Paper microfluidic devices : A review 2017

What are paper microfluidic analytical devices?

Diagnosis is the first step in treating any disease, and paper microfluidic devices can facilitate this crucial step. Most of the time, this necessary task is realized by using conventional laboratory devices providing precise quantitative data from biological samples [1]. Though there are some inexpensive and handy disease detection devices available nowadays, common diagnostic technologies are often very costly, even for an economically developed country that can afford to operate such expensive devices that also require trained personnel [1]. Hence, paper microfluidic devices were developed in an attempt to not only make diagnosis devices environmentally friendly and affordable (above all in in the developing world) but also to be able to instantly conduct tests in urgent situations or remote areas cut out from technology [2]. These diagnostic systems, often called paper microfluidic analytical devices (µPDAs), are made of patterned paper on which a small volume of fluid placed over will move by capillarity [2]. To top it all, paper is not only an inexpensive and handy material but it also is a very light substrate that can be stored and transported easily.

Different applications for paper microfluidic devices

First introduced in 2007 by Martinez et al., paper microfluidic devices are mainly designed for the detection of various types of substances and compounds, though they can perform other specific tasks [3]. In fact, they can not only be used for biochemical analysis but also for medical and forensic diagnostics [3]. There is a wide variety of papers that can be employed to build microfluidic devices, with compositions ranging from cellulose to glass or polymer, and each type of paper can bring different functionalities depending on the applications [4]. One of the first paper diagnostic devices created was for urine analysis [5]. These devices utilize colorimetric assays to measure glucose and protein concentration in urine. Mixing different analytes for different assays is now possible thanks to paper plates that were designed as a low-cost alternative to the conventional plastic microliter plates [5]. Another important application for paper microfluidic devices is pathogen and toxin detection [5]. As affordable and environmentally friendly disease diagnosis tools, these tests can be categorized into biochemical, immunological, and molecular detections according to their reaction mechanisms [6].

µPAD urine test (Lisowski et al. 2013)
Left image : The entrance to the microfluidic device is dipped in urine
Right image : Urine wicks (moves by capillarity) into the assay zones

Devices fabricated by A) photolithography detecting glucose and protein. (adapted from ref. 7); B) plotting detecting glucose and protein (adapted from ref. 30); C) inkjet etching measuring protein, pH, and glucose (adapted from ref. 31); D) plasma etching detecting alkaline phosphatase (adapted from ref. 32); E) cutting detecting protein and glucose (adapted from ref. 29); F) wax printing detecting protein, cholesterol, and glucose (adapted from ref. 34). All the devices are shown on the same size scale except for image B, which is scaled down by 50% compared to the other images. ( Martinez et al. 2010)

Detection and separation methods of paper Microfluidic devices

Detection of analytes in paper microfluidics can be either colorimetric, electrochemical , chemiluminescence, and electrochemiluminescence, but most paper microfluidic analytical devices rely on colorimetric detection, which is generally related to enzymatic or chemical color change reactions [7]. The colorimetric reaction is the most commonly used assay method in the μPADs, due to its easy operation and straightforward signal readout [8]. With reactions color-change reactions, the analysis of results can be directly assessed visually (similar to a pH color test ) and without a deep knowledge of the underlying processes involved in the reaction taking place, which is adequate when a yes/no answer detection is sufficient for diagnosis [9].

Method to create microfluidic channels on paper substrates ( Yamada et al. 2015)

Chematic of a paper microfluidic channel. The channel comprises a porous matrix of hydrophilic cellulose fibers that wick fluids along the path defined by the channel. The sides of the channel are bounded by hydrophobic barriers, and the top and bottom of the channel are open to atmosphere (Martinez et al. 2010)

Fabrication of paper microfluidic devices

Paper microfluidic devices: Wax Printing

Wax printing is a method that requires the fewest number of steps [15]. It is adapted to fabricate large numbers of paper analytical devices in a single batch due to its rapidity (5-10 min) and low cost (∼$0.0001/ cm2 of paper) [16]. The two main steps in the fabrication process are printing patterns of wax (100 μm width) on the paper surface and melting the wax into the paper to form hydrophobic barriers [17].

There are three different ways to realize wax patterning, and direct printing by a wax printer is the most convenient and efficient method [18]. As an alternative, painting with a wax pen is also possible to perform, though some prefer to print patterns with a normal inkjet printer, and then trace and paint with a wax pen for a better resolution of the printed pattern [18].

Quick review of wax printing

Information from Yanyan Xia et al., 2015 / Cai et al., 2013; Li et al., 2014d  /  Lu et al., 2010, 2014  

Resolution : hydrophobic barrier nominal width of 100 μm

Paper microfluidic devices: Inkjet printing

Inkjet printing is a new fabrication method that associates sizing chemistry with digital inkjet printing technique [18]. The main objective here in the fabrication of microfluidic channels is to obtain a contrast between the hydrophobic barrier and the hydrophilic flow channel [19]. Inkjet printing systems employed to print patterns rely on drop-on-demand technology (DOD), which enable the jetting of ink droplets dot-by-dot onto cellulose paper only when it is needed [19]. This make inkjet printing a technique that allows for rapid and flexible high resolution [19]. The Inkjet printing can accurately deliver biomolecules and indicator reagents into the microfluidic patterns, thus forming biological/chemical sensing zones within the patterns [20].

To define channels on the paper substrate, the cellulose fibers can be covalently modified by employing sizing agents such as alkenylsuccinic anhydride (ASA), alkyl ketene dimer (AKD), or rosin [21]. Those hydrophobic reagents render the paper substrate more hydrophobic, and AKD has been particularly used for the microfluidic patterning of paper by inkjet printing [21].

Quick review of Inkjet printing

Information from Yanyan Xia et al., 2015 / Yamada et al., 2014 / Yu and White, 2010 / Lessing et al., 2014

Resolution : hydrophobic barrier nominal width of 550 μm

Paper microfluidic devices: photolithography

Generally, photolithography is the standard method of printed circuit board and it uses light to make the conductive paths on chips [22]. This method involves light exposure through a mask to project the image of a pattern, much like a negative image in standard photography. Photolithography is a convenient, quick, and cheap method, and with this technique, hydrophobic areas that compose the patterns are made of polymeric barriers [22]. Channels created using Photolithography show high background while wax printed channels for instance show very low background. However, photolithography requires organic solvents, expensive photoresists and photolithography equipment, making this method a little more complicated to set up.  Fast Lithographic Activation of Sheets, a variation technique based on photolithography is a rapid method for laboratory prototyping of microfluidic devices on paper. It requires a UV lamp and hotplates only and patterning can even be performed in sunlight when the UV lamp and hotplate are unavailable and no clean room or special facilities are required [22].

Quick review of photolithography

Information from Carrilho et al., 2009 / Dungchai et al., 2011

Resolution : hydrophobic barrier nominal width of 0,5 mm

Paper microfluidic devices: Flexographic printing

This method for patterning is based on flexographic printing of polystyrene, a polymer used to make the paper substrate hydrophobic. This technique leads to the formation of liquid guiding boundaries and layers on paper substrates [23]. As a result, hydrophobic barrier structures are created, and the hydrophobizing inks partially or completely penetrate through the entire depth of the paper substrate [23]. Thus, the structures obtained are very thin fluidic channels on paper with reduced sample volumes [24]. A great advantage of flexographic printing is that biomolecules and other reagents required in analytical and diagnostic tests can easily be transferred by it on paper substrates [25].  The fabrication of paper microfluidic analytical devices using flexographic printing can be done in a single roll-to-roll process, and that’s the reason why this method is ideal for largescale production.

Quick review of Flexographic printing

Information from Olkkonen et al., 2010 / Li et al., 2012

Resolution : hydrophobic barrier nominal width at least 400 μm

Paper microfluidic devices: Plasma treatment

Using plasma treatment can be a method for making microfluidic patterns on a paper substrate. In order to create patterns, the paper used is first hydrophobized via octadecyltrichlorosilane (OTS) silanization and then treated using plasma in combination with a mask presenting channels network [26]. As a result, well defined hydrophilic channels are obtained on the paper with well-defined borders [26]. Paper devices generated by plasma treatment have the advantage to enable the inclusion of simple functional elements such as switches, filters, and separators that can be easily built in the microfluidic system [26]. Unfortunately, this method is not without defect as a well-known problem for plasma treatment is that the substrate under the mask is often over stretched, making the treated pattern bigger than the mask [26]. To remedy to this issue, the treatment process needs to be controlled to generate channels presenting constant width, and this can be done by regulating the intensity and time of the plasma treatment [27].

Quick review of Plasma treatment

Information from Yanyan Xia et al., 2015 / Li et al., 2008

Resolution : hydrophobic barrier nominal width <1,5 mm

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Paper microfluidic devices: Laser treatment

Laser treatment is a method to selectively pattern hydrophilic surfaces on hydrophobic paper. The fabrication procedure relies on polymerization of photopolymers that successfully guides the flow of fluids and allowed containment of fluids in wells [27]. Any paper with a hydrophobic surface coating can be used for this purpose. By modifying the surface structure and property (hydrophobic to hydrophilic) of papers, it exhibits a highly porous structure helping to trap and localize chemical and biological aqueous reagents for analysis [27]. High resolution two-dimensional patterns are created on the hydrophobic surface of the paper using computer-controlled CO2 laser [27]. This laser treatment modifies the surface physically due to melting and re-solidification of the paper coating to create micro/nano-hybrid structure, which enabled the trapping of aqueous reagents [27].

Quick review of Laser treatment

Information from Yanyan Xia et al., 2015 / Sones et al.,2014 / Li et al., 2008 / Spicar-Mihalic et al., 2013

Resolution : hydrophobic barrier nominal width of 120-150 μm

Paper microfluidic devices: Wet etching

This method consists in using hydrophilic filter paper which is hydrophobically patterned employing trimethoxyoctadecylsilane solution as patterning agent [27]. The etching of the silanized filter paper by the etching reagent is enabled by a paper mask that penetrates with NaOH solution (containing 30% glycerol) and aligned onto the hydrophobic filter paper [27]. This causes the masked region to turn into a highly hydrophilic area while the unmasked region still remains highly hydrophobic [27]. The results obtained are hydrophobic barriers that delimit hydrophilic channels that serve as micro-reservoirs and detection zones.

Quick review of Wet etching

Information from Yanyan Xia et al., 2015 / Li et al., 2008

Resolution : hydrophobic barrier nominal width up to 500 μm

Paper microfluidic devices: Screen-printing

To perform bipolar electrochemistry, carbon electrodes screen-printed directly on paper substrate can be employed [27]. In addition, an array of 18 screen-printed bipolar electrodes can be simultaneously controlled using a single pair of driving electrodes [27]. The electrochemical state of the bipolar electrodes is read-out using electro-generated CL. This demonstrates the feasibility of the coupling bipolar electrochemistry for the paper microfluidic analytical devices to perform highly multiplexed and low-cost measurements [27].

Quick review of Screen-printing

Information from Yanyan Xia et al., 2015 / Sun et al.,2015 / Li et al., 2012

Resolution : hydrophobic barrier nominal width of 500 μm

Paper microfluidic devices: Wax screen-printing

Wax screen-printing is a low-cost and simple method for fabricating paper microfluidic analytical devices. Its simple fabricating process includes printing patterns of solid wax on the surface of paper using a simple screen-printing method [27]. The printed wax is then melted into the paper to form hydrophobic barriers using a hot plate [28]. As previously seen, wax is a low-cost material and can be purchased anywhere in the world, and is also environmentally friendly [29]. This method requires a wax printer and easily affordable printing screens. Besides, the wax screen-printing method is accomplished without the use of clean room, UV lamp, organic solvents, or complexed instrumentation [29]. Another major advantage of this method over previous methods is that it requires only a common hot plate (or similar surface) and common printing screen that can be produced anywhere in the world, making it ideal for fabrication of the μPADs in developing countries [29]. Finally, this fabrication method is useful for both colorimetric and electrochemical detection methods [30].

Quick review of Wax screen-printing

Information from Yanyan Xia et al., 2015 / Dungchai et al., 2011

Resolution : hydrophobic barrier nominal width of 1,2 to 6 mm

Fabrication of 3D paper microfluidic devices

3D paper microfluidic devices are fabricated by stacking layers of paper and double-sided adhesive tape impermeable to water, both patterned in ways that guide the flow of fluid within and between layers of paper [31]. 3D paper devices wick fluids (move by capillarity) and distribute microliter volumes of samples from single inlet points into different detection zones. Thus, it is possible to carry out a range of new analytical protocols without external pumps [32]. Fluids can pass vertically (both up and down) and rapidly through multiple layers of paper (each layer is only 100–200 µm thick), and can be distributed, combined with different reagents in different layers, or filtered [32].

The hydrophobic polymer patterned into the paper delimits the channels through which the fluids move laterally, and the layers of water-impermeable double sided tape separates the channels in the neighboring layers of paper [33]. Moreover, holes are cut into the tape and allow fluids to flow vertically. Finally, the layer in the 3D paper microfluidic analytical devices can be made using different papers and the multiple functionalities provided by the different types of paper can be combined into a single device. Besides the material cost is approximately $0.03 or $0.003 per square centimeter per layer of paper [34].

Using the Principles of Origami, paper microfluidic analytical devices can also be built [35]. This origami assembly method does not require adhesive tape, which can lead to contamination and nonspecific adsorption [35]. Avoiding tape also speeds the assembly of the device and eliminates the need for laser cutting [35]. The multilayer device is assembled by simple paper folding, which can be completed in less than 1 min without tools or special alignment techniques. The device can be easily unfolded so that all layers can be used for parallel analysis. Finally, incorporation of additional intermediate layers should not result in much additional fabrication overhead

Three-dimensional paper microfluidic devices assembled using the principles of origami (from Liu and Crooks, 2011)

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References