Editing Microfluidics (section)

=== 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.<ref name="pmid22871959">{{cite journal | vauthors = Kim JY, Cho SW, Kang DK, Edel JB, Chang SI, deMello AJ, O'Hare D | title = Lab-chip HPLC with integrated droplet-based microfluidics for separation and high frequency compartmentalisation | journal = Chemical Communications | volume = 48 | issue = 73 | pages = 9144–9146 | date = September 2012 | pmid = 22871959 | doi = 10.1039/c2cc33774f }}</ref> The early methods had the advantage of easier detection from certain machines like those that measure fluorescence.<ref name="pmid28374582">{{cite journal | vauthors = Ochoa A, Álvarez-Bohórquez E, Castillero E, Olguin LF | title = Detection of Enzyme Inhibitors in Crude Natural Extracts Using Droplet-Based Microfluidics Coupled to HPLC | journal = Analytical Chemistry | volume = 89 | issue = 9 | pages = 4889–4896 | date = May 2017 | pmid = 28374582 | doi = 10.1021/acs.analchem.6b04988 }}</ref> HPLC columns have been integrated 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 [[Polydimethylsiloxane|PDMS]] used in many different droplet-based microfluidic devices.<ref name="pmid29096060">{{cite journal | vauthors = Gerhardt RF, Peretzki AJ, Piendl SK, Belder D | title = Seamless Combination of High-Pressure Chip-HPLC and Droplet Microfluidics on an Integrated Microfluidic Glass Chip | journal = Analytical Chemistry | volume = 89 | issue = 23 | pages = 13030–13037 | date = December 2017 | pmid = 29096060 | doi = 10.1021/acs.analchem.7b04331 }}</ref><ref>{{cite conference|vauthors = Killeen K, Yin H, Sobek D, Brennen R, Van de Goor T|title = Chip-LC/MS: HPLC-MS using polymer microfluidics.|journal = Proc MicroTAS|conference = 7th lnternatonal Conference on Miniaturized Chemical and Blochemlcal Analysts Systems|location = Squaw Valley, Callfornla USA|date = October 2003|pages = 481–484|url = https://www.rsc.org/binaries/LOC/2003/Volume1/120-376.pdf }}</ref> 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.<ref name="pmid16583688">{{cite journal | vauthors = Vollmer M, Hörth P, Rozing G, Couté Y, Grimm R, Hochstrasser D, Sanchez JC | title = Multi-dimensional HPLC/MS of the nucleolar proteome using HPLC-chip/MS | journal = Journal of Separation Science | volume = 29 | issue = 4 | pages = 499–509 | date = March 2006 | pmid = 16583688 | doi = 10.1002/jssc.200500334 }}</ref> 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.<ref name="pmid15859622">{{cite journal | vauthors = Reichmuth DS, Shepodd TJ, Kirby BJ | title = Microchip HPLC of peptides and proteins | journal = Analytical Chemistry | volume = 77 | issue = 9 | pages = 2997–3000 | date = May 2005 | pmid = 15859622 | doi = 10.1021/ac048358r }}</ref> The coupling of HPLC-chip devices with other spectrometry methods like mass-spectrometry allow for enhanced confidence in identification of desired species, like proteins.<ref name="pmid17016832">{{cite journal | vauthors = Hardouin J, Duchateau M, Joubert-Caron R, Caron M | title = Usefulness of an integrated microfluidic device (HPLC-Chip-MS) to enhance confidence in protein identification by proteomics | journal = Rapid Communications in Mass Spectrometry | volume = 20 | issue = 21 | pages = 3236–3244 | date = 2006 | pmid = 17016832 | doi = 10.1002/rcm.2725 | bibcode = 2006RCMS...20.3236H }}</ref> 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.<ref name="pmid17997523">{{cite journal | vauthors = Brennen RA, Yin H, Killeen KP | title = Microfluidic gradient formation for nanoflow chip LC | journal = Analytical Chemistry | volume = 79 | issue = 24 | pages = 9302–9309 | date = December 2007 | pmid = 17997523 | doi = 10.1021/ac0712805 }}</ref> Some other practical applications of integrated HPLC chips include the determination of drug presence in a person through their hair<ref name="pmid22281681">{{cite journal | vauthors = Zhu KY, Leung KW, Ting AK, Wong ZC, Ng WY, Choi RC, Dong TT, Wang T, Lau DT, Tsim KW | display-authors = 6 | title = Microfluidic chip based nano liquid chromatography coupled to tandem mass spectrometry for the determination of abused drugs and metabolites in human hair | journal = Analytical and Bioanalytical Chemistry | volume = 402 | issue = 9 | pages = 2805–2815 | date = March 2012 | pmid = 22281681 | doi = 10.1007/s00216-012-5711-6 | s2cid = 7748546 }}</ref> and the labeling of peptides through reverse phase liquid chromatography.<ref name="pmid22975804">{{cite journal | vauthors = Polat AN, Kraiczek K, Heck AJ, Raijmakers R, Mohammed S | title = Fully automated isotopic dimethyl labeling and phosphopeptide enrichment using a microfluidic HPLC phosphochip | journal = Analytical and Bioanalytical Chemistry | volume = 404 | issue = 8 | pages = 2507–2512 | date = November 2012 | pmid = 22975804 | doi = 10.1007/s00216-012-6395-7 | s2cid = 32545802 }}</ref>