Sericin is a naturally occurring protein produced by the Bombyx mori silkworm, the domesticated species responsible for commercial silk production. It acts as a gelatinous, adhesive substance that encases the two filaments of fibroin, the structural core of the silk thread, holding them together to form the cocoon. Sericin constitutes 20 to 30 percent of the total raw silk mass. Historically, this protein was regarded as a waste byproduct, dissolved and discarded during the textile preparation process known as degumming. Modern scientific investigation has revealed the molecule’s unique chemical structure and biological properties, leading to a resurgence of interest in its potential for high-value applications.
Sericin: The Protein Structure and Key Properties
Sericin is characterized chemically as a globular protein, distinct from the fibrous structure of fibroin, and is composed of 18 different amino acids. Its defining feature is an extremely high concentration of polar amino acids, accounting for up to 75% of its side chains. Serine is the most abundant, often comprising over 30% of the total amino acid content, which is the source of the name “sericin.”
This unique composition, particularly the numerous hydroxyl and carboxyl groups, imparts a strongly hydrophilic nature to sericin. This high affinity for water makes the protein readily soluble and highly effective as a moisturizing agent. The presence of hydroxyl groups also contributes to sericin’s antioxidant properties. These groups can chelate trace metal ions, such as copper and iron, thereby scavenging reactive oxygen species and mitigating oxidative stress.
Modern Uses in Biomedical Science and Cosmetics
The excellent water-binding capacity of sericin has made it a sought-after ingredient in the cosmetic industry for its moisturizing and anti-aging benefits. When applied to the skin, sericin forms a thin, protective film that helps retain natural moisture and enhances elasticity. Its amino acid profile closely resembles the Natural Moisturizing Factor (NMF) found in the skin, allowing it to integrate well and support the skin’s barrier function.
Beyond cosmetics, sericin’s biocompatibility and biodegradability have driven its advancement in biomedical applications. In wound care, sericin is utilized in dressings because it actively promotes the healing process. It stimulates the growth and migration of fibroblasts and keratinocytes, cells fundamental to tissue repair and collagen production.
Sericin’s multifaceted biological activity, which includes anti-inflammatory and antibacterial properties, further supports its use in wound management. This protein is also being researched for use in tissue engineering, where it serves as a scaffold material to support the growth of new tissues. Furthermore, its ability to be fabricated into various forms, such as nanoparticles and hydrogels, makes it a promising biomaterial for advanced drug delivery systems.
Recovering and Preparing Sericin for Industry
The initial step in recovering sericin for industrial use is the degumming process, which separates the protein from the core silk fibroin. Traditionally, this is achieved by boiling the cocoons in hot water or by using alkaline agents such as sodium carbonate or acids. While effective, these harsh methods can degrade the sericin protein, resulting in lower molecular weight and variable properties.
To preserve the protein’s native structure and biological activity, gentler methods are increasingly employed, including enzyme-assisted degumming using enzymes like bromelain. Once dissolved, the sericin solution, often recovered from silk factory wastewater, undergoes further purification. This typically involves centrifugation to remove solid impurities, followed by dialysis to eliminate residual salts and chemicals. The purified liquid sericin is then converted into a stable commercial powder using techniques like lyophilization or spray drying, making it ready for incorporation into various products.