Chong H. Ahn

A review of microvalves

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.

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

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.

An integrated microfluidic biochemical detection system for protein analysis with magnetic bead-based sampling capabilities

This paper presents the development and characterization of an integrated microfluidic biochemical detection system for fast and low-volume immunoassays using magnetic beads, which are used as both immobilization surfaces and bio-molecule carriers. Microfluidic components have been developed and integrated to construct a microfluidic biochemical detection system. Magnetic bead-based immunoassay, as a typical example of biochemical detection and analysis, has been successfully performed on the integrated microfluidic biochemical analysis system that includes a surface-mounted biofilter and electrochemical sensor on a glass microfluidic motherboard. Total time required for an immunoassay was less than 20 min including sample incubation time, and sample volume wasted was less than 50 microl during five repeated assays. Fast and low-volume biochemical analysis has been successfully achieved with the developed biofilter and immunosensor, which is integrated to the microfluidic system. Such a magnetic bead-based biochemical detection system, described in this paper, can be applied to protein analysis systems.

A novel in-plane passive microfluidic mixer with modified Tesla structures

An innovative in-plane passive micromixer using modified Tesla structures, which are used as passive valves, has been designed, simulated, fabricated and successfully characterized in this paper. Simulation and experimental results of the developed novel micromixer have shown excellent mixing performance over a wide range of flow conditions in the micro scale. The micromixer realized in this work has achieved even better mixing performance at a higher flow rate, and its pressure drop is less than 10 KPa at the flow rate of 100 microl min(-1). This micromixer shows characteristics similar to Taylor dispersion, with contributions from both diffusion and convection. The mixer has a diffusion domain region at low flow rate, but it moves to a convection domain region at high flow rate. Due to the simple in-plane structure of the novel micromixer explored in this work, the mixer can be easily realized and integrated with on-chip microfluidic devices and micro total analysis systems (micro-TAS).

A new magnetic bead-based, filterless bio-separator with planar electromagnet surfaces for integrated bio-detection systems

Abstract A new filterless bio-separator separating magnetic microbeads from a carrier fluid has been designed, fabricated, and characterized as a core component of biological cell sampling and detecting systems. To maximize the sampling capability, a planar electromagnet surface with a serpentine coil and semi-encapsulated permalloy has been realized. Using this bio-separator, antibody-coated magnetic beads have been successfully separated from the bio-buffer suspension solution and characterized in dynamic fluid flow. The magnetic characteristics of the bio-separator have been simulated and experimentally studied to establish and validate the design rules for fabrication. The realized filterless bio-separator has a high potential in biomedical and biological detection systems. It is especially useful in selective sampling and separation of small amounts of bio-molecules (e.g., antigen) for on-chip micro total analysis systems (μ-TAS) or remote detection systems.

Micromachined planar inductors on silicon wafers for MEMS applications

This paper describes three micromachined planar inductors (a spiral type, a solenoid type, and a toroidal meander type) with electroplated nickel-iron permalloy cores which have been realized on a silicon wafer using micromachining techniques. The electrical properties among the fabricated inductors are compared and the related fabrication issues are discussed, with emphasis on the low-temperature CMOS-compatible process, the high current-carrying capacity, the high magnetic flux density, the closed magnetic circuits, and the low product cost. The micromachined on-chip inductors can be applied for magnetic microelectromechanical systems devices, such as micromotors, microactuators, microsensors, and integrated power converters, which envisages new micropower magnetics on a chip with integrated circuits.

A tapered hollow metallic microneedle array using backside exposure of SU-8

This paper presents a novel fabrication process for a tapered hollow metallic microneedle array using backside exposure of SU-8, and analytic solutions of critical buckling of a tapered hollow microneedle. An SU-8 mesa was formed on a Pyrex glass substrate and another SU-8 layer, which was spun on top of the SU-8 mesa, was exposed through the backside of the glass substrate. An array of SU-8 tapered pillar structures, with angles in the range of 3.1 ◦ –5 ◦ , was formed on top of the SU-8 mesa. Conformal electrodeposition of metal was carried out followed by a mechanical polishing using a planarizing polymeric layer. All organic layers were then removed to create a metallic hollow microneedle array with a fluidic reservoir on the backside. Both 200 µm and 400 µm tall, 10 by 10 arrays of metallic microneedles with inner diameters of the tip in the range of 33.6–101 µm and wall thickness of 10–20 µm were fabricated. Analytic solutions of the critical buckling of arbitrary-angled truncated cone-shaped columns are also presented. It was found that a single 400 µm tall hollow cylindrical microneedle made of electroplated nickel with a wall thickness of 20 µm, a tapered angle of 3.08 ◦ and a tip inner diameter of 33.6 µ mh as a critical buckling force of 1.8 N. This analytic solution can be used for square or rectangular cross-sectioned column structures with proper modifications.

An on-chip magnetic bead separator using spiral electromagnets with semi-encapsulated permalloy

A new planar bio-magnetic bead separator on a glass chip has been designed, fabricated and tested. The separator is composed of micromachined semi-encapsulated spiral electromagnets and fluid channels, which have been separately fabricated and then bonded. The device was tested with super-paramagnetic beads of mean diameter 1 microm which were suspended in a buffered solution. When a DC current of 300 mA was applied to the inductor, the bio-magnetic beads were successfully separated on the electromagnets, showing a functional capability as a magnetic bead separator. To evaluate separation rate and capability, the inductance measurement method has been introduced and the inductance variation according to the separation rate has been characterized. Using this separator, cells or cell fragments and magnetic beads bonded with protein or enzyme suspended in bio-buffer solutions can be successfully separated from their suspensions, envisaging a filterless bio-separator.

A fully integrated micromachined magnetic particle separator

A prototype micromachined magnetic particle separator that can separate magnetic particles from suspended liquid solutions has been realized on a silicon wafer. The requisite magnetic field gradients are generated by integrated inductive components in place of permanent magnets, which yields several advantages in design flexibility, compactness, electrical and optical monitoring, and integration feasibility (thus enabling mass production). Preliminary experiments have been performed on aqueous suspensions of magnetic beads. At 500 mA of dc current, approximately 0.03 Tesla of magnetic flux density is achieved at the gap between the quadrupoles, and the magnetic particles rapidly move toward the quadrupoles, separate from the buffer solution, and clump on the poles. The magnetic particles clumped on the poles are also easily released when the dc current is removed, achieving the primary purpose of a separator. The device shows that micromachined magnetic components have a high potential in biological or biomedical applications, especially in separating small amounts of cells or DNA that are marked with magnetic beads, especially when close monitoring and control of the process is important.

A planar variable reluctance magnetic micromotor with fully integrated stator and coils

A fully functional electrically excited planar variable reluctance magnetic micromotor has been demonstrated on a silicon wafer. The motor uses a micromachined nickel-iron rotor and a fully integrated stator, in which a toroidal-meander type integrated inductive component is used for flux generations. To reduce the magnetic reluctance in the stator, a modified stator geometry was adopted which removes the yoke used in a conventional magnetic variable reluctance motor. Using polyimide as both an integral structural material as well as an electroplating mold, a 40- mu m-thick nickel-iron rotor 500 mu m in diameter was microassembled onto a fully integrated nickel-iron stator 120 mu m in thickness. When 500 mA of current was applied to each stator, 12 degrees of rotation (1 stroke in this motor) was observed. By applying three phase 200-mA current pulses to the stators, rotation of the rotor was observed. The speed and direction of the rotation could be adjusted by changing the frequency and phase firing order of the power supply, respectively. Continuous rotor rotation was observed at speeds up to 500 rpm; this speed limitation was solely due to the limitation of the maximum frequency of the controller used. The micromotor could be reproducibly started, stopped, reversed, and continuously rotated. The predicted torque for the fabricated micromotor at 500-mA driving current was calculated to be 3.3 nN-m. >