It is not our intent to discuss materials selection as it relates to product function, but rather to discuss manufacturing issues related to these materials. As described previously, lateral-flow assays are generally a combination of filter materials and membranes supported by a plastic backing, all of which are held together by a layer of pressure-sensitive adhesive. These materials are generally selected for their functionality within the test matrix, i.e., filter characteristics, flow rates, and so on. However, when there is a choice between two or more materials, each of which suits these primary product design criteria, then the secondary material selection criteria should be on manufacturing efficiency and the elimination of potential manufacturing problems.
A good example of this is in the selection of the membrane material. Although a large variety of membrane materials are available, nitrocellulose remains the most widely used membrane for lateral-flow assays (see Chapter 4). Nitrocellulose membranes provide a variety of flow-rate options depending on test requirements. The membrane is provided in two different configurations, supported and unsupported (backed and unbacked). Unsupported nitrocellulose is very fragile. It breaks quite easily and is very difficult to guide in reel-to-reel applications, making it extremely difficult to process on automated manufacturing equipment. Additionally, unsupported nitrocellulose typically comes from the manufacturer with an interleaf paper that separates the layers of the roll and must be removed and discarded. This adds complexity and cost to the manufacturing process. Supported nitrocellulose membrane, on the other hand, is cast directly onto a polyester backing web. This gives the membrane structural support and integrity and provides for much more efficient processing, making it the ideal choice for use in lateral-flow assays.
The filters used for sample pads, conjugate pads, and absorbent pads come in a variety of materials. Sample and absorbent pads are usually paper-based products. Conjugate pads may also be paper based, but glass fiber or polypropylene materials are also common. As mentioned previously, it is important to determine first what material performs best within the test-strip matrix. But when optimizing design, the limitations of each material should be carefully evaluated and compared for maximum efficiency on automated equipment. Paper products generally have low tensile properties, especially when wet. This can result in handling problems, especially in web-coating or laminating processes. Glass fiber materials can be difficult to slit or cut and cause significant wear on cutting blades and shears. In addition, nonwoven glass fiber materials generally have poor tensile properties and can be difficult to process on web systems.
The backing material used in the majority of test strips is generally polyvinyl chloride (PVC), polystyrene, or polyester with a pressure-sensitive adhesive layer on one side. The pressure-sensitive adhesive typically includes a release liner that is removed prior to laminating the filter and membrane materials onto it. There are two major considerations when selecting the backing material for the strip—material thickness and adhesive. The backing material should be thick enough to provide structure and support to the strip but should not be so thick as to create cutting problems. Generally speaking, the backing material should be slightly thicker if the strip will not be assembled into a plastic housing, allowing the free strip to have a feeling of quality and integrity. A typical backing material for a free strip might be 0.015 to 0.020 in (0.4 to 0.5 mm) thick. A strip that will be assembled into a plastic housing would have a backing material in the range of 0.008 to 0.015 in (0.2 to 0.4 mm). This thickness should be considered carefully and discussed with a reputable provider of cutting equipment to ensure that the combined thickness and toughness of the strip does not create problems when attempting to cut the material into individual test strips. It should be noted that thicker backing materials tend to result in higher cutting forces, causing undesirable marks along the edges of the strips or improper fluid flow when the test strip is used. Another potential hazard is that the forces required to cut it can actually damage the cutting equipment. This is especially true if rotary shearing processes are used to cut the strips.
Perhaps the most overlooked and most troublesome material in all of teststrip manufacturing is the adhesive itself. Adhesive can frequently build up on the surfaces of the equipment. Strips can stick to each other. Balls of adhesive can collect and interfere with efficient processing or even end up adhered to the strips in the final product. For this reason, it is very important to consider adhe-sives very carefully. It is generally best to use as little as possible, opting for minimal thickness and for adhesives that are the least aggressive but that are sufficient to hold the components together. There are a number of reputable manufacturers of pressure-sensitive adhesives who have extensive knowledge and experience with lateral-flow assays. It is advisable to evaluate alternatives at length and to test them in actual use. Reliable adhesives should hold the strip components together and not tend to ooze out and create sticky strip edges when they are cut.
One final concern regarding material selection is that of material availability. It is best to not become sole-sourced to one supplier. It is disastrous if a test-strip product is in full-scale manufacture and the supplier of one of the components suddenly discontinues the material.
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