Research led by Dr. Fabio Nudelman from the School of Chemistry, University of Edinburgh, has shown a new behaviour of a sea shell of the species Discinisca tenuis. This shell is very hard and stiff when dry, but when it absorbs water it becomes very flexible and can be folded in two without breaking. This work was a collaboration between scientists in UK, Switzerland, Germany, Ireland, Italy and US.
Using a range of characterization techniques including from X-rays, electron microscopy and spectroscopy, it was found that the shells absorb significant amounts of water, which then changes their structure by rearranging the individual mineral crystals and acting as a lubricant. Together, it allows the building blocks of the shell to move more freely when a force is applied. This kind of mechanical adaptation depending on the environment is unheard of for this type of hard materials found in nature. Bone, for example, while it has similar mineral composition, can only absorb small amounts of water and is structured in a way that its building blocks cannot move much when under a force – hence cannot bend as freely as the shells.
As of today, material scientists have a hard time mimicking this type of behaviour in synthetic materials. Learning how the shell is built, and how its structure responds to the uptake of water, will allow us to design and synthesize new materials that adapt to different conditions or react to stimuli. This kind of material could be very useful, for example as bone replacement where it could be shaped to fill a fracture or a defect, and then be strong enough to restore the mechanical properties of the damaged bone.
We discovered this by chance, when I asked a student to wash the shell to clean it. The shell had absorbed water and became very soft and flexible. We now found out how water changes the structure of the shell to make this happen. This kind of behaviour in biological hard materials is very unusual. Bone, for example, has similar composition to the shell but does not change its structure when it gets wet. We can use this knowledge to develop new materials that can be made stiff or flexible in response to certain conditions or stimuli, and used for different applications.