Sample and absorbent pads play different but complementary roles in a lateral flow assay. The sample pad controls how the sample enters the strip and can help condition or filter the sample before it reaches the conjugate pad and nitrocellulose membrane. The absorbent, or wick, pad provides downstream capacity that helps maintain capillary flow through the assay. Selecting the appropriate materials can improve flow consistency, sample compatibility, and overall assay reproducibility.
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Screen sample and wick pad materials for your assay
Compare sample pads, blood separation pads, wick pads, nitrocellulose membranes, and other lateral flow materials with our Material Starter Kit for Lateral Flow.
Sample Pad Selection
The sample pad serves two primary purposes in a lateral flow assay:
- Control sample introduction and flow. Sample composition can vary substantially between individuals and sample types. The sample pad provides an opportunity to condition the sample before it reaches the conjugate pad and other assay reagents.
- Filter unwanted sample components. For matrices such as whole blood or solubilized solids, an appropriate sample or separation pad can retain particulates while allowing the fluid containing the target analyte to continue through the strip.
The sample matrix has a major influence on which material will perform best. Blood, saliva, urine, serum, and other matrices can differ in viscosity, particulate content, pH, ionic strength, and protein composition. For whole-blood assays that require dedicated plasma separation, Fortis Diagnostics also offers custom blood and plasma separation membranes ↗ with configurable pore size, thickness, coatings, and dimensions.
Several material characteristics should be considered during sample pad screening:
| Selection Factor | Why It Matters |
|---|---|
| Material | Cellulose, glass fiber, and synthetic materials differ in sample uptake, release, filtration, and compatibility with pre-treatment reagents. |
| Thickness & Capacity | Higher-capacity materials can accommodate larger sample volumes, but pad thickness must also be compatible with the strip and cassette. Excessive compression can reduce effective absorption and alter flow. |
| Tensile Strength | Materials with low tensile strength can be more difficult to handle, cut, and laminate consistently during manufacturing. |
| Sample Compatibility | The pad must provide appropriate flow and, where necessary, filtration for the intended sample matrix without retaining or interfering with the target analyte. |
Because these properties interact with the sample and the rest of the strip, multiple candidate materials should be evaluated experimentally rather than selecting a pad based on a single physical specification.
Sample Pad Treatment
Sample pads are often treated with buffers, proteins, detergents, surfactants, salts, or polymers to condition the sample before it reaches the reporter conjugate and test line. An optimized treatment can help control pH and ionic strength, reduce non-specific interactions, improve flow, and reduce sample-to-sample variability.
The appropriate treatment depends on the sample matrix and the specific limitation that needs to be addressed. For example, saliva can vary substantially in viscosity due in part to mucins and other proteins. Salt and surfactant concentrations may be adjusted to improve flow and reduce these effects. The same formulation may not be appropriate for whole blood, where overly aggressive conditions can cause hemolysis and interfere with blood separation.
Sample pad treatment can be performed by immersion or by spraying uniformly with an automated dispenser such as IsoFlow, Kinematic, or BioDot equipment. After treatment, we typically dry sample pads in a forced-air convection oven at 37°C for 30–60 minutes and then allow them to dry overnight in a desiccated environment (<20% relative humidity) at 18–25°C.
Treated pads should continue to be stored in a dry environment at room temperature because absorbed moisture can affect reagents dried onto the material and contribute to variability in assay performance.
Absorbent (Wick) Pad Selection
The absorbent pad sits at the distal end of the lateral flow strip and acts as a reservoir for sample and running buffer after they pass through the nitrocellulose membrane. Its capacity helps sustain capillary flow across the test and control lines.
A wick pad with sufficient absorption capacity allows more fluid to move through the assay before the pad becomes saturated. This can help maintain consistent flow, reduce accumulation of non-specifically bound material, and provide a stable window for reading the assay. If the wick pad becomes saturated too early, flow can slow or stop and backflow may occur.
Important wick pad characteristics include:
- Absorption capacity: The pad should have sufficient capacity for the total sample and running-buffer volume used in the assay.
- Thickness and weight: These influence capacity and physical integration into the assembled strip, although thickness alone does not determine absorption capacity.
- Material: Cellulose materials are commonly used because of their relatively high fluid capacity.
- Handling characteristics: The material should tolerate cutting, lamination, and routine manufacturing without excessive tearing or deformation.
As with sample pads, wick materials should be screened experimentally. Two pads with similar thicknesses can have substantially different absorption capacities and flow characteristics, so performance in the complete strip is more informative than material dimensions alone.
Once the sample, conjugate, membrane, and wick materials have been selected and optimized, they can be combined into the complete lateral flow test strip.
Next: Assemble the lateral flow test strip