Microfluidic Paper-based Devices Research Articles

Paper-based optoelectronic nose for identification of indoor air pollution caused by 3D printing thermoplastic filaments

Commercial printers based on fused deposition modeling (FDM) are widely adopted for 3D printing applications. This method consists of the heating of polymeric filaments over the melting point followed by their deposition onto a solid base to create the desirable 3D structure. Prior investigation using chromatographic techniques has shown that chemical compounds (e.g. VOCs), which can be harmful to users, are emitted during the printing process, producing adverse effects to human health and contributing to indoor air pollution. In this study, we present a simple, inexpensive and disposable paper-based optoelectronic nose (i.e. colorimetric sensor array) to identify the gaseous emission fingerprint of five different types of thermoplastic filaments (ABS, TPU, PETG, TRITAN and PLA) in the indoor environment. The optoelectronic nose is comprised of selected 15 dyes with different chemical properties deposited onto a microfluidic paper-based device with spots of 5 mm in diameter each. Digital images were obtained from an ordinary flatbed scanner, and the RGB information collected before and after air exposure was extracted by using an automated routine designed in MATLAB, in which the color changes provide a unique fingerprint for each filament in 5 min of printing. Reproducibility was obtained in the range of 2.5–10% (RSD). Hierarchical clustering analysis (HCA) and principal component analysis (PCA) were successfully employed, showing suitable discrimination of all studied filaments and the non-polluted air. Besides, air spiked with vapors of the most representative VOCs were analyzed by the optoelectronic nose and visually compared to each filament. The described study shows the potential of the paper-based optoelectronic nose to monitor possible hazard emissions from 3D printers.

Dynamic Radial Placement and Routing in Paper Microfluidics

The low cost, simplicity, and ease of use of paper microfluidic devices have made them valuable medical diagnostics for applications from pregnancy testing to COVID-19 screening. Meanwhile, the increasing complexity of paper-based microfluidic devices is driving the need to produce new tools and methodologies that enable more robust biological diagnostics and potential therapeutic applications. A new design framework is being used to facilitate both research and fabrication of paper-based microfluidic biological devices to accelerate the investigative process and reduce material utilization and manpower. In this work we present a methodology for this framework to dynamically place and route microfluidic components in a nondiscrete design space where fluid volume usage, surface area utilization, and the timing required to perform specified biological assays are accounted for and optimized while also accelerating the development of potentially lifesaving new devices.

Zinculose: A new fibrous material with embedded zinc particles

In this paper, we report a simple and inexpensive procedure to make a composite material of cellulose fibers with embedded zinc micoparticles. This fibrous material is produced by sedimentation and is referred to as “Zinculose”. Zinculose increases the surface contact area between a sample fluid and zinc microparticles. The effect of different parameters including fiber content, zinc content, water volume, applied weight and its duration on the thickness of produced Zinculose were investigated. Results show that thickness depends on the amount of initial fiber and zinc while other parameters investigated had little to no effect. Measured porosity values for Zinculose ranged between 0.699 and 0.843. Characterization of flow in Zinculose exhibits a linear relationship between distance and the square root of time which is a distinctive feature of capillary driven flow in porous media. This is an important quality that allows Zinculose to be easily incorporated into any paper-based microfluidic device that requires a sample to flow and interact with zinc microparticles without disrupting the flow path between different sections of the device. An application is presented in which a strip of Zinculose is used to convert nitrate to nitrite. With the use of Zinculose in a paper-based microfluidic device, a conversion efficiency of 27% nitrate to nitrite was achieved. This represents a 36% enhancement over what has been previously published when zinc microparticles were not embedded within the fibers of the paper channel.

A Novel Wick-Like Paper-Based Microfluidic Device for 3D Cell Culture and Anti-Cancer Drugs Screening.

Paper is increasingly recognized as a portable substrate for cell culture, due to its low-cost, flexible, and special porous property, which provides a native cellular 3D microenvironment. Therefore, paper-based microfluidics has been developed for cell culture and biomedical analysis. However, the inability of continuous medium supply limits the wide application of paper devices for cell culture. Herein, a paper-based microfluidic device is developed with novel folded paper strips as wick-like structure, which is used for medium self-driven perfusion. The paper with patterns of hydrophilic channel, culture areas, and hydrophobic barrier could be easily fabricated through wax-printing. After printing, the hydrophilic paper strip at the periphery of the lower layer is then folded at 90° and extended into the medium container for continuous automatic supply of medium to the cell culture area. Tumor cells cultured in the paper device are tested for anti-cancer drug screening. Visualized cell viability and chemical sensitivity testing can be achieved by colorimetry combined with simple smartphone imaging, effectively reducing precision instrument dependence. The wick paper-based microfluidic device for cell culture endows the method the advantages of lower cost, ease-of-operation, miniaturization, and shows a great potential for large-scale cell culture, antibody drug production, and efficient screening.

Hydrophilic patterning of octadecyltrichlorosilane (OTS)-coated paper via atmospheric-pressure dielectric-barrier-discharge jet (DBDjet)

This study demonstrates a method for patterning paper-based microfluidic devices. An octadecyltrichlorosilane (OTS) solution is used to coat a chromatography paper to make it hydrophobic. A low-temperature (~ 32.9 °C) atmospheric-pressure dielectric-barrier-discharge jet with a shadow mask is then used to pattern hydrophilic stripes on the OTS-coated hydrophobic paper. In this manner, 1-mm-wide hydrophilic stripes are patterned successfully. Liquids are demonstrated to be transported and mixed in the hydrophilic stripes. Water contact angle measurement, X-ray photoelectron spectroscopy, surface profiler, optical emission spectroscopy, and scanning electron microscopy are used to characterize effect of pattering.

Flow-based System: A Highly Efficient Tool Speeds Up Data Production and Improves Analytical Performance.

In this review, we cite references from the period between 2015 and 2020 related to the use of a flow-based system as a tool to obtain a modern analytical system for speeding up data production and improving performance. Based on a great deal of concepts for automatic systems, there are several research groups introduced in the development of flow-based systems to increase sample throughput while retaining the reproducibility and repeatability as well as to propose new platforms of flow-based systems, such as microfluidic chip and paper-based devices. Additionally, to apply a developed system for on-site analysis is one of the key features for development. We believe that this review will be very interested and useful for readers because of its impact on developing novel analytical systems. The content of the review is categorized following their applications including quality control and food safety, clinical diagnostics, environmental monitoring and miscellaneous.

A comparative analytical study on reagent-fused silica gel plate and wax painted paper-based microfluidic device for serological testing

The detection and identification of body fluids at a crime scene shed light on the events which might have occurred and the people involved in the crime; the techniques used being lengthy, makes the on-scene detection a tough process to carry out and handle. This study aims to develop an on-spot detection method for serological testing using silica gel encapsulation technique and a modified microfluidic paper-based analytical device (μPADs). Kastle-Meyer reagent was incorporated into the μPAD and Silica Gel plate and was subsequently validated for the detection of blood. This study revealed that the μPAD was a better option. The emerging technique of μPADs allows a cost-effective and simple method of detection for body fluids. The fibrous network of the paper is manipulated to fabricate a guided channel for the fluid flow. The guided channels in the modified μPADs were fabricated by patterning a hydrophobic barrier out of wax. The analyte of interest for this study is blood but the principle can be modified to include other biological fluids.

Effects of wax boundaries in combination with evaporation on dynamics of fluid flow in paper-based devices

Since their introduction, paper-based microfluidic analytical devices (μPADs) have been ubiquitously utilized for different applications. The spontaneous imbibition of liquids in the paper-based devices that eliminates the requirement of an external pumping system has played a primary role in making paper an appropriate alternative for many other materials in the fabrication of microfluidic devices. Wax patterning is one of the most common methods to fabricate μPADs. Dynamics of the flow in channels with wax boundaries deviate from Washburn's law. Despite some research performed to model the effects of wax boundaries, some gaps remain in the presented models. A more general model is needed to account for the effects of wax boundaries and evaporation simultaneously. In this work, we present a general model to model the wax boundaries by introducing a coefficient in the equation of wicking height. We verify our model experimentally for relative humidities of 100% and 20%. Results show that our model can predict wax boundaries effect on imbibition height with considerable accuracy.

Development and Evaluation of Paper-Based Devices for Iron(III) Determination in an Advanced Undergraduate Laboratory

We describe a paper-based microfluidic device for iron(III) assay in an applied instrumental analysis laboratory. Simple and convenient devices were fabricated by the students using patterning of f...

A sample volume independent paper microfluidic device for quantifying glucose in real human plasma

The uncontrolled volume of sample has a significant influence on the outcome of the colorimetric detection method performed on paper-based microfluidic devices. In a reliable colorimetric assay, the results should be completely dependent on the concentration of analyte in the sample rather than its volume. However, this effect is usually overlooked in the development of paper-based microfluidic devices as a simple, inexpensive, portable and user-friendly medical diagnostic system. In this work, a volume-independent micro paper-based analytical device has been introduced in which the need for precise controlling of sample volume is eliminated. We have shown that the proposed device is only sensitive to the concentration of analyte (glucose) in the sample instead of its volume. We have also demonstrated that the proposed device addresses the reported problem of colour heterogeneity and is relatively insusceptible to the variation of reagents volume. An inexpensive smart phone camera is used for capturing the images and a portable 3D printed tool is designed to eliminate the environmental illuminations. To simulate the real condition, human plasma samples are used throughout the experiments.

Creating CRISPR-responsive smart materials for diagnostics and programmable cargo release.

Materials that sense and respond to biological signals in their environment have a broad range of potential applications in drug delivery, medical devices and diagnostics. Nucleic acids are important biological cues that encode information about organismal identity and clinically relevant phenotypes such as drug resistance. We recently developed a strategy to design nucleic acid-responsive materials using the CRISPR-associated nuclease Cas12a as a user-programmable sensor and material actuator. This approach improves on the sensitivity of current DNA-responsive materials while enabling their rapid repurposing toward new sequence targets. Here, we provide a comprehensive resource for the design, synthesis and actuation of CRISPR-responsive hydrogels. First, we provide guidelines for the synthesis of Cas12a guide RNAs (gRNAs) for in vitro applications. We then outline methods for the synthesis of both polyethylene glycol-DNA (PEG-DNA) and polyacrylamide-DNA (PA-DNA) hydrogels, as well as their controlled degradation using Cas12a for the release of cargos, including small molecules, enzymes, nanoparticles and living cells within hours. Finally, we detail the design and assembly of microfluidic paper-based devices that use Cas12a-sensitive hydrogels to convert DNA inputs into a variety of visual and electronic readouts for use in diagnostics. Following the initial validation of the gRNA and Cas12a components (1 d), the synthesis and testing of either PEG-DNA or PA-DNA hydrogels require 3-4 d of laboratory time. Optional extensions, including the release of primary human cells or the design of the paper-based diagnostic, require an additional 2-3 d each.

Modifying Wicking Speeds in Paper-Based Microfluidic Devices by Laser-Etching.

Paper-based microfluidic devices are an attractive platform for developing low-cost, point-of-care diagnostic tools. As paper-based devices’ detection chemistries become more complex, more complicated devices are required, often entailing the sequential delivery of different liquids or reagents to reaction zones. Most research into flow control has been focused on introducing delays. However, delaying the flow can be problematic due to increased evaporation leading to sample loss. We report the use of a CO2 laser to uniformly etch the surface of the paper to modify wicking speeds in paper-based microfluidic devices. This technique can produce both wicking speed increases of up to 1.1× faster and decreases of up to 0.9× slower. Wicking speeds can be further enhanced by etching both sides of the paper, resulting in wicking 1.3× faster than unetched channels. Channels with lengthwise laser-etched grooves were also compared to uniformly etched channels, with the most heavily grooved channels wicking 1.9× faster than the fastest double-sided etched channels. Furthermore, sealing both sides of the channel in packing tape results in the most heavily etched channels, single-sided, double-sided, and grooved, wicking over 13× faster than unetched channels. By selectively etching individual channels, different combinations of sequential fluid delivery can be obtained without altering any channel geometry. Laser etching is a simple process that can be integrated into the patterning of the device and requires no additional materials or chemicals, enabling greater flow control for paper-based microfluidic devices.

Wicking in Paper Strips under Consideration of Liquid Absorption Capacity

Paper-based microfluidic devices have the potential of being a low-cost platform for diagnostic devices. Electrical circuit analogy (ECA) model has been used to model the wicking process in paper-based microfluidic devices. However, material characteristics such as absorption capacity cannot be included in the previous ECA models. This paper proposes a new model to describe the wicking process with liquid absorption in a paper strip. We observed that the fluid continues to flow in a paper strip, even after the fluid reservoir has been removed. This phenomenon is caused by the ability of the paper to store liquid in its matrix. The model presented in this paper is derived from the analogy to the current response of an electric circuit with a capacitance. All coefficients in the model are fitted with data of capillary rise experiments and compared with direct measurement of the absorption capacity. The theoretical data of the model agrees well with experimental data and the conventional Washburn model. Considering liquid absorption capacity as a capacitance helps to explain the relationship between material characteristics and the wicking mechanism.

Solid reference electrode integrated with paper-based microfluidics for potentiometric ion sensing

A new concept of a solid reference electrode integrated with microfluidic paper-based sampling was introduced and applied in potentiometric ion sensing. The new reference electrode consisted of an Ag/AgCl reference element (reusable) that is in contact with a disposable paper substrate (DPS) containing solid KCl. Thus, no KCl reference solution was applied during the analysis step, but the applied sample solution dissolved the solid KCl in the paper providing the electrolyte for the Ag/AgCl reference element. Such reference electrode was found to give a relatively constant potential after less than ca 1 min of equilibration. Moreover, the response of the reference electrode was not affected by concentrations of background electrolytes in the sample. Thus the proposed reference electrode was integrated with a paper-based microfluidic device and solid-contact ion-selective electrodes (ISEs) sensitive to K+, Na+ and Cl−. The optimized systems were characterized with near-Nernstian sensitivities (59.1 ± 1.5, 57.5 ± 0.5 and –56.4 ± 0.6 mV dec−1) and detection limits (10−4.1 ± 0.1, 10−3.3 ± 0.1 and 10−4.1 ± 0.1 mol dm−3) for K+, Na+ and Cl− ions, respectively. The paper-based measurement setup was favourably used for potentiometric determination of ions in environmental (wastewater sludge) and clinically relevant (sweat) samples.

Development of a paper-based microfluidic device for the quantification of ammonia in industrial wastewater

Ammonia is a toxic pollutant increasingly found in urban and industrial wastewater and unprotected surface water. Industry discharges and fertilizer run-off release ammonia into sewers and streams, overloading wastewater treatment plants and causing fish deaths in surface water such as rivers, sea and lakes. The purpose of this study was to develop and evaluate the effectiveness of the microfluidic paper-based device (µPAD) for the quantification of ammonia in wastewater. The µPAD fabricated had an oval-shaped pattern which was designed using CorelDraw software. The hydrophilic zones were created by printing a chromatographic paper with a Xerox wax printer (Xerox colorqube 8570). The modified version of the colorimetric method using Nessler reagent was combined with microfluidic technologies to create a low-cost monitoring system for detection of ammonia in wastewater. The method allows for ammonia determination in the range of 0–5 ppm (mg/L) with a limit of detection of 3.34 ppm. This study indicated that a µPAD was successfully used to quantify the concentration of ammonia in wastewater.

Nanocellulose Films to Improve the Performance of Distance-based Glucose Detection in Paper-based Microfluidic Devices.

We report on a simple, cost-effective, instrument-free, and portable distance-based paper device coupled with NFs for the determination of glucose. The analysis reaction is based upon the oxidative etching reaction of silver nanoparticles (AgNPs) in the presence of H2O2 that is produced from glucose after a glucose oxidase (GOx) catalytic reaction leading to a morphological transformation of AgNPs. A color band length of AgNPs is coated on to a detection channel and then etched by H2O2, and these were changed from a purple color to colorless as a correlate of the glucose concentration. To improve the performance of the enzyme immobilization, NFs, which are biocompatible without compromising their structure and biological activity, were then placed onto the sample zone. The naked-eye detection limit was 0.1 mM for 40 min of analysis time. The recoveries of glucose spiked in the artificial urine samples and control urine samples were then verified by our device and were in the acceptable range of 96 - 100%.

3D Microfluidic Devices in a Single Piece of Paper for the Simultaneous Determination of Nitrite and Thiocyanate.

The concentrations of nitrite and thiocyanate in saliva can be used as the biomarkers of the progression of periodontitis disease and environmental tobacco smoke exposure, respectively. Therefore, it is particularly necessary to detect these two indicators in saliva. Herein, the three-dimensional single-layered paper-based microfluidic analytical devices (3D sl-μPADs) were, for the first time, fabricated by the spraying technique for the colorimetric detection of nitrite and thiocyanate at the same time. The conditions for 3D sl-μPADs fabrication were optimized in order to well control the penetration depth of the lacquer in a paper substrate. Then, the developed 3D sl-μPADs were utilized to simultaneously detect nitrite and thiocyanate and the limits of detection are 0.0096 and 0.074 mM, respectively. What is more, the μPADs exhibited good specificity, good repeatability, and acceptable recoveries in artificial saliva. Therefore, the developed 3D sl-μPADs show a great potential to determine nitrite and thiocyanate for the assessment of the human health.

An Integrated Paper-Based Microfluidic Device for Real-Time Sweat Potassium Monitoring

Wearable electrochemical sensors have attracted tremendous attention in recent years. Here, an integrated three-dimensional paper-based microfluidic electrochemical device (3D-PMED) with flexible wireless circuits is demonstrated for real-time monitoring of sweat potassium. The paper-based microfluidic pad is fabricated by printing wax patterns on cellulose paper and then folding the pre-patterned paper four times to form a five-layer stacked structure: sweat collector, vertical channel, transverse channel, electrode layer, and sweat evaporator. Also, we have discussed the different properties with three swear collector types. The sweat monitoring device is realized by integrating a screen-printed potassium ion-selective sensor on the PET substrate with the fabricated paper microfluidic pad. The sweat flow in 3D-PMED is modeled with red ink to generate the flow pathway of sweat and the capability of sweat storage. The detection range of the potassium ion-selective sensor is 1-32 mM, and the electrode response potential is 61.79 mV per decade of K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> concentration. The device has a small size that is suitable for everywhere on the body, and also have shown good selectivity for both anion and cation, and a stable performance within 1 week. This 3D-PMED has provided a simple, low-cost way for real-time dynamic sweat potassium monitoring when exercising.

Automated pencil electrode formation platform to realize uniform and reproducible graphite electrodes on paper for microfluidic fuel cells

Graphite pencil stroked electrodes for paper-based Microfluidic devices are gaining immense attention due to their electrochemical properties, cost efficiency, and ease-of-use. However, their widespread use has been hindered by the challenges associated with their manual fabrication such as non-uniformity in graphite deposition, applied pressure, etc. This work presents the design and development of an automated graphite pencil stroking device for graphite electrode fabrication with high efficiency through a compact, inexpensive and automatic process, with reduced fabrication time and human intervention leading to more uniformity. The motion platform of Graphtec plotter was used to create multiple strokes with the help of the proposed device. Such inexpensive graphite electrodes (less than the US $1) have been observed to be porous in nature, acting as diffusion agents. The automated graphite electrodes were used to study the performance of microfluidic paper fuel cells (MPFCs) with formic acid, oxygen, and sulphuric acid acting as fuel, oxidising agent and electrolyte respectively. From this configuration, the maximum current density and power density were measured to be 1,305.5 µA cm−2 and 135.5 µW cm−2, respectively at 0.3 V stable OCP at 100 strokes. Overall, the study enumerates the development of an automated pencil stroke device for fabricating graphite electrodes, which can potentially be harnessed in numerous miniaturized paper based applications.

Cross-linked chitosan biofunctionalized paper-based microfluidic device towards long term stabilization of blood typing antibodies

Long term stability of antibodies at room temperature is a major challenge in the commercialization of point-of-care devices for diagnostics. Since chitosan has been proven to be an excellent biofunctionalization material, the effects of four different biofunctionalization processes were studied to improve the room temperature stability of antibodies immobilized on chitosan modified paper-based microfluidic devices using blood typing antibodies as candidates. The devices used in this work have a flower-shaped design with 4 test zones at each corner. In three zones Anti-A, Anti-B, and Anti-D (Anti-Rh) antibodies are immobilized and the fouth zone represents the control (no antibodies) after biofunctionalization. The biofunctionalization of the paper devices was done with chitosan and chitosan cross-linked with sodium triphosphate pentabasic, glutaraldehyde, and sodium hydroxide. These devices were used for blood typing assays using real blood samples. A similar assay was also performed on unmodified (non-biofunctionalized) paper devices for comparison. Chitosan based biofunctionalized paper-devices showed better stability, up to 100 days as compared to 14 days on unmodified paper, at room temperature. Such biofunctionalized paper-based devices will be suitable for on-field and remote testing without any technical expertise and requirement for the cold chain.