Armor inspired by natural microstructures
Bioinspired armor are materials that were inspired by the composition, and most importantly, the microstructures commonly found in nature’s natural defense mechanisms. These microstructures have been evolved by organisms to protect themselves from exterior forces, such as predatory attacks. These materials and their microstructures are optimized to withstand large forces. By taking inspiration from these materials we can design armor that has better penetration resistance and force dissipation properties than previously possible.
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 at producing 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.
Function and microstructure.
Armor for High-Velocity Collision Protection.
Nacre.
Nacre is the composite biological material that makes up the shell of mollusks, featuring high strength and toughness. Layers of nacre work together to protect soft-bodied organisms from external loads, mainly including predatory attack and underwater currents, and can thus dissipate a large amount of energy during impact.
The hierarchical structure of nacre has been observed on many length scales, from the microscale to the nanoscale. On the microscale, the structure of nacre is akin to “brick and mortar”: composed of long, thin brittle plates in a polygonal shape held together by a soft and flexible polymer. The mineral tiles are composed of aragonite, a polymorph of calcium carbonate, and are held together with a polymer of chitin and asparagine- rich protein. The aragonite tiles are 0.5 um thick, while the biopolymer layer is 50 nm. The arrangement of these tiles affects the mechanical properties of the nacre. Columnar nacre occurs when the tablets stack, creating regions of aragonite platelet overlap. Sheet nacre occurs when the tablets are arranged randomly between adjacent layers. Other features on the nanoscale, such as mineral bridges and asperities contribute to the high fracture resistance of nacre.
Nacre has been studied for potential applications in human armor. Engineered nacre-like composites have been shown to possess improved ballistic-penetration resistance compared to monolithic armor of the same area and density.
Turtle Shells.
A turtle shell is a bio-composite consisting of a keratin-coated dorsal shell, or carapace, and a flat interior made of cancellous bone sandwiched between thin layers of cortical bone. The flat, bony configuration allows for significant weight reduction, thereby resulting in higher stiffness, strength, and toughness-to-weight ratios. This design renders the shell particularly effective against sharp, high-strain assaults, providing vital protection from predators such as alligators, jaguars, and birds.
Soft collagenous tissue that joins adjacent interdigitating bone plates, called sutures or interdigitating sutures, allows a turtle to respond to a variety of different loading regimes. Turtle shells were considered to be biological polymers before synthetic polymers were developed, and were thus one of the first natural analogs of early man-made armor.
Armor for Low-Velocity Blunt Impact Loading.
A key application for biological-inspired armor is protection against low-velocity blunt-impact loading. Blunt impact loading refers to the direct contact of a blunt object on a body, resulting in physical force or trauma. It involves the transfer of kinetic energy, without penetration, and usually can result in compression, deformation or material fracture. Common examples would include sports injuries (impacts that result in concussions), assaults (punches, kicks), or falls. Many instances in nature provide organisms with an ability to sustain these loads, and hence provide models for biomimetic armors.
Hooves.
Bovine and equine hooves are highly studied examples of blunt impact loading and shock absorption in nature. This is primarily because the hooves act as a natural energy absorber for animals such as horses and cattle with high force and velocity gait patterns.
"Structure"
Both bovine and equine hooves are made of the protein α-Keratin. α-Keratin is a structural, fibrous protein, the same one found in our hair and nails. The keratin molecules are held together by H-bonding and disulfide cross-linked bonds, which enhance the rigidity of the protein. α-Keratin chains twist together to form coiled-coil dimers. Coiled dimers bind with other coiled dimers to produce photo filaments, which further bind to form a photo fibril, then eventually an intermediate filament (IF). Within the outer wall of the hoof, are tubule structures oriented parallel to the leg (220x140 um, 50 um medullary cavity). Keratin intermediate filaments are organized as circular lamellae that surround these tubules.
"Properties"
Many of the mechanical properties associated with hooves are due to the presence of tubules. The layers of tubules with their lamellar keratin surroundings provide excellent crack deflection and fracture toughness. When a crack is introduced into a material, it will continue to propagate until it hits a new interface. Hooves have layers of keratin IFs surrounding each tubule, each in slightly changing orientations, creating multiple interfaces that the crack will “bump” into, and thus makes the hoof a highly fracture-resistant material. The high fiber alignment and density of keratin as well as the longitudinal orientation of the tubules helps with energy absorption from blunt loading. Finally, the intratubular matrix within the tubules of the hooves is less stiff and dense than that surrounding it, reducing the weight of the hoof. All these properties help hooves support large compressive and impact loads while providing shock absorption from the impact, and make for promising impact loading models.
Horns.
Horns, such as those found in goats, buffalo, and rhinoceroses are another keratin based structure that may provide inspiration for biological armor. Given that horns serve as a defense mechanism for these animals, these structures are well able to withstand significant impacts.
"Structure"
Horn has a very similar structure to hoof, as they are both composed of α-Keratin. The structural makeup of these types of horns involves a lamellar keratin structure, with microtubules radially oriented. These tubules are much larger than those found in hooves, with exact dimensions depending on the species, but are also surrounded by keratin IF lamella. This gives the horns an ability to maintain stiffness in tension while still dissipating kinetic energy from compressive forces and impact.
"Properties"
Given the extremely similar structure of horns to hooves, they share many similar properties. One such property is crack deflection and fracture toughness. Again, the layered lamellar hierarchy of keratin surrounding the tubules creates many interfaces for cracks to run into. One study found that the critical crack length for horn was 60%, meaning that a crack would have to extend through 60% of the transverse direction of a horn before it became critical, which shows extremely high toughness. Horn also has extremely high energy absorption and ability to withstand stress. This is attributed to the tubule orientation and keratin fiber density; unique to certain animals, the spiral geometry of the horn also allows for higher energy absorption. Specifically to bighorn sheep, studies found that their horns had a compressive stress of 4.0 MPa, a tensile stress of 1.4 MPa, and a fighting force of 3400N. The compressive stress is obviously much larger than the tensile stress, but like many other anisotropic materials, this is because an animal such as the bighorn sheep who uses their horns for fighting will be more likely to experience compressive stress and blunt impacts rather than tension. Finally, similar to hooves, horns are extremely tough but also lightweight, due to the lighter, less dense intratubular matrix.
Crustaceans.
Aside from nacre and conch, crustacean exoskeleton and cortical bone are other biological structures that possess intricate features that may be beneficial for ballistic protection and low velocity blunt impact.
"Structure"
The exoskeleton of crustaceans such as crab is made of chitin, which is an amino polysaccharide polymer. A polysaccharide polymer is derived from an amino sugar, a type of sugar where the hydroxyl group is replaced with an amine group. The chitin fibers are embedded in a calcite matrix. The crustacean exoskeleton, like many of the other models discussed previously, is hierarchical. It starts with the chitin polymer fibers arranged in a honeycomb pattern. These “honeycomb” planes are then stacked in a Bouligand pattern, consisting of several layered, rotating planes. This makes up the inner, endocuticle layer of the crustacean exoskeleton and accounts for 90% of the total exoskeleton. The outer exocuticle layer contains more densely packed chitin layers that end up being 200 μm thick. Thickness of the exocuticle and endocuticle layers vary in different parts of crustaceans.
"Properties"
One major mechanical property of the crustacean exoskeleton is high toughness and stiffness. This is attributed to the thick, densely layered exocuticle layer. Energy Dispersive X-Ray mapping found that the exocuticle layer has higher calcite mineralization in its matrix, increasing the “hardness” of the material. The endocuticle layer of the exoskeleton is responsible for energy absorption and crack deflection properties. This layer has a much lower stacking density of the chitin polymer layers than the exocuticle layer. This means that when placed under compressive forces, it is able to dissipate the load. Like mentioned in hoof and horn, the Bouligand pattern provides several interfaces for cracks to run into, creating high crack deflection. Overall, the combination of the hard, stiff exocuticle and the energy absorbing, compressible endocuticle create a structure that is extremely difficult to penetrate and serve as a good model for both blunt impact and ballistic shock applications.
Conch Shell.
"Structure"
Conch is a type of shell composed of calcium carbonate composite material that has a three-order lamellar structure. The first order contains ceramic plates that are 5-60 μm thick. The second order consists of ceramic beams at two different 45º orientations, about 5-30 μm thick, and the third order is made up of thousands of tiny ceramic planks, 75-100 nm thick.
"Properties"
The lamellar layers that make up the conch shell provide strong crack propagation. This structure also gives the conch shell several modes of fracture, which act as toughening mechanisms when put under different loads. Mode 1 fracture, which involves loading orthogonal to the plane of the crack, exists in the third order lamellae due to delamination of the layers. Mode 2 fracture, which involves loading parallel to the plane of the crack, also exist between third order lamella and cause a buckling effect. Compressive and bending tests showed that conch shell was highly anisotropic. It was found that conch is about 60MPa stronger in parallel loading compared to perpendicular loading. This was validated through a Weibull analysis for dynamic compression in surfaces both parallel and perpendicular to the load, the parallel loading was found to fail at higher fracture stresses.
Armor for Sharp Impact Loading.
Crocodiles and Alligators.
Crocodilian skin has the potential to be used as armor for sharp impact loading because its skin is embedded with bony particles (scutes) (i.e., osteoderms) . Dubbed “armored skin”, osteoderms are composed of hydroxyapatite and collagen, the same components found in bone. The scutes have a bony network on their surface and are connected with the collagen fibrils. Osteoderms are present in reptiles (e.g., lizards, crocodilia, dinosaurs), fish, and some mammals (e.g., armadillos, mice), for protection from predators. The scutes on crocodiles have multiple functions, like thermoregulation, calcium storage, and toughening. The low weight and flexibility of the scutes are attributed to their porosity (~12%), while the matrix surrounding the pores gives them their hardness. Compression tests indicate that crocodilian scutes are strongest in the axial direction, with a strength of 67 MPa. The toughness of the scutes can be attributed to pore flattening, mineral bridge formation, and collagen bridge growth as energy dissipation mechanisms upon impact loading.
Crocodiles also have highly irregular polygons of keratinized skin on their head and faces, which offers additional protection. The cracks on the head and face of the crocodilian skin are generated from a cracking response, i.e., “crocodile cracking” that occurs when stress is applied. This cracking mechanism releases deformation energy and helps the skin maintain its hard exterior, with increased flexibility.
Armor for Mobility and Movement.
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,