Sea Urchin Spines Inspire Next Generation Materials Design
- Fig. 1: Radial symmetry of the sea urchin body plan. Sea urchins thrive in typical intertidal environments. © Fons Laure - Fotolia.com
- Fig. 2: Sea urchin spine (a) typically located in the periphery of the test. (b) By pulling off the spine from the pedicle, individual spines can be isolated. (c) Schematic of the typical microstructure of a cross-section through a sea urchin spine.
Sea urchin spines are hard and tough. And would it be surprising if you were told that they are basically made of the same components as chalk? This is an example where nature has been able to "tweak" the orientation at the micro- and nano- structure and create a hierarchy of organization that can be observed at the millimeter length to all the way down to the nanometer length-scales.
Indeed, nature takes on a form of advanced building in materials often used in high-technology companies that make computer chips: the "bottom-up" approach enables nature to construct materials with defined properties and create these materials consistently. Just take a look around yourself, you'll find examples of this building scheme in many biological materials.
Anatomy of the Sea Urchin
Figure 1 shows the sea urchin with its calcareous test. With the radial symmetry, the tissues must be formed in such a way that the sea urchin is able to conduct its "business" in all directions - forward, backward, left, and right. This includes being able to sense, feed, and mate whether it be stuck onto a vertical wall of rock, on the ocean floor, or upside-down.
As another property, it must also be able to defend itself in all directions. The eponymous sea urchin is often defined by the spines that grow and surround the shell. The sea urchin spines are made from the same calcium carbonate material, however, it is structured in a way such that the spines are sharp - so sharp such that any predator will learn a lesson or two before preying on the next sea urchin.
The spine is constructed in such a way that it is able to distribute mechanical load from the tip down to the base; dissipating energy along the way to prevent catastrophic fracture to the main body part, the test.
By examining the sea urchin spine with more powerful methods of microscopy, ever smaller structures in the spine material can be observed, even at length-scales one billionth the size of a single strand of your hair.
You may wonder why these ever smaller structures are important in giving sea urchin spines their strength, toughness, and shapes.
In the same way some people who check the engines of their cars to see how fast the car can go, the ever smaller structures in a material define the properties of the entire material. Any "mis-orientation" of a single element "under the hood" may indeed bring the house of cards tumbling down. In this case, it would mean catastrophic mechanical fracture when loaded in the improper way. Using high resolution transmission electron microscopy techniques, the alignment and orientation of single nanocrystals can be resolved in the spine.
Proteins Make the Difference
The importance of structural orientation can be observed when you bend chalk: it snaps easily. Try the same loading with a spine and you'll find it has some "give" and is able to resist the "snap". Although it is roughly 99.9 % similar in composition as in chalk, the 0.1 % of proteins found in the spine enables it to avoid being snapped like chalk.
These proteins perform jobs as getting the small crystals that compose chalk oriented as well as retaining water at the surfaces of these crystals and keeping an amorphous mineral phase stable. Some scientists even hypothesize that the proteins and this amorphous phase themselves act like a "car bumper" and absorb some of the energy that causes fracture, aiding in keeping the spine intact as well as tough and fracture-resistant.
Calcium Blocks Form Mesocrystals
How these proteins interact with the growing spine and what parts of these proteins specifically keep the nanocrystals aligned and organized - these are the questions materials scientists are actively investigating to understand the source of the materials properties found in a sea urchin spine.
Recent work by researchers at the University of Konstanz have found that some of these proteins are involved in the formation of the space where the nanocrystalline units and their immediate neighboring crystalline units grow to make the spine. Instead of moving millions of small nano-sized crystals around such that the units are aligned, Mother nature has found proteins that will orient crystals by having the crystals themselves grow into place aligned and oriented. Think tiny Lego blocks which "click" together as the individual bricks form to create an array of aligned and oriented crystals that span many millimeters long. Imagine repeating this same "trick" over and over until a large structure emerges like a sea urchin spine. And the structures of these aligned "nanoblocks" form what scientists call a "meso-crystal".
The arrays of the "nano-blocks" in the sea urchin spine are organized along a predominant direction. Ordered in such a way the crystallites are aligned along the growing spine. In addition, the nanocrystallites are also coated on their surfaces with a cement-like layer, acting as an interface between the neighboring crystallites. This layer plays two roles in the structure and function of the spine: (1) mechanically blunting the tip of cracks as fracture occurs and dissipating the fracture energy to prevent complete fracture (2) a repository of amorphous mineral phase that can be shifted to fracture sites to heal the fractured mineral surface.
The organization of all these components together enable for the sea urchin spine to perform and function in the manner it is required to keep predators at bay as well as deal with the normal day-to-day grind.
What we can take away from these strategies in the "building" of a sea urchin spine from nature are ways to make man-made materials with high toughness, high strength and self-regenerating properties. These same strategies can be utilized to make materials that show better absorption of shock in a car bumper or cement that can be tuned to be more resistant to earthquakes - imagine, a concrete that can withstand an 8.0 earthquake In all these instances, inspiration from biological materials can have an impact on the materials that we use to keep us safe, protected, and long-lasting. We should take a look around ourselves and really examine some of Nature's materials more frequently. Maybe we can learn a lesson or two about constructing stronger concrete from Mother Nature herself.