The trigger for the current pandemic is tiny. With a diameter of about 120 nanometres, coronavirus Sars-CoV-2 is several hundred times smaller than the pores of cotton fabric. Home-made face masks therefore only offer partial protection against the pathogen. However, the membranes produced by a team led by TU Professor Wolfgang Ensinger are absolutely virus-proof. They consist of wafer-thin films with nanopores. Viruses do not fit through the small holes – but air, water and other tiny molecules do.
Can the material be used as a filter against coronaviruses? Scientists are now investigating this question. They have already tested the filter force with silicon dioxide nanoparticles. “The membrane is extremely reliable,” says Ensinger. However, it is to be borne in mind that the model particles are very stiff whereas viruses have a certain flexibility. “In order to prevent the pathogens from slipping through, the diameter of the nanopores should be significantly lower than that of the viruses.” Films with pore diameters of 60 nanometres were used in the model test.
In the development of the membranes, Ensinger’s team worked closely with materials scientists from the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt. There, the nanopore membranes are produced in a process called ion beam etching. The technology is only possible at the GSI in Germany because it requires an accelerating system that shoots ions onto a plastic film with extremely high energy. As they pass through the film, the ions destroy chemical bonds and leave straight traces into which fine channels can then be etched with caustic soda.
Ion beam etching may be expensive, but it is very well suited to industrial purposes. One commercial manufacturer of filter materials is already producing membranes for the preparation of blood samples. Polycarbonate films from a roll are first drawn through the accelerator, which fires them with argon ions, and then through the etching bath.
The team at TU Darmstadt uses the extremely stable and heat-resistant plastic polyimide. Heavier artillery is required to perforate this: the foils are irradiated with heavy gold ions rather than light argon ions. Ensinger stresses that this procedure could also be commercialised if a market were to arise. He has primarily air filtration systems in mind, perhaps for virus laboratories or quarantine stations in hospitals.
Protective maskscannot be made (yet) with these membranes as they consist of only ten percent nanopores and not enough air passes through to breathe. The film would have to have more perforations for this purpose – but then there would be the risk of the pores overlapping into bigger holes and no longer stopping viruses. “This is an issue of technical optimisation, a balance between air flow and the frequency of pore clusters,” says Ensinger. It remains to be seen whether the project is a success. If it isn’t, it certainly won’t be because of a lack of demand for anti-virus membranes.
iNAPO: Tiny sensors
At iNAPO at LOEWE, which is part of the Centre for Synthetic Biology at TU Darmstadt, Wolfgang Ensinger and his colleagues provide nanopore membranes with recognition elements to identify biomolecules. They now want to extend this concept to the detection of viruses or antibodies, and to this end are seeking a cooperation with virology experts.