Researchers are getting closer to making artificial nacre

[Cyril Roger Brossard]

Related to this thread.

An article of September 2012.

(Nanowerk Spotlight) The remarkable properties of some natural materials have motivated many researchers to synthesize biomimetic nanocomposites that attempt to reproduce Nature?s achievements and to understand the toughening and deformation mechanisms of natural nanocomposite materials. One of the best examples is nacre, the pearly internal layer of many mollusc shells. It has evolved through millions of years to a level of optimization currently achieved in very few engineered composites. Preparation of artificial analogs of nacre has been approached by using several different methods and the resulting materials capture some of the characteristics of the natural composite (read below: "Nanotechnology inspired by mussels and seashells").
Nacre has a layered structure composed of approximately 95% calcium carbonate (CaCO3) and nearly 5% organics. As depicted in the figure below, single-crystalline calcium carbonate nanotablets (CCNs) are interfaced by entrapped organics. Such a periodic 'bricks and mortar' arrangement is crucial to mechanical and other outstanding properties that nacre possesses. Over the past decades, researchers in this field have devoted significant effort to investigate and mimic such bricks and mortar structures hoping that the understanding on formation of nacreous structure and biological mineralization would lead to new advances in materials technology and related applications.

Formation process of calcium carbonate nanotablets (CCNs) through oriented attachment: small nanoparticles of CaCO3, larger nanoparticles of CaCO3, calcium carbonate nanotablets, and "bricks and mortar" structure of nacre. Background image shows an natural abalone shell. (Image: Dr. Hua Chun Zeng, National University of Singapore)
"CaCO3 has long been considered to be a genuine chemical component to approximate nacre," Hua Chun Zeng, a professor in the Department of Chemical and Biomolecular Engineering at National University of Singapore (NUS), tells Nanowerk. "However, only supported polycrystalline CaCO3 (calcite) films have been reported so far by the research community. With rapid advancement of nanotechnology, it would be highly desirable to synthesize freestanding CaCO3 tablet building blocks in large quantities, ideally identical or similar to those in natural nacre, using solution-based methods."
However, due to lack of methods to prepare CaCO3 into thin tablet form ? i.e., the 'bricks' in nacre's structure ? researchers have adopted various structural substitutes which include clay platelets such as montmorillonite, alumina platelets, graphene or graphene-oxide sheets, metal oxide nanosheets, and layered double hydroxides platelets.
Now, for the first time, a NUS team led by Zeng has devised a facile chemical method to synthesize single-crystalline CaCO3 nanotablets in large quantities and provided genuine primary building blocks for the fabrication of nacreous inorganic-organic hybrids.
Reporting their work in the September 14, 2012 online edition of Advanced Materials ("Calcium Carbonate Nanotablets: Bridging Artificial to Natural Nacre"), the researchers used an effective vacuum evaporation-induced self-assembly method to fabricate films and monoliths with tunable composition and mechanical properties.
"The easy availability of CCNs from our present work will allow the design and synthesis of artificial nacre-like materials more relevant to real nacre, and narrow down the huge compositional gap between them."
The team's synthesis of single-crystalline calcium carbonate nanotablets started with the preparation of CaCO3 nanoparticles with the assistance of two surfactants in a water-ethylene glycol cosolvent.
"We were able to obtain multigram-scale CaCO3 nanoparticles by this low-cost reaction route" says Zeng. "Surprisingly, when the surfactants were removed from the CaCO3 nanoparticles, CCNs could be formed instantaneously."
The pristine CaCO3 nanoparticles were only 1?3 nm in size and they were stable for months in mother liquors.
To prepare single-crystal CCNs into nacre-like structures, an organic matrix has to be incorporated. Here, the NUS researchers chose gelatine as a gluing matrix. Using these gelatine-modified CCNs, they further fabricated nacre-like CCN-gelatine inorganic-organic hybrids as freestanding films with 10 to 50 ?m thickness.
Testing the mechanical properties of their nacre films, the largest Young's modulus of 97 ? 4.6 MPa was obtained from a film with from the film with 33% CaCO3.
"This is quite comparable with the test data for hydrated nacre obtained by different research groups, which range from 80 MPa to 130 MPa," Zeng points out.
So it appears that with the 'bricks and mortar' made from the NUS team's CCNs, artificial nacre-like materials would be closer to natural nacre. Because CaCO3 is nontoxic and biocompatible, such a tablet form of CaCO3 can be used in many existing applications of this material, including calcium supplements, bone substitute, drug delivery vehicle etc. in addition to the nacreous structure which shows outstanding mechanical properties.
"Along with the development of biomimetic materials, fundamentally, we anticipate that "bricks and mortar" interaction can be investigated at molecular level with single-crystalline CCNs, because the cations and anions in these newly available building blocks are chemically identical to those in natural nacre," says Zeng. "In addition to assembling hybrid materials closer to natural nacre, the highly oriented CaCO3 single-crystals may also serve as a starting platform for the investigation of CaCO3 and protein interaction. Many fundamental issues in bioinspired materials can be addressed in the near future with this new form of CaCO3 single-crystals."
By Michael Berger. Copyright ? Nanowerk

Nanotechnology inspired by mussels and seashells
(Nanowerk Spotlight) Super-tough materials with exceptional mechanical properties are in critical need for applications under extreme conditions such as jet engines, power turbines, catalytic heat exchangers, military armors, aircrafts, and spacecrafts. Researchers involved in improving man-made composite materials are trying to understand how some of the amazing high-performance materials found in Nature can be copied or even improved upon. Nature has evolved complex bottom-up methods for fabricating ordered nanostructured materials that often have extraordinary mechanical strength and toughness. One of the best examples is nacre, the pearly internal layer of many mollusc shells. It has evolved through millions of years to a level of optimization currently achieved in very few engineered composites. In a novel approach, scientists have prepared a high-performing nanocomposite material that takes advantage of two different exceptional natural materials - layered nacre and the marine adhesive of mussels. The resulting nanostructured composite film exhibits high strength exceeding that of even nacre.
The remarkable properties of some natural materials have motivated many researchers to synthesize biomimetic nanocomposites that attempt to reproduce Nature?s achievements and to understand the toughening and deformation mechanisms of natural nanocomposite materials. Preparation of artificial analogs of nacre has been approached by using several different methods and the resulting materials capture some of the characteristics of the natural composite.

The iridescent nacre of a Nautilus shell cut in half. (Image: Wiki Commons)
"In our own work, we have used a layer-by- layer (LBL) assembly technique to prepare a nanostructured analogue of nacre from inorganic nanosized sheets of a particular clay and a polyelectrolyte (PDDA)" Nicholas Kotov tells Nanowerk. "The structure, deformation mechanism, and mechanical properties of this material were found to be comparable with those of natural nacre and lamellar bones."
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[Cyril Roger Brossard]

...Kotov, an Associate Professor in Chemical Engineering at the University of Michigan, explains that, contrary to other preparation techniques, the LBL method is relatively simple and highly versatile in merging different functionalities into a single composite. "At the same time" he says, "a vast array of available assembly components allows us to generate alternative designs as a means of understanding the different interactions necessary for preparation of nacre-like composites with application-tailored mechanical responses."
Trying to find ways of improving their composite material further, Kotov and his team turned to another exceptional biomaterial, the unusual protein adhesive secreted by mussels.
"Clay nanosheets possess exceptionally high mechanical properties, with Y (Young's modulus) calculated at ca. 250–260 GPa, which is two orders of magnitude greater than the mechanical properties of most clay nanocomposites achieved thus far" says Kotov (in comparison, the Young's modulus for wrought iron and steel is 190-210 GPa). "We have hypothesized that improving load transfer from the weak polymeric component to the inorganic nanosheets in our artificial nacre should increase the composite’s mechanical properties. This required a polymer that would have a potentially stronger interaction with the clay than the ionic bonds in our clay/PDDA composite."
Mussels secrete remarkable protein-based adhesive materials (mussel adhesive proteins – MAPs) for adherence to the substrates upon which they reside. The protein adhesives are secreted as fluids that undergo a hardening reaction leading to the formation of a solid adhesive plaque – think of cement – with which the mussel bonds to a variety of substrates such as minerals, metal surfaces, and wood. One of the unique structural features of MAP is the presence of DOPA, an amino acid that is believed to be responsible for both adhesive and crosslinking characteristics of MAPs.
Kotov collaborated with the research group of Philip B. Messersmith at Northwestern University who are actively developing synthetic polymers that mimic the composition and properties of adhesive proteins found in nature such as DOPA.
"The simultaneous strong binding, versatility and hardening capability of DOPA prompted us to exploit it for preparing artificial nanostructured nacre in the hope of enhancing the interfacial clay-polymer interaction and to increase mechanical properties of the composite" says Kotov.
In a recent research paper in Advanced Materials, titled "Fusion of Seashell Nacre and Marine Bioadhesive Analogs: High-Strength Nanocomposite by Layer-by-Layer Assembly of Clay and L-3,4-Dihydroxyphenylalanine Polymer", Kotov, Messersmith and collaborators demonstrate, for the first time, preparation of a nanostructured composite having nacre-like architecture, which takes advantage of DOPA adhesion and crosslinking strength.
"Just as in mussels, we found that DOPA molecules impart unusual adhesive strength to the clay composite and the hardening mechanism found in the natural 'cement' plays an equally important role in strengthening of our nanostructured nacre" says Kotov. " In comparison to our previous work with PDDA, we found that even a small amount of DOPA has a dramatic effect on the mechanical properties: the ultimate strength increased by two times and the toughness by approximately eight times."
Overall, this work is a first example of the fusion of two seemingly distinct concepts found in Nature into a unique composite with excellent mechanical properties. It shows that understanding the nanoscale mechanics of materials could open the way for materials engineers to make new materials, based on nanocomposite structures, with exceptional strength.
"However" cautions Kotov, "the greatest challenge is to transfer the unique mechanical properties of nanoscale components such as clay sheets used here for nacre replication into the properties of the actual macroscale materials."
By Michael Berger, Copyright Nanowerk LLC