Droplet trapping and fast acoustic release in a multi-height device with steady-state flow

Simple Method for On-Demand Droplet Trapping in a Microfluidic Device Based on the Concept of Hydrodynamic Resistance

We demonstrate an innovative method to catch the desired droplets from a train of droplets and immobilize them in traps located in an integrated microfluidic device. To this end, water-in-oil droplets are generated in a flow-focusing junction and then guided to a channel connected to chambers designated for on-demand droplet trapping. Each chamber is connected to a side channel through a batch of microposts. The side channels are also connected to the flexible poly(vinyl chloride) tubes, which can be closed by attaching binder clips. The hydrodynamic resistance of the routes in the device can be changed by opening and closing the binder clips. In this way, droplets are easily guided into individual traps based on the user's demand. A set of numerical simulations was also conducted to investigate the authenticity of the employed idea and to find the optimal geometry for implementing our strategy. This simple method can be easily employed for on-demand droplet trapping without using on-chip valves or complex off-chip actuators proposed in previous studies.

On-demand light-driven release of droplets stabilized via a photoresponsive fluorosurfactant

Water-in-oil droplets have emerged as promising microreactors for high-throughput biochemical analysis due to their features of reduced sample consumption and automated operation. For a typical screening application, droplets are often trapped for continuous monitoring of the reaction over an extended period, followed by the selective retrieval of targeted droplets based on the after-effect of biochemical reactions. While techniques for droplet trapping are well developed, retrieval of targeted droplets mainly demands complicated device fabrication or sophisticated control. Herein, facile and rapid selective droplet release is achieved by utilizing a new class of photoresponsive fluorosurfactant based on plasmonic nanoparticles. The intense photothermal response provided by this novel photoresponsive fluorosurfactant is capable of vaporizing the fluorocarbon oil at the droplet interface under laser illumination, resulting in a bubble releasing a trapped droplet on demand. A fully automated fluorescence-activated droplet release platform has also been developed to demonstrate its potential for droplet-based large-scale screening applications.

Recent advances in acoustic microfluidics and its exemplary applications

Acoustic-based microfluidics has been widely used in recent years for fundamental research due to its simple device design, biocompatibility, and contactless operation. In this article, the basic theory, typical devices, and technical applications of acoustic microfluidics technology are summarized. First, the theory of acoustic microfluidics is introduced from the classification of acoustic waves, acoustic radiation force, and streaming flow. Then, various applications of acoustic microfluidics including sorting, mixing, atomization, trapping, patterning, and acoustothermal heating are reviewed. Finally, the development trends of acoustic microfluidics in the future were summarized and looked forward to.

Active single cell encapsulation using SAW overcoming the limitations of Poisson distribution

We demonstrate the use of an acoustic device to actively encapsulate single red blood cells into individual droplets in a T-junction. We compare the active encapsulation with the passive encapsulation depending on the number of loaded cells as well as the created droplet volumes. This method overcomes the Poisson limitation statistical loading of cells for the passive encapsulation. In our experiments we reach a single cell encapsulation efficiency of 97.9 ± 2.1% at droplet formation rates exceeding 15 Hz.

Microfluidics for Biosynthesizing: from Droplets and Vesicles to Artificial Cells

Fabrication of artificial biomimetic materials has attracted abundant attention. As one of the subcategories of biomimetic materials, artificial cells are highly significant for multiple disciplines and their synthesis has been intensively pursued. In order to manufacture robust "alive" artificial cells with high throughput, easy operation, and precise control, flexible microfluidic techniques are widely utilized. Herein, recent advances in microfluidic-based methods for the synthesis of droplets, vesicles, and artificial cells are summarized. First, the advances of droplet fabrication and manipulation on the T-junction, flow-focusing, and coflowing microfluidic devices are discussed. Then, the formation of unicompartmental and multicompartmental vesicles based on microfluidics are summarized. Furthermore, the engineering of droplet-based and vesicle-based artificial cells by microfluidics is also reviewed. Moreover, the artificial cells applied for imitating cell behavior and acting as bioreactors for synthetic biology are highlighted. Finally, the current challenges and future trends in microfluidic-based artificial cells are discussed. This review should be helpful for researchers in the fields of microfluidics, biomaterial fabrication, and synthetic biology.

A programmable microfluidic platform for multisample injection, discretization, and droplet manipulation

A programmable microfluidic platform enabling on-demand sampling, compartmentalization, and manipulation of multiple aqueous volumes is presented. The system provides random-access actuation of a microtrap array supporting selective discretization of picoliter volumes from multiple sample inputs. The platform comprises two interconnected chips, with parallel T-junctions and multiplexed microvalves within one chip enabling programmable injection of aqueous sample plugs, and nanoliter volumes transferred to a second microtrap array chip in which the plugs are actively discretized into picoliter droplets within a static array of membrane displacement actuators. The system employs two different multiplexer designs that reduce the number of input signals required for both sample injection and discretization. This versatile droplet-based technology offers flexible sample workflows and functionalities for the formation and manipulation of heterogeneous picoliter droplets, with particular utility for applications in biochemical synthesis and cell-based assays requiring flexible and programmable operation of parallel and multistep droplet processes. The platform is used here for the selective encapsulation of differentially labeled cells within a discrete droplet array.

Acoustofluidic stick-and-play micropump built on foil for single-cell trapping

The majority of microfluidic devices nowadays are built on rigid or bulky substrates such as glass slides and polydimethylsiloxane (PDMS) slabs, and heavily rely on external equipment such as syringe pumps. Although a variety of micropumps have been developed in the past, few of them are suitable for flexible microfluidics or lab-on-a-foil systems. In this paper, stick-and-play acoustic micropump is built on thin and flexible plastic film by printing microstructures termed defended oscillating membrane equipped structures (DOMES) using two-photon polymerization. Specifically, this new micropump induces rectified flow upon the actuation of acoustic waves, and the flow patterns agree with simulation results very well. More importantly, the developed micropump has the capabilities to generate adjustable flow rates as high as 420 nL min-1, and does not suffer from problems such as bubble instability, gas dissolution, and undesired bubble-trapping that commonly occur in other forms of acoustic micropumps. Since the micropump works in stick-and-play mode, it is reusable after cleaning thanks to the easy separation of covers and substrates. Lastly, the developed micropump is applied for creating a self-pumped single-cell trapping device. The excellent trapping capability of the integrated device proves its potential for long-term studies of biological behaviors of individual cells for biomedical applications.

An acoustofluidic platform for non-contact trapping of cell-laden hydrogel droplets compatible with optical microscopy

Production of cell-laden hydrogel droplets as miniaturized niches for 3D cell culture provides a new route for cell-based assays. Such production can be enabled by droplet microfluidics and here we present a droplet trapping system based on bulk acoustic waves for handling hydrogel droplets in a continuous flow format. The droplet trapping system consists of a glass capillary equipped with a small piezoelectric transducer. By applying ultrasound (4 MHz), a localized acoustic standing wave field is generated in the capillary, trapping the droplets in a well-defined cluster above the transducer area. The results show that the droplet cluster can be retained at flow rates of up to 76 μl/min, corresponding to an average flow speed of 3.2 mm/s. The system allows for important operations such as continuous perfusion and/or addition of chemical reagents to the encapsulated cells with in situ optical access. This feature is demonstrated by performing on-chip staining of the cell nuclei. The key advantages of this trapping method are that it is label-free and gentle and thus well-suited for biological applications. Moreover, the droplets can easily be released on-demand, which facilitates downstream analysis. It is envisioned that the presented droplet trapping system will be a valuable tool for a wide range of multistep assays as well as long-term monitoring of cells encapsulated in gel-based droplets.

Microfluidic on-demand droplet generation, storage, retrieval, and merging for single-cell pairing

A multifunctional microfluidic platform combining on-demand aqueous-phase droplet generation, multi-droplet storage, and controlled merging of droplets selected from a storage library in a single integrated microfluidic device is described. A unique aspect of the technology is a microfluidic trap design comprising a droplet trap chamber and lateral bypass channels integrated with a microvalve that supports the capture and merger of multiple droplets over a wide range of individual droplet sizes. A storage unit comprising an array of microfluidic traps operates in a first-in first-out manner, allowing droplets stored within the library to be analyzed before sequentially delivering selected droplets to a downstream merging zone, while shunting other droplets to waste. Performance of the microfluidic trap is investigated for variations in bypass/chamber hydrodynamic resistance ratio, micro-chamber geometry, trapped droplet volume, and overall flow rate. The integrated microfluidic platform is then utilized to demonstrate the operational steps necessary for cell-based assays requiring the isolation of defined cell populations with single cell resolution, including encapsulation of individual cells within an aqueous-phase droplet carrier, screening or incubation of the immobilized cell-encapsulated droplets, and generation of controlled combinations of individual cells through the sequential droplet merging process. Beyond its utility for cell analysis, the presented platform represents a versatile approach to robust droplet generation, storage, and merging for use in a wide range of droplet-based microfluidics applications.

A Handy Liquid Metal Based Non-Invasive Electrophoretic Particle Microtrap

A handy liquid metal based non-invasive particle microtrap was proposed and demonstrated in this work. This kind of microtrap can be easily designed and fabricated at any location of a microfluidic chip to perform precise particle trapping and releasing without disturbing the microchannel itself. The microsystem demonstrated in this work utilized silicon oil as the continuous phase and fluorescent particles (PE-Cy5, SPHERO^TM Fluorescent Particles, BioLegend, San Diego, CA, USA, 10.5 μm) as the target particles. To perform the particle trapping, the micro system utilized liquid-metal-filled microchannels as noncontact electrodes to generate different patterns of electric field inside the fluid channel. According to the experimental results, the target particle can be selectively trapped and released by switching the electric field patterns. For a better understanding the control mechanism, a numerical simulation of the electric field was performed to explain the trapping mechanism. In order to verify the model, additional experiments were performed and are discussed.

Trapping a moving droplet train by bubble guidance in microfluidic networks

Trapping a train of moving droplets into preset positions within a microfluidic device facilitates the long-term observation of biochemical reactions inside the droplets. In this paper, a new bubble-guided trapping method, which can remarkably improve the limited narrow two-phase flow rate range of uniform trapping, was proposed by taking advantage of the unique physical property that bubbles do not coalescence with two-phase fluids and the hydrodynamic characteristic of large flow resistance of bubbles. The flow behaviors of bubble-free and bubble-guided droplet trains were compared and analyzed under the same two-phase flow rates. The experimental results show that the droplets trapped by bubble-free guided trapping exhibit the four trapping modes of sequentially uniform trapping, non-uniform trapping induced by break-up and collision, and failed trapping due to squeezing through, and the droplets exhibit the desired uniform trapping in a relatively small two-phase flow rate range. Compared with bubble-free guided droplets, bubble-guided droplets also show four trapping modes. However, the two-phase flow rate range in which uniform trapping occurs is increased significantly and the uniformity of the trapped droplet array is improved. This investigation is beneficial to enhance the applicability of microfluidic chips for storing droplets in a passive way.

On-demand acoustic droplet splitting and steering in a disposable microfluidic chip

On-chip droplet splitting is one of the fundamental droplet-based microfluidic unit operations to control droplet volume after production and increase operational capability, flexibility, and throughput. Various droplet splitting methods have been proposed, and among them the acoustic droplet splitting method is promising because of its label-free operation without any physical or thermal damage to droplets. Previous acoustic droplet splitting methods faced several limitations: first, they employed a cross-type acoustofluidic device that precluded multichannel droplet splitting; second, they required irreversible bonding between a piezoelectric substrate and a microfluidic chip, such that the fluidic chip was not replaceable. Here, we present a parallel-type acoustofluidic device with a disposable microfluidic chip to address the limitations of previous acoustic droplet splitting devices. In the proposed device, an acoustic field is applied in the direction opposite to the flow direction to achieve multichannel droplet splitting and steering. A disposable polydimethylsiloxane microfluidic chip is employed in the developed device, thereby removing the need for permanent bonding and improving the flexibility of the droplet microfluidic device. We experimentally demonstrated on-demand acoustic droplet bi-splitting and steering with precise control over the droplet splitting ratio, and we investigated the underlying physical mechanisms of droplet splitting and steering based on Laplace pressure and ray acoustics analyses, respectively. We also demonstrated droplet tri-splitting to prove the feasibility of multichannel droplet splitting. The proposed on-demand acoustic droplet splitting device enables on-chip droplet volume control in various droplet-based microfluidic applications.