Praise be to Allah
Keratin is a substance of natural origin that is found in high amounts in the hair, but they produce it chemically and, according to their claim, it can be used to treat brittle and curly hair.
Yuko involves restructuring curly hair; during this process the hair is exposed to high heat, up to 180 degrees! along with chemical substances.
But both treatments, according to their advertisements, straighten and smooth the hair.
It does not seem to us that these two treatments prevent water from reaching the roots of the hair, or apply an impermeable layer to the hair. Based on that, wudoo’ and ghusl will be valid for the one who uses either treatment on her hair. If there is a period during which the obligatory ghusl may be required but she is not allowed to get her hair wet, then the woman should refuse this treatment, unless that is at the time of her menses. This refusal would not be because of wudoo’ but because of ghusl – because in ghusl it is stipulated that the hair must be wetted with water which should be made to reach the roots of the hair, unlike wudoo’. The presence of an impermeable layer on the hair does not mean that wudoo’ is invalid, because the requirement of wiping the head in wudoo’ is not strict, and what is required is to wipe the head only, not to wash it. Hence it is permissible to wipe over the head cover or turban, and to wipe over the hijab for women. The Prophet (blessings and peace of Allah be upon him) stuck his hair together during Hajj (a process called talbeed - whereby the pilgrim uses some sticky substance to stick his or her hair together in order to keep it orderly and protect against lice), which creates a layer over the hair. This ruling does not appy to other parts of the body that are washed during wudoo’.
Talbeed means sticking the hair together with gum or resin and the like, which holds the hair together and keeps it from getting dirty or dusty.
See also the answer to question no. 39439.
There is no reason why a substance should not be used to straighten the hair, so long as it will not prevent one from doing ghusl as required in sharee‘ah. But it is essential to pay attention to avoiding extravagance and resembling disbelieving women, and to beware of using anything that may be harmful, even if the harm it may cause may not be obvious immediately.
And Allah knows best.