Anyone who has taken a mobile phone to the beach will tell you a universal truth, phone screens scratch very easily. As careful as we might be, inevitably, a single grain of sand will find its way to the delicate screen and leave a permanent reminder of your holiday. Unfortunately, scratching and abrasion is very common on transparent materials and every scratch decreases the transparency. The effects of abrasion on high-tech instruments deployed in harsh environments such as deserts or even on other planets has been a problem that has puzzled scientists for a long time. Recently, researchers at Diamond Light Source looked to the oceans for answers.
Specifically, the research team were interested in seed shrimps, otherwise known as ostracods. These microscopic plankton are abundant in shallow oceanic waters and some have a transparent outer shell known as a carapace which helps them evade predators. Scratches and abrasions to the carapace make it impossible to function normally in their natural habitat. What is interesting however, is that these small creatures come into contact with abrasive material such as sand on a regular basis, and still their carapace remains transparent. It possesses properties to resist abrasion, but the ostracods can also repair themselves when they become damaged. The team's findings were recently published in Marine Biology.
Clearly, technology has a lot to learn from nature and the research team used tools available at Diamond to try and understand how the ostracods were so resistant to abrasion, despite living in harsh environments. They used a variety of techniques to understand the chemical and physical properties of the carapace of ostracods. They performed X‑ray absorption near‑edge structure (XANES), X-ray fluorescence (XRF) and X-ray diffraction (XRD) at Diamond’s Microfocus beamline, I18.
Using XANES and XRD, the team looked for different forms of calcium carbonate in their samples. They knew that calcium carbonate would be highly abundant, but different forms can give different properties to materials. They wanted to understand if the strong and transparent ostracods had a unique calcium carbonate composition.
They discovered that in later stages of development, the ostracods in their samples incorporated a calcium carbonate form called aragonite which produces needle-like crystals. Depending on how they are oriented, these crystals can determine where cracks will propagate. For example, if they are all lined up, cracks will form along the line of the crystals. However, if the needle crystals run perpendicular to one another, then cracks will be stopped. This interesting feature can allow the ostracods to construct their carapace to direct cracks away from areas that are more important.
However, during the experiments, there was an unexpected series of results that led their research team to doubt their data collection, as Cardiff University's Dr Siân Morgan who co-authored the study explained:
As we began our XANES and XRD analysis we found no calcite throughout the entire carapace at any development stage, which is rare in crustaceans. At first, we were worried it may have been an issue with the detection! But as we gained more and more results, we became more and more confident and excited in our findings.
Anything unusual about the composition of the ostracod’s composition could help us understand how the carapace is able to be simultaneously transparent and scratch resistant. This will be the focus of intensive studies moving forward.
Using XRF, the research team also wanted to build up a picture of how abundant different elements were in the carapace, however there was a challenge. Biological samples such as ostracods contain lighter elements that give off low energy secondary fluorescence photons which can be attenuated in air, which is made up of mostly nitrogen and oxygen, before reaching the XRF detector. To counteract this, the team enclosed their samples in a helium-filled bag. As helium is a very light element, more of the signal from the ostracods was then able to reach the detector.
Using this method, they were able to see expression of magnesium, sulphur, phosphorus, chlorine and calcium. Chlorine was always the most abundant, no matter what developmental stage the ostracods were in. Interestingly however, there were significant changes in the amount of magnesium, phosphorus and sulphur when comparing the youngest ostracods to the adults. As adults have a more abrasion resistant coating, these elemental changes will likely impact the mechanical (strength, scratch resistance) and physical properties (transparency) of the carapace.
These data collected at Diamond are helping to build up a picture of what makes ostracods strong and transparent. Now, we can look at transparent materials and see how they will respond if we start to make them more like the ostracod carapace.
However, there is still much to learn from the oceans. The research team plans to be back at Diamond before too long.
We plan to apply for further beamtime to complete a full set of data for all the developmental stages that we could not finish due to time constraints. We would also aim to test modified BMMA [a plastic resin] embedded sections by creating thicker sections to prevent the tearing seen previously and isolated cell samples with improved washing techniques to remove the possibility of contamination by the buffer.
Dr Benjamin Rumney, first author of the study
The team behind this study included researchers from Cardiff University, Swansea University, DSTL Porton Down, University of Oxford and Diamond Light Source.
Rumney B. M. et al. Characterisation of carapace composition in developing and adult ostracods (Skogsbergia lerneri) and its potential for biomaterials. Marine Biology 169, 78 (2022). DOI: 10.1007/s00227-022-04047-6
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