Magnetic forces are now being utilised in an amazing variety of microfluidic applications. Magnetohydrodynamic flow has been applied to the pumping of fluids through microchannels. Magnetic materials such as ferrofluids or magnetically doped PDMS have been used as valves. Magnetic microparticles have been employed for mixing of fluid streams. Magnetic particles have also been used as solid supports for bioreactions in microchannels. Trapping and transport of single cells are being investigated and recently, advances have been made towards the detection of magnetic material on-chip. The aim of this review is to introduce and discuss the various developments within the field of magnetism and microfluidics.

Magnetism and microfluidics

This review gives a brief overview of microvalves, and focuses on the actuation mechanisms and their applications. One of the stumbling blocks for successful miniaturization and commercialization of fully integrated microfluidic systems was the development of reliable microvalves. Applications of the microvalves include flow regulation, on/off switching and sealing of liquids, gases or vacuums. Microvalves have been developed in the form of active or passive microvalves employing mechanical, non-mechanical and external systems. Even though great progress has been made during the last 20 years, there is plenty of room for further improving the performance of existing microvalves.

http://iopscience.iop.org/article/10.1088/0960-1317/16/5/R01/pdf

A review of microvalves

A method of fabricating an elastomeric structure, comprising: forming a first elastomeric layer on top of a first micromachined mold, the first micromachined mold having a first raised protrusion which forms a first recess extending along a bottom surface of the first elastomeric layer; forming a second elastomeric layer on top of a second micromachined mold, the second micromachined mold having a second raised protrusion which forms a second recess extending along a bottom surface of the second elastomeric layer; bonding the bottom surface of the second elastomeric layer onto a top surface of the first elastomeric layer such that a control channel forms in the second recess between the first and second elastomeric layers; and positioning the first elastomeric layer on top of a planar substrate such that a flow channel forms in the first recess between the first elastomeric layer and the planar substrate.

Microfabricated elastomeric valve and pump systems

INTRODUCTION MEMS and NEMS A Capsule History of MEMS and NEMS Dimensional Analysis and Scaling Exercises A REFRESHER ON CONTINUUM MECHANICS Introduction The Continuum Hypothesis Heat Conduction Elasticity Linear Thermoelasticity Fluid Dynamics Electromagnetism Numerical Methods for Continuum Mechanics SMALL IS DIFFERENT The Backyard Scaling Systems Exercises THERMALLY DRIVEN SYSTEMS Introduction Thermally Driven Devices From PDE to ODE: Lumped Models Joule Heating of a Cylinder Analysis of Thermal Data Storage Exercises MODELING ELASTIC STRUCTURES Introduction Examples of Elastic Structures in MEMS/NEMS The Mass on a Spring Membranes Beams Plates The Capacitive Pressure Sensor Exercises MODELING COUPLED THERMAL-ELASTIC SYSTEMS Introduction Devices and Phenomena in Thermal-Elastic Systems Modeling Thermopneumatic Systems The Thermoelastic Rod Revisited Modeling Thermoelastic V-Beam Actuators Modeling Thermal Bimorph Actuators Modeling Bimetallic Thermal Actuators Exercises MODELING ELECTROSTATIC-ELASTIC SYSTEMS Introduction Devices Using Electrostatic Actuation The Mass-Spring Model Modeling General Electrostatic-Elastic Systems Electrostatic-Elastic Systems - Membrane Theory Electrostatic-Elastic Systems - Beam and Plate Theory Analysis of Capacitive Control Schemes Exercises MODELING MAGNETICALLY ACTUATED SYSTEMS Introduction Magnetically Driven Devices Mass-Spring Models A Simple Membrane Micropump Model A Small-Aspect Ratio Model Exercises MICROFLUIDICS Introduction Microfluidic Devices More Fluidic Scaling Modeling Squeeze Film Damping Exercises BEYOND CONTINUUM THEORY Introduction Limits of Contiuum Mechanics Devices and Systems Beyond Continuum Theory Exercises REFERENCES APPENDICES Mathematical Results Physical Constants INDEX Each chapter also contains Related Reading and Notes sections.

Modeling MEMS and NEMS

This paper presents the development of a disposable plastic biochip incorporating smart passive microfluidics with embedded on-chip power sources and integrated biosensor array for applications in clinical diagnostics and point-of-care testing. The fully integrated disposable biochip is capable of precise volume control with smart microfluidic manipulation without costly on-chip microfluidic components. The biochip has a unique power source using on-chip pressurized air reservoirs, for microfluidic manipulation, avoiding the need for complex microfluidic pumps. In addition, the disposable plastic biochip has successfully been tested for the measurements of partial oxygen concentration, glucose, and lactate level in human blood using an integrated biosensor array. This paper presents details of the smart passive microfluidic system, the on-chip power source, and the biosensor array together with a detailed discussion of the plastic micromachining techniques used for chip fabrication. A handheld analyzer capable of multiparameter detection of clinically relevant parameters has also been developed to detect the signals from the cartridge type disposable biochip. The handheld analyzer developed in this work is currently the smallest analyzer capable of multiparameter detection for point-of-care testing.

/pdf/disposable-smart-lab-on-a-chip-for-point-of-care-clinical-46p07tt8gu.pdf

Disposable smart lab on a chip for point-of-care clinical diagnostics

A microactuated device and method of making the same in which an electromagnetic driver, overlapping a magnetically permeable diaphragm, is utilized to drive the microactuated device. Through the use of an electromagnetic driver to provide the motive force for a microactuated device, exceptional performance may be realized, e.g., with a substantially reduced drive voltage for micropumps, microvalves, and the like, and with stronger and more precise sensory outputs for microactuated sensors and the like. Moreover, by overlapping the electromagnetic driver over a diaphragm, a number of batch processing techniques, each of which is well suited for mass production, may be used in the fabrication of extremely compact and cost effective integrated devices.

Electromagnetically driven microactuated device and method of making the same

In this paper, we present a new integrated eddy current sensor for proximity sensing and for the detection of microcracks on the surface of metals. The device consists of two stacked planar coils fabricated onto a glass substrate and encapsulated on one side by a Ni/Fe permalloy magnetic core. Fabrication of the device is achieved by a UV–LIGA thick photoresist lithography process, which involves the lithographic patterning of 15–25 μm thick molds using AZ-4000 series photoresist. The introduction of the permalloy core coupled with the thick conductor lines produces a high inductance, low resistance device capable of generating large magnetic fields at low driving currents. The device has been tested in the frequency range of 10–500 kHz and has been shown to work as both a proximity sensor and crack detector at input powers of 30 mW or less. When used as a proximity sensor, the unamplified output voltage on the sensing coil changes by as much as 75 mV with an aluminum target placed at a distance of 400 μm from the coil. The device has also shown the capability of clearly detecting cracks with depths of as little as 8 mil (200 μm) in both aluminum and titanium. Results show an extremely linear relation between crack depth and output signal voltage with an unamplified signal strength of several millivolts.

On-chip eddy current sensor for proximity sensing and crack detection

In this paper, we describe the design, fabrication, and testing of a prototype microvalvewhich makes use of a novel magnetic microactuator. The completed device consists of three layers, with the bottom two layers making up the normally closed valve. The top layer (actuator) contains the flux generator on its top surface combined with Ni/Fe plated through holes for guiding the flux to the valve. The actuator and valve components are separately fabricated and then mounted onto a glass motherboard which contains both fluid flow channels and patterned gold traces for making electrical connections. In addition to providing an easy means for testing, the motherboard will allow for the later attachment of other microfluidic components to create a complete microfluidic total analysis system ( -TAS) on a single substrate. Preliminary test results show that the valve is capable of controlling gas and liquid flow in the range of submicroliters to hundreds of μ L/min.

/pdf/a-new-magnetically-actuated-microvalve-for-liquid-and-gas-4e150vmkun.pdf

A New Magnetically Actuated Microvalve for Liquid and Gas Control Applications

This paper describes a new universal electromagnetic microactuator that makes use of novel magnetic interconnection concepts. In order to realize the universal actuator, planar microinductors are fabricated on a substrate which already contains anisotropically etched Ni/Fe permalloy-electroplated magnetic vias or through-holes. The inductor, which acts as a flux generator, is physically located on one side of the wafer, but is magnetically connected to the opposite side of the wafer where actuation occurs. This approach to actuator design provides for maximum flexibility in the range of applications. In addition, it allows the actuator to be readily connected to driving circuitry without interfering with the actuating device. Multi-layer 3D inductive components are fabricated using a LIGA-like thick photoresist lithography process. The fabricated coils consist of a horseshoe-shaped permalloy-electroplated magnetic core and electroplated copper conductor lines that form the windings around the core. Initial testing using a prototype cantilever beam structure has proven functionality and indicates that the new device has much potential as a low power magnetic microactuator. Many magnetic MEMS applications require an electromagnetic actuator with high efficiency, and some areas which this device is expected to impact include microfluidics, micromotors, optics, and resonating devices.

A universal electromagnetic microactuator using magnetic interconnection concepts

Integrated planar inductors are fundamentally important to the development of fully integrated magnetic MEMS devices as well as for RF and microwave applications. For many applications, it is important that inductors be CMOS compatible, as this allows for the possible addition of on-chip circuitry. Spiral inductors are typically the simplest inductors to fabricate and are often suited for use as smart sensors and electronic components. In this paper we have designed, fabricated, characterized and compared several different types of spiral inductors, all of which were fabricated by means of a thick photoresist lithography process referred to as UV-LIGA.

Daniel J. Sadler

Papers

Electromagnetically driven microactuated device and method of making the same

On-chip eddy current sensor for proximity sensing and crack detection

A New Magnetically Actuated Microvalve for Liquid and Gas Control Applications

A universal electromagnetic microactuator using magnetic interconnection concepts

Micromachined spiral inductors using UV-LIGA techniques