"How do you get your curl pattern back after straightening your hair?"
It would be beneficial to understand the make up of the hair strand, so that we can see the effects of heat on hair. The hair is made up of the cuticle, cortex and medulla (fine hair may not have a medulla, coarse hair most likely has a medulla.)
The diagram below shows the make up of a single strand.
The cuticle which is the outermost part of the strand protects the cortex from damage. When heat is applied to the hair, the cuticle can be (will discuss in a later post so stay tune) completely removed resulting in heat damage. Once the cuticle layer is damaged, it cannot be repaired. Cuticle damage is most evident in split ends. When the ends of the hair split, the hair takes the form of a "Y". Split ends expose the cortex and the only way to prevent further damage to the strand is to cut off the split ends.
The cortex is made of keratin-which is a protein. When heat is applied to a protein the structure changes through a process called denaturation. For example, when an egg (protein) is scrambled using heat, it cannot return to its original form. The same rules apply to the protein found in hair.
Heat strips away the cuticle that protects the cortex then reforms the cortex by breaking the bonds that make the hair strands curl. Once the characteristics of the cortex are changed, it cannot be undone. Therefore, if you shampoo or wet your hair after it was straightened by a heat styling tool and your hair does not return to its original curl pattern then your hair is permanently heat damaged.