Impact of acid (”progressive brush”) and alkaline straightening on the hair fiber: Differential effects on the cuticle and cortex properties

Glyoxylic acid has emerged as a safe alternative to formol (formaldehyde) use as a hair straightener/relaxer. However, the possible damage to the hair fiber after its application is low known and/or published in the literature.This work aims to characterize hair locks treated with glyoxylic acid compared to traditional alkaline straighteners such as sodium and guanidine hydroxide and ammonium thioglycolate.The morphology of the hair cuticles was observed by scanning electron microscopy. Protein loss was assessed by the Lowry method modified by Peterson and as mechanical properties that were expressed in terms of tensile strength.All products (sodium and guanidine hydroxides and ammonium thioglycolate) caused protein loss of about 2.5 μg/g, except glyoxylic acid that caused the worst damage (3.5 μg/g), in relation to the untreated (virgin) hair (1.12 μg/g), indicating that the chemical treatments can cause hair damage in both cuticles and cortex. The force to break the fibers treated with traditional straighteners based on sodium hydroxide, guanidine hydroxide, and ammonium thioglycolate was statistically the same.The treatment with glyoxylic acid showed rupture tensile statistically equivalent to the alkaline straighteners. The mechanism of action of glyoxylic acid does not appear to be based on breaking and rearrangement of disulfide bridges, but altered them, that influenced the hair strength. However, it is also essential to consider other factors relevant: technical application technique, reaction time, and interval of reapplication of the product, as this can change the pattern of the results obtained.