(Nanowerk Highlights) Microfluidic paper-based analytical device (µPAD) is a type of technology used for fluid analysis that takes advantage of the properties of paper and microfluidic channels. The main advantages of these devices are their low cost, portability, and ease of use, which make them well suited for field testing, particularly in resource-constrained settings. They are capable of performing total (bio)chemical analyses, increasing their utility in a wide variety of fields.
The operation of the µPAD is based on the natural capillary action of paper, which allows liquids to be transported through small channels without the need for an external pump. This capillary action can be guided and controlled by creating a hydrophobic barrier on the paper, usually through methods such as wax printing, photolithography, or plasma treatment. This allows the formation of micro-channels that guide fluid flow.
The most recent advancement in the µPAD field is the integration of electronic components into these devices. Electrode coupling, for example, unlocks new capabilities for µPAD, enabling quantitative electrochemical detection, assay miniaturization, and advanced connectivity via near-field wireless communication. However, the manufacture of electrodes on paper presents a significant challenge due to the coarse and inhomogeneous nature of the cellulose material.
Various techniques have been developed and perfected to address these challenges, including inkjet printing, screen printing, metal sputtering, thermoplastic forming, and pencil drawing. Despite these advances, stored electrodes can still present limitations. They are often hydrophobic, which can impede capillary flow, and they may show limited contact with liquids within the pores of the paper.
Once the liquid is introduced into the device, it can interact with various reagents that are preloaded onto the paper. These reagents can then cause color changes or other observable phenomena based on the presence of specific analytes in the liquid, enabling the detection and measurement of these analytes.
µPAD applications span a wide range of fields, including environmental monitoring, food safety testing and biomedical diagnostics. For example, in the health sector, µPAD has been developed to detect various diseases, including diabetes, malaria and HIV.
Even with these advances and applications, it’s important to note that the µPAD still faces some challenges. These include the cost and complexity of the fabrication technique, the qualitative or semi-quantitative nature of their readability, and problems with the stability and storage of reagents on paper. However, ongoing research is aimed at addressing these challenges and further improving the technology.
In a significant breakthrough for rapid diagnostic tests, a research team at the Institute for Chemical and Bioengineering at ETH Zürich has developed innovative technology for creating diagnostic tests with built-in electrodes. This test allows digital reading of results, providing accurate and quantitative results via electronic signaling.
The scientists report their findings in Advanced Materials (“Paper-Based Laser Pyrolysis Electrofluids: An Electrochemical Platform for Capillary-Based Bio-Diagnostic Testing”).
“This platform has the potential to improve the diagnostic accuracy of existing tests or enable the development of new test functionality that is not currently possible with technologies that rely on color test strips,” Leonard Bezinge, final year PhD student at ETH Zürich, and first author of the work, said Nanowerk.
The heart of the team’s findings lies in a new method for integrating the electrodes into paper-based test strips. They achieved this through a process called laser-induced cellulose paper pyrolysis, which converts cellulose molecules into a conductive graphene-like material. The electrode formed in this way retains the porous and fibrous nature of the paper, important for capillary driven tests such as rapid diagnostic tests.
“The seamless integration of electronic components into paper-based microfluids allows the application of advanced measurement techniques in a simple and cost-effective platform,” commented Bezinge. “It advances the field by increasing the speed, sensitivity and accuracy of rapid tests, facilitating reliable and efficient chemical, biological and medical analyses.”
This development offers a new approach to integrating electronics into paper, opening up new possibilities when coupled with capillarity-based microfluids. Previous methods of preparing the electrodes involved coating the top surface of the paper with inks or conductive metals, which resulted in poor integration in the cellulose paper web and negatively impacted detection efficiency.
“Our motivation stems from a desire to combine the simplicity of existing rapid diagnostic tests with the advantages of electrochemical signaling and digital readout,” Bezinge noted.
This technology provides a solution for integrating electrodes into paper-based microfluids, enabling the combination of certain types of assays, such as flow-through devices, with electrochemical readouts. As the researchers point out, this achievement is significant because certain types of assays previously could not be combined with electrochemical readings.
The team’s work mainly focuses on applications in medical diagnostics. They envision this technology being used as a rapid test technology for in-home testing of nucleic acid, antigen or antibody biomarkers, with results read digitally from a smartphone. Additionally, they saw the potential of this platform to be used for high-throughput analysis of multiple samples on a single device, where test strips could be integrated as single-use cartridges in a portable analyzer.
The common nature and affordability of this fabrication platform anticipates a wide range of applications where quantitative electrochemical readings are required in conjunction with inexpensive, single-use test strips. Bezinge predicts, “We hope to see novel approaches to building and integrating electrodes into complex three-dimensional fluidic architectures for sample processing and detection in the fields of medical diagnostics and health monitoring, environmental testing, and food analysis.”
Going forward, the researchers aim to broaden the potential application fields in disease diagnostics and explore new labeling chemistries and detection pathways that can be combined with these electrodes.
“The key challenge going forward will be the development of a true sample-to-answer tool that integrates all of these steps into one easy-to-use tool,” concluded Bezinge. “This endeavor was extremely challenging and required a multidisciplinary approach. We encourage researchers from various fields to share their design and construction files, especially as we move to digital fabrication tools, to facilitate collaboration and further progress in this area.”
– Michael is the author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: A Small Future And
Nanoengineering: Skills and Tools for Making Technology Invisible
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