These were used at a maternity clinic. They were used for about a month. Pigeon Breastfeeding Realism Direct-Attach Baby Bottle 100ml (for general newborns) Yellow, set of 2 These are made of heat-resistant glass, which is strong and easy to clean, and can be sterilized by boiling or chemical solution. The nipples are designed based on research on breastfeeding animals, allowing babies to feed with a sensation similar to breastfeeding. Regardless of the sterilization time or number of uses, please replace the nipple if you notice any tears, cuts, or deterioration.