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Lo sviluppo di dispositivi a microfluidi su carta si è sviluppato agli inizi del Terzo Millennio per venire incontro alla necessità di sistemi di diagnosi clinica portatili, economici e semplici da utilizzare. Tali dispositivi in genere consistono in una serie di fibre di cellulosa idrofile o di nitrocellulosa che guidano il liquido da un'entrata ad un'apposita uscita tramite imbibizione.

Struttura del dispositivo

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Generalmente un dispositivo su carta di questo tipo è costituito dalle seguenti regioni:[1]

  • Entrata (inlet): un substrato (tipicamente cellulosa) dove i liquidi vengono depositati.
  • Canali (channels): reti idrofile di dimensioni sub-millimetriche che guidano il liquido attraverso il dispositivo.
  • Barriere (barriers): regioni idrofobiche che impediscono al liquido di fuoriuscire dal canale.
  • Uscite (outlets): punti dove una reazione (bio)chimica ha luogo.

Flusso attraverso il dispositivo

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La carta è un mezzo poroso in cui il fluido viene trasportato principalmente tramite traspirazione (wicking) ed evaporazione.[2] Il flusso capillare durante l'umidificazione si può approssimare tramite l'equazione di Washburn,[3] derivata dalla legge di Jurin e dall'equazione di Hagen-Poiseuille.[4] La velocità media del fluido viene generalizzata come segue: dove   è la tensione superficiale,   l'angolo di contatto,   la viscosità e   è la distanza percorsa dal liquido. Modelli più approfonditi tengono conto del raggio dei pori, della tortuosità della carta[5] e della sua deformazione nel tempo.[6]

Una volta che il mezzo è completamente bagnato, il flusso diventa laminare e segue la legge di Darcy.[7] La velocità media del flusso di liquido è generalizzata come: dove   è la permeabilità del mezzo permeability e   è il gradiente di pressione.[8] Una conseguenza del flusso laminare è che il mescolamento è difficile ed è basato solamente sulla diffusione, che è più lenta nei sistemi porosi.[9]

Tecniche di manifattura

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I dispositivi a microfluido si possono preparare usando varie tecniche.

Microfluidic devices can be manufactured using variations of wax printing, inkjet printing, photolithography, flexographic printing, plasma treatment, laser treatment, wet etching[disambiguation needed], screen printing and wax screening.[10] Ogni tecnicha punta a creare barriere fisiche ed idrofobiche su una carta idrofila che trasporta passivamente soluzioni acquose.[11] I reagenti chimici e biologici devono essere depositati selettivamente lungo il dispositivo immergendo il substrato in una soluzione del reagente o depositando un reagente sopra il substrato.[12]

Wax printing

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La stampa a cera usa una semplice stampante per modellare la cera sulla carta a piacimento. La cera viene poi sciolta sopra una piastra riscaldante al fine di creare i canali.[13] Questa tecnica è veloce e a basso costo, ma ha una risoluzione relativamente bassa a causa della non isotropia della cera sciolta.

Inkjet printing

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La stampa ad inchiostro richiede una carta rivestita da un polimero idrofobico. Si piazza poi un inchiostro che incide il polimero rivelando la carta sottostante.[14] Questo metodo è economico e ad alta risoluzione, ma è limitato dalla velocità di deposizione delle gocce di inchiostro (in genere una goccia per volta).

Fotolitografia

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Le tecniche fotolitografiche sono simile alla stampa a getto d'inchiostro e usano una fotomaschera (photomask) per incidere selettivamente un polimero di fotoresist.[15] Questa tecnica garantisce alta risoluzione e velocità, tuttavia l'apparecchiatura richiesta così come i materiali non la rendono molto economica.

Applicazioni

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Panoramica

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Il vantaggio principale di dispisitivi a microfluido su carta rispetto a quelli tradizionali e il loro uso direttamente sul campo piuttosto che in laboratorio.[16][17] La carta da flltro è molto vantaggiosa a questo proposito perché è capace di rimuovere i contaminanti dal campione e prevenire che si diffondano nel microcanale. Questo significa che le particelle non inibiscono l'accuratezza delle analisi su carta quando questi dispositivi sono usati all'aperto.[17] Questi dispositivi sono inoltre molto piccoli (dell'ordine dei centimetri)[17][18][19] rispetto ad altre piattaforme come quelli a goccia di liquido.[20][21] Per via della loro piccola taglia e lunga durabilità, i dispositivi qui descritti sono portatili e convenienti.[16][17] Paper-based devices are also relatively inexpensive. Filter paper is very cheap, and so are most of the patterning agents used in the fabrication of microchannels, including PDMS and wax. Most of the major paper-based fabrication methods also do not require expensive laboratory equipment.[16] These characteristics of paper-based microfluidics make it ideal for point-of-care testing, particularly in countries that lack advanced medical diagnostic tools.[17] Paper-based microfluidics has also been used to conduct environmental and food safety tests.[22][23][24][25]

Test del point of care: rilevamento del glucosio

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Paper-based microfluidic devices have been designed to monitor a wide variety of medical ailments. Glucose plays an important role in diabetes and cancer,[26] and it can be detected through a catalytic cycle involving glucose oxidase, hydrogen peroxide, and horseradish peroxidase that initiates a reaction between glucose and a color indicator, frequently potassium iodide, on a paper-based microfluidic device.[26] This is an example of colorimetric detection. The first paper-based microfluidic device, developed by George Whitesides’ group at Harvard, was able to simultaneously detect protein as well as glucose via color-change reactions (potassium iodide reaction for glucose and tetrabromophenol blue reaction for the protein BSA).[17] The bottom of the paper device is inserted into a sample solution prepared in-lab, and the amount of color change is observed.[17] More recently, a paper-based microfluidic device using colorimetric detection was developed to quantify glucose in blood plasma. Blood plasma is separated from whole blood samples on a wax-printed device, where red blood cells are agglutinated by antibodies and the blood plasma is able to flow to a second compartment for the color-change reaction.[18] Electrochemical detection[27] has also been used in these devices. It provides greater sensitivity in quantification, whereas colorimetric detection is primarily used for qualitative assessments.[16][26] Screen-printed electrodes[28] and electrodes directly printed on filter paper[29] have been used. One example of a paper-based microfluidic device utilizing electrochemical detection has a dumbbell shape to isolate plasma from whole blood.[29] The current from the hydrogen peroxide produced in the aforementioned catalytic cycle is measured and converted into concentration of glucose.[29]

Dispositivi tridimensionali per il rilevamento di glucosio

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Whitesides’ group also developed a 3D paper-based microfluidic device for glucose detection that can produce calibration curves on-chip because of the improved fluid flow design.[30] This 3D device consists of layers of paper patterned with microfluidic channels that are connected by layers of double-sided adhesive tape with holes. The holes in the tape permit flow between channels in alternating layers of paper, so this device allows for more complicated flow paths and enables the detection of multiple samples in a large number (up to ~1,000) of detection zones in the last layer of paper.[30] More recently, 3D paper-based microfluidic devices assembled using origami were developed.[31] Unlike Whitesides’ design, these devices utilize a single layer of patterned paper that is then folded into multiple layers before sample solution is injected into the device.[31] Subsequently, the device can be unfolded, and each layer of the device can be analyzed for the simultaneous detection of multiple analytes.[31] This device is simpler and less expensive to fabricate than the aforementioned device using multiple layers of paper.[30][31] Mixing between the channels in the different layers was not an issue in either device, so both devices were successful in quantifying glucose and BSA in multiple samples simultaneously.[30][31]

Analisi ambientali e di sicurezza alimentare

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Paper-based microfluidic devices have several applications outside of the medical field. For example, paper-based microfluidics has been used extensively in environmental monitoring.[22][23][24][25] Two recent devices were developed for the detection of Salmonella[23] and E. coli[22]. The latter device was specifically used to detect E. coli in seven field water samples from Tucson, Arizona.[22] Antibody-conjugated polystyrene particles were loaded in the middle of the microfluidic channel, after the sample inlet. Immunoagglutination occurs when samples containing Salmonella or E. coli, respectively, come into contact with these particles.[22][23] The amount of immunoagglutination can be correlated with increased Mie scattering of light, which was detected with a specialized smartphone application under ambient light.[22][23] Paper-based microfluidics has also been used to detect pesticides in food products, such as apple juice and milk.[24] A recent design used piezoelectric inkjet printing to imprint paper with the enzyme acetylcholinesterase (AChE) and the substrate indophenyl acetate (IPA), and this paper-based microfluidic device was used to detect organophosphate pesticides (AChE inhibitors) via a decrease in blue-purple color.[24] This device is distinguished by its use of bioactive paper instead of compartments with pre-stored reagents, and it was demonstrated to have good long-term stability, making it ideal for field use.[24] A more recent paper-based microfluidic design utilized a sensor, consisting of fluorescently labeled single-stranded DNA (ssDNA) coupled with graphene oxide, on its surface to simultaneously detect heavy metals and antibiotics in food products.[25] Heavy metals increased fluorescence intensity, whereas antibiotics decreased fluorescence intensity.[25]

Note

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  1. ^ Berthier, Jean; Brakke, Kenneth A.; Berthier, Erwin (2016). Open Microfluidics. John Wiley & Sons, Inc. pp. 229–256. doi:10.1002/9781118720936.ch7/summary. ISBN 9781118720936.
  2. ^ Dixit, Chandra K.; Kaushik, Ajeet (2016-10-13). Microfluidics for Biologists: Fundamentals and Applications. Springer. ISBN 9783319400365.
  3. ^ Masoodi, Reza; Pillai, Krishna M. (2012-10-26). Wicking in Porous Materials: Traditional and Modern Modeling Approaches. CRC Press. ISBN 9781439874325.
  4. ^ Washburn, Edward W. (1921-03-01). "The Dynamics of Capillary Flow". Physical Review. 17 (3): 273–283. doi:10.1103/PhysRev.17.273.
  5. ^ Cai, Jianchao; Yu, Boming (2011-09-01). "A Discussion of the Effect of Tortuosity on the Capillary Imbibition in Porous Media". Transport in Porous Media. 89 (2): 251–263. doi:10.1007/s11242-011-9767-0. ISSN 0169-3913.
  6. ^ Berthier, Jean; Brakke, Kenneth A. The Physics of Microdroplets - Berthier - Wiley Online Library. doi:10.1002/9781118401323.
  7. ^ Bejan, Adrian (2013). Convection Heat Transfer. John Wiley & Sons, Inc. pp. i–xxxiii. doi:10.1002/9781118671627.fmatter/pdf. ISBN 9781118671627.
  8. ^ Darcy, Henry (1856). Les fontaines publiques de la ville de Dijon. Exposition et application des principes à suivre et des formules à employer dans les questions de distribution d'eau: ouvrage terminé par un appendice relatif aux fournitures d'eau de plusieurs villes au filtrage des eaux et à la fabrication des tuyaux de fonte, de plomb, de tole et de bitume (in French). Dalmont.
  9. ^ Diffusion in Natural Porous Media - Contaminant Transport, | Peter Grathwohl | Springer.
  10. ^ "Paper microfluidic devices : A review 2017 - Elveflow". Elveflow. Retrieved 2018-02-06.
  11. ^ Galindo-Rosales, Francisco José (2017-05-26). Complex Fluid-Flows in Microfluidics. Springer. ISBN 9783319595931.
  12. ^ Yamada, Kentaro; Shibata, Hiroyuki; Suzuki, Koji; Citterio, Daniel (2017-03-29). "Toward practical application of paper-based microfluidics for medical diagnostics: state-of-the-art and challenges". Lab on a Chip. 17 (7). doi:10.1039/C6LC01577H. ISSN 1473-0189.
  13. ^ Carrilho, Emanuel; Martinez, Andres W.; Whitesides, George M. (2009-08-15). "Understanding Wax Printing: A Simple Micropatterning Process for Paper-Based Microfluidics". Analytical Chemistry. 81 (16): 7091–7095. doi:10.1021/ac901071p. ISSN 0003-2700.
  14. ^ Yamada, Kentaro; Henares, Terence G.; Suzuki, Koji; Citterio, Daniel (2015-04-27). "Paper-Based Inkjet-Printed Microfluidic Analytical Devices". Angewandte Chemie International Edition. 54 (18): 5294–5310. doi:10.1002/anie.201411508. ISSN 1521-3773.
  15. ^ "Development of paper-based microfluidic analytical device for iron assay using photomask printed with 3D printer for fabrication of hydrophilic and hydrophobic zones on paper by photolithography". Analytica Chimica Acta. 883: 55–60. 2015-07-09. doi:10.1016/j.aca.2015.04.014. ISSN 0003-2670.
  16. ^ a b c d Li, Xu; Ballerini, David R.; Shen, Wei (2012-03-02). "A perspective on paper-based microfluidics: Current status and future trends". Biomicrofluidics. 6 (1): 011301–011301–13. doi:10.1063/1.3687398. ISSN 1932-1058. PMC 3365319. PMID 22662067.
  17. ^ a b c d e f g Martinez, Andres W.; Phillips, Scott T.; Butte, Manish J.; Whitesides, George M. (2007). "Patterned paper as a platform for inexpensive, low-volume, portable bioassays". Angewandte Chemie (International Ed. in English). 46 (8): 1318–1320. doi:10.1002/anie.200603817. ISSN 1433-7851. PMC 3804133. PMID 17211899.
  18. ^ a b Yang, Xiaoxi; Forouzan, Omid; Brown, Theodore P.; Shevkoplyas, Sergey S. (2012-01-21). "Integrated separation of blood plasma from whole blood for microfluidic paper-based analytical devices". Lab on a Chip. 12 (2): 274–280. doi:10.1039/c1lc20803a. ISSN 1473-0189. PMID 22094609.
  19. ^ Yu, Jinghua; Ge, Lei; Huang, Jiadong; Wang, Shoumei; Ge, Shenguang (2011-04-07). "Microfluidic paper-based chemiluminescence biosensor for simultaneous determination of glucose and uric acid". Lab on a Chip. 11 (7): 1286–1291. doi:10.1039/c0lc00524j. ISSN 1473-0189. PMID 21243159.
  20. ^ Clausell-Tormos, Jenifer; Lieber, Diana; Baret, Jean-Christophe; El-Harrak, Abdeslam; Miller, Oliver J.; Frenz, Lucas; Blouwolff, Joshua; Humphry, Katherine J.; Köster, Sarah (May 2008). "Droplet-based microfluidic platforms for the encapsulation and screening of Mammalian cells and multicellular organisms". Chemistry & Biology. 15 (5): 427–437. doi:10.1016/j.chembiol.2008.04.004. ISSN 1074-5521. PMID 18482695.
  21. ^ Baret, Jean-Christophe; Miller, Oliver J.; Taly, Valerie; Ryckelynck, Michaël; El-Harrak, Abdeslam; Frenz, Lucas; Rick, Christian; Samuels, Michael L.; Hutchison, J. Brian (2009-07-07). "Fluorescence-activated droplet sorting (FADS): efficient microfluidic cell sorting based on enzymatic activity". Lab on a Chip. 9 (13): 1850–1858. doi:10.1039/b902504a. ISSN 1473-0197. PMID 19532959.
  22. ^ a b c d e f Park, Tu San; Yoon, Jeong-Yeol (2015-03-01). Smartphone Detection of Escherichia coli From Field Water Samples on Paper Microfluidics. Vol. 15.
  23. ^ a b c d e Park, Tu San; Li, Wenyue; McCracken, Katherine E.; Yoon, Jeong-Yeol (2013-12-21). "Smartphone quantifies Salmonella from paper microfluidics". Lab on a Chip. 13 (24): 4832–4840. doi:10.1039/c3lc50976a. ISSN 1473-0189. PMID 24162816.
  24. ^ a b c d e Hossain, S. M. Zakir; Luckham, Roger E.; McFadden, Meghan J.; Brennan, John D. "Reagentless Bidirectional Lateral Flow Bioactive Paper Sensors for Detection of Pesticides in Beverage and Food Samples". Analytical Chemistry. 81 (21): 9055–9064. doi:10.1021/ac901714h.
  25. ^ a b c d Zhang, Yali; Zuo, Peng; Ye, Bang-Ce (2015-06-15). "A low-cost and simple paper-based microfluidic device for simultaneous multiplex determination of different types of chemical contaminants in food". Biosensors & Bioelectronics. 68: 14–19. doi:10.1016/j.bios.2014.12.042. ISSN 1873-4235. PMID 25558869.
  26. ^ a b c Liu, Shuopeng; Su, Wenqiong; Ding, Xianting (2016-12-08). "A Review on Microfluidic Paper-Based Analytical Devices for Glucose Detection". Sensors. 16 (12): 2086. doi:10.3390/s16122086.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  27. ^ Dungchai, Wijitar; Chailapakul, Orawon; Henry, Charles S. "Electrochemical Detection for Paper-Based Microfluidics". Analytical Chemistry. 81 (14): 5821–5826. doi:10.1021/ac9007573.
  28. ^ Noiphung, Julaluk; Songjaroen, Temsiri; Dungchai, Wijitar; Henry, Charles S.; Chailapakul, Orawon; Laiwattanapaisal, Wanida (2013-07-25). "Electrochemical detection of glucose from whole blood using paper-based microfluidic devices". Analytica Chimica Acta. 788: 39–45. doi:10.1016/j.aca.2013.06.021. ISSN 1873-4324. PMID 23845479.
  29. ^ a b c Li, Zedong; Li, Fei; Hu, Jie; Wee, Wei Hong; Han, Yu Long; Pingguan-Murphy, Belinda; Lu, Tian Jian; Xu, Feng (2015-08-21). "Direct writing electrodes using a ball pen for paper-based point-of-care testing". The Analyst. 140 (16): 5526–5535. doi:10.1039/c5an00620a. ISSN 1364-5528. PMID 26079757.
  30. ^ a b c d Martinez, Andres W.; Phillips, Scott T.; Whitesides, George M. (2008-12-16). "Three-dimensional microfluidic devices fabricated in layered paper and tape". Proceedings of the National Academy of Sciences of the United States of America. 105 (50): 19606–19611. doi:10.1073/pnas.0810903105. ISSN 1091-6490. PMC 2604941. PMID 19064929.
  31. ^ a b c d e Liu, Hong; Crooks, Richard M. "Three-Dimensional Paper Microfluidic Devices Assembled Using the Principles of Origami". Journal of the American Chemical Society. 133 (44): 17564–17566. doi:10.1021/ja2071779.
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