Reduced size and weight with increased capacity and longevity Merino wool helps wick moisture away from feet to promote warmth, and a Surround Heating Element envelopes the toes Bluetooth app controls the 4400 mAh Lithium-Ion Battery Pack from any iOS or Android and offers a category-best 18 hours of constant heating power The safe, intelligent Global Recharger makes it easy to stay warm wherever one travels Compression: soft – PFI 50, 10 – 15 mmHg Material: 74 % nylon, 20 % lycra / spandex, 6 % merino Machine wash warm, air dry The Heat Sock Set XLP 2P BT Surround Comfort is the pinnacle in warmth, performance, and longevity. Combining a pair of HOTRONIC XLP 2P BT Battery Packs with the Soft Compression and Surround Heating Element of the Surround Comfort sock, and then allowing control via a Bluetooth App, HOTRONIC raises the bar with convenient Constant Heating Power when needed most - in the cold! Socks integrated with HOTRONIC‘s heating elements, and 1 Recharger.