Editing Digital microfluidics (section)

== Overview ==
[[File:DMF_Array_pic.pdf|left|thumb|611x611px|Aqueous droplet sitting on top of an open microfluidic system with a cross section view. Device design can be manipulated to fit user's needs (modified electrodes, electrode pattern, materials used, etc.).[3][4]]]
In analogy to digital microelectronics, digital microfluidic operations can be combined and reused within hierarchical design structures so that complex procedures (e.g. chemical synthesis or [[biological assay]]s) can be built up step-by-step. And in contrast to continuous-flow [[microfluidics]], digital microfluidics<ref>{{cite conference | vauthors = Kim CJ | title = Micropumping by Electrowetting | conference = Proc. ASME Int. Mechanical Engineering Congress and Exposition | location = New York, NY | date = November 2001 | id = IMECE2001/HTD-24200 }}</ref> works much the same way as traditional bench-top protocols, only with much smaller volumes and much higher automation. Thus a wide range of established chemical procedures and protocols can be seamlessly transferred to a [[nanoliter]] droplet format. [[Electrowetting]], [[dielectrophoresis]], and immiscible-fluid flows are the three most commonly used principles, which have been used to generate and manipulate microdroplets in a digital microfluidic device.

A digital microfluidic (DMF) device set-up depends on the substrates used, the electrodes, the configuration of those electrodes, the use of a dielectric material, the thickness of that dielectric material, the hydrophobic layers, and the applied voltage.<ref name="Jain_2017">{{cite journal | vauthors = Jain V, Devarasetty V, Patrikar R |date=June 2017|title=Effect of electrode geometry on droplet velocity in open EWOD based device for digital microfluidics applications |journal=Journal of Electrostatics |volume=87 |pages=11–18 |doi=10.1016/j.elstat.2017.02.006 }}</ref><ref name="Choi_2012">{{cite journal | vauthors = Choi K, Ng AH, Fobel R, Wheeler AR | title = Digital microfluidics | journal = Annual Review of Analytical Chemistry | volume = 5 | pages = 413–40 | date = 2012 | pmid = 22524226 | doi = 10.1146/annurev-anchem-062011-143028 | bibcode = 2012ARAC....5..413C }}</ref>
[[File:DMF open and closed system.png|alt=|right|frameless|577x577px|<nowiki>Aqueous droplet sitting on top of an open and closed digital microfluidic system with a cross section view. This shows the movement of the droplet once an electrode is activated. Device design can be manipulated to fit user's needs (modified electrodes, electrode pattern, materials used, etc.).[3][4]]]</nowiki>]]
A common substrate used in this type of system is glass. Depending if the system is open or closed, there would be either one or two layers of glass. The bottom layer of the device contains a patterned array of individually controllable electrodes.<ref name="Jain_2017" /> When looking at a closed system, there is usually a continuous ground electrode found through the top layer made usually of indium tin oxide ([[Indium tin oxide|ITO]]). The [[dielectric]] layer is found around the electrodes in the bottom layer of the device and is important for building up charges and electrical field gradients on the device.<ref name="Choi_2012" /> A hydrophobic layer is applied to the top layer of the system to decrease the surface energy where the droplet will actually we be in contact with.<ref name="Choi_2012" /> The applied voltage activates the electrodes and allows changes in the wettability of droplet on the device’s surface. In order to move a [[droplet]], a control [[voltage]] is applied to an [[electrode]] adjacent to the droplet, and at the same time, the electrode just under the droplet is deactivated. By varying the [[electric potential]] along a linear array of electrodes, [[electrowetting]] can be used to move droplets along this line of electrodes.<ref name="Fair_2007">{{Cite journal| vauthors = Fair RB, Khlystov A, Tailor TD, Ivanov V, Evans RD, Srinivasan V, Pamula VK, Pollack MG, Griffin PB, Zhou J | display-authors = 6 |date=2007-01-01|title=Chemical and Biological Applications of Digital-Microfluidic Devices|journal=IEEE Design and Test of Computers|volume=24|issue=1|pages=10–24|doi=10.1109/MDT.2007.8 |citeseerx=10.1.1.559.1440| s2cid = 10122940 }}</ref>

Modifications to this foundation can also be fabricated into the basic design structure. One example of this is the addition of [[electrochemiluminescence]] detectors within the indium tin oxide layer (the ground electrode in a closed system) which aid in the detection of luminophores in droplets.<ref>{{cite journal | vauthors = Shamsi MH, Choi K, Ng AH, Chamberlain MD, Wheeler AR | title = Electrochemiluminescence on digital microfluidics for microRNA analysis | journal = Biosensors & Bioelectronics | volume = 77 | pages = 845–52 | date = March 2016 | pmid = 26516684 | doi = 10.1016/j.bios.2015.10.036 | url = https://opensiuc.lib.siu.edu/cgi/viewcontent.cgi?article=1004&context=chem_pubs | url-access = subscription }}</ref> In general, different materials may also be used to replace basic components of a DMF system such as the use of [[Polydimethylsiloxane|PDMS]] instead of glass for the substrate.<ref>{{Cite journal| vauthors = Zhao Y, Xu T, Chakrabarty K |date=2011-07-01|title=Broadcast Electrode-Addressing and Scheduling Methods for Pin-Constrained Digital Microfluidic Biochips|journal=IEEE Transactions on Computer-Aided Design of Integrated Circuits and Systems|volume=30|issue=7|pages=986–999|doi=10.1109/TCAD.2011.2116250|s2cid=4159209|issn=0278-0070}}</ref> Liquid materials can  be added, such as oil or another substance, to a closed system to prevent evaporation of materials and decrease surface contamination.<ref name="Fair_2007" /><ref name="Berthier_2008">{{Cite book|title=Microdrops and digital microfluidics| vauthors = Berthier J |publisher=William Andrew Pub|year=2008|isbn=9780815515449|oclc=719878673}}</ref> Also, DMF systems can be compatible with [[ionic liquid]] droplets with the use of an oil in a closed device or with the use of a catena (a suspended wire) over an open DMF device.<ref name="Berthier_2008" />

Digital microfluidics can be light-activated. [[Optoelectrowetting]] can be used to transport [[drop (liquid)|sessile droplets]] around a surface containing patterned [[photoconductivity|photoconductors]].<ref>{{cite journal | vauthors = Chiou PY, Moon H, Toshiyoshi H, Kim CJ, Wu MC | title = Light actuation of liquid by optoelectrowetting. | journal = Sensors and Actuators A: Physical | date = May 2003 | volume = 104 | issue = 3 | pages = 222–8 | doi = 10.1016/S0924-4247(03)00024-4 | bibcode = 2003SeAcA.104..222C }}</ref> The [[photoelectrowetting]] effect<ref name="pmid22355699">{{cite journal | vauthors = Arscott S | title = Moving liquids with light: photoelectrowetting on semiconductors | journal = Scientific Reports | volume = 1 | pages = 184 | date = 2011 | pmid = 22355699 | pmc = 3240946 | doi = 10.1038/srep00184 | bibcode = 2011NatSR...1..184A | arxiv = 1108.4935 }}</ref> can also be used to achieve droplet transport on a silicon wafer without the necessity of patterned electrodes.<ref name="Palma_2018">{{cite journal | vauthors = Palma C, Deegan RD | title = Droplet Translation Actuated by Photoelectrowetting | journal = Langmuir: The ACS Journal of Surfaces and Colloids | volume = 34 | issue = 10 | pages = 3177–3185 | date = March 2018 | pmid = 29457909 | doi = 10.1021/acs.langmuir.7b03340 }}</ref>