Flexi-Kold® Cold Packs are built for durability. The nylon cover makes it better because unlike vinyl or polyurethane cold packs, they will not crack or dry out. Flexi-Kold® packs are constructed of extra-thick, flexible nylon material that is double sealed to prevent leaking. This flexibility allows the packs to maintain complete contact with the treated area and can completely wrap around a 360º area. The advanced gel formula helps the pack stay colder for longer periods of time compared to those using bentonite or glycol formula.