Filtering microplastics like a giant manta ray

Taking nature as a model to solve specific problems in science is nothing new. For example, Velcro was developed thanks to the Velcro plant and the lotus effect thanks to the self-cleaning lotus plant. So it pays to walk through nature with open eyes! Researchers also did this under the surface of the sea - and came across the giant manta ray. Researchers also did this under the surface of the sea - and came across the giant manta ray. Its gills are said to help filter microparticles. K-Mag spoke to Tim Robertino Baumann, who is researching this exciting topic in his doctoral thesis.

Mr. Baumann, you have (further) developed a filter system that is based on a concept inspired by giant manta rays and is designed to filter microplastics effectively. What exactly makes this principle so effective and how have you adapted it for microplastic filter technology?

Tim Robertino Baumann: The rays' feeding mechanism is based on cross-flow filtration at the gill traps. In comparison to the so-called cake filtration, known from coffee grounds in coffee filters, no residues or blockages are formed. It is also a purely passive process that takes place solely through the movement of the rays in the water. The faster the movement, the better the filtration. Not much actually needs to be done to adapt the mechanism for microplastic filtration. The important thing is that the dimensions of the filter match the size of the particles.

How does your system differ from the already established microplastic filters?

Baumann: The method of filtration makes the mechanism effective. The filters have no mechanical wear parts, are inert to clogging (except for particles larger than the channel) and become more efficient the higher the throughput. Compared to other microfluidic systems, our filtration system has the advantage that it works completely passively and offers a high flow rate of 20ml/min. The filter is particularly innovative in that it is only 60µm high and 200µm wide at the widest points. This means that the filter has a volume of just 0.2µl. Depending on the arrangement, there is space for 400 or more individual filters in a one-litre cartridge, which corresponds to a flow rate of up to 8 l/min.

You described how the first few months of research were particularly challenging. What were the biggest hurdles and how did you overcome them?

Baumann: The biggest hurdle was working outside the familiar. In the classic microfluidic laboratory routine, you work with the lowest possible pressures and speeds. However, as the filter is dependent on high speeds, high pressures are currently also required. By working with 6 to 10 bar operating pressure, solutions had to be found for problems that do not occur in this form in the laboratory, e.g:

• The firm and permanent bonding of the connections with an adhesive that is water-resistant, does not flow into the filter and bonds silicone and metal at the same time.
• Determining a suitable operating method. In the meantime, a 50ml container with a screwed stainless steel cap and pressurised connection has proven to be the best option.
• Ensuring the strength of the casting material. The laboratory standard here is PDMS, a silicone that is not designed for such pressures. At present, attempts are being made to reinforce it with glass fibres.

Such hurdles are usually overcome in small steps. Unfortunately, solving one problem often leads to another. To avoid dead ends, you go back to a certain point and think again, just like in real life.

Your filter can currently remove around 81% of microplastic particles. What improvements are you planning to further increase efficiency?

Baumann: We are planning improvements through various approaches. The greatest potential lies in continuously adapting the filter structure. We can evaluate the expected effect fairly accurately by simulating it in advance. Another possibility is to recirculate or string together several individual filters: if three filters in a row each filter 81%, the result is a cleaning performance of 99.31%.

Synthetic clothing is considered to be one of the main causes of microplastics in water. Do you see a possibility of using your filter system directly in the textile industry, e.g. in production processes or washing machines, to catch microplastics at an early stage?

Baumann: Thanks to its simplicity and variable size and flow rate, our filter system can be customised to specific requirements. So a direct application is certainly conceivable. Until then, however, we still need time for research and further development.

You also work as a technology scout at InCamS@BI. What role does the transfer of knowledge from research to industry play in the development of sustainable solutions?

Baumann: Alongside research and teaching, knowledge transfer is the third pillar of Bielefeld University and Bielefeld University of Applied Sciences. The aim is to strengthen the exchange of ideas and innovations between universities, companies and society, or to initiate it in the first place. Cooperations with transfer projects such as InCamS@BI therefore offer absolute added value, especially for small and medium-sized companies that are looking for partners for the sustainable conversion of their production. Together, sustainable solutions can often be initiated.

What next steps are you planning for your filter system and what milestones would you like to achieve in the coming years?

Baumann: There are several plans and ideas. For the time being, however, the focus for this year is on further basic research, such as determining the size, material or shape sensitivity to the microparticles. We also want to increase the throughput and reduce the required pressure.