An intra-droplet particle switch for droplet microfluidics using bulk acoustic waves

Generation, control, and application of stable bubbles in a hypersonic acoustic system

Bubble-based microfluidics has been applied in many fields. However, there remains a need for a facile and flexible method for stable bubble generation and control in a microchannel. This paper reports a hypersonic acoustic system that can generate and release functional stable bubbles in a microchannel in an on-demand manner. It was found that the hypersonic frequency in this system played a vital role in the generation and control of bubbles. Specifically, a nanostructurally enhanced acoustic resonator was used to generate highly localized ultrahigh-frequency acoustic waves that ensured the feasibility and rapidity of bubble generation. Simultaneously, the acoustothermal effect of hypersound was harnessed to effectuate precise control over the bubble size. In addition, high-throughput droplet splitting was performed to confirm the stability of bubbles and their functionality in micromanipulation. The results showed that a mother droplet could be split controllably into a desired number of daughter droplets with specific volume ratios. In summary, the hypersonic acoustic system was demonstrated to be capable of on-demand-generation of stable bubbles in a microfluidic context and thus may extend the bubble-based applications.

Yield stress-enabled microencapsulation of field responsive microparticle suspensions

Try and encapsulate microparticles inside the cores of microcapsules and you will often find that particles adhere to the liquid-liquid interface in a phenomenon known as Pickering stabilization. Particles will remain irreversibly trapped and embedded within the subsequently formed microcapsule membrane. In cases where the encapsulant particles must remain suspended inside the microcapsule core to retain their desired properties or behaviours, Pickering stabilization is detrimental. Here we demonstrate a general procedure using yield stress materials as the core material, where the yield stress of the gel is strong enough to suspend particles against sedimentation, but weak enough to allow spatial manipulation of encapsulant particles using an external field. This external field imparts enough force on particles to disrupt the supporting network and allow particle mobility after encapsulation.

A review on microfluidic-assisted nanoparticle synthesis, and their applications using multiscale simulation methods

Recent years have witnessed an increased interest in the development of nanoparticles (NPs) owing to their potential use in a wide variety of biomedical applications, including drug delivery, imaging agents, gene therapy, and vaccines, where recently, lipid nanoparticle mRNA-based vaccines were developed to prevent SARS-CoV-2 causing COVID-19. NPs typically fall into two broad categories: organic and inorganic. Organic NPs mainly include lipid-based and polymer-based nanoparticles, such as liposomes, solid lipid nanoparticles, polymersomes, dendrimers, and polymer micelles. Gold and silver NPs, iron oxide NPs, quantum dots, and carbon and silica-based nanomaterials make up the bulk of the inorganic NPs. These NPs are prepared using a variety of top-down and bottom-up approaches. Microfluidics provide an attractive synthesis alternative and is advantageous compared to the conventional bulk methods. The microfluidic mixing-based production methods offer better control in achieving the desired size, morphology, shape, size distribution, and surface properties of the synthesized NPs. The technology also exhibits excellent process repeatability, fast handling, less sample usage, and yields greater encapsulation efficiencies. In this article, we provide a comprehensive review of the microfluidic-based passive and active mixing techniques for NP synthesis, and their latest developments. Additionally, a summary of microfluidic devices used for NP production is presented. Nonetheless, despite significant advancements in the experimental procedures, complete details of a nanoparticle-based system cannot be deduced from the experiments alone, and thus, multiscale computer simulations are utilized to perform systematic investigations. The work also details the most common multiscale simulation methods and their advancements in unveiling critical mechanisms involved in nanoparticle synthesis and the interaction of nanoparticles with other entities, especially in biomedical and therapeutic systems. Finally, an analysis is provided on the challenges in microfluidics related to nanoparticle synthesis and applications, and the future perspectives, such as large-scale NP synthesis, and hybrid formulations and devices.

Programmable Droplet Microfluidics Based on Machine Learning and Acoustic Manipulation

Typical microfluidic devices are application-specific and have to be carefully designed to implement the necessary functionalities for the targeted application. Programmable microfluidic chips try to overcome this by offering reconfigurable functionalities, allowing the same chip to be used in multiple different applications. In this work, we demonstrate a programmable microfluidic chip for the two-dimensional manipulation of droplets, based on ultrasonic bulk acoustic waves and a closed-loop machine-learning-based control algorithm. The algorithm has no prior knowledge of the acoustic fields but learns to control the droplets on the fly. The manipulation is based on switching the frequency of a single ultrasonic transducer. Using this method, we demonstrate 2D transportation and merging of water droplets in oil and oil droplets in water, and we performed the chemistry that underlies the basis of a colorimetric glucose assay. We show that we can manipulate drops with volumes ranging from ∼200 pL up to ∼30 nL with our setup. We also demonstrate that our method is robust, by changing the system parameters and showing that the machine learning algorithm can still complete the manipulation tasks. In short, our method uses ultrasonics to flexibly manipulate droplets, enabling programmable droplet microfluidic devices.

A droplet acoustofluidic platform for time-controlled microbead-based reactions

Droplet microfluidics is a powerful method used to characterize chemical reactions at high throughput. Often detection is performed via in-line optical readout, which puts high demands on the detection system or makes detection of low concentration substrates challenging. Here, we have developed a droplet acoustofluidic chip for time-controlled reactions that can be combined with off-line optical readout. The principle of the platform is demonstrated by the enzymatic conversion of fluorescein diphosphate to fluorescein by alkaline phosphatase. The novelty of this work is that the time of the enzymatic reaction is controlled by physically removing the enzymes from the droplets instead of using chemical inhibitors. This is advantageous as inhibitors could potentially interact with the readout. Droplets containing substrate were generated on the chip, and enzyme-coupled microbeads were added into the droplets via pico-injection. The reaction starts as soon as the enzyme/bead complexes are added, and the reaction is stopped when the microbeads are removed from the droplets at a channel bifurcation. The encapsulated microbeads were focused in the droplets by acoustophoresis during the split, leaving the product in the side daughter droplet to be collected for the analysis (without beads). The time of the reaction was controlled by using different outlets, positioned at different lengths from the pico-injector. The enzymatic conversion could be measured with fluorescence readout in a separate PDMS based assay chip. We show the ability to perform time-controlled enzymatic assays in droplet microfluidics coupled to an off-line optical readout, without the need of enzyme inhibitors.

Manipulation of single cells inside nanoliter water droplets using acoustic forces

Droplet microfluidics enables high-throughput screening of single cells and is particularly valuable for applications, where the secreted compounds are analyzed. Typically, optical methods are employed for analysis, which are limited in their applicability as labeling protocols are required. Alternative label-free methods such as mass spectrometry would broaden the range of assays but are harmful to the cells, which is detrimental for some applications such as directed evolution. In this context, separation of cells from supernatant is beneficial prior to the analysis to retain viable cells. In this work, we propose an in-droplet separation method based on contactless and label-free acoustic particle manipulation. In a microfluidic chip, nanoliter droplets containing particles are produced at a T-junction. The particles are trapped in the tip of the droplet by the interplay of acoustic forces in two dimensions and internal flow fields. The droplets are subsequently split at a second T-junction into two daughter droplets-one containing the supernatant and the other containing the corresponding particles. The separation efficiency is measured in detail for polystyrene (PS) beads as a function of droplet speed, size, split ratio, and particle concentration. Further, single-bead (PS) and single-cell (yeast) experiments were carried out. At a throughput of 114 droplets/min, a separation efficiency of 100% ± 0% was achieved for more than 150 droplets. Finally, mammalian cells and bacteria were introduced into the system to test its versatility. This work demonstrates a robust, non-invasive strategy to perform single yeast cell-supernatant sampling in nanoliter volumes.

In-droplet cell separation based on bipolar dielectrophoretic response to facilitate cellular droplet assays

Precise manipulation of cells within water-in-oil emulsion droplets has the potential to vastly expand the type of cellular assays that can be conducted in droplet-based microfluidics systems. However, achieving such manipulation remains challenging. Here, we present an in-droplet label-free cell separation technology by utilizing different dielectrophoretic responses of two different cell types. Two pairs of angled planar electrodes were utilized to generate positive or negative dielectrophoretic force acting on each cell type, which results in selective in-droplet movement of only one specific cell type at a time. A downstream asymmetric Y-shaped microfluidic junction splits the mother droplet into two daughter droplets, each of which contains only one cell type. The capability of this platform was successfully demonstrated by conducting in-droplet separation from a mixture of Salmonella cells and macrophages, two cell types commonly used as a bacterial pathogenicity analysis model. This technology enable the precise manipulation of cells within droplets, which can be exploited as a critical function in implementing broader ranges of droplet-based microfluidics cellular assays.

High-throughput screening for high-efficiency small-molecule biosynthesis

Systems metabolic engineering faces the formidable task of rewiring microbial metabolism to cost-effectively generate high-value molecules from a variety of inexpensive feedstocks for many different applications. Because these cellular systems are still too complex to model accurately, vast collections of engineered organism variants must be systematically created and evaluated through an enormous trial-and-error process in order to identify a manufacturing-ready strain. The high-throughput screening of strains to optimize their scalable manufacturing potential requires execution of many carefully controlled, parallel, miniature fermentations, followed by high-precision analysis of the resulting complex mixtures. This review discusses strategies for the design of high-throughput, small-scale fermentation models to predict improved strain performance at large commercial scale. Established and promising approaches from industrial and academic groups are presented for both cell culture and analysis, with primary focus on microplate- and microfluidics-based screening systems.

Intra-droplet particle enrichment in a focused acoustic field

Particle enrichment is an important preparation or collection process in biomedical and biochemical experiments, but the enrichment process in droplets is harder to realize than in continuous fluid. Here we demonstrate an on-chip, label-free and controllable intra-droplet particle enrichment realized in a focused acoustic field. In this process, droplets containing microparticles are trapped, merged together and split off in the focused acoustic region, resulting in droplets with particles of high concentration. By changing the experimental conditions, the degree of enrichment of this method can be tuned up to 26 times, which enables it to meet requirements for sample preparation in various applications.

Particle enrichment is an important preparation/collection process in biomedical and biochemical experiments, but enrichment in droplets is harder to realize than in continuous fluid. We demonstrate an intra-droplet particle enrichment in a focused acoustic field.

On-chip background dilution in droplets with high particle recovery using acoustophoresis

Droplet microfluidics has shown great potential for on-chip biological and chemical assays. However, fluid exchange in droplet microfluidics with high particle recovery is still a major bottleneck. Here, using acoustophoresis, we present for the first time a label-free method to achieve continuous background dilution in droplets containing cells with high sample recovery. The system comprises droplet generation, acoustic focusing, droplet splitting, picoinjection, and serpentine mixing on the same chip. The capacities of the picoinjection and the droplet split to dilute the background fluorescent signal in the droplets have been characterized. The sample recovery at different droplet split ratios has also been characterized. The results show a maximum of 4.3-fold background dilution with 87.7% particle recovery. We also demonstrated that the system can be used to dilute background fluorescent signal in droplets containing either polystyrene particles or endothelial cells.

In-droplet microparticle washing and enrichment using surface acoustic wave-driven acoustic radiation force

Washing and enrichment of particles and cells are crucial sample preparation procedures in biomedical and biochemical assays. On-chip in-droplet microparticle washing and enrichment have been pursued but remained problematic due to technical difficulties, especially simultaneous and precise control over the droplet interface and in-droplet samples. Here, we have achieved a breakthrough in label-free, continuous, on-demand, in-droplet microparticle washing and enrichment using surface acoustic waves. When exposed to the acoustic field, the droplet and suspended particles experience acoustic radiation force arising from inhomogeneous wave scattering at the liquid/liquid and liquid/solid interfaces. Based on these acoustophoretic phenomena, we have demonstrated in-droplet microparticle washing and enrichment in an acoustofluidic device. We expect that the proposed acoustic method will offer new perspectives to sample washing and enrichment by performing the operation in microscale droplets.

Particle Manipulation Methods in Droplet Microfluidics

This Feature describes the different particle manipulation techniques available in the droplet microfluidics toolbox to handle particles encapsulated inside droplets and to manipulate whole droplets. We address the advantages and disadvantages of the different techniques to guide new users.

In-droplet microparticle separation using travelling surface acoustic wave

Droplets in microfluidic systems can contain microscale objects such as cells and microparticles. The control of the positions of microscale objects within a microchannel is crucial for practical applications in not only continuous-flow-based but also droplet-based systems. This paper proposes an active method for the separation of microparticles inside moving droplets which uses travelling surface acoustic waves (TSAWs). We demonstrate the preconcentration and separation of 5 and 10 μm polystyrene microparticles in moving water-in-oil droplets through the application of TSAWs with two different frequencies. The microparticles inside the droplets are affected by the acoustic radiation force induced by the TSAWs to move laterally in the direction of the TSAW propagation and are thereby separated according to their size. In-droplet separation is then demonstrated through droplet splitting at a Y-junction. Compared to our previous studies, this acoustic approach offers the label-free and on-demand separation of different-sized micro-objects in moving droplets. The present method has potential uses such as in-droplet sample purification and enrichment.