Figure caption: Multiscale quantification of the 3D cellular microstructure of sea urchin spines.The mineralized skeletons of sea urchins, which are light but robust enough to withstand the rough and tumble of ocean waves, hold important clues for analyzing and designing artificial porous materials. Researchers from Virginia Tech have developed an approach for investigating and characterizing these materials using a combination of high-resolution tomography and computational analysis [Yang et al., Acta Biomaterialia 107 (2020) 204-217, https://doi.org/10.1016/j.actbio.2020.02.034; Chen et al., Acta Biomaterialia 107 (2020) 218-231, https://doi.org/10.1016/j.actbio.2020.03.006].
“We are interested in sea urchin spines because they represents a unique group of biological materials that are highly mineralized (composed of calcite, the same material as chalk), lightweight due to their high porosity, and yet highly damage tolerant,” explains Ling Li, who led the work together with Yunhui Zhu.
These natural porous materials are highly complex with huge variations in pore sizes, branch morphology, and three-dimensional organization across different species. Quantifying these elaborate microstructures is the first important step in being to understand how their design leads to their remarkable mechanical properties.
“We [wanted to understand] how can we quantitatively characterize, represent, and rationalize the cellular structural design of sea urchin spines from individual branch and node level to the long-range network level,” says Li.
The researchers gathered data on sea urchin spines using high-resolution synchrotron X-ray tomography and then used computer vision-based analysis and reconstruction approaches to identify, model, and visualize the features of the complex structure in three-dimensions on a multiscale level.
“With this new analysis pipeline, we, for the first time, quantified the network organization of the spines from the sea urchin Heterocentrotus mamillatus from the individual branch and node level to the macroscopic skeletal level,” says Li.
The analysis reveals that within sea urchins spines, some basic design motifs are repeated throughout the structure. The researchers believe that the organism uses a combination of 3- and 4-branch nodes to control the morphology and thickness of individual branches and their alignment and orientation.
“We show that such structural control allows the organism to control the local mechanical properties and anisotropy precisely, leading to optimized mechanical performance with reduced weight at the skeletal level,” points out Li.
A better understanding of the relationship between the porous structure of sea urchin spines and their mechanical properties could hold important lessons for designing novel lightweight and damage tolerant materials. Although the current methodology only works for porous structures, additional algorithms could extend its scope to include the plates and membranes observed in trabecular bone found at the end of long bones like the femur.
“We expect that this approach [will be] generally applicable for open-cell porous materials, either natural or synthetic,” says Li. “We are currently [using it] to analyze other echinoderm structures as well as developing computational design tools to mimic these structures.”
Cellular network analysis algorithm is available for download:
https://github.com/Ziling-Wu/Quantitative-3D-structural-analysis-of-the-cellular-microstructures