From lobster claws and fish scales to conch shells, humans have often been inspired by nature in the creation of protective gear. Recently, a team of researchers hailing from MIT, Virginia Tech, Harvard University, California State University Fullerton, and the Max Planck Institute of Colloids & Interfaces published a paper, titled “Bioinspired design of flexible armor based on chiton scales,” about their work using multimaterial 3D printing and parametric computational modeling to create “a synthetic flexible scaled armor analogue” based on the scaled armors of chitons, a group of marine mollusks.
“This approach allows us to conduct a quantitative evaluation of our chiton-inspired armor to assess its orientation-dependent flexibility and protection capabilities,” the researchers wrote in the abstract.
Biological armor offers mechanical protection from the environment, which includes attacks from predators. Man-made armors use rigid structures for this protection, which the team explained can result “in a trade-off with flexibility and maneuverability.”
“Many chiton species possess hundreds of small, mineralized scales arrayed on the soft girdle that surrounds their overlapping shell plates,” the abstract states. “Ensuring both flexibility for locomotion and protection of the underlying soft body, the scaled girdle is an excellent model for multifunctional armor design.”
Because many biological armors are based on hard and rigid armor plates, flexibility is tough to pair with it. Scale-like armors with many small, repeating elements, like that of chiton, can help maximize the combination of flexibility and protection. The team completed a study of the 3D geometry, interspecific structural diversity, material composition, and nanomechanical properties of chiton girdle scales, focusing on the chiton Rhyssoplax canariensis (Chitonidae: Chitoninae). This species is covered by a total of eight “bilaterally symmetrical overlapping mineralized shell plates,” in addition to the protective scaled girdle.
“In contrast to most shelled mollusks where mobility is limited, as in the single shelled mollusks (gastropods, including snails, scaphopods or tusk shells, and some cephalopods such as Nautilus) or hinge-shelled bivalves (mussels, clams, scallops, etc.), most polyplacophorans (chitons) are characterized by eight overlapping, hard shell plates (Fig. 1a, b), which collectively accommodate a wide range of motion,” the researchers explained. “In addition to the eight overlapping shell plates (which are functionally analogous to the segmented plate-like exoskeleton of many crustaceans), additional protection is provided by a thick leathery girdle that skirts the animal’s periphery.”
Even though the girdle scales are nearly pure mineral and very rigid, they are also very flexible and able to conform to rough surfaces. Chiton scales are also more uniform in composition, with no porosity, sub-layering, or material heterogeneity.
“This observation underlines the suitability of chiton scales as a model for bioinspiration, as the mechanical performance of their armor can be ascribed primarily to geometric considerations, rather than fine scale material variation,” the team noted.
The team used many experimental and modeling approaches, such as mechanical testing, finite element modeling, electron microscopy, synchrotron X-ray micro-computed tomography, and instrumented nanoindentation, to investigate chitons, and the use of chiton-like scales in 3D printed flexible armor.
“Incorporating the physical and functional properties of chiton girdle scales characterized in these investigations, we design a bio-inspired flexible armor system, integrating parametric geometrical modeling and multi-material 3D printing,” the researchers wrote. “We explore the functional trade-offs between protection and flexibility in this model scaled armor system and its potential for informing the design of additional functional prototypes.”
A Connex 500 multi-material 3D printer from Stratasys was used to create prototypes out of both flexible and rigid photopolymers in different colors.
“In order to successfully mimic scale morphology for the production of a 3D-printed structural analogue…quantitative measurements of the scale geometry were conducted by defining several morphometric parameters,” the researchers stated.
The team also took “3D morphometric measurements” of the dorsal girdle scales from chiton species in the Ischnochitonidae and Chitonidae families. In order to reproduce the morphometrics for further modeling of scaled arrays, they created a parametric geometrical model.
“The successful 3D modeling of individual scales allowed us to design a composite scale armor assembly similar to that of chitons,” the team explained. “The bio-inspired armor system included rigid scales embedded in an underlying soft substrate.”
They used materials with moduli of ca. 2 GPa and ca. 0.7 MPa, respectively, to 3D print the scales and matrix, in order to properly replicate how the scales would interact with soft girdle tissue. The scale assembly was very flexible, with a similar range of motion to real chiton scales, and the team was able to efficiently explore a variety of arrangements with the scales due to the “parametric nature of our model.”
They also studied the mechanical performance of their multimaterial 3D printed prototypes, and even 3D printed a scaled kneepad prototype in order to demonstrate the usefulness of the chiton-inspired system for both protective and flexible applications.
“Current kneepad designs often fall in one of two extremes: hard and rigid plates that create heavy protection but limit flexibility, or elastomeric rubbers/foams that provide high flexibility but limited protection (especially against sharp objects). The chiton scale-inspired knee protection pad offers a unique solution to this dilemma,” the researchers noted.
The 3D printed scale assemblies had much higher puncture resistance than typical kneepads with rubber- or foam-based inserts, and also featured good shape-conforming capabilities in extended and bent configurations.
Co-authors of the paper are Matthew Connors, Ting Yang, Ahmed Hosny, Zhifei Deng, Fatemeh Yazdandoost, Hajar Massaadi, Douglas Eernisse, Reza Mirzaeifar, Mason N. Dean, James C. Weaver, Christine Ortiz, and Ling Li.
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