Rapid microscale in-gel processing and digestion of proteins using surface acoustic waves

Acoustohydrodynamic micromixers: Basic mixing principles, programmable mixing prospectives, and biomedical applications

Acoustohydrodynamic micromixers offer excellent mixing efficiency, cost-effectiveness, and flexible controllability compared with conventional micromixers. There are two mechanisms in acoustic micromixers: indirect influence by induced streamlines, exemplified by sharp-edge micromixers, and direct influence by acoustic waves, represented by surface acoustic wave micromixers. The former utilizes sharp-edge structures, while the latter employs acoustic wave action to affect both the fluid and its particles. However, traditional micromixers with acoustic bubbles achieve significant mixing performance and numerous programmable mixing platforms provide excellent solutions with wide applicability. This review offers a comprehensive overview of various micromixers, elucidates their underlying principles, and explores their biomedical applications. In addition, advanced programmable micromixing with impressive versatility, convenience, and ability of cross-scale operations is introduced in detail. We believe this review will benefit the researchers in the biomedical field to know the micromixers and find a suitable micromixing method for their various applications.

Tritoroidal particle rings formation in open microfluidics induced by standing surface acoustic waves

In this paper, the particle movements in a sessile droplet induced by standing surface acoustic waves (SSAWs) are studied. Tritoroidal particle rings are formed under the interaction of acoustic field and electric field. The experimental results demonstrate that the electric field plays an important role in patterning nanoparticles. The electric field can define the droplet shape due to electrowetting. When the droplet approximates a hemisphere, the acoustic radiation force induced by SSAWs drives the particles to form tritoroidal particle rings. When the droplet approximates a convex plate, the drag force induced by acoustic steaming drives the particle to move. The results will be useful for better understanding the nanoparticle movements in a sessile droplet, which is important to explain the mechanism that SSAWs enhance reaction and crystallization in droplet.

High Frequency Sonoprocessing: A New Field of Cavitation-Free Acoustic Materials Synthesis, Processing, and Manipulation

Ultrasound constitutes a powerful means for materials processing. Similarly, a new field has emerged demonstrating the possibility for harnessing sound energy sources at considerably higher frequencies (10 MHz to 1 GHz) compared to conventional ultrasound (⩽3 MHz) for synthesizing and manipulating a variety of bulk, nanoscale, and biological materials. At these frequencies and the typical acoustic intensities employed, cavitation-which underpins most sonochemical or, more broadly, ultrasound-mediated processes-is largely absent, suggesting that altogether fundamentally different mechanisms are at play. Examples include the crystallization of novel morphologies or highly oriented structures; exfoliation of 2D quantum dots and nanosheets; polymer nanoparticle synthesis and encapsulation; and the possibility for manipulating the bandgap of 2D semiconducting materials or the lipid structure that makes up the cell membrane, the latter resulting in the ability to enhance intracellular molecular uptake. These fascinating examples reveal how the highly nonlinear electromechanical coupling associated with such high-frequency surface vibration gives rise to a variety of static and dynamic charge generation and transfer effects, in addition to molecular ordering, polarization, and assembly-remarkably, given the vast dimensional separation between the acoustic wavelength and characteristic molecular length scales, or between the MHz-order excitation frequencies and typical THz-order molecular vibration frequencies.

Joule heating-induced particle manipulation on a microfluidic chip

We develop an electrokinetic technique that continuously manipulates colloidal particles to concentrate into patterned particulate groups in an energy efficient way, by exclusive harnessing of the intrinsic Joule heating effects. Our technique exploits the alternating current electrothermal flow phenomenon which is generated due to the interaction between non-uniform electric and thermal fields. Highly non-uniform electric field generates sharp temperature gradients by generating spatially-varying Joule heat that varies along the radial direction from a concentrated point hotspot. Sharp temperature gradients induce a local variation in electric properties which, in turn, generate a strong electrothermal vortex. The imposed fluid flow brings the colloidal particles at the centre of the hotspot and enables particle aggregation. Furthermore, maneuvering structures of the Joule heating spots, different patterns of particle clustering may be formed in a low power budget, thus opening up a new realm of on-chip particle manipulation process without necessitating a highly focused laser beam which is much complicated and demands higher power budget. This technique can find its use in Lab-on-a-chip devices to manipulate particle groups, including biological cells.

Surface acoustic wave (SAW)-induced synthesis of HKUST-1 with different morphologies and sizes

Metal organic frameworks (MOFs) are porous materials that have wide application prospects.

Surface acoustic wave devices for chemical sensing and microfluidics: A review and perspective

Surface acoustic waves (SAWs), are electro-mechanical waves that form on the surface of piezoelectric crystals. Because they are easy to construct and operate, SAW devices have proven to be versatile and powerful platforms for either direct chemical sensing or for upstream microfluidic processing and sample preparation. This review summarizes recent advances in the development of SAW devices for chemical sensing and analysis. The use of SAW techniques for chemical detection in both gaseous and liquid media is discussed, as well as recent fabrication advances that are pointing the way for the next generation of SAW sensors. Similarly, applications and progress in using SAW devices as microfluidic platforms are covered, ranging from atomization and mixing to new approaches to lysing and cell adhesion studies. Finally, potential new directions and perspectives on the field as it moves forward are offered, with a specific focus on potential strategies for making SAW technologies for bioanalytical applications.

Synchronization of Coupled Oscillators on a Two-Dimensional Plane

The effect of the transfer rate of signal molecules on coupled chemical oscillators arranged on a two-dimensional plane was systematically investigated in this paper. A microreactor equipped with a surface acoustic wave (SAW) mixer was applied to adjust the transfer rate of the signal molecules in the microreactor. The SAW mixer with adjustable input powers provided a simple means to generate different mixing rates in the microreactor. A robust synchronization of the oscillators was found at an input radio frequency power of 20 dBm, with which the chemical waves were initiated at a fixed site of the oscillator system. With increasing input power, the frequency of the chemical waves was increased, which agreed well with the prediction given by the time-delayed phase oscillator model. Results from the finite element simulation agreed well with the experimental results.

An emerging reactor technology for chemical synthesis: surface acoustic wave-assisted closed-vessel Suzuki coupling reactions

In this paper we demonstrate the use of an energy-efficient surface acoustic wave (SAW) device for driving closed-vessel SAW-assisted (CVSAW), ligand-free Suzuki couplings in aqueous media. The reactions were carried out on a mmolar scale with low to ultra-low catalyst loadings. The reactions were driven by heating resulting from the penetration of acoustic energy derived from RF Raleigh waves generated by a piezoelectric chip via a renewable fluid coupling layer. The yields were uniformly high and the reactions could be executed without added ligand and in water. In terms of energy density this new technology was determined to be roughly as efficient as microwaves and superior to ultrasound.

Enabling practical surface acoustic wave nebulizer drug delivery via amplitude modulation

A practical, commercially viable microfluidic device relies upon the miniaturization and integration of all its components--including pumps, circuitry, and power supply--onto a chip-based platform. Surface acoustic waves (SAW) have become popular in microfluidic manipulation, in solving the problems of microfluidic manipulation, but practical applications employing SAW still require more power than available via a battery. Introducing amplitude modulation at 0.5-40 kHz in SAW nebulization, which requires the highest energy input levels of all known SAW microfluidic processes, halves the power required to 1.5 W even while including the power in the sidebands, suitable for small lithium ion batteries, and maintains the nebulization rate, size, and size distributions vital to drug inhalation therapeutics. This simple yet effective means to enable an integrated SAW microfluidics device for nebulization exploits the relatively slow hydrodynamics and is furthermore shown to deliver shear-sensitive biomolecules--plasmid DNA and antibodies as exemplars of future pulmonary gene and vaccination therapies--undamaged in the nebulized mist. Altogether, the approach demonstrates a means to offer truly micro-scale microfluidics devices in a handheld, battery powered SAW nebulization device.

Fluid Mechanics, Arterial Disease, and Gene Expression

This review places modern research developments in vascular mechanobiology in the context of hemodynamic phenomena in the cardiovascular system and the discrete localization of vascular disease. The modern origins of this field are traced, beginning in the 1960s when associations between flow characteristics, particularly blood flow-induced wall shear stress, and the localization of atherosclerotic plaques were uncovered, and continuing to fluid shear stress effects on the vascular lining endothelial) cells (ECs), including their effects on EC morphology, biochemical production, and gene expression. The earliest single-gene studies and genome-wide analyses are considered. The final section moves from the ECs lining the vessel wall to the smooth muscle cells and fibroblasts within the wall that are fluid me chanically activated by interstitial flow that imposes shear stresses on their surfaces comparable with those of flowing blood on EC surfaces. Interstitial flow stimulates biochemical production and gene expression, much like blood flow on ECs.

Hydrogel discs for digital microfluidics

Hydrogels are networks of hydrophilic polymer chains that are swollen with water, and they are useful for a wide range of applications because they provide stable niches for immobilizing proteins and cells. We report here the marriage of hydrogels with digital microfluidic devices. Until recently, digital microfluidics, a fluid handling technique in which discrete droplets are manipulated electromechanically on the surface of an array of electrodes, has been used only for homogeneous systems involving liquid reagents. Here, we demonstrate for the first time that the cylindrical hydrogel discs can be incorporated into digital microfluidic systems and that these discs can be systematically addressed by droplets of reagents. Droplet movement is observed to be unimpeded by interaction with the gel discs, and gel discs remain stationary when droplets pass through them. Analyte transport into gel discs is observed to be identical to diffusion in cases in which droplets are incubated with gels passively, but transport is enhanced when droplets are continually actuated through the gels. The system is useful for generating integrated enzymatic microreactors and for three-dimensional cell culture. This paper demonstrates a new combination of techniques for lab-on-a-chip systems which we propose will be useful for a wide range of applications.

Three-dimensional continuous particle focusing in a microfluidic channel via standing surface acoustic waves (SSAW)

Three-dimensional (3D) continuous microparticle focusing has been achieved in a single-layer polydimethylsiloxane (PDMS) microfluidic channel using a standing surface acoustic wave (SSAW). The SSAW was generated by the interference of two identical surface acoustic waves (SAWs) created by two parallel interdigital transducers (IDTs) on a piezoelectric substrate with a microchannel precisely bonded between them. To understand the working principle of the SSAW-based 3D focusing and investigate the position of the focal point, we computed longitudinal waves, generated by the SAWs and radiated into the fluid media from opposite sides of the microchannel, and the resultant pressure and velocity fields due to the interference and reflection of the longitudinal waves. Simulation results predict the existence of a focusing point which is in good agreement with our experimental observations. Compared with other 3D focusing techniques, this method is non-invasive, robust, energy-efficient, easy to implement, and applicable to nearly all types of microparticles.

Microfluidic devices for bioapplications

Harnessing the ability to precisely and reproducibly actuate fluids and manipulate bioparticles such as DNA, cells, and molecules at the microscale, microfluidics is a powerful tool that is currently revolutionizing chemical and biological analysis by replicating laboratory bench-top technology on a miniature chip-scale device, thus allowing assays to be carried out at a fraction of the time and cost while affording portability and field-use capability. Emerging from a decade of research and development in microfluidic technology are a wide range of promising laboratory and consumer biotechnological applications from microscale genetic and proteomic analysis kits, cell culture and manipulation platforms, biosensors, and pathogen detection systems to point-of-care diagnostic devices, high-throughput combinatorial drug screening platforms, schemes for targeted drug delivery and advanced therapeutics, and novel biomaterials synthesis for tissue engineering. The developments associated with these technological advances along with their respective applications to date are reviewed from a broad perspective and possible future directions that could arise from the current state of the art are discussed.