Array-controlled ultrasonic manipulation of particles in planar acoustic resonator

Ultrasonic particle manipulation tools have many promising applications in life sciences, expanding on the capabilities of current manipulation technologies. In this paper, the ultrasonic manipulation of particles and cells along a microfluidic channel with a piezoelectric array is demonstrated. An array integrated into a planar multilayer resonator structure drives particles toward the pressure nodal plane along the centerline of the channel, then toward the acoustic velocity maximum centered above the subset of elements that are active. Switching the active elements along the array moves trapped particles along the microfluidic channel. A 12-element 1-D array coupled to a rectangular capillary has been modeled and fabricated for experimental testing. The device has a 300-μm-thick channel for a half-wavelength resonance near 2.5 MHz, with 500 μm element pitch. Simulation and experiment confirm the expected trapping of particles at the center of the channel and above the set of active elements. Experiments demonstrated the feasibility of controlling the position of particles along the length of the channel by switching the active array elements.     

Large-area manipulation of microdroplets by holographic optical tweezers based on a hybrid diffractive system

We report on large-area manipulation of microdroplets by holographic optical tweezers based on a hybrid diffractive system, in which a static computer-generated hologram and a spatial light modulator (SLM) are used. The hybrid diffractive system is useful to manipulate microdroplets on distant areas with the same manner. Experimental results demonstrated that microdroplets were transported successfully in parallel with approximately equivalent velocities over the entire manipulation area. Fusion of microdroplets was also achieved at a position where the optical pattern generated by the SLM alone did not reach.     

AC electrothermal manipulation of conductive fluids and particles for lab-chip applications

AC electrokinetics has shown great potential for microfluidic functions such as pumping, mixing and concentrating particles. So far, electrokinetics are typically applied on fluids that are not too conductive (<0.02 S/m), which excludes most biofluidic applications. To solve this problem, this paper seeks to apply AC electrothermal (ACET) effect to manipulate conductive fluids and particles within. ACET generates temperature gradients in the fluids, and consequently induces space charges that move in electric fields and produce microflows. This paper reports two new ACET devices, a parallel plate particle trap and an asymmetric electrode micropump. Preliminary experiments were performed on fluids with conductivity at 0.224 S/m. Particle trapping and micropumping were demonstrated at low voltages, reaching ~100 mum/s for no more than 8 Vrms at 200 kHz. The fluid velocity was found to depend on the applied voltage as V⁴, and the maxima were observed to be ~20 mum above the electrodes     

High trapping forces for high-refractive index particles trapped in dynamic arrays of counterpropagating optical tweezers

We demonstrate the simultaneous trapping of multiple high-refractive index (n > 2) particles in a dynamic array of counterpropagating optical tweezers in which the destabilizing scattering forces are canceled. These particles cannot be trapped in single-beam optical tweezers. The combined use of two opposing high-numerical aperture objectives and micrometer-sized high-index titania particles yields an at least threefold increase in both axial and radial trap stiffness compared to silica particles under the same conditions. The stiffness in the radial direction is obtained from measured power spectra; calculations are given for both the radial and the axial force components, taking spherical aberrations into account. A pair of acousto-optic deflectors allows for fast, computer-controlled manipulation of the individual trapping positions in a plane, while the method used to create the patterns ensures the possibility of arbitrarily chosen configurations. The manipulation of high-index particles finds its application in, e.g., creating defects in colloidal photonic crystals and in exerting high forces with low laser power in, for example, biophysical experiments.     

Rapid rotation of micron and submicron dielectric particles measured using optical tweezers

Abstract

We demonstrate the use of a laser trap (‘optical tweezers’) and back-focal-plane position detector to measure rapid rotation in aqueous solution of single particles with sizes in the vicinity of 1 μm. Two types of rotation were measured: electrorotation of polystyrene microspheres and rotation of the flagellar motor of the bacterium Vibrio alginolyticus. In both cases, speeds in excess of 1000 Hz (rev s^?1) were measured. Polystyrene beads of diameter about 1 μm labelled with smaller beads were held at the centre of a microelectrode array by the optical tweezers. Electrorotation of the labelled beads was induced by applying a rotating electric field to the solution using microelectrodes. Electrorotation spectra were obtained by varying the frequency of the applied field and analysed to obtain the surface conductance of the beads. Single cells of V. alginolyticus were trapped and rotation of the polar sodium-driven flagellar motor was measured. Cells rotated more rapidly in media containing higher concentrations of Na⁺, and photodamage caused by the trap was considerably less when the suspending medium did not contain oxygen. The technique allows single-speed measurements to be made in less than a second and separate particles can be measured at a rate of several per minute.     

Sorting of micron-sized particles using holographic optical Raman tweezers in aqueous medium

We have constructed a holographic optical tweezers system combined with Raman spectroscopy to sort trapped particles. Our software automatically moves the trapped objects to the measurement positions to obtain individual Raman signals from multiple trapped particles. We performed the sorting by comparing their spectra with the previously measured training dataset using the correlation coefficients. We used yeast cells and polystyrene beads as test particles. This study aims to show that biological particles can be separated using single cell analysis with combined holographic optical tweezers and Raman spectroscopy system.     

A time-efficient and accurate strain estimation concept for ultrasonic elastography using iterative phase zero estimation

In ultrasonic elastography, the exact estimation of temporal displacements between two signals is the key to estimating strain. An algorithm was previously proposed that estimates these displacements using phase differences of the corresponding base-band signals. A major advantage of these algorithms compared with correlation techniques is the computational efficiency. In this paper, an extension of the algorithm is presented that iteratively takes into account the time shifts of the signals to overcome the problems of aliasing and accuracy in the estimation of the phase shift. Thus, it can be proven that the algorithm is equivalent to the search of the maximum of the correlation function. Furthermore, a robust logarithmic compression is proposed that only compresses the envelope of the signal. This compression does not introduce systematic errors and significantly reduces decorrelation noise. The resulting algorithm is a computationally simple and very fast alternative to conventional correlation techniques, and the accuracy of strain images is improved.     

超声模拟中医针刺手法量化技术研究进展

传统针灸腧穴定位、行针手法依赖于医生的临床经验,医生不同治疗手段就不同,所造成的治疗效果也不同,具有不可重复性,限制了中医学学术传承。针刺手法是针刺效应系统中的一个组成要素,针刺手法刺激量与针刺效应密切相关,针刺手法量化对于促进针刺技术进步和评价针刺治疗效果具有重要意义。在对针刺手法量化研究的发展、超声在针灸上的应用及相控阵超声技术在针刺手法量化进行综述的基础上,对二维相控阵超声应用于针刺手法模拟及量化分析进行了探索性研究,认为利用二维超声相控阵通过电子控制实现声能在皮下聚焦的深度、角度和强度的任意调节,准确模拟针刺手法,为针刺手法的量化研究、治疗手段的重复再现、治疗效果的科学计量提供了新的思路。     

Controlled modulation of laser beam and dynamic patterning of colloidal particles using optical tweezers

We present controlled generation of complex-structured beam profiles using diffractive optical element and demonstrate multiple dynamic trapping of colloidal particles. The phase element is programmed to generate various tailored optical fields having structures, similar to that of number three, spiral, and circle but in a tractable manner. Thus, the generated spatially tailored optical fields are confined to focal volume in optical tweezers. This enabled real-time trapping of multiple microscopic objects whereby its transverse organization was controlled in a dynamic manner from one structure to another with the help of spatial light modulator. Such a controlled beam shaping finds potential applications in biophotonics, super resolution imaging, and measurement of biophysical parameters, cell sorting, and micro-manipulation of colloidal particles.     

Numerical micromagnetics of small particles

The results of applying the authors' micromagnetic code to a homogeneous spherical particle that is large enough to support inhomogeneous magnetization are given. For smaller particles, magnetization reversal is by coherent rotation. Larger particles initially exhibit curling as the applied magnetic field is reduced from a saturating value. Then one of two new behaviors is observed. For weak crystalline anisotropy, the axis of the curling state rotates and bends, and the magnetization reversal process is reversible, or nearly so. For strong crystalline anisotropy, a sudden discontinuous transition occurs to a vortex state with axis perpendicular to the anisotropy axis, and the vortex moves across the particle as reversal of the applied magnetic field continues. The formation and disappearance of the vortex are irreversible, but all other aspects of the process are reversible     

ECG triggering and gating for ultrasonic small animal imaging

Echocardiography (ECG) is routinely used in the clinical diagnosis of cardiac function. The anatomy of the mouse is similar to that of the human, and thus murine ECG has become an effective tool for the assessment of small animal models of human cardiac diseases. Unfortunately, clinical ultrasonic imaging systems are not suitable for murine cardiac imaging due to their limited spatial and temporal resolutions. Murine ECG requires a spatial resolution better than 100 pim, which mandates the use of high-frequency, ultrasonic imaging (i.e., >20 MHz). High-frequency transducer arrays currently are not available, and so such systems use the mechanical scanning of a single-element transducer for which the frame rate is insufficient for directly monitoring the rapid beating of a mouse heart, and thus retrospective image reconstruction is necessary. This paper presents a high-frequency, ultrasonic imaging system for murine cardiac imaging. Two scanning methods have been developed. One is based on ECG triggering and is called the block scanning mode, in which the murine cardiac images from the isovolumic contraction and isovolumic relaxation phases are retrospectively reconstructed within a relatively short data acquisition time using the ECG R-wave as the trigger to the imaging system. The other method is the line scanning mode based on ECG gating, in which both ECG and ultrasound scan lines are continuously acquired over a longer time, enabling images during the entire cardiac cycle to be obtained. It is demonstrated here that the effective frame rate is determined by the pulse repetition frequency and can be up to 2 kHz in the presented system.     

Optical tweezers for single cells

Optical tweezers (OT) have emerged as an essential tool for manipulating single biological cells and performing sophisticated biophysical/biomechanical characterizations. Distinct advantages of using tweezers for these characterizations include non-contact force for cell manipulation, force resolution as accurate as 100aN and amiability to liquid medium environments. Their wide range of applications, such as transporting foreign materials into single cells, delivering cells to specific locations and sorting cells in microfluidic systems, are reviewed in this article. Recent developments of OT for nanomechanical characterization of various biological cells are discussed in terms of both their theoretical and experimental advancements. The future trends of employing OT in single cells, especially in stem cell delivery, tissue engineering and regenerative medicine, are prospected. More importantly, current limitations and future challenges of OT for these new paradigms are also highlighted in this review.     

A vibrating tube method for evaluating flowability of a small amount of sample particles

To reduce the sample amount required for the measurement and evaluation of particle flowability, a simple method has been developed through experimental research. The principle of this measurement is based on the vibrating tube method. The sample particles used in the experiment were pulverized ZrO₂ and granulated WO₃ with different concentration of fine particles. After a small amount of sample particles were put into a tube, vibration was applied at a constant frequency and amplitude, and the mass of particles discharged from the tube was measured at constant time intervals. The experimental results showed that the minimum sample amount required for the measurement to evaluate the particle flowability was 10 g. The factors to evaluate the flowability in this simple vibrating tube method were the flow time and the characteristic mass flow rate.     

A new ultrasonic technique for detection and sizing of small cracks in studs and bolts

If small cracks in stud bolts are not detected early enough, they can grow rapidly and cause catastrophic disasters in industrial facilities such as nuclear power plants. Their detection, despite its importance, is known to be a very difficult problem due to the complicated structures of the stud bolts. This study shows a method of detecting the existence and determining the size of a small crack in a root between two crests in the bolt threads using ultrasound. The Rayleigh wave propagating from the tip of a crack to the opening of the same crack is utilized. A delayed pulse, due to the Rayleigh wave, is detected between regularly spaced pulses from the threads, with the delay time being proportional to the size of the crack. Theoretical explanation is presented and experimental results demonstrating detection of cracks as small as 0.5 mm are shown.     

Magnetic properties of small acicular particles

Using the optical techniques described previously, the mean dipole moments and anisotropy fields, as well as the switching field distributions were measured on gamma ferric oxide, cobalt-doped gamma ferric oxide and chromium dioxide particles suspended in a liquid. A new method for the determination of switching field distributions is described; its results agree with those of the older one. The particles were about 100 × 20 × 20 nm. The anisotropy, fields (for small reversible changes of the magnetization) ranged from 60 to 100 kA/m, i.e. much lower than expected for shape anisotropy. The observed mean switching fields (for large irreversible changes) were all between 63 and 75 kA/m, and appeared to be independent of particle size, shape or material.     

Convergence criteria for scattering models of ultrasonic wave propagation in suspensions of particles

Scattering models used to simulate the attenuation and phase velocity of an ultrasonic wave propagating through a suspension of particles involve the summation of an infinite series of partial waves. The accuracy of computation is influenced by the number of terms included in the harmonic series, and the number of terms required depends upon the scatterer size compared with wavelength. It is shown that the errors in modelled attenuation and phase velocity resulting from premature truncation can be significant when modelling higher values of particle diameter-frequency product. A useful and simple heuristic is presented, in which the number of terms in the summation of the infinite series needed for satisfactory convergence to a final value is a function of the particle diameter-frequency product and of the compressive wave velocity in the continuous phase