Sample pad is a critical component of the lateral flow assay. It is the starting point of the assay where the sample is placed.
The selection of the sample pad depends on the multitude of functions it performs. With the correct specification and optional treatment, the sample can be used to:
Modulate flow rate
Ensure even flow of sample on the membrane and conjugate pad
Regulate the sample’s ability to solubilize the reagent
Maintain pH of the sample
Prevent non-specific binding on the membrane
Absorption of the sample with different viscosity
Filtration of the sample before reaction with conjugate
It is a common practice to treat the sample pad with buffers of blocking reagents, detergents & surfactants, or proteins. Treating the sample pad with an optimized buffer can improve the flow consistency, normalize the sample pH and salt concentration, act as a blocking agent for any non-specific binding, improve the appearance of lines, regulate flow onto the nitrocellulose membrane and enhance the reproducibility of the assay.
3 key materials used for the purpose are – polyester, glass-fibers, and cellulose.
Polyester screens have good tensile strength and are easy to handle, even for automated reel-to-reel operations. The material is also good in ensuring the en flow of the sample onto the conjugate pad and retains negligible sample volume, given its low bed volume. It both absorbs and releases the sample and reagents quickly, ensuring quick clearance. This is crucial as fast speed of flow prevents many cases of non–specific binding.
Glass-fiber pads and cellulose pads are widely used as sample pads since they are inexpensive and able to regulate non-specific binding well. These pads are also more uniform. They also tend to have good filtration properties in preventing non-specific binding. Cellulose pads tend to have a higher bed volume and high retention of sample volume, making them unsuitable for many applications. However, it works well to regulate test flow speed and lower the background clearance in other tests such as Malaria Ag.
The table below provides Axiflow's range of pads and their applications.
A. Thickness
Average thickness is important to the bed volume of the pad. Variation in thickness affects the consistency of the assay. It is important to suit the bed volume and variation tolerance to suit the application. Thickness is also important for timing retention and release of the volume of sample and buffer. The thickness of the sample pad also depends on the device body’s structure and the sample well diameter.
B. Tensile Strength
Tensile strength is important to ensure easy handling. Automated processing requires a fairly high tensile strength.
C. Extractables
Purity of fibers and use of appropriate binders is required to ensure that the sample & buffer transfer does not impact the macro-structure of the pad. There should be no physical change in pad structure and no leaching of components of the pad. It is important to ensure consistent flow, filtration properties of the pad, and non-specific binding.
D. Pore size
The retention properties of the pad will depend on the pore rating and consistency of the pore size of the material. For reproducible results, the porosity and quality of construction material must be uniform.
From Axiflow Biotech range of sample pad materials, please feel free to reach out to us at Export@axiflow.co.in for guidance on the selection of the right pad for your application.