Optimization of flow path parameters for enhanced sensitivity lateral flow devices

Lateral flow devices (LFDs) or lateral flow tests (LFTs) are one of the most widely used biosensor platforms for point-of-care (POC) diagnostics. The basic LFD design has remained largely unchanged since its first appearance, and this has limited LFD use in clinical applications due to a general lack of analytical sensitivity. We report here a comprehensive study of the use of laser-patterned geometric control barriers that influence the flow dynamics within an LFD, with the specific aim of enhancing LFD sensitivity and lowering the limit of detection (LOD). This control of sample flow produces an increase in the time available for optimizing the binding kinetics of the implemented assay. The geometric modification to the flow path is in the form of a constriction that is produced by depositing a photo-sensitive polymer onto the nitrocellulose membrane which when polymerized, creates impermeable barrier walls through the depth of the membrane. Both the position of the constriction within the flow path and the number of constrictions allow for an increase in the sensitivity because of a slower overall flow rate within the test and a larger volume of sample per unit width of the test line. For these high sensitivity LFDs (HS-LFD), through optimization of the constriction position and addition of a second constriction we attained a 62% increase in test line color intensity for the detection of procalcitonin (PCT) and were also able to lower the LOD from 10 ng/mL to 1 ng/mL. In addition, of relevance for future commercial exploitation, this also significantly decreases the antibody consumption per device leading to reduced costs for test production. We have further tested our HS-LFD with contrived human samples, validating its application for future clinical use.

Laser-patterned paper-based flow-through filters and lateral flow immunoassays to enable the detection of C-reactive protein

We report the use of a laser-based fabrication process in the creation of paper-based flow-through filters that when combined with a traditional lateral flow immunoassay provide an alternative pathway for the detection of a pre-determined analyte over a wide concentration range. The laser-patterned approach was used to create polymeric structures that alter the porosity of the paper to produce porous flow-through filters, with controllable levels of porosity. When located on the top of the front end of a lateral flow immunoassay the flow-through filters were shown to block particles (of known sizes of 200 nm, 500 nm, 1000 nm and 3000 nm) that exceed the effective pore size of the filter while allowing smaller particles to flow through onto a lateral flow immunoassay. The analyte detection is based on the use of a size-exclusive filter that retains a complex (∼3 μm in size) formed by the binding of the target analyte with two antibodies each of which is tagged with different-sized labels (40 nm Au-nanoparticles and 3 μm latex beads), and which is larger than the effective pore size of the filter. This method was tested for the detection of C-reactive protein in a broad concentration range from 10 ng/ml to 100,000 ng/ml with a limit-of-detection found at 13 ng/ml and unlike other reported methods used for analyte detection, with this technique we are able to counter the Hook effect which is a limiting factor in many lateral flow immunoassays.

Semi-quantitative detection of inflammatory biomarkers using a laser-patterned multiplexed lateral flow device

Inflammatory markers including C-reactive protein (CRP) and procalcitonin (PCT) have been shown to be useful biomarkers to improve triage speed and prevent the inappropriate use of antibiotics for infections such as pneumonia. Here, we present a novel and exciting solution to guide the administration of antibiotic treatment via rapid, semi-quantitative and multiplexed detection of CRP and PCT using an advanced lateral flow device (LFD) designed to have multiple parallel flow-paths, produced via the precise laser-based partitioning of the single flow-path of a standard LFD. Each flow-path within this multiplexed LFD has a unique detection capability which permits tailored detection of CRP within a predefined cut-off range (20 μg/mL - 100 μg/mL) and PCT above a pre-defined threshold (0.5 ng/mL). We demonstrate the use of this LFD in the successful detection of CRP and PCT semi-quantitatively within spiked human serum samples. This multiplexed near-patient assay has potential for development into a rapid triage and treatment of patients with suspected pneumonia.

Capillary-based reverse transcriptase loop-mediated isothermal amplification for cost-effective and rapid point-of-care COVID-19 testing

As the SARS-CoV-2 pandemic continues to spread, the necessity for rapid, easy diagnostic capabilities could never have been more crucial. With this aim in mind, we have developed a cost-effective and time-saving testing methodology/strategy that implements a sensitive reverse transcriptase loop-mediated amplification (RT-LAMP) assay within narrow, commercially available and cheap, glass capillaries for detection of the SARS-CoV-2 viral RNA. The methodology is compatible with widely used laboratory-based molecular testing protocols and currently available infrastructure. It employs a simple rapid extraction protocol that lyses the virus, releasing sufficient genetic material for amplification. This extracted viral RNA is then amplified using a SARS-CoV-2 RT-LAMP kit, at a constant temperature and the resulting amplified product produces a colour change which can be visually interpreted. This testing protocol, in conjunction with the RT-LAMP assay, has a sensitivity of ∼100 viral copies per reaction of a sample and provides results in a little over 30 min. As the assay is carried out in a water bath, commonly available within most testing laboratories, it eliminates the need for specialised instruments and associated skills. In addition, our testing pathway requires a significantly reduced quantity of reagents per test while providing comparable sensitivity and specificity to the RT-LAMP kit used in this study. While the conventional technique requires 25 μl of reagent, our test only utilises less than half the quantity (10 μl). Thus, with its minimalistic approach, this capillary-based assay could be a promising alternative to the conventional testing, owing to the fact that it can be performed in resource-limited settings, using readily available apparatus, and has the potential of increasing the overall testing capacity, while also reducing the burden on supply chains for mass testing.

Laser-patterned paper-based sensors for rapid point-of-care detection and antibiotic-resistance testing of bacterial infections

Antimicrobial resistance (AMR) has been identified by the World Health Organisation as a global threat that currently claims at least 25,000 deaths each year in Europe and 700,000 globally; the number is projected to reach 10 million per year between 2015 and 2050. Therefore, there is an urgent need for low-cost but reliable point-of-care diagnostics for early screening of infections especially in developing countries lacking in basic infrastructure and trained personnel. This work is aimed at developing such a device, a paper-based microfluidic device for infection testing by an unskilled user in a low resource setting. Here, we present our work relating to the use of our laser-patterned paper-based devices for detection and susceptibility testing of Escherichia coli, via a simple visually observable colour change. The results indicate the suitability of our integrated paper devices for timely identification of bacterial infections at the point-of-care and their usefulness in providing a hugely beneficial pathway for accurate antibiotic prescribing and thus a novel route to tackling the global challenge of AMR.

Rapid Multiplexed Detection on Lateral-Flow Devices Using a Laser Direct-Write Technique

Paper-based lateral flow devices (LFDs) are regarded as ideal low-cost diagnostic solutions for point-of-care (POC) scenarios that allow rapid detection of a single analyte within a fluidic sample, and have been in common use for a decade. In recent years, there has been an increasing need for rapid and simultaneous detection of multiple analytes present within a single sample and to facilitate this, we report here a novel solution—detection using a multi-path LFD created via the precise partitioning of the single flow-path of a standard LFD using our previously reported laser direct-write (LDW) technique. The multiple flow-paths allow the simultaneous detection of the different analytes individually within each of the parallel channels without any cross-reactivity. The appearance of coloured test lines in individual channels indicates the presence of the different analytes within a sample. We successfully present the use of a LDW-patterned multi-path LFD for multiplexed detection of a biomarker panel comprising C-reactive protein (CRP) and Serum amyloid A-1 (SAA1), used for the diagnosis of bacterial infections. Overall, we demonstrate the use of our LDW technique in the creation of a novel LFD that enables multiplexed detection of two inflammation markers within a single LFD providing a detection protocol that is comparatively more efficient than the standard sequential multiplexing procedure.

Improved sensitivity and limit-of-detection of lateral flow devices using spatial constrictions of the flow-path

We report on the use of a laser-direct write (LDW) technique that allows the fabrication of lateral flow devices with enhanced sensitivity and limit of detection. This manufacturing technique comprises the dispensing of a liquid photopolymer at specific regions of a nitrocellulose membrane and its subsequent photopolymerisation to create impermeable walls inside the volume of the membrane. These polymerised structures are intentionally designed to create fluidic channels which are constricted over a specific length that spans the test zone within which the sample interacts with pre-deposited reagents. Experiments were conducted to show how these constrictions alter the fluid flow rate and the test zone area within the constricted channel geometries. The slower flow rate and smaller test zone area result in the increased sensitivity and lowered limit of detection for these devices. We have quantified these via the improved performance of a C-Reactive Protein (CRP) sandwich assay on our lateral flow devices with constricted flow paths which demonstrate an improvement in its sensitivity by 62x and in its limit of detection by 30x when compared to a standard lateral flow CRP device.

Laser direct-write for fabrication of three-dimensional paper-based devices

We report the use of a laser-based direct-write (LDW) technique that allows the design and fabrication of three-dimensional (3D) structures within a paper substrate that enables implementation of multi-step analytical assays via a 3D protocol. The technique is based on laser-induced photo-polymerisation, and through adjustment of the laser writing parameters such as the laser power and scan speed we can control the depths of hydrophobic barriers that are formed within a substrate which, when carefully designed and integrated, produce 3D flow paths. So far, we have successfully used this depth-variable patterning protocol for stacking and sealing of multi-layer substrates, for assembly of backing layers for two-dimensional (2D) lateral flow devices and finally for fabrication of 3D devices. Since the 3D flow paths can also be formed via a single laser-writing process by controlling the patterning parameters, this is a distinct improvement over other methods that require multiple complicated and repetitive assembly procedures. This technique is therefore suitable for cheap, rapid and large-scale fabrication of 3D paper-based microfluidic devices.

Engineering fluidic delays in paper-based devices using laser direct-writing

We report the use of a new laser-based direct-write technique that allows programmable and timed fluid delivery in channels within a paper substrate which enables implementation of multi-step analytical assays. The technique is based on laser-induced photo-polymerisation, and through adjustment of the laser writing parameters such as the laser power and scan speed we can control the depth and/or the porosity of hydrophobic barriers which, when fabricated in the fluid path, produce controllable fluid delay. We have patterned these flow delaying barriers at pre-defined locations in the fluidic channels using either a continuous wave laser at 405 nm, or a pulsed laser operating at 266 nm. Using this delay patterning protocol we generated flow delays spanning from a few minutes to over half an hour. Since the channels and flow delay barriers can be written via a common laser-writing process, this is a distinct improvement over other methods that require specialist operating environments, or custom-designed equipment. This technique can therefore be used for rapid fabrication of paper-based microfluidic devices that can perform single or multistep analytical assays.

Laser-based patterning for fluidic devices in nitrocellulose

In this report, we demonstrate a simple and low cost method that can be reproducibly used for fabrication of microfluidic devices in nitrocellulose. The fluidic patterns are created via a laser-based direct-write technique that induces polymerisation of a photo-polymer previously impregnated in the nitrocellulose. The resulting structures form hydrophobic barriers that extend through the thickness of the nitrocellulose and define an interconnected hydrophilic fluidic-flow pattern. Our experimental results show that using this method it is possible to achieve microfluidic channels with lateral dimensions of ∼100 μm using hydrophobic barriers that form the channel walls with dimensions of ∼60 μm; both of these values are considerably smaller than those that can be achieved with other current techniques used in the fabrication of nitrocellulose-based fluidic devices. A simple grid patterned nitrocellulose device was then used for the detection of C-reactive protein via a sandwich enzyme-linked immunosorbent assay, which served as a useful proof-of-principle experiment.

Laser-induced photo-polymerisation for creation of paper-based fluidic devices

Paper-based microfluidics is a rapidly progressing inter-disciplinary technology driven by the need for low-cost alternatives to conventional point-of-care diagnostic tools. For transport of reagents/analytes, such devices often consist of interconnected hydrophilic fluid-flow channels that are demarcated by hydrophobic barrier walls that extend through the thickness of the paper. Here, we present a laser-based fabrication procedure that uses polymerisation of a photopolymer to produce the required fluidic channels in paper. Experimental results showed that the structures successfully guide the flow of fluids and allow containment of fluids in wells, and hence the technique is suitable for fabrication of paper-based microfluidic devices. The minimum width for the hydrophobic barriers that successfully prevented fluid leakage was ~120 μm and the minimum width for the fluidic channels that can be formed was ~80 μm, the smallest reported so far for paper-based fluidic patterns.

Paper-based colorimetric enzyme linked immunosorbent assay fabricated by laser induced forward transfer

We report the Laser Induced Forward Transfer (LIFT) of antibodies from a liquid donor film onto paper receivers for application as point-of-care diagnostic sensors. To minimise the loss of functionality of the active biomolecules during transfer, a dynamic release layer was employed to shield the biomaterial from direct exposure to the pulsed laser source. Cellulose paper was chosen as the ideal receiver because of its inherent bio-compatibility, liquid transport properties, wide availability and low cost, all of which make it an efficient and suitable platform for point-of-care diagnostic sensors. Both enzyme-tagged and untagged IgG antibodies were LIFT-printed and their functionality was confirmed via a colorimetric enzyme-linked immunosorbent assay. Localisation of the printed antibodies was exhibited, which can allow the creation of complex 2-d patterns such as QR codes or letters for use in a final working device. Finally, a calibration curve was determined that related the intensity of the colour obtained to the concentration of active antibodies to enable quantitative assessment of the device performance. The motivation for this work was to implement a laser-based procedure for manufacturing low-cost, point-of-care diagnostic devices on paper.