Paper Microfluidics - Yiliang Zhou, Aditi Naik
Paper-based microfluidic devices are devices that can control and manipulate small amount of liquid on the cellulose substrate. Cellulose, as a major component of all plant matter, is one of the most abundant biopolymers in nature.1 Compared to traditional microfluidics, cellulose substrates provide the opportunity for applications to be low-cost, low weight, and flexible.2 Moreover, fluid flow can be easily driven by capillary force within the microchannel, which requires no external power supply. The porous structure of cellulose provides for a high-surface area to volume ratio, which can increase the sensor performance of these types of devices. Cellulose substrates also have ability to store reagents in active form within the channel. All these advantages of using cellulose, makes it as an ideal material for microfluidics devices. As shown in Fig. 1, one of the most well-known paper-based microfluidics device is a simple pregnancy test.
Compared to traditional microfluidics, paper-based microfluidics still have the following limitations:
(1) The small sample retention
(2) For sample with low surface tension, some hydrophobic agents for patterning devices are not strong enough
(3) The limit of detection is usually high for the colorimetric detection method
Cellulose paper is a hydrophilic substrate. In order to fabricate microfluidics channel, specific methods need to be used to tune the hydrophobicity of the cellulose substrate. Two mechanisms were applied to pattern microfluidics channels. One is to selectively fabricate a hydrophobic surface onto cellulose film. Another is used to modify cellulose film as a hydrophobic substrate first, and then, selectively tune part of surface back to hydrophilic surface. Until now, different techniques have been proved to be feasible to create microfluidics channels on the cellulose substrate, such as wax printing,3 inkjet printing,4 photolithography,5 flexographic printing,6 screen printing,7 laser cutting8 and plasma treatment.9 Researchers have divided these methods into four different categories, as shown in Fig. 2.10
Wax printing is a technique to use wax as a hydrophobic agent on the cellulose substrate.3 The mechanism for it is illustrated in Fig.3. Wax can be easily printed by wax pen, inkjet printer or wax printer onto the surface of cellulose membrane. And then, the wax can be melted, allowing it to penetrate through the cellulose under mild heat treatment. The surface with wax turns to a hydrophobic surface. Wax printing is a simple, rapid, and environmentally-friendly method for patterning. The disadvantage of this method is that researchers need expensive wax printer and extra heating process to complete the process.
Inkjet printing is a new type of computer printing method to transfer digital design onto different substrates with droplets of ink. Specific biomolecules can be precisely printed onto the sensing zone of the paper-based microfluidics device.4 As for patterning microfluidics channel, inkjet etching and inkjet printing are developed. For inkjet etching, toluene was used as inkjet agent to selectively remove hydrophobic polystyrene that was prepatterned on the paper. For inkjet printing, hydrophobic agents, such as alkyl ketene dimer, were directly printed on the paper to form hydrophobic barrier.10 The advantage of inkjet printing is high resolution, wide choice of inks. Electrodes can also be printed by this method. The disadvantage of this process is that the speed of printing is not feasible for massive production, unless roll-to-roll fabrication is used.
Photoresist-saturated paper is exposed to UV light through a photomask. After exposure, uncured photoresist can be removed by organic solvent. Cured photoresist form as a hydrophobic barrier on the paper.5 The fabrication process is rapid and allows for high resolution. The drawback is that photolithography needs the use of organic solvents and expensive photoresists.
Flexographic printing is a high throughput fabrication technique. The illustration for flexographic printing is shown in Fig. 4. Olkkonen et al. used flexographic printing with polystyrene ink to fabricate microfluidics channel on paper substrate.6 This method can be easily scale-up, the speed of printing can be greater than 300 m/min. The disadvantage of it is that different printing plates are needed. Meanwhile, it can only print one reagent at one time.
Screen printing is a printing method that use a mesh to transfer ink on the substrate, unless the area that was protected by blocking stencil. The detailed procedure is listed on Fig. 5. The process of screen printing is simple; however, the resolution is low and different blocking stencils are needed for different designs.
The laser cutting used computer to control CO2 laser to cut paper substrate into specific design. No chemicals are needed during process. The disadvantage of this method is expense of laser cutter is far more than a knife and the power of laser is strong that cause paper substrate warping or tearing.
Cellulose substrate was pre-modified via octadecyltrichlorosilane silanization to fabricate a hydrophobic surface. Plasma treatment with a mask will let exposed area of the substrate turn back to hydrophilic, due to degradation of octadecyltrichlorosilane.9 The drawback of this method is that the substrate under a mask is easily over-etched.
The main application for paper-based microfluidics is to provide a low-cost, user-friendly analytic platform for assay diagnosis. Pregnant test is one of well-known products for paper-based microfluidics. Researchers have used paper-based microfluidics for a wide range of applications, such as biochemical detection, immunological detection and molecular detection.11
Many analytes have been proved that they can be detected with paper-based microfluidics. In the detection zone on the paper substrate, the analytes can have a chemical reaction with the immobilized reagent and develop a signal. The signal can be detected by different methods, such as colorimetric, electrochemical, fluorescent, chemiluminiscence (CL). Lopez-Ruiz et al. successfully developed paper microfluidic devices with applications of pH and nitrite colorimetric determination.12
Immunological detection is to detect analytes with immunoassay technique. Antibody or protein can be covalently bonded on the paper substrate with surface modification. One of advantages for paper-based microfluidics in immunological detection is that paper substrate have capability of storage reagent in active form. It is crucial for applications in low resource setting and point of care application.
Sequence-specific detection of nucleic acid hybridization have been proved that they can be detected with paper-based microfluidics. The sequence with a tag or change in the concentration can be targeted by capture probe, letting the reaction visible or measurable.
1 Klemm, D.; Heublein, B.; Fink, H. P.; Bohn, A. Angew. Chem. Int. Ed. 2005, 44, 3358. DOI: 10.1002/anie.200460587
2 Zhou, Y.; Fuentes-Hernandez, C.; Khan, T. M.; Liu, J. C.; Hsu, J.; Shim, J. W.; Dindar, A.; Youngblood, J. P.; Moon, R. J.; Kippelen, B. Scientific reports 2013, 3, 1536. DOI:10.1038/srep01536
3 Lu, Y.,Shi,W.,Qin,J.,Lin,B., Anal.Chem, 2010, 82,329. DOI: 10.1021/ac9020193
4 Abe, K.,Suzuki,K.,Citterio,D., Anal.Chem, 2008, 80,6928. DOI: 10.1021/ac800604v
5 Martinez, A. W.; Phillips, S. T.; Butte, M. J.; Whitesides, G. M. Angew. Chem. Int. Ed. 2007, 46, 1318−1320. DOI: 10.1002/anie.200603817
6 Olkkonen, J.; Lehtinen, K.; Erho, T. Anal. Chem. 2010, 82, 10246−10250. DOI: 10.1021/ac1027066
7 Renault,C.,Scida,K.,Knust,K.N.,Fosdick,S.E.,Crooks,R.M., J. Electrochem. Sci. Te 2013, 4(4),146–152. DOI : 10.5229/JECST.2013.4.4.146
8 Nie, J.; Liang, Y.; Zhang, Y.; Le, S.; Li, D.; Zhang, S. Analyst 2013, 138, 671−676. DOI: 10.1039/C2AN36219H
9 Cai, L.,Xu,C.,Lin,S.,Luo,J.,Wu,M.,Yang,F., Biomicrofluidics 2014, 8, 5. DOI: 10.1063/1.4898096
10 David M. C., Jaclyn A. A., Jaruwan M., Charles S. H. Anal. Chem., 2015, 87 (1), 19–41. DOI: 10.1021/ac503968p
11 Xia, Y., Si, J., & Li, Z. Biosensors and Bioelectronics, 2016, 77, 774-789. DOI:10.1016/j.bios.2015.10.032
12 Lopez-Ruiz, N., Curto, V.F., Erenas, M.M., Benito-Lopez, F., Diamond, D., Palma, A.J., Capitan-Vallvey, L.F., Anal. Chem. 2014, 86 (19), 9554–9562. DOI: 10.1021/ac5019205