Materiomics: Materials Science of Biological Protein Materials - From Nano to Macro

Biology exquisitely creates hierarchical structures, where initiated at nano scales, are exhibited in macro or physiological multifunctional materials to provide structural support, force generation, catalytic properties or energy conversion¹ (see Figure 1). This is exemplified in a wide range of biological materials such as hair, skin, bone, spider silk or cells, which play important roles in providing key functions to biological systems².

Our research focuses on studying the mechanisms of deformation and failure of biological materials by utilizing a computational materials science approach. Our goal is to elucidate nature's design principles that facilitates the formation of materials with exceptional material properties^1,3, despite typically inferior building blocks^4,5.

Figure 1. Three example biological protein materials (A, intermediate filaments, B, collagenous tissues, C, amyloid proteins), revealing their hierarchical makeup. Our research focuses on the development of multiscale material models, specifically focused on their mechanical behavior at deformation and failure (figure taken from Ref. #3).

These efforts are part of a broader field of investigation referred to as materiomics. Materiomics is defined as the study of the material properties of natural and synthetic materials by examining fundamental links between processes, structures and properties at multiple scales, from nano to macro, by using systematic experimental, theoretical or computational methods^6,7.

In order to provide a bottom-up description of materials behavior, our interdisciplinary team of undergraduate students, graduate students and postdocs applies an experimentally validated atomistic based multi-scale simulation approach that considers the structure-process-property paradigm of materials science and the architecture of proteins from the atomistic level up to the overall structure. This novel viewpoint links chemistry and genetics to resulting functional material properties, and provides us with a powerful foundation to ask fundamental questions about the behavior of biological materials.

Moreover, the development of multi-scale, hierarchical models to explain the deformation and failure mechanisms of biological materials provides critical insight into how biological materials are made and how they derive their unique properties. This learning-from-failure-approach has already proven to be useful for engineered materials, and its application to biological materials holds the promise to transform our understanding of key material design principles that have evolved in nature⁸.

In the following sections we review two case studies that demonstrate the application of materiomics.

Hierarchical Structures are Crucial to Provide Multiple, Disparate Properties in a Single Material

Our work has contributed to explain some of the most remarkable properties of biological materials, specifically their ability to provide multiple functions despite severe limitations in the quality of available building blocks, and the ability to adapt to different environmental conditions (for a review, see Ref. #3).

For example, strength, robustness and adaptability are properties of fundamental importance to biological materials and structures, and are crucial to providing functional properties to living systems. Strength is defined as the maximum force (or pressure) a material can withstand before breaking. Robustness is defined as the ability of a material to tolerate flaws and defects in its structural makeup while maintaining its ability to provide functionality.

Adaptability refers to the ability of a material to cope with changing environmental conditions. These properties are crucial for materials in biology (such as skin, bone, spider silk, or cells), which either provide structural support themselves (such as the skeleton formed by bone), or need to withstand mechanical deformation under normal physiologic conditions (such as cells and tissue associated with blood vessels that are exposed to the pressure of the blood).

In engineering, strength and robustness are disparate properties (see Figure 2), and it remains challenging to create materials that combine these two features. Glass or ceramics, for example, are typically very strong materials. However, they are not very robust: Even a small crack in a glass, or an attempt to deform glass considerably will lead to catastrophic failure.

Figure 2. Schematic of the robustness-strength domain, comparing engineered materials and biological materials. The squares represent different hierarchical structures, designed based on more than 16,000 alpha-helical protein filaments. The analysis shows that most random arrangements (98.13%) fall on the so-called banana curve, while fewer, specifically designed structures (1.98%) fall on the inverse banana curve (figure taken from Ref. #3).

In contrast, metals such as copper are very robust; however, they do typically not resist large forces. Yet, these materials allow for large deformation, and even the existence of cracks in the material does not lead to a sudden breakdown. Yet, many biological materials (such as cellular protein filaments, blood vessels, collagenous tissues such as tendon, spider silk, bone, tendon, skin) are capable of providing both properties - strength and robustness, very effectively, and are also combined with the ability to adapt to changes in the environment^1,3.

The key to understand these remarkable properties is the particular structural makeup of biological materials, consisting of few distinct elements (such as alpha-helices or beta-sheet protein domains), but a great diversity in structural arrangement of these elements at multiple material levels, from nano to macro^1,3. Most fibers, tissues, organs and organisms found in nature show a highly hierarchical and organized structure, where features are found at all scales, ranging from protein molecules (≈50 Å), protein assemblies (≈1 to 10 nm), fibrils and fibers (≈10 to 100 µm), to cells (≈50 µm), and to tissues and organs (≈1000s and more µm).

Most early studies have focused on investigations at single scales, or treated tissues or the cellular microenvironment as a continuum medium without heterogeneous structures (e.g. studies that examine isolated effects of material stiffness or the role of chemical cues alone on cell behavior). However, the cause and effect of biological material mechanics is more complex than a singular input at a specific scale, and thus, the examination of how a range of material scales and hierarchies contribute to certain biological function and dysfunction has emerged as a critical aspect in advancing our understanding of the role of materials in biology in both a physiological and pathological context. Specifically, the origin of how naturally occurring biological protein materials are capable of unifying disparate mechanical properties such as strength, robustness and adaptability is of significant interest for both biological and engineering science, and has attracted significant attention.

Through the development of multiscale computational models that provide a representation of multiple material hierarchy levels in a single model, we have elucidated the key role that multiscale mechanics plays in defining a material's ultimate response at failure, and how nature's structural design principles define the hierarchical makeup of biological materials^9,10. This process, likely evolutionarily driven, enables biological materials to combine disparate properties such as strength, robustness and adaptability, and may explain the existence of universal structural features observed in a variety of biological materials, across species (Figure 2)¹¹.

Osteogenesis Imperfecta, Brittle Bone Disease

Materiomics can also be applied to study the catastrophic breakdown of biological materials under disease conditions. Osteogenesis imperfecta is a genetic disorder in collagen characterized by mechanically weakened tendon, fragile bones, skeletal deformities and in severe cases prenatal death¹². Our work has shown that osteogenesis imperfecta mutations severely compromise the mechanical properties of collagenous tissues at multiple scales, from single molecules to collagen fibrils^13,14.

Mutations that lead to the most severe osteogenesis imperfecta phenotype correlate with the strongest effects, leading to weakened intermolecular adhesion, increased intermolecular spacing, reduced stiffness, as well as a reduced failure strength of collagen fibrils that compromises the ability of collagen to provide strength and toughness to connective tissue. Our work has shown that the reason for this is a change in the stress distribution inside collagen fibrils, and is due to the formation of nano-cracks where stress concentrations at the corners develop (Figure 3).

Figure 3. Effect of osteogenesis imperfecta on the stress distribution inside collagen fibrils (left), and resulting change in the stress-strain response (right)14. The formation of nanocracks at the location of mutations result in local stress concentrations (marked with red color), which reduces the overall strength of the fibril as it induces intermolecular shear at moderate applied loads.

Our findings provide insight into the multi-scale mechanisms of this disease, and lead to explanations of characteristic osteogenesis imperfecta tissue features such as reduced mechanical strength, lower cross-link density and changes in the way mineral platelets are distributed. Our results for the first time explain how single point mutations at the nanoscale can lead to catastrophic tissue failure at much larger length-scales.

The key to understand this dramatic change in material behavior is that failure must be understood as a multi-scale phenomenon, where the interplay of mechanisms at multiple scales defines the ultimate material response. Conventional models of failure and disease that consider only one level of the material's structure, do not capture the full range of relevant structures and mechanisms and as such remain limited in their ability to describe their behavior of intervene in material breakdown processes associated with disease. The understanding of failure in the context of defects and mutations could fundamentally change the way diseases are modeled and potentially treated.

Outlook to Future Research

Materiomics is a powerful tool to enhance the understanding of materials in biology, at multiple scales and in a variety of functional contexts. The long-term goal of our research is to develop a new engineering paradigm that encompasses the analysis and design of structures and materials, starting from the molecular level, in order to create new materials that mimic and exceed the properties of biological ones by utilizing material concepts discovered in biological materials.

We envision that our work can lead to the development of a new set of tools that can be applied, together with synthetic biological and self-assembly methods, to select, design, and produce a new class of materials, similar to the approaches used today in computer aided design of buildings, cars and machines. The availability of multifunctional and changeable materials reduces the necessity for the use of different materials to achieve different properties, and as such, may provide significant savings in weight and cost. The utilization of abundant natural building blocks such as organic (e.g. peptides or proteins) or inorganic (e.g. minerals) constituents, combined with novel synthesis techniques based on self-assembly, could lead to new lightweight materials for structural applications in cars, airplanes, and buildings that could reduce the overall energy consumption and ecological footprint of materials.

References

1. Buehler, M.J. and Y.C. Yung, Deformation and failure of protein materials in physiologically extreme conditions and disease. Nature Materials, 2009. 8(3): p. 175-188.
2. Fratzl, P. and R. Weinkamer, Nature's hierarchical materials. Progress in Materials Science, 2007. 52: p. 1263-1334.
3. Buehler, M.J. and Y.C. Yung, How protein materials balance strength, robustness and adaptability. HFSP Journal, 2010: p. doi:10.2976/1.3267779.
4. Keten, S. and M.J. Buehler, Geometric Confinement Governs the Rupture Strength of H-bond Assemblies at a Critical Length Scale. Nano Letters, 2008. 8(2): p. 743 - 748.
5. Keten, S., Z. Xu, B. Ihle, and M.J. Buehler, Nanoconfinement controls stiffness, strength and mechanical toughness of beta-sheet crystals in silk. 2010.
6. Espinosa, H.D., J.E. Rim, F. Barthelat, and M.J. Buehler, Merger of structure and material in nacre and bone - Perspectives on de novo biomimetic materials. Progress in Materials Science, 2009. 54(8): p. 1059-1100
7. Buehler, M.J., S. Keten, and T. Ackbarow, Theoretical and computational hierarchical nanomechanics of protein materials: Deformation and fracture. Progress in Materials Science, 2008 53: p. 1101-1241.
8. Buchanan, M., Learning from failure. Nature Physics, 2009. 5(10): p. 705.
9. Qin, Z., L. Kreplak, and M.J. Buehler, Hierarchical structure controls nanomechanical properties of vimentin intermediate filaments. PLoS ONE, 2009. 4(10): p. e7294.
10. Ackbarow, T., D. Sen, C. Thaulow, and M.J. Buehler, Alpha-Helical Protein Networks are Self Protective and Flaw Tolerant. PLoS ONE, 2009. 4(6): p. e6015.
11. Buehler, M.J. and S. Keten, Failure of molecules, bones, and the earth itself. Rev. Mod. Phys., 2010. in press.
12. Rauch, F. and F.H. Glorieux, Osteogenesis imperfecta. Lancet, 2004. 363(9418): p. 1377-1385.
13. Gautieri, A., S. Vesentini, A. Redaelli, and M.J. Buehler, Single molecule effects of osteogenesis imperfecta mutations in tropocollagen protein domains. Protein Sci, 2009. 18(1): p. 161-8.
14. Gautieri, A., S. Uzel, S. Vesentini, A. Redaelli, and M.J. Buehler, Molecular and mesoscale mechanisms of osteogenesis imperfecta disease in collagen fibrils. Biophys J, 2009. 97(3): p. 857-65.