Drying of Conjugate Pads

The drying process employed for the conjugate pads must be considered alongside the dosing method, as the two processes are inexorably linked. The aim of the drying process is not only to remove the moisture from the pads, but also to do this in a controlled way to ensure a stable, reproducible end product. A key requirement for the dried conjugate is that it can be readily and completely reconstituted following the drying process. The conjugate application medium and choice of pad material are important considerations for both the drying and release of conjugate. The drying process is simple in principle—i.e., the removal of water or moisture from the pad. In practice, this is affected by the normal factors that affect any drying process—temperature, humidity, air flow, and pad thickness. The consistency of the drying process under the selected conditions must be determined and process capability and validation performed to ensure that the drying process is capable of producing consistent product.

The drying process is easiest to perform at a defined temperature around ambient room temperature—e.g., 18-25°C—under a range of ambient humidities—e.g., 30-60% relative humidity (RH). The limitation of this technique is that the ambient conditions will vary, leading to potential variation in dried components.

Control of the temperature and humidity during the drying process is advantageous and will lead to a more stable process to yield a more consistent product. Raised temperature will lead to a more rapid and efficient drying process. Elevated temperature can be achieved using drying cabinets or drying rooms, or by the use of more localized drying tunnels or stacks where the component or product is passed through a series of heaters. Typically, forced air systems are employed in conjunction with elevated temperature in the drying process. The drying tunnels described above will normally be equipped with a fan arrangement to force heated air over the component as it passes through the dryer.

Control of the relative humidity during the drying process is also advantageous. Lowered humidity will lead to a more rapid and efficient drying process. Often, the combination of elevated temperature, use of forced air, and control of relative humidity will be used in the drying process.

There are other methods that can be successfully employed for drying. For example, freeze drying has the advantage of being rapid and in theory more consistent. Freeze drying, however, can be costly and difficult to scale up, and requires a detection reagent that is not damaged by the freezing process.

The storage of the components and/or final product can also contribute to the strip drying process. Typically, intermediate components and final product are stored in pouches with very low vapor transmission rates (e.g., aluminum foil with a desiccant material such as silica gel or molecular sieve). This storage method is designed to protect the dry strip from moisture that could affect performance, and is also conducive to the further drying of the strip.

The reproducibility of the drying process is likely to directly impact the reproducibility of the final assay device; therefore, the drying process conditions, including time, temperature, humidity, and air flow, are key parameters with which to achieve the required control over the process, thus ensuring that a stable and reproducible component is produced.

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