Category: MIT

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.”

Rhyssoplax canariensis (Image: Jose Maria Hernandez Otero, BioLib)

“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.

Figure 1. Biological flexible scaled armor in the girdle of the chiton Rhyssoplax canariensis. a, b Wide-field SEM images of the chiton R. canariensis, which show the dorsal and side view of the primary plates (PP) and peripheral scale-covered girdle (G), respectively. c Enlarged view of the girdle covered with dorsal scales. The image was acquired from the region indicated by the rectangular box in a, highlighting individual overlapping dorsal girdle scales, a fully covered protective armor. d Fractured cross-section of the girdle scaled armor, which consists of three components arranged from dorsal to ventral: (1) dorsal scales (DS); (2) fibrous layer (FL); and (3) ventral scales (VS). The white dashed lines indicate the height of the inter-scale organic matrix, and the red arrow indicates gaps between adjacent scales. e Cross-sectional view of scaled armor based on micro-computed tomography (μ-CT) data. Note the distance between the dorsal and ventral scale layers, which is occupied by the fibrous layer. f SEM image of the rod-shaped ventral scales, where the white arrows indicate small cracks.

“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.

Figure 1. g–i: μ-CT 3D rendering of the chiton R. canariensis girdles in different viewing orientations and modes: g top view of girdle scales, h transparency mode showing the overlapping characteristics among adjacent scales, bottom view i with and j without ventral scales. Local coordinates: N (normal), from ventral to dorsal; R (radial), from proximal to distal; C (circumferential).

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.

3D geometry and surface morphology of individual dorsal scales of the chiton R. canariensis. a–f: μ-CT data-based 3D rendering of individual girdle scales in different view angles and modes: a front view, b top view (yellow arrows indicate pore openings), c bottom view (white arrow shows a depression at the base of the scale), d two side-views (white arrows shows the surface roughness at the lower surface of backside), e back view, and f transparent mode (the yellow arrows show holes in the dorsal surface of scales and the white arrow indicates depression in base). g Projection contours along two orientations (transverse and bottom) are used to describe the geometries of chiton scales. h Top view of a μ-CT data-based reconstruction of the girdle scale assembly of R. canariensis. Three columns of scales used in the geometrical measurement are highlighted in pink color and their positions are indicated. i Variations of geometrical parameters as a function of scale position. The solid line represents the average and the shaded area shows the standard deviation. j SEM image of a scale’s back surface. k Magnified-view of scale surface with microscopic bumps at the underside of the back surfaces of chiton scales, as indicated by the white box in j. l SEM-derived stereographic reconstruction of microscopic bumps in a similar region shown in k. m Backscattered SEM image of a polished cross-section in the region of microscopic bumps of a scale, highlighting the difference in morphology between the front and back surfaces of the dorsal scales.

“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.”

Figure 5. 3D parametric modeling of chiton scale geometries. a Top, the 3D scale model with three principal scaffolding curves. Bottom, 3D scale model highlighted with the central spine for generating the surface meshes. b Three principal curves with geometrical landmarks indicated. c, d Comparison of original chiton scales with corresponding mimicked scale models for two species: c, a single-curved scale from Rhyssoplax canariensis and d, a double-curved scale from Lepidozona mertensii.

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.”

Design and fabrication of bio-inspired flexible scaled armor. a Schematic diagram showing basic components of the armor; (1) scales, (2) matrix, and (3) soft underlying layer. b Side and c bottom view of the armor. d, inter-scale spacing. d Flat panel with uniform scales fabricated through additive manufacturing. e A bent panel showing excellent flexibility. f, g Design of scale pattern with size gradients. h Scale assembly in flat and curved substrate. i Scale assembly on double-curved surfaces. j X-ray projection images of a kneepad based on the bio-inspired scaled protective panel in j extended and k bent positions, demonstrating its conformability and flexibility. l Demonstration of the protection capability of the chiton scale-inspired kneepad on broken glass.

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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