Editing Digital microfluidics (section)

=== Chemical synthesis ===
Digital Microfluidics (DMF) allows for precise manipulation and coordination in small-scale chemical synthesis reactions due to its ability to control micro scale volumes of liquid reagents, allowing for overall less reagent use and waste.<ref name="Geng_2017">{{cite journal | vauthors = Geng H, Feng J, Stabryla LM, Cho SK | title = Dielectrowetting manipulation for digital microfluidics: creating, transporting, splitting, and merging of droplets | journal = Lab on a Chip | volume = 17 | issue = 6 | pages = 1060–1068 | date = March 2017 | pmid = 28217772 | doi = 10.1039/c7lc00006e }}</ref> This technology can be used in the synthesis compounds such as [[peptidomimetic]]s and [[Positron emission tomography|PET]] tracers.<ref name="Jebrail_2012">{{Cite journal| vauthors = Jebrail MJ, Assem N, Mudrik JM, Dryden MD, Lin K, Yudin AK, Wheeler AR |date=2012-08-01|title=Combinatorial Synthesis of Peptidomimetics Using Digital Microfluidics|journal=Journal of Flow Chemistry|volume=2|issue=3|pages=103–107|doi=10.1556/JFC-D-12-00012 |bibcode=2012JFlCh...2..103J |s2cid=34049157}}</ref><ref name="Chen_2014">{{cite journal | vauthors = Chen S, Javed MR, Kim HK, Lei J, Lazari M, Shah GJ, van Dam RM, Keng PY, Kim CJ | display-authors = 6 | title = Radiolabelling diverse positron emission tomography (PET) tracers using a single digital microfluidic reactor chip | journal = Lab on a Chip | volume = 14 | issue = 5 | pages = 902–10 | date = March 2014 | pmid = 24352530 | doi = 10.1039/c3lc51195b | s2cid = 40777981 | url = http://www.escholarship.org/uc/item/44r9z66x }}</ref><ref name="Javed_2014">{{cite journal | vauthors = Javed MR, Chen S, Kim HK, Wei L, Czernin J, Kim CJ, van Dam RM, Keng PY | display-authors = 6 | title = Efficient radiosynthesis of 3'-deoxy-3'-18F-fluorothymidine using electrowetting-on-dielectric digital microfluidic chip | journal = Journal of Nuclear Medicine | volume = 55 | issue = 2 | pages = 321–8 | date = February 2014 | pmid = 24365651 | pmc = 4494735 | doi = 10.2967/jnumed.113.121053 }}</ref> [[Positron emission tomography|PET]] tracers require nanogram quantities and as such, DMF allows for automated and rapid synthesis of tracers with 90-95% efficiency compared to conventional macro-scale techniques.<ref name="Chen_2014" /><ref name="Keng_2012">{{cite journal | vauthors = Keng PY, Chen S, Ding H, Sadeghi S, Shah GJ, Dooraghi A, Phelps ME, Satyamurthy N, Chatziioannou AF, Kim CJ, van Dam RM | display-authors = 6 | title = Micro-chemical synthesis of molecular probes on an electronic microfluidic device | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 109 | issue = 3 | pages = 690–5 | date = January 2012 | pmid = 22210110 | pmc = 3271918 | doi = 10.1073/pnas.1117566109 | bibcode = 2012PNAS..109..690K | doi-access = free }}</ref>

Organic reagents are not commonly used in DMF because they tend to wet the DMF device and cause flooding; however synthesis of organic reagents can be achieved through DMF techniques by carrying the organic reagents through an ionic liquid droplet, thus preventing the organic reagent from flooding the DMF device.<ref name="Dubois_2006">{{cite journal | vauthors = Dubois P, Marchand G, Fouillet Y, Berthier J, Douki T, Hassine F, Gmouh S, Vaultier M | display-authors = 6 | title = Ionic liquid droplet as e-microreactor | journal = Analytical Chemistry | volume = 78 | issue = 14 | pages = 4909–17 | date = July 2006 | pmid = 16841910 | doi = 10.1021/ac060481q | url = https://figshare.com/articles/Ionic_Liquid_Droplet_as_e_Microreactor/3070660 | url-access = subscription }}</ref> Droplets are combined together by inducing opposite charges thus attracting them to each other.<ref name="Um_2016">{{cite journal | vauthors = Um T, Hong J, Im do J, Lee SJ, Kang IS | title = Electrically Controllable Microparticle Synthesis and Digital Microfluidic Manipulation by Electric-Field-Induced Droplet Dispensing into Immiscible Fluids | journal = Scientific Reports | volume = 6 | issue = 1 | pages = 31901 | date = August 2016 | pmid = 27534580 | pmc = 4989170 | doi = 10.1038/srep31901 | bibcode = 2016NatSR...631901U }}</ref> This allows for automated mixing of droplets. Mixing of droplets are also used to deposit [[Metal-organic framework|MOF]] crystals for printing by delivering reagents into wells and evaporating the solutions for crystal deposition.<ref name="Witters_2012">{{cite journal | vauthors = Witters D, Vergauwe N, Ameloot R, Vermeir S, De Vos D, Puers R, Sels B, Lammertyn J | display-authors = 6 | title = Digital microfluidic high-throughput printing of single metal-organic framework crystals | journal = Advanced Materials | volume = 24 | issue = 10 | pages = 1316–20 | date = March 2012 | pmid = 22298246 | doi = 10.1002/adma.201104922 | bibcode = 2012AdM....24.1316W | s2cid = 205244275 }}</ref> This method of [[Metal-organic framework|MOF]] crystal deposition is relatively cheap and does not require extensive robotic equipment.<ref name="Witters_2012" />

Chemical synthesis using digital microfluidics (DMF) has been applied to many noteworthy biological reactions. These include [[polymerase chain reaction]] (PCR), as well as the formation of [[DNA]] and [[peptide]]s.<ref name="Dubois_2006" /><ref name="Jebrail_2010">{{cite journal | vauthors = Jebrail MJ, Ng AH, Rai V, Hili R, Yudin AK, Wheeler AR | title = Synchronized synthesis of peptide-based macrocycles by digital microfluidics | journal = Angewandte Chemie | volume = 49 | issue = 46 | pages = 8625–8629 | date = November 2010 | pmid = 20715231 | doi = 10.1002/anie.201001604 }}</ref> Reduction, alkylation, and enzymatic digestion have also shown robustness and reproducibility utilizing DMF, indicating potential in the synthesis and manipulation of [[proteomics]].<ref>{{cite journal | vauthors = Luk VN, Wheeler AR | title = A digital microfluidic approach to proteomic sample processing | journal = Analytical Chemistry | volume = 81 | issue = 11 | pages = 4524–4530 | date = June 2009 | pmid = 19476392 | doi = 10.1021/ac900522a | hdl-access = free | hdl = 1807/34790 }}</ref> Spectra obtained from the products of these reactions are often identical to their library spectra, while only utilizing a small fraction of bench-scale reactants.<ref name="Jebrail_2012" /> Thus, conducting these syntheses on the microscale has the benefit of limiting money spent on purchasing reagents and waste products produced while yielding desirable experimental results. However, numerous challenges need to be overcome to push these reactions to completion through DMF. There have been reports of reduced efficiency in chemical reactions as compared to bench-scale versions of the same syntheses, as lower product yields have been observed.<ref name="Jebrail_2010" /> Furthermore, since picoliter and nanoliter size samples must be analyzed, any instrument used in analysis needs to be high in sensitivity. In addition, system setup is often difficult due to extensive amounts of wiring and pumps that are required to operate microchannels and reservoirs.<ref name="Jebrail_2010" /> Finally, samples are often subject to solvent evaporation which leads to changes in volume and concentration of reactants, and in some cases reactions to not go to completion.<ref>{{cite journal | vauthors = Sadeghi S, Ding H, Shah GJ, Chen S, Keng PY, Kim CJ, van Dam RM | title = On chip droplet characterization: a practical, high-sensitivity measurement of droplet impedance in digital microfluidics | journal = Analytical Chemistry | volume = 84 | issue = 4 | pages = 1915–1923 | date = February 2012 | pmid = 22248060 | doi = 10.1021/ac202715f | s2cid = 9113055 | url = http://www.escholarship.org/uc/item/4tk0p54d }}</ref>

The composition and purity of molecules synthesized by DMF are often determined utilizing classic analytical techniques. [[Nuclear magnetic resonance]] (NMR) spectroscopy has been successfully applied to analyze corresponding intermediates, products, and reaction kinetics.<ref name="Jebrail_2012" /><ref>{{cite journal | vauthors = Wu B, von der Ecken S, Swyer I, Li C, Jenne A, Vincent F, Schmidig D, Kuehn T, Beck A, Busse F, Stronks H, Soong R, Wheeler AR, Simpson A | display-authors = 6 | title = Rapid Chemical Reaction Monitoring by Digital Microfluidics-NMR: Proof of Principle Towards an Automated Synthetic Discovery Platform | journal = Angewandte Chemie | volume = 58 | issue = 43 | pages = 15372–15376 | date = October 2019 | pmid = 31449724 | doi = 10.1002/anie.201910052 | s2cid = 201728604 }}</ref> A potential issue that arises through the use of NMR is low mass sensitivity, however this can be corrected for by employing [[microcoil]]s that assist in distinguishing molecules of differing masses.<ref name="Jebrail_2012" /> This is necessary since the [[signal-to-noise ratio]] of sample sizes in the microliter to nanoliter range is dramatically reduced compared to bench-scale sample sizes, and microcoils have been shown to resolve this issue.<ref>{{cite journal | vauthors = Peck TL, Magin RL, Lauterbur PC | title = Design and analysis of microcoils for NMR microscopy | journal = Journal of Magnetic Resonance, Series B | volume = 108 | issue = 2 | pages = 114–124 | date = August 1995 | pmid = 7648010 | doi = 10.1006/jmrb.1995.1112 | bibcode = 1995JMRB..108..114P }}</ref> [[Mass spectrometry]] (MS) and [[high-performance liquid chromatography]] (HPLC) have also been used to overcome this challenge.<ref name="Dubois_2006" /><ref name="Jebrail_2012" /> Although MS is an attractive analytical technique for distinguishing the products of reactions accomplished through DMF, it poses its own weaknesses. [[Matrix-assisted laser desorption/ionization|Matrix-assisted laser desorption ionization]] (MALDI) and [[electrospray ionization]] (ESI) MS have recently been paired with analyzing microfluidic chemical reactions. However, crystallization and dilution associated with these methods often leads to unfavorable side effects, such as sample loss and side reactions occurring.<ref name="Kirby_2013" /> The use of MS in DMF is discussed in more detail in a later section.