Steering with magnets
The readout device contains magnets with which the magnetic spheres can be moved. As soon as the spheres have been able to bind enough troponin molecules, they are drawn to the bottom of the cartridge with the help the magnets. The base is also covered with antibodies and so the troponin molecules bind to the base as well. Subsequently the number of spheres attached to the base is taken as a measure for the concentration of troponin molecules in the blood drop. If the non-bound spheres are subsequently drawn upwards by a magnet then only the bound spheres remain on the base. To measure the number of spheres, the base is illuminated from below with light. The amount of shadow on the base finally indicates the quantity of troponin present in the blood.
Measuring minimal concentrations
‘With the magnetic spheres we therefore ensure that the proteins are bound as quickly as possible. With the so-called ‘rinsing step’, we ensure that non-bound spheres are pulled off the base thereby reducing the noise in the measurements. Thanks to these two steps, the sensitivity of this detection method is far better than that of other rapid troponin tests,’ explains Irmscher. ‘With this approach we can measure substances at concentrations of 30 billionths of a gram per litre.’
As the test works with full blood, a number of hurdles had to be overcome. ‘Ultimately we measure the protein concentration in the blood plasma and so we had to develop a filter that separates the plasma from the red and white blood cells.’ That was not easy, explains Irmscher. ‘You need to produce a filter that holds blood cells, for example, while allowing through the proteins you are interested in. The filter must not become clogged up and it must work with very different blood compositions.’
It was therefore pointless working with model liquids, he says. ‘Right from the start we worked with full blood from many different individual patients. The variation in the quantity of red blood cells between different patients is enormous. Producing a filter that is flexible enough to cope with all those extremes was therefore a considerable challenge. Within NanoNextNL researchers produced a theoretical model for that filter.’
Fundamental research has played an important role in the development of the final product, he says. ‘For example within NanoNextNL, a test setup was developed so that we could observe exactly what happened with the spheres. It was like a cross-section of the chamber with an observation window in it so that we could follow the movements of the spheres. This allowed us, for example, to determine the optimum concentration of the spheres. You must have enough spheres in the solution to bind as much of the protein as possible. However, if you use too many spheres then they obstruct each other’s movement and they cannot bind to the base and be detected.’