Due to the passive nature of traditional paper-based microfluidic devices, high precision control of certain parts of multi-step assays can be difficult to acheive. Researchers R. Fobel et al. at the University of Toronto recently published an article in Advanced Materials reporting the invention of a digital paper-based microfluidic device with high precision control of multistep processes on paper.
Digital microfluidics (DMF) is a technology in which nano-to-microliter sized drops are manipulated on an array of electrodes using electric fields. The electrostatic forces can move, split, merge, and dispense drops to and from reservoirs within a device without the use of active pumps or moving parts. Paper-based DMF devices are fabricated by inkjet printing silver electrodes and reservoirs onto smooth paper surfaces. To test the ability of the novel device to perform a multistep process, the group used one of the chips to perform a serial dilution and create a calibration curve for the chemiliuminescence assay of horseradish peroxidase (HRP) mixed with luminol and hydrogen peroxide. Overall, very interesting applications may come from combining these methods with the traditional paper-based diagnostic assay, especially in steps which would be difficult to perform with solely capillary-driven flow.
Rapid and inexpenive nucleic acid assays are challenging to achieve but highly needed for low-cost and point-of-care diagnostics. Yajing Song and co-workers published a paper addressing the above issue by developing a paper-based DNA assay visualized by the naked eye. Their study in the ACS journal Analytical Chemistry demonstrates a novel filter paper-based tool using streptavidin-coated micrometer-sized beads to couple with DNA. Hybridization of the targets was performed by capillary transport through the filter paper array and generated specific signals within 2 min. The resulting signals were detected by the naked eye, as well as measured by a molecular imager. This strategy for visual detection of DNA can be applied not only in a forensic setting but also for point-of-care diagnostics.
In making progress toward an inexpensive paper-based point-of-care device with no electronics required, Scott T. Phillips and coworkers developed a new device that simply relies on keeping track of time. Their study in the ACS journal Analytical Chemistry describes a strategy for quantitative measurement of enzyme analytes in the low to mid femtomolar range. After applying a sample with enzyme analyte to the device, a white assay region turns green, followed by a control region. The user only needs to measure the time for the control region to turn green relative to the assay region. Since the temperature, humidity, and viscosity of the sample will affect the measurement for both the control and assay regions, the control region serves to normalize the output of the assay to account for the effects of these factors. This strategy for a timing-based quantitative assay has great potential for use in remote settings of the world where sophisticated instruments are not options.
In a recent issue of Lab on a Chip, renowned chemist and MF2.0 pioneer George Whitesides provided his "Viewpoint" on the use of sugar delays in paper-based tests. This sugar delay work, performed by our own Yager, Lutz, and Fu labs, was published in Lab on a Chip earlier this year and featured in our blog here. In his current "Viewpoint" article, Dr. Whitesides discusses the need for "simplicity in diagnostics" and commends Lutz et al.'s elegant approach to designing simple but automated paper diagnostics that are actually appropriate for point-of-care settings. He also praises the "quantitative engineering footing" on which the sugar delay work was based. While he notes that further development is of course needed to bring this technology to use, he asserts that this work is a step in the right direction for low-cost testing. Kudos to the authors of the work (Dr. Barry Lutz, Tinny Liang, Dr. Elain Fu, Sujatha Ramachandran, Peter Kauffman, and Dr. Paul Yager), and thank you to Dr. Whitesides for the kind words!
Professors Barry Lutz, Elain Fu, Paul Yager, and colleagues have published their work on sugar-based time delays for paper devices in the most recent issue of Lab on a Chip. This work describes the use of dissolvable sugar barriers to create and control fluidic time delays in paper microfluidic devices. This technique can be used to program multi-step assays that enable automated, easy-to-use paper diagnostic tests. What a “sweet” example of MF2.0!
I am excited to announce that our review article on nitrocellulose is now available in the latest issue of MRS Bulletin! This special issue is devoted to "Paper-Based Technology" and includes several great articles that highlight key aspects of this Microfluidics 2.0 technology! The article written by Gina Fridley, Shefali Oza, Paul Yager, and I describes "The Evolution of Nitrocellulose as a Material for Bioassays," including a brief history of the material, an overview of its physical properties, and considerations for assay development. Since nitrocellulose is one of the most common "paper" substrates for bioassays, we hope you will find this review useful!
Figure from our article illustrating (A) the porous nature of nitrocellulose, (B) the traditional use of nitrocellulose in a lateral flow test, and (C) the emerging use of nitrocellulose in MF2.0-based assays, such as 2DPNs. Fridley et al., MRS Bulletin 38 (4): 326-330 (April 2013).
In making progress towards low cost, almost ‘throw-away’ kind of devices for medical diagnostics, Yildiz and coworkers have come up with a simple and efficient method for the detection of mir21, a micro-RNA sequence associated with lung cancer. An important advantage of this method is that it is non-enzymatic and uses a charged conjugated polyelectrolyte (CPE) as a luminescent reporter. The reporter is impregnated in a robust poly-(vinyledene fluoride) (PVDF) paper. The sensing strategy is based on the color of the triplex species formed between the reporter, mir21, and a peptide nucleic acid (PNA) sequence complementary to mir21. The assay generates an orange color in the presence of mir21 and a purple color in its absence. This platform can detect the presence of mir21 at clinically relevant (nanomolar) levels as well as distinguish mir21 from a single base pair mismatch micro-RNA sequence. The detection can be performed without the use of expensive and complex instruments. The authors have published these results in Analytical Chemistry and their publication may be accessed here: http://pubs.acs.org/doi/abs/10.1021/ac3034008.