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. But...we also wanted to see how eco-friendly that we could make the sleeve. 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 just like traditional neoprene. We have made them in 4 colors. They will not only protect the bottles, but will provide significant insulation.