Adhesive: the chemistry of the bond, and the eye beside it

The molecule

Why a cyanoacrylate grabs in seconds

Lash glue belongs to the family of instant glues. When the glues used by two Finnish lash technicians were analysed, ethyl cyanoacrylate was the main ingredient, and Canada's rules name ethyl, methyl and isopropyl cyanoacrylate as the lash-glue ingredients it regulates.¹⁹²⁹

Each small molecule carries two groups that pull hard on electrons, a nitrile and an ester, on the same carbon. That leaves the far end of its double bond exposed. A trace of water or a weak base is enough to attack it, and the thin film of moisture on almost any surface supplies one.¹

Each addition leaves a negative charge on the end of the chain, shared over those two groups, so the end stays alive and takes the next molecule. The chains grow to between 100,000 and 10 million in molecular weight. Measured in solution, the growth rate is close to a million, against 775 for the acrylic of nail powders: about a thousand times faster.¹

Nothing inside the glue stops the chain; small amounts of water, oxygen or carbon dioxide do not. What keeps it liquid in the bottle is a few parts per million of strong acid, plus inhibitors such as hydroquinone. Canada caps hydroquinone at 0.1 percent in lash and brow adhesives.¹²⁹

The family

Longer side chains: softer, less vapour, the same sting

Cyanoacrylates differ by the side chain on the ester. Methyl and ethyl are the industrial and household instant glues; n-butyl and 2-octyl are the skin and surgical glues; methoxyethyl and ethoxyethyl are sold as low-odour.¹

Cured ethyl glue is a rigid glass up to about 150 degrees. Longer, bulkier chains give softer polymer: 2-octyl softens at about 10 degrees, so it bends with skin. Heated past that point, ethyl glue starts to unzip back into its liquid ingredient.¹

Less vapour is a real gain: the alkoxy glues evaporate less and bloom less. But in mice, the vapour concentration that halved the breathing rate, the standard test of irritation, was 0.7 parts per million for ethyl and 1.0 for methoxyethyl. The authors called the four glues they tested similar in irritant strength.¹²⁵

The eye

What eye clinics actually see

The largest series comes from Japanese eye clinics: 107 women seen between 2007 and 2010 for problems after extensions. Most had glue or remover reach the eye; the next largest group had an allergy of the lids to the glue. All recovered with drops or ointment.¹¹

A 2019 review counted lid allergy as 79 percent of the complications reported in medical journals, though most of those cases come from that one series.¹²

Surveys of students give a broader picture with less certainty. In two Nigerian surveys itching was the most common complaint. In Ghana, 208 wearers examined at the slit lamp reported watery eyes more often than 208 non-wearers, 38.5 against 21.6 percent, and showed more signs of surface disease. That is an association, not proof of cause.¹³¹⁴¹⁵

Followed after a first set, 32 people mostly felt something in the eye at one hour; the tear film was least stable at one week, and symptoms were back to where they started by a month. In people with dry eye, taking extensions off improved symptoms and tear stability over four weeks.¹⁶¹⁷

The glue is not the only chemical in the chair. Under-eye gel pads have been found to contain a preservative known to cause allergy, and a misplaced remover gel once caused a chemical injury in an eye that had had laser surgery.¹²¹⁸

Testing

What a patch test can and cannot tell you

A dermatologist's patch test includes ethyl cyanoacrylate in the standard North American series, tested at 10 percent. It has known blind spots.²¹

Test material can cure in the syringe and read falsely negative. In one shoemaker with glue allergy, glue dissolved in acetone in aluminium test chambers gave false negatives where other chambers gave positives. Medical 2-octyl glue does not cross-react with ethyl glue, so the product itself has to be tested. And because acrylates are strong sensitisers, testing with them can itself cause allergy.²¹²⁰²³

Medical glue is not exempt. A 2026 analysis of 25,442 surgical patients found allergy to 2-octyl glue in about 4 percent, above 20 percent on a second exposure in some groups, and earlier exposure to cosmetic acrylates was a risk factor.³⁸

A salon patch test, a few extensions placed days before a first set, is a different thing. It is widely used. The research for this chapter found no study of how well it predicts a reaction.