Editing Digital microfluidics (section)

==== Immunoassays ====
The advanced fluid handling capabilities of digital microfluidics (DMF) allows for the adoption of DMF as an [[immunoassay]] platform  as DMF devices can precisely manipulate small quantities of liquid reagents. Both heterogeneous immunoassays (antigens interacting with immobilized antibodies) and homogeneous immunoassays (antigens interacting with antibodies in solution) have been developed using a DMF platform.<ref>{{cite journal | vauthors = Ng AH, Uddayasankar U, Wheeler AR | title = Immunoassays in microfluidic systems. Analytical and bioanalytical chemistry | journal = Analytical and Bioanalytical Chemistry | date = June 2010 | volume = 397 | issue = 3 | pages = 991–1007 | doi = 10.1007/s00216-010-3678-8 | pmid = 20422163 | s2cid = 30670634 }}</ref> With regards to heterogeneous immunoassays, DMF can simplify the extended and intensive procedural steps by performing all delivery, mixing, incubation, and washing steps on the surface of the device (on-chip). Further, existing immunoassay techniques and methods, such as magnetic bead-based assays, [[ELISA]]s, and electrochemical detection, have been incorporated onto DMF immunoassay platforms.<ref name="Vergauwe_2011">{{cite journal | vauthors = Vergauwe N, Witters D, Ceyssens F, Vermeir S, Verbruggen B, Puers R, Lammertyn J | title = A versatile electrowetting-based digital microfluidic platform for quantitative homogeneous and heterogeneous bio-assays. | journal = Journal of Micromechanics and Microengineering | date = April 2011 | volume = 21 | issue = 5 | pages = 054026 | doi = 10.1088/0960-1317/21/5/054026 | bibcode = 2011JMiMi..21e4026V | s2cid = 111122895 }}</ref><ref name="Sista_2008">{{cite journal | vauthors = Sista R, Hua Z, Thwar P, Sudarsan A, Srinivasan V, Eckhardt A, Pollack M, Pamula V | title = Development of a digital microfluidic platform for point of care testing | journal = Lab on a Chip | volume = 8 | issue = 12 | pages = 2091–104 | date = December 2008 | pmid = 19023472 | pmc = 2726010 | doi = 10.1039/b814922d }}</ref><ref name="Ng_2012">{{cite journal | vauthors = Ng AH, Choi K, Luoma RP, Robinson JM, Wheeler AR | title = Digital microfluidic magnetic separation for particle-based immunoassays | journal = Analytical Chemistry | volume = 84 | issue = 20 | pages = 8805–12 | date = October 2012 | pmid = 23013543 | doi = 10.1021/ac3020627 }}</ref><ref name="Shamsi_2014">{{cite journal | vauthors = Shamsi MH, Choi K, Ng AH, Wheeler AR | title = A digital microfluidic electrochemical immunoassay | journal = Lab on a Chip | volume = 14 | issue = 3 | pages = 547–54 | date = February 2014 | pmid = 24292705 | doi = 10.1039/c3lc51063h }}</ref>

The incorporation of magnetic bead-based assays onto a DMF immunoassay platform has been demonstrated for the detection of multiple analytes, such as human insulin, [[Wikipedia:Interleukin 6|IL-6]], cardiac marker Troponin I (cTnI), thyroid stimulating hormone (TSH), sTNF-RI, and 17β-estradiol.<ref name="Ng_2012" /><ref name="Sista_2008b">{{cite journal | vauthors = Sista RS, Eckhardt AE, Srinivasan V, Pollack MG, Palanki S, Pamula VK | title = Heterogeneous immunoassays using magnetic beads on a digital microfluidic platform | journal = Lab on a Chip | volume = 8 | issue = 12 | pages = 2188–96 | date = December 2008 | pmid = 19023486 | pmc = 2726047 | doi = 10.1039/b807855f }}</ref><ref name="Tsaloglou_2014">{{cite journal | vauthors = Tsaloglou MN, Jacobs A, Morgan H | title = A fluorogenic heterogeneous immunoassay for cardiac muscle troponin cTnI on a digital microfluidic device | journal = Analytical and Bioanalytical Chemistry | volume = 406 | issue = 24 | pages = 5967–76 | date = September 2014 | pmid = 25074544 | doi = 10.1007/s00216-014-7997-z | s2cid = 24266593 }}</ref><ref name="Huang_2016">{{cite journal | vauthors = Huang CY, Tsai PY, Lee IC, Hsu HY, Huang HY, Fan SK, Yao DJ, Liu CH, Hsu W | title = A highly efficient bead extraction technique with low bead number for digital microfluidic immunoassay | journal = Biomicrofluidics | volume = 10 | issue = 1 | pages = 011901 | date = January 2016 | pmid = 26858807 | pmc = 4714987 | doi = 10.1063/1.4939942 }}</ref> For example, a magnetic bead-based approached has been used for the detection of cTnI from whole blood in less than 8 minutes.<ref name="Sista_2008b" /> Briefly, magnetic beads containing primary antibodies were mixed with labeled secondary antibodies, incubated, and immobilized with a magnet for the washing steps. The droplet was then mixed with a chemiluminescent reagent and detection of the accompanying enzymatic reaction was measured on-chip with a [[photomultiplier]] tube.

The ELISA template, commonly used for performing immunoassays and other enzyme-based biochemical assays, has been adapted for use with the DMF platform for the detection of analytes such as IgE and IgG.<ref name="Zhu_2012">{{cite journal | vauthors = Zhu L, Feng Y, Ye X, Feng J, Wu Y, Zhou Z | title = An ELISA chip based on an EWOD microfluidic platform. | journal = Journal of Adhesion Science and Technology | date = September 2012 | volume = 26 | issue = 12–17 | pages = 2113–24 | doi = 10.1163/156856111x600172 | s2cid = 136668522 }}</ref><ref name="pmid21057776">{{cite journal | vauthors = Miller EM, Ng AH, Uddayasankar U, Wheeler AR | title = A digital microfluidic approach to heterogeneous immunoassays | journal = Analytical and Bioanalytical Chemistry | volume = 399 | issue = 1 | pages = 337–45 | date = January 2011 | pmid = 21057776 | doi = 10.1007/s00216-010-4368-2 | s2cid = 2809777 }}</ref> In one example,<ref name="Vergauwe_2011" /> a series of bioassays were conducted to establish the quantification capabilities of DMF devices, including an ELISA-based immunoassay for the detection of IgE. Superparamagnetic nanoparticles were  immobilized with anti-IgE antibodies and fluorescently labeled aptamers to quantify IgE using an ELISA template. Similarly, for the detection of IgG, IgG can be immobilized onto a DMF chip, conjugated with horseradish-peroxidase (HRP)-labeled IgG, and then quantified through measurement of the color change associated with product formation of the reaction between HRP and tetramethylbenzidine.<ref name="Zhu_2012" />

To further expand the capabilities and applications of DMF immunoassays beyond [[Colorimetry|colorimetric]] detection (i.e., ELISA, magnetic bead-based assays), electrochemical detection tools (e.g., microelectrodes) have been incorporated into DMF chips for the detection of analytes such as TSH and rubella virus.<ref name="Shamsi_2014" /><ref name="Rackus_2015">{{cite journal | vauthors = Rackus DG, Dryden MD, Lamanna J, Zaragoza A, Lam B, Kelley SO, Wheeler AR | title = A digital microfluidic device with integrated nanostructured microelectrodes for electrochemical immunoassays | journal = Lab on a Chip | volume = 15 | issue = 18 | pages = 3776–84 | date = 2015 | pmid = 26247922 | doi = 10.1039/c5lc00660k }}</ref><ref name="Dixon_2016">{{cite journal | vauthors = Dixon C, Ng AH, Fobel R, Miltenburg MB, Wheeler AR | title = An inkjet printed, roll-coated digital microfluidic device for inexpensive, miniaturized diagnostic assays | journal = Lab on a Chip | volume = 16 | issue = 23 | pages = 4560–4568 | date = November 2016 | pmid = 27801455 | doi = 10.1039/c6lc01064d | url = https://authors.library.caltech.edu/71665/4/c6lc01064d.pdf }}</ref> For example, Rackus et al.<ref name="Rackus_2015" /> integrated microelectrodes onto a DMF chip surface and substituted a previously reported chemiluminescent IgG immunoassay<ref name="Ng_2015b">{{cite journal | vauthors = Ng AH, Lee M, Choi K, Fischer AT, Robinson JM, Wheeler AR | title = Digital microfluidic platform for the detection of rubella infection and immunity: a proof of concept | journal = Clinical Chemistry | volume = 61 | issue = 2 | pages = 420–9 | date = February 2015 | pmid = 25512641 | doi = 10.1373/clinchem.2014.232181 | doi-access = free }}</ref> with an electroactive species, enabling detection of rubella virus. They coated magnetic beads with rubella virus, anti-rubella IgG, and anti-human IgG coupled with alkaline phosphatase, which in turn catalyzed an electron transfer reaction that was detected by the on-chip microelectrodes.