Droplet-based microfluidics

[3][4] Microdroplets offer the feasibility of handling miniature volumes (μL to fL) of fluids conveniently, provide better mixing, encapsulation, sorting, sensing and are suitable for high throughput experiments.

[30] For example, a triblock copolymer surfactant containing two perfluoropolyether (PFPE) tails and a polyethylene glycol (PEG) block head group is a fluorosurfactant with great biocompatibility and excellent droplet stability against coalescence.

In order to make droplet-based microfluidics a viable technique for carrying out chemical reactions or working with living cells on the microscale, it is necessary to implement methods allowing for droplet incubation.

[87] Microcapsules that respond to magnetic and photo stimuli, such as these constructed of graphene oxide, are useful for biomedical applications that require in vivo, contact-free manipulation of cellular structures such as stem cells.

[113] Secondly, the cellular proliferation and behavior may differ depending on the microfluidic systems, a determining factor is the culture surface area to media volume, which vary from one device to another.

[115][116] Polymerase chain reaction (PCR) has been a vital tool in genomics and biological endeavors since its inception as it has greatly sped up production and analysis of DNA samples for a wide range of applications.

[128] A recent droplet-PCR PDMS device allowed for higher accuracy and amplification of small copy numbers in comparison to traditional quantitative PCR experiments.

[146] In 2017, S. S. Terekhov et al. developed monodisperse microfluidic double water-in-oil-in-water emulsion (MDE) sorting, which they combined with FACS followed by liquid chromatography-mass spectrometry (LC-MS) and next-generation sequencing (NGS).

[158] In 2020, Haidas et al. presented a microfluidic approach that uses both matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) and fluorescence microscopy, which the authors used to measure the concentration and activity of phytase, respectively, in yeast cells.

[160] Experts anticipate that future microfluidic-based innovations for directed evolution campaigns will be driven in the commercial space, resulting in more simple and less expensive methods and tools that can be applied to biotechnologically-relevant enzymes.

[162] Microdispersed droplets created by droplet-based chemistry are capable of acting as environments in which chemical reactions occur, as reagent carriers in the process of generating complex nanostructures.

[173] The first particles incorporated in droplet-based systems were silica gels in the micrometer size range in order to test their applications in the manufacturing of displays and optical coatings.

Techniques such as the controlled encapsulation of individual gas bubbles to create hollow nanoparticles for synthesizing microbubbles with specific contents are vital for drug delivery systems.

Recent advancements on the nanoscale such as devices that fabricate both spherical and non-spherical droplets that are ultrafast and homogeneous mixed are being produced for large scale production of powdered particles in industrial applications.

This microfluidic method differs from the traditional wet-spinning synthesis route through the use of aqueous droplets in an immiscible oil stream rather than the extrusion of a bulk solution of the same composition mixed off site.

[199] The ability to control the size, flow rate, and composition of droplets provides an option to fine tune the morphology of fibers to fit a specific use in bioanalysis and emulation of anatomical functions.

[214] HPLC columns can also be built directly into microfluidic lab-chips creating monolithic hybrid devices capable of chemical separation as well as droplet formation and manipulation.

[218][219][220] Droplet-based microfluidic devices coupled to HPLC have high detection sensitivity, use low volumes of reagents, have short analysis times, and minimal cross-contamination of analytes, which make them efficient in many aspects.

[215] The additional strength of TPE made it capable of supporting the higher pressures needed for HPLC such that a single, microfluidic lab-chip could perform chemical separation, fractionation, and further droplet manipulation.

[225] Similarly to HPLC, fluorescence based detection techniques are used for capillary electrophoresis, which make these methods practical and can be applied to fields such as biotechnology, analytical chemistry, and drug development.

This makes MS, unlike other detection techniques (such as fluorescence), label-free; i.e. there is no need to bind additional ligands or groups to the molecule of interest in order to receive a signal and identify the compound.

This makes MS, unlike other detection techniques (such as fluorescence), label-free; i.e. there is no need to bind additional ligands or groups to the molecule of interest in order to receive a signal and identify the compound.

[232][233][234] Difficulty of separation/purification make entirely microfluidic scale systems coupled to mass spectrometry ideal in the fields of proteomics,[235][63][236][237] enzyme kinetics,[238] drug discovery,[239] and newborn disease screening.

[223][235] Droplet size, Taylor cone shape, and flow rate can be controlled by varying the potential differential and the temperature of a drying (to evaporate analyte-surrounding solvent) stream of gas (usually nitrogen).

[250] MALDI is typified by the use of an ultraviolet (UV) laser to trigger ablation of analyte species that are mixed with a matrix of crystallized molecules with high optical absorption.

[292] Electrochemical detection serves as an inexpensive alternative to not only measure chemical composition in certain cases, but also droplet length, frequency, conductivity, and velocity at high speeds and usually with very little space compensation on the chip.

[283][294] Another group displayed, with a series of controls, that mixed droplet composition involving potassium iodide was detected accurately on the time scale of seconds with optimal voltage, velocity, and pH ranges.

[297] This method is enhanced into a digital microfluidic (DMF) setting, where gold and silver electrodes in junction with dissolved magnetic microparticles in the fluids replaced the typical fluorescence-based detection of droplets in the immunoassay of biomarker analytes.

[298] The above experiment by Shamsi et al, alludes to the main use for electrochemical detection in microfluidics; biosensing for various measurements such as enzyme kinetics and biological assays of many other types of cells.

[301] Use of this detection method is primarily found in the electrowetting of dielectric DMFs, where the sensing electrode apparatus can be reconfigurable and has a longer lifetime while still producing accurate results.

Droplet formation using a T-junction. [ 9 ]
Formation of droplets with a T junction microfluidic device. The break off of a droplet comes from a drop in pressure as a drop emerges. [ 10 ]
Droplet formation using a flow focusing device. [ 17 ]
Diagram of flow focusing droplet formation device commonly used in microfluidic devices. Liquid flowing in from the left is pinched off into droplets by an oil flowing in from the top and bottom. [ 10 ]
Two stream reagent addition using a flow focusing approach with a planar chip format. [ 18 ]
The surfactant stabilizes the interface between the continuous oil phase and the aqueous droplet. [ 29 ] Also, the surfactants can reduce the adhesion of aqueous droplets to the channel wall, by decreasing the surface tension at the aqueous-oil interface. [ 30 ]
The hydrophobic tails on the surfactant keep the emulsion stable and prevent droplet coalescence together during the incubation time. [ 29 ] The larger/longer the hydrophobic tails are, the better the biocompatibility and the better the solubility in oil phase, also the better the insolubility in aqueous phase. [ 29 ] < [ 32 ]
Simplified Gibbs adsorption isotherm visualization- displaying model Critical Micelle Concentration (left). Simplified micelle and vesicle structure, with size ranges (right). [ 40 ]
Controlled electrocoalesence of two droplets. A smaller droplet flows faster than a larger one, catching up to the larger droplet. Upon application of an electric field, as the droplet pair flows through the electrodes, the droplets fuse. [ 52 ] [ 53 ]
Two droplets come together in this device. The pillars divide the flow into three channels: two side branches on the top and bottom, and a middle branch, through which the entire merged droplet flows to. The continuous phase between adjacent droplets is effectively filtered out by being allowed to flow through the top and bottom branches. The removal of the continuous phase between droplets facilitates droplet fusion. [ 57 ]
Picoinjection reagent addition method, showing the addition of aqueous fluid (Aq 2 – dark blue), into previously formed droplets (Aq 1 – light blue) flowing through a channel. [ 58 ] [ 59 ]
On-chip reservoir designed to house droplets for longer term storage. [ 70 ]
Mixing by chaotic advection in a winding channel. As the droplets proceed in the winding channel, unsteady fluid flow is caused by co- and counter-rotating vortices (see inset). [ 1 ]