Microfluidics

Microfluidics refers to a system that manipulates a small amount of

The behaviour of fluids at the microscale can differ from "macrofluidic" behaviour in that factors such as surface tension, energy dissipation, and fluidic resistance start to dominate the system. Microfluidics studies how these behaviours change, and how they can be worked around, or exploited for new uses.^{[2][3][4][5][6]}

At small scales (channel size of around 100

High specificity of chemical and physical properties (concentration, pH, temperature, shear force, etc.) can also be ensured resulting in more uniform reaction conditions and higher grade products in single and multi-step reactions.[8]^[9]

Various kinds of microfluidic flows

Microfluidic flows need only be constrained by geometrical length scale – the modalities and methods used to achieve such a geometrical constraint are highly dependent on the targeted application.^[10] Traditionally, microfluidic flows have been generated inside closed channels with the channel cross section being in the order of 10 μm x 10 μm. Each of these methods has its own associated techniques to maintain robust fluid flow which have matured over several years.^{[citation needed]}

Open microfluidics

The behavior of fluids and their control in open microchannels was pioneered around 2005^[11] and applied in air-to-liquid sample collection^[12][13] and chromatography.^[14] In open microfluidics, at least one boundary of the system is removed, exposing the fluid to air or another interface (i.e. liquid).^[15][16][17] Advantages of open microfluidics include accessibility to the flowing liquid for intervention, larger liquid-gas surface area, and minimized bubble formation.^{[18][15][17][19]} Another advantage of open microfluidics is the ability to integrate open systems with surface-tension driven fluid flow, which eliminates the need for external pumping methods such as peristaltic or syringe pumps.^[20] Open microfluidic devices are also easy and inexpensive to fabricate by milling, thermoforming, and hot embossing.^{[21][22][23][24]} In addition, open microfluidics eliminates the need to glue or bond a cover for devices, which could be detrimental to capillary flows. Examples of open microfluidics include open-channel microfluidics, rail-based microfluidics, paper-based, and thread-based microfluidics.^[15][20][25] Disadvantages to open systems include susceptibility to evaporation,^[26] contamination,^[27] and limited flow rate.^[17]

Continuous-flow microfluidics

Continuous flow microfluidics rely on the control of a steady state

Process monitoring capabilities in continuous-flow systems can be achieved with highly sensitive microfluidic flow sensors based on

Droplet-based microfluidics is a subcategory of microfluidics in contrast with continuous microfluidics; droplet-based microfluidics manipulates discrete volumes of fluids in immiscible phases with low Reynolds number and laminar flow regimes. Interest in droplet-based microfluidics systems has been growing substantially in past decades. Microdroplets allow for handling miniature volumes (μl to fl) of fluids conveniently, provide better mixing, encapsulation, sorting, and sensing, and suit high throughput experiments.^[34] Exploiting the benefits of droplet-based microfluidics efficiently requires a deep understanding of droplet generation^[35] to perform various logical operations^[36][37] such as droplet manipulation,^[38] droplet sorting,^[39] droplet merging,^[40] and droplet breakup.^[41]

Digital microfluidics

Alternatives to the above closed-channel continuous-flow systems include novel open structures, where discrete, independently controllable droplets are manipulated on a substrate using

Paper-based microfluidic devices fill a growing niche for portable, cheap, and user-friendly medical diagnostic systems.^[48] Paper based microfluidics rely on the phenomenon of capillary penetration in porous media.^[49] To tune fluid penetration in porous substrates such as paper in two and three dimensions, the pore structure, wettability and geometry of the microfluidic devices can be controlled while the viscosity and evaporation rate of the liquid play a further significant role. Many such devices feature hydrophobic barriers on hydrophilic paper that passively transport aqueous solutions to outlets where biological reactions take place.^[50] Paper-based microfluidics are considered as portable point-of-care biosensors used in a remote setting where advanced medical diagnostic tools are not accessible.^[51] Current applications include portable glucose detection^[52] and environmental testing,^[53] with hopes of reaching areas that lack advanced medical diagnostic tools.

Particle detection microfluidics

One application area that has seen significant academic effort and some commercial effort is in the area of particle detection in fluids. Particle detection of small fluid-borne particles down to about 1 μm in diameter is typically done using a Coulter counter, in which electrical signals are generated when a weakly-conducting fluid such as in saline water is passed through a small (~100 μm diameter) pore, so that an electrical signal is generated that is directly proportional to the ratio of the particle volume to the pore volume. The physics behind this is relatively simple, described in a classic paper by DeBlois and Bean,^[54] and the implementation first described in Coulter's original patent.^[55] This is the method used to e.g. size and count erythrocytes (red blood cells [wiki]) as well as leukocytes (white blood cells) for standard blood analysis. The generic term for this method is resistive pulse sensing (RPS); Coulter counting is a trademark term. However, the RPS method does not work well for particles below 1 μm diameter, as the signal-to-noise ratio falls below the reliably detectable limit, set mostly by the size of the pore in which the analyte passes and the input noise of the first-stage amplifier.^{[citation needed]}

The limit on the pore size in traditional RPS Coulter counters is set by the method used to make the pores, which while a trade secret, most likely^{[according to whom?]} uses traditional mechanical methods. This is where microfluidics can have an impact: The lithography-based production of microfluidic devices, or more likely the production of reusable molds for making microfluidic devices using a molding process, is limited to sizes much smaller than traditional machining. Critical dimensions down to 1 μm are easily fabricated, and with a bit more effort and expense, feature sizes below 100 nm can be patterned reliably as well. This enables the inexpensive production of pores integrated in a microfluidic circuit where the pore diameters can reach sizes of order 100 nm, with a concomitant reduction in the minimum particle diameters by several orders of magnitude.

As a result, there has been some university-based development of microfluidic particle counting and sizing^{[56][57][58][59][60][61][62][63][64][65] [excessive citations]}with the accompanying commercialization of this technology. This method has been termed microfluidic resistive pulse sensing (MRPS).

Microfluidic-assisted magnetophoresis

One major area of application for microfluidic devices is the separation and sorting of different fluids or cell types. Recent developments in the microfluidics field have seen the integration of microfluidic devices with

Other, more research-oriented applications of microfluidic-assisted magnetophoresis are numerous and are generally targeted towards cell separation. The general way this is accomplished involves several steps. First, a paramagnetic substance (usually micro/nanoparticles or a paramagnetic fluid)^[68] needs to be functionalized to target the cell type of interest. This can be accomplished by identifying a transmembranal protein unique to the cell type of interest and subsequently functionalizing magnetic particles with the complementary antigen or antibody.^{[67][69][70][71][72]} Once the magnetic particles are functionalized, they are dispersed in a cell mixture where they bind to only the cells of interest. The resulting cell/particle mixture can then be flowed through a microfluidic device with a magnetic field to separate the targeted cells from the rest.

Conversely, microfluidic-assisted magnetophoresis may be used to facilitate efficient mixing within microdroplets or plugs. To accomplish this, microdroplets are injected with paramagnetic nanoparticles and are flowed through a straight channel which passes through rapidly alternating magnetic fields. This causes the magnetic particles to be quickly pushed from side to side within the droplet and results in the mixing of the microdroplet contents.^[71] This eliminates the need for tedious engineering considerations that are necessary for traditional, channel-based droplet mixing. Other research has also shown that the label-free separation of cells may be possible by suspending cells in a paramagnetic fluid and taking advantage of the magneto-Archimedes effect.^[73][74] While this does eliminate the complexity of particle functionalization, more research is needed to fully understand the magneto-Archimedes phenomenon and how it can be used to this end. This is not an exhaustive list of the various applications of microfluidic-assisted magnetophoresis; the above examples merely highlight the versatility of this separation technique in both current and future applications.

Key application areas

Microfluidic structures include micropneumatic systems, i.e. microsystems for the handling of off-chip fluids (liquid pumps, gas valves, etc.), and microfluidic structures for the on-chip handling of nanoliter (nl) and picoliter (pl) volumes.

Advances in microfluidics technology are revolutionizing molecular biology procedures for enzymatic analysis (e.g., glucose and lactate assays), DNA analysis (e.g., polymerase chain reaction and high-throughput sequencing), proteomics, and in chemical synthesis.^[28][79] The basic idea of microfluidic biochips is to integrate assay operations such as detection, as well as sample pre-treatment and sample preparation on one chip.^[80][81]

An emerging application area for biochips is

Microfluidic technology has led to the creation of powerful tools for biologists to control the complete cellular environment, leading to new questions and discoveries. Many diverse advantages of this technology for microbiology are listed below:

• General single cell studies including growth^[84][34]
• Cellular aging: microfluidic devices such as the "mother machine" allow tracking of thousands of individual cells for many generations until they die^[84]
• Microenvironmental control: ranging from mechanical environment^[85] to chemical environment^[86][87]
• Precise spatiotemporal concentration gradients by incorporating multiple chemical inputs to a single device^[88]
• Force measurements of adherent cells or confined chromosomes: objects trapped in a microfluidic device can be directly manipulated using optical tweezers or other force-generating methods^[89]
• Confining cells and exerting controlled forces by coupling with external force-generation methods such as
  optical tweezer, or controlled deformation of the PDMS (Polydimethylsiloxane) device^[89][90][91]
• Electric field integration[91]
• Plant on a chip and plant tissue culture^[92]
• Antibiotic resistance: microfluidic devices can be used as heterogeneous environments for microorganisms. In a heterogeneous environment, it is easier for a microorganism to evolve. This can be useful for testing the acceleration of evolution of a microorganism / for testing the development of antibiotic resistance.

Some of these areas are further elaborated in the sections below:

DNA chips (microarrays)

Early biochips were based on the idea of a

Molecular biology

In addition to microarrays, biochips have been designed for two-dimensional

The ability to create precise and carefully controlled

Microfluidics has also greatly aided the study of durotaxis by facilitating the creation of durotactic (stiffness) gradients.

Cellular biophysics

By rectifying the motion of individual swimming bacteria,^[100] microfluidic structures can be used to extract mechanical motion from a population of motile bacterial cells.^[101] This way, bacteria-powered rotors can be built.^[102][103]

Optics

The merger of microfluidics and optics is typical known as optofluidics. Examples of optofluidic devices are tunable microlens arrays^[104][105] and optofluidic microscopes.

Microfluidic flow enables fast sample throughput, automated imaging of large sample populations, as well as 3D capabilities.^[106][107] or superresolution.^[108]

Photonics Lab on a Chip (PhLOC)

Due to the increase in safety concerns and operating costs of common analytic methods (ICP-MS, ICP-AAS, and ICP-OES^[109]), the Photonics Lab on a Chip (PhLOC) is becoming an increasingly popular tool for the analysis of actinides and nitrates in spent nuclear waste. The PhLOC is based on the simultaneous application of Raman and UV-Vis-NIR spectroscopy,^[110] which allows for the analysis of more complex mixtures which contain several actinides at different oxidation states.^[111] Measurements made with these methods have been validated at the bulk level for industrial tests,^[109][112] and are observed to have a much lower variance at the micro-scale.^[113] This approach has been found to have molar extinction coefficients (UV-Vis) in line with known literature values over a comparatively large concentration span for 150 μL^[111] via elongation of the measurement channel, and obeys Beer's Law at the micro-scale for U(IV).^[114] Through the development of a spectrophotometric approach to analyzing spent fuel, an on-line method for measurement of reactant quantities is created, increasing the rate at which samples can be analyzed and thus decreasing the size of deviations detectable within reprocessing.^[112]

Through the application of the PhLOC, flexibility and safety of operational methods are increased. Since the analysis of spent nuclear fuel involves extremely harsh conditions, the application of disposable and rapidly produced devices (Based on castable and/or engravable materials such as PDMS, PMMA, and glass^[115]) is advantageous, although material integrity must be considered under specific harsh conditions.^[114] Through the usage of fiber optic coupling, the device can be isolated from instrumentation, preventing irradiative damage and minimizing the exposure of lab personnel to potentially harmful radiation, something not possible on the lab scale nor with the previous standard of analysis.^[111] The shrinkage of the device also allows for lower amounts of analyte to be used, decreasing the amount of waste generated and exposure to hazardous materials.^[111]

Expansion of the PhLOC to miniaturize research of the full nuclear fuel cycle is currently being evaluated, with steps of the PUREX process successfully being demonstrated at the micro-scale.^[110] Likewise, the microfluidic technology developed for the analysis of spent nuclear fuel is predicted to expand horizontally to analysis of other actinide, lanthanides, and transition metals with little to no modification.^[111]

High Performance Liquid Chromatography (HPLC)

HPLC in the field of microfluidics comes in two different forms. Early designs included running liquid through the HPLC column then transferring the eluted liquid to microfluidic chips and attaching HPLC columns to the microfluidic chip directly.^[116] The early methods had the advantage of easier detection from certain machines like those that measure fluorescence.^[117] More recent designs have fully integrated HPLC columns into microfluidic chips. The main advantage of integrating HPLC columns into microfluidic devices is the smaller form factor that can be achieved, which allows for additional features to be combined within one microfluidic chip. Integrated chips can also be fabricated from multiple different materials, including glass and polyimide which are quite different from the standard material of PDMS used in many different droplet-based microfluidic devices.^[118][119] This is an important feature because different applications of HPLC microfluidic chips may call for different pressures. PDMS fails in comparison for high-pressure uses compared to glass and polyimide. High versatility of HPLC integration ensures robustness by avoiding connections and fittings between the column and chip.^[120] The ability to build off said designs in the future allows the field of microfluidics to continue expanding its potential applications.

The potential applications surrounding integrated HPLC columns within microfluidic devices have proven expansive over the last 10–15 years. The integration of such columns allows for experiments to be run where materials were in low availability or very expensive, like in biological analysis of proteins. This reduction in reagent volumes allows for new experiments like single-cell protein analysis, which due to size limitations of prior devices, previously came with great difficulty.^[121] The coupling of HPLC-chip devices with other spectrometry methods like mass-spectrometry allow for enhanced confidence in identification of desired species, like proteins.^[122] Microfluidic chips have also been created with internal delay-lines that allow for gradient generation to further improve HPLC, which can reduce the need for further separations.^[123] Some other practical applications of integrated HPLC chips include the determination of drug presence in a person through their hair^[124] and the labeling of peptides through reverse phase liquid chromatography.^[125]

Acoustic droplet ejection (ADE)

Microfluidic

To understand the prospects for life to exist elsewhere in the universe, astrobiologists are interested in measuring the chemical composition of extraplanetary bodies.^[129] Because of their small size and wide-ranging functionality, microfluidic devices are uniquely suited for these remote sample analyses.^{[130][131][132]} From an extraterrestrial sample, the organic content can be assessed using microchip capillary electrophoresis and selective fluorescent dyes.^[133] These devices are capable of detecting amino acids,^[134] peptides,^[135] fatty acids,^[136] and simple aldehydes, ketones,^[137] and thiols.^[138] These analyses coupled together could allow powerful detection of the key components of life, and hopefully inform our search for functioning extraterrestrial life.^[139]

Food science

Microfluidic techniques such as droplet microfluidics, paper microfluidics, and lab-on-a-chip are used in the realm of food science in a variety of categories.^[140] Research in nutrition,^[141][142] food processing, and food safety benefit from microfluidic technique because experiments can be done with less reagents.^[140]

Food processing requires the ability to enable shelf stability in foods, such as emulsions or additions of preservatives. Techniques such as droplet microfluidics are used to create emulsions that are more controlled and complex than those created by traditional homogenization due to the precision of droplets that is achievable. Using microfluidics for emulsions is also more energy efficient compared to homogenization in which “only 5% of the supplied energy is used to generate the emulsion, with the rest dissipated as heat” .^[143] Although these methods have benefits, they currently lack the ability to be produced at large scale that is needed for commercialization.^[144] Microfluidics are also used in research as they allow for innovation in food chemistry and food processing.^[140][144] An example in food engineering research is a novel micro-3D-printed device fabricated to research production of droplets for potential food processing industry use, particularly in work with enhancing emulsions.^[145]

Paper and droplet microfluidics allow for devices that can detect small amounts of unwanted bacteria or chemicals, making them useful in food safety and analysis.^[146] Paper-based microfluidic devices are often referred to as microfluidic paper-based analytical devices (µPADs) and can detect such things as nitrate,^[147] preservatives,^[148] or antibiotics^[149] in meat by a colorimetric reaction that can be detected with a smartphone. These methods are being researched because they use less reactants, space, and time compared to traditional techniques such as liquid chromatography. µPADs also make home detection tests possible, which is of interest to those with allergies and intolerances.^[147] In addition to paper-based methods, research demonstrates droplet-based microfluidics shows promise in drastically shortening the time necessary to confirm viable bacterial contamination in agricultural waters in the domestic and international food industry.^[146]

Future directions

Microfluidics for personalized cancer treatment

Personalized cancer treatment is a tuned method based on the patient's diagnosis and background. Microfluidic technology offers sensitive detection with higher throughput, as well as reduced time and costs. For personalized cancer treatment, tumor composition and drug sensitivities are very important.^[150]

A patient's drug response can be predicted based on the status of

Microfluidics is also suitable for circulating tumor cells (CTCs) and non-CTCs liquid biopsy analysis. Beads conjugate to anti‐epithelial cell adhesion molecule (EpCAM) antibodies for positive selection in the CTCs isolation chip (iCHIP).^[154] CTCs can also be detected by using the acidification of the tumor microenvironment and the difference in membrane capacitance.^[155][156] CTCs are isolated from blood by a microfluidic device, and are cultured on-chip, which can be a method to capture more biological information in a single analysis. For example, it can be used to test the cell survival rate of 40 different drugs or drug combinations.^[157] Tumor‐derived extracellular vesicles can be isolated from urine and detected by an integrated double‐filtration microfluidic device; they also can be isolated from blood and detected by electrochemical sensing method with a two‐level amplification enzymatic assay.^[158][159]

Tumor materials can directly be used for detection through microfluidic devices. To screen

Microfluidics devices also can simulate the tumor microenvironment, to help to test anticancer drugs. Microfluidic devices with 2D or 3D cell cultures can be used to analyze spheroids for different cancer systems (such as lung cancer and ovarian cancer), and are essential for multiple anti-cancer drugs and toxicity tests. This strategy can be improved by increasing the throughput and production of spheroids. For example, one droplet-based microfluidic device for 3D cell culture produces 500 spheroids per chip.^[163] These spheroids can be cultured longer in different surroundings to analyze and monitor. The other advanced technology is organs‐on‐a‐chip, and it can be used to simulate several organs to determine the drug metabolism and activity based on vessels mimicking, as well as mimic pH, oxygen... to analyze the relationship between drugs and human organ surroundings.^[163]

A recent strategy is single-cell chromatin immunoprecipitation (ChiP)‐Sequencing in droplets, which operates by combining droplet‐based single cell RNA sequencing with DNA‐barcoded antibodies, possibly to explore the tumor heterogeneity by the genotype and phenotype to select the personalized anti-cancer drugs and prevent the cancer relapse.^[164]

See also

• Advanced Simulation Library
• Droplet-based microfluidics
• Fluidics
• Induced-charge electrokinetics
• Lab-on-a-chip
• Microfluidic cell culture
• Microfluidic modulation spectroscopy
• Microphysiometry
• Micropumps
• Microvalves
• uFluids@Home
• Paper-based microfluidics

References

Further reading

Review papers

• Yetisen AK, Akram MS, Lowe CR (June 2013). "Paper-based microfluidic point-of-care diagnostic devices". Lab on a Chip. 13 (12): 2210–2251.
  S2CID 17745196
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• Whitesides GM (July 2006). "The origins and the future of microfluidics". Nature. 442 (7101): 368–373.
  S2CID 205210989
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• Seemann R, Brinkmann M, Pfohl T, Herminghaus S (January 2012). "Droplet based microfluidics". Reports on Progress in Physics. 75 (1): 016601.
  S2CID 5206697
  .
• Squires TM, Quake SR (2005). "Microfluidics: Fluid physics at the nanoliter scale" (PDF). Reviews of Modern Physics. 77 (3): 977–1026.
  doi:10.1103/RevModPhys.77.977
  .
• Yetisen AK, Volpatti LR (July 2014). "Patent protection and licensing in microfluidics". Lab on a Chip. 14 (13): 2217–2225.
  S2CID 8669721
  .
• Chen K (2011). "Microfluidics and the future of drug research". Journal of Undergraduate Life Sciences. 5 (1): 66–69. Archived from the original on 2012-03-31. Retrieved 2011-08-30.
• Angell JB, Terry SC, Barth PW (April 1983). "Silicon Micromechanical Devices".
  doi:10.1038/scientificamerican0483-44
  .
• Carugo D, Bottaro E, Owen J, Stride E, Nastruzzi C (May 2016). "Liposome production by microfluidics: potential and limiting factors". Scientific Reports. 6: 25876.
  PMID 27194474
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• Chossat JB, Park YL, Wood RJ, Duchaine V (September 2013). "A Soft Strain Sensor Based on Ionic and Metal Liquids".
  S2CID 14492585
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• Tseng TM, Li M, Freitas DN, Mongersun A, Araci IE, Ho TY, Schlichtmann U (2018). Columba S: a scalable co-layout design automation tool for microfluidic large-scale integration (PDF). Proceedings of the 55th Annual Design Automation Conference. p. 163. Archived from the original (PDF) on April 9, 2023.

Books

• Bruus H (2008). Theoretical Microfluidics. Oxford University Press.
  ISBN 978-0199235094
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• Folch, Albert. Hidden in Plain Sight: The History, Science, and Engineering of Microfluidic Technology (MIT Press, 2022) online review
• Herold KE, Rasooly A (2009). Lab-on-a-Chip Technology: Fabrication and Microfluidics. Caister Academic Press.
  ISBN 978-1-904455-46-2
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• Kelly R, ed. (2012). Advances in Microfluidics. Richland, Washington, USA: Pacific Northwest National Laboratory.
  ISBN 978-953-510-106-2
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• Jenkins G, Mansfield CD (2012). Microfluidic Diagnostics. Humana Press.
  ISBN 978-1-62703-133-2
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• Li X, Zhou Y, eds. (2013). Microfluidic devices for biomedical applications. Woodhead Publishing.
  ISBN 978-0-85709-697-5
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• Tabeling P (2006). Introduction to Microfluidics. Oxford UP.
  ISBN 978-0-19-856864-3
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