Polyester insert with super thin embedded brass coils connects by flexible wires to two AA batteries. The batteries fit in a small pouch and attach to your ankle or calf with a Velcro strap. Provides immediate relief in seconds by heating to a maximum 98 degrees by way of a safe shock proof and low voltage: 3 v, 2 w. Portable inserts go anywhere, unnoticeable in any shoe, boot, or slipper. Move as much as you’d like, you’re not tied to an electrical outlet. Battery powered so it doesn’t lose its effectiveness like microwavable or reactive chemical heating elements. Velcro is a registered trademark of Velcro Industries B.V.).