Eun Sok Kim

University of California, Los Angeles, Los Angeles, California, United States

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Publications (127)121.07 Total impact

  • Qian Zhang, Yufeng Wang, Eun Sok Kim
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    ABSTRACT: This article presents a hand-held electromagnetic energy harvester which can be used to harvest tens of mW power level from human body motion. A magnet array, aligned to a coil array for maximum magnetic flux change, is suspended by a magnetic spring for a resonant frequency of several Hz and is stabilized horizontally by graphite sheets for reducing the friction. An analytical model of vibration-driven energy harvester with magnetic spring through magnet and coil arrays is developed to explore the power generation from vibrations at low frequency and large amplitude. When the energy harvester (occupying 120 cc and weighing 180 g) is placed in a backpack of a human walking at various speeds, the power output increases as the walking speed increases from 0.45 m/s (slow walking) to 3.58 m/s (slow running), and reaches 32 mW at 3.58 m/s.
    Journal of Applied Physics 01/2014; 115(6). · 2.19 Impact Factor
  • Youngki Choe, Shih-Jui Chen, Eun Sok Kim
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    ABSTRACT: This paper describes the synthesis of a 9-mers-long peptide ladder structure of glycine on a modified glass surface using a nano-liter droplet ejector. To synthesize peptide on a glass substrate, SPOTTM peptide synthesis protocol was followed with a nozzleless acoustic droplet ejector being used to eject about 300 droplets of pre-activated amino acid solution to dispense 60 nL of the solution per mer. The coupling efficiency of each mer was measured with FITC fluorescent tag to be 96 %, resulting in net 70 % efficiency for the whole 9-mer-long peptide of glycine. Usage of a nano-liter droplet ejector for SPOTTM peptide synthesis increases the density of protein array on a chip.
    IEEE transactions on bio-medical engineering 10/2013; · 2.15 Impact Factor
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    ABSTRACT: Biophysical effects of ultrasonic energy in tissue include changes induced by heat, cavitation, and body force (radiation energy). Conventional acoustic devices generate low-frequency (1-4 MHz) high-intensity acoustic waves (> 103 W/cm2), which cause tissue destruction primarily through thermal or cavitation effects. However, these effects may be difficult to precisely control and not specific for cancerous cells over normal tissue. Here, we describe the design, fabrication, and therapeutic potential of high-frequency (18-MHz) acoustic irradiation with a self-focusing acoustic transducer (SFAT). A surface micromachining technique was used on a piezoelectric substrate to produce a SFAT device capable of focusing acoustic energy within an area of 100 μm in diameter at the 800-μm focal length. As we sought to minimize potentially nonspecific heat or cavitation effects by acoustic irradiation, operational parameters were chosen to study bioeffects of the device in the absence of tissue heating or biological effects due to cavitation. By varying the acoustic energy, we identified an acoustic intensity threshold (AIT) of 0.15 W/cm2 at 17.3 MHz, sufficient to cause this cytolysis effect in human prostate cancer cells 22RV1 without heat or cavitation. Next, we compared the AIT in various cell lines representative of benign and malignant prostate, breast, and skin cells and observed lower AITs in cancer cells over nonmalignant variants. As decreased stiffness (increased compliance) is a biomechanical characteristic, which differs between malignant and nonmalignant cell lines, we hypothesized that a less organized actin cytoskeletal pattern, which is known to be associated with decreased cell stiffness, would correlate with changes in the AIT. Actin staining of cytoskeletal structures confirmed an association between a pattern of diffuse and less organized actin filaments with decreased AIT. Moreover, the same trend of dec- eased actin organization and decreased AIT was observed following treatments that changed actin patterns in the MCF-10A breast epithelial cell line. These results suggest that biomechanical properties make malignant cells specifically sensitive to cytolysis caused by this form of acoustic energy. In summary, we describe a miniaturized acoustic transducer capable of producing a heatless and cavitation-free, cancer-specific focused cytolysis by direct body force (radiation pressure) effects alone. Ultimately, this device may lead to a miniaturized cancer-treatment system that can be used to focally and specifically ablate cancerous tissue with microscopic precision.
    Journal of Microelectromechanical Systems 06/2013; 22(3):542-552. · 1.92 Impact Factor
  • Youngki Choe, Eun Sok Kim
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    ABSTRACT: This paper describes two valveless micropumps built on a 260 µm thick PZT with 20 µm thick parylene acoustic Fresnel lenses with air cavities. The micropumps produce in-plane body force through acoustic streaming effect of high-intensity acoustic beam that is generated by acoustic wave interference. The fabricated micropumps were shown to move microspheres, which have a diameter of 70–90 µm and a density of 0.99 g cm−3, on the water surface to form U-shape streams of microspheres with a drift velocity of 7.3 cm s−1 when the micropumps were located 4 mm below the water surface and driven by 160 Vpeak-to-peak pulsed sinusoidal waves. The driven microspheres formed U-shape streaming even without any fluidic channel according to the serial connection of the pie-shaped lenses and top electrodes. A micropump with a straight-lined fluidic channel was also fabricated and tested to show a 9.2 cm s−1 microspheres' drift velocity and a 9.5 mL min−1 volume pumping rate when combined with the acrylic acoustic wave reflector. Both the Fresnel lens and top electrode were patterned in a pie-shape with its apex angle of 90° to form asymmetric acoustic pressure distribution at the focal plane of the acoustic Fresnel lenses in order to push water in one direction.
    Journal of Micromechanics and Microengineering 02/2013; 23(4):045005. · 1.73 Impact Factor
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    ABSTRACT: We report an array of piezoelectric monocrystalline silicon microphones for audio-range acoustic sensing. Thirteen cantilever-type diaphragm transducers make up the array, each having a closely spaced and precisely controlled resonant frequency. These overlapping resonances serve to greatly boost the sensitivity of the array when the signals are added; if the signals are individually taken, the array acts as a physical filter bank with a quality factor over 40. Such filtering would enhance the performance and the efficiency of speech-recognition systems. In the “summing mode,” the array demonstrates high response over a large bandwidth, with unamplified sensitivity greater than 2.5 mV/Pa from 240 to 6.5 kHz. Both modes of operation rely on the precise control of resonant frequencies, often a challenge with large compliant microelectromechanical-system (MEMS) structures, where residual stress causes deformation. We mitigate these ill effects through the use of stress-compensating layer thicknesses and a stress-free monocrystalline diaphragm. For determining device geometry, we develop a simple analytical method that yields excellent agreement between designed and measured resonant frequency; all devices are within 4.5%, and four are within 0.5% (just several hertz). The technique could be useful not only for microphones but also for other low-frequency MEMS transducers designed for resonance operation at a specific frequency.
    Journal of Microelectromechanical Systems 02/2013; 22(1):107-114. · 1.92 Impact Factor
  • Source
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    ABSTRACT: Similar to optical tweezers, a tightly focused ultrasound microbeam is needed to manipulate microparticles in acoustic tweezers. The development of highly sensitive ultrahigh frequency ultrasonic transducers is crucial for trapping particles or cells with a size of a few microns. As an extra lens would cause excessive attenuation at ultrahigh frequencies, two types of 200-MHz lensless transducer design were developed as an ultrasound microbeam device for acoustic tweezers application. Lithium niobate single crystal press-focused (PF) transducer and zinc oxide self-focused transducer were designed, fabricated and characterized. Tightly focused acoustic beams produced by these transducers were shown to be capable of manipulating single microspheres as small as 5 µm two-dimensionally within a range of hundreds of micrometers in distilled water. The size of the trapped microspheres is the smallest ever reported in the literature of acoustic PF devices. These results suggest that these lensless ultrahigh frequency ultrasonic transducers are capable of manipulating particles at the cellular level and that acoustic tweezers may be a useful tool to manipulate a single cell or molecule for a wide range of biomedical applications. Biotechnol. Bioeng. © 2012 Wiley Periodicals, Inc.
    Biotechnology and Bioengineering 10/2012; · 4.16 Impact Factor
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    ABSTRACT: A sensitive, broad-bandwidth piezoelectric microelectromechanical systems (MEMS) transducer based on frequency interleaving of resonant transducers was designed and fabricated. A sputter-deposited piezoelectric zinc oxide (ZnO) thin film on the diaphragm is used to sense and generate acoustic pressure. A high compliance cantilever and spiral-beam-supported diaphragms are designed and built on the edge-released MEMS structure to release initial residual stress and to avoid in-plane tension when bent. Stress compensation has been achieved by adjusting the thickness of each layer of the cantilever and by compensating for the ZnO film's compressive stress with the bimorph structure of the spiral-beam. For a given pressure level and diaphragm size, the maximum strain on the spiral-beam-supported diaphragm is about an order of magnitude larger than that of a rectangular cantilever diaphragm. Also, the acoustic transducer built on the spiral-beam-supported diaphragm has a much higher sensitivity (but with less tolerance on the fabrication process variation and at the cost of lower usable bandwidth) than the one built on a rectangular cantilever diaphragm. By connecting many transducers in parallel, both the sensitivity and acoustic output were improved about 30 times. The interleaving of the transducers increased not only the sensitivity, but also broadened the useable bandwidth.
    Journal of Micromechanics and Microengineering 01/2012; 22(2):025005. · 1.73 Impact Factor
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    ABSTRACT: This paper reports combinatory localized-cytolysis by an array of MEMS ultrasonic transducers for fast screening of drug-induced cytoskeleton variation with fluorescence-stained cytolysis assay. An array of 6×6 Self Focused Acoustic transducers (SFATs) and disposable cell culture microwells were fabricated for the cytolysis and the fluorescence stain analysis. Cells were cultured in the microwells, and different drugs were applied to the cells to modify cell cytoskeletons. Multi-spot, localized cytolysis with micron precision was carried out with the SFAT array. Experimental results show that the SFAT array produced localized cytolysis with a focal spot of about 100–300 microns in diameter in multiple microwells, and the changes of the acoustic intensity threshold (AIT) for cytolysis were in accord with the alterations of the cytoskeleton induced by the drug treatments. Therefore, cytoskeleton-specific fast drug screening can be realized by observing the variation of the AIT. Since the SFAT array can lyse multiple cell samples within 3 minutes, it is easy to discern the lysed cells under fluorescent microscope, and the SFAT array system improves the efficiency and simplicity of the drug screening greatly.
    Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS) 01/2012;
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    ABSTRACT: This paper describes an acoustic trap consisting of a multi-foci Fresnel lens on 127 μm thick lead zirconate titanate sheet. The multi-foci Fresnel lens was designed to have similar working mechanism to an Axicon lens and generates an acoustic Bessel beam, and has negative axial radiation force capable of trapping one or more microparticle(s). The fabricated acoustic tweezers trapped lipid particles ranging in diameter from 50 to 200 μm and microspheres ranging in diameter from 70 to 90 μm at a distance of 2 to 5 mm from the tweezers without any contact between the transducer and microparticles.
    Applied Physics Letters 12/2011; 99(23):233704-2337043. · 3.52 Impact Factor
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    ABSTRACT: Piezoelectric microelectromechanical systems (MEMS) resonant sensors, known for their excellent mass resolution, have been studied for many applications, including DNA hybridization, protein-ligand interactions, and immunosensor development. They have also been explored for detecting antigens, organic gas, toxic ions, and explosives. Most piezoelectric MEMS resonant sensors are acoustic sensors (with specific coating layers) that enable selective and label-free detection of biological events in real time. These label-free technologies have recently garnered significant attention for their sensitive and quantitative multi-parameter analysis of biological systems. Since piezoelectric MEMS resonant sensors do more than transform analyte mass or thickness into an electrical signal (e.g., frequency and impedance), special attention must be paid to their potential beyond microweighing, such as measuring elastic and viscous properties, and several types of sensors currently under development operate at different resonant modes (i.e., thickness extensional mode, thickness shear mode, lateral extensional mode, flexural mode, etc.). In this review, we provide an overview of recent developments in micromachined resonant sensors and activities relating to biochemical interfaces for acoustic sensors.
    Lab on a Chip 11/2011; 12(1):29-44. · 5.70 Impact Factor
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    ABSTRACT: This study describes a tested prototype for a controllable directional underwater thruster with no moving parts. During operation, a high-intensity acoustic wave creates directional water jets and the device moves itself in the opposite direction. When the underwater thruster moves along a non-vertical angle, it can produce straight backward thrust of 2.3 mN and lateral thrust of 0.6 mN in parallel with the device surface, with a total thrust-to-weight ratio of 2:1. To enhance the acoustic streaming effect, a self-focusing acoustic transducer (SFAT) with air reflectors is used to focus the acoustic wave.
    IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 07/2011; · 1.50 Impact Factor
  • Lingtao Wang, Youngki Choe, Eun Sok Kim
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    ABSTRACT: paper describes a novel design of a multi-directional acoustic ejector with capability of electrical control on the droplet ejection angle by changing the operating frequency. The newly developed ejector consistently ejects uniform droplets in diameter of 70 µm, with electrical control of the directional angle from -30° to 35° (with respect to normal direction of liquid surface plane) as the operating frequency is varied from 16.78 MHz to 19.08 MHz. To produce the electrically adjustable oblique ejections of nano-liter droplets, destructive wave interference is intentionally introduced through a phase-varied lens. With the novel lens, the direction of the droplet ejection depends monotonically on the operating frequency of the driving signal. This paper presents the experimental results, as well as the theoretical analysis and simulation verification of the phase-varied lens design that gives the electrical control on the direction of ejected droplets.
    Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS) 01/2011;
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    ABSTRACT: This paper presents the design, fabrication, and characterization of a contactless radio frequency (RF) microelectromechanical system (MEMS) switch, composed of two surface-micromachined piezoelectric tunable capacitors and two bonded-wire inductors. The measured insertion loss and power isolation of the fabricated switch are 2.2 and 10.1dB, respectively, with a capacitance variation of 4:1 over a narrow bandwidth near 2.2GHz. This novel approach of using inductors eases the deflection requirement for the deformable bridge of the variable capacitor, and allows piezoelectric ZnO film to be used to deflect the capacitor bridge to vary the air gap, thus yielding a contactless RF switch.
    Sensors and Actuators A Physical 01/2011; 165(1):73-78. · 1.94 Impact Factor
  • Youngki Choe, Lingtao Wang, Eun Sok Kim
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    ABSTRACT: This paper describes an on-chip acoustic ejector array consisting of eight directional droplet ejectors that was designed to ink a spot with eight different droplets without having to move the ejectors. Each of the eight directional ejectors consistently ejects uniform droplets in diameter of 51 μm with a directional angle about 17° (with respect to the normal direction of the liquid surface). When a glass substrate was placed 8 mm away from the ejector array chip, all the ejected droplets from the 8 ejectors were placed within 399 x 1080 μm 2 area. If we exclude two ejectors which had bad alignment with the others, all the 6 droplets were placed within 238 x 380 μm 2 area.
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    ABSTRACT: This paper reports a real-time, label-free detection of prostate-specific-antigen (PSA) through mass sensing with film bulk acoustic resonator (FBAR) coated with an oriented antibody layer. The oriented antibody layer is produced with protein A forming a polarized bond between antibody, at the Fc (fragment crystallizable) region of an IgG (immunoglobulin G), and a gold layer. The first prototype sensor is capable of detecting PSA concentration in hundreds of nanogram per milliliter range, and can be regenerated with 0.1 M glycine pH 2.5 that strips off only the antibody-antigen complex. Also, preliminary testing shows the absence of cross-reaction with bovine serum albumin (BSA). An improved version of the sensor utilizing DTSSP (3,3'-Dithiobis[sulfosuccinimidylpropionate]) crosslinker shows a better response, and increases the sensitivity to a level where PSA detection down to nanograms per milliliter range is possible.
    Ultrasonics Symposium (IUS), 2011 IEEE International; 01/2011
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    ABSTRACT: This paper describes an acoustic tweezers consisting of a multi-foci Fresnel lens on 127 µm thick PZT sheet, designed to capture micron-sized particles. The multi-foci Fresnel lens was designed to have similar working mechanism as that of an axicon lens to generate an acoustic Bessel beam and, correspondingly, to generate negative axial radiation force capable of trapping one or more microparticle(s). The fabricated acoustic tweezers successfully trapped lipid particles ranging in diameter from 50 to 200 µm and microspheres ranging in diameter from 70 to 90 µm at a distance of 2 to 5 mm from the tweezers without any contact between the transducer and microparticles.
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    ABSTRACT: This paper describes a novel, highly-sensitive ultrasonic Doppler velocity sensing system for low velocity measurement in portable navigation systems. It is a compact velocity sensing system, in which MEMS ultrasonic transducers are incorporated with phase-locked-loop (PLL) circuitry for frequency detection and signal processing. The achieved voltage-velocity sensitivity is 0.22 V/(mm/s) and the minimum detectable velocity is 0.67 mm/s, corresponding to 0.11 Hz in Doppler frequency. To the best of our knowledge, the minimum detectable velocity is the best reported in literature. Also, the output of the PLL is a DC voltage linearly related to the velocity, and there is no need to convert the frequency shift to analog voltage.
    Solid-State Sensors, Actuators and Microsystems Conference (TRANSDUCERS), 2011 16th International; 01/2011
  • Hao Zhang, Wei Pang, Eun Sok Kim
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    ABSTRACT: In this paper, micromachined longitudinal wave resonant mass sensors operating near 1 GHz in a liquid environment are investigated and characterized. Mass sensitivities of the film bulk acoustic resonator (FBAR) and high-tone bulk acoustic resonator (HBAR) microbalances with small size are measured to be 782.7 cm(2)/g (50 times larger than that of conventional bulky quartz crystal microbalance) and 9.3 cm(2)/g, respectively. Based on the mass sensitivities and frequency noise floor, the minimum detectable mass of the FBAR and HBAR are estimated to be 2.8 ng/cm(2) and 11.9 ng/cm(2) in liquid, respectively.
    IEEE transactions on ultrasonics, ferroelectrics, and frequency control 01/2011; 58(1):255-8. · 1.80 Impact Factor
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    ABSTRACT: In this study, we designed and fabricated Self Focused Acoustic Transducer (SFAT) for micron-sized localized cytolysis. Monolayer 22RV1 prostate cancer cells were cultured in the cell culture chamber and locally lysed by the SFAT. Various electric powers and operating frequencies of actuating pulsed signal were applied to characterize the localized cell lysis effects. The cell lysis area was around 2.28×10-9 m2 and 1.64×10-9 m2, when the acoustic waves produced by the transducer were 17.3 and 52 MHz, respectively. The minimum electric power required for the cell lysis of 22RV1 is as low as 9 mW, which produces an acoustic intensity 0.15 W/cm2 at the focal spot. The amount of mRNA released in the culture media was increased more than 10 times after the cytolysis. According to experiment results, the size of lysed cells area is determined by the acoustic-wave frequency, and very little by the electric power applied to the device above a threshold. Signs of inertia cavitation phenomena such as bubble generation or temperature raise were not observed. Therefore, low-power micron-sized cell lysis without cavitation may have practical applications relating to cancer diagnosis and therapeutics.
    Ultrasonics Symposium (IUS), 2011 IEEE International; 01/2011
  • Hao Zhang, Wei Pang, Eun Sok Kim, Hongyu Yu
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    ABSTRACT: This paper presents a novel micromachined probe with a miniature film-bulk-acoustic-resonator (FBAR) mass sensor integrated at the tip. The detailed fabrication processes for a silicon microprobe and an SU-8 microprobe are described and discussed. The electrical performances of FBARs integrated on both the microprobes were characterized and compared. The semiconducting silicon was found to degrade the FBAR's quality factor (Q) significantly, due to the GHz electromagnetic wave's energy loss in the low-resistivity silicon. Yet, the FBAR on the SU-8 probe was measured to have a higher Q than that on the silicon probe, as the SU-8 polymer material is highly resistive and electrically insulating. As an application demonstration, the fabricated SU-8 probe was successfully used in detecting Hg2+ concentration in liquids. The integration of a resonant mass sensor with a micromachined probe offers easy access to sensing environment with minimal disturbance to the environment.
    Journal of Micromechanics and Microengineering 11/2010; 20(12):125008. · 1.73 Impact Factor

Publication Stats

719 Citations
121.07 Total Impact Points


  • 2014
    • University of California, Los Angeles
      Los Angeles, California, United States
  • 2011
    • Tianjin University
      • State Key Laboratory of Precision Measurement Technology and Instruments
      T’ien-ching-shih, Tianjin Shi, China
  • 2010–2011
    • Wuhan University
      • • Department of Physics
      • • School of Electronic Information
      Wuhan, Hubei, China
  • 2002–2011
    • University of Southern California
      • Department of Electrical Engineering
      Los Angeles, CA, United States
  • 2009
    • Arizona State University
      Phoenix, Arizona, United States
  • 2006
    • University of Missouri
      • Department of Electrical and Computer Engineering
      Columbia, MO, United States
  • 1996–2004
    • University of Hawaiʻi at Mānoa
      • Department of Electrical Engineering
      Honolulu, HI, United States
  • 2000
    • Korea Advanced Institute of Science and Technology
      • Department of Mechanical Engineering
      Seoul, Seoul, South Korea