuators. The book starts with the emerging field of bioMEMS, including MEMS coil for retinal prostheses, DNA extraction by micro/bio-fluidics devices and acoustic biosensors. MEMS characterization, micromachining, macromodels, RF and Optical MEMS switches are discussed in next sections. The book concludes with the emphasis on MEMS based actuators

Micro/Nano-Chip Electrokinetics

Micro/nanofluidic chips have found increasing applications in the analysis of chemical and biological samples over the past two decades. Electrokinetics has become the method of choice in these micro/nano-chips for transporting, manipulating and sensing ions, (bio)molecules, fluids and (bio)particles, etc., due to the high maneuverability, scalability, sensitivity, and integrability. The involved phenomena, which cover electroosmosis, electrophoresis, dielectrophoresis, electrohydrodynamics, electrothermal flow, diffusioosmosis, diffusiophoresis, streaming potential, current, etc., arise from either the inherent or the induced surface charge on the solid-liquid interface under DC and/or AC electric fields. To review the state-of-the-art of micro/nanochip electrokinetics, we welcome, in this Special Issue of Micromachines, all original research or review articles on the fundamentals and applications of the variety of electrokinetic phenomena in both microfluidic and nanofluidic devices