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Molecular to macroscale energy absorption mechanisms in biological body armour illuminated by scanning x-ray diffraction with in situ compression, Zhang, Garrevoet, Wang, Roeh, Terrill, Falkenberg, Dong, Gupta, ACS Nano, 2020

Novelty/impact/significance:

A new experimental method is created to simultaneously determine the in situ strains at the molecular, meso- and macro-levels in the impact-resistant stomatopod cuticle under compression. This reveals new findings such as the molecular-level prestrain gradient in the chitin fibers, a decrease in strain-levels by over a factor of 10 from the tissue to the molecular-scale, evidence that the chitin nanofibers act as an elastic spring-like network, thus helping explain how the large strains and energy absorption are transduced down to the nanofiber level.

The approach is applicable to other biological/engineering composites in measuring the strains from multiple length scales, e.g., metallic nanofibers, carbon nanotubes.   

Scientific question:

How to experimentally characterize the complex, in-situ strain/displacement mechanisms concurrently ranging from molecular to macroscale in a full-field manner?

Key of how:

A combination of micrometer-scale synchrotron x-ray diffraction, in situ compressive loading and sample rotation to retrieve different slices of the 3D reciprocal space intensity, coupled with 2D scanning to resolve the strains at different mesoscale tissue locations are used to measure the molecular-, meso-, and macro-level strains. The method depends on the molecular-level fiber symmetry; the 1D gradient in fiber orientation characteristic of the Bouligand plywood structure affects the diffraction intensity, the fiber-angle dependent intensity to map the lamellar (meso) strains, the peak shifts of the on- and off-axis reflections to measure the molecular-level strain.  

Major points:

1. Although extensively studied, the structure of impact-resistant stomatopod cuticle has been examined mostly from pre- and postloading combining electron microscopies and nanoindentation. Also, the tissue involves growth-induced chemical and structural gradients at molecular and micrometer scales, which usually associate with prestrain and residual stress that are important in influencing the macroscale mechanical behaviors.  

2. Measuring the cuticle’s multiscale deformation is challenging. The hierarchical structure consists of molecular to nanoscale, chitin fibrils (3-4 nm) self-assemble with proteins into fibers (20-100 nm), fibers aggregate in a protein matrix with partial mineralization form twisted plywood structure/lamellae (which is called Bouligand structure, microscale). Most current characterization techniques (already very advanced) can only determine the microscale strains (not molecular level) or have restricted field of view of ~10s of nanometer.

3. Here by using scanning synchrotron x-ray diffraction during in situ compression with sample rotation and 3D reciprocal space modeling, the strains at different hierarchical levels are resolved. The molecular-level fiber symmetry is the basis; the 1D gradient in fiber orientation within the Bouligand structure affects the diffraction intensity of fibers oriented at different angles, while the fiber-angle dependent intensity maps the lamellar strains and the peak shifts of the on-and off-axis reflections measure the molecular strain.   

4. The peak shifts in the axial (002) and equatorial (110) reflections can be used as molecular-level strain sensors. The intensity of these reflections depends on the fiber angle, which varies periodically within the Bouligand structure; the intensity variations can be predicted by a 3D diffraction model (developed by the authors previously), which agree well with experimental diagonal banding. Then the (110) diffraction intensity can be linked to the fiber angle/periodic lamellar structure. Two effects (the Ewald-sphere curvature effect and the change in scattering volume when the rectangular sample is rotated) have been neglected.

5. The discovery of a prestrain spatial gradient at the nanofiber level in chitin. The intensity of the axial (002) wide angle x-ray diffraction (WAXD) reflections show a most striking feature: the wavevector corresponding to the (002) lattice spacing (D(002)) in the exoxuticle is larger than that in the endocuticle, specifically, the D(002) increase by 0.37% (~5.155 to 5.174 Å) from exocuticle to endocuticle region. This demonstrates the existence of a prestrain gradient in the chitin network, explaining the relatively high modulus of chitin fibers in experiment and simulation reported by another paper. The possibility of drying effect is eliminated by the similar finding from measurements on samples kept in a fluid chamber.

6. Concurrent strain-determination at macro-, meso-, and molecular-scales. From the lamellar structure by the (110) intensity map and the D-period by the q(002) value map (both from raster scans with the rotation of the sample), it is possible to calculate the meso (lamellar-level) and nano/molecular (fiber-level) displacements and strains with sublamellar level resolution, jointly with macroscopic (tissue-level) strains. The lamellar-level displacements from in situ compression of cuticle are from the abbreviated scans without ϕ-rotation; the molecular-level strains (full scans including ϕ-rotation) are obtained from percentage shifts in the (002) (for axial fiber strain) and (110) (for transverse fiber strain) peak positions.

7. The nanofibre network in cuticle behaves like a well-connected 3D elastic foam, retaining the structural integrity and sustaining impact loads, while the highly mineralized matrix enables excellent dynamic mechanical performance by dissipating energy through plastic deformation.

8. Main findings on the small-scale mechanics of cuticle: (1) a molecular-level prestrain gradient in the chitin nanofibers from exo- to endocuticle, (2) a decrese in strain-levels by over a factor of 10 from the tissue to the molecular-scale, (3) higher degree of radial compaction of chitin nanofibers in the endocuticle versus the exocuticle, (4) under unloading, a different response of the chitin fibers in the exocuticle versus the endocuticle, (5) evidence that the chitin nanofibers act as an elastic spring-like network even under irreversible macroscopic deformation (the molecular strains return to zero while the macroscale strains remain ~8% residual strain). These account for a balance between structural stability, high compressibility and energy dissipation efficiency at low metabolic costs for tissue repair and renewal.

9. Considering the finding of the prestrain gradient and the mineralization differences in the exo- and endocuticle, it is possible that the lower (002) D-spacing in the exocuticle is linked to its higher degree in mineralization, while whether to understand this as a compressive prestrain or a loss of tensile prestrain is not clear.

It’s not easy for me to fully understand the content and then summarize the core clearly and concisely. Any comments or corrections are welcome. Overall, the work presents a promising methodology for probing and interpreting the in situ micromechanical behaviors that are extremely difficult to achieve.

Here is the link of the fulltext: https://pubs.acs.org/doi/10.1021/acsnano.0c02879