Science Week: How Covid lateral flow tests work

Flow processes like those in Covid antigen and pregnancy tests, have become a major area of development in the pharmaceutical industry, writes PhD student Adam McCormack and Prof  John Stephens, Department of Chemistry

Many of us will have experienced the "flow state" at different times in our lives. Sportsmen, artists, and musicians might describe the flow state as being in "the zone" or "in flow". For them, the flow state is a place of supreme confidence, where the execution of their skill flows almost effortlessly from one action to the next, delivering a complete performance.

In science, the use of flow and flowing systems is becoming an established strategy for technological developments. Often this involves scientists seeking to develop technology that allows chemicals and/or samples to flow effortlessly from one environment to another in a continuous fashion. This allows them to avoid multiple, and potentially troublesome, start stop processes. In effect, achieving the "flow state" for a given technology process.

An example of such technology can be found in lateral flow tests. These are devices that aim to detect the presence of a target substance/molecule in a liquid sample without the need for specialised and expensive equipment. As many of us know, this approach is now used as a quick and accessible method for Covid-19 antigen testing and household pregnancy tests.

The flow test detects a target molecule in the sample, an antigen in the case of Covid-19 and a hormone in the case of a pregnancy test. These operate via the test liquid "flowing" along the surface of a pad, by a process known as capillary flow or action. This is the movement of water within the spaces of a porous material due to the forces of adhesion, cohesion, and surface tension. In other words, water is sticky, and sticks to other water molecules and to surfaces/walls. In essence, capillary action occurs when the water sticks more to the surface/walls than with other water molecules.

The capillary action in the lateral flow test causes the reactive molecules present in the sample (the antigen for Covid-19 tests, the hormone for pregnancy tests) to flow along the pad and interact with the antibodies present further up, and on the surface, of the pad. The antibodies are specific for the target molecule in the sample and will only bind to that target molecule. Hence, the test line will only appear if the target molecule is present and binds to the antibody, indicating a positive result. For the user of these tests, a complex series of scientific events appear as a simple flow process were they just wait for the test line to appear.

Continuous flow processes are also becoming a major area of development in the pharmaceutical industry and the production of active pharmaceutical ingredients. To understand continuous flow in this setting, we first need to consider batch reactions, where chemical products such as pharma ingredients are generated in large batches.

When one thinks of a chemical reaction, one typically imagines a flask or large reactor, into which your reacting materials are placed and the reaction mixture is stirred, heated, or cooled depending on the reaction in question. Traditionally, the pharmaceutical industry employed these large batch reactors to generate their active ingredients. Multiple large batch reactions may be needed to generate one, with large quantities of material generated in each reaction, with that material often requiring purification and transfer to a new clean reactor before the next step, another batch reaction, can begin.

Scale is very important for the pharmaceutical industry and preforming large scale batch reactions has its problems. Considerable amounts of chemicals are present within the reaction vessel at any one time, which can be challenging when dealing with reactive or hazardous chemicals or reactions that require or generate large amounts of heat.

The synthesis of active pharmaceutical ingredients using continuous flow is a lean method where a series of tubular reactors and purification systems are set up in a single linear series. Chemicals can then be pumped into, and through, the tubular system in a continuous fashion. The product from each reaction/step can flow directly to next tubular reaction, there can be multiple tubular reactors, until the end of the process is reached where the final product is collected. Smaller amounts of chemicals, compared to batch reactors, are in use at any given moment, but as it is a continuous process large amounts of final product can still be generated.

In many cases, continuous flow processes offer several advantages for the pharmaceutical industry over batch processes. It allows them to overcome problems of excessive heat production, scale issues, highly reactive materials, and minimising employees interaction with hazardous material. This can all result in a leaner, more efficient, and in some cases safer process for the generation of a given active pharmaceutical ingredient.

Continuous flow processes are potentially the most appropriate method to produce large volumes of a product uniformly, consistently, and with reliability. However, to date continuous flow methods are not as widely used in the manufacture of these ingredients as one might have expected.

Continuous flow is not without its engineering challenges and requires a detailed understanding of the chemical processes involved, which can be quite complex. As such, for some companies, there has not been sufficient motivation to move existing batch process, which are working well, to a continuous flow process. However, the US Food and Drug Administration stated in 2011 that continuous manufacturing in the next 25 years will make current methods "obsolete". We are beginning to see that happen as the pharmaceutical industry are increasingly looking to the future, to "go with the flow" and apply continuous flow process to the manufacture of new active ingredients.

(Front photo: by Annie Spratt on Unsplash)