Sericin, long dismissed as the sticky coating on silk fibers, actively organizes silk fibroin into ordered nanofibrils through liquid-liquid phase separation, researchers in Zhejiang and Hong Kong report.

For thousands of years, people have prized silk for its strength, luster, and versatility. Today, silk proteins are showing up in far less traditional places — flexible bioelectronics, sustainable water-treatment materials, and 3D-bioprinted tissues. Yet one basic question about silk has lingered for centuries: how does a silkworm turn a liquid protein solution stored in its silk glands into one of nature’s toughest fibers without the protein solidifying too early?²

A new study published in Science Bulletin suggests that part of the answer lies with an ingredient long treated as an afterthought. Researchers from Zhejiang University and the Chinese University of Hong Kong report that silk sericin, the protein that coats silk fibers like glue, actively directs how the fiber’s structural protein assembles.^1,2

Two proteins, one fiber

Silk is made of two main proteins. Silk fibroin forms the strong structural core of the fiber. Silk sericin wraps around the fibroin. For decades, sericin was viewed as a passive, secondary component — useful for sticking fibers together, but not much more. Scattered evidence hinted at a bigger role. Higher sericin concentrations seemed to slow fibroin aggregation, for example, but contradictory reports left the picture muddy.²

To sort it out, a team led by Professors Mingying Yang and Yajun Shuai at Zhejiang University, along with Professor Chuanbin Mao at the Chinese University of Hong Kong, built a biomimetic system that recreated the chemical environment inside a silkworm’s silk gland during the fifth larval instar, the stage when silk production peaks. Using regenerated fibroin and sericin, they tracked how the two proteins interacted over six days of incubation.²

Droplets as reaction chambers

Under salt-assisted conditions that mimicked the weakly alkaline environment of the silk gland, sericin promoted liquid-liquid phase separation, a process in which a protein solution spontaneously splits into dense, liquid-like droplets called coacervates. Cryo-electron microscopy showed these droplets stayed fully hydrated and dynamic, able to fuse, bud, and reorganize within seconds. Confocal imaging revealed a core-shell structure, with fibroin concentrated at the center and sericin around the edges — a natural reaction chamber.²

Liquid-liquid phase separation has become one of the hottest topics in cell biology over the past decade, as researchers have realized that cells use it to create membrane-less compartments that concentrate specific molecules. Finding the same physics at work in silk production ties an ancient material to a frontier of molecular biology.

Even without added salts, sericin quickly formed aggregate compartments that captured fibroin nanoparticles. Over six days, the confined fibroin matured into highly ordered, parallel-aligned nanofibrils. Their beta-sheet content — the stacked protein structure responsible for much of silk’s strength — nearly tripled, rising from about 11.2 percent to 27.1 percent, and their crystallinity increased markedly.²

When the researchers applied mechanical shear, mimicking the forces silk proteins experience as they are squeezed through the silkworm’s spinneret, the compartments broke apart and reorganized, producing ultra-long nanofibrils.²

From lubricant to regulator

The study revises the textbook view of sericin. Rather than a passive lubricant, the authors argue, it is an internal regulator with two jobs. First, by confining fibroin within compartments, it concentrates and stabilizes the protein, preventing it from clumping prematurely and chaotically. Second, through the maturation of those compartments, it guides fibroin nanofibrils into an orderly, aligned arrangement.² That dual role helps connect what happens at the scale of individual molecules to the properties of the finished fiber.

“Sericin is far more than glue—it’s the conductor of a symphony,” said Shuai, the study’s corresponding author. “Our work shows that by creating confined, dynamic compartments, sericin provides the spatial and temporal control needed to transform disordered protein solutions into the exquisite hierarchical structure of silk.”²

A blueprint for engineered materials

The implications extend beyond silkworms. The researchers describe a multi-stage sequence of phase transitions — from dissolved individual proteins, to coacervates, to aggregates, to gels, and finally to ordered fibrils — that lets nature avoid messy, disordered precipitation by stepping down the energy landscape through a series of intermediate states.²

The same principles, the team suggests, could guide the design of synthetic protein fibers, smart hydrogels, and hierarchically structured biomaterials for tissue engineering and other applications.² “Understanding this natural assembly line opens the door to engineering materials with similarly elegant architectures,” Shuai said.²

Silk has been studied for as long as it has been worn. That one of its two main ingredients was misunderstood for so long is a reminder that even the most familiar materials can still hold basic surprises.

---

Notes

1. Science Bulletin (2026). https://doi.org/10.1016/j.scib.2026.06.045
2. Science China Press. (2026, July 23). Silkworm silk’s hidden conductor: How sericin orchestrates fibroin into ordered nanofibrils [Press release]. EurekAlert! https://www.eurekalert.org/news-releases/1137492