Olaplex vs. K18: How Bond Builders Work and What the Evidence Shows

Few hair products have generated as much salon talk as bond builders. Olaplex made the category famous in the 2010s by promising to protect hair during bleaching. K18 arrived later with a peptide it says "clicks into" hair's keratin. Both have devoted followings, both are expensive per ounce, and both make claims at the molecular level that you can't check in a mirror.

We read the patents, the brand science pages and the independent lab studies. Here's what each product is supposed to do, what has been measured and how to decide which one fits your routine.

What "bond damage" means

Hair is mostly keratin, a protein rich in the amino acid cysteine. Neighboring cysteines link through their sulfur atoms to form disulfide bonds, which act like cross-braces between protein chains and give hair much of its strength and shape ^[3].

Those bonds break under stress. Lab Muffin's chemistry explainer lists high-pH relaxers, repeated heat, sunlight and chemical treatments such as coloring among the culprits. Bleaching is especially hard on hair because peroxide can turn broken sulfur sites into forms that no longer re-link ^[3]. The result is the familiar picture of over-processed hair: stretchy when wet, rough, prone to snapping and slow to hold a style.

Dermatologists' first advice is to reduce the damage in the first place: space out color, perm and relaxer services, use conditioner after each shampoo, and turn down the heat ^[10]. Bond builders sit on top of those basics. They don't replace them.

How Olaplex says it works

Olaplex's active ingredient is bis-aminopropyl diglycol dimaleate, which the brand describes as a single patented molecule that works at the molecular level to improve hair strength ^[1]. The underlying patent, Keratin treatment formulations and methods, lays out the proposed chemistry ^[2].

In plain terms, the molecule has a reactive maleate group at each end. Chemists call these Michael acceptors, and they can react with free sulfur (thiol) groups left behind when a disulfide bond breaks. If both ends react, the molecule bridges two sulfur sites, forming an extended, artificial link in place of the lost bond ^[3]. Lab Muffin's Michelle Wong, a cosmetic chemist, found the proposed mechanism scientifically plausible after reading the patent, while noting at the time that there were no published independent trials ^[3].

The salon system uses this active at different strengths. No. 1 is mixed into lightener or color, and No. 2 follows as an in-salon treatment. No. 3 is the take-home treatment, applied to towel-dried hair and left at least 10 minutes before shampooing, and the numbered shampoos, conditioners and stylers come after it ^[3].

How K18 says it works

K18's active is a short, lab-designed peptide the brand calls K18PEPTIDE. K18 says it analyzed 1,235 peptide sequences from the human hair keratin genome to arrive at it ^[7]. On ingredient lists it appears as sh-oligopeptide-78 ^[8].

According to the brand, the peptide "uniquely matches and clicks into" hair's keratin structure. It says the peptide reconnects damage sites across polypeptide chains and disulfide bonds, restoring strength and elasticity, and that it works in four minutes on towel-dried hair without rinsing ^[6]. K18's pages describe internal testing but don't cite a peer-reviewed publication of the peptide's mechanism ^[6][7].

Side by side

What independent studies have found

The 2021 spectroscopy study

The most rigorous independent look so far comes from University of Bologna researchers, published in the International Journal of Biological Macromolecules in 2021 ^[4]. They treated bleached hair with commercial restoring products and with two model compounds that mimic the chemistry: dimethyl maleate, standing in for the maleate approach, and shikimic acid. They then examined the fibers with infrared and Raman spectroscopy and electron microscopy.

The findings were mixed:

• The treatments did interact with the hair and changed both its cortex and cuticle.
• They did not increase disulfide bridges in the cortex, though an increase in the cuticle couldn't be ruled out.
• The data did not directly demonstrate the cross-linking reaction the patents describe.
• Under the microscope, treated hair looked smoother, with cuticle scales lying flatter ^[4].

That last point matters to anyone who uses these products. Smoother cuticles reflect more light and tangle less, which fits what people report: softer, shinier hair that feels less fragile. It just doesn't prove the molecular story on the label.

The 2024 head-to-head study

A 2024 paper in the journal Cosmetics compared Olaplex No. 0 and K18 on bleached hair using Raman spectroscopy, electron and atomic force microscopy, and a chemical test called Ellman's assay ^[5]. The authors concluded that Olaplex formed more new sulfur bonds than K18, and that K18 acted mainly at the surface while Olaplex reached deeper.

Lab Muffin's detailed critique pushes back hard. Ellman's assay measures free thiol groups, not the sulfur-carbon links Olaplex is proposed to form. The two test conditions also used different reagent volumes and very different amounts of sodium hydroxide, and the study assumed a product works at only one depth ^[5]. Wong's conclusion is that the paper doesn't support the patent's disulfide-repair mechanism, and that bond builders may help through other interactions, such as hydrogen and ionic bonding ^[5].

Bottom line: Independent chemistry confirms that both product types change damaged hair. It hasn't confirmed that either one rebuilds disulfide bonds the way its marketing describes, and there is no independent clinical trial comparing the two on people.

The lilial episode, briefly

In early 2022, social posts claimed Olaplex No. 3 was being "banned" for causing infertility. The real story was narrower. Lilial (butylphenyl methylpropional) is a fragrance ingredient. EU regulators announced in 2020, following a 2019 scientific safety opinion, that it would be banned in cosmetics from March 1, 2022 ^[9]. The concern came from studies in which rats were fed lilial at 5 to 50 mg per kilogram of body weight a day. Cosmetic chemists interviewed at the time stressed that oral animal doses don't translate directly to a rinse-off hair product ^[9].

Olaplex told TODAY that lilial had been used "in small amounts as a fragrance" in No. 3 and was not an active or functional ingredient. The company said it had stopped selling lilial-containing products in the UK and EU from January 2022 and had removed the ingredient from No. 3 worldwide ^[9]. If you have an older bottle, the INCI list will show whether it contains butylphenyl methylpropional.

How to choose

Neither product has independent evidence of superiority, so match the product to how you live with your hair.

Olaplex may suit you if:

• You bleach or color in a salon and want the bond-building step built into the service.
• You like a pre-shampoo treatment and are happy to use a coordinated shampoo and conditioner.
• You want a wide range of formats, from oils to styling creams.

K18 may suit you if:

• You want one short step: a small amount on clean, towel-dried hair, left in ^[6].
• Your hair gets weighed down by multiple conditioning layers.
• You prefer not to add a separate pre-wash step.

Either way:

• Use less product than you think. Both are concentrated, and over-application mostly makes hair feel coated.
• Judge by your hair over a few weeks: fewer snapped ends in the brush, easier detangling, better curl or wave pattern.
• Keep the basics: lower heat, a conditioner every wash and spacing between chemical services ^[10]. Our guide to heat styling without damage covers temperatures and protectants.

Tip: Price per use matters more than price per bottle. A product you need only a fingertip of can cost less over a year than a cheaper one you use by the palmful. Our value scores in hair treatments are built on that math; see how we score.

Red flags in bond-builder marketing

The category has grown fast, and many products now say "bond" on the label. A few things to check:

When to see a professional

Bond builders are cosmetic products for damaged hair fibers. They don't address what happens at the scalp. If you notice patchy loss, increased shedding, scalp pain, itching or scaling, see a board-certified dermatologist rather than switching treatments ^[10]. And if hair is breaking badly after a chemical service, a stylist can judge whether the safest step is a trim and a pause.

Bottom line: Olaplex and K18 can both make over-processed hair look and feel healthier, and plenty of users swear by each. The molecular claims are better documented in patents and brand pages than in independent research, so buy for the routine you'll actually keep.