Tear-off patterning: a simple method for patterning nitrocellulose membranes to improve the performance of point-of-care diagnostic biosensors

Recent Advances In the development of enzymatic paper-based microfluidic biosensors

Using microfluidic paper-based analytical devices has attracted considerable attention in recent years. This is mainly due to their low cost, availability, portability, simple design, high selectivity, and sensitivity. Owing to their specific substrates and catalytic functions, enzymes are the most commonly used bioactive agents in μPADs. Enzymatic μPADs are various in design, fabrication, and detection methods. This paper provides a comprehensive review of the development of enzymatic μPADs by considering the methods of detection and fabrication. Particularly, techniques for mass production of these enzymatic μPADs for use in different fields such as medicine, environment, agriculture, and food industries are critically discussed. This paper aims to provide a critical review of μPADs and discuss different fabrication methods as the central parts of the μPADs production categorized into printable and non-printable methods. In addition, state-of-the-art technologies such as fully printed enzymatic μPADs for rapid, low-cost, and mass production and improvement have been considered.

Modern evolution of paper-based analytical devices for wearable use: from disorder to order

Paper devices have attracted great attention for their rapid development in multiple fields, such as life sciences, biochemistry, and materials science. When manufacturing paper chips, flexible materials, such as cellulose paper or other porous flexible membranes, can offer several advantages in terms of their flexibility, lightweight, low cost, safety and wearability. However, traditional cellulose paper sheets with chaotic cellulose fiber constitutions do not have special structures and optical characteristics, leading to poor repeatability and low sensitivity during biochemical sensing, limiting their wide application. Recent evidence showed that the addition of ordered structure provides a promising method for manufacturing intelligent flexible devices, making traditional flexible devices with multiple functions (microfluidics, motion detection and optical display). There is an urgent need for an overall summary of the evolution of paper devices so that readers can fully understand the field. Hence, in this review, we summarized the latest developments in intelligent paper devices, starting with the fabrication of paper and smart flexible paper devices, in the fields of biology, chemistry, electronics, etc. First, we outlined the manufacturing methods and applications of both traditional cellulose paper devices and modern smart devices based on pseudopaper (order paper). Then, considering different materials, such as cellulose, nitrocellulose, nature sourced photonic crystals (photonic crystals sourced from nature directly) and artificial photonic crystals, we summarized a new type of smart flexible device containing an ordered structure. Next, the applications of paper devices in biochemical sensing, wearable sensing, and cross-scale sensing were discussed. Finally, we summarized the development direction of this field. The aim of this review is to take an integral cognition approach to the development of smart flexible paper devices in multiple fields and promote communications between materials science, biology, chemistry and electrical science.

Programmable Paper-Based Microfluidic Devices for Biomarker Detections

Recent advanced paper-based microfluidic devices provide an alternative technology for the detection of biomarkers by using affordable and portable devices for point-of-care testing (POCT). Programmable paper-based microfluidic devices enable a wide range of biomarker detection with high sensitivity and automation for single- and multi-step assays because they provide better control for manipulating fluid samples. In this review, we examine the advances in programmable microfluidics, i.e., paper-based continuous-flow microfluidic (p-CMF) devices and paper-based digital microfluidic (p-DMF) devices, for biomarker detection. First, we discuss the methods used to fabricate these two types of paper-based microfluidic devices and the strategies for programming fluid delivery and for droplet manipulation. Next, we discuss the use of these programmable paper-based devices for the single- and multi-step detection of biomarkers. Finally, we present the current limitations of paper-based microfluidics for biomarker detection and the outlook for their development.

Advanced Colorimetric Paper Sensors Using Color Focusing Effect Based on Asymmetric Flow of Fluid

Although paper-based colorimetric sensors utilizing enzymatic reactions are well suited for real-field diagnosis, their widespread use is hindered by signal blurring at the detection spot due to the action of capillary forces on the liquid and the corresponding membrane. In this study, we eliminated signal losses commonly observed during enzyme-mediated colorimetric sensing and achieved pattern-free quantitative analysis of glucose and uric acid by mixing enzymes and color-forming reagents with chitosan oligosaccharide lactate (COL), which resulted in perfectly focused colorimetric signals at the detection spot, using asymmetric flow induced by changing the flow rate of the COL-treated paper. The targets were calibrated with 0-500 mg/dL of glucose and 0-200 mg/dL of uric acid, and the limit of detection was calculated to be 0.6 and 0.03 mg/dL, respectively. In human urine, the correlation has a high response between the measured and spiked concentrations, and the stability of the enzyme mixture including COL increased by 41% for glucose oxidase mixture and 29% for uricase mixture, compared to the corresponding mixtures without COL. Thus, the color focusing and pattern-free sensor, which have the advantages of easy fabrication, easy handling, and high stability, should be applied to real-field diagnosis.

Emerging paper microfluidic devices

Paper has unique advantages over other materials, including low cost, flexibility, porosity, and self-driven liquid pumping, thus making it widely used in various fields in biology, chemistry, physics and materials science.

Paper Microfluidics for Cell Analysis

Paper microfluidics has attracted much attention since its first introduction around one decade ago due to the merits such as low cost, ease of fabrication and operation, portability, and facile integration with other devices. The dominant application for paper microfluidics still lies in point-of-care testing (POCT), which holds great promise to provide diagnostic tools to meet the ASSURED criteria. With micro/nanostructures inside, paper substrates provide a natural 3D scaffold to mimic native cellular microenvironments and create excellent biointerfaces for cell analysis applications, such as long-term 3D cell culture, cell capture/phenotyping, and cell-related biochemical analysis (small molecules, protein DNA, etc.). This review summarizes cell-related applications based on various engineered paper microdevices and provides some perspectives for paper microfluidics-based cell analysis.

A Paper-Based Device for Performing Loop-Mediated Isothermal Amplification with Real-Time Simultaneous Detection of Multiple DNA Targets

Paper-based diagnostic devices have many advantages as a one of the multiple diagnostic test platforms for point-of-care (POC) testing because they have simplicity, portability, and cost-effectiveness. However, despite high sensitivity and specificity of nucleic acid testing (NAT), the development of NAT based on a paper platform has not progressed as much as the others because various specific conditions for nucleic acid amplification reactions such as pH, buffer components, and temperature, inhibitions from technical differences of paper-based device. Here, we propose a paper-based device for performing loop-mediated isothermal amplification (LAMP) with real-time simultaneous detection of multiple DNA targets. We determined the optimal chemical components to enable dry conditions for the LAMP reaction without lyophilization or other techniques. We also devised the simple paper device structure by sequentially stacking functional layers, and employed a newly discovered property of hydroxynaphthol blue fluorescence to analyze real-time LAMP signals in the paper device. This proposed platform allowed analysis of three different meningitis DNA samples in a single device with single-step operation. This LAMP-based multiple diagnostic device has potential for real-time analysis with quantitative detection of 10²-10⁵ copies of genomic DNA. Furthermore, we propose the transformation of DNA amplification devices to a simple and affordable paper system approach with great potential for realizing a paper-based NAT system for POC testing.