A researcher from Queen Mary University of London (QMUL) was part of a team, led by the Department of Materials Science and Engineering at the Technion-Israel Institute of Technology, that has figured out the unique protective mechanism of highly resistant lenses located on the arms of a species of brittle star called Ophiocoma wendtii.
The team’s findings, published in Science, might one day open new bio-inspired routes for toughening brittle ceramics in various applications.
As part of the study Professor Nicola Pugno, from QMUL’s School of Engineering and Materials Science and the University of Trento, led the coupled analytical and experimental mechanical analysis.
Professor Pugno said: "One of the main findings of this paper is the observation of the toughening mechanisms of this natural material; in order to rationalize them we have developed the simplest fracture mechanics model able to describe the role of both the pre-stress and of the multilayered microstructure and ad hoc microindentation experiments realised at different temperatures in order to discriminate between and quantify the two different mechanisms. These discovered strategies are of interest per se but also for designing new advanced bio-inspired tough composites."
Lead researcher Professor Boaz Pokroy researches materials created by living organisms, and produces analogous synthetic materials using methods inspired by nature. Mollusk shells, pearls, bones, and turtle shells are some examples of biominerals – minerals produced by the creature itself. For the past three years, his research has focused the deciphering of the crystalline and nanoscale structure of the brittle star’s lenses, of which there are hundreds.
“What is impressive about biominerals is that they are made from materials available to the organism, for example, chalk,” says Prof. Pokroy. “Engineers would never choose chalk as a durable building material, but nature does, and successfully, using different toughening and strengthening strategies. Moreover, the creatures produce these biominerals under available conditions, without furnaces and pressure-generating devices available in research laboratories. It is therefore clear that, as scientists and engineers, there is much for us to learn from these processes.”
This team’s findings focus on the structure of the lens system on the arms of the Ophiocoma wendtii, an echinoderm that lives in coral reefs and feeds on organic matter it collects using its five flexible arms. In a 2001 article in the Nature, a team led by Prof. Joanna Aizenberg (who later advised Prof. Pokroy during his postdoctoral research at Harvard University), first reported on the creature’s elaborate optical system made up of microscopic lenses made of a material called calcite. These lenses focus sunlight and project it on nerve centers that transfer the information to the rest of the body through the nervous system. (Note: This brittle star does not have a brain, so this process does not include image processing akin to what we are familiar with from the human visual system.)
“Ophiocoma wendtii lenses are created in the open sea, not in a laboratory, and in effect we have discovered a strategy for making brittle material much more tough and durable under natural conditions,” said Prof. Pokroy. “It is ‘crystal engineering’ and tempering without heating and quenching – a process that could be very useful in materials science and engineering.”
For the sake of comparison, tempered glass and prestressed concrete are produced by exerting pressure that compresses the material and leaves it more compact than its natural state. Glass tempering, for example, is performed by rapidly heating and then cooling the material. In this process, the outside of the material cools more quickly than the inside and thereby compresses the inside. Prestressing concrete is a similar process, with the insertion of steel rods into the liquid concrete at the beginning of the process. Before the concrete hardens the rods are stretched, and after solidification the tension is released and the rods apply pressure on the concrete.
The Technion researchers discovered that the crucial stage in the process of lens formation is the transition from the amorphous phase – the phase between liquid and solid – to the crystalline phase. At this stage calcite nanoparticles (which are rich in magnesium and characterized by a relatively low density) separate from the rest of the material. The difference in concentration of magnesium in the calcite particles causes various degrees of hardness, density, and pressure in different regions of the material. Magnesium-rich particles press on the inner part of the lens and as it crystallizes, and “temper” it into a clear and tough crystalline material.
“Just as nature exhibits creativity in improving an organism’s abilities in various contexts such as strength, sensing, and self-defense, we see very high efficiency by the brittle star’s use of existing raw materials under natural conditions to create hardy and precise transparent lenses,” says Prof. Pokroy “We as scientists and engineers can utilize this newly discovered biostrategy in toughening and strengthening synthetic ceramic materials utilized in various applications that span from optical lenses to automotive turbochargers and even biomaterial implants.”
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