Dyson Chitosan Post-style serum is a lightweight post-styling hair serum that provides flexible, all day hold for smooth, shiny, frizz-free hair. Formulated for all hair types, this lightweight serum simultaneously Nourishes individual hairs, while locking their shapes with a flexible bond structure without compromising your hair s natural feel. For use on dry and styled hair to define style. vs. untreated.²vs. untreated hair. Tested at relative 80% humidity..