A simple method to produce 2D and 3D microfluidic paper-based analytical devices for clinical analysis
Graphical abstract
Introduction
In recent years, the interest in the development of microfluidic paper-based analytical devices (μPADs) has grown exponentially [1], [2]. μPADs are a new class of point-of-care devices, which combine the capabilities of conventional microfluidic devices with simple tests on paper strips. Aspects such as low cost, easy use, low sample consumption, simple fabrication, and high potential for use in remote locations are some of the main reasons behind the interest in μPADs. They have been applied as accessible tools for chemical, biological, or clinical analyses in developing countries or in remote areas as well as in emergency situations [1], [3], [4]. The advantage of using μPADs is the possibility of handling small sample volumes, thereby saving valuable reagents, reducing cost per analysis and providing rapid diagnosis requiring no trained personnel [5].
The use of cellulose paper as a substrate for creating μPADs presents several advantages including: (i) it is thin and light (∼10 mg/cm2), is available in a wide range of thicknesses (0.25–1.50 mm) [5], and is easy to storage and transport; (ii) biocompatibility with biological samples [6]; (iii) paper can be easily printed, coated, modified, and cut [6], [7]; (iv) it is biodegradable or can be easily burned, what is the ideal case for safe single usages [6]; (v) cellulose paper is inexpensive (US$ 0.15/m2–US$ 1.20/m2) [6]; and (vi) it can be used for sample pretreatment, since filter papers with well-defined pores can separate suspended solids from samples [1], [6]. μPADs have received enormous attention in recent years with promising applications in immunoassays [8], urine [9], and saliva analysis [10], environmental monitoring [11], [12], and blood tests, using colorimetric [11], [13], [14], electrochemical [15], [16], [17], chemiluminescent [8], [18], and other detection methods [19].
Photolithography [20] and wax printing [9], [21], [22] are among the main techniques used for fabricating μPADs [23], [24], [25]. Other techniques for creating hydrophilic channels in paper include the use of desktop plotter to print [26], stamp [27] or draw using commercial and in-house inks [28], poly(dimethylsiloxane) (PDMS), plasma treatment of hydrophobized paper [29], [30] laser cutting [31], and printer cutting [32], [33]. These manufacturing techniques still present many drawbacks for production of μPADs at large scale, including the use of expensive equipment, as those used in photolithography or the CO2 laser for cutting, as well as, the needed of too many steps as, e.g. the washing cycles to remove non-crosslinked polymer in the photolithography method [33]. Clipping methods involving nitrocellulose [33] or glass fiber membranes [34], which are harder and more expensive than usual cellulose-based filter paper, have been suitable for fabricating μPADs. Recently, it was reported that cellulose-based filter paper cannot be properly cut using knife cutters [34]. Yet Fenton et al. fabricated 2D lateral-flow devices cut from cellulose paper or nitrocellulose membrane previously surrounded by a polyester tape using a “kiss cuts” procedure, which was based on three sequential overlapping cuts with adjustment of knife height to gradually increase cutting deep, however, the costly nitrocellulose membrane was preferentially used [33]. Cassano and Fan developed 2D μPADs based on chromatographic paper cut using a digital craft cutter and thermal roll laminator to seal the devices [32]. To avoid tearing, the paper was covered with a sacrificial polyester film and cut in two steps using a high-force-cutting. Although the cellulose paper was cut without tearing using high-force-cutting, this cut mode cannot be found in low-cost home cutter printer.
In this paper, we describe an alternative method for fabricating 2D and 3D μPADs platforms based on a simple procedure to cut soft cellulose filter paper with an inexpensive home cutter printer and plastic adhesives to seal the microfluidic devices, thereby allowing ease and safe handling and transportation of the μPADs. The 2D and 3D μPADs produced were evaluated in bioassays applications encompassing colorimetric detection of glucose, nitrite, and total protein in simulated blood serum and urine.
Section snippets
Chemicals and materials
(+) Glucose (99.5%), glucose oxidase (from Aspergillus Niger, 215 U mg−1), peroxidase (from horseradish, 113 U mg−1), potassium iodate, bovine serum albumin (98%), bromophenol blue, potassium chloride, citric acid, sodium citrate, sodium nitrite, n-(1-napthyl)ethylenediamine, and sulphanilamide were purchased from Sigma-Aldrich (St. Louis, MO). Sodium carbonate, sodium bicarbonate, sodium phosphate dibasic, magnesium chloride, sodium chloride, urea, calcium chloride, magnesium sulphate, sodium
Fabrication of μPADs
One of the major challenges to produce μPADs is developing a simplest procedure capable of creating hydrophilic microfluidic channels in inexpensive cellulose-paper substrates, which could be easily applied for large scale production. A series of techniques have been used to overcome this challenge, e.g., photolithography, wax printing, PDMS stamp, and knife and laser cutting. Regarding the use of knife cutters, some authors have described the difficulty to cut filter papers. Harder and costly
Conclusions
This study describes a simple, easily, and cost-effective method to fabricate completely sealed 2D and 3D μPADs using filter papers and an inexpensive home cutter printer. We demonstrated that soft filter paper can be easily cut in any desirable pattern and size without tearing. The design of the 3D μPAD proposed has the advantage of keeping the paper layers closely attached allowing the solution easily flow eliminating the needed of hydrophilic materials filling the gap between the paper
Acknowledgements
The authors thank the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP – Proc. No 2015/19890-1), and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq Proc. No 427727/2016-2 and 140484/2013-2), for all financial support.
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