Patent ID: 7316815

Claim:
A method for strengthening keratin fibers, comprising applying to said keratin fibers a cosmetic composition comprising at least one polyamino acid of formula (I): wherein X is chosen from —O—, —S— and —NR 5 —; R 1 is wherein m is 1 to 5; R 2 is chosen from a hydrogen atom, C 1 -C 8 alkyl groups, a —CH 2 C 6 H 5 group, a —CH 2 C 6 H 5 pOH group, wherein pOH is para-OH, a —CH 2 OH group, a —CHOH—CH 3 group and —(CH 2 ) t —NH 2 groups; wherein t is chosen from 3, 4 and 5; R 3 and R 5 which may be identical or different, are each chosen from a hydrogen atom and C 1 -C 6 alkyl groups; and n is an average number greater than 1 such that the molecular weight of the polyamino acid of formula (I) ranges from 200 to 200,000, wherein the n repeating units may be identical or different; or a salt of said polyamino acid.