Bond builders (Olaplex, K18, maleic acid, "acidic bonding")

"Bond builder" (also "plex", "bond multiplier", "bond repair") is a marketing category, not a chemical class. It covers at least four unrelated chemistries that all claim to protect or restore the internal bonds of hair damaged by bleach, colour, perms, relaxers or heat. The evidence for all of them is laboratory work on cut hair, mostly produced or funded by the companies that sell them. We found no published controlled study on people's heads for any of them.

Key takeaways

• Bleach damage is mostly irreversible chemistry. Bleaching turns the disulfide bonds of cystine into cysteic acid, mainly in the cortex, and washes protein out of the fibre in proportion to severity. No cosmetic can turn cysteic acid back into a disulfide. Grosvenor 2018; Kuzuhara 2006
• Olaplex's claimed mechanism, maleate "bridges" re-linking broken disulfides, is unproven. It comes from the patent. The only independent spectroscopy test found no increase in cortical disulfide bonds and "no direct evidence" of the claimed reaction, although treated hair looked smoother under the microscope. Di Foggia 2021
• Olaplex still strengthens bleached hair in the lab. One independent university study found a single leave-in application of Olaplex No. 0 raised the tensile strength of bleached fibres by 47.6% versus untreated bleached hair (significant), while K18's mask gave +19.5% (not significant). Only 3 tresses per group were used. Martins 2024
• A more plausible mechanism is ionic. Bleached hair is full of negatively charged sulfonate groups, and molecules with several positive charges, such as the diamine in Olaplex, can bind to them like staples. Kang 2025
• K18's peptide has a real academic origin (University of Minho) but little independent testing. The one independent comparison found K18 acted mostly as a surface coating. Martins 2024
• "Acidic bonding" (citric acid) has a mechanism paper, from its own maker. L'Oreal researchers reported that citric-acid post-treatments raised fatigue resistance of chemically treated hair by 56-124% and cut calcium in the fibre by 21-42%, probably through calcium chelation, low pH and reinforcing weak bonds rather than forming new covalent bonds. Zhang 2025
• Bond builders reduce harm; they do not make bleaching safe. Lifting less, using lower developer strength and avoiding overlap matter more (see bleach and colour damage).
• Using two bond builders together "for complementary mechanisms" has never been tested.

What actually breaks

Hair keratin is held together by covalent disulfide bonds and isopeptide bonds plus many weaker hydrogen bonds, salt bridges and hydrophobic interactions.

• Bleaching (oxidation): alkaline hydrogen peroxide plus persulfate reaches the cortex quickly, destroys melanin, oxidises sulfur amino acids and leaches cuticle and cortex protein. Grosvenor 2018 Raman spectroscopy shows the disulfide signal falling and the cysteic-acid signal rising through the whole cross-section, more so in dark hair. Kuzuhara 2006 Bleaching also strips the fatty outer layer and leaves a surface dominated by negatively charged sulfonate groups, about 2.2 per square nanometre. Korte 2014
• Perming (reduction): thioglycolate breaks disulfides into thiols and the neutraliser re-forms them. Disulfides can reform after reduction, but not after oxidation to cysteic acid.
• Relaxers use a very high pH to rearrange the hair's cross-links permanently (see relaxers, perms and smoothing).

The four chemistries

Olaplex and bis-aminopropyl diglycol dimaleate

• The patent says agents with maleic-acid-type groups form a carbon-sulfur covalent bond with free thiols in hair. Its examples are qualitative ("more shine", "little or no signs of breakage").
• What is in the bottle: a competitor's lab (Coty) measured 9.15% of the diamine in Olaplex No. 1. In a model bleach, about 17.5% of the diamine degraded over a 40-minute service into a by-product the authors consider relevant for safety evaluation. Gargano 2018
• Independent test 1 (Bologna): infrared and Raman spectroscopy on bleached hair treated with commercial maleate products. None increased cortical disulfide content, and "none of our results can be interpreted as direct evidence" of the claimed cross-linking. Di Foggia 2021
• Independent test 2 (Porto): Olaplex No. 0 applied once as a leave-in gave +47.6% tensile strength on single bleached fibres (p < 0.05). A chemical test found more sulfur-linked bonds, but the Raman disulfide band did not rise. Only 3 tresses per condition. Martins 2024
• Why the thiol story is shaky: the patent mechanism needs free thiols, but bleaching produces mainly cysteic acid, not thiols. Free thiols exist mainly in reduced hair (perms, relaxers), which is where newer thiol-based chemistry is being developed. Liu 2026; Liu 2026
• The ionic alternative: a 2025 Korean industry-university study used the same diamine as its starting "hair-rebonding compound" and modified it to penetrate deeper, which improved tensile strength. Kang 2025 Our inference is that Olaplex's measured strengthening may come from ionic cross-linking and water-content changes, not re-formed disulfides.
• Newer thiol-reactive cross-linkers made in Lebanon and Kuwait outperformed a reference bond builder in tensile tests on cut hair. El Khatib 2025

K18 peptide

• K18 says its peptide technology came from Prof. Artur Cavaco-Paulo's group at the University of Minho. The licensed patent covers keratin-derived peptides of 6-12 amino acids with 2-5 cysteines. Its examples show stiffer bleached and relaxed hair, but these are inventor data without statistics. The commercial sequence is not public.
• The group's peer-reviewed work screened 1,235 keratin-derived peptides and found binding driven by cysteine and hydrophobic content Cruz 2017, showed a keratin peptide strengthened relaxed hair in vitro Fernandes 2012, and found small peptides penetrate damaged hair better than large cysteine-rich proteins, which bind mainly to the cuticle. Carvalho 2026 Charged peptides penetrate only damaged hair. Silva 2007
• Independent test: K18's leave-in mask (which also contains behentrimonium chloride, cetearyl alcohol and hydrolysed wheat protein) gave +19.5% tensile strength (not significant) and formed "an external layer above the fiber". Martins 2024

Maleic acid products

Maleic acid is the same reactive group as in Olaplex, without the diamine. The US cosmetic ingredient review covered it only as a low-level pH adjuster, and 20% maleic acid applied daily for 6 weeks caused acute dermatitis in 17 of 50 volunteers. CIR 2007 We found no peer-reviewed efficacy study of maleic-acid bond builders on bleached human hair. See maleic acid.

"Acidic bonding" and other organic acids

• Citric acid (L'Oreal data): applied after high-lift bleach, mid-lift bleach, permanent colour, or perm plus colour, it raised keratin denaturation temperature by 6.7-15.0%, stiffness by 7.5-23.5% and median cycles to break by 56-124%, and cut fibre calcium by 21-42%. The size of the effect depended on the prior damage. Zhang 2025 This is manufacturer research, but it is the most mechanistic paper on any bond-builder category, and it suggests "acidic bonding" works partly by removing hard-water minerals (see acidic rinses and pH).
• A typical "acidic bonding" product lists silicones, a fatty alcohol and cationic polymers ahead of citric acid, so much of the feel is classic conditioning with an acid on top (inference from a manufacturer's ingredient list).
• Itaconic acid complex (ingredient supplier data): a blend of itaconic acid, arginine, panthenol and plant polysaccharides improved break stress, shine and combing when added to bleach. Camargo 2025

Why lab wins may not show on a head

• Studies use single-fibre tensile tests, cyclic fatigue (cycles to break), calorimetry, Raman spectroscopy and electron microscopy.
• Fatigue testing, which applies many small repeated loads, finds bigger and more realistic differences than a single pull to break, because real breakage comes from repeated grooming. Evans 2009; Evans 2010
• In straight hair, combing breakage is controlled mostly by friction, so any good conditioner cuts early fragment formation. A "less breakage" claim does not prove internal repair. Davies 2026
• None of the studies above is randomised on people, blinded on heads, or long term.

What it means for your hair

• Bleached, highlighted or high-lift blonde hair (any curl pattern) is the main use case, and where the lab data are strongest.
• Relaxed or permed hair contains reduced or lanthionised cystine. Peptide and thiol-based approaches were designed for it Fernandes 2012, but there is no head study.
• Heat-styled but uncoloured hair: no bond-builder evidence; heat protectants and lower temperatures are better supported (see heat damage).
• Virgin low-porosity hair has no reason to use one.
• In hard-water areas, acid-based products may help partly by removing calcium.

Myths and facts

Bottom line

Before bleaching, a bond-building additive in the lightener is low risk but supported only by lab data. Fewer lifts, lower developer strength, no overlapping and longer gaps between sessions do more. After chemical services, a weekly bond-builder or acid-based treatment is reasonable on bleached or relaxed hair. Budget option: an acidic conditioner containing citric acid. Mid-range: a product such as Redken Acidic Bonding Concentrate or Olaplex No. 3. Premium: K18 or salon Olaplex No. 1 and 2. No head-to-head data justify the price differences. Do not skip ordinary cationic conditioner, which has far more data behind it (see how conditioner works). Patch-test maleic-acid products if your scalp is sensitive. CIR 2007