Double Wall Vacuum Insulated. Keeps Hot for 24 hours, Cool for 12. Unlike other leading brands, [proof] products are built using super high-grade MEDICAL GRADE STEEL. This means not only superior corrosion resistance, but products that will last longer. Our products are also insulated using our own EcoProof(TM) technology, a non-toxic insulation technique that uses safe materials through our ENTIRE production process. No copper coatings, contaminants or other hidden insulating elements here and of course everything is BPA FREE. [proof] we care about your future.