Silk is built from protein. Specifically, a silk fiber from the domesticated silkworm (Bombyx mori) consists of two main proteins: fibroin, which forms the structural core of each filament, and sericin, which coats the fibroin strands and glues them together. That two-component system accounts for virtually everything in a raw silk cocoon, but the molecular details of each protein, and the biological process that converts a liquid protein solution into a solid fiber, turn out to be far more interesting than the simple label “protein fiber” suggests.
The Two Proteins in a Silk Fiber
When you look at a single silk filament under magnification, you see a pair of fibroin cores running side by side, wrapped in a sticky layer of sericin. The fibroin is the part that gives silk its strength and sheen; the sericin is essentially biological adhesive that holds the twin filaments together and bonds layers of the cocoon to one another. In a raw cocoon, sericin makes up roughly a quarter to a third of the total weight, with fibroin accounting for the rest.
Sericin is not a single uniform substance. Research on silkworm sericins has identified multiple types that vary in their amino acid makeup, how water-attracting they are, and how sticky they are. In the cocoon, sericin’s properties change from layer to layer: the adhesive strength and water affinity increase from the inner layers (closest to the fibroin) toward the outer surface of the cocoon.1PubMed. Identification and characterization of sericin5 reveals non-cocoon silk sericin components with high β-sheet content and adhesive strength Like fibroin, sericin is rich in the amino acids glycine and serine, though their exact proportions differ between species of silkworm.2PubMed. Isolation, purification and characterization of silk protein sericin from cocoon peduncles of tropical tasar silkworm, Antheraea mylitta
Inside Fibroin at the Molecular Level
Fibroin is the molecule that deserves most of the credit for silk’s remarkable properties. It is not a single chain but a complex of three components: a heavy chain weighing roughly 390 kilodaltons, a light chain of about 26 kilodaltons, and a small glycoprotein called P25 at around 25 kilodaltons. The heavy and light chains are linked by a chemical bond at the end of the heavy chain, and these paired units associate with P25 in a ratio of six heavy-light pairs to one P25 molecule.3American Journal of Polymer Science. Molecular Structure and Potential of Silk Fibroin as a Biomaterial: A Review
The heavy chain is the workhorse. Most of its length consists of repetitive stretches dominated by the amino acids glycine and alanine, interspersed with short, less ordered segments of about 30 amino acid residues each.4PubMed. Fibroin heavy chain gene replacement with a highly ordered synthetic repeat sequence in Bombyx mori Those glycine-alanine-rich stretches are the regions that fold into tightly packed crystalline sheets when the silk solidifies, while the short intervening segments stay more flexible and disordered. This alternation between rigid crystal zones and flexible amorphous zones is the fundamental architectural trick that makes silk simultaneously strong and stretchy. The crystalline regions resist pulling forces; the amorphous regions let the fiber deform without snapping.
How a Silkworm Turns Liquid Protein Into Solid Fiber
A silkworm does not simply extrude a pre-made thread. It manufactures silk proteins as a concentrated liquid solution in specialized glands, then converts that solution into a solid fiber through a carefully controlled process involving chemistry, physics, and plumbing.
The silk gland itself is divided into distinct regions. Fibroin proteins are made in the middle and posterior sections of the gland. As they move forward through the gland toward the silkworm’s head, the proteins enter the anterior section, where the silk is processed, stored, and eventually spun out.5PubMed Central. Transcriptomic Analysis of the Anterior Silk Gland in the Domestic Silkworm (Bombyx mori) – Insight into the Mechanism of Silk Formation and Spinning The gland narrows progressively along this path, and it is within that narrowing tube that the actual magic happens.
The protein starts out highly soluble at an alkaline pH. As it flows forward, enzymes and proton pumps create a pH gradient that becomes increasingly acidic. The drop in pH, combined with the physical forces of being squeezed through a progressively narrower tube, triggers a dramatic change in the protein’s shape. The silk molecules shift from loose, soluble configurations into tightly stacked crystalline sheets.6PubMed Central. Silk Spinning in Silkworms and Spiders Recent work has shown that calcium ions also play a role: they trigger a phase separation in the fibroin solution, creating protein-rich droplets that, once the pH drops below about 4.0, can be drawn out into fibrils by shear force.7Nature Communications. Calcium ion-triggered liquid-liquid phase separation of silk fibroin and spinning through acidification and shear stress
So three things happen almost simultaneously during spinning: the pH drops, metal ions help the protein concentrate into droplets, and the physical stretching through the narrow spinneret forces those molecules to align and lock into place. The result is a solid fiber that, moments earlier, was a viscous liquid. Researchers have compared the underlying mechanism to how certain disease-related proteins (like those in prion diseases) convert from soluble forms into insoluble aggregates, though in silk the process is precisely controlled rather than pathological.
Why Silk Is So Strong
Silk’s mechanical performance comes directly from its nanostructure. The fiber is a composite: stiff crystalline domains embedded in a softer, more mobile matrix of amorphous protein. This is broadly similar to how fiberglass or carbon fiber composites work, but at a molecular scale and with no synthetic resins involved. The crystalline regions are held together primarily by dense networks of hydrogen bonds between the stacked protein sheets.
Spider dragline silk takes this even further. With a higher density of hydrogen bonds and other non-covalent interactions, spider silk achieves tensile strength around 1,300 megapascals and a toughness that surpasses Kevlar by roughly sixfold.8Matter. What Is Silk Made Of? A Biological & Chemical Breakdown Silkworm silk is somewhat less impressive on those absolute numbers but remains extraordinarily strong for a biological material, and it has the advantage of being much easier to farm.
Humidity plays a surprisingly large role in silk’s behavior. In dry conditions, the amorphous regions of the fiber are essentially frozen in place, and the molecules barely move. Add moisture, and water molecules slip into those amorphous zones, acting as a plasticizer that loosens the structure and lets the protein chains shift around.9PubMed. Increased molecular mobility in humid silk fibers under tensile stress That is why silk feels different on a humid day: the fiber’s stiffness drops because water is softening the amorphous matrix.10Communications Materials. Simultaneous effect of strain rate and humidity on the structure and mechanical behavior of spider silk Spider dragline silk can even “supercontract” in high humidity, shrinking substantially as the water-loosened amorphous regions relax and coil up.
Light exposure also degrades silk over time, but in a specific way. Ultraviolet light breaks the hydrogen bonds that hold the crystalline regions together, converting those orderly stacked sheets back into disordered random coils. The crystalline fraction gradually shrinks, and the amorphous fraction grows, which is why old silk textiles become brittle and lose their luster.11Heritage Science. Study on the photodegradation behaviors of thermal-aged silk
Spider Silk Versus Silkworm Silk
Silkworms and spiders both produce silk from protein, and the broad architecture is recognizably similar: large, highly repetitive proteins stored as concentrated solutions and converted to solid fibers through pH changes and physical forces. But the details diverge in ways that matter.
Spiders are far more versatile. A single orb-weaving spider can produce up to seven distinct types of silk from different glands, each with its own protein composition and mechanical profile. Dragline silk (for the structural frame of a web) is stiff and enormously tough. Flagelliform silk (the spiral capture threads) is far stretchier and coated with sticky glue droplets. Egg-case silk is yet another formulation. Phylogenetic analysis suggests that the most recent common ancestor of all living spiders likely had a single silk protein gene, and the modern diversity arose through gene duplication events that occurred before the major spider lineages split apart.12PLOS ONE. Early Events in the Evolution of Spider Silk Genes More recent work on long-read transcripts has traced modern spider silk proteins back to two ancestral types: one rich in alanine and serine, and one rich in glycine and serine. The alanine-serine type stayed relatively conserved and gave rise to egg-case silk, while the glycine-serine type diversified extensively to produce the high-performance dragline and minor ampullate silks.13New insights into the evolution of spider silk proteins illuminated by long-read transcriptomes. New insights into the evolution of spider silk proteins illuminated by long-read transcriptomes
Silkworm silk, by contrast, comes in essentially one variety per species. The domesticated Bombyx mori produces a cocoon silk optimized for protecting the pupa during metamorphosis. It is strong but not as tough as spider dragline silk, and it lacks the capture-thread stretchiness. What silkworm silk offers instead is volume: a single cocoon contains over a kilometer of continuous filament, and the worms are easy to raise in captivity, which is why sericulture has thrived for thousands of years while spider farming has never scaled.
At the gene level, both silkworm and spider fibroin genes share the basic pattern of hierarchically organized, repetitive amino acid sequences. However, the genetic architecture differs: silkworm fibroin genes tend to be built from just two coding sections separated by a single non-coding region, while spider silk genes and sericin genes contain multiple coding and non-coding segments.14PubMed. Comparative architecture of silks, fibrous proteins and their encoding genes in insects and spiders
Caddisfly Silk and Underwater Alternatives
Silkworms and spiders get all the attention, but caddisfly larvae quietly produce one of the most unusual silks in nature: a fiber that works underwater. These aquatic insects spin silk to build protective tube cases around their bodies or to construct nets that filter food from flowing water.
Caddisfly silk contains recognizable versions of the heavy-chain and light-chain fibroin found in moth silk, but lacks the P25 glycoprotein that silkworms use to stabilize their fibroin complex.15PubMed. Protein composition of silk filaments spun under water by caddisfly larvae Instead, the heavy-chain fibroin in caddisfly silk is loaded with phosphoserine residues arranged in repeating patterns. These phosphorylated amino acids bond with metal ions like calcium absorbed from the surrounding water after the silk is extruded, forming cross-links that harden the fiber in an aquatic environment where the silkworm strategy of air-drying would obviously fail.16PubMed Central. Exploring the underwater silken architectures of caddisworms: comparative silkomics across two caddisfly suborders Research on the two main caddisfly suborders has found that while the general features of their silk are conserved across roughly 200 million years of divergence, the number and arrangement of those phosphoserine-rich blocks differ between species, suggesting the molecules have been tuned through evolution to match different aquatic lifestyles.17Trends in Genetics. Unraveling the genetics of underwater caddisfly silk
The broader picture is that silk production has evolved independently many times across arthropods. Spiders, silkworms, caddisflies, and various other insects have converged on the same general idea of spinning protein fibers but arrived at it through different genetic and biochemical routes.18PubMed. Evolution of arthropod silks
From Cocoon to Usable Fiber
Raw silk straight off the cocoon is stiff and dull, largely because of the sericin coating. Removing that coating, a process called degumming, has been done for millennia, traditionally by boiling cocoons in an alkaline solution like soap or sodium carbonate. The trouble is that degumming is chemically harsh. Not only does it strip away sericin, but it also damages the fibroin. Studies using gel analysis have shown that fibroin molecules are degraded at each step of conventional processing: during the initial reeling of the filament off the cocoon, during degumming, and again during dissolution if the silk is being prepared for further use.19Materials Science and Engineering: C. Preparation of undegraded native molecular fibroin solution from silkworm cocoons
This matters because fibroin’s usefulness in advanced applications depends on keeping its molecular weight high. An intact heavy chain is a massive molecule; once chopped into smaller fragments by harsh degumming, it loses mechanical strength and behaves differently in solution. Newer degumming approaches try to preserve more of that molecular weight. One method uses the enzyme papain, which targets specific sites in sericin without attacking fibroin as aggressively, yielding fibroin with significantly higher molecular weight and better tensile strength than conventional alkali degumming.20PubMed Central. High Molecular Weight Silk Fibroin Prepared by Papain Degumming
Even dissolving fibroin for research or biomedical use is tricky. The standard approach involves concentrated salt solutions that break apart the crystalline regions so the protein can go back into solution. Recent computational work has revealed that one common dissolution agent, zinc chloride, works not by the zinc ions directly grabbing the protein but through water molecules that are electrically rearranged by the zinc ions forming strong new hydrogen bonds with the fibroin backbone.21Colloids and Surfaces A: Physicochemical and Engineering Aspects. Dissolution of silk fibroin in zinc chloride: Molecular weight preservation and mechanism insights
Silk in Biomedicine and the Challenge of Recombinant Production
Silk fibroin has become one of the most studied natural polymers in biomedical engineering, largely because of a combination of traits that is hard to find elsewhere: it is strong, biodegradable, and remarkably well tolerated by the human body. Once dissolved into solution, fibroin can be reshaped into films, mats, hydrogels, sponges, and three-dimensional scaffolds through various fabrication methods.22PubMed Central. Silk Fibroin as a Functional Biomaterial for Tissue Engineering Researchers are exploring these formats for bone and cartilage repair, cardiovascular grafts, nerve regeneration, skin wound healing, and even pancreatic tissue engineering.23PubMed Central. Silk Fibroin Materials: Biomedical Applications and Perspectives
Spider silk, with its superior mechanical properties, would be the dream material for many of these applications, but spiders cannot be farmed efficiently. They are territorial and cannibalistic, and a single spider produces minuscule quantities of silk. This has driven extensive efforts to produce spider silk proteins in bacteria, yeast, and other organisms using genetic engineering. The proteins can be manufactured this way, but getting them to form into fibers that match real spider silk has proven stubbornly difficult. The standard method, called wet spinning, involves pushing a recombinant protein solution through a narrow opening into a chemical bath that forces the protein to solidify. The fibers that come out are typically weaker and less tough than the real thing, requiring additional stretching and treatment to improve their properties.24Trends in Biotechnology. What Is Silk Made Of? A Biological & Chemical Breakdown
The gap between lab-made and natural spider silk highlights just how much of silk’s performance comes from the spinning process itself, not just the protein sequence. A spider’s gland manages pH, ion concentrations, flow rate, and physical stretching with a precision that artificial spinning setups have yet to replicate fully. That is one reason why understanding the biology of silk production in fine detail remains an active research priority, and why the question of what silk is “made of” extends well beyond a list of amino acids into the physics and chemistry of how those amino acids are assembled into a fiber.