Sample, Conjugate, and Absorbent Pad Materials

Depending on wherein pad products are sourced, castle may absence specifications the pertain come lateral flow tests and also exhibit level of variability the are greater than desired, an especially at the dimensions that they space being supplied in lateral circulation tests. Various other materials, such together woven meshes and also synthetic nonwovens, are likewise used as pad materials, although at a much lower frequency.

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Chemistries ~ above SureWick™ Pad Materials

There may be areas within the liquid stream where the concentration that the chemistry is not conducive to interaction in between the antibodies and also analyte.

Sample Pad choice and Specifications

The sample pad (Figure 1) can be used to do multiple tasks, foremost of i beg your pardon is to encourage the even and also controlled circulation of the sample ~ above the conjugate pad. The may additionally control the rate at which fluid enters the conjugate pad, staying clear of flooding that the device. When impregnated with contents such together proteins, detergents, viscosity enhancers, and buffer salts, the sample pad can likewise be offered to:

Increase sample viscosity (modulate flow properties).Enhance the ability of the sample come solubilize the detector reagent.Prevent the conjugate and also analyte indigenous binding non-specifically to any type of of the downstream materials.Modify the chemistry nature of the sample so the it is compatible through immunocomplex development at the check line.Promote also flow the the sample along the membrane.

Figure 1.Sample Pad

By including blocking agents to the sample pad, it may be feasible to get rid of blocking that the membrane. This approach may be lot easier and considerably an ext cost-effective 보다 attempting to block the membrane directly. Unless the antibody (or antigen) is covalently attached come the detector particle, the is no advisable to dried the detector reagent right into the conjugate pad in the presence of blocking proteins or detergents. Exogenous proteins, specifically in the presence of detergents, can displace the antibody or the antigen from the detector bit during prolonged storage. Thus, the sample pad might be the only ar in the test maker other 보다 the membrane whereby blocking and also resolubilization agents have the right to be added safely.

Some tests require samples that exhibit vast variation in chemistry composition. Human urine, for instance, have the right to have a pH in between 5 and also 10. Distinctions in pH and ionic toughness may change the specificity and also sensitivity that capture and detector reagents and promote varying degrees of non-specific binding of detector reagents due to alters in fee densities. Adding a reasonably high concentration the buffer salt to the sample pad (for example, by pre-treating with 1.0 M borate buffer, pH 9.5) can minimize variation by controlling the pH and ionic stamin of the systems that increase from the sample pad.

There room two species of materials that are generally used together sample pads: cellulose fiber filters and woven meshes.

Woven meshes, sometimes called screens, generally work an extremely well to distribute the sample volume evenly end the conjugate pad. They likewise typically have an excellent tensile strength and handle well, also when wet. Meshes have very low bed volumes, definition that they retain very tiny sample volume, typically 1 – 2 μL/ cm2. ~ above the other hand, the is impractical to treat them v the intentionally of loading lock with enough solutes to modify the protein content, pH, ionic strength, or viscosity of the check sample. Meshes can also be expensive relative to various other porous media and difficult to procedure through strip cutting machinery.

Cellulose filters have actually properties that are practically the opposite of woven meshes. They room thick (> 250 μm), weak, and fairly inexpensive. Cellulose filters likewise have large bed volumes (> 25 μL/ cm2). File can it is in very an overwhelming to handle, particularly when wet. Cellulosic filters are the most typically used materials to do the sample pad because they can be loaded v a vast array of prevent agents, detector reagent release agents, pH and ionic strength modifiers, and viscosity enhancers. As soon as using cellulose filters, care must be taken to ensure sufficient and consistent call with the basic conjugate pad material. Failure to achieve good contact and adequate compression deserve to lead to interrupted or inconsistent carry of fluid into the conjugate pad.

For numerous urine-based assays, particularly pregnancy and also ovulation tests, porous plastic wicks protrude from the finish of the cassette. Their primary function is to collection liquid indigenous the to pee stream so the it have the right to be moved to the test strip within the cassette. The size of the wick space tailored to the style of the test strip cassette and also the need to absorb sufficient liquid to run the test. The wick might or might not be chemically treated, depending upon the chemistries that the other products within the cassette. The is conceivable that the plastic wick deserve to serve the duty of a sample pad.

Specifying the Sample Pad

Specification of the sample pad depends significantly on that intended objective in the test device. If the sample pad is being used mainly to modify the sample, the following qualities should it is in specified:

Thickness (Mean and Variability)

Mean thickness might be given as microns, millimeters, or thousandths the an inch (mils). The selection of variability is also critical since this will influence the quantity of bed volume and also the consistency that compression in a housing. For strips placed in housings, the sample is typically applied to a port the exposes a small region of the sample pad. If the pad product is as well thick, the fibers may be compressed so the absorption of liquid right into the pad is greatly lessened or prevented. If the pad product is as well thin, there may be small or no contact with the housing. This enables the sample to go into the housing unobstructed, flooding the interior, and also significantly altering the circulation dynamics that the test strip.

Basis weight (Mean and also Variability)

The basis load is the massive of fibers every unit area. In the paper and non-woven industries, that is most often expressed as g/cm2. This value is of little relevance for lateral circulation tests. Using the basis weight, the thickness the the material, and the thickness of the polymer, the bed volume and also porosity can be calculated. Bed volume is directly proportional to thickness at consistent porosity. Similarly, bed volume is straight proportional come porosity at consistent thickness. Thus, variability in basis weight have the right to be equated to variability in bed volume. Bed volume is actually the an important performance parameter, but it is rarely provided.

Tensile Strength

Tensile stamin for sample pad products is important for the same reason as for membranes. Some products are as weak together unbacked membranes. Due to the fact that they may be slit come widths the 1 cm or less, web handling can it is in very difficult in a continuous processing operation.


Materials used to make sample pads contain binder to hold the fibers together. In addition, few of the fibers might break or no be interconnected with the pad’s macrostructure. Consequently, a significant percentage of the pad’s materials may it is in dislodged during various processing steps. As soon as a test piece is run, this deserve to lead to plugging and also poor fluid transfer together the sample wets out the pad and also moves downstream.

Dimensions (Length, Width)

The dimensions and tolerances have to be fully defined.

Packaging, Labeling, and Certification

The considerations pertaining to membranes likewise apply come sample pad materials.

Particle Retention Rating

In some applications, the sample pad is provided as a filter to remove particles native the sample prior to the fluid enters the conjugate pad. Thus, the is essential to recognize the particle retention rating. Due to the fact that these products are depth filters, they do not exhibit 100% retention capability. Because of the way that papers and also non-woven products are manufactured, changing the bit retention rating often involves far-ranging changes to the thickness and basis weight.

Conjugate Pad choice and Specifications

The conjugate pad (Figure 2) can perform multiple tasks, the most essential of i beg your pardon is uniform move of the detector reagent and test sample top top the membrane. When sample flows right into the conjugate pad, the detector reagent solubilizes, lifts off the pad material, and also moves v the sample front into the membrane. The appropriate conjugate pad material has the following attributes:

Low non-specific binding: If the detector reagent or analyte binds to the conjugate pad, it will be unavailable to form the immunocomplex in ~ the test line, in order to reducing signal intensity and also sensitivity.Consistent circulation characteristics: Inconsistent circulation properties can reason serious power problems. If the product does no deposit the sample uniformly top top the membrane, the detector reagent might be channeled top top the membrane, showing up as streaks as the sample migrates along the membrane. Consequently, there will be uneven signal development at the test and also control lines.Consistent bed volume: when loaded right into the conjugate pad through dipping, the amount of detector reagent in each test strip depends on the bed volume the the material. If the bed volume different significantly, variable signal intensities may be observed also though every other components of the strip room constant.Low extractables: In addition to chemistry extractables, the product should be cost-free of corpuscle that can clog the membrane in ~ the conjugate pad/membrane interface.

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Good web managing characteristics and also consistent compressibility: These qualities are important for incorporation right into test piece manufacture and for regular liquid carry onto the membrane.