Manipulation and Mixing of 200 Femtoliter Droplets in Nanofluidic Channels Using MHz-Order Surface Acoustic Waves

Acoustofluidic manipulation for submicron to nanoparticles

Particles, ranging from submicron to nanometer scale, can be broadly categorized into biological and non-biological types. Submicron-to-nanoscale bioparticles include various bacteria, viruses, liposomes, and exosomes. Non-biological particles cover various inorganic, metallic, and carbon-based particles. The effective manipulation of these submicron to nanoparticles, including their separation, sorting, enrichment, assembly, trapping, and transport, is a fundamental requirement for different applications. Acoustofluidics, owing to their distinct advantages, have emerged as a potent tool for nanoparticle manipulation over the past decade. Although recent literature reviews have encapsulated the evolution of acoustofluidic technology, there is a paucity of reports specifically addressing the acoustical manipulation of submicron to nanoparticles. This article endeavors to provide a comprehensive study of this topic, delving into the principles, apparatus, and merits of acoustofluidic manipulation of submicron to nanoparticles, and discussing the state-of-the-art developments in this technology. The discourse commences with an introduction to the fundamental theory of acoustofluidic control and the forces involved in nanoparticle manipulation. Subsequently, the working mechanism of acoustofluidic manipulation of submicron to nanoparticles is dissected into two parts, dominated by the acoustic wave field and the acoustic streaming field. A critical analysis of the advantages and limitations of different acoustofluidic platforms in nanoparticles control is presented. The article concludes with a summary of the challenges acoustofluidics face in the realm of nanoparticle manipulation and analysis, and a forecast of future development prospects.

A Miniaturized Wireless Micropump Enabled by Confined Acoustic Streaming

Miniaturization of health care, biomedical, and chemical systems is highly desirable for developing point-of-care testing (POCT) technologies. In system miniaturization, micropumps represent one of the major bottlenecks due to their undesirable pumping performance at such small sizes. Here, we developed a microelectromechanical system fabricated acoustic micropump based on an ultrahigh-frequency bulk acoustic wave resonator. The concept of an inner-boundary-confined acoustic jet was introduced to facilitate unidirectional flow. Benefitting from the high resonant frequency and confined acoustic streaming, the micropump reaches 32.620 kPa/cm3 (pressure/size) and 11.800 ml/min∙cm3 (flow rate/size), showing a 2-order-of-magnitude improvement in the energy transduction efficiency compared with the existing acoustic micropumps. As a proof of concept, the micropump was constructed as a wearable and wirelessly powered integrated drug delivery system with a size of only 9×9×9 mm3 and a weight of 1.16 g. It was demonstrated for ocular disease treatment through animal experimentation and a human pilot test. With superior pumping performance, miniaturized pump size, ultralow power consumption, and complementary metal-oxide-semiconductor compatibility, we expect it to be readily applied to various POCT applications including clinical diagnosis, prognosis, and drug delivery systems.

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.

Acoustofluidic patterning in glass capillaries using travelling acoustic waves based on thin film flexible platform

Surface acoustic wave (SAW) technology has been widely used to manipulate microparticles and biological species, based on acoustic radiation force (ARF) and drag force induced by acoustic streaming, either by standing SAWs (SSAWs) or travelling SAWs (TSAWs). These acoustofluidic patterning functions can be achieved within a polymer chamber or a glass capillary with various cross-sections positioned along the wave propagating paths. In this paper, we demonstrated that microparticles can be aligned, patterned, and concentrated within both circular and rectangular glass capillaries using TSAWs based on a piezoelectric thin film acoustic wave platform. The glass capillary was placed at different angles along with the interdigital transducer directions. We systematically investigated effects of tilting angles and wave characteristics using numerical simulations in both circular and square shaped capillaries, and the patterning mechanisms were discussed and compared with those agitated under the SSAWs. We then experimentally verified the particle patterns within different glass capillaries using thin film ZnO SAW devices on aluminum (Al) sheets. Results show that the propagating SAWs can generate acoustic pressures and patterns in the fluid due to the diffractive effects, drag forces and ARF, as functions of the SAW device's resonant frequency and tilting angle. We demonstrated potential applications using this multiplexing, integrated, and flexible thin film-based platform, including patterning particles (1) inside multiple and successively positioned circular tubes; (2) inside a solidified hydrogel in the glass capillary; and (3) by wrapping a flexible ZnO/Al SAW device around the glass capillary.

Multifunctional Droplets Formed by Interfacially Self-Assembled Fluorinated Magnetic Nanoparticles for Biocompatible Single Cell Culture and Magnet-Driven Manipulation

The ability to encapsulate and manipulate droplets with a picoliter volume of samples and reagents shows great potential for practical applications in chemistry, biology, and materials science. Magnetic control is a promising approach for droplet manipulation due to its ability for wireless control and its ease of implementation. However, it is challenged by the poor biocompatibility of magnetic materials in aqueous droplets. Moreover, current droplet technology is problematic because of the molecule leakage between droplets. In the paper, we propose multifunctional droplets with the surface coated by a layer of fluorinated magnetic nanoparticles for magnetically actuated droplet manipulation. Multifunctional droplets show excellent biocompatibility for cell culture, nonleakage of molecules, and high response to a magnetic field. We developed a strategy of coating the F-MNP@SiO2 on the outer surface of droplets instead of adding magnetic material into droplets to enable droplets with a highly magnetic response. The encapsulated bacteria and cells in droplets did not need to directly contact with the magnetic materials at the outer surface, showing high biocompatibility with living cells. These droplets can be precisely manipulated based on magnet distance, the time duration of the magnetic field, the droplet size, and the MNP composition, which well match with theoretical analysis. The precise magnetically actuated droplet manipulation shows great potential for accurate and sensitive droplet-based bioassays like single cell analysis.

On-demand liquid microlens arrays by non-contact relocation of inhomogeneous fluids in acoustic fields

Microlens arrays (MLAs) are key micro-optical components that possess a high degree of parallelism and ease of integration. However, rapid and low-cost fabrication of MLAs with flexible focusing remains a challenge. Herein, liquid MLAs with dynamic tunability are presented using non-contact acoustic relocation of inhomogeneous fluids. By designing ring-shaped acoustic pressure node (PN) arrays, the denser fluid of miscible liquids is relocated to PNs, and liquid MLAs with ideal morphology are obtained. The experimental results demonstrate that the liquid MLAs possess a powerful reconfigurability with long-term stability and sharp imaging that can conveniently switch between the on and off state and can dynamically magnify by simply adjusting the acoustic amplitude. Moreover, the high biocompatibility inherited from liquids accompanied by the acoustic treatment allows cells to be within working distance of the MLAs without immersion, as would be required for a solid lens. This innovative liquid MLA is inexpensive to manufacture and possesses continuous focus, fast response, and satisfactory bioaffinity, and thus offers promising potential for microfluidic adaptive imaging and biomedical sensing, especially for live cell imaging.

Droplet transportation by adjusting the temporal phase shift of surface acoustic waves in the exciter-exciter mode

Droplet actuation using Surface Acoustic Wave (SAW) technology has been widely employed in 'lab-on-a-chip' applications, such as for on-chip Polymerase Chain Reactions. The current strategy uses the exciter-absorber mode (exciting a single InterDigital Transducer, IDT) to form a pure Travelling Surface Acoustic Wave (TSAW) and to actuate the droplet, where the velocity and direction of the droplet can be adjusted by controlling the on-off and amplitude of the excitation signals applied to a pair of IDTs. Herein, in a way that is different from using the exciter-absorber mode, we propose a method of actuating droplets by using the exciter-exciter mode (exciting a pair of IDTs simultaneously), where the velocity and directional adjustment of the droplet can be realized by changing only one excitation parameter for the signals (the temporal phase shift, θ), and the droplet velocity can also be significantly improved. Specifically, we report for the first time the equation of the vibration of the mixed waves (TSAW and Standing Surface Acoustic Wave (SSAW)) formed on the substrate surface using the exciter-exciter mode. This is analyzed theoretically, where it is shown in this work that the amplitude and direction of the TSAW component of the mixed waves can be adjusted by changing θ. Following that, the velocity and directional adjustment of the droplet has been realized by changing θ and the improvement of the droplet velocity has been verified on a one-dimensional SAW device, using this exciter-exciter mode. Moreover a series of experiments on droplet transportation, along different trajectories in an x-y plane, has been carried out using a two-dimensional SAW device and this has demonstrated the effectiveness of the θ changing-based approach. Here this exciter-exciter mode provides an alternative method for the transportation of droplets in 'lab-on-a-chip' applications.

In vivo acoustic manipulation of microparticles in zebrafish embryos

In vivo micromanipulation using ultrasound is an exciting technology with promises for cancer research, brain research, vasculature biology, diseases, and treatment development. In the present work, we demonstrate in vivo manipulation of gas-filled microparticles using zebrafish embryos as a vertebrate model system. Micromanipulation methods often are conducted in vitro, and they do not fully reflect the complex environment associated in vivo. Four piezoelectric actuators were positioned orthogonally to each other around an off-centered fluidic channel that allowed for two-dimensional manipulation of intravenously injected microbubbles. Selective manipulation of microbubbles inside a blood vessel with micrometer precision was achieved without interfering with circulating blood cells. Last, we studied the viability of zebrafish embryos subjected to the acoustic field. This successful high-precision, in vivo acoustic manipulation of intravenously injected microbubbles offers potentially promising therapeutic options.

An investigation of maximum particle velocity as a universal invariant-Defined by a statistical measure of failure or plastic energy loss for acoustofluidic applications

Materials under vibration experience internal stress waves that can cause material failure or energy loss due to inelastic vibration. Traditionally, failure is defined in terms of material acceleration, yet this approach has many drawbacks, principally because it is not invariant with respect to scale, type of vibration, or material choice. Here, the likelihood of failure is instead considered in terms of the maximum vibration or particle velocity for various metals, polymers, and structural materials. The exact relationship between the maximum particle velocity and the maximum induced stress may be derived, but only if one knows the details of the vibration, material, flaws, and geometry. Statistical results with over thousands of individual trials are presented here to demonstrate a wide variety of vibrations across a sufficient variety of these choices. Failure in this context is defined as either fracture or plastic yield, the latter associated with inelastic deformation and energy loss during vibration. If the maximum permissible cyclical stress in material vibration is known, to at least an order of magnitude, the probability of this type of failure may be computed for a range of vibration velocities in each material. The results support the notion that a maximum particle velocity on the order of 1 m/s is a universal and critical limit that, upon exceeding, causes the probability of failure to become significant regardless of the details of the material, geometry, or vibration. We illustrate this in a specific example relevant to acoustofluidics, a simple surface acoustic wave device. The consequences of particle velocity limit analysis can effectively be used in materials and structural engineering to predict when dynamic material particle velocity can cause inelastic losses or failure via brittle fracture, plastic deformation, or fatigue failure.