In creating a sleeve for Thinkbaby bottles, we looked carefully at the solutions on the market. We wanted a material that would provide maximum insulation and protection. Our investigation took us to our friends at Patagonia. Patagonia largely pioneered the use of Limestone based neoprene to replace traditional neoprene for their wetsuits. Thinkbaby's sleeves are made from Limestone. They feel traditional neoprene. We have made them in 4 colours. They will not only protect the bottles, but will provide significant insulation. Please note that we made them fit tight, so they cannot be easily removed by a toddler.