Surface acoustic waves enable rotational manipulation of Caenorhabditis elegans

Design and optimization of an octuple-electrode array for micro-particle chain rotation via electrorotation integrated with machine vision technology

Microparticle rotation is an important process in biomedical engineering, such as biosensors, cell injection or cell morphology. Single particle rotation has been widely investigated, while rotation of particle chains has gained rare attention. In this paper, we utilize a noncontact manipulation method to rotate microparticle chains via electrorotation by designing an octuple-electrode array (OEA). Finite element simulations were conducted for analyzing the desired electrode field and optimizing the structure of microelectrode pairs. The direction of the electric field in the workspace is investigated with different voltage signal inputs through specially designed circuits. In the experiment, microparticles are driven to form several chains in the proposed electrode fields. With the rotation of the electric field, particle chains could be rotated synchronously. Automated rotation and detection of polystyrene microspheres and yeast cell chains are achieved using machine vision technology. Results show that the proposed method could be utilized to rotate ordered microparticles with an appropriate input signal.

The effect of common paralytic agents used for fluorescence imaging on redox tone and ATP levels in Caenorhabditis elegans

One aspect of Caenorhabditis elegans that makes it a highly valuable model organism is the ease of use of in vivo genetic reporters, facilitated by its transparent cuticle and highly tractable genetics. Despite the rapid advancement of these technologies, worms must be paralyzed for most imaging applications, and few investigations have characterized the impacts of common chemical anesthetic methods on the parameters measured, in particular biochemical measurements such as cellular energetics and redox tone. Using two dynamic reporters, QUEEN-2m for relative ATP levels and reduction-oxidation sensitive GFP (roGFP) for redox tone, we assess the impact of commonly used chemical paralytics. We report that no chemical anesthetic is entirely effective at doses required for full paralysis without altering redox tone or ATP levels, and that anesthetic use alters the detected outcome of rotenone exposure on relative ATP levels and redox tone. We also assess the use of cold shock, commonly used in combination with physical restraint methods, and find that cold shock does not alter either ATP levels or redox tone. In addition to informing which paralytics are most appropriate for research in these topics, we highlight the need for tailoring the use of anesthetics to different endpoints and experimental questions. Further, we reinforce the need for developing less disruptive paralytic methods for optimal imaging of dynamic in vivo reporters.

Acoustofluidic Actuation of Living Cells

Acoutofluidics is an increasingly developing and maturing technical discipline. With the advantages of being label-free, non-contact, bio-friendly, high-resolution, and remote-controllable, it is very suitable for the operation of living cells. After decades of fundamental laboratory research, its technical principles have become increasingly clear, and its manufacturing technology has gradually become popularized. Presently, various imaginative applications continue to emerge and are constantly being improved. Here, we introduce the development of acoustofluidic actuation technology from the perspective of related manipulation applications on living cells. Among them, we focus on the main development directions such as acoustofluidic sorting, acoustofluidic tissue engineering, acoustofluidic microscopy, and acoustofluidic biophysical therapy. This review aims to provide a concise summary of the current state of research and bridge past developments with future directions, offering researchers a comprehensive overview and sparking innovation in the field.

Ultrasound-induced reorientation for multi-angle optical coherence tomography

Organoid and spheroid technology provide valuable insights into developmental biology and oncology. Optical coherence tomography (OCT) is a label-free technique that has emerged as an excellent tool for monitoring the structure and function of these samples. However, mature organoids are often too opaque for OCT. Access to multi-angle views is highly desirable to overcome this limitation, preferably with non-contact sample handling. To fulfil these requirements, we present an ultrasound-induced reorientation method for multi-angle-OCT, which employs a 3D-printed acoustic trap inserted into an OCT imaging system, to levitate and reorient zebrafish larvae and tumor spheroids in a controlled and reproducible manner. A model-based algorithm was developed for the physically consistent fusion of multi-angle data from a priori unknown angles. We demonstrate enhanced penetration depth in the joint 3D-recovery of reflectivity, attenuation, refractive index, and position registration for zebrafish larvae, creating an enabling tool for future applications in volumetric imaging.

Cavity-agnostic acoustofluidic manipulations enabled by guided flexural waves on a membrane acoustic waveguide actuator

This article presents an in-depth exploration of the acoustofluidic capabilities of guided flexural waves (GFWs) generated by a membrane acoustic waveguide actuator (MAWA). By harnessing the potential of GFWs, cavity-agnostic advanced particle manipulation functions are achieved, unlocking new avenues for microfluidic systems and lab-on-a-chip development. The localized acoustofluidic effects of GFWs arising from the evanescent nature of the acoustic fields they induce inside a liquid medium are numerically investigated to highlight their unique and promising characteristics. Unlike traditional acoustofluidic technologies, the GFWs propagating on the MAWA's membrane waveguide allow for cavity-agnostic particle manipulation, irrespective of the resonant properties of the fluidic chamber. Moreover, the acoustofluidic functions enabled by the device depend on the flexural mode populating the active region of the membrane waveguide. Experimental demonstrations using two types of particles include in-sessile-droplet particle transport, mixing, and spatial separation based on particle diameter, along with streaming-induced counter-flow virtual channel generation in microfluidic PDMS channels. These experiments emphasize the versatility and potential applications of the MAWA as a microfluidic platform targeted at lab-on-a-chip development and showcase the MAWA's compatibility with existing microfluidic systems.

Bidirectional and Stepwise Rotation of Cells and Particles Using Induced Charge Electroosmosis Vortexes

The rotation of cells is of significant importance in various applications including bioimaging, biophysical analysis and microsurgery. Current methods usually require complicated fabrication processes. Herein, we proposed an induced charged electroosmosis (ICEO) based on a chip manipulation method for rotating cells. Under an AC electric field, symmetric ICEO flow microvortexes formed above the electrode surface can be used to trap and rotate cells. We have discussed the impact of ICEO and dielectrophoresis (DEP) under the experimental conditions. The capabilities of our method have been tested by investigating the precise rotation of yeast cells and K562 cells in a controllable manner. By adjusting the position of cells, the rotation direction can be changed based on the asymmetric ICEO microvortexes via applying a gate voltage to the gate electrode. Additionally, by applying a pulsed signal instead of a continuous signal, we can also precisely and flexibly rotate cells in a stepwise way. Our ICEO-based rotational manipulation method is an easy to use, biocompatible and low-cost technique, allowing rotation regardless of optical, magnetic or acoustic properties of the sample.

A vibrating capillary for ultrasound rotation manipulation of zebrafish larvae

Multifunctional micromanipulation systems have garnered significant attention due to the growing interest in biological and medical research involving model organisms like zebrafish (Danio rerio). Here, we report a novel acoustofluidic rotational micromanipulation system that offers rapid trapping, high-speed rotation, multi-angle imaging, and 3D model reconstruction of zebrafish larvae. An ultrasound-activated oscillatory glass capillary is used to trap and rotate a zebrafish larva. Simulation and experimental results demonstrate that both the vibrating mode and geometric placement of the capillary contribute to the developed polarized vortices along the long axis of the capillary. Given its capacities for easy-to-operate, stable rotation, avoiding overheating, and high-throughput manipulation, our system poses the potential to accelerate zebrafish-directed biomedical research.

Acoustic Actuators for the Manipulation of Micro/Nanorobots: State-of-the-Art and Future Outlooks

Compared to other actuating methods, acoustic actuators offer the distinctive capability of the contactless manipulation of small objects, such as microscale and nanoscale robots. Furthermore, they have the ability to penetrate the skin, allowing for the trapping and manipulation of micro/nanorobots that carry therapeutic agents in diverse media. In this review, we summarize the current progress in using acoustic actuators for the manipulation of micro/nanorobots used in various biomedical applications. First, we introduce the actuating method of using acoustic waves to manipulate objects, including the principle of operation and different types of acoustic actuators that are usually employed. Then, applications involving manipulating different types of devices are reviewed, including bubble-based microrobots, bubble-free robots, biohybrid microrobots, and nanorobots. Finally, we discuss the challenges and future perspectives for the development of the field.

Cell-Based Micro/Nano-Robots for Biomedical Applications: A Review

Micro/nano-robots are powerful tools for biomedical applications and are applied in disease diagnosis, tumor imaging, drug delivery, and targeted therapy. Among the various types of micro-robots, cell-based micro-robots exhibit unique properties because of their different cell sources. In combination with various actuation methods, particularly externally propelled methods, cell-based microrobots have enormous potential for biomedical applications. This review introduces recent progress and applications of cell-based micro/nano-robots. Different actuation methods for micro/nano-robots are summarized, and cell-based micro-robots with different cell templates are introduced. Furthermore, the review focuses on the combination of cell-based micro/nano-robots with precise control using different external fields. Potential challenges, further prospects, and clinical translations are also discussed.

Aerosol jet printing of surface acoustic wave microfluidic devices

The addition of surface acoustic wave (SAW) technologies to microfluidics has greatly advanced lab-on-a-chip applications due to their unique and powerful attributes, including high-precision manipulation, versatility, integrability, biocompatibility, contactless nature, and rapid actuation. However, the development of SAW microfluidic devices is limited by complex and time-consuming micro/nanofabrication techniques and access to cleanroom facilities for multistep photolithography and vacuum-based processing. To simplify the fabrication of SAW microfluidic devices with customizable dimensions and functions, we utilized the additive manufacturing technique of aerosol jet printing. We successfully fabricated customized SAW microfluidic devices of varying materials, including silver nanowires, graphene, and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). To characterize and compare the acoustic actuation performance of these aerosol jet printed SAW microfluidic devices with their cleanroom-fabricated counterparts, the wave displacements and resonant frequencies of the different fabricated devices were directly measured through scanning laser Doppler vibrometry. Finally, to exhibit the capability of the aerosol jet printed devices for lab-on-a-chip applications, we successfully conducted acoustic streaming and particle concentration experiments. Overall, we demonstrated a novel solution-based, direct-write, single-step, cleanroom-free additive manufacturing technique to rapidly develop SAW microfluidic devices that shows viability for applications in the fields of biology, chemistry, engineering, and medicine.

The effect of common paralytic agents used for fluorescence imaging on redox tone and ATP levels in Caenorhabditis elegans

One aspect of Caenorhabditis elegans that makes it a highly valuable model organism is the ease of use of in vivo genetic reporters, facilitated by its transparent cuticle and highly tractable genetics. Despite the rapid advancement of these technologies, worms must be paralyzed for most imaging applications, and few investigations have characterized the impacts of common chemical anesthetic methods on the parameters measured, in particular biochemical measurements such as cellular energetics and redox tone. Using two dynamic reporters, QUEEN-2m for relative ATP levels and reduction-oxidation sensitive GFP (roGFP) for redox tone, we assess the impact of commonly used chemical paralytics. We report that no chemical anesthetic is entirely effective at doses required for full paralysis without altering redox tone or ATP levels, though 100 mM 2,3-Butadione monoxime appears to be the least problematic. We also assess the use of cold shock, commonly used in combination with physical restraint methods, and find that cold shock does not alter either ATP levels or redox tone. In addition to informing which paralytics are most appropriate for research in these topics, we highlight the need for tailoring the use of anesthetics to different endpoints and experimental questions. Further, we reinforce the need for developing less disruptive paralytic methods for optimal imaging of dynamic in vivo reporters.

Acoustic tweezers for high-throughput single-cell analysis

Acoustic tweezers provide an effective means for manipulating single cells and particles in a high-throughput, precise, selective and contact-free manner. The adoption of acoustic tweezers in next-generation cellular assays may advance our understanding of biological systems. Here we present a comprehensive set of instructions that guide users through device fabrication, instrumentation setup and data acquisition to study single cells with an experimental throughput that surpasses traditional methods, such as atomic force microscopy and micropipette aspiration, by several orders of magnitude. With acoustic tweezers, users can conduct versatile experiments that require the trapping, patterning, pairing and separation of single cells in a myriad of applications ranging across the biological and biomedical sciences. This procedure is widely generalizable and adaptable for investigations in materials and physical sciences, such as the spinning motion of colloids or the development of acoustic-based quantum simulations. Overall, the device fabrication requires ~12 h, the experimental setup of the acoustic tweezers requires 1-2 h and the cell manipulation experiment requires ~30 min to complete. Our protocol is suitable for use by interdisciplinary researchers in biology, medicine, engineering and physics.

Manipulation with sound and vibration: A review on the micromanipulation system based on sub-MHz acoustic waves

Manipulation of micro-objects have been playing an essential role in biochemical analysis or clinical diagnostics. Among the diverse technologies for micromanipulation, acoustic methods show the advantages of good biocompatibility, wide tunability, a label-free and contactless manner. Thus, acoustic micromanipulations have been widely exploited in micro-analysis systems. In this article, we reviewed the acoustic micromanipulation systems that were actuated by sub-MHz acoustic waves. In contrast to the high-frequency range, the acoustic microsystems operating at sub-MHz acoustic frequency are more accessible, whose acoustic sources are at low cost and even available from daily acoustic devices (e.g. buzzers, speakers, piezoelectric plates). The broad availability, with the addition of the advantages of acoustic micromanipulation, make sub-MHz microsystems promising for a variety of biomedical applications. Here, we review recent progresses in sub-MHz acoustic micromanipulation technologies, focusing on their applications in biomedical fields. These technologies are based on the basic acoustic phenomenon, such as cavitation, acoustic radiation force, and acoustic streaming. And categorized by their applications, we introduce these systems for mixing, pumping and droplet generation, separation and enrichment, patterning, rotation, propulsion and actuation. The diverse applications of these systems hold great promise for a wide range of enhancements in biomedicines and attract increasing interest for further investigation.

Microfluidic viscometer by acoustic streaming transducers

Measurement of fluid viscosity represents a huge need for many biomedical and materials processing applications. Sample fluids containing DNA, antibodies, protein-based drugs, and even cells have become important therapeutic options. The physical properties, including viscosity, of these biologics are critical factors in the optimization of the biomanufacturing processes and delivery of therapeutics to patients. Here we demonstrate an acoustic microstreaming platform termed as microfluidic viscometer by acoustic streaming transducers (μVAST) that induces fluid transport from second-order microstreaming to measure viscosity. Validation of our platform is achieved with different glycerol content mixtures to reflect different viscosities and shows that viscosity can be estimated based on the maximum speed of the second-order acoustic microstreaming. The μVAST platform requires only a small volume of fluid sample (∼1.2 μL), which is 16-30 times smaller than that of commercial viscometers. In addition, μVAST can be scaled up for ultra-high throughput measurements of viscosity. Here we demonstrate 16 samples within 3 seconds, which is an attractive feature for automating the process flows in drug development and materials manufacturing and production.

Acoustic streaming enabled moderate swimming exercise reduces neurodegeneration in C. elegans

Regular physical exercise has been shown to delay and alleviate neurodegenerative diseases. Yet, optimum physical exercise conditions that provide neuronal protection and exercise-related factors remain poorly understood. Here, we create an Acoustic Gym on a chip through the surface acoustic wave (SAW) microfluidic technology to precisely control the duration and intensity of swimming exercise of model organisms. We find that precisely dosed swimming exercise enabled by acoustic streaming decreases neuronal loss in two different neurodegenerative disease models of Caenorhabditis elegans, a Parkinson's disease model and a tauopathy model. These findings highlight the importance of optimum exercise conditions for effective neuronal protection, a key characteristic of healthy aging in the elderly population. This SAW device also paves avenues for screening for compounds that can enhance or replace the beneficial effects of exercise and for identifying drug targets for treating neurodegenerative diseases.

A spiral microfluidic device for rapid sorting, trapping, and long-term live imaging of Caenorhabditis elegans embryos

Caenorhabditis elegans embryos have been widely used to study cellular processes and developmental regulation at early stages. However, most existing microfluidic devices focus on the studies of larval or adult worms rather than embryos. To accurately study the real-time dynamics of embryonic development under different conditions, many technical barriers must be overcome; these can include single-embryo sorting and immobilization, precise control of the experimental environment, and long-term live imaging of embryos. This paper reports a spiral microfluidic device for effective sorting, trapping, and long-term live imaging of single C. elegans embryos under precisely controlled experimental conditions. The device successfully sorts embryos from a mixed population of C. elegans at different developmental stages via Dean vortices generated inside a spiral microchannel and traps the sorted embryos at single-cell resolution through hydrodynamic traps on the sidewall of the spiral channel for long-term imaging. Through the well-controlled microenvironment inside the microfluidic device, the response of the trapped C. elegans embryos to mechanical and chemical stimulation can be quantitatively measured. The experimental results show that a gentle hydrodynamic force would induce faster growth of embryos, and embryos developmentally arrested in the high-salinity solution could be rescued by the M9 buffer. The microfluidic device provides new avenues for easy, rapid, high-content screening of C. elegans embryos.

Surface acoustic wave microfluidics for repetitive and reversible temporary immobilization of C. elegans

Caenorhabditis elegans is an important genetic model for neuroscience studies, used for analyses of how genes control connectivity, neuronal function, and behavior. To date, however, most studies of neuronal function in C. elegans are incapable of obtaining microscopy imaging with subcellular resolution and behavior analysis in the same set of animals. This constraint stems from the immobilization requirement for high-resolution imaging that is incompatible with behavioral analysis using conventional immobilization techniques. Here, we present a novel microfluidic device that uses surface acoustic waves (SAW) as a non-contact method to temporarily immobilize worms for a short period (30 seconds). We optimize the SAW based protocol for rapid switching between free-swimming and immobilized states, facilitating non-invasive analysis of swimming behavior as well as high-resolution synaptic imaging in the same animal. We find that the coupling of heat and acoustic pressure play a key role in the immobilization process. We introduce a proof-of-concept longitudinal study, illustrating that the device enables repeated imaging of fluorescently tagged synaptic receptors in command interneurons and analysis of swimming behavior in the same animals for three days. This longitudinal approach provides the first correlative analysis of synaptic glutamatergic receptors and swimming behavior in aging animals. We anticipate that this device will enable further longitudinal analysis of animal motility and subcellular morphological changes during development and aging in C. elegans.

Sonoporation: Past, Present, and Future

A surge of research in intracellular delivery technologies is underway with the increased innovations in cell-based therapies and cell reprogramming. Particularly, physical cell membrane permeabilization techniques are highlighted as the leading technologies because of their unique features, including versatility, independence of cargo properties, and high-throughput delivery that is critical for providing the desired cell quantity for cell-based therapies. Amongst the physical permeabilization methods, sonoporation holds great promise and has been demonstrated for delivering a variety of functional cargos, such as biomolecular drugs, proteins, and plasmids, to various cells including cancer, immune, and stem cells. However, traditional bubble-based sonoporation methods usually require special contrast agents. Bubble-based sonoporation methods also have high chances of inducing irreversible damage to critical cell components, lowering the cell viability, and reducing the effectiveness of delivered cargos. To overcome these limitations, several novel non-bubble-based sonoporation mechanisms are under development. This review will cover both the bubble-based and non-bubble-based sonoporation mechanisms being employed for intracellular delivery, the technologies being investigated to overcome the limitations of traditional platforms, as well as perspectives on the future sonoporation mechanisms, technologies, and applications.

Acousto-Pi: An Opto-Acoustofluidic System Using Surface Acoustic Waves Controlled With Open-Source Electronics for Integrated In-Field Diagnostics

Surface acoustic wave (SAW) devices are increasingly applied in life sciences, biology, and point-of-care applications due to their combined acoustofluidic sensing and actuating properties. Despite the advances in this field, there remain significant gaps in interfacing hardware and control strategies to facilitate system integration with high performance and low cost. In this work, we present a versatile and digitally controlled acoustofluidic platform by demonstrating key functions for biological assays such as droplet transportation and mixing using a closed-loop feedback control with image recognition. Moreover, we integrate optical detection by demonstrating in situ fluorescence sensing capabilities with a standard camera and digital filters, bypassing the need for expensive and complex optical setups. The Acousto-Pi setup is based on open-source Raspberry Pi hardware and 3-D printed housing, and the SAW devices are fabricated with piezoelectric thin films on a metallic substrate. The platform enables the control of droplet position and speed for sample processing (mixing and dilution of samples), as well as the control of temperature based on acousto-heating, offering embedded processing capability. It can be operated remotely while recording the measurements in cloud databases toward integrated in-field diagnostic applications such as disease outbreak control, mass healthcare screening, and food safety.

Acoustics at the nanoscale (nanoacoustics): A comprehensive literature review.: Part I: Materials, devices and selected applications

In the past decade, acoustics at the nanoscale (i.e., nanoacoustics) has evolved rapidly with continuous and substantial expansion of capabilities and refinement of techniques. Motivated by research innovations in the last decade, for the first time, recent advancements of acoustics-associated nanomaterials/nanostructures and nanodevices for different applications are outlined in this comprehensive review, which is written in two parts. As part I of this two part review, firstly, active and passive nanomaterials and nanostructures for acoustics are presented. Following that, representative applications of nanoacoustics including material property characterization, nanomaterial/nanostructure manipulation, and sensing, are discussed in detail. Finally, a summary is presented with point of views on the current challenges and potential solutions in this burgeoning field.

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.

Controllable fusion of human brain organoids using acoustofluidics

The fusion of human organoids holds promising potential in modeling physiological and pathological processes of tissue genesis and organogenesis. However, current fused organoid models face challenges of high heterogeneity and variable reproducibility, which may stem from the random fusion of heterogeneous organoids. Thus, we developed a simple and versatile acoustofluidic method to improve the standardization of fused organoid models via a controllable spatial arrangement of organoids. By regulating dynamic acoustic fields within a hexagonal acoustofluidic device, we can rotate, transport, and fuse one organoid with another in a contact-free, label-free, and minimal-impact manner. As a proof-of-concept to model the development of the human midbrain-to-forebrain mesocortical pathway, we acoustically fused human forebrain organoids (hFOs) and human midbrain organoids (hMOs) with the controllable alignment of neuroepithelial buds. We found that post-assembly, hMO can successfully project tyrosine hydroxylase neurons towards hFO, accompanied by an increase of firing rates and synchrony of excitatory neurons. Moreover, we found that our controllable fusion method can regulate neuron projection (e.g., range, length, and density), projection maturation (e.g., higher firing rate and synchrony), and neural progenitor cell (NPC) division in the assembloids via the initial spatial control. Thus, our acoustofluidic method may serve as a label-free, contact-free, and highly biocompatible tool to effectively assemble organoids and facilitate the standardization and robustness of organoid-based disease models and tissue engineering.

Acoustofluidic rotational tweezing enables high-speed contactless morphological phenotyping of zebrafish larvae

Modern biomedical research and preclinical pharmaceutical development rely heavily on the phenotyping of small vertebrate models for various diseases prior to human testing. In this article, we demonstrate an acoustofluidic rotational tweezing platform that enables contactless, high-speed, 3D multispectral imaging and digital reconstruction of zebrafish larvae for quantitative phenotypic analysis. The acoustic-induced polarized vortex streaming achieves contactless and rapid (~1 s/rotation) rotation of zebrafish larvae. This enables multispectral imaging of the zebrafish body and internal organs from different viewing perspectives. Moreover, we develop a 3D reconstruction pipeline that yields accurate 3D models based on the multi-view images for quantitative evaluation of basic morphological characteristics and advanced combinations of metrics. With its contactless nature and advantages in speed and automation, our acoustofluidic rotational tweezing system has the potential to be a valuable asset in numerous fields, especially for developmental biology, small molecule screening in biochemistry, and pre-clinical drug development in pharmacology.

A Pot-Like Vibrational Microfluidic Rotational Motor

Constructing a micro-sized microfluidic motor always involves the problem of how to transfer the mechanical energy out of the motor. The paper presents several experiments with pot-like microfluidic rotational motor structures driven by two perpendicular sine and cosine vibrations with amplitudes around 10 μm in the frequency region from 200 Hz to 500 Hz. The extensive theoretical research based on the mathematical model of the liquid streaming in a pot-like structure was the base for the successful real-life laboratory application of a microfluidic rotational motor. The final microfluidic motor structure allowed transferring the rotational mechanical energy out of the motor with a central axis. The main practical challenge of the research was to find the proper balance between the torque, due to friction in the bearings and the motor’s maximal torque. The presented motor, with sizes 1 mm by 0.6 mm, reached the maximal rotational speed in both directions between −15 rad/s to +14 rad/s, with the estimated maximal torque of 0.1 pNm. The measured frequency characteristics of vibration amplitudes and phase angle between the directions of both vibrational amplitudes and rotational speed of the motor rotor against frequency of vibrations, allowed us to understand how to build the pot-like microfluidic rotational motor.

Motile cells as probes for characterizing acoustofluidic devices

Acoustic microfluidics has emerged as a versatile solution for particle manipulation in medicine and biology. However, current technologies are largely confined to specialized research laboratories. The translation of acoustofluidics from research to clinical and industrial settings requires improved consistency and repeatability across different platforms. Performance comparisons will require straightforward experimental assessment tools that are not yet available. We introduce a method for characterizing acoustofluidic devices in real-time by exploiting the capacity of swimming microorganisms to respond to changes in their environment. The unicellular alga, Chlamydomonas reinhardtii, is used as an active probe to visualize the evolving acoustic pressure field within microfluidic channels and chambers. In contrast to more familiar mammalian cells, C. reinhardtii are simple to prepare and maintain, and exhibit a relatively uniform size distribution that more closely resembles calibration particles; however, unlike passive particles, these motile cells naturally fill complex chamber geometries and redistribute when the acoustic field changes or is turned off. In this way, C. reinhardtii cells offer greater flexibility than conventional polymer or glass calibration beads for in situ determination of device operating characteristics. To illustrate the technique, the varying spatial density and distribution of swimming cells are correlated to the acoustic potential to automatically locate device resonances within a specified frequency range. Peaks in the correlation coefficient of successive images not only identify the resonant frequencies for various geometries, but the peak shape can be related to the relative strength of the resonances. Qualitative mapping of the acoustic field strength with increasing voltage amplitude is also shown. Thus, we demonstrate that dynamically responsive C. reinhardtii enable real-time measurement and continuous monitoring of acoustofluidic device performance.

A microfluidic chip for on-line derivatization and application to in vivo neurochemical monitoring

Microfluidic chips can perform a broad range of automated fluid manipulation operations for chemical analysis including on-line reactions. Derivatization reactions carried out on-chip reduce manual sample preparation and improve experimental throughput. In this work we develop a chip for on-line benzoyl chloride derivatization coupled to microdialysis, an in vivo sampling technique. Benzoyl chloride derivatization is useful for the analysis of small molecule neurochemicals in complex biological matrices using HPLC-MS/MS. The addition of one or more benzoyl groups to small, polar compounds containing amines, phenols, thiols, and certain alcohols improves reversed phase chromatographic retention, electrospray ionization efficiency, and analyte stability. The current derivatization protocol requires a three-step manual sample preparation, which ultimately limits the utility of this method for rapid sample collection and large sample sets. A glass microfluidic chip was developed for derivatizing microdialysis fractions on-line as they exit the probe for collection and off-line analysis with HPLC-MS/MS. Calibration curves for 21 neurochemicals prepared using the on-chip method showed linearity (R² > 0.99), limits of detection (0.1-500 nM), and peak area RSDs (4-14%) comparable to manual derivatization. Method temporal resolution was investigated both in vitro and in vivo showing rapid rise times for all analytes, which was limited by fraction length (3 min) rather than the device. The platform was applied to basal measurements in the striatum of awake rats where 19 of 21 neurochemicals were above the limit of detection. For a typical 2 h study, a minimum of 120 pipetting steps are eliminated per animal. Such a device provides a useful tool for the analysis of small molecules in biological matrices which may extend beyond microdialysis to other sampling techniques.

Deterministic droplet coding via acoustofluidics

Droplet microfluidics has become an indispensable tool for biomedical research and lab-on-a-chip applications owing to its unprecedented throughput, precision, and cost-effectiveness. Although droplets can be generated and screened in a high-throughput manner, the inability to label the inordinate amounts of droplets hinders identifying the individual droplets after generation. Herein, we demonstrate an acoustofluidic platform that enables on-demand, real-time dispensing, and deterministic coding of droplets based on their volumes. By dynamically splitting the aqueous flow using an oil jet triggered by focused traveling surface acoustic waves, a sequence of droplets with deterministic volumes can be continuously dispensed at a throughput of 100 Hz. These sequences encode barcoding information through the combination of various droplet lengths. As a proof-of-concept, we encoded droplet sequences into end-to-end packages (e.g., a series of 50 droplets), which consisted of an address barcode with binary volumetric combinations and a sample package with consistent volumes for hosting analytes. This acoustofluidics-based, deterministic droplet coding technique enables the tagging of droplets with high capacity and high error-tolerance, and can potentially benefit various applications involving single cell phenotyping and multiplexed screening.

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.

Generating multifunctional acoustic tweezers in Petri dishes for contactless, precise manipulation of bioparticles

Acoustic tweezers are a promising technology for the biocompatible, precise manipulation of delicate bioparticles ranging from nanometer-sized exosomes to millimeter-sized zebrafish larva. However, their widespread usage is hindered by their low compatibility with the workflows in biological laboratories. Here, we present multifunctional acoustic tweezers that can manipulate bioparticles in a disposable Petri dish. Various functionalities including cell patterning, tissue engineering, concentrating particles, translating cells, stimulating cells, and cell lysis are demonstrated. Moreover, leaky surface acoustic wave-based holography is achieved by encoding required phases in electrode profiles of interdigitated transducers. This overcomes the frequency and resolution limits of previous holographic techniques to control three-dimensional acoustic beams in microscale. This study presents a favorable technique for noncontact and label-free manipulation of bioparticles in commonly used Petri dishes. It can be readily adopted by the biological and medical communities for cell studies, tissue generation, and regenerative medicine.

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.

Comprehensive Analysis of SiNPs on the Genome-Wide Transcriptional Changes in Caenorhabditis elegans

Background

Large-scale production and application of amorphous silica nanoparticles (SiNPs) have enhanced the risk of human exposure to SiNPs. However, the toxic effects and the underlying biological mechanisms of SiNPs on Caenorhabditis elegans remain largely unclear.

Purpose

This study was to investigate the genome-wide transcriptional alteration of SiNPs on C. elegans.

Methods and Results

In this study, a total number of 3105 differentially expressed genes were identified in C. elegans. Among them, 1398 genes were significantly upregulated and 1707 genes were notably downregulated in C. elegans. Gene ontology analysis revealed that the significant change of gene functional categories triggered by SiNPs was focused on locomotion, determination of adult lifespan, reproduction, body morphogenesis, multicellular organism development, endoplasmic reticulum unfolded protein response, oocyte development, and nematode larval development. Meanwhile, we explored the regulated effects between microRNA and genes or signaling pathways. Pathway enrichment analysis and miRNA-gene-pathway-network displayed that 23 differential expression microRNA including cel-miR-85-3p, cel-miR-793, cel-miR-241-5p, and cel-miR-5549-5p could regulate the longevity-related pathways and inflammation signaling pathways, etc. Additionally, our data confirmed that SiNPs could disrupt the locomotion behavior and reduce the longevity by activating ins-7, daf-16, ftt-2, fat-5, and rho-1 genes in C. elegans.

Conclusion

Our study showed that SiNPs induced the change of the whole transcriptome in C. elegans, and triggered negative effects on longevity, development, reproduction, and body morphogenesis. These data provide abundant clues to understand the molecular mechanisms of SiNPs in C. elegans.

Pitch-rotational manipulation of single cells and particles using single-beam thermo-optical tweezers

3D pitch rotation of microparticles and cells assumes importance in a wide variety of applications in biology, physics, chemistry and medicine. Applications such as cell imaging and injection benefit from pitch-rotational manipulation. Generation of such motion in single beam optical tweezers has remained elusive due to the complexities of generating high enough ellipticity perpendicular to the direction of propagation. Further, trapping a perfectly spherical object at two locations and subsequent pitch rotation hasn't yet been demonstrated to be possible. Here, we use hexagonal-shaped upconverting particles and single cells trapped close to a gold-coated glass cover slip in a sample chamber to generate complete 360 degree and continuous pitch motion even with a single optical tweezer beam. The tweezers beam passing through the gold surface is partially absorbed and generates a hot-spot to produce circulatory convective flows in the vicinity which rotates the objects. The rotation rate can be controlled by the intensity of the laser light. Thus such a simple configuration can turn the particle in the pitch sense. The circulatory flows in this technique have a diameter of about 5 μm which is smaller than those reported using acousto-fluidic techniques.

Acoustic Microfluidics

Acoustic microfluidic devices are powerful tools that use sound waves to manipulate micro- or nanoscale objects or fluids in analytical chemistry and biomedicine. Their simple device designs, biocompatible and contactless operation, and label-free nature are all characteristics that make acoustic microfluidic devices ideal platforms for fundamental research, diagnostics, and therapeutics. Herein, we summarize the physical principles underlying acoustic microfluidics and review their applications, with particular emphasis on the manipulation of macromolecules, cells, particles, model organisms, and fluidic flows. We also present future goals of this technology in analytical chemistry and biomedical research, as well as challenges and opportunities.

Determination of Dielectric Properties of Cells using AC Electrokinetic-based Microfluidic Platform: A Review of Recent Advances

Cell dielectric properties, a type of intrinsic property of cells, can be used as electrophysiological biomarkers that offer a label-free way to characterize cell phenotypes and states, purify clinical samples, and identify target cancer cells. Here, we present a review of the determination of cell dielectric properties using alternating current (AC) electrokinetic-based microfluidic mechanisms, including electro-rotation (ROT) and dielectrophoresis (DEP). The review covers theoretically how ROT and DEP work to extract cell dielectric properties. We also dive into the details of differently structured ROT chips, followed by a discussion on the determination of cell dielectric properties and the use of these properties in bio-related applications. Additionally, the review offers a look at the future challenges facing the AC electrokinetic-based microfluidic platform in terms of acquiring cell dielectric parameters. Our conclusion is that this platform will bring biomedical and bioengineering sciences to the next level and ultimately achieve the shift from lab-oriented research to real-world applications.

Fluorescence-based sorting of Caenorhabditis elegans via acoustofluidics

Effectively isolating and categorizing large quantities of Caenorhabditis elegans (C. elegans) based on different phenotypes is important for most worm research, especially genetics. Here we present an integrated acoustofluidic chip capable of identifying worms of interest based on expression of a fluorescent protein in a continuous flow and then separate them accordingly in a high-throughput manner. Utilizing planar fiber optics as the detection unit, our acoustofluidic device requires no temporary immobilization of worms for interrogation/detection, thereby improving the throughput. Implementing surface acoustic waves (SAW) as the sorting unit, our device provides a contact-free method to move worms of interest to the desired outlet, thus ensuring the biocompatibility for our chip. Our device can sort worms of different developmental stages (L3 and L4 stage worms) at high throughput and accuracy. For example, L3 worms can be processed at a throughput of around 70 worms per min with a sample purity over 99%, which remains over 90% when the throughput is increased to around 115 worms per min. In our acoustofluidic chip, the time period to complete the detection and sorting of one worm is only 50 ms, which outperforms nearly all existing microfluidics-based worm sorting devices and may be further reduced to achieve higher throughput.

Development and characterisation of acoustofluidic devices using detachable electrodes made from PCB

Acoustofluidics has been increasingly applied in biology, medicine and chemistry due to its versatility in manipulating fluids, cells and nano-/micro-particles. In this paper, we develop a novel and simple technology to fabricate a surface acoustic wave (SAW)-based acoustofluidic device by clamping electrodes made using a printed circuit board (PCB) with a piezoelectric substrate. The PCB-based SAW (PCB-SAW) device is systematically characterised and benchmarked with a SAW device made using the conventional photolithography process with the same specifications. Microparticle manipulations such as streaming in droplets and patterning in microchannels were demonstrated in the PCB-SAW device. In addition, the PCB-SAW device was applied as an acoustic tweezer to pattern lung cancer cells to form three or four traces inside the microchannel in a controllable manner. Cell viability of ∼97% was achieved after acoustic manipulation using the PCB-SAW device, which proved its ability as a suitable tool for acoustophoretic applications.

Acoustic tweezers based on circular, slanted-finger interdigital transducers for dynamic manipulation of micro-objects

Acoustic tweezing technologies are gaining significant attention from the scientific communities due to their versatility and biocompatibility. This study presents acoustic tweezers based on circular, slanted-finger interdigital transducers (CSFITs), which can steer the propagation direction of surface acoustic waves (SAWs) by tuning the excitation frequency. The CSFITs based acoustic tweezers enable dynamic and reconfigurable manipulation of micro-objects using multi-tone excitation signals. Compared to traditional interdigital transducers that generate and control SAWs along one axis, the CSFITs allow for simultaneously generating and independently controlling SAWs propagating along multiple axes by changing the frequency composition and the phase information in a multi-tone excitation signal. Moreover, the CSFITs based acoustic tweezers can be used for patterning cells/particles in various distributions and translating them along complex paths. We believe that our design is valuable for cellular-scale biological applications, in which on-chip, contactless, biocompatible handling of bioparticles is needed.

Dancing with the Cells: Acoustic Microflows Generated by Oscillating Cells

The interaction of a sound or ultrasound wave with an elastic object, such as a microbubble, can give rise to a steady-state microstreaming flow in its surrounding liquid. Many microfluidic strategies for cell and particle manipulation, and analyte mixing, are based on this type of flow. In addition, there are reports that acoustic streaming can be generated in biological systems, for instance, in a mammalian inner ear. Here, new observations are reported that individual cells are able to induce microstreaming flow, when they are excited by controlled acoustic waves in vitro. Single adherent cells are exposed to an acoustic field inside a microfluidic device. The cell-induced microstreaming is then investigated by monitoring flow tracers around the cell, while the structure and extracellular environment of the cell are altered using different chemicals. The observations suggest that the maximum streaming flow induced by an MDA-MB-231 breast cancer cell can reach velocities on the order of mm s^-1 , and this maximum velocity is primarily governed by the overall cell stiffness. Therefore, such cell-induced microstreaming measurements, including flow pattern and velocity magnitude, may be used as label-free proxies of cellular mechanical properties, such as stiffness.

Modeling and Analysis of the Two-Dimensional Axisymmetric Acoustofluidic Fields in the Probe-Type and Substrate-Type Ultrasonic Micro/Nano Manipulation Systems

The probe-type and substrate-type ultrasonic micro/nano manipulation systems have proven to be two kinds of powerful tools for manipulating micro/nanoscale materials. Numerical simulations of acoustofluidic fields in these two kinds of systems can not only be used to explain and analyze the physical mechanisms of experimental phenomena, but also provide guidelines for optimization of device parameters and working conditions. However, in-depth quantitative study and analysis of acoustofluidic fields in the two ultrasonic micro/nano manipulation systems have scarcely been reported. In this paper, based on the finite element method (FEM), we numerically investigated the two-dimensional (2D) axisymmetric acoustofluidic fields in the probe-type and substrate-type ultrasonic micro/nano manipulation systems by the perturbation method (PM) and Reynolds stress method (RSM), respectively. Through comparing the simulation results computed by the two methods and the experimental verifications, the feasibility and reasonability of the two methods in simulating the acoustofluidic fields in these two ultrasonic micro/nano manipulation systems have been validated. Moreover, the effects of device parameters and working conditions on the acoustofluidic fields are clarified by the simulation results and qualitatively verified by the experiments.

A Microfluidic Rotational Motor Driven by Circular Vibrations

Constructing micro-sized machines always involves the problem of how to bring the energy (electric, magnetic, light, electro wetting, vibrational, etc.) source to the device to produce mechanical movements. The paper presents a rotational micro-sized motor (the diameter of the rotor is 350 µm) driven by low frequency (200–700 Hz) circular vibrations, made by two piezoelectric actuators, through the medium of a water droplet with diameter of 1 mm (volume 3.6 µL). The theoretical model presents how to produce the circular streaming (rotation) of the liquid around an infinitely long pillar with micro-sized diameter. The practical application has been focused to make a time-stable circular stream of the medium around the finite long vibrated pillar with diameter of 80 µm in the presence of disturbances produced by the vibrated plate where the pillar is placed. Only the time-stable circular stream in the water droplet around the pillar produces enough energy to rotate the micro-sized rotor. The rotational speed of the rotor is controlled in both directions from −20 rad/s to +26 rad/s. 3D printed mechanical amplifiers of vibrations, driven by piezoelectric actuators, amplify the amplitude of the piezoelectric actuator up to 20 µm in the frequency region of 200 to 700 Hz.

Modelling of SAW-PDMS acoustofluidics: physical fields and particle motions influenced by different descriptions of the PDMS domain

In modelling acoustofluidic chips actuated by surface acoustic waves (SAWs) and using polydimethylsilane (PDMS) as a channel material, reduced models are often adopted to describe the acoustic behaviors of PDMS. Here, based on a standing SAW (SSAW) acoustophoresis chip, we compared three different descriptions of a PDMS chamber and looked into in-chamber physical fields and ensuing particle motion processes through finite element (FE) simulations. Specifically, the PDMS domain was considered as an elastic solid material, a non-flow fluid, and a lossy wall, respectively. The major findings include: (a) the shear waves that propagated in a solid PDMS wall did not influence the in-chamber pressure and ARF fields severely, but induced an observable difference in the acoustic streaming (AS) patterns, and distinctly changed the trajectories of polystyrene particles, especially those whose radii were below 1.5 μm; and (b) the equivalent damping coefficients were linearly dependent on the SAW frequency, characterized by a fixed loss per wavelength, indicating the wave leakage at the interface being the main source of the transmission loss of SAWs. Meanwhile, the acoustic radiation force (ARF) can be overestimated when describing PDMS as a lossy wall, especially at the bottom corners of the chamber, which could cause inaccurate predictions of the motion of big particles. Based on the damping mechanism, a rough protocol is provided for scaling of pressure fields between different models. Some suggestions for device designs and operations are also given based on the obtained findings.

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.