Nanostructure will become design variable for materials engineers

(Nanowerk Spotlight) Bone is one of the most fascinating materials that has evolved in nature. There are 206 bones in your body - did you know that a newborn has 350 bones but they fuse together as you grow? - more than half of them in your hands and feet. These bones not only protect your organs, support your body against gravity's pull and allow you to move but they also are living tissues that produce blood cells and store important minerals. Not only important for humans, bones are the essential part of the endoskeleton of all vertebrates. Bone is a composite material of the mineral calcium hydroxyapatite and tropocollagen molecules (the fragile and soluble form of collagen when first synthesized by fibroblasts). It forms an extremely tough, yet lightweight material and its properties and behavior are of great interest to scientists and materials engineers. For instance, very little is known about the fracture behavior of bone and all such studies have been conducted at scales much larger than the nanoscale that explicitly considers individual tropocollagen molecules and mineral particles. New research at MIT has discovered a previously unknown toughening mechanism of bone that operates at the nanoscale - the level of individual collagen molecules and nano-platelets of hydroxyapatite. This breakthrough work lays the foundation for new materials design that includes the nanostructure as a specific 'design variable' and may help engineers to design materials from the bottom up by including hierarchies as a design parameter.
"We have discovered the role of the characteristic collagen/mineral nanocomposite structure in making bone tough, that is, increase bone's resistance to fracture" Prof. Markus J. Buehler explains to Nanowerk. "All bone starts to develop as a pure collagen phase. Addition of a second, hydroxyapatite mineral phase creates the characteristic material properties of bone, that is, it is strong, stiff but yet very tough (highly resistant against failure). Notably, all bone features a very similar characteristic nanostructure. This nanostructure has thus far been unexplained. We have shown that this feature is vital for several physiologically important properties of bone."
In particular, Buehler, who is an Assistant Professor at MIT's Department of Civil and Environmental Engineering, and his group have discovered that the nanostructure is indeed crucial for bone's properties. Their atomistic scale analysis further elucidates the actual mechanisms of how this happens. The study reveals how a highly dissipative, yet strong material can be formed out of a soft polymeric collagen phase and hard, brittle hydroxyapatite by arranging molecules and crystals at characteristic nanostructured length scales.
The nanostructure of bone
Geometry of the nanostructure of bone, showing several hierarchical features from atomic scale to microscale. A simple schematic diagram is given of the hierarchical structure of mineralized collagen fibrils, forming the most basic building block of bone. Three polypeptide strands arrange to form a triple helical tropocollagen molecule. Tropocollagen (TC) molecules assemble into collagen fibrils in a hydrated environment, which mineralize by formation of hydroxyapatite (HA) crystals in the gap regions that exist due to the staggered geometry. Mineralized collagen fibrils combine with the extrafibrillar matrix to fibril arrays, which form fibril array patters. Typically, a total of seven hierarchical levels are found in bone. Buehler's work is limited to the scale of mineralized fibrils, with the objective of providing insight into the most fundamental scales of bone and its deformation mechanics under tensile loading. (Reprinted with permission from IOP Publishing)
In his research ("Molecular nanomechanics of nascent bone: fibrillar toughening by mineralization"), Buehler utilizes a simple molecular model of mineralized collagen fibrils that provides a fundamental description of its nanomechanical properties. He systematically compares the small and large deformation mechanics of a pure collagen fibril and a mineralized collagen fibril. By comparing the deformation mechanisms, stress-strain behavior and energy dissipation, Buehler found that mineralization leads to an increase in stiffness, yield stress, fracture stress and energy dissipation.
Buehler points out that previously known toughening mechanisms have all been observed at larger length-scales. In contrast, his analysis represents a major breakthrough since it explains that such toughening occurs at various hierarchical scales, including the nanoscale.
"Our finding has major implications on the mechanical integrity of bone" says Buehler: "It is known that bone needs to be constantly remodeled in biological organisms. This remodeling happens through so-called Bone Molecular Units (BMUs) – which are tiny crack-like defects (or elongated elliptical inclusions) that include bone cells that on the one end dissolve bone and on the other end create new bone. Previously it has been unclear how these BMUs can exist in bone, since these BMUs are actually small cracks in bone! How can one survive with a large number of small cracks in bone? Think of having a crack in glass – the smallest load will surely break glass since the crack starts to propagate. Cracks are fatal in most engineering materials, but they form a part of materials in biology. How this can be the case has been very puzzling for decades."
The effect of the characteristic nanostructure of bone on the fracture behavior
Illustration of the effect of the characteristic nanostructure of bone on the fracture behavior. A crack in a mineralized collagen fibril does not extend; instead, the material forms local "yield regions" (top). This represents a strategy to allow local failure in order to save the entire structure. In contrast, a pure hydroxyapatite crystal fails rapidly due to extension of the crack (bottom). (Image: M.J. Buehler, MIT)
The novel insight that the MIT researchers gained is that the characteristic nanostructure of bone is crucial in making this possible, since it allows the material to actually tolerate cracks of several hundred micrometer size, without causing any fracture.
Buehler explains the atomistic mechanism: "The intrinsic nanostructure of bone shields any kind of major damage due to large stresses. Bone's strategy thereby is quite perplexing: In case of emergent damage, bone fails locally, that is, a local piece of the material is sacrificed in order to protect the larger structure. In other words, a small failure is tolerated for the 'greater good'."
There are several very important scientific and medical problems that can now be better understood.
Most importantly, a consequence of this discovery is that bone can actually sustain cracks inside the material without breaking (see the discussion of BMUs above).
Secondly, Buehler's study suggests that bone toughening occurs at multiple scales. "From what we understand today, each scale of bone has its own toughening mechanism" he says. "This hierarchical distribution of toughening may be critical to explain the intriguing properties of bone. We now believe that this is also due to the universally observed tendency of biological systems to increase robustness — in case one of the toughening mechanisms fails, there are still others available. Our results strongly suggest that all hierarchical scales in bone are crucial."
Intriguingly, this work at MIT lays the foundation for new materials design that includes the nanostructure as a specific "design variable".
According to Buehler, the results clearly show that interesting properties can be generated due to the hierarchical design of the material. In fact, this appears that nature uses this to overcome certain physical limitations: "operating at multiple, hierarchical scales enables to reach new physical realities that would not be accessible to single scale design" he says. "This is potentially the most important contribution to new technology development. Our insight may help engineers design materials from the bottom up, through the inclusion of hierarchies as a design parameter. This is a very new approach in making materials."
Another aspect of this work with potential consequences for materials design is robustness. Today, engineers typically over-dimension structures in order to make them robust against failure, that is, they make walls more massive, trusses stronger and airplane tubes thicker. This often leads to significant increases in weight of the structures. "In the case of airplanes or cars, such weight increase leads to significant energy waste, contributing eventually to emission of CO2 with negative environmental impact," says Buehler. Nature creates robustness by hierarchical structures. This is much more efficient since it saves resources but actually creates structures and materials that are much more robust.
Many materials and structures engineered by humans bear a conflict between strength and robustness; strong materials are often fragile, while robust materials are soft. Fragility occurs due to the high sensitivity to material instabilities such as formation of fractures. Consequently, only high safety factors and thus bigger amount of resources can guarantee the strength of engineered materials, if extreme conditions are expected. It is very difficult to combine strength and robustness at a single scale; instead, structures with multiple scales must be introduced, where universal and divers patterns are unified hierarchically.
"This may be an important principle that may apply to most biological materials," says Buehler. "In biology, universality generates robustness, while diversity enables optimality. Materials like bone, being a nano-composite of strong but brittle and soft but ductile materials, illustrate this unification of components with disparate properties within a hierarchical structure."
This finding is important because it could explain why so many protein motifs and nanostructures are universally found in nature: At nanoscale, robustness is linked to universality, and optimality is linked to diversity.
Materials engineers are trying to learn from biological structures how one can achieve highly robust, yet highly adapted materials, without wasting resources. Nature is very frugal, and resources are not wasted. This represents a different paradigm from the one used in engineering right now.
Buehler and his group are very heavily involved in developing theories that capture the properties of hierarchical systems, not only for bone, but for other protein materials: "Protein materials are a new frontier of materials science, whose understanding will not only impact medical and biological sciences, but also technology development."
One important problem is the role of hydrogen bonds in biology – hydrogen bonds are very weak chemical bonds, yet they can create very strong materials (some of which are stronger than steel). "We are close to a major breakthrough in understanding how systems of hydrogen bonds behave, and how it is possible to make spider silk so strong, and how cells retain their structure" says Buehler. "Again, similar as in bone – the nanoscale and hierarchical structures are crucial for strength and robustness."
This research has been supported by the Army Research Office and the National Science Foundation.
By Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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