1.durable plastic with steel axle 2.no any battery, powered by rotating the wrist 3.Perfect exercise gadget for sports NEW Fitness LED Wrist Strengthener workout Ball,Power Grip Ball,Power Ball,POWERBALL It is a application by centrifugal force & inertia like a gyroscapeMaterial: durable plastic with steel axle Function:1.Perfect exercise gadget for sports like golf, tennis, squash, cricket, ...