Hair Bonds: The Three Types, What Breaks Them, and What Bond Repair Does

What are hair bonds?

Hair bonds are the chemical connections that hold a strand's proteins in place, along each protein chain and between neighboring chains. They decide how strong a hair is, how far it stretches and which shape it keeps.

Hair is mostly keratin. Under the cuticle, the overlapping outer scales, lies the cortex, which carries most of the load when hair is pulled. Its keratin filaments sit in a matrix of smaller proteins rich in cysteine, a sulfur-carrying amino acid; in some of them, about one building block in three is cysteine (Robbins 2012).

Picture a rope ladder. The rails are protein chains. A few rungs are welded in place: the disulfide bonds. Many more are tied with thread that water, heat and pH loosen and retie: the hydrogen and salt bonds. Bleach, perms and relaxers do much of their damage by breaking the welds.

What are the three types of hair bonds?

The three types of hair bonds are disulfide bonds, hydrogen bonds and salt bonds, also called ionic bonds. Cosmetology courses call them side bonds, because they link neighboring keratin chains side by side. They differ in strength, in number and in what breaks them.

Disulfide bonds in hair

A disulfide bond forms when the sulfur atoms of two cysteines join, a pair chemists call cystine. The bond is covalent, so water at everyday temperatures does not break it (Robbins 2012). It ties keratin chains together, and it is the bond a perm cuts and re-forms on purpose.

Its loss shows most in wet hair. In one person's frosted (heavily bleached) hair, with 48% of the disulfide bonds lost, the force needed to stretch a wet hair fell by close to 60%, while dry it fell by less than 10% (Robbins 2012). That is why heavily lightened hair can feel acceptable dry and stretch like chewing gum when wet; see gummy hair.

Hydrogen bonds in hair

A hydrogen bond is a weak attraction between a hydrogen atom and a nearby oxygen or nitrogen atom, roughly a tenth as strong as a covalent bond. Hair holds far more of them than disulfide bonds (Robbins 2012). Water breaks them. As wet hair dries in a new shape, they re-form in that shape, which is how a blow-dry holds, and moisture moving in or out breaks them again.

What undoes a set is a change in humidity, not humidity alone. In one lab test, single hairs set at 60% humidity kept 73.5% of their curl after two hours at that humidity, but only 61.3% after two hours in dry air. Damage adds a second effect, by the textbook explanation: once bleach or a perm has cut disulfide bonds, water reaches hydrogen bonds it could not reach before, so damaged hair loses even more strength wet (Robbins 2012).

Salt bonds in hair

Salt bonds form between negatively charged acidic groups and positively charged basic groups on keratin (Robbins 2012). pH decides which groups carry a charge, so pH decides how many salt bonds hold. Hair swells least between about pH 4 and 9 and sharply above pH 10, the range of relaxers and lighteners (Robbins 2012).

Their strength is the hardest to pin down. In water, the pull between two charges is partly cancelled by the energy it takes to strip away the water around each one. A University of Zurich review calls precise energies for salt bridges, as chemists call them, "very difficult" to determine (Bosshard 2004). Bleach also redraws the charge map. Its cysteic acid is strongly negative, so bleached hair binds more conditioner and more tap-water metals (Robbins 2012; see is bleached hair high porosity?).

Are there only three types of hair bonds?

No. "Three types" is salon and dermatology shorthand (Draelos 2025). Textbooks add peptide bonds, which form the chain itself and which strong alkali can cut, isopeptide crosslinks in the cuticle, and van der Waals and hydrophobic attractions, which are weaker still (Robbins 2012; Daniels 2016). The common claim that each provides "one-third of hair's strength" comes from cosmetology training; we found no measurement behind it.

What breaks hair bonds?

Water, humidity and everyday styling heat break hydrogen bonds, and those re-form. Disulfide bonds are broken by chemistry, and some of those breaks never close.

• Bleach turns disulfide bonds into cysteic acid, a chemical dead end (Robbins 2012; Daniels 2016). Normal bleaching breaks about 15 to 25% of them; frosting, or lifting black hair to light blond, up to about 45%. In one person's frosted hair, cystine fell by half and cysteic acid rose about 24-fold (Robbins 2012). See how to repair bleach-damaged hair.
• Perms and reductive straighteners break about 20% of disulfide bonds on purpose, reshape the hair, then re-form most of them with a neutralizer. In data reported by the hair chemist Clarence Robbins, tensile strength ended about 5 to 20% lower after a home perm (Robbins 2012).
• Alkaline relaxers convert up to about a third of hair's cystine into lanthionine, a single-sulfur link that cannot return to a disulfide, and they cut some peptide bonds. That is part of why the result lasts, and why a standard textbook ranks them the most damaging straighteners (Robbins 2012; Gavazzoni Dias 2015).
• Heat from dryers and irons resets hydrogen bonds. Very high heat goes deeper: in one lab study, repeated passes of a 235 °C (455 °F) flat iron weakened the disulfide signal (Essendoubi 2022), and steam can cleave these bonds (Robbins 2012). Ironing damp hair is the clearest avoidable risk; more in hair breakage.
• Sunlight oxidizes cystine too, one reason the ends of long hair hold more cysteic acid than the roots (Robbins 2012).

What does bond repair do for hair?

"Bond repair" means a product aims to make damaged hair stronger and less likely to break by acting on its internal bonds. The phrase names a goal; it does not say which chemistry. Products pursue the goal by at least four routes (Draelos 2025; TRI Princeton 2024).

1. Bridging with acids. Di-acids such as maleic or succinic acid are proposed to link neighboring keratin chains through salt and hydrogen bonds. A trade-journal article, not peer-reviewed, reported strength, grooming and calorimetry (heat-flow) tests in support (Förster 2018).
2. Reacting with sulfur groups. Patents describe maleic-acid-based ingredients reacting with sulfur-hydrogen (thiol) groups to reconstruct the crosslinks (Di Foggia 2021). In bleached hair, though, most broken disulfide bonds end as cysteic acid, and remaining thiols are likely to be oxidized during bleaching, which leaves little for such a reaction (Robbins 2012; TRI Princeton 2024).
3. Adding peptides. Small keratin fragments penetrate damaged hair and bind through salt and hydrogen bonds. In lab studies they reached deep into the cortex (Malinauskyte 2021) and raised wet breaking force (Daniels 2016).
4. Building new covalent crosslinks. Some chemistries form fresh bridges between protein chains rather than restoring the old ones. In one industry study, a plant polyphenol with a copper catalyst crosslinked bleached hair until it swelled less than untreated hair in a harsh test (Yoshida 2023).

Conditioning improves slip and feel, a different thing from strength: in one peptide study, the conditioner base alone did not make bleached hair significantly stronger (Daniels 2016).

Does bond repair work?

Some treatments measurably change how damaged hair behaves. A dermatologist reviewing the category put the limit plainly: "None of these methods can recreate the strength of virgin, newly grown, unprocessed hair" (Draelos 2025).

Independent evidence is thin and mixed: one university study found a maleate-based treatment left on bleached hair raised the breaking strength of single fibers tested dry (Martins 2024), while another found no increase in disulfide bonds in the cortex after maleate-type treatments (Di Foggia 2021). The first, from Portugal, used hair bleached once and did not test wet strength or durability through washing. The second, from the University of Bologna, measured sulfur bonds by spectroscopy. It could not rule out an increase in the cuticle and found no direct evidence of the crosslinking reaction the patents describe, although treated hair looked more regular under the electron microscope.

Two studies do not settle the question, but the second shows why a smoother photograph is not a stronger fiber: electron microscopy "cannot prove that a bond-builder has strengthened hair" (TRI Princeton 2024). Good evidence means strength or fatigue tests on single fibers, wet and dry, against an untreated control (see hair tensile testing). And because the hair shaft is not alive, any treatment works only while it remains in or on the fiber.

How do I know if my hair needs bond repair?

Your hair's history is the best guide. A wet stretch test on a single hair then gives a rough sense of how much has been lost, because wet strength tracks disulfide bonds.

1. Check the history of your lengths

Look at your lengths, not your roots. Have they been lightened, even once? Relaxed or permed? Flat-ironed or curled several times a week? Are the ends several years old? Each yes makes damage likely there; for lightening, relaxing and perming, that means disulfide loss. New growth near the scalp is your comparison.

2. Stretch one wet hair

Soak one shed hair in lukewarm water for five minutes. Stretch it gently between your fingertips, first near the root end, which is the newest growth, then along the lengths. Undamaged wet hair can stretch by about 30% of its length without harm (Gavazzoni Dias 2015). Hair that stretches far and stays long, or feels gummy, has probably lost many disulfide bonds, and hair that snaps at once may have little reserve left. The full method, and what each result means, is in our gummy hair guide. It is a rough check, not a validated measurement.

3. Compare wet with dry

Hair that feels acceptable dry but limp or stretchy when wet shows the pattern of disulfide loss (Robbins 2012).

One hair is a sample of one. It tells you how much wet strength that fiber has kept, but nothing about how the rest of your head compares or what to change first. The free hair consultation starts from the other end, with what your hair has been through: about a minute of questions on color, heat, washing and how your hair behaves. It scores porosity and damage, asks for your email, and then shows a Damage Profile with a routine matched to it.

Take the free hair consultation

How do you repair damaged hair bonds?

You cannot bring back bonds that bleach has turned into cysteic acid. You can stop adding damage, protect hair while it is wet or hot, and choose any strengthening treatment by its evidence. The order matters, because the first steps protect every length, treated or not.

1. Space out chemical services. Hair that is already bleached or permed reacts faster with perm chemicals (Robbins 2012), so avoid stacking bleach and a relaxer or perm on the same lengths.
2. Be gentle while hair is wet, when damaged hair is weakest. Detangle with conditioner in, starting at the ends.
3. Let hair dry before heat, and turn the heat down. Water lowers the temperature at which keratin's ordered structure melts, from about 205 °C (401 °F) in dry hair to about 155 °C (311 °F) in damp hair (Cao and Leroy 2005).
4. Condition every wash. Conditioners bind most strongly to damaged hair and cut friction, but they partly wash out and do not restore strength alone (Robbins 2012; Daniels 2016). More in high porosity hair.
5. Choose a strengthening treatment by its evidence, and use it consistently: benefits last only while it stays in the fiber.
6. Trim what cannot be saved. A split end cannot be fused back into one fiber, and trimming is the only way to remove it. See split ends.

A note on ANATOMY

Think of the two halves of a buckle, shaped for each other and joined in one movement. Click chemistry, the reaction class recognized by the 2022 Nobel Prize in Chemistry, joins matched molecular parts in a similarly efficient way (Walter 2022). ANATOMY is built on click chemistry. It is patented and made in Switzerland, and it was independently tested at SGS proderm in Germany, on bleached hair. None of the studies cited above tested ANATOMY. The Complete Reconstruction System has three steps, and the Leave-In is the one that stays on the hair.

Where should you start?

Two things decide what your hair needs: which bonds it has lost, and what keeps breaking them. The free hair consultation asks about the second in about a minute, then requests your email before it returns your Damage Profile.

Frequently asked questions

Which hair bond is the strongest?

The disulfide bond. It is covalent, roughly ten times stronger than a hydrogen bond (Robbins 2012). Salt bonds are hard to rank because their strength depends on water and pH. Hydrogen bonds are the weakest of the three individually, and the most numerous.

Can hair bonds repair themselves?

Hydrogen and salt bonds re-form on their own as hair dries or returns to its usual pH. Disulfide bonds lost to bleach do not. They become cysteic acid, and the hair shaft is not alive, so it cannot rebuild them. New growth arrives intact.

Does water damage hair bonds?

Water breaks hydrogen bonds temporarily, which is how a wet set works, and they re-form as hair dries. At everyday temperatures it does not break disulfide bonds; steam can. The real risk is mechanical: hair that has lost disulfide bonds is weakest wet, so detangle it gently.

Is a protein treatment the same as bond repair?

Not quite. Protein and peptide treatments add small protein fragments that bind to hair through salt and hydrogen bonds, and some raise wet strength in lab tests (Daniels 2016). Bond repair aims at the crosslinks between chains, though a label alone will not tell you which a product does.

Can bond repair fix split ends?

No. A split end cannot be fused back into one fiber, and trimming is the only way to remove it. Treatments can at most hold split ends together for a while and help reduce further splitting.

Which bond repair treatment is best?

The one with evidence for the result you want. Ask which bond it acts on, by what chemistry, and whether strength was measured on single wet fibers against untreated hair, by someone independent. Expect any benefit to last only while the treatment stays in the hair.

Related reading

Read next