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Structural orientation and anisotropy in biological materials: Functional designs and mechanics, Liu, Zhang, Ritchie, Advanced Functional Materials, 2020

Novelty/impact/significance:

Based on the fast expanding knowledge of biological materials, this review presents a new perspective looking into the design of nature: structural orientation (predetermined) and reorientation (in-situ change with loading) for diverse favorable mechanical functions.

This essential principle could account for properties that are categorized into several groups including toughening, adhesion, dynamic deformation, damage resistance, and adaptation to load, and promotes a further understanding of the biological materials. Moving forward, the implementation of the mechanisms will facilitate the creation of bioinspired, high-performance materials.  A very good read.   

Major points:

1. Biological materials differ from engineering counterparts in displaying exquisite architectures that greatly influence properties/functions. General principles from these could guide the structural design of advanced engineering materials.

2. Biological materials show characteristic hierarchical organization of structural elements (SE), e.g., fibers, tubules, lamellae, into specific directions, thus being property-dependent on structural orientations.

This structural anisotropy allows a large structural design space, strengthening along the required/desired directions, and multifunctionalities. A comprehensive, deep understanding of the mechanisms is needed to optimize materials properties via manipulating the nano/microscale structural orientations.

3. Interfaces in biological materials are crucial in mechanical properties, featuring more compliant/deformable and thus good shearing ability, weaker than other SE and prone to cracking, and more hygroscopic/responsive to stimuli.

4. Mechanics of this structural orientation/anisotropy can be analyzed by a 2D elementary composite model, aligned 1D stiff constituent embedded in a soft matrix assuming ideal bonding, with the off-axis angle being the angle between the loading axis and the stiff phase. This model can describe various biological materials at different length scales, their properties of stiffness, strength, and fracture toughness, exemplified by bone, the nanoscale mineralized collagen fibrils and microscale osteons, and by wood.

The stiffness, characterized by Young’s modulus, generally decreases with the increase of the off-axis angle, which has been studied considering shear and normal deformation modes. The relationship describes natural materials well at each length scale ranging from the nano to the macro.

The tensile strength decreases with increasing off-axis angle, while compressive strength shows similar dependence but increases in the range of larger angles. The compressive strength behavior of porous trabecular bone anisotropy can also be explained.

Fracture toughness increases with increasing off-axis angle in mode I (increase angle between the crack path and the stiff constituent). Crack deflection is an effective extrinsic toughening mechanism.

5. Toughening v.s. structural orientation: varying structural orientations creates a complex cracking path along the weak matrix/interface for toughening, thereby impeding damage and resisting impact. Hierarchical structure inherently possesses this advantage, such like the multi-scale cross-lamellar and Bouligand structures. They show intricate structural orientations and interfaces to create considerable spatial tortuosity for crack propagation.

6. Robust and releasable adhesion v.s. structural orientation: a good example is the gecko feet adhesion. The detachment and attachment are closely associated with the asymmetrical setae, as different inclination/pulling angles lead to different adhesion forces. The anisotropy of adhesion (adhesion strength varies with pulling directions) enables reversible switching between attachment and detachment by tuning the pulling directions.  

7. Programmable dynamic deformation v.s. structural orientation: by differently aligned cellulose microfibrils in different layers within one component, some plants show predefined deformations of bending and twisting to expose/protect seeds upon water desorption/absorption. This is due to the different hygroscopic behaviors of the cellulose microfibrils and the matrix, usually explained by a bilayer theory: the component has two layers in which the cellulose align along and perpendicular to the axis; upon hydration, the layer with perpendicularly aligned cellulose swells longitudinally while the other layer does not, therefore causing bending deformation toward the other layer. Manipulating the cellulose alignment leads to desired swelling and thus overall deformation behaviors, which can be quantitatively established.

8. Contact damage resistance v.s. structural orientation: a variation of the structural orientation/reinforcement alignment from parallel-to to perpendicular-to the external load (meaning an increasing off-axis angle) with distance from the surface creates a mechanical gradient of high-to-low strength and stiffness and low-to-high fracture toughness. This hard surface with a tough base feature through the structural orientation leads to improved contact damage and wear resistances.

9. In-situ adaption to load v.s. structural reorientation: the soft interfacial matrix shears to enable the in-situ structural reorientation (changing of the off-axis angle of the reinforcement), thus adapting and optimizing the mechanical properties of strength and deformability to the applied load. Tensile strength and stiffness both increase due to the realignment of reinforcement along the loading axis (decreasing off-axis angle), and the reorientation also generates toughening via crack deflection; in compression, structural reorientation increases buckling resistance and decreases interfacial splitting.

This should be the most beautiful part of the structural reorientation.

10. Characterizing the structural orientation and anisotropy needs the attention to the specific length scale of the structural features that provide the properties, e.g., the fracture toughness usually derives a major part from the micro to mesa scales. In-situ SEM, SAXS, WAXS are capable of real-time imaging and quantifying the detailed structural reorientation mechanisms.

11. Numerous natural designs display the structural (re)orientation for tailoring and optimizing a wide spectrum of mechanical & functional properties. Inspiration from these could provide new avenues for developing advanced composites with enhanced performances for diverse applications. There is a lot more on structural anisotropy and (re)orientation to be understood from the mechanics and materials science, and exploring the common principles among differing biological systems is a central task. Along with the challenges such as considering the combining factors of structural hierarchy and time-scale effect, and more advanced processing technologies needed for controlling the fine structure, the bright prospect of implementing the above design principles for high-performance composite materials is exciting.

A review paper not only summarizes relevant research studies in a logical manner, unfolding the panorama of current knowledge of certain fields from different aspects, but also presents novel perspectives/views through integrating what has been discovered, thus providing and provoking new thoughts.  

Here is the link of the full text: https://onlinelibrary.wiley.com/doi/full/10.1002/adfm.201908121