Patented Cobra Cuffs are made with Cross-Linked Rubber. Why Rubber? The rubber helps prevent the stress cracking common with nylon zip ties when exposed to cold, heat, or creasing. Superior shelf life is an added benefit with Cobra Cuffs (3-5 yrs, depending upon storage conditions). The Super Tough Rubber Based Polymer used in Cobra Cuffs absorbs the shock and stress during a struggle-making the 250-300 lb Tensile Strength is more than sufficient to contain the most violent prisoners. Independent tensile testing for Cobra Cuffs is conducted with locked straps loops. Many zip tie manufacturers only report the tensile strength of the strap alone, which does not include the locking mechanism or ratchet, typically the weak link.