Lab on a Chip PAPER Cite this: Lab Chip, 2020, 20, 1845 Received 24th March 2020, Accepted 21st April 2020 DOI: 10.1039/d0lc00302f rsc.li/loc Direct loading of blood for plasma separation and diagnostic assays on a digital microfluidic device† Christopher Dixon, a Julian Lamanna ab and Aaron R. Wheeler * abc Finger-stick blood sampling is convenient for point of care diagnostics, but whole blood samples are problematic for many assays because of severe matrix effects associated with blood cells and cell debris. We introduce a new digital microfluidic (DMF) diagnostic platform with integrated porous membranes for blood-plasma separation from finger-stick blood volumes, capable of performing complex, multi-step, diagnostic assays. Importantly, the samples can be directly loaded onto the device by a finger “dab” for user-friendly operation. We characterize the platform by comparison to plasma generated via the “gold standard” centrifugation technique, and demonstrate a 21-step rubella virus (RV) IgG immunoassay yielding a detection limit of 1.9 IU mL -1 , below the diagnostic cut-off. We propose that this work represents a critical next step in DMF based portable diagnostic assays—allowing the analysis of whole blood samples without pre-processing. Introduction There is great interest in the development of portable disease- diagnostic assays that can be used at the “point of care” (POC). These systems are particularly attractive for application in remote settings in which centralized laboratory testing facilities are not available. 1–5 Finger-stick (and heel-stick) whole-blood sampling methods are convenient for such systems as they are non-invasive and can be used with infants and young children —cases in which an intravenous draw may not be safe to perform. However, whole-blood, a complex fluid that contains substantial volumes of liquid (plasma) and solids (cells and debris), is problematic for many assays because of the severe matrix effects associated with blood cells and their contents. 6 Plasma is thus preferred for POC assays, but plasma generation from whole blood by the conventional “gold standard” method —centrifugation 7 —requires access to external equipment and materials, thus being inconsistent with the POC testing philosophy. 6,8 Microchannel-based methods have been developed to separate plasma from whole blood, relying on diverse mechanisms including hydrodynamic focusing, 9–11 application of external forces (e.g., acoustic, electrical, etc.), 12–15 and microfiltration. 16–18 While these are promising alternatives, such systems can become highly complex, particularly when combined with in-line multi-step diagnostic assays. Digital microfluidics (DMF) is an alternative to conventional microfluidics that is characterized by the manipulation of individual picolitre to microlitre sized droplets on an open surface, often via application of electrostatic forces to a planar array of electrodes. When used in the two-plate format, droplets are sandwiched between two substrates—typically a bottom plate consisting of individual driving electrodes—and a top plate made up of a single continuous counter-electrode. The most important advantage of digital microfluidics is its ‘reconfigurability’, which allows the implementation of diverse applications 19–23 on devices with generic architecture (with little or no change to device design between them). For this reason digital microfluidics has emerged as a popular mechanism for miniaturizing and automating human health-related diagnostic assays, 24–27 and there is at least one report 28 describing the use of a portable DMF diagnostic system at the point of care. We are aware of a single previous paper 29 describing blood-plasma separation in a digital microfluidic device, which relies on a lectin-based agglutination step to immobilize cells, allowing the collection of plasma in a separate droplet for further analysis. The previous report 29 does not include much detail about the blood-plasma separation (e.g., there is no enumeration of cells remaining in the plasma), and samples in this technique were first metered in a “sample transfer pipette” prior to loading onto the device. This is not ideal for POC applications, which Lab Chip, 2020, 20, 1845–1855 | 1845 This journal is © The Royal Society of Chemistry 2020 a Department of Chemistry, University of Toronto, 80. St. George Street, Toronto, Ontario, M5S 3H6, Canada. E-mail: aaron.wheeler@utoronto.ca; Fax: +1 416 946 3865; Tel: +1 416 946 3864 b Donnelly Centre for Cellular and Biomolecular Research, University of Toronto, 160 College Street, Toronto, Ontario, M5S 3E1, Canada c Institute for Biomaterials and Biomedical Engineering, University of Toronto, 164 College Street, Toronto, Ontario, M5S 3G9, Canada † Electronic supplementary information (ESI) available: ESI includes a document featuring three figures (Fig. S1–S3) and a note (N1), as well a movie file, M1. See DOI:10.1039/d0lc00302f Published on 27 April 2020. Downloaded by University of Toronto on 5/26/2020 4:49:59 PM. View Article Online View Journal | View Issue