In creating a sleeve for Thinkbaby bottles, they looked carefully at the solutions on the market. They wanted a material that would provide maximum insulation and protection. But...they also wanted to see how eco-friendly that we could make the sleeve. Thinkbaby's sleeves are made from Limestone. They feel just like traditional neoprene. They will not only protect the bottles, but will provide significant insulation. Please note that they are made to fit tight, so they cannot be easily removed by a toddler.