Ecotoxicological studies on the effects of microplastics on human health and the environment have, until now, frequently produced contradictory results. In relevant studies on the health risks due to microplastics, human tissue such as intestinal cells or marine organisms such as crabs and mussels were subjected to various microplastic particle concentrations in order to research the interactions with cell tissue. These microparticles were previously categorised according to the type of plastic, the shape and the size. Their chemical and physical properties were, however, seldom taken into account. An interdisciplinary research team from the University of Bayreuth headed by Prof. Dr. Christian Laforsch has now established that the characteristics of the conventional polystyrene that is frequently used in these studies differ significantly depending on the manufacturer, and that it also affects cell tissue differently. Generalised statements on health hazards or ecological risks from microplastics are therefore considered by the scientists to be problematical.
The research team at the University of Bayreuth initially studied the surface properties and chemical composition of polystyrene particles that are frequently used in impact studies. Standard polystyrene spheres with a size of 3 micrometres from two producers were analysed. Depending on the source, there were significant differences in particular as regards the surface properties and monomer content of the polystyrene particles. An analysis by scanning electron microscopy (SEM) showed, according to the study, that the surface structures of the two polystyrene types differ (photo). It could be seen that the polystyrene from producer 1 has a rougher surface than that from producer 2. To examine the chemical composition of the two polystyrene types, use was made of magnetic resonance, X-ray and chromatographic procedures, which showed differences in the surface charge of the microparticles and the monomer content of the polymer. These differences between the polystyrene pellets of the two manufacturers are attributed by the researchers to different so-called radical initiators, which were used in the production of the plastics to trigger the process of polymerisation. Both the surface structure and the monomer content of microparticles could, according to the study, change the interaction with cells. For this reason, tests were also carried out on how both particle types affect the metabolism of immune cells. After the immune cells (macrophages of mouse cell lines) were exposed to the two particle types, differences showed up in the metabolic activity and in the cell growth. The polystyrene type with the rougher surface structure also had an electric potential of approx. minus 80 microvolts on the surface. This type interacted more significantly with the cell tissue: The particles were in some cases completely surrounded by cell structures, which, as the study showed, disrupted both the cell metabolism and the growth. The particles from the other manufacturer were almost uncharged and docked only externally to the cells.
"Our study shows impressively how problematical it is to make generalised statements on the health-related or ecological effects of microplastics," says Professor Laforsch. He added that if equal-sized particles of the same type of plastic in the same shape show surprising chemical and physical differences when subjected to in-depth analysis, and when these differences affect interactions with living cells, then care must be taken not to draw premature conclusions. "The replicability of experiments must have maximum priority in microplastic research – especially when it is a matter of examining health-related effects," adds co-author Anja Ramsperger. "In the meantime, There are numerous toxicological studies that aim to track down the effects of microplastic on living organisms. But only when we know in detail the chemical composition and the surface properties of the used particles can we compare these studies scientifically. Only on this basis will it be possible to decipher the properties than make certain types of microplastic potentially dangerous for the environment and for human beings," says Ramsperger.
The study results from Prof. Laforsch's team agree with those of an earlier study by the Leibniz Institute IOW, in which microplastic of other polymer types was examined with regard to its behaviour in seawater. The authors, including Dr. Ingo Sartorius as co-author, therefore recommend first gaining an exact understanding of the microplastic particles under investigation, not only to make the studies comparable, but above all to obtain representative statements in the first place.
More information: to the study „Supposedly identical microplastic particles substantially differ in their material properties influencing particle-cell interactions and cellular responses“