Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

How seashells get their strength

11.01.2016

Study shows how calcium carbonate forms composites to make strong materials such as in shells and pearls

Seashells and lobster claws are hard to break, but chalk is soft enough to draw on sidewalks. Though all three are made of calcium carbonate crystals, the hard materials include clumps of soft biological matter that make them much stronger. A study today in Nature Communications reveals how soft clumps get into crystals and endow them with remarkable strength.


When calcium carbonate crystallizes into hard shells, it incorporates soft bits of proteins to add strength. Research done in the lab shows how this might happen, and why it works.

Credit: Wikipedia Public Domain

The results show that such clumps become incorporated via chemical interactions with atoms in the crystals, an unexpected mechanism based on previous understanding. By providing insight into the formation of natural minerals that are a composite of both soft and hard components, the work will help scientists develop new materials for a sustainable energy future, based on this principle.

"This work helps us to sort out how rather weak crystals can form composite materials with remarkable mechanical properties," said materials scientist Jim De Yoreo of the Department of Energy's Pacific Northwest National Laboratory.

"It also provides us with ideas for trapping carbon dioxide in useful materials to deal with the excess greenhouse gases we're putting in the atmosphere, or for incorporating light-responsive nanoparticles into highly ordered crystalline matrices for solar energy applications."

Beautiful and functional

Calcium carbonate is one of the most important materials on earth, crystallizing into chalk, shells, and rocks. Animals from mollusks to people use calcium carbonate to make biominerals such as pearls, seashells, exoskeletons, or the tiny organs in ears that maintain balance. These biominerals include proteins or other organic matter in the crystalline matrix to convert the weak calcium carbonate to hard, durable materials.

Scientists have been exploring how organisms produce these biominerals in the hopes of determining the basic geochemical principles of how they form, and also how to build synthetic materials with unique properties in any desired shape or size.

The strength of a material depends on how easy it is to disrupt its underlying crystal matrix. If a material is compressed, then it becomes harder to break the matrix apart. Proteins trapped in calcium carbonate crystals create a compressive force -- or strain -- within the crystal structure.

Unlike the strain that makes muscles sore, this compressive strain is helpful in materials, because it makes it harder to disrupt the underlying crystal structure, thereby adding strength. Scientists understand how forces, stress and strain combine to make strong materials, but they understand less about how to create the materials in the first place.

Pearls of wisdom

The leading explanation for how growing crystals incorporate proteins and other particles is by simple mechanics. Particles land on the flat surface of calcium carbonate as it is crystallizing, and units of calcium carbonate attach over and around the particles, trapping them.

"The standard view is that the crystal front moves too fast for the inclusions to move out of the way, like a wave washing over a rock," said De Yoreo.

That idea's drawback is that it lacks the details needed to explain where the strain within the material comes from. The new results from De Yoreo and colleagues do, however.

"We've found a completely different mechanism," he said.

To find out how calcium carbonate incorporates proteins or other strength-building components, the team turned to atomic force microscopy, also known as AFM, at the Molecular Foundry, a DOE Office of Science User Facility at Lawrence Berkeley National Laboratory. In AFM, the microscope tip delicately runs over the surface of a sample like a needle running over the grooves in a vinyl record. This creates a three-dimensional image of a specimen under the scope.

The team used a high concentration of calcium carbonate that naturally forms a crystalline mineral known as calcite. The calcite builds up in layers, creating uneven surfaces during growth, like steps and terraces on a mountainside. Or, imagine a staircase. A terrace is the flat landing at the bottom; the stair steps have vertical edges from which calcite grows out, eventually turning into terraces too.

For their inclusions, the team created spheres out of organic molecules and added them to the mix. These spheres called micelles are molecules that roll up like roly-poly bugs based on the chemistry along their bodies -- pointing outwards are the parts of their molecules that play well chemically with both the surrounding water and the calcite, while tucked inside are the parts that don't get along with the watery environment.

Better composites through chemistry

The first thing the team noticed under the microscope is that the micelles do not randomly land on the flat terraces. Instead they only stick to the edges of the steps.

"The step edge has chemistry that the terrace doesn't," said De Yoreo. "There are these extra dangling bonds that the micelles can interact with."

The edges hold onto the micelles as the calcium carbonate steps close around them, one after another. The team watched as the growing steps squeezed the micelles. As the step closed around the top of the micelle, first a cavity formed and then it disappeared altogether under the surface of the growing crystal.

To verify that the micelles were in fact buried within the crystals, the team dissolved the crystal and looked again. Like running a movie backwards, the team saw micelles appear as the layers of crystal disappeared.

Finally, the team recreated the process in a mathematical simulation. This showed them that the micelles -- or any spherical inclusions -- are compressed like springs as the steps close around them. These compressed springs then create strain in the crystal lattice between the micelles, leading to enhanced mechanical strength. This strain likely accounts for the added strength seen in seashells, pearls and similar biominerals.

"The steps capture the micelles for a chemical reason, not a mechanical one, and the resulting compression of the micelles by the steps then leads to forces that explain where the strength comes from," said De Yoreo.

###

This work was supported by the Department of Energy Office of Science, National Institutes of Health.

Reference: Kang Rae Cho, Yi-Yeoun Kim, Pengcheng Yang, Wei Cai, Haihua Pan, Alexander N. Kulak, Jolene L. Lau, Prashant Kulshreshtha, Steven P. Armes, Fiona C. Meldrum & James J. De Yoreo. Direct observation of mineral-organic composite formation reveals occlusion mechanism, Nature Communications January 6, 2016, doi:10.1038/NCOMMS10187.

Interdisciplinary teams at Pacific Northwest National Laboratory address many of America's most pressing issues in energy, the environment and national security through advances in basic and applied science. Founded in 1965, PNNL employs 4,400 staff and has an annual budget of nearly $1 billion. It is managed by Battelle for the U.S. Department of Energy's Office of Science. As the single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time. For more information on PNNL, visit the PNNL News Center, or follow PNNL on Facebook, Google+, LinkedIn and Twitter.

Media Contact

Mary Beckman
mary.beckman@pnnl.gov
509-375-3688

 @PNNLab

http://www.pnnl.gov/news 

Mary Beckman | EurekAlert!

Further reports about: PNNL biominerals calcite calcium carbonate crystalline crystals micelles

More articles from Materials Sciences:

nachricht Interfacial Superconductivity: Magnetic and superconducting order revealed simultaneously
17.01.2017 | Sonderforschungsbereich 668

nachricht Manchester scientists tie the tightest knot ever achieved
13.01.2017 | University of Manchester

All articles from Materials Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Interfacial Superconductivity: Magnetic and superconducting order revealed simultaneously

Researchers from the University of Hamburg in Germany, in collaboration with colleagues from the University of Aarhus in Denmark, have synthesized a new superconducting material by growing a few layers of an antiferromagnetic transition-metal chalcogenide on a bismuth-based topological insulator, both being non-superconducting materials.

While superconductivity and magnetism are generally believed to be mutually exclusive, surprisingly, in this new material, superconducting correlations...

Im Focus: Studying fundamental particles in materials

Laser-driving of semimetals allows creating novel quasiparticle states within condensed matter systems and switching between different states on ultrafast time scales

Studying properties of fundamental particles in condensed matter systems is a promising approach to quantum field theory. Quasiparticles offer the opportunity...

Im Focus: Designing Architecture with Solar Building Envelopes

Among the general public, solar thermal energy is currently associated with dark blue, rectangular collectors on building roofs. Technologies are needed for aesthetically high quality architecture which offer the architect more room for manoeuvre when it comes to low- and plus-energy buildings. With the “ArKol” project, researchers at Fraunhofer ISE together with partners are currently developing two façade collectors for solar thermal energy generation, which permit a high degree of design flexibility: a strip collector for opaque façade sections and a solar thermal blind for transparent sections. The current state of the two developments will be presented at the BAU 2017 trade fair.

As part of the “ArKol – development of architecturally highly integrated façade collectors with heat pipes” project, Fraunhofer ISE together with its partners...

Im Focus: How to inflate a hardened concrete shell with a weight of 80 t

At TU Wien, an alternative for resource intensive formwork for the construction of concrete domes was developed. It is now used in a test dome for the Austrian Federal Railways Infrastructure (ÖBB Infrastruktur).

Concrete shells are efficient structures, but not very resource efficient. The formwork for the construction of concrete domes alone requires a high amount of...

Im Focus: Bacterial Pac Man molecule snaps at sugar

Many pathogens use certain sugar compounds from their host to help conceal themselves against the immune system. Scientists at the University of Bonn have now, in cooperation with researchers at the University of York in the United Kingdom, analyzed the dynamics of a bacterial molecule that is involved in this process. They demonstrate that the protein grabs onto the sugar molecule with a Pac Man-like chewing motion and holds it until it can be used. Their results could help design therapeutics that could make the protein poorer at grabbing and holding and hence compromise the pathogen in the host. The study has now been published in “Biophysical Journal”.

The cells of the mouth, nose and intestinal mucosa produce large quantities of a chemical called sialic acid. Many bacteria possess a special transport system...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

12V, 48V, high-voltage – trends in E/E automotive architecture

10.01.2017 | Event News

2nd Conference on Non-Textual Information on 10 and 11 May 2017 in Hannover

09.01.2017 | Event News

Nothing will happen without batteries making it happen!

05.01.2017 | Event News

 
Latest News

Water - as the underlying driver of the Earth’s carbon cycle

17.01.2017 | Earth Sciences

Interfacial Superconductivity: Magnetic and superconducting order revealed simultaneously

17.01.2017 | Materials Sciences

Smart homes will “LISTEN” to your voice

17.01.2017 | Architecture and Construction

VideoLinks
B2B-VideoLinks
More VideoLinks >>>