The natural world has produced biological materials, or biomaterials, with properties that often surpass what human engineers can create. These materials exhibit an impressive combination of strength, flexibility, and light weight. Identifying the “strongest” biological material is complex because the term “strength” has multiple definitions in materials science. The superlative title depends entirely on the specific mechanical property being measured, leading to different record-holders for different types of stress.
Understanding Measures of Mechanical Strength
Materials scientists use distinct metrics to quantify how materials react to applied forces. Tensile Strength quantifies the maximum stress a material can withstand while being stretched or pulled apart before it breaks, measuring absolute load-bearing capacity. Stiffness describes a material’s resistance to elastic deformation; stiff materials resist bending or stretching and return to their original form once the force is removed. Conversely, Toughness measures a material’s ability to absorb energy before fracturing. This property is crucial for resisting impacts and preventing the spread of cracks. Biological structures are often finely tuned to excel in one of these areas, making a direct comparison difficult.
Materials Excelling in Tensile Strength and Stiffness
For sheer absolute tensile strength, the limpet, a small aquatic snail, holds the record. The teeth of the common limpet, used to scrape algae off rocks, are reinforced with a mineral that allows them to withstand immense pulling forces. The ultimate tensile strength of this material ranges between 3.0 and 6.5 Gigapascals (GPa), which is higher than many high-performance man-made carbon fibers.
The limpet tooth achieves this strength through a distinctive composite nanostructure. It is composed of high-volume fractions of goethite, an iron-bearing mineral, which forms reinforcing nanofibres. These mineral nanofibres are embedded within a softer protein matrix, creating a composite material optimized for maximum load-bearing capacity.
Spider dragline silk is celebrated for its superior combination of tensile strength and light weight. Dragline silk, which forms the radial spokes and frame of an orb web, has a tensile strength of up to 1.5 GPa, making it stronger than steel on a pound-for-pound basis. This protein-based fiber derives its performance from its molecular architecture.
The high stiffness and strength come from tiny crystalline regions made of beta-sheet stacks of protein subunits. These crystalline stacks are embedded within a matrix of flexible, amorphous protein chains containing glycine-rich motifs that provide elasticity. This dual-phase structure allows the silk to absorb significant energy before breaking. The silk’s ability to withstand substantial force while remaining lightweight makes it a standout material for applications requiring both strength and flexibility.
Biological Structures Designed for Extreme Toughness
While high tensile strength resists breaking under a steady pull, high toughness resists fracturing under impact, a property achieved through complex, layered composite structures. The gold standard for biological toughness is nacre, or mother-of-pearl, which forms the iridescent inner layer of certain mollusk shells. Nacre exhibits a fracture resistance almost 3,000 times greater than the brittle mineral it is primarily made of.
The structure of nacre is described as a “brick-and-mortar” arrangement, which resists cracking. The “bricks” are polygonal platelets of aragonite, a rigid form of calcium carbonate, making up about 95% of the material. These mineral layers are separated by a thin layer of organic biopolymer, the “mortar.”
When a crack attempts to propagate, it encounters the soft organic layer, which dissipates the energy. The crack is forced to deflect and meander around the staggered layers, allowing the aragonite tablets to slide slightly against one another and absorb impact energy.
Translating Nature’s Materials into New Technology
The high-performance characteristics of these biological materials are inspiring the field of biomimicry, where scientists translate nature’s designs into new technologies. The strength of the limpet tooth is driving research into creating new lightweight composite materials for high-performance engineering applications, such as components in aircraft, racing cars, and specialized protective armor.
The combination of strength and elasticity in spider silk has spurred commercial interest in developing synthetic versions. Companies are producing artificial spider silk proteins for use in advanced textiles, lightweight body armor, and medical devices like surgical sutures. Similarly, the layered “brick-and-mortar” structure of nacre is being replicated to create nacre-like ceramics and polymers. These synthetic composites aim to achieve the same balance of rigidity and toughness for use in applications like protective shields and blast-resistant components.