The Castelli Nano Flex 3G Armwarmer. Choose Nano Flex for warmers that are great in dry conditions but also give some extra protection and warmth in wet conditions. Castelli invented the category of water-protective thermal fabrics and Castelli’s proprietary Nano Flex is often imitated but never equaled. No compromises for any conditions. Water-repellent thermal and breathable Nano Flex fabric. Flat-lock stitching with single seam. Double-sided silicone grippers to keep warmers in place. 8deg-20degC 46deg-68degF. Weight 66 g.