Controlled Three-dimensional Rotation of Single Cells Using Acoustic Waves

Non-adhesive contrast substrate for single-cell trapping and Raman spectroscopic analysis

Droplet splitting by exploiting tailored surface wettability is emerging as an important pathway to creating ultralow volumes of samples that can have applications in bioassays, tissue engineering, protein chips, and material synthesis. Reduction of droplet volumes enables the encapsulation of single biological cells which allows high throughput screening. In this work, we demonstrate a facile fabrication approach to create a non-adhesive contrast quartz substrate that allows droplet splitting under gravitational force and its utilization to trap single biological cells for Raman spectroscopic studies. The non-adhesive contrast surface is created by low-power continuous-wave laser-assisted removal of the region of interest from a biocompatible non-adhesive silicone oil grafted quartz substrate. For a given laser spot dimension, the hydrophilic zone dimension is controlled via irradiation with varying laser powers. The fabricated non-adhesive contrast surface can split a microliter droplet into pico- and sub-picolitre daughter droplets. By using the substrate, the trapping of a single polystyrene bead is demonstrated and the recording of Raman spectra is carried out. Additionally, the Raman spectra of two biological cells, yeast cells and human mononuclear cells (MNCs), from a daughter droplet are recorded independently and from a mixture of the solutions. This single-cell Raman analysis could find applications in cell identification and type discrimination, biochemical imaging, metabolic and functional characterization, and clinical and toxicity studies.

Acoustic Propulsion of Nano- and Microcones: Dependence on the Viscosity of the Surrounding Fluid

This article investigates how the acoustic propulsion of cone-shaped colloidal particles that are exposed to a traveling ultrasound wave depends on the viscosity of the fluid surrounding the particles. Using acoustofluidic computer simulations, we found that the propulsion of such nano- and microcones decreases strongly and even changes sign for increasing shear viscosity. In contrast, we found only a weak dependence of the propulsion on the bulk viscosity. The obtained results are in line with the findings of previous theoretical and experimental studies.

Accurate and Automatic Extraction of Cell Self-Rotation Speed in an ODEP Field Using an Area Change Algorithm

Cells are complex biological units that can sense physicochemical stimuli from their surroundings and respond positively to them through characterization of the cell behavior. Thus, understanding the motions of cells is important for investigating their intrinsic properties and reflecting their various states. Computer-vision-based methods for elucidating cell behavior offer a novel approach to accurately extract cell motions. Here, we propose an algorithm based on area change to automatically extract the self-rotation of cells in an optically induced dielectrophoresis field. To obtain a clear and complete outline of the cell structure, dark corner removal and contrast stretching techniques are used in the pre-processing stage. The self-rotation speed is calculated by determining the frequency of the cell area changes in all of the captured images. The algorithm is suitable for calculating in-plane and out-of-plane rotations, while addressing the problem of identical images at different rotation angles when dealing with rotations of spherical and flat cells. In addition, the algorithm can be used to determine the motion trajectory of cells. The experimental results show that the algorithm can efficiently and accurately calculate cell rotation speeds of up to ~155 rpm. Potential applications of the proposed algorithm include cell morphology extraction, cell classification, and characterization of the cell mechanical properties. The algorithm can be very helpful for those who are interested in using computer vision and artificial-intelligence-based ideology in single-cell studies, drug treatment, and other bio-related fields.

Embedded Microbubbles for Acoustic Manipulation of Single Cells and Microfluidic Applications

Acoustically excited microstructures have demonstrated significant potential for small-scale biomedical applications by overcoming major microfluidic limitations. Recently, the application of oscillating microbubbles has demonstrated their superiority over acoustically excited solid structures due to their enhanced acoustic streaming at low input power. However, their limited temporal stability hinders their direct applicability for industrial or clinical purposes. Here, we introduce the embedded microbubble, a novel acoustofluidic design based on the combination of solid structures (poly(dimethylsiloxane)) and microbubbles (air-filled cavity) to combine the benefits of both approaches while minimizing their drawbacks. We investigate the influence of various design parameters and geometrical features through numerical simulations and experimentally evaluate their manipulation capabilities. Finally, we demonstrate the capabilities of our design for microfluidic applications by investigating its mixing performance as well as through the controlled rotational manipulation of individual HeLa cells.

3D mechanical characterization of single cells and small organisms using acoustic manipulation and force microscopy

Quantitative micromechanical characterization of single cells and multicellular tissues or organisms is of fundamental importance to the study of cellular growth, morphogenesis, and cell-cell interactions. However, due to limited manipulation capabilities at the microscale, systems used for mechanical characterizations struggle to provide complete three-dimensional coverage of individual specimens. Here, we combine an acoustically driven manipulation device with a micro-force sensor to freely rotate biological samples and quantify mechanical properties at multiple regions of interest within a specimen. The versatility of this tool is demonstrated through the analysis of single Lilium longiflorum pollen grains, in combination with numerical simulations, and individual Caenorhabditis elegans nematodes. It reveals local variations in apparent stiffness for single specimens, providing previously inaccessible information and datasets on mechanical properties that serve as the basis for biophysical modelling and allow deeper insights into the biomechanics of these living systems.

Contactless acoustic micro/nano manipulation: a paradigm for next generation applications in life sciences

Acoustic actuation techniques offer a promising tool for contactless manipulation of both synthetic and biological micro/nano agents that encompass different length scales. The traditional usage of sound waves has steadily progressed from mid-air manipulation of salt grains to sophisticated techniques that employ nanoparticle flow in microfluidic networks. State-of-the-art in microfabrication and instrumentation have further expanded the outreach of these actuation techniques to autonomous propulsion of micro-agents. In this review article, we provide a universal perspective of the known acoustic micromanipulation technologies in terms of their applications and governing physics. Hereby, we survey these technologies and classify them with regards to passive and active manipulation of agents. These manipulation methods account for both intelligent devices adept at dexterous non-contact handling of micro-agents, and acoustically induced mechanisms for self-propulsion of micro-robots. Moreover, owing to the clinical compliance of ultrasound, we provide future considerations of acoustic manipulation techniques to be fruitfully employed in biological applications that range from label-free drug testing to minimally invasive clinical interventions.

Rotational manipulation of a microscopic object inside a microfluidic channel

Observations and analyses of a microscopic object are essential processes in various fields such as chemical engineering and life science. Microfluidic techniques with various functions and extensions have often been used for such purposes to investigate the mechanical properties of microscopic objects such as biological cells. One of such extensions proposed in this context is a real-time visual feedback manipulation system, which is composed of a high-speed camera and a piezoelectric actuator with a single-line microfluidic channel. Although the on-chip manipulation system enables us to control the 1 degree-of-freedom position of a target object by the real-time pressure control, it has suffered from unintended changes in the object orientation, which is out of control in the previous system. In this study, we propose and demonstrate a novel shear-flow-based mechanism for the control of the orientation of a target object in addition to the position control in a microchannel to overcome the problem of the unintended rotation. We designed a tributary channel using a three-dimensional hydrodynamic simulation with boundary conditions appropriate for the particle manipulation to apply shear stress to the target particle placed at the junction and succeeded in rotating the particle at an angular velocity of 0.2 rad/s even under the position control in the experiment. The proposed mechanism would be applied to feedback controls of a target object in a microchannel to be in a desired orientation and at a desired position, which could be a universally useful function for various microfluidic platforms.

On-chip rotational manipulation of microbeads and oocytes using acoustic microstreaming generated by oscillating asymmetrical microstructures

The capability to precisely rotate cells and other micrometer-sized biological samples is invaluable in biomedicine, bioengineering, and biophysics. We propose herein a novel on-chip cell rotation method using acoustic microstreaming generated by oscillating asymmetrical microstructures. When the vibration is applied to a microchip with our custom-designed microstructures, two different modes of highly localized microvortices are generated that are utilized to precisely achieve in-plane and out-of-plane rotational manipulation of microbeads and oocytes. The rotation mechanism is studied and verified using numerical simulations. Experiments of the microbeads are conducted to evaluate the claimed functions and investigate the effects of various parameters, such as the frequency and the driving voltage on the acoustically induced flows. Accordingly, it is shown that the rotational speed and direction can be effectively tuned on demand in single-cell studies. Finally, the rotation of swine oocytes is involved as further applications. By observing the maturation stages of M2 after the exclusion of the first polar body of operated oocytes, the proposed method is proved to be noninvasive. Compared with the conventional approaches, our acoustofluidic cell rotation approach can be simple-to-fabricate and easy-to-operate, thereby allowing rotations irrespective of the physical properties of the specimen under investigation.

Automatic System for the Blastocyst Embryo Manipulation and Rotation

Cell manipulation plays a vital role in the success rate and efficiency of the cell microsurgical operations, including biopsy of cell internal organelles such as the embryo biopsy, in which the embryo is manipulated and reoriented safely to a predefined desired position and orientation. In this paper, a simplified approach for the blastocyst embryo reorientation is proposed. It utilizes conventional tools and techniques currently in use in manual approaches in research labs and In Vitro Fertilization clinics, and controls the process using a vision feedback system. An experimental setup is developed to verify the dynamic behavior of the proposed approach, in which a stationary holding micropipette is used to hold the embryo, which is then rotated in two coordinate directions through friction contact with a moving substrate, in our case a glass microscope slide. The embryo rotates on the holding micropipette tip, due to the relatively low friction of this contact. A computer vision algorithm is used to estimate the embryo orientation coordinates, and use this information as a feedback signal to a simple proportional controller to control the embryo rotation angle. Experimental results demonstrate that the system is capable of cell rotation in two independent coordinates, suitable for embryo microsurgical task execution.

Assembling and rotating erythrocyte aggregates by acoustofluidic pressure enabling full phase-contrast tomography

The combined use of ultrasound radiation and microfluidics is a promising tool for aiding the development of lab-on-a-chip devices. In this study, we show that the rotation of linear aggregates of micro-particles can be achieved under the action of acoustic field pressure. This novel manipulation is investigated by tracking polystyrene beads of different sizes through the 3D imaging features of digital holography (DH). From our analysis it is understood that the positioning of the micro-particles and their aggregations are associated with the effect of bulk acoustic radiation forces. The observed rotation is instead found to be compatible with the presence of acoustic streaming patterns as evidenced by our modelling and the resulting numerical simulation. Furthermore, the rotation frequency is shown to depend on the input voltage applied on the acoustic device. Finally, we demonstrate that we can take full advantage of such rotation by combining it with quantitative phase imaging of DH for a significant lab-on-a-chip biomedical application. In fact, we demonstrate that it is possible to put in rotation a linear aggregate of erythrocytes and rely on holographic imaging to achieve a full phase-contrast tomography of the aforementioned aggregate.

Acoustofluidic methods in cell analysis

Cellular analysis is a central concept for both biology and medicine. Over the past two decades, acoustofluidic technologies, which marry acoustic waves with microfluidics, have significantly contributed to the development of innovative approaches for cellular analysis. Acoustofluidic technologies enable precise manipulations of cells and the fluids that confine them, and these capabilities have been utilized in many cell analysis applications. In this review article, we examine various applications where acoustofluidic methods have been implemented, including cell imaging, cell mechanotyping, circulating tumor cell phenotyping, sample preparation in clinics, and investigation of cell-cell interactions and cell-environment responses. We also provide our perspectives on the technological advantages, limitations, and potential future directions for this innovative field of methods.

A Review of Automated Microinjection of Zebrafish Embryos

Cell microinjection is a technique of precise delivery of substances into cells and is widely used for studying cell transfection, signaling pathways, and organelle functions. Microinjection of the embryos of zebrafish, the third most important animal model, has become a very useful technique in bioscience. However, factors such as the small cell size, high cell deformation tendency, and transparent zebrafish embryo membrane make the microinjection process difficult. Furthermore, this process has strict, specific requirements, such as chorion softening, avoiding contacting the first polar body, and high-precision detection. Therefore, highly accurate control and detection platforms are critical for achieving the automated microinjection of zebrafish embryos. This article reviews the latest technologies and methods used in the automated microinjection of zebrafish embryos and provides a detailed description of the current developments and applications of robotic microinjection systems. The review covers key areas related to automated embryo injection, including cell searching and location, cell position and posture adjustment, microscopic visual servoing control, sensors, actuators, puncturing mechanisms, and microinjection.

On-Chip Tunable Cell Rotation Using Acoustically Oscillating Asymmetrical Microstructures

The precise rotational manipulation of cells and other micrometer-sized biological samples is critical to many applications in biology, medicine, and agriculture. We describe an acoustic-based, on-chip manipulation method that can achieve tunable cell rotation. In an acoustic field formed by the vibration of a piezoelectric transducer, acoustic streaming was generated using a specially designed, oscillating asymmetrical sidewall shape. We also studied the nature of acoustic streaming generation by numerical simulations, and our simulation results matched well with the experimental results. Trapping and rotation of diatom cells and swine oocytes were coupled using oscillating asymmetrical microstructures with different vibration modes. Finally, we investigated the relationship between the driving voltage and the speed of cell rotation, showing that the rotational rate achieved could be as large as approximately 1800 rpm. Using our device, the rotation rate can be effectively tuned on demand for single-cell studies. Our acoustofluidic cell rotation approach is simple, compact, non-contact, and biocompatible, permitting rotation irrespective of the optical, magnetic, or electrical properties of the specimen under investigation.

Enhancement of plant leaf transpiration with effective use of surface acoustic waves: effect of wave frequency

Water transport in vascular plants provides remarkable opportunities for various engineering applications due to its highly efficient and powerless transportability. Several previous studies were conducted to regulate the biological responses of plants using noninvasive audible or ultrasound waves. However, the control mechanism of acoustic stimuli applied to plants has not been investigated yet. Thus, the practical application of these stimuli to real plants still exhibits technological limitations. This study experimentally investigated the effects of surface acoustic wave (SAW) frequency on plant transpiration to understand the acoustic-activated leaf transpiration and utilize the advantages of SAW. We captured consecutive images of the enhanced water transport in the test plant (Epipremnum aureum) by SAW at three different frequencies (10, 15, and 20 MHz). The dye solution at 15 MHz SAW presented the highest intensity value after 40 min of SAW stimulation. The excitation areas for 15 and 20 MHz SAWs were decreased to 42.3% and 22.6%, respectively, compared with that of 10 MHz SAW. The transpiration rates were directly measured to compare water transport enhancement quantitatively when different SAW frequencies were applied to the same plant leaves. The water transport in the leaves was maximized at 15 MHz SAW, regardless of excitation area.

Plant leaf transpiration was enhanced with effective use of SAW activation. The effects on leaf transpiration enhancement were analyzed in detail. The results could be applied to regulate temperature and relative humidity effectively.

Accurate Extraction of the Self-Rotational Speed for Cells in an Electrokinetics Force Field by an Image Matching Algorithm

We present an image-matching-based automated algorithm capable of accurately determining the self-rotational speed of cancer cells in an optically-induced electrokinetics-based microfluidic chip. To automatically track a specific cell in a video featuring more than one cell, a background subtraction technique was used. To determine the rotational speeds of cells, a reference frame was automatically selected and curve fitting was performed to improve the stability and accuracy. Results show that the algorithm was able to accurately calculate the self-rotational speeds of cells up to ~150 rpm. In addition, the algorithm could be used to determine the motion trajectories of the cells. Potential applications for the developed algorithm include the differentiation of cell morphology and characterization of cell electrical properties.