Younghun Kim
Postdoctoral Scholar, Electrical Engineering
Contact
https://orcid.org/0000-0003-0854-0266All Publications
-
A Transparent Acoustic Lens for Integrated Ultrasound-Optical Systems
IEEE. 2025
View details for DOI 10.1109/IUS62464.2025.11201618
View details for Web of Science ID 001719884800350
-
High-spatial-resolution transcranial focused ultrasound neuromodulation using frequency-modulated pattern interference radiation force.
Ultrasonics
2024; 140: 107298
Abstract
Stimulating the brain in a precise location is crucial in ultrasound neuromodulation. However, improving the resolution proves a challenge owing to the characteristics of transcranial focused ultrasound. In this paper, we present a new neuromodulation system that overcomes the existing limitations based on an acoustic radiation force with a frequency-modulated waveform and standing waves. By using the frequency-modulated pattern interference radiation force (FM-PIRF), the axial spatial resolution can be reduced to a single wavelength level and the target location can be controlled in axial direction electronically. A linear frequency-modulated chirp waveform used in the experiment was designed based on the simulation results. The displacement of the polydimethylsiloxane (PDMS) cantilever was measured at intervals of 0.1 mm to visualize the distribution of radiation force. These results and methods experimentally show that FM-PIRF has improved spatial resolution and capability of electrical movement.
View details for DOI 10.1016/j.ultras.2024.107298
View details for PubMedID 38531115
-
Real-time breath monitoring using multi-channel MEMS resonator
SENSORS AND ACTUATORS A-PHYSICAL
2023; 363
View details for DOI 10.1016/j.sna.2023.114703
View details for Web of Science ID 001160688000001
-
3D volumetric imaging system based on a carbon-based light-induced ultrasound transmitter
JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY
2023; 37 (9): 4455-4465
View details for DOI 10.1007/s12206-023-0804-z
View details for Web of Science ID 001056320100003
-
Acoustic radiation force for analyzing the mechanical stress in ultrasound neuromodulation.
Physics in medicine and biology
2023; 68 (13)
Abstract
Objective. Although recent studies have shown that mechanical stress plays an important role in ultrasound neuromodulation, the magnitude and distribution of the mechanical stress generated in tissues by focused ultrasound transducers have not been adequately examined. Various acoustic radiation force (ARF) equations used in previous studies have been evaluated based on the tissue displacement results and are suitable for estimating the displacement. However, it is unclear whether mechanical stress can be accurately determined. This study evaluates the mechanical stress predicted by various AFR equations and suggests the optimal equation for estimating the mechanical stress in the brain tissue.Approach. In this paper, brain tissue responses are compared through numerical finite element simulations by applying the three most used ARF equations-Reynolds stress force ((RSF)), momentum flux density tensor force, and attenuation force. Three ARF fields obtained from the same pressure field were applied to the linear elastic model to calculate the displacement, mechanical stress, and mean pressure generated inside the tissue. Both the simple pressure field using a single transducer and the complex standing wave pressure field using two transducers were simulated.Main results. For the case using a single transducer, all three ARFs showed similar displacement. However, when comparing the mechanical stress results, only the results using the RSF showed a strong stress tensor at the focal point. For the case of using two transducers, the displacement and stress tensor field of the pattern related to the standing wave were calculated only from the results using the RSF.Significance. The model using RSF equation allows accurate analysis on stress tensor inside the tissue for ultrasound neuromodulation.
View details for DOI 10.1088/1361-6560/acdbb5
View details for PubMedID 37366067
-
Measurement of the acoustic reflectance function based on Lamb wave using high f-number transducers.
Ultrasonics
2023; 132: 107003
Abstract
Ultrasonic reflectivity using a V(z) technique is a powerful characterization method in acoustic microscopy to measure the elastic properties of materials. Conventional techniques generally use a low f-number with high frequency; however, to measure the reflectance function of the highly attenuative material, a low frequency is essential. In this study, the transducer-pair method based on Lamb waves is used to measure the reflectance function of a highly attenuative material. The results demonstrate the feasibility of the proposed method using a commercial ultrasound transducer with high f-number.
View details for DOI 10.1016/j.ultras.2023.107003
View details for PubMedID 37099939
-
Design of Piezoelectric Acoustic Transducers for Underwater Applications
SENSORS
2023; 23 (4)
Abstract
Interest in underwater transducers has persisted since the mid-1900s. Underwater transducers are designed in various shapes using various materials depending on the purpose of use, such as to achieve high power, improve broadband, and enhance beam steering. Therefore, in this study, an analysis is conducted according to the structural shape of the transducer, exterior material, and active material. By classifying transducers by structure, the transducer design trends and possible design issues can be identified. Researchers have constantly attempted new methods to improve the performance of transducers. In addition, a methodology to overcome this problem is presented. Finally, this review covers old and new research, and will serve as a reference for designers of underwater transducer.
View details for DOI 10.3390/s23041821
View details for Web of Science ID 000940047400001
View details for PubMedID 36850418
View details for PubMedCentralID PMC9966007
-
Particle Manipulation in 2D Space Using a Capacitive Micromachined Ultrasonic Transducer
MICROMACHINES
2022; 13 (4)
Abstract
Ultrasonic particle manipulation is a noncontact method for controlling microscale objects, such as cells or microparticles, using an acoustic field. In this study, a 2D array of capacitive micromachined ultrasonic transducers (CMUTs), placed horizontally in immersion, generated ultrasonic waves in the vertical direction, and the oil's surface increased due to the radiation force of the ultrasonic waves. In addition, the radiation force directly exerted a force on a floating particle. By measuring the movement of the reflected laser light by the moving oil surface, the height of the oil's surface deformed by the acoustic radiation force (ARF) was measured. The ARF made a floating particle, as well as the oil's surface, move. The particle moved radially away from the surface position above the transducer, and its velocity was determined by its position on the fluid's surface. When a single channel was operated, it moved 0.4 mm at an average speed of 90 μm/s, and when two adjacent channels were operated, it moved 1.2 mm at a speed of 272 μm/s. The particles moved in any direction on the surface of the oil by controlling the actuation channel using an electrical switch.
View details for DOI 10.3390/mi13040534
View details for Web of Science ID 000785325800001
View details for PubMedID 35457839
View details for PubMedCentralID PMC9032542
-
Increasing the transmission efficiency of transcranial ultrasound using a dual-mode conversion technique based on Lamb waves.
The Journal of the Acoustical Society of America
2022; 151 (3): 2159
Abstract
Transcranial focused ultrasound (FUS) is a noninvasive treatment for brain tumors and neuromodulation. Based on normal incidence, conventional FUS techniques use a focused or an array of ultrasonic transducers to overcome the attenuation and absorption of ultrasound in the skull; however, this remains the main limitation of using FUS. A dual-mode conversion technique based on Lamb waves is proposed to achieve high transmission efficiency. This concept was validated using the finite element analysis (FEA) and experiments based on changes in the incident angle. Aluminum, plexiglass, and a human skull were used as materials with different attenuations. The transmission loss was calculated for each material, and the results were compared with the reflectance function of the Lamb waves. Oblique incidence based on dual-mode conversion exhibited a better transmission efficiency than that of a normal incidence for all of the specimens. The total transmission losses for the materials were 13.7, 15.46, and 3.91dB less than those associated with the normal incidence. A wedge transducer was designed and fabricated to implement the proposed method. The results demonstrated the potential applicability of the dual-mode conversion technique for the human skull.
View details for DOI 10.1121/10.0009849
View details for PubMedID 35364946
-
Feasibility Study on Multi-Touch Ultrasound Large-Panel Touchscreen Using Guided Lamb Waves
MEASUREMENT
2022; 190
View details for DOI 10.1016/j.measurement.2022.110755
View details for Web of Science ID 000750642300004
-
Comparison of Waveform Modulation Methods Used in Pattern Interference Radiation Force Neuromodulator
IEEE. 2022
View details for DOI 10.1109/IUS54386.2022.9957675
View details for Web of Science ID 000896080400183
-
Patterned Interference Radiation Force for Transcranial Neuromodulation.
Ultrasound in medicine & biology
1800
Abstract
Compared with the conventional method of transcranial focused ultrasound stimulation using a single transducer or a focused beam, the compression and tensile forces are generated from the high-pressure gradient of a standing wave that can generate increased stimulation. We experimentally verified a neuromodulation system using patterned interference radiation force (PIRF) and propose a method for obtaining the magnitude of the radiation force, which is considered the main factor influencing ultrasound neuromodulation. The radiation forces generated using a single focused transducer and a standing wave created via two focused transducers were compared using simulations. Radiation force was calculated based on the relationship between the acoustic pressure, radiation force and time-averaged second-order pressure obtained using an acoustic streaming simulation. The presence of the radiation force was verified by measuring the time-averaged second-order pressure generated due to the radiation force, by using a glass tube.
View details for DOI 10.1016/j.ultrasmedbio.2021.11.006
View details for PubMedID 34955292
-
Design and Analysis of an Active Reflection Controller That Can Reduce Acoustic Signal Refer to the Angle of Incidence
SENSORS
2021; 21 (17)
Abstract
Techniques for reducing the reflection of acoustic signals have recently been actively studied. Most methods for reducing acoustic signals were studied using the normal-incidence wave reduction technique. Although the technique of canceling an object from the normal incidence wave is essential, research on reducing acoustic signals according to the angle of incidence is required for practical applications. In this study, we designed, fabricated, and experimented with an active reflection controller that can reduce acoustic signals according to the angle of incidence. The controller consists of a transmitter on one layer, a receiver sensor on two layers, and an acoustic window on three layers. To reduce the reflected signal, a combination of the time delay and phase was applied to the controller to minimize the acoustic signal by up to -23 dB at an angle of 10°. A controller array simulation was performed based on the results of a controlled experiment. In conclusion, our proposed controller can reduce acoustic signals according to the angle of incidence, which makes it suitable for many applications.
View details for DOI 10.3390/s21175793
View details for Web of Science ID 000694457100001
View details for PubMedID 34502684
View details for PubMedCentralID PMC8433782
-
Comparison of Fresnel lens-based focused vortex transducer and focused transducer for neurostimulation
IEEE. 2021
View details for DOI 10.1109/IUS52206.2021.9593624
View details for Web of Science ID 000832095000292
-
Transmission efficiency comparison between dual-mode conversion incidence and normal incidence
IEEE. 2021
View details for DOI 10.1109/IUS52206.2021.9593823
View details for Web of Science ID 000832095000470
-
Single-Shot Near-Field Volumetric Imaging System for Optical Ultrasound and Photoacoustics Using Capacitive Micromachined Ultrasonic Transducer Without Transmission Mode
IEEE TRANSACTIONS ON ULTRASONICS FERROELECTRICS AND FREQUENCY CONTROL
2020; 67 (6): 1151-1158
Abstract
In this article, we present a single-shot dual-mode imaging system that uses optical ultrasound (US) as an ultrasonic pulser without a transmission circuit. The ultrasonic pulse-echo system comprises an optical US pulser generated by carbon nanotubes (CNTs), which generate a high-power photoacoustic (PA) signal and a capacitive micromachined ultrasonic transducer (CMUT) receiver. By fabricating a thin CNT-polydimethylsiloxane (PDMS) composite capable of semiabsorption of the laser, a single-shot imaging system was developed. By transmitting a semipenetration light to the object, US and PA imaging were performed in a single shot. A CNT thickness of [Formula: see text] produced a maximum pressure of 154 kPa, and US was received by CMUT with a 2-MHz center frequency in PDMS. Additionally, a low-profile and near-depth imaging system was constructed with an intermediate layer of the 6-mm PDMS for the dry contact method. We performed a single-shot dual-mode imaging experiment on point and line phantoms, as well as the particle spread in the soft tissue. Thus, we examined the feasibility of the near-depth and single-shot dual-mode (US and PA) imaging system capable of a dry contact.
View details for DOI 10.1109/TUFFC.2020.2965600
View details for Web of Science ID 000538144900006
View details for PubMedID 31976884
-
Comparison of Si<sub>3</sub>N<sub>4</sub>-SiO<sub>2</sub> and SiO<sub>2</sub> Insulation Layer for Zero-Bias CMUT Operation Using Dielectric Charging Effects
IEEE TRANSACTIONS ON ULTRASONICS FERROELECTRICS AND FREQUENCY CONTROL
2020; 67 (4): 879-882
Abstract
In this letter, we report the characteristics of zero-bias capacitive micromachined ultrasonic transducers (CMUTs) in various aspects, considering the transmission and reception sensitivity and evaluation of the long-term stability with ac transmission in immersion. The main idea of the zero-bias CMUT implementation is that the charge is injected by the dielectric charging effects in an insulation layer in the pull-in state. The CMUT was fabricated by a local oxidation of silicon (LOCOS) process, and the insulation layer consists of Si3 N4 -SiO2 and SiO2, which have been commonly used in previous studies. A study on the charging effects is reported to quantitatively observe the voltage shift by charge transfer with time dependence at different temperatures and collapsing time dependence. Therefore, we successfully implemented a zero-bias CMUT with a transmission efficiency of 4.62 kPa/V at a center frequency of 7.53 MHz in Si3 N4-SiO2 and a transmission efficiency of 6.78 kPa/V at a center frequency of 7.86 MHz in SiO2 immersion.
View details for DOI 10.1109/TUFFC.2019.2950902
View details for Web of Science ID 000522170700021
View details for PubMedID 31689188
-
Comparison between Resonance and Non-Resonance Type Piezoelectric Acoustic Absorbers
SENSORS
2020; 20 (1)
Abstract
In this study, piezoelectric acoustic absorbers employing two receivers and one transmitter with a feedback controller were evaluated. Based on the target and resonance frequencies of the system, resonance and non-resonance models were designed and fabricated. With a lateral size less than half the wavelength, the model had stacked structures of lossy acoustic windows, polyvinylidene difluoride, and lead zirconate titanate-5A. The structures of both models were identical, except that the resonance model had steel backing material to adjust the center frequency. Both models were analyzed in the frequency and time domains, and the effectiveness of the absorbers was compared at the target and off-target frequencies. Both models were fabricated and acoustically and electrically characterized. Their reflection reduction ratios were evaluated in the quasi-continuous-wave and time-transient modes.
View details for DOI 10.3390/s20010047
View details for Web of Science ID 000510493100047
View details for PubMedID 31861792
View details for PubMedCentralID PMC6982962
-
Pattern Interference Radiation Force (PIRF) Based on Large-scale Focused Transducer Using Fresnel Lens
IEEE. 2020
View details for Web of Science ID 000635688900453
-
Transcranial Ultrasound Using Leaky Lamb Waves by Wedge Transducer Array
IEEE. 2020
View details for Web of Science ID 000635688900530
-
Multipoint-Detection Strain Sensor with a Single Electrode Using Optical Ultrasound Generated by Carbon Nanotubes
SENSORS
2019; 19 (18)
Abstract
With the development of wearable devices, strain sensors have attracted large interest for the detection of human motion, movement, and breathing. Various strain sensors consisting of stretchable conductive materials have been investigated based on resistance and capacitance differences according to the strain. However, this method requires multiple electrodes for multipoint detection. We propose a strain sensor capable of multipoint detection with a single electrode, based on the ultrasound pulse-echo method. It consists of several transmitters of carbon nanotubes (CNTs) and a single polyvinylidene fluoride receiver. The strain sensor was fabricated using CNTs embedded in stretchable polydimethylsiloxane. The received data are characterized by the different times of transmission from the CNTs of each point depending on the strain, i.e., the sensor can detect the positions of the CNTs. This study demonstrates the application of the multipoint strain sensor with a single electrode for measurements up to a strain of 30% (interval of 1%). We considered the optical and acoustic energy losses in the sensor design. In addition, to evaluate the utility of the sensor, finger bending with three-point CNTs and flexible phantom bending with six-point CNTs for the identification of an S-curve having mixed expansion and compression components were carried out.
View details for DOI 10.3390/s19183877
View details for Web of Science ID 000489187800063
View details for PubMedID 31505727
View details for PubMedCentralID PMC6767607
-
Single-Shot Waterless Low-Profile Photoacoustic System: Near-Field Volumetric Imaging In Vivo for Blood Vessels Based on Capacitive Micromachined Ultrasonic Transducer (CMUT)
SENSORS
2019; 19 (5)
Abstract
Intensive research on photoacoustics (PA) for imaging of the living human body, including the skin, vessels, and tumors, has recently been conducted. We propose a PA measurement system based on a capacitive micromachined ultrasonic transducer (CMUT) with waterless coupling, short measurement time (<1 s), backward light irradiation, and a low-profile ultrasonic receiver unit (<1 cm). We fabricate a 64-element CMUT ring array with 6.2 mm diameter and 10.4 MHz center frequency in air, and 100% yield and uniform element response. To validate the PA tissue characterization, we employ pencil lead and red ink as solid and liquid models, respectively, and a living body to target moles and vessels. The system implements a near-field imaging system consisting of a 6 mm polydimethylsiloxane (PDMS) matching layer between the object and CMUT, which has a 3.7 MHz center frequency in PDMS. Experiments were performed in a waterless contact on the PDMS and the laser was irradiated with a 1 cm diameter. The experimental results show the feasibility of this near-field PA imaging system for position and depth detection of skin, mole, vessel cells, etc. Therefore, a system applicable to a low-profile compact biomedical device is presented.
View details for DOI 10.3390/s19050995
View details for Web of Science ID 000462540400014
View details for PubMedID 30813583
View details for PubMedCentralID PMC6427691
-
Design and Fabrication of Piezoelectric Transducers Separating Incident and Reflected Wave
JOURNAL OF THE KOREAN SOCIETY FOR NONDESTRUCTIVE TESTING
2019; 39 (1): 29-36
View details for DOI 10.7779/JKSNT.2019.39.1.29
View details for Web of Science ID 000461804800005
-
Non-contact leaky Lamb wave imaging based on pulsed laser and ultrasound microphone
IEEE. 2019: 1571-1574
View details for DOI 10.1109/ultsym.2019.8925599
View details for Web of Science ID 000510220100401