Bond Builders and Hair Proteins: What Actually Repairs Damaged Hair?
Bleach, colour, heat tools, sun, swimming pools: hair goes through a lot. And since hair is no longer alive once it leaves the scalp, it can't heal itself. That's why "bond repair" and "protein treatment" have become two of the biggest phrases in hair care. Let's look at what's really going on inside a strand.
What's inside a hair strand
Hair is mostly keratin, a protein whose chains are held together by several kinds of bonds. The strongest are disulfide bonds, sulfur-to-sulfur bridges between cystine units. Think of them as the steel cables. Weaker hydrogen and salt bonds act more like Velcro: they break when hair gets wet and re-form as it dries.
What bleaching does
Bleach (hydrogen peroxide) has to get into the cortex to lighten pigment, and on the way it does collateral damage. A study using electron microscopy and protein analysis showed bleaching turns cystine disulfide bonds into cysteic acid, and causes protein loss that grows with bleaching severity (Grosvenor et al., International Journal of Cosmetic Science, 2018). Broken cables mean weaker, more porous, frizzier hair.
Bond builders: what does the research say?
Many bond-building products are based on maleic acid derivatives, marketed as re-linking broken bonds. Independent research paints a more nuanced picture:
- A University of Bologna study found that maleate-type repair agents did not increase disulfide content in the cortex, and found no direct evidence of the claimed cross-linking reaction, though they did see a smoother cuticle (Di Foggia et al., 2021).
- A 2024 study comparing commercial repair products on bleached hair found benefits, but attributed them to non-specific bonding rather than true disulfide repair (Martins et al., Cosmetics, 2024).
What this means: bond builders can make bleached hair feel and behave better, especially during colouring. But "rebuilds broken bonds" is a big claim, so keep your wording to visible results like "helps reduce breakage" or "strengthens the feel of damaged hair", supported by your own testing. Our Bond-Building Agent (Maleic Acid) comes with supplier guidance on use level and pH.
Proteins: size changes everything
Hydrolysed proteins are proteins chopped into smaller pieces. How small decides where they go. A study on textured hair found small keratin peptides (around 0.2–2.6 kDa) penetrated deep into the cortex, while larger ones (~75 kDa) mostly stayed on the surface (note: run by a testing institute with an ingredient-company co-author) (Malinauskyte et al., 2021).
- Small proteins and amino acids work inside the fibre: Silk Amino Acids, Hydrolyzed Keratin.
- Larger proteins coat and smooth the surface and add body: Hydrolyzed Wheat Protein, Hydrolyzed Rice Protein, Hydrolyzed Quinoa.
- Vegan option: Vegan Keratin Alternative.
Protein overload is real. Very protein-heavy routines can leave some hair, especially fine or low-porosity hair, feeling stiff and straw-like. Balance protein with moisture and conditioning.
A repair mask blueprint
- Conditioning base: BTMS with Cetearyl Alcohol
- A blend of small and large proteins
- Moisture: Panthenol and Glycerin
- Slip and shine: a little Amodimethicone (it targets damaged areas) or Argan Oil
- pH around 4–5, which helps keep the cuticle flat, plus a broad-spectrum preservative
Pair it with the right scalp routine from our scalp care post and gentle oiling from our champi post.
Explore Proteins & Hydrolysates, Hair Conditioning Polymers & Quats and Shampoo & Conditioner Ingredients.