Table 7.

Studies about prevention of structural damage during the use of hair straighteners.

Author	Study type	Population	Hair straightener	Effects on hairs
De Sá Dias et al.,40 2008.	Clinical trial	Afro hair samples	Ammonia thioglycolate	Conditioning agents added to the straightener resulted in less protein loss, in fiber protection and in increased strength to breakage. However, it may reduce the effectiveness of straightening.
Bernard et al.,41 2002.	Clinical trial	African-American hair samples	Guanidine hydroxide or sodium hydroxide	The use of ceramides (in this case, C18-dhCer) prevented damage to straightened hair.
Vermeulen et al.,42 2004.	Clinical trial	Hair samples	Sodium hydroxide, lithium hydroxide	Addition of polymethylene waxes to a straightening formula resulted in a better appearance compared to commercial formulas, without minimizing performance.