Paper microfluidic devices : A review 2017

 

What are paper microfluidic analytical devices?


MICROFLUIDIC PAPER-BASED ANALYTICAL DEVICES-What are microfluidic paper-based analytical devicesDiagnosis 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 dection devices available nowadays , common diagnostic technologies are often very costly, even for economically developed country that can afford to operate such expensive devices that also require trained personal[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 inexpesive and  handy material but it also constitutes  a very light substrate that can be stored 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 paper 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 functionality 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].

MICROFLUIDIC PAPER-BASED ANALYTICAL DEVICES-DIFFERENT APPLICATIONS FOR MICROFLUIDIC PAPER-BASED DEVICES-urine test

µPAD urine test (Lisowski et al. 2013)

 

Left image : The entrance to the microfluidic device is dipped into urine

Right image : Urine wicks (moves by capillarity) into the assay zones

MICROFLUIDIC PAPER-BASED ANALYTICAL DEVICES-DIFFERENT APPLICATIONS FOR MICROFLUIDIC PAPER-BASED DEVICES

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


MICROFLUIDIC PAPER-BASED ANALYTICAL DEVICES-DETECTION AND SEPARATION METHODS OF PAPER-BASED MICROFLUIDIC DEVICES

Image of a 96-zone plate after application of a range of volumes (1-55 µL) of solutions of aqueous dyes – Martinez et al. 2010

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].

One of the colorimetric system created utilizes cross-linked siloxane 3-aminopropyltriethoxysilane (APTMS) as reactant for the detection of a broad range of targets, including H2O2 and glucose. When the APTMS is cross-linked with glutaraldehyde (GA), the resulting complex (APTMS– GA) displays brick-red color, and this visual color change is observed when the complex reacts with H2O2[10].

Chemiluminescence and electrochemiluminescence are performed in the dark and are independent of ambient light[11]. They are also the most common optical detection methods in microfluidics area. However, they have not been widely used in paper microfluidic analytical devices due to the simplicity and convenience of colorimetric systems. EC detection has higher sensitivity, enabling the detection and quantification of analytes even in the lower range (nM)[11].

Fabrication of 2D paper microfluidic devices


Due to its functioning principles and the materials employed, the fabrication process of a paper microfluidic device is cheap and not so complex, in fact its estimated price is below $10 per square meter[11]. They are composed of hydrophilic/hydrophobic micro-channel networks in which fluids can move by capillarity in both horizontal and vertical dimensions depending on the diagnosis application[12]. There are many techniques for fabricating paper microfluidic devices including the detection methods associated with these techniques. This short review will focus in-depth on those different fabrication methods by mostly relying on Yanyan Xia et al., 2015 commendable article (“Fabrication techniques for paper microfluidic analytical devices and their applications for biological testing: A review”).

The first step of this treatment generally consists in turning some areas of the hydrophilic paper into hydrophobic ones by printing parallel hydrophobic lines defining the micro-channels[12]. This is the basic approach to print patterns on the cellulose paper sheet though it is not the only one. The height of micro-channels is determined by the thickness of the paper used[13]. The channels hence formed prevents the hydrophilic sample solution from spreading due to hydrophobic barriers, and the movement of fluids inside the micro-channels is based on capillary flow.

There are many methods available for the patterning process that defines the width and length of paper microfluidic channels, with each method having its own set of advantages and limitations. They include wax printing, inkjet printing, photolithography, flexographic printing, plasma treatment, laser treatment, wet etching, screen-printing and wax screening[14].

MICROFLUIDIC PAPER-BASED ANALYTICAL DEVICES-Fabrication of 2D microfluidic paper-based devices-creation of micro-channelsMethod to create microfluidic channels on paper substrates ( Yamada et al. 2015)

MICROFLUIDIC PAPER-BASED ANALYTICAL DEVICES-Fabrication of 2D microfluidic paper-based devices-celluloseChematic 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)

Paper microfluidic devices: Wax Printing


MICROFLUIDIC-PAPER-BASED-ANALYTICAL-DEVICES-Microfluidic-paper-based-devices-Wax-PrintingWax 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

Advantages Downsides
  • Simple method
  • Rapid process (5-10 min)
  • Adequate for most applications of µPADs
  • Environmentally friendly (no use of organic solvents in the fabrication process, the paper and wax can be easily disposed of by burning)

 

  • Requires expensive wax
  • Printers and patterns with a not so high resolution (freshly printed wax spreads vertically, thus increasing the hydrophobic barriers width)
  • Necessity of an extra heating step

Paper microfluidic devices: Inkjet printing


Channel patterning by inkjet printing (yamada et al. 2015)

Channel patterning by inkjet printing (yamada et al. 2015)

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

Advantages Downsides
  • Low cost and commercial volume
  • Can be scaled up
  • High resolution enabled by dot-by-dot deposition of ink droplets
  • Digital control of the printing process allows for rapid and flexible modification of print patterns
  • Possibility of depositing picoliter volumes of material ink

 

  • Quite slow compared to other major printing techniques (dot-by-dot deposition versus entire pattern printed in one step)
  • Not suitable for mass fabrication and reproduction of large amounts of identical patterns that do not require high print resolution
  • The limited window of ink viscosity and surface tension occasionally diminish the versatility of the inkjet printing technique
  • Using surfactants (the ink) in some cases might be problematic when it comes detect proteins prone to denaturation.

 

Paper microfluidic devices: photolithography


Fabrication of paper-based microfluidic device using photolithography technique (described by Martinez et al. 2007)

Fabrication of paper microfluidic device using photolithography technique (described by Martinez et al. 2007)

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 photolithographyhph

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

 

Resolution : hydrophobic barrier nominal width of 0,5 mm

Advantages Downsides
  • Rapid method (15 min)
  • high resolution of microfluidic channels
  • Presents variation techniques based on it
  • Requires organic solvents
  • Requires expensive equipment
  • Requires an extra washing step to remove  un-crosslinked polymer; devices are
  • vulnerable to bending

Paper microfluidic devices: Flexographic printing


tape strip from the ink adhesion test

Tape strip from  ink adhesion test using flexography (Rentzhog et al. 2006)

This method for patterning is based on flexographic printing of polystyrene, 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

Advantages Downsides
  •  Thin fluidic channels
  • Reduced sample volumes  allow direct roll-to-roll production in many existing printing houses
  • Ideal for largescale production
  • avoids the heat treatment of printed patterns
  •  Requires two prints of polystyrene solution
  • Different printing plates needed
  • Print quality relies on the smoothness of paper surface

Paper microfluidic devices: Plasma treatment


µPAD obtained by plasma treatment

Paper microfluidic device obtained by 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 remediate to this issue, the treatment process need to be controlled to generate channel 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

Advantages Downsides
  • Cheap patterning agent
  • simple functional elements can be easily built into the microfluidic system using this method
  • The substrate under a mask is often over-etching, making it difficult to control the channels width
  • Requires different masks for creating different microfluidic patterns on paper

Paper microfluidic devices: Laser treatment


Laser treatment on different hydrophobic papers

Laser treatment on different hydrophobic papers (Chitnis et al. 2011)

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.,2014Li et al., 2008 / Spicar-Mihalic et al., 2013

 

Resolution : hydrophobic barrier nominal width of 120-150 μm

Advantages Downsides
  • Simple and inexpensive method
  • High resolution
  • Any paper with a hydrophobic surface coating can be used
  • Versatile, easy controlled
  • Selectively modify the surface structure and property of several papers
  • Do not allow lateral flow of fluids
  • Requires extra coating for liquid flow
  •  Not well suited for a scale up to very high throughput mass production of devices

Paper microfluidic devices: Wet etching


Optical picture of intersection of two channels (through glass) and holes performed by wet etching in 49% HF solution

Optical picture of intersection of two channels (through glass) and holes performed by wet etching in 49% HF solution (Tay et al. 2006)

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

Advantages Downsides
  •  No expensive facilities and materials are used
  • The printing apparatus must be customized

Paper microfluidic devices: Screen-printing


SEM photos of a bare screen printed carbon electrode (x200 and x10 000); Dong et al. 2007

SEM photos of a bare screen printed carbon electrode (x200 and x10 000); Dong et al. 2007

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

Advantages Downsides
  • Produces devices with simple process
  • microfluidic channels with low resolution
  • requires different printing screens for creating different patterns

Paper microfluidic devices: Wax screen-printing


Hydrophobic channels printed with wax screen-printing

Hydrophobic channels printed using wax screen-printing (Dungchai et al. 2011)

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

Advantages Downsides
  • Printing screens are cheap
  • Environmentally friendly
  • Requires only a common hot plate
  • Does not suffer from problems of interference from residues remaining in the hydrophilic channel after fabrication
  • Patterned mesh is necessary
  •  Inadequate for prototyping

Fabrication of 3D paper microfluidic devices


Schematic diagrams and photographe of the  unfolded 3-Ddevice ( glucose and BSA assay )after completion of theassay.Thecolor change from colorless to brown indicated the presence of glucose in samples (Liu and al. 2011)

Schematic diagrams and photographs of unfolded 3-D devices ( glucose and BSA assay ) after completion of an assay. The color change from colorless to brown indicates the presence of glucose in samples (Liu and al. 2011)

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

 

MICROFLUIDIC PAPER-BASED ANALYTICAL DEVICES-FABRICATION OF 3D PAPER-BASED MICROFLUIDIC DEVICES-origamiThree-dimensional paper microfluidic devices assembled using the principles of origami (from Liu and Crooks, 2011)

REFERENCES

[1] «Patterned Paper as a Platform for Inexpensive, Low-Volume, Portable Bioassays     Andres W. Martinez et al.»

[2] «Diagnostics for the Developing World: Microfluidic Paper-Based Analytical Devices     Andres W. Martinez et al.»

[3] «Microfluidic Paper-Based Analytical Devices (µPADs) and Micro Total Analysis Systems (µTAS): Development, Applications a».

[4] «Fabrication techniques for microfluidic paper-based analytical devices and their applications for biological testing: A review      Yanyan Xia et al. ».

[5] «Microfluidic Paper-Based Analytical Devices (µPADs) and Micro Total Analysis Systems (µTAS): Development, Applications a».

[6] «Fabrication techniques for microfluidic paper-based analytical devices and their applications for biological testing: A review      Yanyan Xia et al.».

[7] «Microfluidic Paper-Based Analytical Devices (µPADs) and Micro Total Analysis Systems (µTAS): Development, Applications a».

[8] «Fabrication techniques for microfluidic paper-based analytical devices and their applications for biological testing: A review      Yanyan Xia et al. ».

[9] «A perspective on paper-based microfluidics: Current status and future trends      Li et al.»

[10] «Fabrication techniques for microfluidic paper-based analytical devices and their applications for biological testing: A review      Yanyan Xia et al. ».

[11] «Microfluidic Paper-Based Analytical Devices (µPADs) and Micro Total Analysis Systems (µTAS): Development, Applications a».

[12] «Fabrication techniques for microfluidic paper-based analytical devices and their applications for biological testing: A review      Yanyan Xia et al. ».

[13] «Diagnostics for the Developing World: Microfluidic Paper-Based Analytical Devices     Andres W. Martinez et al.»

[14] «Fabrication techniques for microfluidic paper-based analytical devices and their applications for biological testing: A review      Yanyan Xia et al. ».

[15] «Understanding Wax Printing: A Simple Micropatterning Process for Paper-Based Microfluidics     Emanuel Carrilho et al.»

[16] «Fabrication techniques for microfluidic paper-based analytical devices and their applications for biological testing: A review      Yanyan Xia et al. ».

[17] «Understanding Wax Printing: A Simple Micropatterning Process for Paper-Based Microfluidics     Emanuel Carrilho et al.»

[18] «Fabrication techniques for microfluidic paper-based analytical devices and their applications for biological testing: A review      Yanyan Xia et al. ».

[19] «Paper-Based Inkjet-Printed Microfluidic Analytical Devices      Kentaro Yamada et al.»

[20] «Fabrication techniques for microfluidic paper-based analytical devices and their applications for biological testing: A review      Yanyan Xia et al. ».

[21] «Paper-Based Inkjet-Printed Microfluidic Analytical Devices      Kentaro Yamada et al.»

[22] «Fabrication techniques for microfluidic paper-based analytical devices and their applications for biological testing:  ».

[23] «Flexographically Printed Fluidic Structures in Paper     Juuso Olkkonen et al.»

[24] «Fabrication techniques for microfluidic paper-based analytical devices and their applications for biological testing: A review      Yanyan Xia et al. ».

[25] «Flexographically Printed Fluidic Structures in Paper     Juuso Olkkonen et al.»

[26] «Paper-Based Microfluidic Devices by Plasma Treatment      Xu Li et al.»

[27] «Fabrication techniques for microfluidic paper-based analytical devices and their applications for biological testing: A review      Yanyan Xia et al.».

[28] «A low-cost, simple, and rapid fabrication method for paper-based microfluidics using wax screen-printing      Dungchai et al.».

[29] «Fabrication techniques for microfluidic paper-based analytical devices and their applications for biological testing: A ».

[30] «A low-cost, simple, and rapid fabrication method for paper-based microfluidics using wax screen-printing      Dungchai et al.».

[31] «Fabrication techniques for microfluidic paper-based analytical devices and their applications for biological testing: A ».

[32] «Three-dimensional microfluidic devices fabricated in layered paper and tape      Martinez et al.»

[33] «Fabrication techniques for microfluidic paper-based analytical devices and their applications for biological testing: A ».

[34] «Three-dimensional microfluidic devices fabricated in layered paper and tape      Martinez et al.»

[35] «Three-Dimensional Paper Microfluidic Devices Assembled Using the Principles of Origami Liu et al.»

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