Microfluidic barcode assay for antibody-based confirmatory diagnostics.

Confirmatory diagnostics offer high clinical sensitivity and specificity typically by assaying multiple disease biomarkers. Employed in clinical laboratory settings, such assays confirm a positive screening diagnostic result. These important multiplexed confirmatory assays require hours to complete. To address this performance gap, we introduce a simple 'single inlet, single outlet' microchannel architecture with multiplexed analyte detection capability. A streptavidin-functionalized, channel-filling polyacrylamide gel in a straight glass microchannel operates as a 3D scaffold for a purely electrophoretic yet heterogeneous immunoassay. Biotin and biotinylated capture reagents are patterned in discrete regions along the axis of the microchannel resulting in a barcode-like pattern of reagents and spacers. To characterize barcode fabrication, an empirical study of patterning behaviour was conducted across a range of electromigration and binding reaction timescales. We apply the heterogeneous barcode immunoassay to detection of human antibodies against hepatitis C virus and human immunodeficiency virus antigens. Serum was electrophoresed through the barcode patterned gel, allowing capture of antibody targets. We assess assay performance across a range of Damkohler numbers. Compared to clinical immunoblots that require 4-10 h long sample incubation steps with concomitant 8-20 h total assay durations; directed electromigration and reaction in the microfluidic barcode assay leads to a 10 min sample incubation step and a 30 min total assay duration. Further, the barcode assay reports clinically relevant sensitivity (25 ng ml(-1) in 2% human sera) comparable to standard HCV confirmatory diagnostics. Given the low voltage, low power and automated operation, we see the streamlined microfluidic barcode assay as a step towards rapid confirmatory diagnostics for a low-resource clinical laboratory setting.

Microfluidic validation of diagnostic protein markers for spontaneous cerebrospinal fluid rhinorrhea.

Cerebrospinal fluid (CSF) rhinorrhea is a potentially dangerous condition identified by CSF leakage into the nasal cavity. This malady stands to benefit from rapid and noninvasive screening diagnostics to complement low-throughput imaging based methods currently in use. To address this gap, we demonstrate on-chip immunosubtraction to accelerate biomarker validation and immunoassay development for a putative CSF rhinorrhea diagnostic marker, transthyretin, by combining high-specificity immunoaffinity capture with subsequent polyacrylamide gel electrophoresis (PAGE). We demonstrate the on-chip assay using photopatterned polyacrylamide immunofilters. The filter consists of polymer with controlled pore-sizes to size-exclude (i.e., "subtract") large antibody-target immune complexes from downstream PAGE separation. A control PAGE separation is also performed for comparison without immunoaffinity capture (i.e., no antibody present). We compare on-chip immunosubtraction to Western blotting and ELISA to validate CSF rhinorrhea biomarkers from nasal surgery samples. For samples representative of spontaneous rhinorrhea, the 5 min on-chip assay achieved clinical specificity of 100%, compared to 50% for ELISA which required 6 h. On-chip immunosubtraction also generated results for clinical samples not assayable via ELISA due to matrix protein spurious signals. The pilot study suggests the capability of a rapid on-chip validation tool to expedite scrutiny of putative protein markers for new clinical assays.

Chip-based immunoassays.

Microfluidic immunoassay techniques offer advantages in speed, automation, and portability over -bench-top gold standard counterparts. In particular, on-chip immunosubtraction is a rapid homogeneous immunoassay used for reporting both protein native mobility and binding specificity. Immunosubtraction is performed by removing antibody-bound target proteins from electrophoretic detection via a size-based exclusion filter, while unbound nontarget proteins are able to pass through the filter for downstream detection. Immunosubtraction is achieved on-chip by fabrication of discrete patterned polyacrylamide (PA) gel regions. Additionally, PA gel regions are used to define on-chip sample preparation regions for protein enrichment, fluorescent labeling, and antibody-target binding prior to immunosubtraction. Here we describe the immunosubtraction device fabrication technique as well as the electrophoretic assay protocol for determining target protein mobility and binding specificity within complex biological samples including cerebrospinal fluid.

Homogeneous immunosubtraction integrated with sample preparation enabled by a microfluidic format.

Immunosubtraction is a powerful and resource-intensive laboratory medicine assay that reports both protein mobility and binding specificity. To expedite and automate this electrophoretic assay, we report on advances to the electrophoretic immunosubtraction assay by introducing a homogeneous, not heterogeneous, format with integrated sample preparation. To accomplish homogeneous immunosubtraction, a step-decrease in separation matrix pore-size at the head of a polyacrylamide gel electrophoresis (PAGE) separation channel enables "subtraction" of target analyte when capture antibody is present (as the large immune-complex is excluded from PAGE), but no subtraction when capture antibody is absent. Inclusion of sample preparation functionality via small pore size polyacrylamide membranes is also key to automated operation (i.e., sample enrichment, fluorescence sample labeling, and mixing of sample with free capture antibody). Homogeneous sample preparation and assay operation allows on-the-fly, integrated subtraction of one to multiple protein targets and reuse of each device. Optimization of the assay is detailed which allowed for ~95% subtraction of target with 20% non-specific extraction of large species at the optimal antibody-antigen ratio, providing conditions needed for selective target identification. We demonstrate the assay on putative markers of injury and inflammation in cerebrospinal fluid (CSF), an emerging area of diagnostics research, by rapidly reporting protein mobility and binding specificity within the sample matrix. We simultaneously detect S100B and C-reactive protein, suspected biomarkers for traumatic brain injury (TBI), in ~2 min. Lastly, we demonstrate S100B detection (65 nM) in raw human CSF with an estimated lower limit of detection of 3.25 nM, within the clinically relevant concentration range for detecting TBI in CSF. Beyond the novel CSF assay introduced here, a fully automated immunosubtraction assay would impact a spectrum of routine but labor and time-intensive laboratory medicine assays.