Spider silk has long captured the human imagination as a natural biomaterial. This fiber is incredibly lightweight yet possesses a remarkable durability that exceeds many synthetic materials. The unique combination of low density and high mechanical performance makes it a subject of intense scientific study. Determining which spider produces the “strongest” silk, however, requires a precise understanding of how material strength is scientifically measured.
Defining “Strongest”: Metrics Used in Silk Science
The term “strongest” is complex in materials science, requiring a distinction between two primary performance metrics: tensile strength and toughness. Tensile strength measures the maximum stress a material can endure before it breaks. This metric is measured as force per unit area and indicates how much pulling a fiber can withstand, often compared to high-grade steel. While some spider silks exhibit a tensile strength comparable to steel, they are not necessarily stronger than all engineered steels.
A more appropriate measure for spider silk’s superiority is toughness, which defines the total amount of energy a material can absorb before fracturing. Toughness is a combination of both strength and elasticity. This means a material must be strong but also able to stretch significantly without snapping. This ability to absorb massive amounts of kinetic energy, such as that from a flying insect, is where spider silk truly excels when compared to synthetic fibers like Kevlar.
The Reigning Champion: The World’s Toughest Silk
The definitive answer to the question of the strongest silk, when measured by the more encompassing metric of toughness, belongs to the Darwin’s Bark Spider (Caerostris darwini). This species, discovered in Madagascar in 2009, produces a dragline silk that is recognized as the toughest biological material ever studied. Its silk has an average toughness measured around 350 megajoules per cubic meter (MJ/m³), with some fibers reaching up to 520 MJ/m³. This makes it more than ten times tougher than a similarly sized piece of Kevlar.
The spider’s massive webs are the reason for this material superiority, as they span large bodies of water like rivers and streams. The webs can cover an area of up to 2.8 square meters and are anchored by bridge lines that stretch over 25 meters. This extreme structural demand necessitates a fiber that can withstand the high impact of large prey, like dragonflies, hitting the web at high speeds. The silk’s extraordinary elasticity allows it to stretch over 90% of its original length before breaking, enabling it to absorb the kinetic energy of the impact without failure.
The Unique Biochemistry of Super-Strong Silk
The remarkable mechanical performance of spider silk is rooted in the molecular architecture of its constituent proteins, known as spidroins. These proteins are exceptionally large, with molecular weights ranging up to 700 kilodaltons, and feature a highly repetitive central core. The structure of the silk fiber itself is a composite material, seamlessly blending two distinct phases.
The first phase consists of rigid crystalline regions formed by blocks of alanine amino acids that stack together into strong beta-sheets. These organized, hard segments are responsible for providing the fiber with its considerable tensile strength. Interspersed between these crystalline regions are amorphous, flexible sections that are rich in glycine amino acid motifs. These glycine-rich regions act like microscopic, spring-like coils, giving the silk its exceptional elasticity and extensibility.
During the spinning process, the aqueous protein solution is funneled through the spider’s silk gland. Changes in acidity and shear stress force the proteins to self-assemble into this semi-crystalline structure. The resulting composite material is a fiber that is both strong like the crystalline regions and incredibly stretchy like the amorphous ones, which is the definition of high toughness.
Real-World Applications and Synthetic Production
The superior properties of super-tough dragline silk have positioned it as a highly desirable target for material engineers. Potential applications leverage the silk’s strength, lightness, and biocompatibility, including lightweight body armor and high-performance textiles. In medicine, the non-toxic, biodegradable nature of the protein makes it suitable for use in advanced applications:
- Advanced surgical sutures
- Artificial tendons
- Frameworks for tissue engineering
- Drug delivery systems
Despite the clear benefits, harvesting natural spider silk for industrial use is impractical because spiders are territorial, carnivorous, and cannot be efficiently farmed like silkworms. This challenge has driven the development of synthetic production methods, primarily using genetically engineered microorganisms. Scientists insert the spider’s spidroin genes into bacteria or yeast, which then produce the silk proteins in large, scalable fermentation tanks.
While this recombinant production has been successful, the resulting synthetic silk fibers often lack the identical mechanical performance of the natural material. A persistent challenge involves mimicking the spider’s spinning duct. This duct precisely controls the chemical and physical conditions necessary to assemble the protein solution into a fiber with the perfect crystalline and amorphous regions.