Bottles are made of PPSU, a lightweight FDA-grade material known for its high heat resistance, similar to glass. Simply Press-to-Close, Twist-to-Open™ (PCTO™) to secure and remove the World s first patented no screw-thread closure lid. Hegen’s signature Sqround™ shape allows for interchangeable lids, making it a breeze to pump, store & feed breastmilk in a single bottle with no transfer needed. Plus they are stackable & easy to clean!