Jae Wan Kwon

A micromachined energy harvester from a keyboard using combined electromagnetic and piezoelectric conversion

This paper describes a novel micromachined energy harvester, which can harness energy from typing motions on a computer keyboard. It converts mechanical energy into electrical energy by utilizing piezoelectric and electromagnetic conversion simultaneously for reciprocating z-directional motions of each key. We have fabricated and tested a micro power harvester chip integrated into a computer keyboard. Experimental results show that the maximum harvested power is about 40.8 µW with a 3 MΩ load from piezoelectric conversion, and 1.15 µW with a 35 Ω load from electromagnetic conversion. The micro power harvesters can be extended into the form of an array.

Microfluidic mixer and transporter based on PZT self-focusing acoustic transducers

This paper describes noninvasive acoustic micromixer and transporter as well as a microfluidic subsystem (on a single silicon chip) based on the two microfluidic actuators. The actuators use a piezoelectric PZT sheet to generate about 10-12.5MHz acoustic waves (corresponding to the PZT thickness), an array of sectored Fresnel annular rings [for the electrodes sandwiching the piezoelectric transducer (PZT)] to self-focus the acoustic waves, and acoustic streaming effect to produce a body force for fluidic motion. The self-focused waves from the PZT array propagate through 200-mum-thick silicon without any significant energy loss, and move the liquid contained in the channels and chambers (without a cover) on a micromachined silicon chip, up to 123 mm/s in-plane speed. The PZT array (consisting of a PZT sheet with an array of electrode patterns for sectored Fresnel rings) is bonded to the micromachined silicon chip to produce a fast in-plane liquid motion in the open channels and chambers. When the liquid motion is produced in a channel, we have a liquid transporter; while when the motion is produced in a large area chamber, we have a micromixer. Various channel and chamber structures have been fabricated to verify the efficacies of the liquid transport and mixing. A microfluidic subsystem integrating a micromixer, transporters and reservoirs has been shown to transport two types of liquids from their reservoirs to a mixer and to mix the liquids together within 2 s. The experimental results agree well with theoretical studies and simulation results

Multi-level microfluidic channel routing with protected convex corners

This paper describes a novel design of microfluidic channel routing technology with well-protected convex corners. For a generic microfluidic channel routing, we have invented and demonstrated two innovative ideas: (1) a complete convex corner protection with which an indefinite over-etching can be performed without damaging the convex corners and (2) channel crossings of two micro-channels (carrying different liquids at different heights) that do not interfere with each other at the cross point. These micromachined 3D structures are made through photolithography on the surfaces of anisotropically etched grooves of a silicon wafer.

Fine ZnO patterning with controlled sidewall-etch front slope

This paper describes a wet-etching technique that solves the major difficulty of fine patterning a c-axis oriented polycrystalline ZnO film. The technique uses aqueous NH/sub 4/Cl with electrolytically added copper ions and convection flow, and for the first time, allows the ZnO film to be etched 1) with controlled etch rate ratio between the vertical and horizontal etch rates and 2) with controlled etch-front slope. The ratio between the vertical and horizontal etch rates is as high as 20 to 1, while the angle between the sidewall etch-front surface and the substrate surface can be electrically controlled between 73/spl deg/ and 106/spl deg/. Also, ZnO films can now be patterned to fine features (even sub-/spl mu/m level) with a wet etchant. The electroless galvanic etching technique described in this paper produces uniform etching over a large area (larger than 3 in diameter).

Virtual Walls Based on Oil-Repellent Surfaces for Low-Surface-Tension Liquids

Manipulating and controlling water-based aqueous solutions with the use of virtual walls is relatively simple compared to that of nonaqueous low-surface-tension liquids, which pose greater challenges to microfluidic devices. This letter reports a novel technique to form a virtual wall for various low-surface-tension liquids. A microfluidic channel with virtual walls has been made to guide low-surface-tension liquids by using a specially designed oil-repellent surface. Unlike generic superoleophobic surfaces, our oil-repellent surface exhibited strong repellency to the lateral flow of low-surface-tension liquids such as hexadecane and dodecane. A plasma-assisted surface micromachining process has been utilized to form the oil-repellent surface. The use of combined features of re-entrant geometries on the surface played an important role in promoting its repellence to the lateral flow of low-surface-tension liquids. We have successfully demonstrated how low-surface-tension liquids can be well confined by the virtual walls.

Directional droplet ejection by nozzleless acoustic ejectors built on ZnO and PZT

This paper describes a technique to eject liquid droplets in almost any direction with a nozzleless self-focusing acoustic transducer (SFAT) built on a ZnO thin film as well as on a thick PZT substrate. Sectoring of the SFAT annular rings of half-wave-band sources to create a piezoelectrically inactive area causes the droplet ejections to be directed non-perpendicular (i.e., oblique) to the liquid surface. The direction of the droplet ejections depends on the size of the piezoelectrically inactive area within the area of the half-wave-band sources. Droplets are ejected from the center part of the annular rings toward the open inactive area. Various openings up to 90° of pie shape have been made and tested to show that the ejection direction becomes less vertical as the piezoelectrically inactive area in the transducer increases. Additionally, a multi-directional ejector built on ZnO film has been demonstrated to eject micron-sized liquid droplets (several microns in diameter) in any of eight predetermined directions on demand. Larger size liquid droplets (about a hundred microns in diameter) have also been directionally ejected from a sectored SFAT built on a PZT substrate.

In situ DNA synthesis on glass substrate for microarray fabrication using self-focusing acoustic transducer

This paper presents a droplet-ejection-based technique for synthesizing deoxyribonucleic acid (DNA) sequences on different substrates, such as glass, plastic, or silicon. Any DNA sequence can be synthesized by ejecting droplets of DNA bases by a self-focusing acoustic transducer (SFAT) that does not require any nozzles. An SFAT can eject liquid droplets around 3-5 /spl mu/m in diameter, which is significantly smaller than those ejected by commercial ink jet printers and reduces the amount of reagents needed for the synthesis. An array of SFATs is integrated with microchannels and reservoirs for delivery of DNA bases to the SFATs. Poly-l-lysine-coated glass slide is patterned, and is used as a target substrate for in situ synthesis of multiple T bases. The significant advantage of this scheme over some of the existing commercial solutions is that it can allow geneticists to synthesize any DNA sequence within hours using a computer program at an affordable cost in their own labs. This paper describes the concept and scheme of the on-demand DNA synthesis (with an acoustic ejector integrated with microfluidic components) along with the results of an actual DNA synthesis by an SFAT. Note to Practitioners-Deoxyribonucleic acid (DNA) microarrays allow geneticists to monitor the interactions among thousands of genes simultaneously in a chip. There are commercial systems for producing DNA microarrays, but none of them give flexibility to synthesize DNA microarrays on-demand in the geneticists own lab. Affymetrixs GeneChip technology produces DNA probe sequences premade at Affymetrix with a set of 4n photomasks for n-mers. Other techniques transfer premade DNA sequences to a substrate (glass, plastic, or silicon) through ink-jet printing or contact dispensing. Agilent and Rosetta use their ink-jet printing technology to produce DNA probe sequences at their factories. The ink-jet print heads used for printing microarrays use either piezoelectric or thermal actuation, and eject liquid droplets through nozzles. Thus, the smallest droplet size ejected from these devices depends on the size of the nozzle. The small nozzles are difficult to construct with good uniformity and tend to get clogged. The idea presented in this paper is to develop a microelectromechanical-system (MEMS)-based portable system for synthesizing DNA on different substrates, using nozzleless, heatless, lensless, acoustic droplet ejectors. The future research is to synthesize longer DNA sequences with a combination of different bases, using directional droplet ejectors.

Film transfer and bonding techniques for covering single-chip ejector array with microchannels and reservoirs

This paper describes a novel covering technique for an MEMS ejector array that is integrated with liquid reservoirs and microchannels on a single chip. The covering technique is based on wicking of a low viscous epoxy through the gap between the ejector wafer and a plate containing a parylene film, and allows the integrated ejector array to be fully covered by the parylene film with excellent uniformity, repeatability and yield. The technique is batch-processible and is suitable to cover many microfluidic systems with a thin film. The parylene film is tightly attached to the ejector array chip (with excellent bonding strength owing to the epoxy), so that liquid is automatically brought into the ejectors from the reservoirs through the microchannels (due to capillary force), as the ejectors shoot out liquid droplets. This automatic liquid supply makes the liquid level (in the ejector) be maintained constant throughout the entire ejection process until more than 90% of the liquid stored in the reservoir is delivered to the ejector through the microchannel. This paper describes also a number of other covering methods that we have experimentally tried, and compares those with the new covering technique. [1459].

On-Demand DNA Synthesis on Solid Surface by Four Directional Ejectors on a Chip

This paper presents a synthesis technique for any random deoxyribonucleic acid (DNA) sequences on different substrates such as glass, plastic or silicon by an array of directional droplet ejectors. Any DNA sequence can be synthesized by ejecting droplets of DNA bases by an ultrasonic transducer having lens with air-reflectors (LWARs) that requires no nozzle. The LWAR is capable of ejecting liquid droplets around 80 mum in diameter, and reduces the amount of reagents needed for the synthesis from most of conventional microarray techniques. One major advantage of the nozzleless ejector is that it can eject droplets in any direction, so that a spot can be inked by four ejectors (carrying four DNA bases) without moving the ejector. The directional ejection of the droplets removes the need for aligning the substrate with the ejector, and minimizes the automation and control circuitry. To demonstrate the DNA synthesis capability of the directional droplet ejectors, four LWAR ejectors were used to synthesize a 15-mer 5-CGCCAAGCAGTTCGT-3 on a substrate surface. This paper describes the concept and scheme of the on-demand DNA synthesis (with MEMS ejector integrated with microfluidic components) along with experimental results of an actual DNA synthesis by four directional droplet ejectors.

Film transfer and bonding technique to cover lab on a chip

This paper describes a novel film transfer technique with a new bonding technique to cover microfluidic components, (on a silicon chip) with a 6 /spl mu/m thick parylene layer. We have developed a batch-process technique for covering microfluidic systems such as a lab on a chip with a thin film, and applied it to packaging an integrated microfluidic system containing a liquid-droplet ejector array, microchannels and reservoirs on a chip. The well-attached cover without any leaks makes it possible for capillary force to bring liquid into the ejectors from the reservoirs through microchannels, so that the ejectors are supplied with the liquid(s) automatically from the reservoirs. The new bonding technique reported in this paper accomplishes a bonding at room temperature by using capillary flow and surface tension of low viscous aqueous bonding materials without any added pressure, and can also bond many different materials at a very low processing cost.