What is Silk Protein?

Silk, a luxurious and prized fabric, has captivated humankind for centuries. While its beauty and sheen are well-known, the complex biological structures that give silk its remarkable properties are often overlooked. At the heart of this wonder lies silk protein, which is not just one substance, but primarily two key proteins: sericin and fibroin. Understanding these proteins provides insight into the diverse characteristics and applications of silk, from luxurious textiles to innovative biomedical applications.

1. Sericin: The Sticky Outer Layer

Sericin is a glue-like protein that coats the silk fibroin fibers. It makes up roughly 20-30% of the silk produced by the silkworm (Bombyx mori). Unlike the structured nature of fibroin, sericin has a more amorphous, irregular structure. It is primarily characterized by its high proportion of polar amino acids like serine and aspartic acid, making it soluble in hot water and enabling the removal process called “degumming.”

Here’s a more detailed breakdown of sericin:

The role of sericin is not only to provide a sticky casing. It acts as a protective agent for fibroin, shielding it from external stressors and acting as a scaffold within the silk cocoon. The presence of hydroxyl and carboxyl groups in its molecular makeup means it can absorb water, therefore improving moisturization. Sericin has demonstrated remarkable skin hydration and wound-healing properties. While historically discarded as a byproduct of the silk production process, scientists and innovators now recognize the vast potential of sericin in various industries, such as in cosmetics, pharmaceuticals, and even advanced biomaterials.

2. Fibroin: The Strong Inner Strand

Fibroin constitutes the majority of silk and is the core component that gives silk its exceptional strength, durability, and shine. It’s comprised of primarily repeated amino acid sequences such as glycine, alanine, and serine, allowing it to fold into a beta-sheet crystalline structure. Fibroin has very specific arrangements that make it strong and rigid, enabling it to maintain high strength at both high and low temperatures and makes it flexible when subjected to shear stress. It accounts for around 70 to 80% of the weight of the raw silk.

Fibroin’s remarkable strength-to-weight ratio, and high tensile strength is attributable to its robust molecular structure and tight hydrogen bonds. Fibroin’s beta-sheet arrangement gives it stability and rigidity while it can withstand high tension and stress due to hydrogen bonds and molecular elasticity, giving it its tensile properties. Unlike sericin, fibroin is highly resistant to breakdown, with its crystalline beta-sheets preventing penetration by solvents and enzymes, and therefore very good for a vast array of applications ranging from fabrics and wound dressings to biomedical scaffolds. The unique arrangement of its amino acid sequence forms into crystalline areas separated by amorphous regions, further conferring stability and flexibility to its final application.

3. Extraction and Processing

The processing of raw silk, notably for textiles, traditionally focuses on extracting the highly valued fibroin while removing the sericin layer. This degumming process, which is most commonly done with hot water, alkali solutions or enzymatic solutions removes sericin while the water is further treated for environmentally safe methods of reuse and treatment. Following this extraction process the extracted fibroin is processed by spinning and weaving techniques into various forms, and its properties tailored to end product such as luxurious fabrics, apparel and wound dressings. Sericin, once considered a waste product, now increasingly utilized for its various therapeutic properties, such as it use in cosmetics and medical industry, through purification methods or encapsulation, further reducing the textile production wastage, to meet modern industry goals and sustainable development initiatives. This efficient management of silk’s protein fractions improves production process management by offering a sustainable end-to-end approach.

4. Applications Across Industries

Silk proteins have been a marvel of textile applications for centuries, primarily through fibroin-based luxurious silk fabrics like PandaSilk. Yet these versatile materials now stretch well beyond their role in textiles:

• Biomedical Applications: In the medical field, silk fibroin’s biocompatibility and non-toxic characteristics allow its use in tissue engineering for scaffolds in wound dressings, bone regeneration, drug delivery and medical sutures, whilst sericin is used for its wound healing properties and also its anti-inflammation applications.
• Cosmetics: The high moisture retention capacity of sericin and skin penetration properties along with it antioxidant features make sericin well-suited for use in many skincare applications and anti-ageing formulas as well as hair products. Sericin gives hair a natural sheen by enhancing hair protein’s ability to bind water and reduce static and friction to improve ease of styling.
• Other Advanced Materials: Given silk’s molecular strength it provides advanced use cases including flexible electronics and biosensors. Silk fibroin’s mechanical resilience, combined with ease of fabrication into thin films and coatings has helped make new materials with unique properties.

In summary, the power of silk is more complex than its soft surface; it relies on the complementary properties of its major structural components, sericin and fibroin. Fibroin provides silk with tensile strength while sericin adds beneficial hydration, making the material ideal for the development of luxurious materials, and sustainable practices in textiles and beyond. Ongoing innovation and exploration continue to drive new, sophisticated applications of these unique biomaterials, and the industry constantly seeks more innovative sustainable and economically sound practices for extracting silk proteins that contribute to responsible manufacturing of all downstream applications and help reduce impact of production process.