If we pay close attention, the elements that make up silk fibers, such as silkworms, silk excrement, mulberry leaves, and mulberry trees (including stems, fruits, and roots), are highly beneficial to humans. Each of these components serves a purpose, and silk fibers go beyond the realm of textiles; they find applications in cosmetics, pharmaceuticals, and biotechnology. What gives silk fibers these remarkable properties? Let’s dissect and explore the natural silk fiber’s structure.

When a silkworm matures, it secretes a pair of liquid protein fibers from its mouth, which, upon exposure to air, forms a pair of silk threads. The silk threads consist of two main proteins: Sericin and Fibroin. Sericin acts as a glue covering the fibroin threads, aiding in the bonding of fibroin threads into a cocoon. Here’s a breakdown:

Sericin Protein

Sericin is a water-loving protein containing approximately 18 amino acids. Among them, serine accounts for 32%, aspartic acid for 18%, and glycine for 16%, along with other compounds in lower proportions. These are familiar names in the ingredient lists of specialized cosmetics used for moisturizing and anti-aging. Sericin’s primary function is moisture retention due to its serine content. Serine, an amino acid, proves highly effective in moisturizing the skin. Skin regularly moisturized is a key factor in preventing dryness and wrinkles.

Sericin also has a structure similar to the Natural Moisturizing Factor (NMF) of the skin. Therefore, this natural conditioning agent is gentle. Some studies suggest that sericin can stimulate collagen production, heal acne scars, enhance elasticity, and contribute to improving skin tone.

Sericin is an essential component in various shampoos and conditioners, offering protection and restoration to damaged hair.

Fibroin Protein

Fibroin is the primary protein forming 75% of the silk fiber, acting as the core axis of the thread. The crystalline structure of fibroin comprises amino acids (-Gly-Ser-Gly-Ala-Gly-Ala-) repeated along the length, forming a substantial amount of beta-sheet-shaped crystal structures. The high glycine content (and, to a lesser extent, alanine) allows tight packing of these sheets, contributing to the silk’s rigid structure and tensile strength. The combination of hardness and flexibility makes it a material with applications in various fields, including biomedical and textiles.

Let’s look at the three amino acids with the highest content in silk fiber:

Serine: Used as a natural moisturizing agent in some cosmetics and skincare products.

Glycine: Found in collagen, a structural protein, and supplementing collagen has proven benefits for skin health, joint pain reduction, and preventing bone loss. Adequate glycine is necessary to support collagen production.

Alanine: A non-essential amino acid synthesized by the body, used in protein synthesis.

In fact, the crystalline structure of fibroin threads contributes to their high mechanical strength, making them challenging to decompose. This leads to certain limitations in using silk fibroin for cosmetic, pharmaceutical, and biomedical applications, as it is considered a non-degradable material according to the U.S. Pharmacopeia.

From here, we can synthesize the benefits of fabrics woven from natural silk fibers:

Silk fabric is naturally odor-resistant, anti-fungal, and mildly antibacterial.

Silk fibers have a protein structure compatible with human skin and tissues. Therefore, silk fabric, silk scarves, and silk face masks can soothe damaged skin areas, reduce inflammation, and benefit sensitive skin.

Sericin in silk stimulates collagen production, promoting radiant and firm skin, combating aging. Silk face masks, in particular, contain a significant amount of sericin.

Silk fabric has excellent sun protection capabilities.

Silk doesn’t tangle hair during sleep when used as a pillowcase, unlike other fabrics.

What makes silk fiber so naturally durable?

Silk fiber is a thread produced by silkworms feeding on mulberry leaves. It stands out as one of the most durable natural fibers, defying attempts to replicate its qualities.

For over a thousand years, silk has maintained its dominance in the textile industry, despite the emergence and disappearance of various fiber types. So, what sets silk apart and earns it such reverence? The answer lies in its exceptional durability; silk fibers are remarkably long, extending for hundreds of meters. Notably, these fibers boast a structure made up of proteins that are highly beneficial for both the skin and hair.

When a silkworm reaches maturity, it secretes a pair of liquid protein fibers through its mouth, which, upon exposure to air, solidify into silk threads. These threads comprise two main proteins: Sericin and Fibroin. Sericin acts as a sticky coating that binds the fibroin threads together, creating a strong network. Specifically:

Sericin Protein: This is a water-loving protein containing about 18 amino acids, with serine making up 32%, aspartic acid 18%, and glycine 16%, along with other compounds. These familiar names appear in the ingredient lists of specialized skincare products, known for moisturizing and anti-aging effects. Sericin’s primary function is moisture retention due to its serine content, a potent amino acid for skin hydration. Regular moisturizing is crucial for combating dryness and wrinkles.

Sericin also shares a structure similar to the Natural Moisturizing Factor (NMF) of the skin. Therefore, products containing sericin contribute gentle and natural hydration. Some studies suggest that sericin can stimulate collagen production, aiding in scar healing. Additionally, it enhances elasticity and plays a role in improving skin tone.

Sericin is a vital component in various products such as shampoos and conditioners, contributing to hair care by preserving and restoring damaged hair.

Fibroin Protein: Fibroin makes up 75% of the silk fiber and acts as the core structure of the thread. Its crystalline structure consists of repeating amino acid sequences (-Gly-Ser-Gly-Ala-Gly-Ala-), forming numerous beta-sheet-like crystalline structures. The high glycine content (and, to a lesser extent, alanine) allows tight packing of these sheets, contributing to the rigid structure and tensile strength of silk. The combination of stiffness and flexibility makes silk a material suitable for various applications, including both medical and textile manufacturing.

In fact, the highly crystalline structure of fibroin silk fibers makes them mechanically robust, rendering them resistant to degradation. This property poses certain limitations in using fibroin silk for cosmetic, pharmaceutical, and biomedical applications, as it is considered a non-degradable material in the U.S. Pharmacopeia. In summary, the crystalline structure of fibroin contributes to the exceptional durability of silk fibers, making them an ideal natural material for textiles