Low Megahertz Frequency Research Articles

A PVDF Receiver for Acoustic Monitoring of Microbubble-Mediated Ultrasound Brain Therapy.

The real-time monitoring of spectral characteristics of microbubble (MB) acoustic emissions permits the prediction of increases in blood-brain barrier (BBB) permeability and of tissue damage in MB-mediated focused ultrasound (FUS) brain therapy. Single-element passive cavitation detectors provide limited spatial information regarding MB activity, greatly affecting the performance of acoustic control. However, an array of receivers can be used to spatially map cavitation events and thus improve treatment control. The spectral content of the acoustic emissions provides additional information that can be correlated with the bio-effects, and wideband receivers can thus provide the most complete spectral information. Here, we develop a miniature polyvinylidene fluoride (PVDF thickness = 110 μm, active area = 1.2 mm2) broadband receiver for the acoustic monitoring of MBs. The receiver has superior sensitivity (2.36-3.87 V/MPa) to those of a commercial fibre-optic hydrophone in the low megahertz frequency range (0.51-5.4 MHz). The receiver also has a wide -6 dB acceptance angle (54 degrees at 1.1 MHz and 13 degrees at 5.4 MHz) and the ability to detect subharmonic and higher harmonic MB emissions in phantoms. The overall acoustic performance of this low-cost receiver indicates its suitability for the eventual use within an array for MB monitoring and mapping in preclinical studies.

Laser-generated focused ultrasound transmitters with frequency-tuned outputs over sub-10-MHz range

Previous laser-generated focused ultrasound (LGFU) systems have been operated with >15 MHz frequency, allowing for high spatial precision (<100 μm). However, they have been limited only to proximal biomedical applications ex vivo with treatment depths smaller than 10 mm from the lens surface. Although the low-megahertz frequency operation has the advantage of a longer range of therapy, this requires a proper photoacoustic lens made of a nanocomposite coating over a spherically curved substrate whose transmission layer is physically designed for frequency-tuned outputs. This demands a fabrication method that can provide such a nanocomposite structure. We demonstrate photoacoustic lenses operated in an unexplored frequency range of 1–10 MHz, which can simultaneously produce high-amplitude pressure outputs sufficient for pulsed acoustic cavitation. We physically design a spatially elongated photoacoustic output and then fabricate a transmitter by controlling the density of light-absorbing nanoscale elements in a solution form and by using a replica mold to shape the lens curvature. Our approach is validated by fabricating and characterizing planar transmitters and then applied to focal configurations. This offers various possibilities for LGFU-based treatments (e.g., pulsed cavitational therapy such as histotripsy) over the low-megahertz frequency range, which has not been realized by conventional LGFU systems.

Wave propagation control and switching for wireless power transfer using tunable 2-D magnetic metamaterials

Abstract Magnetic metamaterials operating at low megahertz frequencies provide various important commercial and research applications. In this work, we investigate the control of wave propagation in two-dimensional (2-D) tunable magnetic metamaterials for wireless power transfer (WPT). The propagation control is achieved by using reconfigurable defect cavities formed in the metamaterial, which allows for the dynamic creation of various waveguide configurations with switching control. The physical mechanism for creating the cavity is described using Fano interference, in which the resonant frequency of the cavity falls into the bandgap of the metasurface. And the routing and transmission control of the proposed waveguide is easily achieved by resonant switching. The proposed approach allows highly localized, strong field confinement in the deep subwavelength scale of 2.6λ × 10−3. The transmission losses and bandwidths of various dynamically tunable metamaterial waveguides are experimentally characterized. This result can find useful applications for integrated surface wave devices and planar WPT.

Evaluation of Near-Field Electromagnetic Shielding Effectiveness at Low Frequencies

Magnetic induction tomography (MIT) is a novel technology for flow measurement offering significant promise in the measurement of multiphase flows containing low-conductivity fluids such as saline water. Such measurements rely on optimal effective shielding to avoid external field interference and extraneous capacitive coupling that can lead to false readings and overestimations of the eddy current-induced fields. The performance of various attenuation materials in the low megahertz frequency spectrum is presented and compared with outcomes from a numerical computational method. The results demonstrate that the shielding mechanism that prevails at low frequencies is that of reflection. Consequently, hard shields such as metals show superior wave attenuation performance for MIT systems operating below 13 MHz. For higher frequencies, the absorption effect on the incident wave path within soft electromagnetic shields presents enhanced shielding properties. This paper also explores the limitations of traditional testing geometry for shielding effectiveness and proposes an alternative approach to near-field, free-space measurement for MIT sensors. The proposed semi-enclosed approach shows enhanced shielding effectiveness measurements compared with the traditional transversal barrier method. The proposed method was used to assess the electromagnetic shielding effectiveness of ferromagnetic and various metallic materials.

Impedance matching control method for an underwater magnetic resonance-based wireless power transfer system with metamaterials

In this paper, a new method of improving the efficiency of the underwater magnetic resonance-based wireless power transfer (MRWPT) system is proposed. A kind of the negative permeability (MNG) metamaterials with low megahertz frequency is designed, which can be adjusted to work well in the system. The S-parameters, resonant frequency, and permeability of metamaterials are worked out for analysis. A new impedance matching control method was also proposed for reducing the reflection loss in a way to help the sea water MRWPT system to achieve the maximum efficiency. This method is also able to reduce the unnecessary power loss caused by power fluctuation. The measured results show that the performance of the proposed metamaterials and the proposed control method are perfect in improving its efficiency in sea water.

Ultrasonic Relaxation Study of 1-Alkyl-3-methylimidazolium-Based Room-Temperature Ionic Liquids: Probing the Role of Alkyl Chain Length in the Cation

Ultrasound absorption spectra of four 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imides were determined as a function of the alkyl chain length on the cation from 1-propyl to 1-hexyl from 293.15 to 323.15 K at ambient pressure. Herein, the ultrasound absorption measurements were carried out using a standard pulse technique within a frequency range from 10 to 300 MHz. Additionally, the speed of sound, density, and viscosity have been measured. The presence of strong dissipative processes during the ultrasound wave propagation was found experimentally, i.e., relaxation processes in the megahertz range were observed for all compounds over the whole temperature range. The relaxation spectra (both relaxation amplitude and relaxation frequency) were shown to be dependent on the alkyl side chain length of the 1-alkyl-3-methylimidazolium ring. In most cases, a single-Debye model described the absorption spectra very well. However, a comparison of the determined spectra with the spectra of a few other imidazolium-based ionic liquids reported in the literature (in part recalculated in this work) shows that the complexity of the spectra increases rapidly with the elongation of the alkyl chain length on the cation. This complexity indicates that both the volume viscosity and the shear viscosity are involved in relaxation processes even in relatively low frequency ranges. As a consequence, the sound velocity dispersion is present at relatively low megahertz frequencies.

Experiments on multiple-receiver magnetic resonance-based wireless power transfer in low megahertz with metamaterials

In this paper, an efficient magnetic resonance-based wireless power transfer (MRWPT) system with metamaterials is proposed. The negative permeability (MNG) metamaterials for this system with low-megahertz frequency is designed, which can be adjusted to work well at a variable receiving angle ranging from 0° to 45° along z-direction. The S-parameters, resonant frequency and permeability of metamaterials are computed for analysis. The transmission efficiency of the multiple-receiver MRWPT system in free space is compared to that in the presence of metamaterials placed in front of transmission and receive coils. The measured results show that the performance of the proposed metamaterials is perfect in improving the efficiency with incident electromagnetic waves from various directions.

3-D Microtransformers for DC–DC On-Chip Power Conversion

We address the miniaturization of power converters by introducing novel 3-D microtransformers with magnetic core for low-megahertz frequency applications. The core is fabricated by lamination and microstructuring of Metglas 2714A magnetic alloy. The solenoids of the microtransformers are wound around the core using a ball-wedge wirebonder. The wirebonding process is fast, allowing the fabrication of solenoids with up to 40 turns in 10 s. The fabricated devices yield the high inductance per unit volume of 2.95 μH/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and energy per unit volume of 133 nJ/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> at the frequency of 1 MHz. The power efficiency of 64-76% is measured for different turns ratio with coupling factors as high as 98%. To demonstrate the applicability of our passive components, two PWM controllers were selected to implement an isolated and a nonisolated switch-mode power supply. The isolated converter operates with overall efficiency of 55% and maximum output power of 136 mW; then, we experimentally demonstrate how we increased this efficiency to 71% and output power to 408 mW. The nonisolated converter can deliver an overall efficiency of 81% with a maximum output power of 515 mW. Finally, we benchmarked the results to underline the potential of the technology for power on-chip applications.

A novel 6 × 6 element MEMS capacitive ultrasonic transducer with multiple moving membranes for high performance imaging applications

A 6×6 element multiple moving membrane capacitive micromachined ultrasonic transducer (M3-CMUT) array has been developed for air-coupled detection purposes in the low megahertz frequency range. Unlike conventional capacitive MEMS transducers, the developed transducer array employs a novel cell configuration where more than one flexible membrane contributes in the transducer signal transmission and detection. In this structure, a stack of two vibrating membranes is suspended over a fixed grounded bottom electrode. The transducer array has been fabricated through employing a MEMS sacrificial technique, with the top and middle membrane radii of 65 and 42μm, respectively. The electrical, optical and acoustic evaluation results of this transducer array are presented in this paper. It is shown that using this configuration, the amplitude of membrane displacement is increased which enhances the transducer power output and sensitivity for a given bias condition. The experimental results demonstrate that this transducer array structure can be highly beneficial in high performance imaging application systems.

A02 低MHz超音波によるU937細胞への生物学的効果(口頭発表)

A02 低MHz超音波によるU937細胞への生物学的効果(口頭発表)

Enhancement of Ostwald Ripening by Depletion Flocculation

The time dependence of the dynamic mobility and the ultrasonic attenuation of octane and decane oil-in-water emulsions stabilized by sodium dodecyl sulfate (SDS) was measured. The emulsions grew to larger droplets due to Ostwald ripening. The growth rate measured by attenuation depends on the surfactant concentration and the polydispersity of the emulsion. At surfactant concentrations below the critical micelle concentration (cmc) of SDS, the growth was linear with time and the rate was dependent on the polydispersity of the drops; the rate was several times faster than that predicted on the basis of a diffusion growth mechanism. Above the cmc, however, as the droplets grew in size there was a point at which the rate of growth increased, which corresponds to the droplet size at which depletion forces due to the surfactant micelles become significant. Under these conditions both the electroacoustic dynamic mobility and the acoustic attenuation spectra displayed characteristics of flocs: a large decrease in the phase lag at higher frequencies in the dynamic mobility spectrum and a decrease in the attenuation coefficient at low-megahertz frequencies with an increase at higher frequencies. This depletion flocculation enhancement in ripening rates in the presence of SDS micelles provides another, alternative, and self-consistent mechanism for the effect of surfactant micelles on Ostwald ripening.

1P-278 低MHz超音波によるU937細胞への生体影響(放射線生物/活性酸素,第46回日本生物物理学会年会)

1P-278 低MHz超音波によるU937細胞への生体影響(放射線生物/活性酸素,第46回日本生物物理学会年会)

Plane wave source with minimal harmonic distortion for investigating nonlinear acoustic properties

The objective of this investigation is to introduce and validate a practical ultrasound source to be used in the investigation of the nonlinear material properties of liquids and soft tissues studied in vitro. Methods based on the progressive distortion of finite amplitude ultrasonic waves in the low megahertz frequency range are most easily implemented under the assumption of plane wave propagation. However, achieving an approximately planar ultrasonic field over substantial propagation distances can be challenging. Furthermore, undesired harmonic distortion of the ultrasonic field prior to insonification of the specified region of interest represents another serious limitation. This paper introduces an approach based on the use of the ultrasonic field emanating from a stainless-steel delay line. Both simulation and direct experimental measurement demonstrate that such a field exhibits relatively planar wave fronts to a good approximation (such that a 3-mm-diam receiver would be exposed to no more than 3 dB of loss across its face) and is free from the significant harmonic distortion that would occur in a conventional water path.

Standard-target calibration of active sonars used to quantify aquatic organisms

The span of sonars being calibrated by the standard-target method continues to grow. In this retrospective of the past 25 years, the essence of the method is reviewed for both narrow-band and broadband systems. Applications to scientific echo sounders with operating frequencies from 18 kHz to 3.2 MHz are summarized. Applications to multibeam sonars that provide the water-column signal, with frequencies of 90, 200, 240, and 400 kHz, are also summarized. A planned application to a parametric sonar, with primary frequencies in the range 15–21 kHz and difference-frequency band 0.5–6 kHz, is described. The method is applicable to ordinary midfrequency sonars. In each example, particular standard targets are specified. Diameters vary from 10 mm for a sphere of tungsten carbide with 6% cobalt binder for use at low megahertz frequencies, to 60 mm for a sphere of electrolytic-grade copper for use at 38 kHz, to 280 mm for a sphere of aluminum alloy for use at low-kilohertz frequencies.

High-temperature piezoelectric film ultrasonic transducers by a sol-gel spray technique and their application to process monitoring of polymer injection molding

Thick-film (90 /spl mu/m) piezoelectric ceramic high-temperature ultrasonic transducers (HTUTs) have been successfully deposited on metallic substrates by a sol-gel spray technique. The gel is composed of fine powders of bismuth titanate dispersed in a lead-zirconate-titanate solution. The films with desired thickness have been obtained through multilayer coating approach. Piezoelectricity is achieved using the corona discharge poling method. The center frequencies of ultrasonic signals generated by these HTUTs are around 10 MHz and their signal-to-noise ratio (SNR) is more than 30 dB in pulse-echo mode at 500/spl deg/C. The main advantages of these new HTUTs are that they 1) are applicable at temperatures higher than 500/spl deg/C, 2) are miniature, 3) can be coated on flat and curved surfaces, 4) do not need ultrasonic couplant, 5) can be operated at low and medium megahertz frequency range with sufficient frequency bandwidth, and 6) have sufficient piezoelectric strength and SNR. The ability of the HTUTs to monitor the polymer injection molding process in real-time at the mold insert of the machine is demonstrated.

Application of Ultrafast Schottky Diodes to High Megahertz Chaotic Oscillators

The Schottky-barrier diodes are commonly classified as low-noise high-speed semiconductor devices for high frequency and microwave applications. When reverse-biased at the breakdown voltages they can be employed as noise sources characterized by an effective noise temperature from 10 K to 10 K [1]. In the present paper we describe high-power noise generator with a Schottky diode. Actually it is a dynamical noise (chaos) generator similar to the tunnel diode oscillator driven by external sinusoidal voltage [2, 3]. The main shortcoming of the externally driven (non-autonomous) systems is that sharp peaks do appear in the power spectra at the drive frequency and its higher harmonics. In contrast, the dynamical noise generator suggested in this paper is an autonomous one, providing essentially smoother power spectra. Two experimental prototypes of the oscillator operating at different fundamental frequencies f∗ = 1/2π √ LC have been investigated. The first one has been set at low megahertz frequency (f∗ ≈ 1 MHz), meanwhile the second one has been tuned to operate at high megahertz frequency (f∗ ≈ 100 MHz).

Vertically Laminated Magnetic Cores by Electroplating Ni-Fe Into Micromachined Si

The fabrication and characterization of vertically laminated, electrodeposited Ni-Fe magnetic cores in micromachined silicon for the low megahertz frequency range is presented. Laminated cores were fabricated by etching vertical trenches (60-180 /spl mu/m wide and 525 /spl mu/m deep) through silicon wafers and directly plating Ni-Fe onto the etched sidewalls. The Ni-Fe was plated over a range of thicknesses (3-53 /spl mu/m) in order to study the influence of eddy currents. The measured impedances in the range 10 kHz-40 MHz confirm the reduction of eddy current losses. This work demonstrates a simple fabrication method for achieving high-aspect-ratio, vertically laminated magnetic cores where the geometry of the structures can be easily tailored for various applications.

Acoustic impedance matching of piezoelectric transducers to the air

The purpose of this work is threefold: to investigate material requirements to produce impedance matching layers for air-coupled piezoelectric transducers, to identify materials that meet these requirements, and to propose the best solution to produce air-coupled piezoelectric transducers for the low megahertz frequency range. Toward this end, design criteria for the matching layers and possible configurations are reviewed. Among the several factors that affect the efficiency of the matching layer, the importance of attenuation is pointed out. A standard characterization procedure is applied to a wide collection of candidate materials to produce matching layers. In particular, some types of filtration membranes are studied. From these results, the best materials are identified, and the better matching configuration is proposed. Four pairs of air-coupled piezoelectric transducers also are produced to illustrate the performance of the proposed solution. The lowest two-way insertion loss figure is -24 dB obtained at 0.45 MHz. This increases for higher frequency transducers up to -42 dB at 1.8 MHz and -50 at 2.25 MHz. Typical bandwidth is about 15-20%.

The application of the acoustic spectrophonometer to biomolecular spectrometry: a step towards acoustic "fingerprinting".

A tunable acoustic biosensor for investigating the properties of biomolecules at the solid-liquid interfaces is described. In its current, format the device can be tuned to frequencies between 6.5 MHz and 1.1 GHz in order to provide a unique detection feature: a variable evanescent wave thickness at the sensor surface. The key to its successful implementation required the careful selection of antennae designs that could induce shear acoustic waves at the solid-liquid interface. This non-contact format makes it possible to recover resonant shear acoustic waves over 100 different harmonic frequencies as a result of the electrical characteristics of the spiral coil. For testing this multifrequency sensing concept the surface of a quartz disc was exposed to solutions of immunoglobulin G (IgG) to form an adsorbed monolayer, whence protein A and IgG were added again in order to form multilayers. Spectra at frequencies between 6 and 600 MHz were generated for each successive layer and revealed two characteristic phases: an initial phase at the low megahertz frequencies consistent with the conventional Sauerbrey relation, and a possible additional phase towards the high megahertz to gigahertz frequencies, that we believe relates to the structure of the biomolecular film. This two-phase behaviour evident from differences between high and low frequencies, rather than from any distinct frequency transition, was anticipated from the reduction in evanescent wave thickness down to nanometre dimensions, and thin film resonance phenomena that are known to occur for film and fluid systems. These measurements suggested that the single element acoustic biosensor we present here may form the basis from which to generate acoustic molecular spectra, or "acoustic fingerprints", in a manner akin to optical spectroscopy.

Microwave Dielectric Relaxation, Electrical Conductance, and Ultrasonic Relaxation of LiPF6 in Poly(ethylene oxide) Dimethyl Ether-500

Microwave dielectric complex permittivity ε* = ε‘ − Jε‘ ‘ in the frequency range ∼1−130 GHz and at temperatures of 25, 40, and 55 °C for the system LiPF6 dissolved in poly(ethylene oxide) dimethyl ether of average molar mass 500 (PEO-500) are reported. The dielectric spectra are interpreted by a Cole−Davidson distribution function for both the solvent and the solutions. The activation parameters ΔH⧧ and ΔS⧧ for the dielectric process have been determined for both solvent and solutions. Audio frequency electrical conductance of solutions of LiPF6 in PEO-500, in the concentration range ∼5 × 10-4 to 1.0 M (M = mol/dm3) and temperatures 25, 40, and 55 °C, have been determined. The conductance data in the diluted range have been analyzed by two forms of the Fuoss-Onsager conductance theory. Despite the presence of a minimum in the conductance vs concentration functions, the presence of conducting triple ions has not been postulated, because the minimum is predicted by the functional form of the conductance theory at low solvent permittivities, for moderate ion-pair associations. Ultrasonic relaxation spectra in the frequency range ∼1 to ∼400 MHz and LiPF6 concentration range 0.1−0.5 M at 25 °C are reported and analyzed in terms of the sum of two debye relaxation processes. The upper one centered around 150−200 MHz and common to the solvent PEO-500 is attributed to polymer-chain rearrangements, also influenced by the presence of the electrolyte. The lower relaxation process in the low megahertz frequency range is interpreted in terms of formation of dimers-ion pairs according to the process 2LiPF6 ⇌ (LiPF6)2, thus rationalizing the presence of a maximum in the static dielectric permitivity vs concentration.