Hydrolyzed Silk

Hydrolysed silk is a water-soluble protein ingredient made from the cocoon fibre spun by the silkworm, most commonly Bombyx mori. Raw silk fibre is built from two proteins: fibroin, the strong structural core, and sericin, the gum-like coating that binds the cocoon together. Fibroin makes up the bulk of the cocoon, with sericin accounting for most of the remainder alongside small amounts of waxes, pigments and sugars. In its natural state silk is far too large and too insoluble to be of any use in a serum or a cream, so it is broken down using acid, alkali or enzymes. That breakdown is the hydrolysis the name refers to, and what it leaves behind is a mixture of short peptides and free amino acids that dissolves readily in water.

On a label it appears as Hydrolyzed Silk — INCI convention keeps the American spelling — and you may also meet closely related entries such as Silk Amino Acids, Sericin and Hydrolyzed Sericin, which differ mainly in which part of the fibre they are taken from. The EU cosmetic ingredient database classifies it as a skin conditioning agent, humectant, hair conditioning agent and antistatic agent, which is a fair summary of the work it does. It is a comfort-and-texture ingredient rather than a headline active, and it turns up in serums, essences, sheet masks, mists, light creams, cleansers and a great deal of hair care. Because it comes from silkworm cocoons it is animal-derived, and so not vegan.

Two things happen once hydrolysed silk is spread across the skin. First, the peptides and free amino acids it contains are hygroscopic: they attract water molecules and hold onto them. Silk proteins are unusually rich in the small amino acids glycine, alanine and serine, and serine is one of the recognised components of the skin's own natural moisturising factor — the mixture of tiny water-binding molecules in the outermost layer that keeps skin looking supple. Putting similar molecules onto the surface tops up that water-holding capacity while they remain there.

Second, as the water in the formula evaporates, the protein fragments are left behind as an extremely thin, flexible film. That film gives silk its signature slip and soft, powdery finish, and slows the rate at which moisture escapes from the surface. The fragments carry both positive and negative charges, which is why they cling to keratin.

It is worth being honest about the limits. Peptides of this size sit at or very near the surface, so the effect is temporary and fades as the film is washed or worn away. Small in vivo studies on silk sericin gels have reported improved surface hydration and reduced water loss, but the human evidence for hydrolysed silk itself is thin, and several published trials tested it inside multi-ingredient complexes.

Hydrolysed silk is one of the easiest ingredients to slot into a routine, because it is water-soluble, low-irritation and has no pH requirements of its own.

• Use it in the water phase. It works best in the lightweight steps — toners, essences, hydrating serums, mists and sheet masks — applied to skin that is still slightly damp.
• Seal it in. On its own the film is very light, so follow with a moisturiser containing occlusives or emollients if your skin is genuinely dry.
• Layer freely. There are no known conflicts with niacinamide, hyaluronic acid, ceramides, peptides, vitamin C, exfoliating acids or retinoids, so it can sit anywhere in a routine.
• Morning and evening is fine. Nothing about it limits how often you use it.
• Check where it sits on the list. It usually appears in the lower half of an ingredients list, which suggests a modest inclusion level — treat it as a supporting player, not the reason to buy a product.
• Great under make-up. The smooth, non-greasy finish helps base products sit more evenly.
• Patch test on the inner forearm for a couple of days if your skin is reactive or you have multiple allergies.