Lateral Flow Test Strip Assembly
The assembly of a lateral flow test strip can range from fully automated to completely manual where the driving force for the manufacturing choice is often time and cost.
At large scale manufacturing (e.g. >1 million units per year) the use of automation can significantly cut down the overall cost per strip. One example of this type of automation is the BioDot Reel-to-Reel Dispensing Systems, as shown in figure 1.
In this module, we are focusing on the main steps to consider during a typical lateral flow assembly line.
Lamination
Lamination is the term given to the assembly of all the lateral flow strip components into a single item, beginning with the backing card. Backing cards are typically made of vinyl with an acrylic adhesive on one side that is covered with a precision cut release liner. Each component being assembled onto the backing card will typically have its own dedicated release liner.
Figure 2 shows an example of the release liner for the nitrocellulose membrane, which is typically in the center of the backing card, being pulled back prior to adhering the membrane onto the backing card.
The order that a backing card is laminated with the lateral flow strip components is very important.
Typically, the nitrocellulose membrane will be laminated onto the backing first, as it will need to be underneath both the wick pad and sample pad in order for the capillary flow to work as desired. Once the nitrocellulose membrane has been laminated onto the backing card, you can remove the adhesive liner for either the conjugate pad or absorbent pad, and laminate both respectively. Each of these components will need some overlap with the nitrocellulose membrane.
Once these three components have been laminated, the sample pad is applied. The sample pad needs to overlap with the conjugate pad in order for the sample to transition to the conjugate pad, after which the conjugate will release and flow onto the membrane together with the sample.
Although lamination can be done by hand, it is common to use a lamination machine like the Kinematic Matrix 2210, Figure 3.
With the use of vacuums to hold material in place, and a lamination key to ensure materials are being laminated at precisely the correct position, the backing cards are assembled with high reproducibility.
Once the backing card has been fully assembled, it is now referred to as a "master card".
Cutting
After all the lateral flow strip components have been laminated on the backing card, the master card is now ready to be cut into strips. An automated guillotine, such as that shown in Figure 4, can be used to accurately cut strips with a high degree of reproducibility.
The two most common problems during cutting are strip width variability and frayed edges. Strip width can be verified with precision calipers, and should be done so consistently, both when beginning to cut a mastercard and throughout cutting to ensure consistency.
Frayed edges are typically a byproduct of the guillotine blade dulling over time, and will negatively impact running conditions. It is important to clean and sharpen the guillotine blade regularly.
Strip widths may vary depending on the specific assay requirements, with thinner strips being more cost effective (higher number of strips per card) but potentially less accurate due to edge effects.
Quantitative assays are most commonly cut to 5-6 mm widths.
Cassette
The cassette that houses the test strip can be one of the most critical components to achieve a reproducible and reliable lateral flow assay, and is especially important for quantitative tests.
The cassette provides flow control by applying pressure at appropriate points on the strip to ensure that all the fluid passes through the strip assembly at the same flow rate. It also ensures that the fluid flows through the test strip materials rather than passing over just the top of the strip or flowing along the edges.
Typically, cassettes are designed after all materials have been selected and optimized, and it is important to customize the cassette to the lengths, widths, and thicknesses of each component. Pressure points at material junctions control the flow rate of the sample fluid, allowing for longer or shorter incubation times of conjugate with sample analyte.
For large scale production, a custom designed cassette from an experienced industrial design company is necessary, preferably a company that already has lateral flow cassette expertise, but for initial testing, existing generic cassettes may be sufficient.
Automated cassette assembly is possible, but traditionally assembly is performed manually with the help of a cassette assembly roller. The assembly roller uses a belt feeder to move cassettes through a roller that will apply consistent pressure across the top housing to close the cassette. It is always best to visually check all end products to ensure the strips have been assembled correctly (e.g. the strip was placed in the cassette in the correct orientation), and the nitrocellulose has no blemishes or scratches that may have occurred during the assembly process.
For some assays, a quantitative read-out is not required and a dipstick format may be sufficient for running the strip. In this format, there is no sample or conjugate pad, and the strip is placed upright directly into the liquid sample. Strips in this format are not placed inside a plastic cassette, and instead cover tape is utilized to provide flow control and to hold all of the components of the strip together.
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