About

Eun Sok Kim is associated with the USC + Amazon Center on Secure & Trusted Machine Learning, a joint research center focused on developing new approaches to machine learning (ML) privacy, security, and trustworthiness. The center aims to unleash a new line of fundamental research on privacy, security, and trustworthiness in machine learning, addressing the crucial challenges posed by the proliferation of artificial intelligence across society. The center organizes annual research festivals and targeted workshops, fostering collaboration and innovation in these critical areas.

Research topics

• Physics
• Acoustics
• Engineering
• Materials science
• Computer Science
• Artificial Intelligence
• Electrical engineering
• Electronic engineering
• Speech recognition
• Computer vision
• Optics
• Optoelectronics

Selected publications

• Patch clamp recordings of action potentials from pyramidal neuron in hippocampus CA1 under focused ultrasound neurostimulation with MEMS self-focusing acoustic transducer

  Journal of Neural Engineering · 2025-06-24 · 1 citations

  articleOpen accessSenior author

  Abstract Objective. This study aims to investigate the modulatory effects of focused ultrasound (FUS) on neuronal activity at the single-cell level, using whole-cell patch clamp recordings in hippocampal slices. Approach. A self-focused acoustic transducer (SFAT) was designed and fabricated on a 127 µ m-thick translucent lead zirconate titanate substrate to allow infrared light transmission for visualizing neurons during patch clamp experiments. The SFAT operates at 18.4 MHz, producing low-intensity FUS with a 46 µ m focal diameter at a depth of 400 µ m. Three types of SFAT—active, FUS-blocking control, and low-electromagnetic interference (EMI) versions—were developed to assess the effects of acoustic stimulation, thermal heating, and EMI. Neuronal responses were recorded across 78 tissue samples from 29 animals using 48 combinations of acoustic parameters, including peak-to-peak voltage, pulse repetition frequency (PRF), and pulse duration. Main results. Whole-cell patch clamp recordings from CA1 pyramidal neurons in rat hippocampal slices revealed that FUS induces both inhibitory and excitatory effects on action potential firing, depending on the stimulation parameters. Inhibition was found to be the dominant response, while excitation was mainly attributable to thermal effects. Optimal inhibition was achieved with 60 Vpp (ISAPA = 2.11 W cm −2 ), 35 kCycles/pulse (1.90 ms), and 100 Hz PRF, yielding a 60% success rate. Conversely, excitation was observed in 60% of trials using 120 Vpp (ISAPA = 8.44 W cm −2 ), 50 kCycles/pulse (2.72 ms), and 20 Hz PRF. Significance. This work presents a novel neuromodulation platform that combines high-frequency focused ultrasound with real-time whole-cell patch clamp recording at single-neuron resolution. The results provide direct electrophysiological evidence of parameter-dependent, bidirectional modulation of neuronal activity by FUS, offering new insights into its underlying mechanisms and helping define stimulation protocols for future neurotherapeutic applications.

  PublisherDOI

• Wearable Stethoscope Based On Resonant Microphone Array With Wireless Data Transfer

  2024-01-21 · 2 citations

  articleSenior author

  RMA is built on a silicon-on-insulator (SOI) wafer using a shared window for fitting two width-stepped cantilevers in one window to reduce pressure leakage through the air gaps around the cantilever edges. Four resonant microphones are used in the array to cover the entire frequency range. The highest unamplified sensitivity in the array is measured to be 94.4 mV/Pa at 330 Hz, which is more than 4 times that of any commercially available condenser microphone. Experimental results show more prominent wheezing signatures in the spectrogram of the signals obtained with RMA compared to those with a reference microphone. A folded printed circuit board (PCB) houses ultra-low-power op Amps, filters, a commercial wireless chipset (CYBLE-416045-02) with a built-in antenna for Bluetooth communication, and a battery. Data transfer is obtained through Bluetooth low-energy (BLE) communication from the wireless chipset to a mobile phone. The wearable and wireless stethoscope (including the RMA, electronics, and battery) weighs 33.2 g, and can continuously pick up sounds with the RMA and send the data through BLE for several hours.

  PublisherDOI

• MEMS piezoelectric resonant microphone array for lung sound classification

  Journal of Micromechanics and Microengineering · 2023 · 16 citations

  Senior authorCorresponding
  • Acoustics
  • Materials science
  • Engineering

  , while the SNRs are between 79 and 98 dBA. Wheezing feature in lung sounds recorded by the RMA is more distinguishable than that recorded by a reference microphone with traditional flat sensitivity, and thus, the automatic classification accuracy of wheezing is higher with the lung sounds recorded by the RMA than with those by the reference microphone, when tested with deep learning algorithms on computer or with simple machine learning algorithms on low-power wireless chip set for wearable applications.

  PublisherOA PDFDOI

• Simple sacrificial-layer-free microfabrication processes for air-cavity Fresnel acoustic lenses (ACFALs) with improved focusing performance

  Microsystems & Nanoengineering · 2022-07-05 · 12 citations

  articleOpen accessSenior author

  Focused ultrasound (FUS) is a powerful tool widely used in biomedical therapy and imaging as well as in sensors and actuators. Conventional focusing techniques based on curved surfaces, metamaterial structures, and multielement phased arrays either present difficulties in massively parallel manufacturing with high precision or require complex drive electronics to operate. These difficulties have been addressed by microfabricated self-focusing acoustic transducers (SFATs) with Parylene air-cavity Fresnel acoustic lenses (ACFALs), which require a time-demanding step in removing the sacrificial layer. This paper presents three new and improved types of ACFALs based on polydimethylsiloxane (PDMS), an SU-8/PDMS bilayer, and SU-8, which are manufactured through simple sacrificial-layer-free microfabrication processes that are two to four times faster than that for the Parylene ACFALs. Moreover, by studying the effect of the lens thickness on the acoustic transmittance through the lens, the performance of the transducers has been optimized with improved thickness control techniques developed for PDMS and SU-8. As a result, the measured power transfer efficiency (PTE) and peak output acoustic pressure are up to 2.0 and 1.8 times higher than those of the Parylene ACFALs, respectively. The simple microfabrication techniques described in this paper are useful for manufacturing not only high-performance ACFALs but also other miniaturized devices with hollow or suspended structures for microfluidic and optical applications.

  PublisherOA PDFDOI

• Wireless and Battery-Less Tamper Detection With Pyroelectric Energy Converter and High-Overtone Bulk Acoustic Resonator

  IEEE Sensors Journal · 2022-06-20 · 6 citations

  articleSenior author

  This paper describes a proof-of-concept sensor (1) that can detect and record (without battery) a semiconductor-chip tamper activity (i.e., de-soldering followed by mechanical banging) which a counterfeiter does to scavenge semiconductor chips from a printed circuit board and (2) that can be wirelessly interrogated without need to open semiconductor packages. The sensor is based on a High-overtone-Bulk-Acoustic-Resonator (HBAR) working as a Radio-Frequency Identification (RFID) tag, which can be permanently broken down by the voltage and charge generated by a pyroelectric-energy-converter (PEC). The concept is demonstrated through connecting a 7.5 GHz HBAR (based on a 350 nm-thick ZnO thin film deposited on sapphire) to a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5\times 5\times0.15$ </tex-math></inline-formula> mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> lithium niobate-based PEC, via a Kapton-copper cantilever switch. When the switch is turned on (due to a mechanical shock emulating a counterfeiter’s mechanical banging), the accumulated charge on the PEC (produced by 250 °C temperature rise emulating a counterfeiter’s de-soldering) generates an electrical pulse with 9.6 V peak voltage and 1.36 nC charge transfer to an 8 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{M}\Omega $ </tex-math></inline-formula> load (close to the HBAR’s resistance). The PEC’s voltage and charge are shown to permanently breakdown the ZnO film on the HBAR ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.1\times 0.1\times0.33$ </tex-math></inline-formula> mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) having a quality factor (Q) of about 2,500, so that the Q drops to near zero, drastically changing the RF absorption/scatter characteristics. A wireless interrogation is demonstrated by having a pair of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times 2\times0.2$ </tex-math></inline-formula> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> microstrip patch antennas, one of which is connected to HBAR while the other one is connected to a network analyzer working as a wireless interrogator.

  PublisherDOI

• Subminiature Underwater Propeller with Electrical Controllability of Steering

  2021-09-11 · 2 citations

  articleSenior author

  This paper describes a subminiature underwater ultrasonic propeller with electrically controllability over its propulsion direction. Built on a 200-micron thick nickel-coated lead zirconate titanate (PZT) substrate, the propeller consists of 28 sectors of individually accessible Fresnel lens that are composed of Parylene air-cavity-reflectors on top of the frontside nickel electrode. A backside Fresnel air-reflector is added to prevent any propulsion from the backside that may cancel the propulsion from the front side. The fabricated propeller (4 × 4 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in size and 37.5 mg in weight) is demonstrated to have control over its advancing direction when assembled on an air-filled capsule.

  PublisherDOI

• Phase Array Ultrasonic Transducer Based on a Flip Chip Bonding with Indium Solder Bump

  2021-09-11 · 5 citations

  articleSenior author

  This paper presents the design, simulation and fabrication process of a phase array ultrasonic transducer based on 1mm-thick lead zirconate titanate (PZT) substrate with 128 independently accessible electrodes through flip chip bonding with indium solder bump. The reflow processes of indium have been explored under air, nitrogen, hydrogen, liquid metal flux, and formic acid (H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> ). Among these, formic acid is found to be the best ambient for forming indium bump, as it removes indium oxide and clean out residuals most effectively. Also, different sizes of under bump metallization (UBM) and indium pattern have been tested, and the measured heights of indium bumps after a reflow show that the lateral dimension ratio of 1:2.5 between the UBM and indium patterns offers the best wettability over the UBM with the smallest residual.

  PublisherDOI

• Patch Clamp Technology for Focused Ultrasonic (FUS) Neuromodulation

  Methods in molecular biology · 2021-11-26 · 7 citations

  article1st author
  PublisherDOI

• Nozzleless Acoustic Droplet Ejector With Electrically Tunable Droplet Size for Picking and Placing Semiconductor Chips

  Journal of Microelectromechanical Systems · 2021-01-09 · 12 citations

  articleSenior author

  This paper presents a microfabricated nozzleless acoustic droplet ejector with electrically tunable droplet size as a new tool for on-demand picking and placing semiconductor chips for semiconductor packaging. The ejector is based on a 2-mm-thick lead zirconate titanate (PZT) sheet with a planar annular-ring air-cavity acoustic Fresnel lens on top. When driven with sinusoidal pulsed voltage signals of 1.16 MHz, the ejector generates focused ultrasound with 1-mm focal diameter and 5-mm focal depth at 22 mm focal length in sodium polytungstate (SPT) solution. A finite-element-method (FEM) simulation model calculating the acoustic-field-induced fluid motion during the droplet ejection process has been developed, and verified by experiments, in which the device ejects SPT droplets whose diameter is from 850 to 2, 490 μm, controlled by the driving pulse width and voltage. The ejected droplets are able to carry 400- μm-thick square silicon chips with side length from 700 to 3, 100 μm. A polyester channel-embedded guiding cover for semi-automatic loading of silicon chips to the ejection site is designed to avoid manual placement of the silicon chips. With the proof-of-concept system, we demonstrate ejecting silicon chips out of SPT surface onto a nearby paper, assembling them into arrays with good repeatability. [2020-0337].

  PublisherDOI

• In Vivo Non-Thermal, Selective Cancer Treatment With High-Frequency Medium-Intensity Focused Ultrasound

  IEEE Access · 2021-01-01 · 7 citations

  articleOpen accessSenior author

  Focused ultrasound (FUS) has proven its efficacy in non-invasive, radiation-free cancer treatment. However, the commonly used low-frequency high intensity focused ultrasound (HIFU) destroys both cancerous and healthy tissues non-specifically through extreme heat and inertial cavitation with low spatial resolution. To address this issue, we evaluate the therapeutic effects of pulsed (60 Hz pulse repetition frequency, 1.45 ms pulse width) high-frequency (20.7 MHz) medium-intensity (spatial-peak pulse-average intensity ISPPA < 279.1 W/cm2, spatial-peak temporal-average intensity ISPTA < 24.3 W/cm2) focused ultrasound (pHFMIFU) for selective cancer treatment without thermal damage and with low risk of inertial cavitation (mechanical index < 0.66), in an in vivo subcutaneous B16F10 melanoma tumor growth model in mice. The pHFMIFU with 104 μm focal diameter is generated by a microfabricated self-focusing acoustic transducer (SFAT) with a Fresnel acoustic lens. A three-axis positioning system has been developed for automatic scanning of the transducer to cover a larger treatment volume, while a water-cooling system is custom-built for dissipating non-acoustic heat from the transducer surface. Initial testing revealed that pHFMIFU treatment can be applied to a living animal while maintaining skin temperature under 35.6 °C without damaging normal skin and tissue. After eleven days of treatment with pHFMIFU, the treated tumors were significantly smaller with large areas of necrosis and apoptosis in the treatment field compared to untreated controls. Potential mechanisms of this selective, non-thermal killing effect, as well as possible causes of and solutions to the variation in treatment results, have been analyzed and proposed. The pHFMIFU could potentially be used as a new therapeutic modality for safer cancer treatment especially in critical body regions, due to its cancer-specific effects and high spatial resolution.

  PublisherOA PDFDOI

Recent grants

• Contactless RF MEMS Switches

  NSF · $240k · 2006–2009

• NIH Grant R21HG005118

  NIH · $438k · 2012

• Electromagnetic Vibration-Energy Harvesters

  NSF · $359k · 2013–2017

• Selective Cancer Treatment Based on High Frequency Ultrasonic Beam

  NIH · $439k · 2016–2018

• Ultrasonic Neural Stimulation for Neuromodulation Therapeutics

  NIH · $1.3M · 2018–2024

Frequent coauthors

• Hongyu Yu

  Jinan University

  47 shared

• Youngki Choe

  20 shared

• Wei Kong Pang

  University of Wollongong

  18 shared

• Yongkui Tang

  University of California, Santa Barbara

  17 shared

• Chuang-Yuan Lee

  17 shared

• Wei Pang

  Tianjin University

  17 shared

• Lurui Zhao

  15 shared

• Hao Zhang

  Hanyang University

  14 shared

Education

• Ph.D., Electrical Engineering

  University of Southern California

  1995

• M.S., Electrical Engineering

  University of Southern California

  1991

• B.S., Electrical Engineering

  Seoul National University

  1988

Awards & honors

• National Academy of Inventors (NAI) Fellow (2023)
• Institute of Electrical and Electronics Engineers (IEEE) Fel…
• IEEE Transactions on Automation Science and Engineering Best…
• Institute of Physics (IOP) Fellow (1996)
• IEEE Student Chapter of U. of Hawaii Teaching Award (1996)