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The assembly of a lateral flow test strip can range from fully automated to completely manual. The driving force for the manufacturing choice is time and cost. At large scale manufacturing (e.g. >1 million 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.

Biodot Lateral Flow Automated Manufacturing

Lamination

Lamination is the term given to the assembly of all the lateral flow strip components into a single item. This assembly begins 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. 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 is shown below.

Lamination backing card for nitrocellulose assay

The order that a backing card is laminated with the lateral flow strip components is very crucial. 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 sample 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 conjugate pad is applied. The conjugate pad needs to overlap with the sample pad in order for the conjugate to transition between the two materials. Although lamination can be done by hand, it is common to use a lamination machine like the Kinematic Matrix 2210.

Kinematic Matrix 2210 Laminator

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 can be used to accurately cut strips with a high degree of reproducibility.

Biodot Guillotine CM5000

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 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. Frayed edges are typically a byproduct of the guillotine blade dulling over time. It is important to clean, and sharpen the guillotine blade regularly.  Strip widths may vary depending on the specific assay requirements. Thinner strips are more cost effective (higher number of strips per card) but can be 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 later 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 who already has lateral flow cassette expertise.  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 in running the strip.  In this format, the strip is 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.