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Biological/Bioinspired armor are materials that were inspired by the composition, and most importantly, the microstructures commonly found in natural defense mechanisms. These microstructures have been evolved by organisms to protect themselves from exterior forces, such as predatory attack.

Nature uses abundantly available materials to develop structures that have the most efficiently aimed mechanical properties. By examining the microstructures produced in nature, scientists can engineer these structures with more optimal materials to produce the most mechanically robust version of these structures. Biological/Bioinspired armor is specifically aimed to produce protective materials by optimizing naturally occurring defensive materials.

This article will cover common types of defensive materials observed in nature, how these microstructures contribute to the impressive material properties, and how scientists have used this knowledge to develop novel protective materials.

D. Mobility and movement - Ely

In nature an efficient biological armor needs to be able to protect the organism while introducing the least amount of hindrance on its function. For many organisms' movement is one of their most important abilities, therefore, the armor cannot limit their movement mechanics [].

Unlike the continuous structure used to protect stationary organisms, like clam shells, these types of armor are usually composed of many separate structures to allow for the elongation and contraction required for movement, while maintaining complete protection []. There are two major classes of biological armor found in nature, these are scale armors and osteoderm armor. Both of these biological armors have specialized microstructures and macrostructures that produce the impressive properties of these materials.


 * 1) Scales

Scale armor is the most widely expressed armor in nature []. It is mostly seen on aquatic animals; however, it is also seen on some land animals such as pangolins. This armor is known for its impressive flexibility, while also its impressive compressive and puncture resistance []. There are many subcategories of fish scales, but the main three are Elasmoid, Ganoid, and Placoid. These scale types are distinguished by their mechanical properties, geometric shape, and macroscopic alignments. Elasmoids are the typical oval shaped scales found on ray-finned fish []. These scales are known for their ultra lightweight, puncture resistant, and flexibility which allows for the propulsion movements required for swimming []. Ganoid scales are rhomboid in shape and exhibit enhanced stiffness, which is due to their thicker layer of mineralized material []. Lastly, Placoid scales are best known by their shape. They have spines that run against the pattern of the scale which results in a sharp or rough feeling to the touch []. These scales are found on animals like sharks and stingrays [].

Structure

Elasmoid scale strength against force loading are significant due to their macrostructure. The macrostructure is composed of their positioning, shape, and scale features. Elasmoid scales are oval in shape, three quarters of which is covered by neighboring scales []. This overlapping does not only allow for the movement of the organism while maintaining complete coverage, but also aids in the compressive force resistance []. When the scales are loaded with a compressive force, it is distributed across all the neighboring scales []. As these scales are bent into each other, in compressive loading or in natural movement, the scales exhibit a material hardening effect []. This allows for each scale to be produced below the required stiffness to protect against an attack, however when joined together in an overlapping pattern, the material is able to resist large compressive forces []. This is one structure that this armor design uses to maintain light weight.

Another major macroscale system of the elasmoid scales are the physical scale features. The scales have grooves running from the focal point of the scale towards the edge, known as radii, as well as rings around the focal point in a concentric pattern, known as circuli []. Both the radii and circuli are hypothesized to help in the bending mechanisms of the scale as well as aid in anchoring the scale into the dermis [].

The macroscale structures of the scales are largely important for the armors function; however, the microstructures give the materials their impressive properties. These scales are made from composite materials []. These are mineralized protein matrices that allow for the strength and toughness of the mineral while reducing the brittle effects of these materials with protein components []. In the elasmoid scale there are three main layers []. The Limiting layer, which is the outermost layer; the elasmodine outer, and the elasmodine inner layers which are defined by their level of mineralization [].

This layer is found just on the surface of the scale where it is the first line of defense against puncture []. This layer varies its thickness depending where it is located on the scale as well as which species scale it is []. It is typically on the 10-1000 micrometer scale in thickness []. The limiting layer also forms various shapes depending on if it is posterior or anterior on the scale []. Posterior limiting layer commonly forms varying pillar structures assumed to be for varying water interface functions, while the anterior portion is most commonly formed into the circuli shape discussed earlier []. As seen in Figure …. the cross-section of the anterior region shows a saw tooth shape. This is assumed to help with dermal integration [].

This composite material is almost completely mineral apatite with small amounts of collagen []. Between varying fish species researchers noted a carbonate substitution in the apatite structure []. This material's structure is well developed for its application. This layer of the scales needs to be tough and stiff to help defend from punctures which explains the high level of mineral in this layer. The apatite percent volume in the limiting layer was around 65% [].

Below the limiting layer there is a thicker basal plate composed of larger collagen fibrils, called the elasmodine layer []. The elasmodine layer is split into the external elasmodine and internal elasmodine which are distinguished by the difference in their mineral content within the composite material []. The external layer contains higher concentration of the mineral component while the internal layer contains almost only collagen []. This variation between slightly mineralized to almost completely unmineralized composite leads to the great flexibility of the scales []. As a force is applied to the surface of the scale, as seen in predatory attack, the outermost layer is put under compressive force and needs to resist puncture, while the innermost layer is experiencing tensile forces []. The high mineral concentration of the limiting layer and the external elasmodine are better suited for dealing with compressive forces, while the almost completely unmineralized material of the inner elasmodine is better suited for stretching forces []. The outer elamsodine layer is around 35% mineralized [].

The collagen fibril alignment in the composite material plays a large role in the material properties. These collagen fibrils form a structure known as the Bouligand structure []. The Bouligand structure is a rotated plywood design that imparts multidirectional strength. The collagen fibers in one layer are all aligned linearly in a single direction to give strengthening in that one direction. These unidirectional collagen fibril plies are then slightly offset from their neighboring layers to help with the materials overall multidirectional strength. The scale’s strength and elastic modulus correlates to the number of elasmodine layers and the thickness of those layers compared to the overall scale thickness []. Additionally, the collagen fibrils within each lamella layer do not demonstrate grouping. Each fibril is isolated and connected to its neighbors through sacrificial bonds []. This connection allows for another level of force dissipation as the sacrificial bonds will break first under force instead of the macroscale material.

The structure of the scale armor is specifically designed to provide protection during movement, dissipate forces, and maintain lightweight. This is then combined with the microstructures of the scale to produce a material that is best suited for the required parameters. The composite material provides force resistance and puncture resistance, while still providing the flexibility required for the macrostructure movements. This combination of aspects provides great protective armor for many animals on earth.

An example of this armor structure in nature are the Arapaimas fish. These are large fish that exhibit these elasmoid scales and heavily rely on their microstructure and macrostructures to protect them []. These fish live in the same water as piranhas, so having effective armor to protect themselves from possible piranha attacks are vital to their survival. The overlapping of the scales allows for the armor to absorb kinetic energy by transmitting the impact energy to adjacent discs []. A larger number of scale layers can protect from larger forced attacks. For example, the Arapaima have an average of three scale layers which is capable of protecting from piranha attack []. Without this efficiently designed armor these fish would not be protected in their habitats.

Osteoderms

Osteoderms are bony deposits formed inside the dermis, commonly found in lizard species and alligators []. Osteoderms form inside the dermis with or without skeletal connection []. The level of osteoderm distribution varies heavily between species. Some animals are completely covered, while others only have the armor in certain areas of the body as seen in the image.

Additionally, the size of the osteoderms varies highly depending on species and area of the body []. There are typically larger plates on the back, side and belly, then smaller plates around the head and tail []. There are also many types of osteoderm structures. They can form in isolated groups creating a partial coverage of the organism [], but they can also form a structure more similar to the fish scales, where they overlap each other to form a more complete armor []. Depending on the macroscale structure of armor the osteoderm morphology changes []. The overlapping layers are typically thinner than standalone plates seen in the isolated groups.

Osteoderms are made of mineral composite materials. They include various types of bone, mineralized and unmineralized collagen bundles, as well as blood vessels. The composite material that makes up osteoderms are made from calcium phosphate and collagen. Due to the bone-like material structure, these materials are much more stiff than the fish scales previously discussed []. With a complete coating of this armor, it would inhibit the mobility of the organism. However, these structures are connected by stiff fibers and dermal tissue which allow for the movement of the osteoderms []. The soft regions, however, are not protected by the armor.

Osteoderm structures are formed similarly to bone; the outer layer is made of parallel fibered bone, with a cancellous core lined with lamellar bone []. These bone structures have very similar mechanical properties to skeletal bone. By producing this dermal layer of bone, the organism creates a sacrificial layer of stiff bone at the surface of the skin to defend against point force attack. This structure is then surrounded by flexible dermal material to allow for movement, however as stated before, the gaps between the osteoderms are not protected leaving some vulnerable areas. This armor combines larger and more mechanically robust materials in a macrostructure design that still allows for movement.