Bond Repair Treatments: We Know What the Brands Say, What Does the Science Say? by Charlie Westphal

If you have ever bleached, colored, relaxed, flat-ironed, or even just brushed your hair too aggressively, you have probably seen the phrase “bond repair” somewhere in your feed. In the past five years, bond repair has moved from a niche salon treatment to a category now with shelves of products promising to “rebuild,” “restore,” or “relink” the bonds inside your strands. Some of the most familiar treatments are from brands such as K18, Olaplex, Epres, Living Proof, Curlsmith, Innersense, and As I Am.

The question worth actually asking is what is inside those bottles, and which products do what they say. The science of how hair bonds work and how they can (and cannot) be repaired is more interesting, and a little less tidy, than the marketing suggests. Independent, peer-reviewed studies of these products are rare, and the few that exist do not always agree with brand claims. This article walks through the science of hair bonds, the two distinct categories that get marketed under the same “bond repair” umbrella, treatments from the seven products above, what the published research actually shows, and the personal factors that should shape which one (if any) belongs in your routine.

The Science of Hair Bonds

Hair is roughly 90% keratin, held together by three primary kinds of bonds : hydrogen, ionic, & disulfide bonds. Hydrogen bonds, which are weak — break and reform with water and heat, which is how wetting hair and blow-drying it can temporarily change its shape. Ionic, or salt, bonds break and reform with shifts in pH, which is why alkaline products like high-pH shampoos disrupt them. Disulfide bonds are different. They are covalent links between cysteine amino acids on adjacent keratin chains, and they are what give hair its tensile strength and its natural curl pattern. They can be broken by chemical processes like bleach, relaxers, and by extreme heat. Unlike hydrogen and ionic bonds, disulfide bonds do not reform on their own. Rebuilding the connection requires a separate molecule that can react with the damaged sulfur and form a new covalent link in its place.² 

Hair damage is, at the molecular level, the breaking of these disulfide bonds. Bleach and color services oxidize disulfide bonds into cysteic acid, which cannot spontaneously reform.¹ Heat styling, mechanical stress, and UV exposure compound that damage. Chemical relaxers, used predominantly by women of African descent, work by deliberately breaking disulfide bonds so that the hair can be reshaped into a straighter pattern.

The goal of a bond repair product is, in theory, to reverse that molecular damage. The question is whether the chemistry inside the bottle can actually do it.

Two Categories That Get Marketed Under the Same Name

Almost every product currently sold as a “bond repair” treatment falls into one of two categories, and the distinction matters more than the marketing usually admits.

The first category is true covalent-bond chemistry. These are products built around a specific molecule that is intended to form new covalent bonds inside the hair shaft. They are the products that can, at least in principle, address the actual molecular damage from bleach, color, and chemical processing. Olaplex, Epres, K18, and Living Proof Triple Bond Complex all sit in this category, though each uses a different active ingredient, and not all of them work the same way.

The second category is protein and botanical strengtheners that are marketed in the bond-repair conversation but rely on different mechanisms. These products typically use hydrolyzed proteins (wheat, moringa, keratin, wool peptides), amino acids, and conditioning agents to fill in damaged areas of the cuticle, smooth the hair surface, and improve tensile strength without forming new covalent bonds. Innersense, Curlsmith, and As I Am all fall here. This category can be useful, but it is doing something fundamentally different from bond repair.

Covalent-bond products are designed to address the strongest, hardest-to-repair damage, while protein and botanical strengtheners are closer to high-quality conditioners that support hair from the outside. Both can be useful. They are not interchangeable.

Tier One: Products with Covalent-Bond Chemistry

Olaplex is the original. It was developed in 2014 by chemists Eric Pressly and Craig Hawker and is built around a single patented molecule, bis-aminopropyl diglycol dimaleate (BADDM). The molecule works as an acceptor in what chemists call a Michael addition reaction: it reacts with the free sulfur groups left behind when disulfide bonds are broken, forming new covalent links between keratin chains.¹ The brand sells a multi-step system (No.1, No.2, and others) intended to be used together, which is part of why the regimen can feel involved and expensive.

Independent peer-reviewed data on Olaplex is limited but supportive on this specific point. A 2024 study in Cosmetics compared Olaplex and K18 on standardized bleached hair samples and found that both products improved hair brightness and lightness compared to bleached controls.¹ Where the two products diverged was on what happened inside the hair shaft. Olaplex produced a significant increase in measurable disulfide-type bonds (p < 0.0001 versus the bleached control) and penetrated into the cortex.¹ K18, by contrast, did not produce a meaningful disulfide-bond signal on Raman spectroscopy and instead deposited as a coating on the surface of the cuticle rather than penetrating the hair shaft.¹ The K18 product does smooth and protect the hair surface, and provides optical improvements, but the mechanism appears to be different from what the brand's marketing implies.  One important caveat from the same study is that the bonds Olaplex forms are not strictly the original disulfide bonds but a broader class of S–R (sulfur-to-other-atom) covalent links — a new bond forms between sulfur and a piece of the Olaplex molecule itself, rather than directly between the two original sulfur atoms that were broken. So the brand's diagrams of "rebuilt disulfide bridges" are a simplification. The peer-reviewed evidence supports the formation of new covalent bonds where old ones were broken; it does not show that the original disulfide bonds themselves are restored. The new bonds appear to perform the same structural job of holding adjacent keratin chains together.  

Epres was launched in 2022 by Eric Pressly, the same chemist who invented Olaplex. The active molecule is different (diethylhexyl maleate rather than BADDM), but the underlying maleate chemistry is closely related. The pitch is essentially “Olaplex, simplified”, a four-ingredient, waterless concentrate that mixes with water at home, applied as a single 10-minute treatment rather than a multi-product regimen.

There is no independent peer-reviewed study of Epres comparable to the 2024 Olaplex/K18 paper, so claims about its efficacy currently rest on the brand’s own clinical data and on the fact that its inventor designed the original mechanism. If you find Olaplex’s multi-step routine unmanageable but want to stay in the maleate-chemistry category, Epres is the simpler version of the same idea. The trade-off is that it is newer and less independently studied.

K18 takes a different approach. Its active ingredient is a patented peptide (INCI: sh-oligopeptide-78), a short chain of amino acids designed to insert into damaged sections of the keratin chain and “click” into place, restoring some of the original structure. Unlike Olaplex and Epres, which target only disulfide bonds, K18 markets itself as reconnecting both disulfide bonds and broken polypeptide chains (long strands of amino acids linked by peptide bonds) ¹³. 

The 2024 Cosmetics study did test K18 head-to-head with Olaplex and found a meaningfully different result. K18’s lipophilic (oil-loving) ingredients caused the formula to deposit as an external coating on the hair surface rather than penetrating the cortex.¹ Ellman’s test, which measures sulfur-containing bonds, did show some new bond formation with K18, but at a substantially lower level than Olaplex, and Raman spectroscopy did not detect the specific disulfide-bond signal that the brand’s marketing implies. The product does appear to smooth and protect the hair surface, just not in the way the marketing copy describes. K18 is also among the most expensive products on this list per ounce.

Living Proof Triple Bond Complex is one of the newer entrants in the covalent-bond tier. The brand describes a patent-pending “3D fortifying technology” that claims to build new connections across all three bond types (disulfide, hydrogen, and ionic) rather than focusing on one. Clinical claims from the brand include eight times stronger hair after one use and heat protection up to 450°F.

Independent peer-reviewed validation of Triple Bond Complex specifically is limited, which is a recurring problem in this category. The technology is patented (so the broad chemistry is on the record), but as with Epres, the published efficacy data largely come from the brand. What makes Triple Bond Complex worth including alongside Olaplex, Epres, and K18 is that it is one of the few products that explicitly markets itself as targeting more than just disulfide bonds, which is useful framing for damage that is not strictly chemical (like heat styling, weather, and mechanical breakage).

Tier 2: Products with Protein and Botanical Strengtheners

Innersense launched its Repair Mask in 2025, marketing it as the “first clean mask with disulfide bond repair technology.” The active ingredient is hydrolyzed moringa protein, supported by vitamin C and tamanu oil. There is peer-reviewed evidence that hydrolyzed proteins of appropriate molecular weight can penetrate the hair cortex and reduce breakage. A 2021 study in the International Journal of Cosmetic Science tested hydrolyzed keratin peptides on relaxed African-descent hair and found that low- and mid-molecular-weight peptides penetrated deep into the cortex, and that mid- and high-molecular-weight peptides reduced breakage at both 20% and 80% humidity.³ The Innersense formula is in line with that research. The claim that it specifically rebuilds disulfide bonds, though, is a marketing leap. This is a sophisticated protein treatment, not a covalent-bond product in the Olaplex sense. 

Curlsmith Bond Curl Rehab Salve is a pre-shampoo treatment built around chia seed protein, creatine, and an array of plant extracts (shiitake, prickly pear, pumpkin, guava, kale). The brand markets it as targeting “all three types of bonds,” but the chemistry is firmly in the protein and conditioner category rather than the covalent one.

The product has a reasonable scientific underpinning, creatine and amino acid-rich extracts can support hair-shaft strength, but it is more accurately a strengthening and conditioning treatment that performs in the same category as a high-end protein mask. If you are choosing it because you specifically want covalent disulfide-bond chemistry, the label’s use of the word “bond” may be doing more work than the chemistry actually does.

As I Am Bond sits at the intersection of two categories. The Bond line (which includes a mask, shampoo, conditioner, leave-in, oil, and water) is built around a patent-pending "Bond Complex Technology" that the brand describes as penetrating the hair fiber to reconnect broken disulfide bonds caused by chemical processing and mechanical damage. That is a covalent-bond claim, in the same language as Olaplex and Epres. The mechanism, according to the brand's published materials, uses a combination of ceramides, copper tripeptide-1, moringa, and amla to deliver what they call “sub-micron penetration” into the hair shaft.

The formulation is protein-free and built around ingredients common like moringa and amla, in particular, which have long-standing use in natural-hair routines, and the protein-free formulation matters for the substantial portion of natural-hair consumers who experience protein overload from other treatments. 

However, the brand publishes in-house clinical results (improvements of 123% in hair strength, 129% in damage repair, and 141% reduction in hair porosity), but these come from the brand's own testing rather than from a third party. As I Am has been a respected name in hair care for decades, founded by Dr. Ali N. Syed, who holds 45 patents in the field. But for a reader trying to weigh the strength of the bond-repair claim, the published evidence base is thinner than it is for Olaplex.

What Does Independent Science Actually Say?

If you survey the peer-reviewed literature on this category rather than the brand-funded studies, three patterns emerge.

First, covalent-bond chemistry is real but more limited than marketing implies. The 2024 Cosmetics study is currently the strongest independent comparison of the two best-known products in this category.¹ Olaplex produced measurable increases in covalent S–R bonds, penetrated the cortex, and improved hair brightness and lightness. K18 also improved hair brightness and lightness, but its lipophilic ingredients deposited on the surface of the cuticle rather than penetrating the cortex, and Raman spectroscopy did not detect the internal disulfide-bond signal the brand's marketing implies. The surface conditioning effect K18 produces is real, but it appears to be a different mechanism than what the marketing describes. Brand pictograms of "rebuilt disulfide bridges" for both products are, at best, a simplification. 

Second, most of the bond-repair research has been done on bleached Caucasian hair. The Martins' study, like nearly every other product-efficacy paper in this category, explicitly used bleached hair of European descent as its standard template. That decision is methodologically defensible (it produces a uniform damage baseline), but it has a real downstream consequence: the published data does not reliably predict outcomes for hair types with different cortical density, follicle curvature, or chemical history.⁷¹¹ The one major exception is the Malinauskyte 2021 study, which specifically tested hydrolyzed keratin peptides on chemically relaxed Afro-textured hair and found measurable improvements in tensile strength and breakage resistance.³

Third, most products marketed as “bond” treatments are not actually bond-repair products in the covalent sense. The word “bond” has been adopted across the industry, but the chemistry varies dramatically. Protein-based strengthening can be excellent, especially for hair that has been relaxed, color-treated, or mechanically stressed, but they work by reinforcing the existing protein structure, not by repairing nor forming new bonds.

The Factors That Should Drive Your Decision

Age

Hair changes structurally as we age. Older hair tends to be more porous, has thinner cuticle layers, and shows accumulated lifetime damage from heat, color, and environmental exposure.¹⁰ Consumers with one or two bleach sessions in their history may see dramatic results from a single treatment; while consumers with cumulative damage in their history may need multiple sessions and may benefit more from pairing a covalent-bond product with a regular protein treatment. 

Race and Ethnicity

Studies discussed looked at damage caused by bleach on hair of European descent, but are marketed as repairing bond damage on hair types from people of all descents and from chemical damage other than bleach, like permanent straighteners. The process of permanently straightening hair using alkaline chemicals (which work by deliberately breaking disulfide bonds) are used overwhelmingly by women of African descent. A 2024 review in JAAD Reviews notes that roughly 94% of Black women under 45 in the United States have reported relaxer use.⁴ 

So, a population that may be highly exposed to disulfide-bond damage is also the population most underrepresented in the research that validates the products designed to repair that damage. Most published bond-repair studies, including the 2024 Martins paper, used bleached European-descent hair as the standardized test material.¹⁷ A 2023 review of 246 U.S. dermatology trials found that Black and African American participants were the single most underrepresented racial group in these trials, even after adjusting for population prevalence of the conditions studied.⁸ The Malinauskyte 2021 study that tested protein peptides specifically on relaxed Afro-textured hair is one of the rare exceptions.³

Practically, this means that if you have kinky, coily, or curly hair that has been chemically processed, the published efficacy data for most products on this list were not generated on hair like yours. The peer-reviewed evidence that does exist supports hydrolyzed-protein treatments (Innersense & Curlsmith) for chemically relaxed African-descent hair. Olaplex and K18 use chemistry that should work on any hair type, but the clinical validation is in European-descent samples.

Hair Texture and Pattern

Kinky, coily, and curly hair is structurally more fragile at the bend points of each strand and tends to be lower-density and slower-growing than straighter hair patterns.⁷¹¹ The cuticle layer is generally thinner and more easily lifted, which means products can penetrate more readily, but also that the hair may be more easily over-treated with strong actives. Finer strands, regardless of pattern, can be similarly easier to penetrate than coarse ones which means two people with the same hair pattern can respond differently to the same product based on strand thickness alone. 

Damage Type

If the damage is from bleach, color, relaxer, or other chemical processing, a covalent-bond product (Olaplex or K18) has the strongest mechanistic justification, because the underlying damage is broken disulfide bonds, and the chemistry can address that directly.¹

If the damage is from mechanical breakage or environmental stress alone (without chemical processing) a protein-based strengthener with deep hydration (Innersense, Curlsmith) is often more appropriate. The disulfide network in the cortex is likely largely intact in this scenario; the damage is at the cuticle and the protein surface structure, and that is where protein-based products do their best work.³

If the damage is mixed, this means that chemical processing is on top of heat styling, mechanical stress, or both at once. The two categories are not redundant. They address different structural problems, and the chemistry supports using them together: a covalent-bond treatment to address the cortex, followed by a protein mask in the next wash cycle to address the cuticle, with regular hydration in between. For most readers with a real damage history (color plus heat, relaxer plus heat, multiple stacked services over years), this combination approach is closer to what the hair actually needs than a single product.

If the damage involves active scalp issues (burning, itching, persistent inflammation, scarring at the hairline or crown), no bond-repair product is the right starting place. A dermatologist is.

The answer to “which bond repair product should I use?” depends on what kind of damage you are actually trying to address.

The Bottom Line

“Bond repair” as a category is important, but it is not one thing. Olaplex has peer-reviewed mechanistic support for forming new covalent bonds inside the hair shaft and based on the available research, is the strongest option for chemically processed hair. K18, despite its marketing, appears in independent testing to function more as a sophisticated surface treatment than as a true intracortical bond builder. Living Proof and Epres sit in the covalent tier with newer chemistry. Innersense, Curlsmith, and As I Am, are protein and botanical strengtheners that can genuinely improve hair strength and resilience. 

Before you spend the time and money on a bond repair treatment, ask which kind of damage you have, whether the published evidence reflects hair like yours, and whether the source of damage can be reduced by the treatment you choose. Healthy hair treatments are built on personalization and a good read of what the science says, not on packaging.

References

¹ Martins, E., Castro, P., Ribeiro, A. B., Pereira, C. F., Casanova, F., Vilarinho, R., Moreira, J., & Ramos, Ó. L. (2024). Bleached Hair as Standard Template to Insight the Performance of Commercial Hair Repair Products. Cosmetics, 11(5), 150. doi.org/10.3390/cosmetics11050150

² Breakspear, S., Nöcker, B., & Popescu, C. (2024). Chemical bonds and hair behaviour—A review. International Journal of Cosmetic Science. onlinelibrary.wiley.com/journal/14682494

³ Malinauskyte, E., Shrestha, R., Cornwell, P. A., Gourion-Arsiquaud, S., & Hindley, M. (2021). Penetration of different molecular weight hydrolysed keratins into hair fibres and their effects on the physical properties of textured hair. International Journal of Cosmetic Science, 43(1), 26–36. doi.org/10.1111/ics.12663

⁴ Tisdale, B., et al. (2024). Safety of chemical hair relaxers: A review article. JAAD Reviews. jaadreviews.org/article/S2950-1989(24)00033-3/fulltext

⁵ Chang, C.-J., O’Brien, K. M., Keil, A. P., Gaston, S. A., Jackson, C. L., Sandler, D. P., & White, A. J. (2022). Use of Straighteners and Other Hair Products and Incident Uterine Cancer. JNCI: Journal of the National Cancer Institute, 114(12), 1636–1645. doi.org/10.1093/jnci/djac165

⁶ Stenn, K. S., & Paus, R. (2001). Controls of Hair Follicle Cycling. Physiological Reviews, 81(1), 449–494. doi.org/10.1152/physrev.2001.81.1.449

⁷ Maguire, A. K., et al. (2021). Hair Aging in Different Races and Ethnicities. Journal of Clinical and Aesthetic Dermatology. jcadonline.com/hair-aging-races-ethnicities-review

⁸ Charrow, A., Xia, F. D., Joyce, C., & Mostaghimi, A. (2023). Racial and ethnic underrepresentation in dermatology clinical trials. Journal of the American Academy of Dermatology. pubmed.ncbi.nlm.nih.gov/37062462

⁹ Lebari, D., et al. (2024). The Genomic Variation in Textured Hair: Implications in Developing a Holistic Hair Care Routine. Cosmetics, 11(6), 183. doi.org/10.3390/cosmetics11060183

¹⁰ Villani, A., Fabbrocini, G., Cacciapuoti, S., & Verde, L. (2022). Hair Aging and Hair Disorders in Elderly Patients. International Journal of Trichology, 14(6), 191–196. doi.org/10.4103/ijt.ijt_72_22

¹¹ Westgate, G. E., Botchkareva, N. V., & Tobin, D. J. (2017). The biology and genetics of curly hair. Experimental Dermatology, 26(6), 483–490. doi.org/10.1111/exd.13347

¹² As I Am Naturally. (n.d.). Bond Hair Mask: Product Information and Ingredients [Manufacturer source]. asiamnaturally.com/products/bond-repair-hair-mask 

¹³ K18 Hair. (n.d.). Wait, what's a bond? And what does molecular mean? K18 Hair. Retrieved June 25, 2026, from https://www.k18hair.com/blogs/consumer/wait-whats-a-bond-and-what-does-molecular-mean